Enantio- and Diastereoselective Catalytic Carboalumination of 1-Alkenes and α,ω-Dienes with Cationic Zirconocenes: Scope and Mechanism

Abstract

Bis(pentamethylcyclopentadienyl) dimethylzirconium (Cp*2ZrMe2) and bis(1-(−)-neomenthylindenyl) dimethylzirconium ((NMIn)2ZrMe2) upon activation by tris(pentafluorophenyl)borane are efficient catalysts for the carboalumination of a variety of 1-alkenes and nonconjugated dienes. The organoalane products can be oxidized to alcohol products, which are isolated in 50−90% yield. (NMIn)2ZrMe2 carboaluminates 1-alkenes and dienes with moderate (55−65%) enantioselectivities. In contrast, ethylenebis(tetrahydroindenyl) dimethylzirconium ((EBTHI)ZrMe2) and related zirconocenes give low activities and selectivities in the carboalumination reaction. Study of the interaction of zirconocene methyl cations with trimethylaluminum by 1H NMR shows that [(EBTHI)ZrMe][MeB(C6F5)3] forms a stable bis(μ-methyl) aluminum adduct. Zirconocene methyl cations derived from Cp*2ZrMe2 and (NMIn)2ZrMe2 react slowly with large excesses of trimethylaluminum to form weakly bound trimethylaluminum adducts. The difference in trimethylaluminu...