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Experimental Evidence for γ-Agostic Assistance in β-Methyl Elimination, the Microscopic Reverse of α-Agostic Assistance in the Chain Propagation Step of Olefin Polymerization

Chirik, Paul J. and Dalleska, Nathan F. and Henling, Lawrence M. and Bercaw, John E. (2005) Experimental Evidence for γ-Agostic Assistance in β-Methyl Elimination, the Microscopic Reverse of α-Agostic Assistance in the Chain Propagation Step of Olefin Polymerization. Organometallics, 24 (11). pp. 2789-2794. ISSN 0276-7333. http://resolver.caltech.edu/CaltechAUTHORS:20150622-144456252

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Abstract

Isotopically labeled zirconocene methyl neopentyl complexes of the formula (CpR_n)_2Zr(CH_3)(CH_2C(CH_3)2CD_3) are obtained via reaction of LiCH_2C(CH_3)_2CD_3 with (CpR_n)_2Zr(CH_3)(Cl). Addition of B(C_6F_5)_3 to (CpR_n)_2Zr(CH_3)(CH_2C(CH_3)_2CD_3) results in β-methyl elimination, forming the ion-paired species [(CpR_n)_2Zr(CH_3)][CH_3B(C_6F_5)_3] along with isotopologs of isobutene. The relative amounts of d_3- and d_0-isobutene afford the isotope effect for β-methyl elimination. For Cp_2Zr(CH_3)(CH_2C(CH_3)_2CD_3) (Cp = (η^5-C_5H_5)), a kinetic deuterium isotope effect of 1.40(2) has been measured at 23 °C. Comparable deuterium kinetic isotope effects have been observed for four other zirconocene methyl neopentyl compounds:  [Cp^*(C_5Me_4H)Zr] (Cp^* = (η^5-C_5Me_5)), [Cp^*_2Zr], [rac-(EBI)Zr] (EBI = ethylenebis(indenyl)), and [(THP)Zr] (THP = 1,2-(SiMe_2)_2(η^5-3,5-C_5H(CHMe_2)_2)(η^5-C_5H_3)). The direction and magnitude of these effects are consistent with γ-agostic assistance in the transition state for β-methyl elimination, the microscopic reverse of α-agostic assistance in the transition state for olefin insertion into the Zr−methyl bond.


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Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/om058002lDOIArticle
http://pubs.acs.org/doi/abs/10.1021/om058002lPublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/om058002l/suppl_file/om058002lPublisherSupporting Information
ORCID:
AuthorORCID
Dalleska, Nathan F.0000-0002-2059-1587
Additional Information:© 2005 American Chemical Society. Received January 14, 2005. Publication Date (Web): April 23, 2005. This work has been supported by the USDOE Office of Basic Energy Sciences (Grant No. DE-FG03-85ER13431), by Exxon Chemicals America, and by the National Science Foundation (Grant No. CHE-0131180). We also thank Dr. Peter Green for assistance with GC/MS analysis and for helpful discussions.
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG03-85ER13431
Exxon Chemicals AmericaUNSPECIFIED
NSFCHE-0131180
Record Number:CaltechAUTHORS:20150622-144456252
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20150622-144456252
Official Citation:Experimental Evidence for γ-Agostic Assistance in β-Methyl Elimination, the Microscopic Reverse of α-Agostic Assistance in the Chain Propagation Step of Olefin Polymerization Paul J. Chirik, Nathan F. Dalleska, Lawrence M. Henling, and John E. Bercaw Organometallics 2005 24 (11), 2789-2794 DOI: 10.1021/om058002l
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58414
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:23 Jun 2015 21:55
Last Modified:11 Apr 2017 16:22

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