Handbook of Metathesis, Volume 1 E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
The second edition of the Handbook of Metathesis, edited by Nobel Prize Winner Robert H. Grubbs and his team, is available as a 3 Volume set as well as individual volumes.
Volume 1, edited by R. H. Grubbs together with A. G. Wenzel focusses on Catalyst Development and Mechanism.
The new edition of this set is completely updated (more than 80% new content) and expanded, with a special focus on industrial applications. Written by the "Who-is-Who" of metathesis, this book gives a comprehensive
and high-quality overview. It is the perfect and ultimate one-stop-reference source in this field and indispensable for chemists in academia and industry alike.
View the set here - http://www.wiley.com/WileyCDA/WileyTitle/productCd-3527334246.html
Other available volumes:
Volume 2: Applications in Organic Synthesis, Editors: R. H. Grubbs and D. J. O´Leary - http://www.wiley.com/WileyCDA/WileyTitle/productCd-3527339493.html
Volume 3: Polymer Synthesis, Editors: R. H. Grubbs and E. Khosravi - http://www.wiley.com/WileyCDA/WileyTitle/productCd-3527339507.html
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Veröffentlichungsjahr: 2015
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Table of Contents
Cover
Title Page
Copyright
Preface
References
List of Contributors
Chapter 1: High-Oxidation State Molybdenum and Tungsten Complexes Relevant to Olefin Metathesis
1.1 Introduction
1.2 New Imido Ligands and Synthetic Approaches
1.3 Bispyrrolide and Related Complexes
1.4 Monoalkoxide Pyrrolide (MAP) Complexes
1.5 Reactions of Alkylidenes with Olefins
1.6 Olefin and Metallacyclopentane Complexes
1.7 Tungsten Oxo Complexes
1.8 Bisaryloxides
1.9 Other Constructs
1.10 Conclusions
Acknowledgments
References
Chapter 2: Alkane Metathesis
2.1 Introduction
2.2 Alkane Metathesis by Single-Catalyst Systems
2.3 Alkane Metathesis by Tandem, Dual-Catalytic Systems
2.4 Conclusion
References
Chapter 3: Diastereocontrol in Olefin Metathesis: the Development of Z-Selective Ruthenium Catalysts
3.1 Introduction
3.2 The Challenge of
Z
-Selective Olefin Metathesis
3.3 Previous Strategies
3.4 A Serendipitous Discovery
3.5 Catalyst Studies
3.6 Applications of
Z
-Selective Ru Metathesis Catalysts
3.7 Conclusion
References
Chapter 4: Ruthenium Olefin Metathesis Catalysts Supported by Cyclic Alkyl Aminocarbenes (CAACs)
4.1 Introduction
4.2 Properties and Preparation of CAAC Ligands
4.3 CAAC-Supported, Ruthenium Olefin Metathesis Catalysts
4.4 Summary
References
Chapter 5: Supported Catalysts and Nontraditional Reaction Media
5.1 Introduction
5.2 Supported Catalyst Systems
5.3 Olefin Metathesis in Nontraditional Media
5.4 Conclusions
References
Chapter 6: Insights from Computational Studies on d0 Metal-Catalyzed Alkene and Alkyne Metathesis and Related Reactions
6.1 Introduction
6.2 Alkene Metathesis
6.3 Alkyne Metathesis
6.4 Alkane Metathesis
6.5 Outlook
References
Chapter 7: Computational Studies of Ruthenium-Catalyzed Olefin Metathesis
7.1 Introduction
7.2 Computational Investigations of Non-Chelated Ruthenium Catalysts
7.3 Computational Investigations of Chelated,
Z
-Selective Ruthenium Catalysts
7.4 Accuracy of the Computational Methods
References
Chapter 8: Intermediates in Olefin Metathesis
8.1 Introduction
8.2 Metathesis-Active, Early-Metal Metallacycles
8.3 Intermediates in Ruthenium-Catalyzed Olefin Metathesis
8.4 Future Directions
References
Chapter 9: Factors Affecting Initiation Rates
9.1 Introduction
9.2 Grubbs Second-Generation Catalyst
9.3 Grubbs–Hoveyda-Type Precatalysts
9.4 Pyridine Solvates
9.5 Piers Catalysts
9.6 Indenylidene Carbene Precatalysts
9.7
Z
-Selective Catalysts
9.8 Herrmann-Type, BisNHCs
9.9 Conclusions
Acknowledgments
References
Chapter 10: Degenerate Metathesis
10.1 Introduction
10.2 Degenerate Metathesis Mechanisms
10.3 Degenerate Metathesis with Early Transition-Metal Catalysts
10.4 Degenerate Metathesis with Ruthenium Catalysts
10.5 Beneficial Effects of Degenerate Metathesis
10.6 Conclusions
References
Chapter 11: Mechanisms of Olefin Metathesis Catalyst Decomposition and Methods of Catalyst Reactivation
11.1 Introduction
11.2 Decomposition of Mo and W Imido Alkylidene Catalysts and Related Complexes
11.3 Decomposition of Ru Alkylidene Catalysts and Related Complexes
11.4 Conclusions
References
Chapter 12: Solvent and Additive Effects on Olefin Metathesis
12.1 General Introduction
12.2 Solvent Effects on Olefin Metathesis
12.3 Additive Effects in Olefin Metathesis
12.4 Summary
References
Chapter 13: Metathesis Product Purification
13.1 Introduction
13.2 Chromatographic and Chemical Removal of Ruthenium
13.3 Removal by Complexation
13.4 Conclusion
References
Chapter 14: Ruthenium Indenylidene Catalysts for Alkene Metathesis
14.1 Introduction
14.2 The Initial Development of Indenylidene Metal Complexes for Alkene Metathesis
14.3 Binuclear Indenylidene Ruthenium Catalysts Arising from Ruthenium(arene) Complexes
14.4 Preparation of Ruthenium Indenylidene Catalysts from RuCl
2
(PPh
3
)
3
14.5 Ruthenium Catalysts Bearing a Chelating Indenylidene Ligand
14.6 Conclusion
References
Index
End User License Agreement
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Guide
Cover
Table of Contents
Table of Contents
Begin Reading
List of Illustrations
Scheme 1.1
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Scheme 1.2
Scheme 1.3
Scheme 2.1
Figure 2.1
Figure 2.2
Scheme 2.2
Figure 2.3
Scheme 2.3
Figure 2.4
Scheme 2.4
Scheme 2.5
Scheme 2.6
Scheme 2.7
Scheme 2.8
Figure 2.5
Figure 2.6
Figure 2.7
Scheme 2.9
Figure 2.8
Scheme 2.10
Scheme 2.11
Scheme 2.12
Figure 2.9
Scheme 2.13
Figure 2.10
Figure 2.11
Figure 2.12
Scheme 2.14
Figure 2.13
Scheme 2.15
Scheme 2.16
Figure 2.14
Figure 2.15
Figure 2.16
Scheme 2.17
Figure 2.17
Figure 2.18
Scheme 2.18
Figure 2.19
Figure 2.20
Figure 2.21
Figure 2.22
Scheme 2.19
Scheme 2.20
Scheme 3.1
Scheme 3.2
Scheme 3.3
Scheme 3.4
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Scheme 3.5
Scheme 3.6
Figure 3.5
Scheme 3.7
Scheme 3.8
Figure 4.1
Figure 4.2
Scheme 4.1
Figure 4.3
Scheme 4.2
Scheme 4.3
Scheme 4.4
Scheme 4.5
Scheme 4.6
Figure 5.1
Figure 5.2
Scheme 5.1
Figure 5.3
Figure 5.4
Scheme 5.2
Figure 5.5
Scheme 5.3
Figure 5.6
Figure 5.7
Figure 5.8
Figure 5.9
Scheme 5.4
Figure 5.10
Scheme 5.5
Figure 5.11
Scheme 5.6
Figure 5.12
Figure 5.13
Figure 5.14
Scheme 5.7
Figure 5.15
Scheme 5.8
Figure 5.16
Scheme 5.9
Figure 5.17
Figure 5.18
Scheme 5.10
Figure 5.19
Figure 5.20
Figure 5.21
Figure 5.22
Figure 5.23
Figure 5.24
Figure 5.25
Figure 5.26
Figure 5.27
Figure 5.28
Figure 5.29
Scheme 5.11
Scheme 5.12
Figure 5.30
Figure 5.31
Figure 5.32
Figure 5.33
Scheme 6.1
Scheme 6.2
Scheme 6.3
Scheme 6.4
Scheme 6.5
Scheme 6.6
Scheme 6.7
Figure 6.1
Scheme 6.8
Scheme 6.9
Scheme 6.10
Scheme 6.11
Scheme 6.12
Scheme 6.13
Scheme 6.14
Scheme 6.15
Scheme 6.16
Scheme 6.17
Scheme 6.18
Scheme 6.19
Scheme 6.20
Scheme 6.21
Scheme 6.22
Figure 6.23
Scheme 6.24
Scheme 6.25
Scheme 6.26
Figure 7.1
Figure 7.2
Figure 7.3
Figure 7.4
Figure 7.5
Figure 7.6
Figure 7.7
Figure 7.8
Figure 7.9
Figure 7.10
Figure 7.11
Figure 7.12
Figure 7.13
Figure 7.14
Figure 7.15
Figure 7.16
Figure 7.17
Scheme 7.1
Scheme 7.2
Scheme 7.3
Scheme 7.4
Scheme 7.5
Scheme 7.6
