Science of Synthesis Knowledge Updates 2013 Vol. 3 E-Book

• Herausgeber: Thieme
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 965

Veröffentlichungsjahr: 2014

Beschreibung

Science of Synthesis is a reference work for preparative methods in synthetic chemistry. Its product-based classification system enables chemists to easily find solutions to their synthetic problems.

Key Features:

• Critical selection of reliable synthetic methods, saving the researcher the time required to find procedures in the primary literature.
• Expertise provided by leading chemists.
• Detailed experimental procedures.
• The information is highly organized in a logical format to allow easy access to the relevant information.

The Science of Synthesis Editorial Board, together with the volume editors and authors, is constantly reviewing the whole field of synthetic organic chemistry as presented in Science of Synthesis and evaluating significant developments in synthetic methodology. Four annual volumes updating content across all categories ensure that you always have access to state-of-the-art synthetic methodology.

Leseprobe

Science of Synthesis

Science of Synthesis is the authoritative and comprehensive reference work for the entire field of organic and organometallic synthesis.

Science of Synthesis presents the important synthetic methods for all classes of compounds and includes:

Methods critically evaluated by leading scientists

Background information and detailed experimental procedures

Schemes and tables which illustrate the reaction scope

Preface

As the pace and breadth of research intensifies, organic synthesis is playing an increasingly central role in the discovery process within all imaginable areas of science: from pharmaceuticals, agrochemicals, and materials science to areas of biology and physics, the most impactful investigations are becoming more and more molecular. As an enabling science, synthetic organic chemistry is uniquely poised to provide access to compounds with exciting and valuable new properties. Organic molecules of extreme complexity can, given expert knowledge, be prepared with exquisite efficiency and selectivity, allowing virtually any phenomenon to be probed at levels never before imagined. With ready access to materials of remarkable structural diversity, critical studies can be conducted that reveal the intimate workings of chemical, biological, or physical processes with stunning detail.

The sheer variety of chemical structural space required for these investigations and the design elements necessary to assemble molecular targets of increasing intricacy place extraordinary demands on the individual synthetic methods used. They must be robust and provide reliably high yields on both small and large scales, have broad applicability, and exhibit high selectivity. Increasingly, synthetic approaches to organic molecules must take into account environmental sustainability. Thus, atom economy and the overall environmental impact of the transformations are taking on increased importance.

The need to provide a dependable source of information on evaluated synthetic methods in organic chemistry embracing these characteristics was first acknowledged over 100 years ago, when the highly regarded reference source Houben–Weyl Methoden der Organischen Chemie was first introduced. Recognizing the necessity to provide a modernized, comprehensive, and critical assessment of synthetic organic chemistry, in 2000 Thieme launched Science of Synthesis, Houben–Weyl Methods of Molecular Transformations. This effort, assembled by almost 1000 leading experts from both industry and academia, provides a balanced and critical analysis of the entire literature from the early 1800s until the year of publication. The accompanying online version of Science of Synthesis provides text, structure, substructure, and reaction searching capabilities by a powerful, yet easy-to-use, intuitive interface.

From 2010 onward, Science of Synthesis is being updated quarterly with high-quality content via Science of Synthesis Knowledge Updates. The goal of the Science of Synthesis Knowledge Updates is to provide a continuous review of the field of synthetic organic chemistry, with an eye toward evaluating and analyzing significant new developments in synthetic methods. A list of stringent criteria for inclusion of each synthetic transformation ensures that only the best and most reliable synthetic methods are incorporated. These efforts guarantee that Science of Synthesis will continue to be the most up-to-date electronic database available for the documentation of validated synthetic methods.

Also from 2010, Science of Synthesis includes the Science of Synthesis Reference Library, comprising volumes covering special topics of organic chemistry in a modular fashion, with six main classifications: (1) Classical, (2) Advances, (3) Transformations, (4) Applications, (5) Structures, and (6) Techniques. Titles will include Stereoselective Synthesis, Water in Organic Synthesis, and Asymmetric Organocatalysis, among others. With expertevaluated content focusing on subjects of particular current interest, the Science of Synthesis Reference Library complements the Science of Synthesis Knowledge Updates, to make Science of Synthesis the complete information source for the modern synthetic chemist.

The overarching goal of the Science of Synthesis Editorial Board is to make the suite of Science of Synthesis resources the first and foremost focal point for critically evaluated information on chemical transformations for those individuals involved in the design and construction of organic molecules.

Throughout the years, the chemical community has benefited tremendously from the outstanding contribution of hundreds of highly dedicated expert authors who have devoted their energies and intellectual capital to these projects. We thank all of these individuals for the heroic efforts they have made throughout the entire publication process to make Science of Synthesis a reference work of the highest integrity and quality.

The Editorial Board

July 2010

E. M. Carreira (Zurich, Switzerland)

C. P. Decicco (Princeton, USA)

A. Fuerstner (Muelheim, Germany)

G. A. Molander (Philadelphia, USA)

P. J. Reider (Princeton, USA)

E. Schaumann (Clausthal-Zellerfeld, Germany)

M. Shibasaki (Tokyo, Japan)

E. J. Thomas (Manchester, UK)

B. M. Trost (Stanford, USA)

Abstracts

4.4.43 Product Subclass 43: Silylium Ions and Stabilized Silylium Ions

T. Müller

This chapter describes methods for the synthesis of silylium ions and silylium ions stabilized by direct interaction with solvents or counteranions. The applications of these species in Lewis acid catalysis and in bond-activation processes are also summarized.

Keywords: C—F bond activation · borates · Brønsted acids · carbocations · C—Si bonds · hydrosilylation · Lewis acid catalysts · onium ions · silanes · silicon compounds · silyl cations · solvent effects

4.4.44 Product Subclass 44: Silyl Radicals

Y. Landais

Silyl radicals are short-lived species that have found widespread use in various areas, including organic and polymer chemistry and, more recently, material science. These silicon-centered radicals are generated from various sources, including silyl hydrides, disilanes, allylsilanes, silyl halides, and silylenes, and by carbon—heteroatom bond cleavage. Silyl radicals are intermediates in important transformations such as hydrosilylation and reduction processes. They add to unsaturated systems (including alkenes, alkynes, arenes, and carbonyl derivatives) with high rate constants, generating carbon-centered radicals which are then involved in subsequent transformations. The understanding of steric and electronic properties of silyl radicals now allows a better prediction of their reactivity. Silyl radical precursors, such as silyl hydrides, are thus commonly used in the synthesis of complex targets including natural products. These radicals efficiently trigger complex radical cascades as well as rearrangements processes, opening an access to elaborate architectures that would be otherwise difficult to access. Finally, silyl radicals are key intermediates in the functionalization of silicon surfaces, which have recently received a lot of interest due to the importance of organic films for applications as biomaterials and biochips.

Keywords: radicals · silyl hydrides · abstraction · disilanes · allylsilanes · hydrogen transfer · homolytic substitution · silylenes · silyliums · polarity-reversal catalysis

4.4.45 Product Subclass 45: Silanecarboxylic Acids and Esters

K. Igawa and K. Tomooka

Silanecarboxylic acids having a carboxy group on the silicon atom are synthesized from chlorosilanes via their reductive lithiation and subsequent carboxylation with carbon dioxide. Silanecarboxylic acid esters are synthesized from silanecarboxylic acids by O-alkylation with diazoalkanes or by the Mitsunobu reaction with alcohols.

Keywords: silanecarboxylic acids · silanecarboxylic acid esters · reductive lithiation · carboxylation · esterification

6.1.6 Product Subclass 6: Haloborates

G. A. Molander and F. Beaumard

This chapter is a revision of the earlier Science of Synthesis contribution describing methods for the synthesis of haloborates. It focuses on the synthesis of organotrifluoroborates and highlights methods published between 1999 and 2013.

Keywords: organotrifluoroborates · organoboron compounds · C—B bonds · C—H bond activation · hydroboration · transmetalation · borylation

14.1.5 Pyrylium Salts

A. T. Balaban and T. S. Balaban

This update covers the literature from 2000 to the end of 2011; it also includes a few references from 1999 that were not discussed in the original Science of Synthesis review of pyrylium salts. In addition to methodologies for preparing pyrylium salts, some new applications of these compounds are also described.

Keywords: pyrylium salts · aldehydes · ketones · 1,5-diones · cyclization · aromatization

18.8.22 Acyclic and Cyclic Ureas

S. Kubik

This update summarizes synthetic approaches to acyclic and cyclic ureas, as well as nonfunctionalized and functionalized derivatives. Syntheses of various urea derivatives are presented that were either not covered, or not treated in such detail, in the earlier Science of Synthesis contribution. For example, syntheses of imidazolidine-2,4-diones (hydantoins), 3,4-dihydropyrimidin-2(1H)-ones (Biginelli products), and pyrimidine-2,4,6(1H,3H,5H)-triones (barbiturates) are presented. The literature is covered between the years 2001 and 2012.

