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Solution Structures and Dynamic Properties of Chelated d0 Metal Olefin Complexes {η5: η1-C5R4SiMe2NtBu}Ti(OCMe2CH2CH2CHCH2)+ (R = H, Me):  Models for the {η5: η1-C5R4SiMe2NtBu}Ti(R‘)(olefin)+ Intermediates in “Constrained Geometry” Catalysts

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journal contribution
posted on 2001-01-09, 00:00 authored by Jean-François Carpentier, Vladimir P. Maryin, Jeffrey Luci, Richard F. Jordan
To model the Ti−olefin interaction in the putative {η5: η1-C5R4SiMe2NtBu}Ti(R‘)(olefin)+ intermediates in “constrained geometry” Ti-catalyzed olefin polymerization, chelated alkoxide olefin complexes {η5: η1-C5R4SiMe2NtBu}Ti(OCMe2CH2CH2CHCH2)+ have been investigated. The reaction of {η5: η1-C5R4SiMe2NtBu}TiMe2 (1a,b; R = H, Me) with HOCMe2CH2CH2CHCH2 yields mixtures of {η5-C5R4SiMe2NHtBu}TiMe2(OCMe2CH2CH2CHCH2) (2a,b) and {η5: η1-C5R4SiMe2NtBu}TiMe(OCMe2CH2CH2CHCH2) (3a,b). The reaction of 2a/3a and 2b/3b mixtures with B(C6F5)3 yields the chelated olefin complexes [{η5: η1-C5R4SiMe2NtBu}Ti(OCMe2CH2CH2CHCH2)][MeB(C6F5)3] (4a,b; 71 and 89% NMR yield). The reaction of 2b/3b with [Ph3C][B(C6F5)4] yields [{η5: η1-C5Me4SiMe2NtBu}Ti(OCMe2CH2CH2CHCH2)][B(C6F5)4] (5b, 88% NMR yield). NMR studies establish that 4a,b and 5b exist as mixtures of diastereomers (isomer ratios:  4a/4a‘, 62/38; 4b/4b‘, 75/25; 5b/5b‘, 75/25), which differ in the enantioface of the olefin that is coordinated. NMR data for these d0 metal olefin complexes show that the olefin coordinates to Ti in an unsymmetrical fashion primarily through Cterm such that the CC π bond is polarized with positive charge buildup on Cint. Dynamic NMR studies show that 4b/4b‘ undergoes olefin face exchange by a dissociative mechanism which is accompanied by fast inversion of configuration at Ti (“O-shift”) in the olefin-dissociated intermediate. The activation parameters for the conversion of 4b to 4b‘ (i.e., 4b/4b‘ face exchange) are:  ΔH = 17.2(8) kcal/mol; ΔS = 8(1) eu. 4a/4a‘ also undergoes olefin face exchange but with a lower barrier (ΔH = 12.2(9) kcal/mol; ΔS = −2(3) eu), for the conversion of 4a to 4a‘.