Scheme 7.7
Scheme 7.8
Scheme 7.9
Scheme 7.10
Scheme 7.11
Scheme 7.12
Scheme 7.13
Scheme 7.14
Scheme 7.15
Scheme 7.17
Scheme 7.17
Scheme 7.18
Figure 7.18
Figure 7.19
Figure 7.20
Scheme 7.19
Scheme 7.20
Scheme 7.21
Figure 7.21
Figure 7.22
Scheme 8.1
Figure 8.1
Figure 8.2
Scheme 8.2
Scheme 8.3
Figure 8.3
Scheme 8.4
Figure 8.4
Scheme 8.5
Figure 8.5
Figure 8.6
Scheme 8.6
Scheme 8.7
Figure 8.7
Scheme 8.8
Scheme 8.9
Figure 8.8
Scheme 8.10
Scheme 8.11
Scheme 8.12
Figure 9.1
Scheme 9.1
Figure 9.2
Figure 9.3
Figure 9.4
Figure 9.5
Figure 9.6
Scheme 9.2
Figure 9.7
Scheme 9.3
Figure 9.1
Figure 9.8
Figure 9.2
Scheme 9.4
Scheme 10.1
Scheme 10.2
Scheme 10.3
Scheme 10.4
Scheme 10.5
Scheme 10.6
Scheme 10.7
Scheme 10.8
Scheme 10.9
Scheme 10.10
Scheme 10.11
Scheme 10.12
Scheme 10.13
Scheme 10.14
Scheme 10.15
Scheme 10.16
Scheme 10.17
Scheme 10.18
Scheme 10.19
Figure 10.1
Scheme 10.20
Figure 10.2
Figure 10.3
Scheme 10.21
Scheme 10.22
Scheme 10.23
Scheme 10.24
Scheme 10.25
Scheme 11.1
Scheme 11.2
Scheme 11.3
Scheme 11.4
Scheme 11.5
Scheme 11.6
Scheme 11.7
Scheme 11.8
Scheme 11.9
Figure 11.1
Scheme 11.10
Scheme 11.11
Figure 11.2
Figure 11.3
Scheme 11.12
Scheme 12.1
Scheme 12.2
Scheme 12.3
Scheme 12.4
Scheme 12.5
Scheme 12.6
Figure 12.1
Scheme 12.7
Figure 12.2
Figure 12.3
Scheme 12.8
Scheme 12.9
Scheme 12.10
Scheme 12.11
Scheme 12.12
Scheme 12.13
Scheme 12.14
Scheme 12.15
Scheme 12.16
Scheme 12.17
Scheme 12.18
Scheme 12.19
Scheme 12.20
Scheme 12.21
Scheme 12.22
Scheme 12.23
Scheme 12.24
Scheme 12.25
Scheme 12.26
Scheme 12.27
Scheme 12.28
Scheme 12.29
Scheme 12.30
Schemes 12.31
Figure 12.32
Scheme 12.33
Scheme 12.34
Scheme 12.35
Scheme 12.36
Scheme 13.1
Figure 13.1
Figure 13.2
Figure 13.3
Figure 13.4
Figure 13.5
Scheme 14.1
Scheme 14.2
Scheme 14.3
Scheme 14.4
Scheme 14.5
Scheme 14.6
Scheme 14.7
Scheme 14.8
Scheme 14.9
Scheme 14.10
Scheme 14.11
Scheme 14.12
Scheme 14.13
Scheme 14.14
Scheme 14.15
Scheme 14.16
Scheme 14.17
Scheme 14.18
Scheme 14.19
Scheme 14.20
Scheme 14.21
Scheme 14.22
Scheme 14.23
Scheme 14.24
Scheme 14.25
Scheme 14.26
Scheme 14.27
Scheme 14.28
Scheme 14.29
Scheme 14.30
Scheme 14.31
List of Tables
Table 1.1
Table 4.1
Table 7.1
Table 7.2
Table 8.1
Table 8.2
Table 9.1
Table 9.2
Table 9.3
Table 9.4
Table 9.5
Table 9.6
Table 9.7
Table 9.8
Table 9.9
Table 9.10
Table 10.1
Edited by Robert H. Grubbs and Anna G. Wenzel
Handbook of Metathesis
Volume 1: Catalyst Development and Mechanism
Second Edition
Editors
Prof. Dr. Robert H. Grubbs
California Institute of Technology
Division of Chemistry and Chemical Engineering
E. California Blvd 1200
Pasadena, CA 91125
United States
Prof. Anna G. Wenzel
W.M. Keck Science Center
The Claremont Colleges
925 N. Mills Avenue
Claremont, CA 91711
United States
Handbook of Metathesis
Second Edition
Set ISBN (3 Volumes): 978-3-527-33424-7
oBook ISBN: 978-3-527-67410-7
Vol 2: Applications in Organic Synthesis,
Editors: R. H. Grubbs and D. J. O'Leary
ISBN: 978-3-527-33949-5
Vol 3: Polymer Synthesis,
Editors: R. H. Grubbs and E. Khosravi
ISBN: 978-3-527-33950-1
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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Preface
In 2003, the first edition of the Handbook of Metathesis comprehensively covered the origins of the olefin metathesis reaction and the myriad of applications blossoming from the development of robust, homogeneous transition-metal catalysts. In the intervening 10 years, applications and advances in this field have continued to exponentially increase. To date, 3732 publications regarding olefin metathesis have been reported; of these, 2292 have been reported since 2003! By 2005, olefin metathesis had become so integral to the field of organic synthesis that the Nobel Prize in Chemistry was awarded to the field (Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock) [1, 2].
In light of these many advancements, a second edition of the Handbook is quite timely. Early on in the planning, it was decided that rather than simply updating the 2003 edition, the second edition would instead emphasize important advancements (e.g., new ligands, diastereoselective metathesis, alkyne metathesis, industrial applications, self-healing polymers) that have occurred during the past decade. In addition, the past 10 years have seen important developments in our understanding of the metathesis mechanism utilizing both computational and mechanistic studies. A greater knowledge of catalyst decomposition, product purification, and the use of supported catalysts and nontraditional reaction media have further enhanced the utility of metathesis systems. A number of new applications are now becoming commercialized based on these new catalyst systems. For example, the first pharmaceutical that uses olefin metathesis in a key step is now commercially available, and a biorefinery that utilizes a homogeneous catalyst is now in production.
Similar to the first edition of this Handbook, contributions have been arranged into three volumes. Volume I (Anna Wenzel, coeditor) emphasizes recent catalyst developments and mechanism and is intended to provide a foundation for the applications discussed throughout the rest of the Handbook. Volume II (Dan O'Leary, coeditor) covers synthetic applications of the olefin metathesis reaction, and polymer chemistry is the topic of Volume III (Ezat Khosravi, coeditor). Chapter topics have been selected to provide comprehensive coverage of these areas of olefin metathesis. Contributors, many of whom are pioneers in the field, were chosen based on their firsthand experience with the topics discussed.
We wish to sincerely thank all the contributors for their diligence in writing and editing their chapters. Our goal was to comprehensively cover the complete breadth of the olefin metathesis reaction – this Handbook would not have been possible without all their time and effort! It was truly a pleasure and an honor to work with everyone!
Claremont, CAAnna G. Wenzel, Daniel J. O'Leary
Durham, UKEzat Khosravi, and
Pasadena, CARobert H. Grubbs
November 20th, 2014
1 Data obtained from keyword searches conducted within the ISI Web of Science (accessed 1/18/2015).
References
1. Nobel Prizes.org Development of the Metathesis Method in Organic Synthesis,
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2005/advanced-chemistryprize2005.pdf
(accessed 18 January 2015).
2. Rouhi, M. (2005)
Chem. Eng. News
,
83
, 8.
List of Contributors
Daryl P. Allen
Materia Inc.
North San Gabriel Boulevard
Pasadena, CA 91107
USA
Jean-Marie Basset
King Abdullah University of Science and Technology
Catalysis Research Center
Q8 P.O. Box 55455
Thuwal, 23955-6900
Saudi Arabia
Christian Bruneau
UMR 6226 : CNRS - Université de Rennes 1
Institut des Sciences Chimiques
Organométalliques : Matériaux et Catalyse
Centre de Catalyse et Chimie Verte
Campus de Beaulieu
Bât 10C, 35042 Rennes
France
Emmanuel Callens
King Abdullah University of Science and Technology
Catalysis Research Center
Q8 P.O. Box 55455
Thuwal, 23955-6900
Saudi Arabia
Shawn K. Collins
University of Montreal
Department of Chemistry
Édouard-Montpetit Boulevard
V-325, Montréal
QC H3T 1J4
Canada
Christophe Copéret
ETH Zürich
Department of Chemistry and Applied Biosciences
Vladimir Prelog Weg 2.