Keywords: barbiturates · Biginelli reaction · carbamates · carbon dioxide · carbonylation · 1,2-diamination · hydantoins · isocyanates · multicomponent reactions · ureas

19.5.14.15 Synthesis from Nitriles with Retention of the Cyano Group

N. Mase

This is an update to the original Section 19.5.14, which deals with synthesis from nitriles with retention of the cyano group. In order to cover significant recent developments, this update focuses on organocatalytic reactions of nitriles. These reactions are classified into two reaction modes: (1) reactions of nucleophiles containing a cyano group with electrophiles, and (2) reactions of nucleophiles with electrophiles containing a cyano group. In this update, significant achievements made employing asymmetric organocatalysts from the years 2000–2012 are highlighted.

Keywords: organocatalysis · nitriles · cyanides · isocyanides · cyanation · nucleophilic addition · nucleophilic substitution · one-pot processes

Science of Synthesis Knowledge Updates 2013/3

Preface

Abstracts

Table of Contents

4.4.43 Product Subclass 43: Silylium Ions and Stabilized Silylium Ions

T. Müller

4.4.44 Product Subclass 44: Silyl Radicals

Y. Landais

4.4.45 Product Subclass 45: Silanecarboxylic Acids and Esters

K. Igawa and K. Tomooka

6.1.6 Product Subclass 6: Haloborates

G. A. Molander and F. Beaumard

14.1.5 Pyrylium Salts (Update 2013)

A. T. Balaban and T. S. Balaban

18.8.22 Acyclic and Cyclic Ureas (Update 2013)

S. Kubik

19.5.14.15 Synthesis from Nitriles with Retention of the Cyano Group (Update 2013)

N. Mase

Author Index

Abbreviations

Table of Contents

Volume 4: Compounds of Group 15 (As, Sb, Bi) and Silicon Compounds

4.4 Product Class 4: Silicon Compounds

4.4.43 Product Subclass 43: Silylium Ions and Stabilized Silylium Ions

T. Müller

4.4.43 Product Subclass 43: Silylium Ions and Stabilized Silylium Ions

Synthesis of Product Subclass 43

4.4.43.1 Method 1: Heterolytic Cleavage of Si—Hal Bonds

4.4.43.2 Method 2: Heterolytic Cleavage of Si—H Bonds

4.4.43.2.1 Variation 1: By Hydride Transfer in Aromatic Hydrocarbons

4.4.43.2.2 Variation 2: By Hydride Transfer in Silanes

4.4.43.2.3 Variation 3: By Hydride Transfer in Halogenated Hydrocarbons

4.4.43.2.4 Variation 4: By Hydride Transfer with Subsequent Intermolecular Substituent Exchange

4.4.43.3 Method 3: Heterolytic Cleavage of Si—C Bonds

4.4.43.4 Method 4: Heterolytic Cleavage of Si—Si Bonds

4.4.43.5 Method 5: Oxidation of Disilanes and Silyl Radicals

4.4.43.6 Method 6: Addition of Electrophiles to Silylenes

Applications of Product Subclass 43 in Organic Synthesis

4.4.43.7 Method 7: Addition Reactions

4.4.43.7.1 Variation 1: Addition to Lewis Bases with Formation of Onium Ions

4.4.43.7.2 Variation 2: Addition to Lewis Bases with Formation of Brønsted Superacids and Strong Alkylating Reagents

4.4.43.7.3 Variation 3: Addition to C—C Unsaturated Compounds

4.4.43.8 Method 8: Hydrosilylation Reactions

4.4.43.9 Method 9: C—F Bond Activation Reactions

4.4.43.10 Method 10: Lewis Acid Catalysis

4.4.44 Product Subclass 44: Silyl Radicals

Y. Landais

4.4.44 Product Subclass 44: Silyl Radicals

Synthesis of Product Subclass 44

4.4.44.1 Method 1: Synthesis from Silanes

4.4.44.1.1 Variation 1: Hydrogen Abstraction Using Carbon-Centered Radicals

4.4.44.1.2 Variation 2: Hydrogen Abstraction through [1,5]-Hydrogen Transfer

4.4.44.1.3 Variation 3: Hydrogen Abstraction Using Oxygen-Centered Radicals

4.4.44.1.4 Variation 4: Polarity-Reversal Catalysis

4.4.44.2 Method 2: Synthesis from Disilanes

4.4.44.2.1 Variation 1: Photolysis of the Silicon—Silicon Bond

4.4.44.2.2 Variation 2: Thermal Cleavage of the Silicon—Silicon Bond

4.4.44.3 Method 3: Synthesis from Silyl Metal Compounds

4.4.44.3.1 Variation 1: From Silyllithiums and Silylsodium Reagents

4.4.44.3.2 Variation 2: From Silylaluminum Reagents

4.4.44.3.3 Variation 3: From Silylmercury Reagents

4.4.44.4 Method 4: Synthesis from Carbosilanes

4.4.44.4.1 Variation 1: From Allylsilanes

4.4.44.4.2 Variation 2: From 1-Silylcyclohexa-2,5-dienes

4.4.44.4.3 Variation 3: From Benzylsilanes

4.4.44.5 Method 5: Synthesis through Cleavage of Silicon—Heteroatom Bonds

4.4.44.5.1 Variation 1: Homolytic Cleavage of the Silicon—Boron Bond

4.4.44.5.2 Variation 2: Homolytic Cleavage of the Silicon—Phosphorus Bond

4.4.44.5.3 Variation 3: Homolytic Cleavage of the Silicon—Selenium Bond

4.4.44.6 Method 6: Single-Electron Reduction of Silyl Halides

4.4.44.7 Method 7: Reduction of Silylium Ions

4.4.44.8 Method 8: Addition of Radicals to Silylenes

4.4.44.9 Method 9: Reduction of Disilenes

4.4.44.10 Method 10: Rearrangements

4.4.44.10.1 Variation 1: 1,4- and 1,5-Aryl Migration from Silicon to Carbon

4.4.44.10.2 Variation 2: 1,2-Aryl Migration from Silicon to Oxygen

4.4.44.10.3 Variation 3: 1,2-Silicon Group Migration from Silicon to Carbon

4.4.44.10.4 Variation 4: 1,2- and 1,3-Silicon Group Migration from Silicon to Oxygen

4.4.44.10.5 Variation 5: 1,2- and 1,3-Silicon Group Migration from Silicon to Sulfur

4.4.44.10.6 Variation 6: Intramolecular Homolytic Substitution

Applications of Product Subclass 44 in Organic Synthesis

4.4.44.11 Method 11: Silyl Radicals in Reduction Processes

4.4.44.12 Method 12: Addition of Silyl Radicals to Unsaturated Systems

4.4.44.13 Method 13: Formation of Carbon—Carbon Bonds

4.4.44.14 Method 14: Formation of Carbon—Heteroatom Bonds

4.4.44.15 Method 15: Radical Cascades Mediated by Silyl Radicals

4.4.44.16 Method 16: Rearrangement Processes

4.4.45 Product Subclass 45: Silanecarboxylic Acids and Esters

K. Igawa and K. Tomooka

4.4.45 Product Subclass 45: Silanecarboxylic Acids and Esters

