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, John E. Bercaw

Research output: Contribution to journalArticle

31 Scopus citations

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 = (n 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* = (n 5-C 5Me 5)), [Cp* 2Zr], [rac-(EBI)Zr] (EBI - ethylenebis(indenyl)), and [(THP)Zr] (THP = 1,2-(SiMe 2) 2(n 5-3,5-C 5H(CHMe 2) 2)(n 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.

Original languageEnglish (US)
Pages (from-to)2789-2794
Number of pages6
JournalOrganometallics
Volume24
Issue number11
DOIs
StatePublished - May 23 2005
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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