CH-8093 Zürich
Switzerland
Steven T. Diver
University of Buffalo
Department of Chemistry
Natural Sciences Complex
Buffalo, NY 14260-3000
USA
Pierre H. Dixneuf
UMR 6226 : CNRS - Université de Rennes 1
Institut des Sciences Chimiques
Organométalliques : Matériaux et Catalyse
Centre de Catalyse et Chimie Verte
Campus de Beaulieu
Bât 10C, 35042 Rennes
France
Odile Eisenstein
Institut Charles Gerhardt
CNRS UMR 5253
Université Montpellier 2
Place Eugène Bataillon
Montpellier
France
Jesus Garcia-Lopez
University of California
Los Angeles, Department of Chemistry and Biochemistry
Charles E. Young Dr. East
Los Angeles, CA 90095-1569
USA
Justin R. Gri{f}fiths
University at Buffalo
Department of Chemistry
Natural Sciences Complex
Buffalo, NY 14260-3000
USA
Kendall N. Houk
University of California
Los Angeles, Department of Chemistry and Biochemistry
Charles E. Young Dr. East
Los Angeles, CA 90095-1569
USA
Benjamin K. Keitz
Division of Chemistry and Chemical Engineering
California Institute of Technology, 1200
East California Boulevard
Pasadena, CA 91125
USA
David W. Knight
Cardiff University
School of Chemistry
Main Building
Park Place
Cardiff CF10 3AT
UK
Vincent Lavallo
University of California
Riverside
Department of Chemistry
Big Springs Road
Riverside, CA 92521
USA
Peng Liu
University of California
Los Angeles, Department of Chemistry and Biochemistry
Charles E. Young Dr. East
Los Angeles, CA 90095-1569
USA
Nassima Riache
King Abdullah University of Science and Technology
Catalysis Research Center
Q8 P.O. Box 55455
Thuwal, 23955-6900
Saudi Arabia
Richard R. Schrock
Department of Chemistry
Massachusetts Institute of Technology
Massachusetts Avenue
Cambridge, MA 02139
USA
Yann Schrodi
California State University Northridge
Department of Chemistry and Biochemistry
Magnolia Hall
Northridge, CA 91330-8262
USA
Xavier Solans-Monfort
Departament de Química
Universitat Autònoma de Barcelona
Campus Bellaterra
08193-Cerdanyola del Vallès
Spain
Ian C. Stewart
ExxonMobil Chemical Company
Bayway Drive
Baytown, TX 77520
USA
Buck L. H. Taylor
University of California
Los Angeles, Department of Chemistry and Biochemistry
Charles E. Young Dr. East
Los Angeles, CA 90095-1569
USA
David S. Weinberger
University of California
Riverside
Department of Chemistry
Big Springs Road
Riverside, CA 92521
USA
Anna G. Wenzel
Claremont McKenna, Pitzer, and Scripps Colleges
Keck Science Department
North Mills Avenue
Claremont
California 91711
USA
Chapter 1High-Oxidation State Molybdenum and Tungsten Complexes Relevant to Olefin Metathesis
Richard R. Schrock
1.1 Introduction
The first examples of high-oxidation state (“d0”) alkylidene and alkylidyne complexes (of tantalum) were published in 1974 and 1975 [1]. Several years of research were required to show that the principles behind tantalum chemistry could be employed to prepare alkylidene and alkylidyne complexes of Mo, W, and Re in their highest oxidation states (counting the alkylidene as a dianionic ligand and the alkylidyne as a trianionic ligand), and that these high-oxidation state complexes, especially those that contain one or more alkoxide ligands, are efficient catalysts for alkene and alkyne metathesis reactions, respectively. This process has been described in previous reviews [2]. Applications of high-oxidation state catalysts for alkene and alkyne metathesis in organic chemistry have also been reviewed [3], although each of these subjects is reviewed again elsewhere in this series in view of the many recent advancements.
This review will focus on isolated and characterized high-oxidation state molybdenum and tungsten alkylidene and metallacyclobutane complexes. Attention will be directed largely toward monoalkoxide pyrrolide (MAP) complexes because they have yielded the majority of new results in the last several years. MAP species have been found to be especially efficient in several Z-selective olefin metathesis reactions, such as homocoupling, cross-coupling, ethenolysis, and ROMP (see Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7). Most of what is presented here has appeared since a review in 2009 [4].
In the last decade, impressive advances have been made in the synthesis of alkylidene and alkylidyne complexes that contain metals from groups 4 [5] and 5 [5]i,j,m, [6], especially Ti and V, but – except for the ROMP of norbornene by vanadium complexes – group 4 and 5 metals have not shown wide-ranging activity for olefin metathesis. Well-characterized rhenium(VII) complexes are known to be active for metathesis, and many rhenium(VII) alkylidene and alkylidyne complexes have been isolated [2]a, but little attention has been paid to the syntheses of rhenium alkylidene or alkylidyne complexes in the last decade. Theoretical calculations have been carried out on M(X)(CHR′)(Y)(Z) complexes and their reactions with olefins, where X is imido (primarily) or oxo, and Y and Z are monoanionic ligands [7]; the results of these calculations are discussed in Chapter 6. Basic principles of Mo and W olefin metathesis catalysts will be discussed only if the new data have shed light on the basics. Advances in attaching Mo or W catalysts to solid supports, such as silica [8], alumina [9], or organic polymers [10], will also not be reviewed here, as it is discussed in Chapter 5. Transferring the knowledge gained from the studies of homogeneous catalysts to the synthesis of supported catalysts, especially those in which specificity is retained, is one of the remaining challenges in the field.
Table 1.1, located at the end of this chapter, provides a ready reference to Mo and W compounds that have been prepared since about 2007 that are relevant to olefin metathesis studies. Abbreviations can be found in the footnote to Table 1.1. Some entries in Table 1.1 are not discussed in the text since they have not been central to recent olefin metathesis studies. X-ray structures (indicated with an asterisk (*) in Table 1.1) will not be described in detail unless some unusual features warrant discussion.
Table 1.1 Tabulation of Isolated Neutral Alkylidene Complexes
Mo(NR)(CHR′)X
2
(X = pyrrolide, indolide, or pyrazolide)
References
Mo(NAr)(CHR′)(Pyr)
2
R′ =
t
-Bu or CMe
2
Ph
[11]
Mo(N-2,6-Br
2
-4-MeC
6
H
2
)(CH-
t
-Bu)(Pyr)
2
[11]
Mo(NAd)(CHCMe
2
Ph)(Pyr)
2
[11]
Mo(NAd)(CHCMe
2
Ph)(Pyr)
2
(PMe
3
)*
[12]
Mo(NR)(CHCMe
2
Ph)(Pyr)
2
(bipy)
R = Ar*, Ad, Ar
Me2
, Ar
Cl
, Ar
i
Pr
, Ar
t
Bu
, Ar
Mes
[55]
Mo(NAr)(CHCMe
2
Ph)X
2
X = Me
4
Pyr*,
i
-Pr
2
Pyr*, Ph
2