Synthesis of Product Subclass 45

4.4.45.1 Method 1: Carboxylation of Silyl Anions

4.4.45.1.1 Variation 1: Reduction of Chlorosilanes with Lithium Metal and Subsequent Carboxylation

4.4.45.1.2 Variation 2: Reduction of Chlorosilanes with Lithium Arenides and Subsequent Carboxylation

Applications of Product Subclass 45 in Organic Synthesis

4.4.45.2 Method 2: Decomposition of Silanecarboxylic Acids as a Source of Carbon Monoxide

4.4.45.3 Method 3: Esterification of Silanecarboxylic Acids

4.4.45.3.1 Variation 1: Esterification of Silanecarboxylic Acids with Diazoalkanes

4.4.45.3.2 Variation 2: Esterification of Silanecarboxylic Acids by Mitsunobu Reaction

Volume 6: Boron Compounds

6.1 Product Class 1: Boron Compounds

6.1.6 Product Subclass 6: Haloborates

G. A. Molander and F. Beaumard

6.1.6 Product Subclass 6: Haloborates

Synthesis of Product Subclass 6

6.1.6.1 Aryl- and Hetaryltrifluoroborates

6.1.6.1.1 Method 1: Transmetalation

6.1.6.1.1.1 Variation 1: Metalation/Electrophilic Borylation

6.1.6.1.1.2 Variation 2: Directed Metalation/Electrophilic Borylation

6.1.6.1.2 Method 2: Miyaura Borylation

6.1.6.1.2.1 Variation 1: Using Dialkoxyboranes

6.1.6.1.2.2 Variation 2: Using Bisboronates

6.1.6.1.2.3 Variation 3: Using Tetrahydroxydiboron

6.1.6.1.2.4 Variation 4: Using Tetrakis(dimethylamino)diboron

6.1.6.1.3 Method 3: C—H Activation/Electrophilic Borylation

6.1.6.2 Alkenyltrifluoroborates

6.1.6.2.1 Method 1: Metalation/Electrophilic Borylation

6.1.6.2.2 Method 2: Hydroboration

6.1.6.2.2.1 Variation 1: Using Diorganoboranes

6.1.6.2.2.2 Variation 2: Using Diorganooxyboranes

6.1.6.2.2.3 Variation 3: Using Dihaloboranes

6.1.6.3 Alkynyltrifluoroborates

6.1.6.3.1 Method 1: Deprotonation of Terminal Alkynes/Borylation of Alkynylmetals

6.1.6.3.2 Method 2: Dehydrohalogenation of Alkenes

6.1.6.3.3 Method 3: Dehydrohalogenation of Alkanes

6.1.6.4 Alkyltrifluoroborates

6.1.6.4.1 Method 1: Metalation/Electrophilic Borylation

6.1.6.4.2 Method 2: Hydroboration

6.1.6.4.3 Method 3: C—H Activation/Borylation

6.1.6.4.4 Method 4: 1,4-Addition

6.1.6.4.5 Method 5: C-Alkylation of Enolates

6.1.6.4.6 Method 6: α-Halo Transfer of Nitrogen-Based Nucleophiles

6.1.6.4.6.1 Variation 1: (Carboxamidomethyl)trifluoroborates

6.1.6.4.6.2 Variation 2: (Sulfonamidomethyl)trifluoroborates

6.1.6.4.6.3 Variation 3: (Carbamatomethyl)trifluoroborates

6.1.6.4.7 Method 7: Borylation of Alkyl Halides

6.1.6.4.8 Method 8: Borylation of Aldehydes

6.1.6.5 Allyltrifluoroborates

6.1.6.5.1 Method 1: Metalation/Electrophilic Borylation

6.1.6.5.2 Method 2: Palladium-Catalyzed Reactions

6.1.6.5.2.1 Variation 1: Using Baylis–Hillman Adducts

6.1.6.5.2.2 Variation 2: Using Allylic Alcohols

6.1.6.6 Benzyltrifluoroborates

6.1.6.6.1 Method 1: Transmetalation and Metalation/Electrophilic Borylation

6.1.6.7 Propargyltrifluoroborates

6.1.6.7.1 Method 1: Metalation/Transmetalation

6.1.6.8 Acyltrifluoroborates

6.1.6.8.1 Method 1: Transmetalation from Metalated Alkenyl Ethers and Hydrolysis

6.1.6.8.2 Method 2: Alkylation of Functionalized Carbanions

Applications of Product Subclass 6 in Organic Synthesis

6.1.6.9 Modification of Potassium Trifluoroborates

Volume 14: Six-Membered Hetarenes with One Chalcogen

14.1 Product Class 1: Pyrylium Salts

14.1.5 Pyrylium Salts

A. T. Balaban and T. S. Balaban

14.1.5 Pyrylium Salts

14.1.5.1 Synthesis by Ring-Closure Reactions

14.1.5.1.1 By Formation of One O—C and Two C—C Bonds

14.1.5.1.1.1 Method 1: [C1 + C3 + C1] Diacylation of Acyclic Propene Derivatives

14.1.5.1.1.2 Method 2: [C2 + C1 + C2] Condensation of Methyl(ene) Ketones with Aldehydes

14.1.5.1.1.3 Method 3: Formation of Metalated Pyrylium Salts from Metalated Ortho Esters and Enol Ethers of Methyl(ene) Ketones

14.1.5.1.2 By Formation of Two O—C Bonds

14.1.5.1.2.1 Method 1: Formation of Pyrylium Rings from Substituted Cyclopentadienes

14.1.5.1.3 By Formation of One O—C and One C—C Bond

14.1.5.1.3.1 Method 1: [C3 + C2] Synthesis from Methyl(ene) Ketones and α,β-Unsaturated Ketones Followed by Dehydrocyclization

14.1.5.1.3.2 Method 2: From Ketones or Their Synthetic Equivalents and β-Chloro α,β-Unsaturated Ketones or Aldehydes

14.1.5.1.3.3 Method 3: Formation of Metalated Pyrylium Salts from Alkynes and Propargylic Esters

14.1.5.1.3.4 Method 4: Formation of Pyrylium Pseudobases by Alkene Cross Metathesis

14.1.5.1.4 [C4 + C1]: Formation of a C5 Chain from Two Synthons

14.1.5.1.4.1 Method 1: From α,β -or β, γ-Unsaturated Ketones and Carboxylic Acid Derivatives

14.1.5.1.4.2 Method 2: 2,4,6-Triarylpyrylium Salts from Acetophenone Derivatives by Cleavage of an Aroyl Group from an Initially Formed 1,3-Diarylbut-2-en-1-one Derivative

14.1.5.1.5 By Formation of One O—C Bond

14.1.5.1.5.1 Formation of Pyrylium Rings by Closure of a Preformed Acyclic C5 Chain

14.1.5.1.5.1.1 Method 1: Formation of Pyrylium Salts from Pent-2-ene-1,5-diones (Pyrylium Pseudobases) and 1,5-Dioates

14.1.5.1.5.1.2 Method 2: Formation of Pyrylium Salts by Dehydrocyclization of Penta-2,4-dienals

14.1.5.1.5.1.3 Method 3: Formation of Pyrylium Salts by Dehydrocyclizations of Pentane-1,5-diones

14.1.5.2 Aromatization

14.1.5.2.1 By Hydride Abstraction from a Preformed Pyran System

14.1.5.2.1.1 Method 1: Formation of Pyrylium Salts by Hydride Abstraction from a 4H-Pyran

14.1.5.2.1.2 Method 2: Formation of Pyrylium Salts by Addition to 2,6-Disubstituted Pyrylium Cations

14.1.5.2.1.3 Method 3: Benzotriazole-Mediated Derivatization of 2,6-Disubstituted Pyrylium Salts in the 4-Position by Electrophiles

14.1.5.2.1.4 Method 4: Formation of Pyrylium Salts by Wittig Reaction between 2,6-Disubstituted Pyrylium Cations and Aldehydes

