The Mechanism of Methane Elimination in B(C6F5)3-Initiated Monocyclopentadienyl-Ketimide Titanium and Related Olefin Polymerization Catalysts

Abstract

A new class of monocyclopentadienyl titanium olefin polymerization catalysts and their activation with B(C6F5)3 is reported herein. Dichlorides Cp[tBu(R)CN]TiCl2 {Cp = C5H5, R = tBu (1a); Cp = C5Me5, R = tBu (2a); Cp = C5Me4SiMe3, R = tBu (3a); Cp = C5Me5, R = CH2SiMe3 (4a); Cp = C5Me5, R = Me (5a)} were prepared in 50−92% yield from CpTiCl3 and tBu(R)CNLi. Analogous dimethyl compounds 1b−5b were prepare via methylation of dichlorides a using MeMgBr in 89−92% yield. Dimethyl compound 6b (L = C5Me5, R = CH(SiMe3)2) was prepared directly from Cp*TiMe3 and tBu[(Me3Si)2CH]CNH in 40% yield. Dynamic 1H NMR studies showed that the ketimide ligands in compounds b rotate rapidly about Ti−N on the NMR time scale, with a ΔG‡ of 9.6(6) kcal mol-1 or less. The mixed alkyl compound Cp*[tBu(R)CN]Ti(CH3)CH2SiMe3 {R = tBu (7)} was prepared via alkylation of the corresponding methyl chloride derivative with BrMgCH2SiMe3. When treated with B(C6F5)3, compounds 1b−6b are rapidly converted into the ion pairs {Cp[tBu(R)CN]TiCH3...