Pyr*, Indolide*
[12]
Mo(NAr)(CH-
t
-Bu)(Ph
2
Pyr)
2
[12]
Mo(NAr)(CHCMe
2
Ph)(indolide)
2
(THF)
[12]
Mo(NAr)(CHCMe
2
Ph)(R
2
Pz)
2
R
2
Pz = 3,5-diphenylpyrazolide* or 3,5-di-
t
-butylpyrazolide
[8]g
Mo(NR)(CHCMe
2
Ph)(Me
2
Pyr)
2
R = Ar, Ar
Me2
, Ar
CF3
[13]
Mo(NAd)(CHCMe
2
Ph)(Me
2
Pyr)
2
*
[12]
Mo(NAr)(CH-
t
-Bu)(Me
2
Pyr)
2
[14]
Mo(NAr)(CHCMe
2
Ph)(MesPyr)
2
*
[15]
Mo(NAd)(CH-
t
-Bu)(MesPyr)
2
[16]
Mo(NAd)(CHCMe
2
Ph)(indolide)
2
[12]
Mo(NAd)(CHCMe
2
Ph)(MesPyr)
2
[17]
Mo(NAd)(CHCMe
2
Ph)(2-CNPyr)
2
*
[17]
Mo(NC
6
F
5
)(CHCMe
2
Ph)(Me
2
Pyr)
2
[18]
Mo(NAr
X
)(CHCMe
2
Ph)(Me
2
Pyr)
2
X = Cl,
i
-Pr, Mes*
[19]
Mo(NAr
X
)(CH-
t
-Bu)(Me
2
Pyr)
2
X = CF
3
,
t
-Bu, Trip
[19]
Mo(NAr
Mes2
)(CHCMe
2
Ph)(Me
2
pyr)
2
[20]
Mo(NAr
Mes2
)(CHCMe
2
Ph)(Pyr)
2
(py)
[21]
Mo(NR)(CHR′)(pyrrolide)(OR″) (Mo MAP)
Mo(NAr)(CHCMe
2
Ph)(X)(OR
F6
)
X = Me
4
Pyr,
i
-Pr
2
Pyr, Ph
2
Pyr
[12]
Mo(NAr)(CHCMe
2
Ph)(X)(OR
F6
)(PMe
3
)
X = Me
2
Pyr, Me
4
Pyr,
i
-Pr
2
Pyr, Ph
2
Pyr*
[12]
Mo(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OR″)
OR″ = O-
t
-Bu, OCHMe
2
, OAr,*OCH(CF
3
)
2
, OR
F6
[22]
Mo(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OR″)
OR″ = OTPP*, ODPP*, OR
F6
*
[23]
Mo(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OR″)
OR″ = O-1-PhC
6
H
10
, OSi(O-
t
-Bu)
3
, OSiPh
3
[24]
Mo(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OSiPh
3
)
[25]
(
R
)-Mo(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OBr
2
Bitet)*
[26]
(
R
)-Mo(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OBr
2
Bitet)(PMe
3
)*
[27]
(
S
)-Mo(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OBr
2
Bitet)*
[26]
Mo(NAr)(CH
2
)(Pyr)(OHIPT)*
[28]
Mo(NAr)(CH
2
)(Me
2
Pyr)(OTPP)
[36]
Mo(NAr)(CHCMe
2
Ph)(Pyr)(OTPP)
[29]
Mo(NAr)(CHCMe
2
Ph)(Pyr)[OSi(
t
-Bu)
3
]*
[30]
Mo(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OCPh
3
)
[31]
Mo(NAr)(CHCMe
2
Ph)(Pyr)(OR)
OR = ODPP
Ph
* or ODPP
i
Pr
*
[32]
Mo(NAd)(CHCMe
2
Ph)(Pyr)(OR)
OR = ODPP
Ph
or ODPP
i
Pr
[32]
Mo(NAd)(CHCMe
2
Ph)(Me
2
Pyr)(OR)
OR = ODPP
Ph
or ODPP
i
Pr
[32]
Mo(NAd)(CHCMe
2
Ph)(Me
2
Pyr)(OTPP)
[33]
Mo(NAd)(CHCMe
2
Ph)(Pyr)(OHIPT)
[34]
Mo(NAd)(CH-
t
-Bu)(Pyr)(OHIPT)
[34]
Mo(NAd)(CHCMe
2
Ph)(Pyr)(OR)
OR = O-(3,5-R′
2
C
6
H
3
)
2
C
6
H
3
(R′ = Me or
t
-Bu), OCPh
3
, OSiTMS
3
[31]
Mo(NAd)(CH-
t
-Bu)(Pyr)(OHIPT)*
[28]
Mo(NAd)(CHCMe
2
Ph)(MesPyr)(OR)
OR = OTPP*, OBr
2
Bitet*, OHIPT*
[17]
Mo(NAd)(CHCMe
2
Ph)(CNPyr)(OHIPT)*
[17]
Mo(NAd)(CHCMe
2
Ph)(Pyr)(OHMT)
[35]
Mo(NAr
Me2
)(CHCMe
2
Ph)(Pyr)(OHIPT)
[14]
Mo(NR)(CHCMe
2
Ph)(Pyr)(OHMT)
R = Ar, Ar
Me2
, Ar
Cl
, Ar
i
Pr
*, Ar
t
Bu
, Ar
Mes
[55]
Mo(NAr
X
)(CHCMe
2
Ph)(Me
2
Pyr)(OHMT)
X = Cl,
i
-Pr, Mes*
[19]
Mo(NAr
X
)(CH-
t
-Bu)(Me
2
Pyr)(OHMT)
X = CF
3
,
t
-Bu, TRIP*
[19]
Mo(NAr
Mes
)(CHCMe
2
Ph)(Me
2
Pyr)(OHIPT)
[19]
Mo(NAr)(CH-
t
-Bu)(Me
2
Pyr)(OHMT)
[14]
Mo(NAr)(CHCHCHMe)(Me
2
Pyr)(OHMT)
[14]
Mo(NAr)[CHCHCMe
2
](Me
2
Pyr)(OHMT)*
[14]
Mo(NAr
Trip
)(CH-
t
-Bu)(Me
2
Pyr)(OTPP)
[19]
Mo(NAr
Mes2
)(CHCMe
2
Ph)(Me
2
pyr)(O-
t
-Bu)
[20]
Mo(NAr
Mes2
)(CHCMe
2
Ph)(Me
2
pyr)(OR)
R = CMe(CF
3
)
2
, OSiPh
3
, OArMe
2
[21]
Mo(NAr
Mes2
)(CHCMe
2
Ph)(OR)(Pyr)(py)
R = OCMe(CF
3
)
2
, OCHMe
2
, OCH(CF
3
)
2
, OAr
Me2
, OSi(
i
-Pr)
3
, OSiPh
3
, OSi(SiMe
3
)
3
[21]
Mo(NR)(CHCMe
2
Ph)(Me
2
Pyr)(OR′)
R = C
6
F
5
, OR′ = OHMT or ODFT; R = Ar′, OR′ = ODFT
[46]
Mo(NAr)(CHX)(Me
2
Pyr)(OTPP)
X = B(pin),* SiMe
3
,* Carbazole, Pyrrolidinone,* PPh
2
,* OPr,* or SPh*
[36]
W(NR)(CHR′)(pyrrolide)
2
W(NAr)(CHCMe
2
Ph)(Pyr)
2
(DME)*
[37]
W(NAr)(CHR′)(Me
2
Pyr)
2
R′ = CMe
2
Ph*,
t
-Bu
[37]
W(NR)(CH
2
)(Me
2
Pyr)
2
R = Ar or Ar
Cl2
*
[37]
W(NAr
Cl2
)(CH-
t
-Bu)(Pyr)
2
(DME)
[37]
W(NAr
Cl2
)(CH-
t
-Bu)(Me
2
Pyr)
2
[37]
W(NAr
Me2
)(CHCMe
2
Ph)(Me
2
Pyr)
2
[34]
W(NAr
Me2
)(CHCMe
2
Ph)(Pyr)
2
(DME)
[34]
W(NAr
t
Bu
)(CHCMe
2
Ph)(Me
2
Pyr)
2
[28]
W(NAr
3,5Me2
)(CHCMe
2
Ph)(Pyr)
2
(DME)
[38]
W(NAr
3,5Me2
)(CHCMe
2
Ph)(Me
2
Pyr)
2
[38]
W(NAr
3,5Me2
)(CHCMe
2
Ph)(MesPyr)
2
[38]
W(NC
6
F
5
)(CH-
t
-Bu)(Pyr)
2
(DME)
[18]
W(N-
t
-Bu)(CH-
t
-Bu)(Me
2
Pyr)
2
[39]
W(N-
t
-Bu)(CH-
t
-Bu)(Pyr)
2
(bipy)
[39]
W(NAr
Mes2
)(CHCMe
2
Ph)(Me
2
Pyr)
2
(py)
[21]
W(NAr
Mes2
)(CHCMe
2
Ph)(Pyr)
2
(py)
[21]
W(NR)(CHR′)(pyrrolide)(OR″) (W MAP)
W(NAr)(CHCMe
2
Ph)(Pyr)(OMes
2
Bitet)
[34]
W(NAr)(CHR′)(Me
2
Pyr)(OBr
2
Bitet)
R′ = H, CMe
2
Ph
[40]
W(NAr)(CHR′)(Me
2
Pyr)(OTPP)
R′ = H, CMe
2
Ph
[40]
W(NAr)(CH
2
)(Me
2
Pyr)(OTPP)*
[28]
W(NAr)(CH
2
)(Me
2
Pyr)(OR″)(PMe
3
)
OR″ = OBr
2
Bitet or OTPP
[40]
W(NAr)(CHCMe
2
Ph)(Me
2
Pyr)(OR″)
OR″ = OAr, OR
F6
, OSiPh
3
[24]
W(NAr)(CHCMe
2
Ph)(Pyr)(OHIPTNMe
2
)
[9]a
W(NAr
Cl
)(CH-
t
-Bu)(Pyr)(OHIPT)
[34]
W(NAr
t
Bu
)(CHCMe
2
Ph)(Me
2
Pyr)(OTPP)
[28]
W(NAr
3,5-Me2
)(CHCMe
2
Ph)(Me
2
Pyr)(OHIPT)
[38]
W(NAr
3,5-Me2
)(CHCMe
2
Ph)(Me
2
Pyr)(OTPP)
[38]
W(NAr
3,5Me2
)(CH
2
)(Me
2
Pyr)(OHIPT)
[38]
W(NR)(CHCMe
2
Ph)(Pyr)(OHMT)
R = Ar, Ar
Me2
[14]
W(NAr)(CH
2
)(Me
2
Pyr)(OTPP)(L)
L = THF*, PMe
3
*
[40]
W(N-
t
-Bu)(CH-
t
-Bu)(Me
2
pyr)(OHMT)
[39]
W(N-
t
-Bu)(CH-
t
-Bu)(pyr)(OHIPT)
[39]
W(N-
t
-Bu)(CH-
t
-Bu)(pyr)(OHMT)*
[39]
W(NAr
Mes2
)(CHCMe
2
Ph)(Me
2
pyr)(OR)
R = CMe(CF
3
)
2
, OSiPh
3
, OArMe
2
[21]
W Oxo-Alkylidene Complexes
W(O)(CH-
t
-Bu)(Me
2
Pyr)(OHIPT)*
[41]
W(O)(CH-
t
-Bu)(Me
2
Pyr)(OHMT)(PMe
2
Ph)*
[41]
W(O)(CH-
t
-Bu)(Me
2
Pyr)(OHMT)[B(C
6
F
5
)
3
]*
[41]
W(O)(CH-
t
-Bu)(Cl)(OHIPT)(PMe
2
Ph)*
[41]
W(O)(CH-
t
-Bu)(Ph
2
Pyr)(OHMT)*
[42]
W(O)(CH-
t
-Bu)[N(C
6
F
5
)
2
](OHMT)(PMe
2
Ph)*
[42]
W(O)(CH
2
)(OHMT)
2
*
[42]
W(O)(CH-
t
-Bu)(OHMT)
2
[42]
Metallacyclobutanes
Mo(NAr)(C
3
H
6
)(Me
2
Pyr)(OBr
2
Bitet)*
[25]
Mo(NAr)(C
3
H
6
)(Pyr)(OHIPT)
[28]
Mo(NAr)(C
3
H
6
)(OSiPh
3
)
2
[43]
W(NAr)(C
3
H
6
)(Pyr)(OHIPT)*
[33]
W(NAr)(C
3
H
6
)(Me
2
Pyr)(OBr
2
Bitet)*
[40]
W(NAr)(C
3
H
6
)(Me
2
Pyr)(OTPP)*
[40]
W(NAr)(C
3
H
6
)(Me
2
Pyr)(ODPP
Ph
)*
[32]
W(NAr
Me2
)(C
3
H
6
)(Pyr)(OHIPT)
[34]
W(NAr
t
Bu
)(C
3
H
6
)(Me
2
Pyr)(OTPP)
[28]
W(NAr
3,5Me2
)(C
3
H
6
)(Pyr)(OHIPT)*
[38]
W(NAr
3,5Me2
)(C
3
H
6
)(MesPyr)(OTPP)*
[38]
W(NC
6
F
5
)(C
3
H
6
)[OC(CF
3
)
3
]
2
*
[18]
W(NAr)(C
3
H
6
)[OC(CF
3
)
3
]
2
[24]
W(NAr)(C
3
H
6
)(Pyr)(OHIPTNMe
2
)
[9]a
W(NAr)[CH
2
CH(CMe
2
Ph)CH
2
](Pyr)(OHIPTNMe
2
)
[9]a
W(O)(C
3
H
6
)(OHMT)[OSi(
t
-Bu)
3
]*
[42]
W(O)(C
3
H
6
)(OHMT)
2
[42]
SAM's
Mo(NAd)(CHCMe
2
Ph)(OHIPT)(OR)
OR = OTf, O-
t
-Bu*
[17]
Mo(NAd)(CHCMe
2
Ph)(OHIPT)(OTf)(PMe
3
)*
[17]
Mo(NR)(CHCMe
2
Ph)(OHMT)(OR
F6
)
R = Ar, Ar
Me2
, Ar
i
Pr
, Ad*
[17]
Mo(NR)(CHCMe
2
Ph)[N(H)HMT](OR
F6
)
R = Ar
Me2
, Ar
i
Pr
*
[17]
Mo(NAd)(CHCMe
2
Ph)(HMT)(OR
F6
)*
[17]
Mo(NAr
Mes2
)(CHCMe
2
Ph)Cl(OR)(py)
OR = OR
F6
, O-
t
-Bu, OAr
Me2
, OHMT*
[20]
Mo(NAr)(CHCMe