14.1.5.3 Synthesis by Substituent Modification

14.1.5.3.1 Method 1: Modification of 2- or 4-(Arylvinyl)pyrylium Salts

14.1.5.3.2 Method 2: Modification of Pyrylium Salts by Metalation of 2- or 4-Methyl Groups

14.1.5.3.3 Method 3: Formation of 4- or 2-(Azulen-1-yl)pyrylium Salts

14.1.5.3.4 Method 4: Suzuki–Miyaura Reactions

Volume 18: Four Carbon—Heteroatom Bonds: X—C≡X, X=C=X, X2C=X, CX4

18.8 Product Class 8: Acyclic and Cyclic Ureas

18.8.22 Acyclic and Cyclic Ureas

S. Kubik

18.8.22 Acyclic and Cyclic Ureas

18.8.22.1 Acyclic Unfunctionalized Ureas

18.8.22.1.1 Synthesis of Acyclic Unfunctionalized Ureas

18.8.22.1.1.1 Method 1: From Isocyanates or Isothiocyanates

18.8.22.1.1.1.1 Variation 1: From Isocyanates and Amines

18.8.22.1.1.1.2 Variation 2: From Isothiocyanates, Amines, and Hydrogen Peroxide

18.8.22.1.1.1.3 Variation 3: From Isocyanates via Carbodiimides

18.8.22.1.1.2 Method 2: From Cyanate Salts

18.8.22.1.1.3 Method 3: From Carboxylic Acids

18.8.22.1.1.3.1 Variation 1: Without Trapping of the Isocyanate

18.8.22.1.1.3.2 Variation 2: With Trapping of the Isocyanate

18.8.22.1.1.4 Method 4: From Amides

18.8.22.1.1.5 Method 5: From Hydroxamic Acids

18.8.22.1.1.6 Method 6: From Phosgene or Bis(trichloromethyl) Carbonate

18.8.22.1.1.7 Method 7: From Chloroformates

18.8.22.1.1.8 Method 8: From Carbamoylimidazoles or Carbamoylimidazolium Salts

18.8.22.1.1.9 Method 9: From Carbonates or Dithiocarbonates

18.8.22.1.1.9.1 Variation 1: From 1,3-Dioxolan-2-one

18.8.22.1.1.9.2 Variation 2: From S,S-Dimethyl Dithiocarbonate

18.8.22.1.1.10 Method 10: From Carbamates

18.8.22.1.1.11 Method 11: From Ureas or Thioureas

18.8.22.1.1.11.1 Variation 1: By Alkylation of Ureas

18.8.22.1.1.11.2 Variation 2: By Arylation of Ureas

18.8.22.1.1.11.3 Variation 3: By Transamidation of Ureas

18.8.22.1.1.11.4 Variation 4: By Desulfurization of Thioureas

18.8.22.1.1.12 Method 12: From Carbon Monoxide

18.8.22.1.1.12.1 Variation 1: With Catalysis by Transition Metals

18.8.22.1.1.12.2 Variation 2: With Catalysis by Main Group Elements

18.8.22.1.1.13 Method 13: From Carbon Dioxide

18.8.22.1.1.14 Method 14: Miscellaneous Methods

18.8.22.2 Imidazolidin-2-ones and Other Unfunctionalized Five-Membered Cyclic Ureas

18.8.22.2.1 Synthesis of Imidazolidin-2-ones and Other Unfunctionalized Five-Membered Cyclic Ureas

18.8.22.2.1.1 Method 1: From Alkenes

18.8.22.2.1.2 Method 2: Miscellaneous Methods

18.8.22.3 Tetrahydropyrimidin-2(1H)-ones and Other Unfunctionalized Six-Membered Cyclic Ureas

18.8.22.3.1 Synthesis of Tetrahydropyrimidin-2(1H)-ones and Other Unfunctionalized Six Membered Cyclic Ureas

18.8.22.3.1.1 Method 1: From 1,3-Diamines

18.8.22.3.1.2 Method 2: From a β-Oxo Ester, an Aldehyde, and Urea (Biginelli Reaction)

18.8.22.3.1.3 Method 3: Miscellaneous Methods

18.8.22.4 1,3-Diazepan-2-one and Other Unfunctionalized Seven-Membered Cyclic Ureas

18.8.22.4.1 Synthesis of 1,3-Diazepan-2-one and Other Unfunctionalized Seven-Membered Cyclic Ureas

18.8.22.5 Acyclic N-Acyl-, N,N-Diacyl-, and N,N′-Diacylureas

18.8.22.5.1 Synthesis of Acyclic N-Acyl-, N,N-Diacyl-, and N,N′-Diacylureas

18.8.22.5.1.1 Method 1: From Acyl Isocyanates

18.8.22.5.1.2 Method 2: From Carboxylic Acids

18.8.22.5.1.3 Method 3: From Ureas or Thioureas

18.8.22.6 Imidazolidine-2,4-diones (Hydantoins) and Other Five-Membered Cyclic N-Acylureas

18.8.22.6.1 Synthesis of Imidazolidine-2,4-diones (Hydantoins) and Other Five-Membered Cyclic N-Acylureas

18.8.22.6.1.1 Method 1: From α-Amino Amides

18.8.22.6.1.2 Method 2: From 1,2-Dicarbonyl Compounds and Urea (Biltz Synthesis)

18.8.22.6.1.3 Method 3: From Carbonyl Compounds, Cyanide Salts, and Ammonium Carbonate (Bucherer–Bergs Synthesis)

18.8.22.6.1.4 Method 4: From Amino Acids and Cyanate Salts (Read Synthesis)

18.8.22.6.1.5 Method 5: From Amino Acid Esters and Isocyanates

18.8.22.6.1.6 Method 6: From Amino Acid Amides and Carbamates

18.8.22.6.1.7 Method 7: From Other Heterocycles

18.8.22.6.1.8 Method 8: Cycloadditions

18.8.22.6.1.9 Method 9: Multicomponent Reactions

18.8.22.6.1.10 Method 10: Miscellaneous Methods

18.8.22.7 Dihydropyrimidine-2,4(1H,3H)-diones, Pyrimidine-2,4,6(1H,3H,5H)-triones (Barbiturates), and Other Six-Membered Cyclic N-Acyl- or N,N′-Diacylureas

18.8.22.7.1 Synthesis of Dihydropyrimidine-2,4(1H,3H)-diones, Pyrimidine-2,4,6(1H,3H,5H)-triones (Barbiturates), and Other Six-Membered Cyclic N-Acyl- or N,N′-Diacylureas

18.8.22.7.1.1 Method 1: From Uracil Derivatives

18.8.22.7.1.2 Method 2: From α-Amino Acids

18.8.22.7.1.3 Method 3: Multicomponent Reactions

18.8.22.7.1.4 Method 4: From Malonic Acid Monoesters and Carbodiimides

18.8.22.7.1.5 Method 5: Miscellaneous Methods

18.8.22.8 1,3-Diazepane-2,4-diones, 1,3-Diazepane-2,4,7-triones, and Other Seven-Membered Cyclic N-Acyl- or N,N′-Diacylureas

18.8.22.8.1 Synthesis of 1,3-Diazepane-2,4-diones, 1,3-Diazepane-2,4,7-triones, and Other Seven-Membered Cyclic N-Acyl- or N,N′-Diacylureas

18.8.22.8.1.1 Method 1: From Pyroglutamates

18.8.22.9 Biurets and Triurets

18.8.22.9.1 Synthesis of Biurets and Triurets

18.8.22.10 N-(Iminomethyl)ureas

18.8.22.10.1 Synthesis of N-(Iminomethyl)ureas

18.8.22.10.1.1 Method 1: From Isocyanides and Ureas

18.8.22.11 N-Carbamimidoylureas

18.8.22.11.1 Synthesis of N-Carbamimidoylureas

18.8.22.11.1.1 Method 1: From Guanidines

18.8.22.12 Semicarbazides and Carbonohydrazides

18.8.22.12.1 Synthesis of Semicarbazides and Carbonohydrazides

18.8.22.13 N-Nitroso- and N-Nitroureas

18.8.22.13.1 Synthesis of N-Nitroso- and N-Nitroureas

18.8.22.14 N-Chloroureas

18.8.22.14.1 Synthesis of N-Chloroureas

18.8.22.14.1.1 Method 1: From Hydantoins

18.8.22.15 N-Hydroxyureas

18.8.22.15.1 Synthesis of N-Hydroxyureas

18.8.22.16 N-Alkoxy-N-chloro-, N-Acyloxy-N-alkoxy-, and N,N-Dialkoxyureas

18.8.22.16.1 Synthesis of N-Alkoxy-N-chloro-, N-Acyloxy-N-alkoxy-, and N,N-Dialkoxyureas

18.8.22.17 N-Sulfonylureas

18.8.22.17.1 Synthesis of N-Sulfonylureas

18.8.22.18 N-Phosphorylureas

18.8.22.18.1 Synthesis of N-Phosphorylureas

Volume 19: Three Carbon—Heteroatom Bonds: Nitriles, Isocyanides, and Derivatives

19.5 Product Class 5: Nitriles

19.5.14.15 Synthesis from Nitriles with Retention of the Cyano Group

N. Mase

19.5.14.15.1 Organocatalytic Reactions of Nucleophiles Containing a Cyano Group with Electrophiles

19.5.14.15.1.1 Nucleophilic Additions of Cyanide to Double Bonds

19.5.14.15.1.1.1 Method 1: Nucleophilic Additions of Cyanide to Carbonyl and Imino Groups

19.5.14.15.1.1.2 Method 2: Michael Addition of Cyanide to Electron-Deficient Alkenes

19.5.14.15.1.1.2.1 Variation 1: Michael Addition of Cyanide to Enones

19.5.14.15.1.1.2.2 Variation 2: Michael Addition of Cyanide to Nitroalkenes

19.5.14.15.1.2 Addition of Carbon Nucleophiles to Various Electrophiles

19.5.14.15.1.2.1 Method 1: Cyanomethylations

19.5.14.15.1.2.1.1 Variation 1: Cyanomethylation of Aldehydes and Ketones

19.5.14.15.1.2.1.2 Variation 2: Cyanomethylation of Imines

19.5.14.15.1.2.2 Method 2: Knoevenagel Condensations

19.5.14.15.1.2.2.1 Variation 1: Knoevenagel Condensation of an α-Cyano Ester with Aldehydes and Ketones

19.5.14.15.1.2.2.2 Variation 2: Knoevenagel Condensation of α-Cyanoamides with Aldehydes

19.5.14.15.1.2.2.3 Variation 3: Knoevenagel Condensation of Malononitrile with Aldehydes and Ketones

19.5.14.15.1.2.2.4 Variation 4: Knoevenagel Condensation Using a Heterogeneous Organocatalyst

19.5.14.15.1.2.3 Method 3: Mannich Reaction

19.5.14.15.1.2.3.1 Variation 1: Mannich Reactions with Imines

19.5.14.15.1.2.3.2 Variation 2: Mannich Reactions with Imines Generated In Situ

19.5.14.15.1.2.3.3 Variation 3: Vinylogous Mannich Reactions

19.5.14.15.1.2.3.4 Variation 4: One-Pot Mannich/Cyclization/Tautomerization Reactions of Malononitrile with Imines Generated In Situ

