Enantio- and diastereoselective catalytic carboalumination of 1-alkenes and α,ω-dienes with cationic zirconocenes:: Scope and mechanism

Bis(pentamethylcyclopentadienyl) dimethylzirconium (Cp-2*ZrMe2) and bis(1-(-)-neomenthylindenyl) dimethylzirconium ((NMIn)(2)ZrMe2) upon activation by tris(pentafluorophenyl)borane are efficient catalysts for the carboalumination of a variety of l-alkenes and nonconjugated dienes. The organoalane products can be oxidized to alcohol products, which are isolated in 50-90% yield. (NMIn)(2)ZrMe2 carboaluminates l-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 H-1 NMR shows that [(EBTHI)ZrMe][MeB(C6F5)(3)] forms a stable bis(mu-methyl) aluminum adduct. Zirconocene methyl cations derived from Cp-2*ZrMe2 and (NMIn)(2)ZrMe2 react slowly with large excesses of trimethylaluminum to form weakly bound trimethylaluminum adducts. The difference in trimethylaluminum binding to the cationic zirconocene centers is consistent with the significantly lower activity observed when (EBTHI)ZrMe2 is used as the precatalyst.