2
Ph)(Pyr)(OTf)(DME)
[34]
Mo(NAr)(CHCMe
2
Ph)[OSi(
t
-Bu)
3
](OTf)
[44]
Bisalkoxide and bisaryloxide alkylidenes
Mo(NC
6
F
5
)(CHCMe
2
Ph)[OC(CF
3
)
3
]
2
[18]
W(NC
6
F
5
)(CH-
t
-Bu)[OC(CF
3
)
3
]
2
(DME)
[18]
Mo(NC
6
F
5
)(CHCMe
2
Ph)[OC(C
6
F
5
)
3
]
2
*
[18]
W(NC
6
F
5
)(CH-
t
-Bu)[OC(C
6
F
5
)
3
]
2
[18]
Mo(NC
6
F
5
)(CHCMe
2
Ph)(ODFT)
2
*
[18]
W(NC
6
F
5
)(CH-
t
-Bu)(ODFT)
2
[18]
Mo(NAr
Mes
)(CHCMe
2
Ph)(OTPP)
2
*
[19]
Mo(NAr)(CHCMe
2
Ph)(OBr
2
Bitet)
2
*
[45]
W(NAr)(CHCMe
2
Ph)(OR)
2
OR = OCMe(C
6
F
5
)
2
, OAr, OBINAP-TBS, OCMe
2
(CF
3
), OC(CF
3
)
3
, OSiPh
3
[24]
Mo(NAr)(CHCMe
2
Ph)(OR)
2
OR = OCMe(C
6
F
5
)
2
, OBINAP-TBS
[24]
OR = OSiMe
2
(
t
-Bu), OSiPh
3
[30]
Mo(NAd)(CHCMe
2
Ph)(OR)
2
OR = OSi(
t
-Bu)
3
, OSi(TMS)
3
, OSi(O-
t
-Bu)
3
[30]
Mo(NAr)(CHCMe
2
Ph)(ODFT)
2
R = Ar, Ar′, Ad
[46]
Olefin complexes
Mo(NAr)(alkene)(OSiPh
3
)
2
alkene = C
2
H
4
, styrene*,
trans
-3-hexene*
[43]
Mo(NAr)(C
2
H
4
)(OSiPh
3
)
2
(Et
2
O)*
[43]
Mo(NAr)(C
2
H
4
)(Pyrrolide)
2
Pyrrolide = Me
2
Pyr,* MesPyr
[43]
Mo(NAr)(C
2
H
4
)(Me
2
Pyr)(OR)
OR = OSiPh
3
,* OAr, OR
F6
[43]
Mo(NR)(C
2
H
4
)(OTf)
2
(dme)
NR = NAr, NAd
[43]
Mo(NAr)(C
2
H
4
)[OCH(CF
3
)
2
)
2
](Et
2
O)
[43]
Mo(NAr
Cl
)(C
2
H
4
)(Biphen)(Et
2
O)*
[47]
[Mo(NAr)(C
2
H
4
)(OR
F6
)(THF)
3
][B(3,5-(CF
3
)
2
C
6
H
3
)
4
]*
[15]
Mo(NC
6
F
5
)(CH
2
CH
2
)(DFTO)
2
*
[46]
Mo(NC
6
F
5
)(CH
2
CH
2
)(DCMNBD)(DFTO)
2
*
[46]
W(NAr
Cl
)(C
2
H
4
)(Biphen)(THF)*
[48]
W(NPh)(C
2
H
4
)[o-(Me
3
Si)
2
C
6
H
4
](PMe
3
)
2
*
[49]
Metallacyclopentanes
Mo(NAr)(C
4
H
8
)(Biphen)
[47]
Mo(NAr)(C
4
H
8
)(OSiPh
3
)
2
*
[43]
W(NAr
Cl
)(C
4
H
8
)(Biphen)*
[48]
*An X-ray structure was obtained for this compoundbipy, 2,2-bipyridyl; DME, 1,2-dimethoxyethane; Mes, 2,4,6-trimethylphenyl; NAd, N-1-Admantyl; NAr, N-2,6-i-Pr2C6H3; NArR2, N-2,6-R2C6H3; NArR, N-2-RC6H4; NAr3,5Me2, N-3,5-Me2C6H3; Biphen2−, 3,3′-di-t-Bu-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate; OBr2Bitet, 3,3′-dibromo-2′-(tert-butyldimethylsilyloxy)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl-2-olate; OHIPT, hexaisopropylterphenoxide = O-2,6-(2,4,6-i-Pr3C6H2)2C6H2; OHMT, hexamethylterphenoxide = O-2,6-Mes2C6H2; ODFT, decafluoroterphenoxide = O-2,6-(C6F5)2C6H3; ODPP, 2,6-diphenylphenoxide; ODIPP, O-2,6-(i-Pr)2C6H3; ORF6, OCMe(CF3)2; ORF3, OCMe2(CF3); OBiphenTMS, O-3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-2′-(trimethylsilyloxy)biphenyl-2-olate; OBINAP-TBS, O-2′-(tert-butyldimethylsilyloxy)-1,1′-binaphthyl-2-olate; ODPPR, 2,6-(2,5-R2pyrrolyl)2Phenoxide (R = i-Pr or Ph); Pyr, pyrrolide; MesPyr, 2-mesitylpyrrolide; CNPyr, 2-cyanopyrrolide; Me4Pyr, 2,3,4,5-tetramethylpyrrolide; R2Pyr, 2,5-R2pyrrolide; TBS, dimethyl-t-butylsilyl; Trip, 2,4,6-triisopropylphenyl.
1.2 New Imido Ligands and Synthetic Approaches
High-oxidation state alkylidenes are usually generated through α-hydrogen abstraction reactions [1], most efficiently from dineopentyl or dineophyl complexes [2]. The resulting neopentylidene or neophylidene complexes are the most stable terminal alkylidenes toward bimolecular coupling to give olefins, which is one of the main modes of decomposition of alkylidenes, especially methylidenes.
Since adamantylimido alkylidene complexes of molybdenum have been the catalysts of choice in some metathesis reactions, and since tungsten arylimido alkylidene complexes are often more selective for forming Z products than the analogous molybdenum complexes, a route to tungsten-based adamantyl and t-butylimido catalysts was sought.
A convenient route to tungsten t-butylimido and adamantylimido complexes employs the reaction between t-BuNH(TMS) and WCl6 to give {W(N-t-Bu)2(μ-Cl)(t-BuNH2)Cl}2 [50]. An analogous adamantylimido complex, {W(NAd)2(μ-Cl)(t-BuNH2)Cl}2 (NAd, N-1-admantyl), can also be formed in high yield [39]. These species can be alkylated directly with a neopentyl or neophyl Grignard reagent to give the W(NR)2(CH2R′)2 complexes (R = t-Bu or Ad; R′ = t-Bu or CMe2Ph). The addition of triflic acid to W(NR)2(CHR′)2 in the presence of 1,2-dimethoxyethane (DME), the standard method of making imido alkylidene complexes, did not lead to the expected W(NR)(CHR′)(OTf)2(DME) complexes. However, the addition of 3 equiv. of pyridinium chloride to W(NR)2(CH2R′)2 complexes led to the W(NR)(CHR′)Cl2(py)2 complexes in good yield (Eq. (1.1a)). The structure of W(N-t-Bu)(CH-t-Bu)Cl2(py)2 was confirmed in an X-ray study. The reaction of
W(N-t-Bu)(CH-t-Bu)Cl2(py)2 with 2 equiv. of lithium 2,5-dimethylpyrrolide led to W(N-t-Bu)(CH-t-Bu)(2,5-Me2pyr)2 (Eq. (1.1b)), which is a precursor to various MAP species (see later section). Although reactions analogous to that in Eq. (1.1a), in which NR is an arylimido ligand, have not yet been successful, the addition of 1 equiv. of 2,2′-bipyridine (bipy) to W(NR)2(CH2CMe2Ph)2 complexes (R = Ar, ArMe2, ArCl2, or AriPr; see Table 1.1), followed by 2 equiv. of HCl in diethyl ether, led to the formation of W(NR)(CHCMe2Ph)Cl2(bipy) complexes. HCl is an inexpensive alternative to triflic acid as a means of inducing α-hydrogen abstraction in a dialkyl complex to form an imido alkylidene complex.
Since no imido alkylidene catalysts had been prepared in which the imido ligand is highly electron withdrawing, attention turned to the synthesis of Mo and W catalysts that contain a pentafluorophenylimido ligand [18]. The addition of 3 equiv. of triflic acid to Mo(NC6F5)2(CH2CMe2Ph)2 gave Mo(NC6F5)(CHCMe2Ph)(OTf)2(DME) as a mixture of cis and trans isomers. Only 2 equiv. of triflic acid were required in the reaction with W(NC6F5)2(CH2-t-Bu)2 in a 5 : 1 mixture of diethyl ether and DME. The product was an insoluble, ivory-colored solid that was analyzed as W(NC6F5)(CH-t-Bu)(DME)(OTf)2. All evidence suggests that this product is an unusual polymer,
[W(NC6F5)(CH-t-Bu)(μ-DME)(OTf)2]x, in which the DME oxygen atom donors (monodentate to each metal) are located trans to one another (Eq. (1.2)). The formation of an insoluble polymer greatly simplified the isolation of the bistriflate derivative, from which a bisdimethylpyrrolide complex, Mo(NC6F5)(CHCMe2Ph)(Me2Pyr)2 (Pyr, pyrrolide) (see next section), was readily prepared. The addition of (CF3)3COH, (C6F5)3COH, or DFTOH (2,6-(C6F5)2C6H3OH) to Mo(NC6F5)(CHCMe2Ph)(Me2Pyr)2 led largely to bisalkoxide complexes, rather than Mo(NC6F5)(CHCMe2Ph)(Me2Pyr)(OR) complexes, although the use of acetonitrile as a solvent allowed for Mo(NC6F5)(CHCMe2Ph)(Me2Pyr)(OR) complexes to be prepared (see later section).