19.5.14.15.1.2.4 Method 4: Amination Reactions

19.5.14.15.1.2.4.1 Variation 1: Amination Reactions of α-Cyano Carbonyl Compounds

19.5.14.15.1.2.4.2 Variation 2: Vinylogous Amination Reactions

19.5.14.15.1.2.5 Method 5: Nitroso-Aldol Reactions

19.5.14.15.1.2.6 Method 6: Michael Reactions between Nucleophiles Containing a Cyano Group and Enals or Enones

19.5.14.15.1.2.6.1 Variation 1: Michael Reactions of α-Cyano Esters with Enals

19.5.14.15.1.2.6.2 Variation 2: Michael/Hemiaminal Reactions of N-Protected α-Cyano Esters with Enals

19.5.14.15.1.2.6.3 Variation 3: Michael Reactions of α-Cyano Esters with Enones

19.5.14.15.1.2.6.4 Variation 4: Michael Reactions of α-Cyano Sulfones with Enones

19.5.14.15.1.2.6.5 Variation 5: Michael Reactions of α-Cyanophosphonates with Enones

19.5.14.15.1.2.6.6 Variation 6: Michael Reactions of Malononitriles with Enones

19.5.14.15.1.2.6.7 Variation 7: Michael/Hemiacetal Reactions of α-Cyano Ketones with Enones

19.5.14.15.1.2.6.8 Variation 8: Michael/Cyclization/Tautomerization Reactions of Malononitrile with Enones

19.5.14.15.1.2.7 Method 7: Michael Reactions between Nucleophiles Containing a Cyano Group and α,β-Unsaturated Carboxylic Acid Derivatives

19.5.14.15.1.2.7.1 Variation 1: Michael Reactions with α,β-Unsaturated Esters

19.5.14.15.1.2.7.2 Variation 2: Michael Reactions with Acyclic Imides

19.5.14.15.1.2.7.3 Variation 3: Michael Reactions with Cyclic Imides

19.5.14.15.1.2.8 Method 8: Michael Reactions between Nucleophiles Containing a Cyano Group and Various Other Electrophiles

19.5.14.15.1.2.8.1 Variation 1: Michael Reactions with Alkynyl Electrophiles

19.5.14.15.1.2.8.2 Variation 2: Michael Reactions of Malononitrile with Dienones

19.5.14.15.1.2.8.3 Variation 3: Michael/Cyclization/Tautomerization Reactions with Dienones

19.5.14.15.1.2.8.4 Variation 4: Michael Reactions with a Vinylogous Imine

19.5.14.15.1.2.8.5 Variation 5: Michael Reactions of Cyano Compounds with Nitroalkenes

19.5.14.15.1.2.8.6 Variation 6: Michael/Cyclization/Tautomerization Reactions of Malononitrile with Nitroalkenes

19.5.14.15.1.2.8.7 Variation 7: Michael Reactions with Vinyl Sulfones

19.5.14.15.1.2.8.8 Variation 8: Michael Reactions with Vinyl Selenones

19.5.14.15.1.2.9 Method 9: Vinylogous Michael Reactions of Nucleophiles Containing a Cyano Group

19.5.14.15.1.2.9.1 Variation 1: Vinylogous Michael Reactions with Enals

19.5.14.15.1.2.9.2 Variation 2: Vinylogous Michael Reactions with Enones

19.5.14.15.1.2.9.3 Variation 3: Vinylogous Michael Reactions with Quinones

19.5.14.15.1.2.9.4 Variation 4: Vinylogous Michael Reactions with Dienones

19.5.14.15.1.2.9.5 Variation 5: Vinylogous Michael Reactions with Imides

19.5.14.15.1.2.9.6 Variation 6: Vinylogous Michael Reactions with Nitroalkenes

19.5.14.15.1.2.10 Method 10: Michael Reactions of Isocyanide Derivatives with Imides

19.5.14.15.1.2.11 Method 11: [3 + 2] Cycloadditions of Cyano Derivatives with Enals

19.5.14.15.1.3 Substitutions of Carbon Nucleophiles by Various Electrophiles

19.5.14.15.1.3.1 Method 1: Alkylation

19.5.14.15.1.3.2 Method 2: Allylation

19.5.14.15.1.3.2.1 Variation 1: Allylation of α-Cyano Esters

19.5.14.15.1.3.2.2 Variation 2: Allylation of α-Hydroxy Nitrile Derivatives

19.5.14.15.1.3.2.3 Variation 3: Allylation of Aminonitriles

19.5.14.15.1.3.2.4 Variation 4: Allylation of Vinylogous Nucleophiles

19.5.14.15.1.3.3 Method 3: Vinylic Substitutions

19.5.14.15.2 Organocatalytic Reactions of Nucleophiles with Electrophiles Containing Cyano Groups

19.5.14.15.2.1 Michael Reactions with Electrophiles Containing Cyano Groups

19.5.14.15.2.1.1 Method 1: Michael Reactions of Carbon Nucleophiles

19.5.14.15.2.1.1.1 Variation 1: Michael Reactions of Carbonyl Compounds with Monocyano Electrophiles

19.5.14.15.2.1.1.2 Variation 2: Michael Reactions of Carbonyl Compounds with Dicyano Electrophiles

19.5.14.15.2.1.1.3 Variation 3: Michael/Cyclization Reactions of Nucleophiles Containing a Carbonyl Group with Cyano Electrophiles

19.5.14.15.2.1.1.4 Variation 4: Michael/Cyclization/Tautomerization Reactions of Carbonyl Compounds with Dicyano Electrophiles

