Transition metal alkylidene complexes. Pathways in their formation and tautomerization between bis-alkylidenes and alkyl alkylidynes

Abstract

Studies from authors’ group (at the University of Tennessee) on alkylidene complexes and α-H migration in alkyl alkylidyne complexes, leading to unusual tautomerization equilibria between bis-alkylidenes and alkyl alkylidynes, are reviewed. Preparation of silyl alkylidene complexes (Me 3ECH 2) 2Ta( CHEMe 3)(SiR 3) [R 3 = (SiMe 3) 3, E = C, 3a, Si, 3b; R 3 = Bu t Ph 2, E = C, 4a, Si, 4b] and the pathway in the formation of 3b are discussed first. Pathways in the formation of archetypical Schrock-type alkyl alkylidenes (Me 3ECH 2) 3Ta CHEMe 3 (E = C, 5a; Si, 5b), including the work using Ta(CD 2CMe 3) 5 ( 21- d 10) to confirm that it is the precursor to (Me 3CCD 2) 3Ta CDCMe 3 ( 5a- d 7), are then considered. Tautomerization of silyl alkylidyne (Me 3CCH 2) 2W( CCMe 3)(SiBu t Ph 2) ( 6a) with bis-alkylidene (Me 3CCH 2)W( CHCMe 3) 2(SiBu t Ph 2) ( 6b) as well as (Me 3SiCH 2) 3W( CSiMe 3)(PR 3) [R 3 = Me 3, 7a; Me 2Ph, 8a; Me 2(CH 2) 2PMe 2 (DMPE- P), 9a] with (Me 3SiCH 2) 2W( CHSiMe 3) 2(PR 3) (R 3 = Me 3, 7b; Me 2Ph, 8b; DMPE- P, 9b) [ P refers to a dangling P atom in Me 2P(CH 2) 2PMe 2] is covered next. Finally the conversion of the tungsten phosphine tautomerization mixtures to alkyl alkylidene alkylidyne (Me 3SiCH 2)W( CHSiMe 3)( CSiMe 3)(PR 3) 2 [(PR 3) 2 = (PMe 3) 2, 10; (PMe 2Ph) 2, 11; DMPE, 12], including its pathway, is presented.