Much of the chemistry of MAP species involves aryloxides that have large aryl substituents in the 2 and 6 positions, namely 2,6-mesityl2C6H3O (HMTO, 2,6-dimesitylphenoxide) or 2,6-(2,4,6-i-Pr3C6H2)2C6H3O (HIPTO). Therefore, it was desirable to explore the properties of complexes in which the imido group was substituted at the 2 and 6 positions with mesityl or Trip (2,4,6-i-Pr3C6H2) substituents (Trip, 2,4,6-triisopropylphenyl). The 2,6-dimesitylphenylimido complexes became targets [20], since Ta [51] and Ni [52] compounds that contained NArMes2 and NHArMes2 ligands had been reported. All attempts to prepare Mo or W species that contain two NArMes2 ligands (i.e., Mo(NArMes2)2Cl2) by the standard methods employed to make other bisarylimido complexes failed, presumably because of the significant steric demands of an NArMes2 ligand. Therefore, a new synthetic route had to be developed (Scheme 1.1), one that was based on the work by Gibson [53]. Key steps included: (i) the conversion of Mo(N-t-Bu)2(NHArMes2)Cl into Mo(N-t-Bu)(NArMes2)(NH-t-Bu)Cl upon the addition of a catalytic amount of base; (ii) the synthesis of the “mixed” bisimido dialkyl species, Mo(N-t-Bu)(NArMes2)(CH2CMe2Ph)2(NH2-t-Bu); and (iii) the selective protonation of the t-butylimido ligand in Mo(N-t-Bu)(NArMes2)(CH2CMe2Ph)2(NH2-t-Bu) with LHCl (L = pyridine or lutidine) to give Mo(NArMes2)(CHCMe2Ph)Cl2(L). The Mo(NArMes2)(CHCMe2Ph)(Me2pyr)2 complex can be readily prepared from Mo(NArMes2)(CHCMe2Ph)Cl2(L).
Scheme 1.1 Synthesis of Mo=NArMes2 complexes.
An approach similar to that shown in Scheme 1.1 that begins with W(N-t-Bu)2Cl2(py)2 [54] led to W(NArMes2)(N-t-Bu)(CH2CMe2Ph)2, which could subsequently be converted into W(NArMes2)(CHCMe2Ph)Cl2(py) upon the addition of 1 equiv. of pyridine and 3 equiv. of HCl [21]. The W(NArMes2)(CHCMe2Ph)Cl2(bipy) complex was also synthesized in a reaction between W(NArMes2)(N-t-Bu)(CH2CMe2Ph)2 and bipy, followed by the addition of 3 equiv. of HCl.
1.3 Bispyrrolide and Related Complexes
Bispyrrolide alkylidene complexes were prepared with the intent of employing them as precursors to bisalkoxide or biphenolate and binaphtholate complexes through the addition of the corresponding alcohol. Catalysts could then be generated and evaluated in situ, at least initially, instead of having to be isolated in pure form and individually evaluated. Metathesis is unaffected by the pyrrole that is generated as a consequence of forming the catalyst in situ, since a pyrrole is a poor ligand for electron-poor transition metals. The pyrrolide ligand is isoelectronic with the cyclopentadienide ligand, and can therefore bind to a metal in an η1 or η5 fashion, with the two binding modes being relatively close in energy.
The first bispyrrolide imido alkylidene complexes to be prepared contained the parent pyrrolide ligand, [NC4H4]− (Pyr), in which the NR group was NR = NAr, NAd, or N-2,6-Br2-4-MeC6H2 [11]. An X-ray structure showed that the NAr species was the unsymmetric dimer, {Mo(NAr)(syn-CHCMe2Ph)(η5-NC4H4)(η1-NC4H4)}{Mo(NAr)(syn-CHCMe2Ph)(η1-NC4H4)2}, in which the nitrogen in the η5-pyrrolyl bound to one of the Mo atoms behaves as a donor to the other Mo, as schematically shown in Figure 1.1. Pyrrolide complexes are usually fluxional on the NMR time scale at room temperature; therefore, pyrrolide resonances in the spectra of these complexes are broad at room temperature. However, spectra consistent with the dimeric structure of [Mo(NAr)(CHR)(Pyr)2]2 shown in Figure 1.1 were found at low temperatures.
Figure 1.1 Schematic of the structure of [Mo(NAr)(CHR)(Pyr)2]2.
Bipyridine adducts of imido alkylidene bispyrrolide complexes have proven useful as intermediates in the often-unsuccessful synthesis and isolation of complexes that contain the parent pyrrolide. For example, Mo(NR)(CHCMe2R′)(Pyr)2(bipy) (NR = NAr, NAd, NArMe2, NAriPr, NArCl, NArtBu, and NArMes; R′ = Me, Ph) can be prepared and readily isolated [55]. The sonication of a mixture containing the bispyrrolide bipy adduct, HMTOH, and ZnCl2(dioxane) (to remove the bipy) led to formation of MAP species of the type Mo(NR)(CHCMe2R′)(Pyr)(OHMT) (OHMT, hexamethylterphenoxide) (Eq. (1.3)). The W(N-t-Bu)(CH-t-Bu)(Pyr)(OR) complex could be similarly prepared from W(N-t-Bu)(CH-t-Bu)(Pyr)2(bipy) (OR, OHMT or OHIPT, hexaisopropylterphenoxide) [39]. Interestingly, attempts to prepare bipy adducts of bisdimethylpyrrolide complexes led to the formation of imido alkylidyne complexes of the type Mo(NR)(CCMe2R′)(Me2Pyr)(bipy) through the ligand-induced migration of an alkylidene α proton to a dimethylpyrrolide ligand (Eq. (1.4)) [55].
Bis(2,5-dimethylpyrrolide) imido alkylidene complexes are monomeric, fluxional 18-electron species [13]. They were first employed as precursors to Mo(NR)(CHCMe2Ph)(diolate) complexes that contain the relatively electron-withdrawing binaphtholate (3,3-bis-(9-anthracenyl), 3,3′-bispentafluorophenyl, 3,3′-bis(3,5-bis(trifluoromethyl)phenyl), or biphenolate (3,3′-di-tert-butyl-5,5′-bistrifluoromethyl-6,6′-dimethyl-1,1′-biphenyl-2,2′-diolate) ligands. In one case, the Mo(NR)(CHCMe2Ph)(diolate) complex could be prepared only through the reaction of 3,3′-bis(pentafluorophenyl)binaphthol with a bis(2,5-dimethylpyrrolide) complex.
Other substituted pyrrolides form monomeric η1,η5-bispyrrolides, except when the pyrrolides: (i) can only bind in an η1 fashion, as is the case of the 2-mesitylpyrrolide (MesPyr) in Mo(NAr)(CHCMe2Ph)(η1-MesPyr)2 [15], or (ii) are much more prone to bind in an η1 fashion than an η5 fashion, as in Mo(NAr)(CH2CMe2Ph)(η1-indolide)2 [12]. If a donor functionality is present in the pyrrolide, then it can bind to another metal to form oligomers. For example, Mo(NAd)(CHCMe2Ph)(2-CNPyr)2 is an octamer in which two η1-pyrrolides are trans to one another at each metal center, and the cyano groups from neighboring Mo centers bind trans to the alkylidene and imido ligands [17]. Bis-η1-pyrrolides are also found as adducts, as in W(NAr)(CHCMe2Ph)(η1-MesPyr)2(dme) [37], Mo(NAd)(CHCMe2Ph)(η1-NC4H4)2(PMe3) [12], or one of the metals shown in Figure 1.1. The bispyrazolide complexes Mo(NAr)(CHCMe2Ph)(dppz)2 and Mo(NAr)(CHCMe2Ph)(dtpz)2 (dppz, 3,5-diphenylpyrazolide; dtpz, 3,5-di-t-butylpyrazolide) could be readily prepared from Mo(NAr)(CHCMe2Ph)(OTf)2(dme) [8]g. An X-ray structure of Mo(NAr)(CHCMe2Ph)(dppz)2 showed that one dppz was η1-bound, while the other was η2-bound through the two nitrogen atoms. Both bispyrazolide complexes were found to rapidly react with 2 equiv. of ROH (OR = O-t-Bu, OCMe2(CF3), OCMe(CF3)2, OC(CF3)3, and O-2,6-diisopropylphenyl) to give the bisalkoxide complexes, Mo(NAr)(CHCMe2Ph)(OR)2.
The syntheses of bispyrrolides are complicated by steric issues that lead to the α-hydrogen in the neopentylidene or neophylidene ligand being removed by a pyrrolide more rapidly than the pyrrolide can attack the metal center. The deprotonation of the alkylidene by the incoming pyrrolide nucleophile is especially problematic when one or two triflates is (are) present, and the pyrrolide is relatively sterically demanding [17]. Nevertheless, several bispyrrolides have been prepared in which the pyrrolides are relatively sterically demanding, for example, 2-mesityl [15–17], 2,5-diisopropyl [12], 2,5-diphenyl [12], or 2,3,4,5-tetramethyl [12]. The chemistry of complexes that contain one or two sterically demanding pyrrolides is underdeveloped with respect to the chemistry of the parent pyrrolide or 2,5-dimethylpyrrolide complexes, but the steric differences between various substituted pyrrolides could have significant consequences worthy of study.
The addition of 2 equiv. of LiPyr to Mo(NArMes2)(CHCMe2Ph)Cl2(py) led to the formation of Mo(NArMes2)(CHCMe2Ph)(Pyr)2(py) in good yield [21]. The W(NArMes2)(CHCMe2Ph)(Pyr)2(py) and W(NArMes2)(CHCMe2Ph)(Me2Pyr)2 complexes could be prepared in a similar manner [21]. Therefore, all four M(NArMes2)(CHCMe2Ph)(Pyr)2(py) and M(NArMes2)(CHCMe2Ph)(Me2Pyr)2 complexes (M = Mo or W) [20, 21] became available for the syntheses of MAP complexes that contain the NArMes2 ligand.
