STRUCTURE AND PROPERTIES OF SOME α-, β- AND γ-AGOSTIC ALKYL-TITANOCENE COMPLEXES

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Date
2014-12-23
Authors
Dunlop-Brière, Alexandre
Keyword
titanocene , olefin polymerization , alpha-agostic , agostic , beta-agostic , gamma-agostic , H-tunnelling , intermediate
Abstract
In chapter 2, the synthesis and NMR characterization of [Cp2TiCH3(H2O)]+ (I) is presented. In the presence of excess water, there is evidence of rapid H-exchange and H-bonding between coordinated and free water. At higher temperatures, I undergoes hydrolysis to produce methane. In chapter 3, formation of [Cp2TiCH2CHMeCMe3]+ (II) via a controlled single 1,2-migratory insertion reaction of [Cp2TiMe]+ and 3,3-dimethyl-1-butene (3,3-DMB) at 205 K is presented. II exchanges between its two α-agostic isomers, with a preference for one agostic isomer over the other and also remains α-agostic under coordination of Et2O. In chapter 4, [Cp2TiCH2CHMeSiMe3]+ (III), a complex that is exchanging between γ- (ground state) and β- agostic isomers, is prepared by reaction of [Cp2TiMe]+ with 1 equivalent of trimethylvinylsilane at 205 K. There is exchange between β-H and γ-H from 215 K to 260 K but 1H NMR data suggests no or very little increase in rate. [Cp2TiCH2CHCD3SiMe3]+ undergoes a reversible and exclusive isotopomeric rearrangement to form specifically [Cp2TiCD2CDCH3SiMe3]+ through a series of reversible β-H elimination, 2,1- and 1,2- migratory-insertion reactions (process 1). Deuteration of III at β- or γ- positions essentially shuts down the β-H/γ-H exchange, with a kH/kD of at least 13000 at 225 K. These results indicate that this latter exchange process is driven by H quantum mechanical tunneling (QMT). Finally, chapter 5 presents is a re-investigation of II that unravels some exchange processes that were not detected in previous studies of II. EXSY and D-labeling (of β-Me in II) experiments suggest that II also undergoes process 1. Olefin dissociation from [Cp2TiH(CH2=CMeCMe3)]+ is faster than process 1 and EXSY correlations are seen intermolecularly between II and 2,3,3-TMB, as well as intramolecularly in 2,3,3-TMB (fastest exchange). In II-d3, this rapid reversible olefin dissociation from the olefin-hydride complex coupled to process 1 leads to multiple isotopologuous rearrangements in 2,3,3-TMB before it can be re-incorporated in II, with similar overall rates of deuterium incorporation at all α- and β- sites in II-dx (x = 0-6, statistical distribution with an average value of 3).
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