19.5.14.15.2.1.1.5 Variation 5: Michael Reactions of Phosphonates with Cyano Electrophiles

19.5.14.15.2.1.1.6 Variation 6: Michael Reactions of Nitroalkanes with Cyano Electrophiles

19.5.14.15.2.1.2 Method 2: Aza-Michael Reactions

19.5.14.15.2.1.3 Method 3: Oxa-Michael Reactions

19.5.14.15.2.1.4 Method 4: Thia-Michael Reactions

19.5.14.15.2.2 Cyclopropanation

19.5.14.15.2.2.1 Method 1: Cyclopropanation of Acrylonitrile Derivatives

19.5.14.15.2.3 Epoxidation

19.5.14.15.2.3.1 Method 1: Epoxidation of α-Acylacrylonitriles

19.5.14.15.2.4 Morita–Baylis–Hillman Reactions

19.5.14.15.2.4.1 Method 1: Morita–Baylis–Hillman Reactions with Acrylonitrile

19.5.14.15.2.4.2 Method 2: Aza-Morita–Baylis–Hillman Reactions

19.5.14.15.2.5 Cycloaddition Reactions

19.5.14.15.2.5.1 Method 1: [4 + 2] Cycloadditions with Cyano Dienophiles

19.5.14.15.3 One-Pot Sequential Reactions

19.5.14.15.3.1 One-Pot Reactions via Knoevenagel Condensations

19.5.14.15.3.1.1 Method 1: Knoevenagel/Hydrogenation Reactions

19.5.14.15.3.1.2 Method 2: Knoevenagel/Hydrogenation/Alkylation Reactions

19.5.14.15.3.1.3 Method 3: Knoevenagel/Michael Reactions

19.5.14.15.3.1.4 Method 4: Knoevenagel/Michael/Cyclization Reactions

19.5.14.15.3.1.5 Method 5: Knoevenagel/Michael/Cyclization/Tautomerization Reactions

19.5.14.15.3.1.6 Method 6: Knoevenagel/Friedel–Crafts/Cyclization/Tautomerization Reactions

19.5.14.15.3.1.7 Method 7: Knoevenagel/Cycloaddition Reactions

19.5.14.15.3.1.8 Method 8: Deacetalization/Knoevenagel Reactions

19.5.14.15.3.1.9 Method 9: Michael/Cyclization/Tautomerization/Knoevenagel Reactions

19.5.14.15.3.1.10 Method 10: Multicomponent Reactions

19.5.14.15.3.2 One-Pot Reactions via Allylation

19.5.14.15.3.2.1 Method 1: Cyanation/Allylation Reactions of Aldehydes

19.5.14.15.3.2.2 Method 2: Cyanation/Allylation Reactions of Imines

19.5.14.15.3.3 One-Pot Michael/Michael/Cyclization Reactions

19.5.14.15.3.3.1 Method 1: Michael/Michael/Cyclization Reactions with a Brønsted Base Catalyst

19.5.14.15.3.3.2 Method 2: Michael/Michael/Cyclization Reactions with Nucleophilic Catalysis

19.5.14.15.3.4 Michael/Michael Reactions Using Enamine and/or Iminium Catalysis

19.5.14.15.3.4.1 Method 1: Enamine/Iminium Catalysis

19.5.14.15.3.4.2 Method 2: Iminium/Enamine Catalysis

19.5.14.15.3.4.3 Method 3: Iminium/Iminium Catalysis

19.5.14.15.3.4.4 Method 4: Enamine/Iminium/Enamine Catalysis

19.5.14.15.3.5 Singly Occupied Molecular Orbital (SOMO) Catalysis

19.5.14.15.3.5.1 Method 1: Polycyclization of Polyenals

19.5.14.15.3.6 Reaction Sequences Involving Metal Catalysts and Organocatalysts

19.5.14.15.3.6.1 Method 1: Hydroformylation/Knoevenagel Reaction

19.5.14.15.3.6.2 Method 2: Michael/Cyclization/Isomerization Reaction

19.5.14.15.3.6.3 Method 3: Knoevenagel/Hydrogenation/Cyclization Reaction

Author Index

Abbreviations

4.4.43 Product Subclass 43: Silylium Ions and Stabilized Silylium Ions

T. Müller

General Introduction

▶ Scheme 1 Stable Silylium Ions[12,16–23]

The synthetic efforts toward the isolation of silylium salts with an ideal trigonal planar coordinated positively charged silicon atom created a series of stabilized silyl cations in which either the interaction with the solvent, the counteranion, or intramolecular donor groups pacifies the high reactivity of the silyl cations. This electron donation leads to cationic species 7 in which the silicon atom adopts a distorted tetrahedral coordination environment (▶ Scheme 2). Siliconium ions 8, in which the silicon atom has expanded its coordination number to 5 by addition of two solvent molecules, have been structurally characterized. Intermolecular species 7 and 8 as well as intramolecular variants 9 and 10, which have both modes of stabilization, have been characterized.

▶ Scheme 2 Different Modes of Stabilization for Silylium Ions

▶ Scheme 3 Examples of Solvent-Stabilized Silylium Ions[8]

▶ Scheme 4 Anion-Stabilized Silylium Ion and Conventional Silyl Lewis Acids[8,24,25]

In cases in which the high Lewis acidity of the cationic silicon is pacified by an intramolecular Lewis basic group (LB), as in the tetracoordinated 9 (and likewise in the siliconium ion 10; ▶ Scheme 2), interactions between the positively charged silicon atom and the solvent and/or anion are of minor importance. The structures, spectroscopic properties, and reactivities of these silyl cations are greatly determined by the electron-donating ability of the Lewis basic groups. As a consequence, the structural and spectroscopic features of silyl cations 21 and 22, which are stabilized by intramolecular electron donation from an aryl substituent, closely resemble those of benzenium ions 12 (▶ Scheme 5).[26,27] The intramolecular stabilization operative in silylium ions 9 offers the intriguing possibility of controlling the Lewis acidity of the positively charged silicon atom by adjusting the electron-donating ability of the Lewis basic group. Several model systems for such tunable silyl Lewis acids have been suggested.[27,28]

▶ Scheme 5 Inter- and Intramolecular Stabilization of Silylium Ions by Arenes[26,27]

▶ Scheme 6 Synthesis of Silylium Ions[8]

The reactivity of silylium ions is dominated by the extreme Lewis acidity of these species. Their beneficial application in organic synthesis is surveyed in ▶ Sections 4.4.43.7–4.4.43.10. Important for the understanding of the chemistry of silylium ions is also the high stability of Si—F and Si—O bonds. The combination of both factors provides opportunities for catalytic bond-activation processes (see ▶ Sections 4.4.43.9 and 4.4.43.10) and for the preparation of extremely potent Brønsted acids or alkylating reagents (see ▶ Section 4.4.43.7.2). Stabilized silylium ions have also found application in ring-opening polymerization processes of phosphazene trimers,[29] cyclosiloxanes,[30] and lactones.[31] In addition, silylium ions are efficient catalysts in the polymerization of methyl methacrylate.[32–34]

Synthesis of Product Subclass 43

4.4.43.1 Method 1: Heterolytic Cleavage of Si—Hal Bonds

▶ Scheme 7 Preparation of a Siliconium Ion by Cleavage of a Si—Hal Bond; Example of Intermolecular Stabilization by Solvent Molecules[35,36]

▶ Scheme 8 Preparation of a Siliconium Ion by Cleavage of a Si—Hal Bond; Example of Intramolecular Stabilization[37]

Hydridodimethylbis(1-methyl-1H-imidazole-κN3)siliconium Chloride (23):[35,36]

1-Methyl-1H-imidazole (2 equiv) was added to Me2SiHCl (1 equiv). A white precipitate immediately formed, which was dried for 6 h under vacuum. Crystals of 23 suitable for Xray diffraction were obtained by sublimation under reduced pressure; yield: quant; mp 90 °C (dec);29Si NMR (CDCl3, rt, δ): –81.0.

Bis[2-(methoxy-κO-methyl)phenyl-κC1]siliconium Trifluoromethanesulfonate (25); Typical Procedure:[37]

4.4.43.2 Method 2: Heterolytic Cleavage of Si—H Bonds

4.4.43.2.1 Variation 1: By Hydride Transfer in Aromatic Hydrocarbons

The hydride-transfer reaction between trialkylsilanes and triphenylcarbenium tetrakis(pentafluorophenyl)borate in aromatic hydrocarbons results in the formation of (trialkylsilyl)arenium salts, such as 26, which can be isolated at room temperature and used for further reactions (▶ Scheme 9).[44–46] The triphenylcarbenium borate usually forms a biphasic mixture with aromatic hydrocarbons that consists of an upper nonpolar phase and a lower ionic phase. The byproduct of the hydride transfer, triphenylmethane, is not reactive toward the formed products and is efficiently removed by separating the phases and washing the ionic phase with arene. The hydride transfer from bulky triaryl-substituted silanes with the triphenylcarbenium cation is severely hampered and sterically hindered triarylsilylium salts cannot be prepared by this route.[12,16] Ethylbis(2,4,6-triisopropylphenyl)silylium (2), however, is obtained by stirring the corresponding silane with triphenylcarbenium tetrakis(pentafluorophenyl)borate in benzene at room temperature for 5 hours (see also ▶ Section 4.4.43.2.4).[18] While cation 2 is a true silylium ion, silyl cations [R3Si+] possessing smaller substituents R, which are obtained in this reaction under these conditions, are best termed as silylated arenium ions. Their spectroscopic properties, most notably the 29Si NMR chemical shift, depends on the arene solvent, indicating the formation of the tetracoordinated silyl cationic species in which the fourth coordination site is occupied by the arene solvent. These arenium salts (e.g., 26) are perfect silylating reagents and strong silyl Lewis acids. In general, they serve in many chemical applications as synthetic equivalents to silylium ions and therefore the term “solvent-stabilized silylium ions” seems to be well-justified.

▶ Scheme 9 Preparation of a Silylarenium Ion (Arene-Stabilized Silylium Ion)[44–46]

The hydride-transfer reaction between trialkylsilanes and halogenated closo-borates[24,25] or closo-carborates[47,48] of the triphenylcarbenium cation in aromatic hydrocarbons results in the formation of trialkylsilyl compounds, such as 28, that exist in solution and in the solid state as ion pairs with close halogen–silicon contacts (▶ Scheme 10).[47] In this case, the hydride transfer is very slow due to the low solubility of the triphenylcarbenium carborate in aromatic hydrocarbons.