1.4 Monoalkoxide Pyrrolide (MAP) Complexes
Bispyrrolide complexes have been primarily employed as precursors to monoalkoxide/monoaryloxide pyrrolide (MAP) species through the addition of a monoalcohol or (usually) monophenol (Eq. (1.5)) [22]. MAP species virtually form exclusively when R′OH is HMTOH or HIPTOH, presumably because the formation of the bisaryloxide is prevented for steric reasons. No imido alkylidene complexes have as yet been prepared that contain two HMTO or HIPTO ligands, either through the protonation of bispyrrolides or through nucleophilic attack on a bistriflate. The formation of MAP species is often complicated by the protonation of both pyrrolide ligands to give the bisalkoxide or bisaryloxide complexes, even when only 1 equiv. of R′OH is added, particularly if R′ (Eq. (1.5)) is not sterically demanding enough.
When the combined steric demands of the pyrrolide (e.g., MesPyr), imido substituent, R, and R′ are too great, then the reaction fails even at elevated temperatures. Another concern is that the MAP species may not be stable toward a disproportionation to the bispyrrolide and bisalkoxide. So far, the only MAP species that has been shown to be unstable with respect to disproportionation to give mixtures that contain the bispyrrolide and bisalkoxide/aryloxide species is Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OC6F5) [56]. All MAP species that have been crystallographically characterized have been found to contain a η1-pyrrolide instead of a η5-pyrrolide. MAP species that contain a “large” aryloxide ligand in combination with a “small” imido ligand (e.g., Mo(NAd)(CHCMe2Ph)(Pyr)(OHMT)) have led to the development of metathesis catalysts that selectively produce Z olefins [35]. “Smaller” imido ligands (e.g., 3,5-dimethylphenylimido [38]) have proven useful in this regard. Early-metal Z-selective reactions are reviewed in Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7.
The synthesis of MAP species in which the OR* group is enantiomerically pure (e.g., (R)-3,3′-dibromo-2′-(tert-butyldimethylsilyloxy)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl-2-olate, OBr2Bitet) resulted in the formation of the diastereomers, (R)- and (S)-Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OR*). The formation of diastereomers can complicate the interpretation of enantioselective reactions [26]. Trimethylphosphine was found to bind to Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OR*) to give square-pyramidal PMe3 adducts, which were found to catalytically interconvert (R)- and (S)-Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OR*) via five-coordinate phosphine adducts [27]. Both (R)-Mo(NAr)(CHCMe2Ph)(Me2Pyr) and (S)-Mo(NAr)(CHCMe2Ph)(Me2Pyr) have been crystallographically characterized [26].
The anti isomers have been observed in the solution for bisalkoxide complexes when the photolysis of syn isomers was conducted at low temperatures [57]. syn Isomers are the norm for MAP species, although anti isomers have now been observed through the aforementioned photolysis of the syn species. For example, the photolysis of syn-Mo(NAr)(CHCMe2Ph)(Pyr)(OTPP) [29] (δHα = 12.1 ppm, 1JCH = 122 Hz) at −78 °C (366 nm) for 3 h led to the formation (up to 20% of the total) of the anti isomer (δHα = 13.6 ppm, 1JCH = 143 Hz). Rate constants for the conversion of anti to syn were obtained at several temperatures (ΔH‡ = 19.9 ± 2 kcal mol−1 and ΔS‡ = 8 ± 2 eu). The rate constant for the conversion of anti to syn (ka/s) at 298 K was found to be 3 s−1, which can be compared with those obtained for Mo(NAr)(CHCMe2Ph)[OCMe2(CF3)]2 (ka/s = 6.8 s−1) and Mo(NAr)(CHCMe2Ph)[OCMe(CF3)2]2 (ka/s = 0.10 s−1) at 298 K in previous studies [57]. Since the equilibrium constant ([syn]/[anti]) was estimated to be > 1000, the rate constant for the conversion of syn to anti (ks/a) at 298 K is < 3 × 10−3 s−1. Similar studies of Mo(NAd)(CHCMe2Ph)(Pyr)(OHIPT) showed that ka/s at 298 K is 1 s−1, with ΔH† = 17.51 kcal mol−1 and ΔS† = 0.36 eu. The equilibrium constant ka/s/ks/a was estimated to be of the order of 4000 or more, so the value of ks/a is of the order of 2.5 × 10−4 s−1 or less.
Molybdenum and tungsten MAP 2,5-dimethylpyrrolide complexes that contain O-t-Bu, OCMe(CF3)2, or O-2,6-Me2C6H3 ligands were found to have approximately equal amounts of syn- and anti-alkylidene isomers, which allowed for a study of the interconversion of the two employing 1H-1H EXSY methods. The Keq values ([syn]/[anti]) were all found to be two to three orders of magnitude smaller than those observed for a large number of Mo bisalkoxide imido alkylidene complexes, presumably as a consequence of the destabilization of the syn isomer by the sterically demanding NAr* ligand. The rates of interconversion of the syn and anti isomers were found to be one to two orders of magnitude faster for W MAP complexes than for Mo MAP complexes. Little is known about the rates of interconversion of syn- and anti-protons for MAP species [2]c, although the rates of syn/anti interconversions in bisalkoxide Mo imido alkylidene complexes have been found to vary over approximately six orders of magnitude [57]. It is not surprising that the sterically demanding NArMes2 ligand would destabilize the syn isomer for steric reasons and lead to mixtures that contain both syn and anti species, as observed.
The addition of ROH to Mo(NArMes2)(CHCMe2Ph)(Pyr)2(py) (R = OCMe(CF3)2, OCHMe2, OCH(CF3)2, OArMe2, OSi(i-Pr)3, OSiPh3, OSi(SiMe3)3; py = pyridine) led to the formation of the MAP species Mo(NArMes2)(CHCMe2Ph)(OR)(Pyr)(py). In contrast, the pyridine-free species, Mo(NArMes2)(CHCMe2Ph)(OR′)(Me2Pyr)2, could be isolated upon the treatment of Mo(NArMes2)(CHCMe2Ph)(Me2Pyr)2 with R′OH (R′ = O-t-Bu, OCMe(CF3)2, OArMe2, and OSiPh3). All Mo(NArMes2)(CHCMe2Ph)(Me2Pyr)2 species could be isolated through crystallization from acetonitrile. Molybdenum and tungsten 2,5-dimethylpyrrolide MAP complexes that contain O-t-Bu, OCMe(CF3)2, or O-2,6-Me2C6H3 ligands were found to have approximately equal amounts of syn- and anti-alkylidene isomers in solution, which allowed for a study of the interconversion of the two by employing 1H–1H EXSY methods. The Keq values ([syn]/[anti]) were all found to be two to three orders of magnitude smaller than those observed for a large number of Mo bisalkoxide imido alkylidene complexes, presumably as a consequence of a destabilization of the syn isomer by the sterically demanding NArMes2 ligand. The rates of interconversion of syn and anti isomers were found to be one to two orders of magnitude faster for W MAP complexes than for Mo MAP complexes.
An exploration of molybdenum MAP complexes of the type Mo(NArX)(CHCMe2R)(Me2Pyr)(OR′), in which NArX is an ortho-substituted phenylimido group (NArX = NArCl, NArCF3, NAriPr, NArtBu, NArMes, or NArTrip) and OR′ = OTPP, OHMT, or OHIPT [19], suggested that a single phenylimido ortho-substituent does not cause the imido group to behave as an especially bulky ligand. Even NArMes and NArTrip groups do not behave as especially sterically demanding imido ligands. What appears to be much more important in Z-selective reactions, at least so far, is that the OR′ group is a sterically demanding terphenoxide.
The monoaryloxide monopyrrolide complexes Mo(NR)(CHCMe2Ph)(Me2Pyr)(OAr) (Me2Pyr = 2,5-dimethylpyrrolide; R = C6F5, OAr = DFTO or HMTO; R = 2,6-Me2C6H3, OAr = DFTO) have been prepared in good yields [46]. The DFTO complexes had to be prepared in acetonitrile in order to prevent overprotonation to give the bisDFTO complexes. The polymerization of DCMNBD (dicarbomethoxynorbornadiene) by Mo(NC6F5)(CHCMe2Ph)(Me2Pyr)(HMTO) gave a polymer with the expected cis,syndiotactic structure, but the polymerization of DCMNBD by Mo(NR)(CHCMe2Ph)(Me2Pyr)(DFTO) (R = C6F5 or 2,6-Me2C6H3) generated a polymer with a cis,isotactic structure, the first prepared structure employing a MAP initiator. Norbornene was polymerized to give what was proposed to be highly tactic cis-polyNBE, although the tacticity is not known.
1.5 Reactions of Alkylidenes with Olefins
The reactions between an alkylidene and a terminal olefin are routinely employed to prepare new alkylidenes, usually from a neopentylidene or neophylidene. In all such reactions, the intermediate that leads to the new alkylidene, an α,α′-disubstituted metallacyclobutane (Eq. (1.6)), must be formed, but it is rarely stable enough to be observed. However, a 14-electron Mo vinylalkylidene MAP complex, syn-Mo(NAr)(CHCH=CMe2)(Me2Pyr)(OHMT), was successfully prepared and isolated by treating Mo(NAr)(CH-t-Bu)(Me2Pyr)(OHMT) with excess 4-methyl-1,3-pentadiene [14].
Mo(NAr)(CHCH=CMe2)(Me2Pyr)(OHMT) is relevant to the Z-selective homocoupling of 1,3-dienes by molybdenum and tungsten MAP complexes. The formation of the desired M=CHR′ complex can be complicated by the reformation of M=CHR or the metathesis of R′CH=CH2 (when added in large excess) to give R′CH=CHR′ and ethylene, which lead to the formation of methylidenes and unsubstituted metallacyclobutane complexes.