▶ Scheme 10 Preparation of a Silyl Carborate (Anion-Stabilized Silylium Ion)[47]

The hydride-transfer reaction between the triphenylcarbenium cation and silanes that possess donor substituents results in the formation of intramolecularly stabilized silyl cations. These cations are free from additional coordination to anions and arene solvents. The coordination number of the silicon atom in those silyl cations is greater than three, the additional coordination sites being occupied by the Lewis basic donor substituents. The formation of these intramolecularly stabilized silylium ions in arene solution is favored by entropy over the formation of arene-stabilized cations. The use of solvents with higher donation ability than the intramolecular donor results in the formation of solvent-stabilized silylium ions. Typical intramolecular donor substituents are amino, alkoxy, and phosphino groups. The resulting tetra- or pentacoordinated species have found some interest from a structural point of view. For the synthesis of more reactive intramolecularly stabilized cations, species with weak donor substituents such as aryl[26,27,49] and Si—H groups[50–52] (▶ Scheme 11) have found application. By this approach, fundamentally new structural motifs were found, like the Si—H—Si three-center, two-electron bond in silyl cations such as 29.[50,51] A particularly interesting example of intramolecular aryl stabilization is provided by terphenyl-substituted silylium ions such as 30 (▶ Scheme 12).[27] The housetop-like terphenyl scaffold provides the desired potential for tunable π-electron donation and silyl Lewis acid character. Variation of the 2,6-diarylphenyl substituent offers the possibility to control the electron deficiency of the silyl cation fragment.

▶ Scheme 11 Preparation of a Bissilylhydronium Ion (Intramolecularly Stabilized Silylium Ion)[51]

▶ Scheme 12 Preparation of a meta-Terphenyl-Substituted Silylium Ion (Intramolecularly Stabilized Silylium Ion)[27]

(Triethylsilyl)benzenium Tetrakis(pentafluorophenyl)borate (26); Typical Procedure:[45]

Et3SiH (116 mg, 1 mmol) was added at rt by syringe to a well-stirred biphasic soln of [Ph3C][B(C6F5)4] (462 mg, 0.5 mmol) in benzene (1.5 mL) (CAUTION:carcinogen). The orange color of the triphenylcarbenium salt disappeared immediately. The mixture was stirred for 5 min at rt, the two phases were allowed to separate, and the upper phase was removed. The lower phase was washed with benzene (2 × 2 mL) to remove excess silane and the byproduct Ph3CH. The solvent was evaporated and silylarenium borate 26 was obtained as a colorless solid; yield: quant;29Si NMR (benzene-d6, rt, δ): 97.4.

7-[(Triisopropylsilyl)iodo]-8,9,10,11,12-pentaiodo-1-carba-closo-dodecaborane (28); Typical Procedure:[47]

To a slurry of [Ph3C][HCB11H5I6] 27 (0.05 g, 0.044 mmol) in toluene (20 mL) was added iPr3SiH (0.028 g, 0.18 mmol) and then the mixture was stirred for 1 week at rt. After filtration through a fine frit, hexane diffusion over a period of 10 d led to pale yellow crystals of 28 suitable for X-ray diffraction;29Si NMR (CPMAS, δ): 97.0.

[8-(Dimethylsilyl)-1-naphthyl]dimethylsilylium Tetrakis(pentafluorophenyl)borate (29); Typical Procedure:[51]

(2,2″,4,4″,6,6″-Hexamethyl-1,1′:3′, 1″-terphenyl-2′-yl)dimethylsilylium Tetrakis(pentafluorophenyl)borate (30); Typical Procedure:[27]

A suspension of [Ph3C][B(C6F5)4] (221 mg, 0.24 mmol) and (2,2′,4,4′,6,6′-hexamethyl-1,1′:3′,1″-terphenyl-2′-yl)dimethylsilane (89.4 mg 0.24 mmol) in dry benzene (1 mL) (CAUTION:carcinogen) was prepared. The oily brown mixture was stirred for 24 h at rt; two layers formed: a dark brown oil at the bottom and a clear yellow upper layer. The brown oil, containing mainly the ionic product, was examined by NMR spectroscopy. The product was isolated by addition of pentane to the oil, removal of the supernatant, and drying of the residue under vacuum. Repeated addition of hexane and vigorous stirring afforded the borate 30 as a solid; yield: 240 mg (95%);29Si NMR (benzene-d6, rt, δ): 79.1.

4.4.43.2.2 Variation 2: By Hydride Transfer in Silanes

The use of excess silane [R3SiH] as the solvent in the hydride-transfer reaction with triphenylcarbenium tetrakis(pentafluorophenyl)borate results in the formation of bissilylhydronium ions {[R3Si—H—SiR3]+} that might be regarded as silane-stabilized silylium ions.[53] These cations are characterized by a Si—H—Si three-center, two-electron bond and feature tetracoordinated silicon atoms (▶ Scheme 13). In publications prior to 2011, the obtained colorless salts were often erroneously identified as silylium salts {[R3Si]+[B(C6F5)4]–}.[53–55] The silane is easily replaced by even weak donors such sulfur dioxide or benzene. Therefore, these bissilylhydronium borates can be used as synthetic equivalents for silylium borates. In cases where triphenylcarbenium carborates are applied, the anion-stabilized species are isolated (▶ Scheme 4).[56] Related to these bissilylhydronium ions {[R3Si—H—SiR3]+} are species formed by the interaction of tris(pentafluorophenyl)borane with silanes [R3SiH].[57] The neutral complexes of composition R3Si—HB(C6F5)3, which are in equilibrium with the starting silane [R3SiH] and tris(pentafluorophenyl)borane, act as powerful silylating agents and consequently they can be regarded as hydridoborate-stabilized silylium ions. Such species are suggested to be intermediates in hydrosilylation reactions of carbonyl compounds catalyzed by triarylboranes. A stereochemical analysis of the course of this reaction reveals that no free silylium ions are involved.[58] Stable compounds with intramolecular variants of Si—H—B bridges have also been reported.[59]

▶ Scheme 13 Preparation of a Bissilylhydronium Ion[53,55]

Bis(trimethylsilyl)hydronium Tetrakis(pentafluorophenyl)borate (31); Typical Procedure:[53,55]

A large excess of Me3SiH (30 mL) was condensed onto solid [Ph3C][B(C6F5)4] (4.612 g, 5.0 mmol) under vacuum at –196 °C in a sealed tube (equipped with a PTFE valve). The resulting yellow suspension was warmed to rt and stirred for 10 h, resulting in a colorless suspension. Excess Me3SiH was then slowly removed under reduced pressure, and the resulting colorless residue was suspended in pentane, and filtered through an F4-filter frit. The colorless residue was washed by several back distillations of solvent, and dried under vacuum to give 31 as a colorless solid; yield: 3.723 g (90%); mp 137 °C (dec); IR (ATR) : (Si—H—Si) 1941 (m, br) cm–1;29Si NMR (Me3SiH, benzene-d6 ext, δ): 84.5;29Si NMR (CPMAS, δ): 84.8.

4.4.43.2.3 Variation 3: By Hydride Transfer in Halogenated Hydrocarbons

The most widely used hydride-transfer reagent is triphenylcarbenium tetrakis(pentafluorophenyl)borate, and the produced silyl cation borates in arene solvents form a biphasic mixture. The ionic phase solidifies at temperatures not much lower than –20 °C. Therefore, for low-temperature work, it is necessary to change the solvent to halogenated hydrocarbons. The most popular solvents for these cases are chlorobenzene and dichloromethane. Although in general nonstabilized silylium ions form stable solvent complexes with aromatic halohydrocarbons,[45] special care must be taken when aliphatic chlorides are used. Thus, stabilized silylium ions can be prepared in dichloromethane at lower temperatures, but undergo decomposition in these solvents at ambient temperatures (▶ Scheme 14).[60] Nevertheless, using carefully controlled conditions these silylium ions can be isolated and even crystallized, revealing intriguing structural features caused by the high electron deficiency of the positively charged silicon atom. A particular interesting example is provided by the ferrocenylsilylium borate 32, in which the silylium ion is stabilized by the interaction with the transition metal.[61] Related ferrocenylsilylium borate 34 is applied as a Lewis acid catalyst in several chemical transformations (▶ Scheme 15).[9] The Lewis acidity of the silicon cation 36 is conserved by the interaction with the ferrocene backbone and it is available for the activation of the Lewis base substrate (S) and its transformation into the Lewis base product (P). The intramolecular electron donation from the ferrocenyl group has two functions: it pacifies the electron demand of the silicon atom to some extent and it supports the decomplexation of the Lewis basic product to regenerate the cation.