Ethylene, the most reactive simple olefin, is present in a large fraction of metathesis reactions, either as a reactant (in ethenolysis reactions) or as a product. Therefore, reactions involving ethylene have attracted significant attention. Although bispyrrolides, in general, do not readily react with olefins, W(NR)(CHCMe2Ph)(η1-Me2Pyr)(η5-Me2Pyr) (R = Ar or ArCl) will react with ethylene at 60 °C to produce the methylidene species, W(NR)(CH2)(η1-Me2Pyr)(η5-Me2Pyr) [37]. No metallacyclobutane complex that contains two pyrrolides has been observed.
Ethylene will react with tungsten (primarily) or molybdenum MAP species in which the aryloxide is OHIPT, OHMT, OTPP, or OBr2Bitet to yield metallacyclobutane complexes that are stable enough to be isolated and crystallographically characterized. For example, Mo(NAr)(CHCMe2Ph)(OR)(Pyr) and W(NAr)(CHCMe2Ph)(OTPP)(Me2Pyr) react with ethylene to yield Mo(NAr)(C3H6)(OHIPT)(Pyr) and W(NAr)(C3H6)(OTPP)(Me2Pyr), respectively. A key to the isolation of unsubstituted metallacyclobutane complexes prepared from MAP species is to capitalize on their lower solubilities and therefore preferential crystallization from, for example, pentane. Similar reactions have yielded Mo(NAr)(C3H6)(OHIPT)(Pyr) and W(NAr)(C3H6)(OHIPT)(Pyr).
Two diastereomers of the MAP species, W(NAr)(CH2)(Me2Pyr)(OR*) (OR* = OBr2Bitet), were generated through the addition of R*OH to W(NAr)(CH2)(Me2Pyr)2 [40]. The unsubstituted tungstacyclobutane species, W(NAr)(C3H6)(Me2Pyr)(OR*), was isolated by treating a mixture of the diastereomers of W(NAr)(CH2)(Me2Pyr)(OR*) with ethylene. A variety of NMR experiments revealed: (i) that the methylidene ligands in the two diastereomers of W(NAr)(CH2)(Me2Pyr)(OR*) readily rotate about the W=C bond with k = 2–7 s−1 at 22 °C and (ii) that what is believed to be an intermediate alkylidene/ethylene complex is formed in the process of loss of ethylene from W(NAr)(C3H6)(Me2Pyr)(OR*). The W(NAr)(CH2)(Me2Pyr)(OTPP) complex can be heated to 80 °C, where methylidene rotation about the W=C bond is facile and observable in a variable temperature 1H-NMR spectrum; at 20 °C, the methylidene protons exchange at a rate of 90 s−1. It is not yet known whether the rotation of a methylidene ligand tends to be inherently faster than the rotation of a monosubsubstituted alkylidene ligand.
Detailed NMR studies on Mo(NAr)(C3H6)(OBr2Bitet)(Me2Pyr) have been compared with similar studies conducted on W(NAr)(C3H6)(OBr2Bitet)(Me2Pyr). In these studies, it was observed that Mo(NAr)(C3H6)(OBr2Bitet)(Me2Pyr) forms what can be described as an ethylene/methylidene intermediate at 20 °C. The rate of formation was determined to be ∼4500 times faster than the rate at which W(NAr)(C3H6)(OBr2Bitet)(Me2Pyr) forms what has been proposed to be an ethylene/methylidene intermediate (Eq. (1.7)). The equilibrium for the Mo complex was found to lie toward the ethylene/methylidene intermediate. The stability of the methylidene complexes, coupled with their high reactivity, is likely to be responsible – at least to some degree – for the high efficiency of many olefin metathesis processes that employ MAP catalysts.
In some cases, metallacyclobutane complexes can lose ethylene and yield relatively stable methylidene species. Single crystal X-ray studies of Mo(NAr)(CH2)(OHIPT)(Pyr) and W(NAr)(CH2)(OTPP)(Me2Pyr) have shown that the M–C–Hanti angle in the methylidene is smaller than the M–C–Hsyn angle, which is consistent with an agostic interaction between CHanti and the metal. The loss of ethylene from tungstacyclobutane complexes is slow and incomplete, in general, compared to the loss of ethylene from molybdacyclobutane complexes. This is likely the consequence of the greater stability of the tungstacycles, as previously discussed for the Mo and W OBr2Bitet species.
A tungsten alkylidene complex that contains a dimethylamino group in the para position of an OHIPT ligand, W(NAr)(CHCMe2Ph)(OHIPTNMe2)(Pyr) (OHIPTNMe2 = O-2,6-(2,4,6-i-Pr3C6H2)2-4-NMe2-C6H2), was prepared in order to attach the intact complex to an acidic site on the alumina. In the process of exploring the reaction between W(NAr)(CHCMe2Ph)(OHIPTNMe2)(Pyr) and ethylene, the β-substituted tungstacyclobutane complex, W(NAr)[CH2CH(CMe2Ph)CH2](OHIPTNMe2)(Pyr), was isolated as a consequence of a back reaction between the initial metathesis product, Me2PhCCH=CH2, and W(NAr)(CH2)(OHIPTNMe2)(Pyr), as ethylene was removed from the reaction system (Eqs. (1.8a) and (1.8b)). An X-ray study of W(NAr)[CH2CH(CMe2Ph)CH2](OHIPTNMe2)(Pyr) showed that it was essentially a square pyramid (SP), with the imido group in the apical position (τ [58] = 0.060)
and the CMe2Ph group pointed away from the imido ligand. The W(NAr)[CH2CH(CMe2Ph)CH2](Pyr)(OHIPTNMe2) complex is the first substituted metallacyclobutane derived from a MAP species to be crystallographically characterized. Since much evidence, both experimental and theoretical, suggests that metallacyclobutane complexes are highly fluxional, and that the transition state for loss of olefin is closer to a trigonal bipyramidal (TBP) species than to a SP species, it is difficult to assess the significance of this particular metallacyclobutane complex to a metathesis reaction involving W(NAr)(alkylidene)(Pyr)(OHIPTNMe2) intermediates. Most likely, it must convert to a TBP species before it can lose olefin.
1.6 Olefin and Metallacyclopentane Complexes
In many cases, the addition of ethylene to an alkylidene complex produces neither an unsubstituted metallacyclobutane complex nor a methylidene complex, but an ethylene complex. For example, the Mo(NAr)(CHR)(Me2Pyr)(OR′) complexes (R = t-Bu or CMe2Ph; OR′ = OAr, OCMe(CF3)2, or OSiPh3) react with ethylene (1 atm) to yield the ethylene complexes, Mo(NAr)(CH2CH2)(Me2Pyr)(OR′) [43]. The reaction between Mo(NAd)(CHCMe2Ph)(OHIPT)(Pyr) and ethylene also yielded an ethylene complex, Mo(NAd)(C2H4)(OHIPT)(Pyr). The addition of triphenylsilanol to Mo(NAr)(CHR)(Me2Pyr)(OSiPh3) yielded Mo(NAr)(CH2CH2)(OSiPh3)2. Interestingly, Mo(NAr)(CHCMe2Ph)(OTf)2(dme) reacted slowly with ethylene (60 psi) in toluene at 80 °C to give cis and trans isomers of Mo(NAr)(CH2CH2)(OTf)2(dme) in the ratio of ∼2(cis) : 1, from which Mo(NAr)(CH2CH2)(η1-Me2Pyr)(η5-Me2Pyr) was readily prepared. The Mo(NAr)(CHCMe2Ph)(η1-MesPyr)2 complex also reacted cleanly with ethylene in benzene at 60 °C over a period of 4 d to give Mo(NAr)(CH2CH2)(MesPyr)2.
The ethylene in Mo(NAr)(CH2CH2)(OSiPh3)2 can be replaced by styrene or trans-3-hexene. Although Mo(NAr)(trans-3-hexene)(OSiPh3)2 could be isolated and characterized crystallographically, bound trans-3-hexene was slowly isomerized (cis → trans and through double-bond migration) to yield a complex mixture of all possible Mo(NAr)(hexene)(OSiPh3)2 complexes. When Mo(NAr)(CH2CH2)(OSiPh3)(Pyr) was heated in 1-decene, internal olefins were catalytically formed. All evidence suggests that alkene exchange at the Mo(IV) center is the most facile, followed by olefin isomerization. When Mo(NAr)(CH2CH2)[OCH(CF3)2](Et2O) was heated in 1-octene, a distribution of internal olefins was formed, with the maximum chain length being C13 or C14. This olefin distribution must arise from metathesis of the olefins that are formed through double-bond isomerization. It has been proposed that olefins are metathesized by traces of Mo(NAr)(CHR)(OSiPh3)2 complexes, but how these alkylidenes are formed from olefins remains unknown.
Ethylene complexes arise from either the formation and bimolecular decomposition of an intermediate methylidene complex or the rearrangement of a metallacyclobutane complex to propylene (Eq. (1.9)). The mechanistic details of the rearrangement of the unsubstituted metallacyclobutane complex to propylene are not
known. A Hβ from the metallacycle may be transferred to the metal to give an intermediate allyl hydride complex, although the transfer of Hβ to another ligand (e.g., the imido nitrogen) or even ethylene itself cannot be discounted [7]. Propylene has been detected upon the decomposition of the metallacyclobutane in several cases. The presence of bulky ligands (e.g., OHMT, OHIPT, or NArMes2) can virtually eliminate the bimolecular decomposition of alkylidenes (especially a methylidene) to give dimers that contain metal–metal multiple bonds [59, 60]. At this stage, it is not understood how to slow or eliminate the rearrangement of a metallacyclobutane to prolong metathesis activity to a significant degree.
Metallacyclopentane complexes can form through the addition of an olefin to an olefin complex. Unsubstituted metallacyclopentane complexes have been observed in solutions of biphenolate complexes under ethylene [47]. One biphenolate complex, W(NArCl)(Biphen)(C4H8) [48], and one disiloxide, Mo(NAr)(C4H8)(OSiPh3)2 [43], have been structurally characterized. Substituted metallacyclopentanes have not yet been identified.