▶ Scheme 14 Preparation of an Internally Stabilized Silylium Ion in Dichloromethane[60]

▶ Scheme 15 Chemical Transformation of Substrate to Product Mediated by a Ferrocenyl-Stabilized Silylium Ion[9]

In some cases only the use of dichloromethane allows the successful synthesis of silylium ions, due to the moderately strong coordination of dichloromethane to the silicon atom. For example, aromatic silatropylium cations decompose in toluene at temperatures as low as –50 °C, but can be detected at the same temperature in dichloromethane.[62,63] Simple silylium ions, however, cannot be prepared in dichloromethane even at temperatures as low as –78 °C.[64]

tert-Butyl(ferrocenyl)methylsilylium Tetrakis(pentafluorophenyl)borate (34); Typical Procedure:[60]

In a glovebox, a flame-dried 10-mL Schlenk tube equipped with a magnetic stirrer bar was charged with [Ph3C][B(C6F5)4] (46 mg, 0.050 mmol). Then, the Schlenk tube was transferred to a fume hood and connected to an argon–vacuum manifold. Addition of dry CH2Cl2(1.5 mL) resulted in a yellow soln, which was subsequently cooled to –78 °C, followed by addition of ferrocenylsilane 33 (13 mg, 0.055 mmol). The soln was maintained at –78 °C for 10 min whereupon the color changed to deep red, indicating the formation of the stabilized silylium ion 34;29Si NMR (CD2Cl2, –40 °C, δ): 114.5. Subsequent warming to rt gave ferrocenylsilyl chloride 35;29Si NMR (CD2Cl2, rt, δ): 30.1.

4.4.43.2.4 Variation 4: By Hydride Transfer with Subsequent Intermolecular Substituent Exchange

The hydride transfer between the triphenylcarbenium cation and bulky triarylsilanes is severely hampered by steric effects. Bisarylsilanes, however, readily undergo a hydridetransfer reaction in benzene or toluene. In some cases, subsequent intermolecular substituent exchange to give triarylsilylium ions (e.g., 38) follows (▶ Scheme 16). A series of nonstabilized triarylsilylium ions has been prepared using this reaction.[18] The rate-determining step for this reaction is the initial hydride transfer to produce an alkyldiarylsilylium ion. In subsequent intermolecular reactions with the starting silane, aryl and alkyl group exchange reactions occur. There are specific requirements for the steric bulkiness of the aryl substituents at the silicon atom. The substituents at the alkyldiarylsilane (e.g., 37) must be large enough to prevent the formation of solvent-stabilized silyl cations, but their steric bulkiness must be limited so as not to prevent the subsequent bimolecular reaction between the formed diarylalkylsilylium ion und unreacted silane to give the triarylsilylium ion (e.g., 38). The substituent-exchange reaction does not occur when the primarily formed alkyldiarylsilyl cation forms stable solvent complexes.

▶ Scheme 16 Preparation of a Triarylsilylium Ion from an Alkyldiarylsilane[18]

Tris(pentamethylphenyl)silylium Tetrakis(pentafluorophenyl)borate (38); Typical Procedure:[18]

All manipulations were conducted under exclusion of air and moisture. [Ph3C][B(C6F5)4] (500 mg, 0.54 mmol) was dissolved in benzene (2 mL) (CAUTION:carcinogen), resulting in a biphasic soln. Diarylsilane 37 (292.2 mg, 0.864 mmol) was added via syringe. The resulting soln was stirred at rt for 1 h. Stirring was stopped and the two phases were allowed to separate. The completeness of the reaction could be assessed by the disappearance of the orange color of the upper phase. The upper phase was removed, the lower phase was washed with benzene (2 ×), and the volatiles were evaporated under reduced pressure to give silylium borate 38; yield: quant;29Si NMR (benzene-d6, δ): 216.2.

4.4.43.3 Method 3: Heterolytic Cleavage of Si—C Bonds

The heterolytic cleavage of the significantly stronger Si—C bonds by potent electrophiles to liberate silyl cations has found far less synthetic use compared to the corresponding reaction of hydrogen-substituted silanes. Nevertheless, this method is applicable when silanes with good leaving groups are used.[65] Historically, it was the method of choice for the preparation of the first silylium ion not stabilized by inter- or intramolecular interaction with donors, i.e. trimesitylsilylium as its tetrakis(pentafluorophenyl)borate salt 40 (▶ Scheme 17)[12,16] or as its 7,8,9,10,11,12-hexabromo-2,3,4,5,6-pentamethyl-1-carba-closododecaborate {[HCB11Me5Br6]–} salt.[17] In all reported cases, very potent electrophiles, such as carbocations, protonated ethers, or silylarenium ions were used to achieve Si—C bond cleavage.[16,19,66] The cleavage of Si—C bonds with the formation of silyl cationic species is thought to be the initial step in Wagner–Meerwein-type rearrangements of peralkylated polysilanes catalyzed by Lewis acids.[67]

▶ Scheme 17 Preparation of a Triarylsilylium Ion from an Allyltriarylsilane[16]

Trimesitylsilylium Tetrakis(pentafluorophenyl)borate (40):[16]

In a N2-filled glovebox, [Ph3C][B(C6F5)4] (160 mg, 0.17 mmol) was dissolved in dry benzene-d6 (0.7 mL) (CAUTION:carcinogen) in a valved 5-mm NMR tube. Addition of Et3SiH (25 mg, 0.22 mmol) produced two layers, the lower one consisting of a light brown oil. The colorless top phase was taken up in a syringe to remove the Ph3CH byproduct. 1,1-Diphenylethene (40 mg, 0.22 mmol) was added, and the oil phase became deep green. Allyltrimesitylsilane (39; 80 mg, 0.19 mmol) in benzene-d6 (0.5 mL) was added to again create two layers. The lower layer was a deep red oil. The light orange top phase was removed, and the remaining oil was examined by NMR spectroscopy;29Si NMR (benzene-d6, δ): 225.5.

4.4.43.4 Method 4: Heterolytic Cleavage of Si—Si Bonds

The cleavage of Si—Si bonds by strong electrophiles has been applied in one special case for the synthesis of an aromatic trisilacyclopropenium ion as its borate salt 42 (▶ Scheme 18).[20] Related reactions have been used previously for the generation of a trigermacyclopropenium ion, and the method has the potential to develop into an important synthetic route for the generation of silyl cations from electron-rich polysilanes. As an electrophile, the triphenylcarbenium cation paired with a weakly coordinating borate anion is applied. It has been claimed that the triphenylcarbenium cation acts as a one-electron oxidant and that the resulting radical cation fragments into the silyl cation and a silyl radical.[68] The fate of both radicals produced, trityl and silyl, is, however, not clear.

▶ Scheme 18 Preparation of a Cyclotrisilenylium Cation[20]

[Di-tert-butyl(methyl)silyl]bis(tri-tert-butylsilyl)cyclotrisilenylium Tetrakis[4-(tert-butyldimethylsilyl)-2,3,5,6-tetrafluorophenyl]borate (42):[20]

To a mixture of cyclotrisilene 41 (150 mg, 0.19 mmol) and triphenylcarbenium tetrakis[4-(tert-butyldimethylsilyl)-2,3,5,6-tetrafluorophenyl]borate (221 mg, 0.18 mmol) was added carefully dried and degassed toluene (1.5 mL) by vacuum transfer. Then the toluene suspension was stirred at rt for 8 h. The mixture separated into two liquid phases, accompanied by a color change from red-orange to dark brown (lower phase). The lower phase was separated and washed with hexane to remove neutral materials, giving 42 as an air- and moisture-sensitive yellow solid; yield: quant; mp 240–242 °C (dec); 29Si NMR (toluened8, rt, δ): 5.3, 43.3, 48.4, 284.6, 288.1.

4.4.43.5 Method 5: Oxidation of Disilanes and Silyl Radicals

The oxidation of distannanes to give stannyl cations by one-electron oxidants in strongly coordinating solvents such as acetonitrile has been firmly established since the early 1990s.[69,70] The use of weakly coordinating anions allows this synthetic methodology to be adapted to the synthesis of stabilized silylium ions by the oxidation of disilanes.[71] In the case of disilanes, triphenylcarbenium borates are useful one-electron oxidants. The reaction is thought to proceed via one-electron oxidation of the disilane by the triphenylcarbenium cation. The resulting disilane radical cation undergoes fragmentation into a silyl cation and a silyl radical. The latter is oxidized to the silyl cation by a second equivalent of triphenylcarbenium cation. It is noteworthy, however, that only highly sterically congested disilanes such as hexa-tert-butyldisilane and hexaisopropyldisilane undergo the oxidation reaction. Obviously, the oxidation potential of hexaethyldisilane and other sterically less congested disilanes is too high. Silylium ions that are stabilized by the nitrile solvent, i.e. silylnitrilium ions such as 43, are obtained by this method (▶ Scheme 19).[71] In view of the problems with bulky silanes in the hydride-transfer reaction (see ▶ Section 4.4.43.2