Hydroboration of Imines and Alkynes Catalyzed by Electronically Unsaturated Aluminum Hydride and Methyl Aluminum Cations

Aluminum hydride cations, [LAlH]+[HB(C6F5)3]- (1) and [LAlH]+[B(C6F5)4]- (2), and the methyl aluminum cation, [LAlMe]+[B(C6F5)4]- (3) (L = [{(2,6-iPr2C6H3N)P(Ph2)}2N]), due to their electronic and coordinative unsaturation at the Al center, exhibit high Lewis acidity and have been exploited for catalytic hydroboration (using HBpin/HBcat) of a variety of imines and alkynes. These catalysts, under mild reaction conditions, afford excellent yields of the respective products. Thorough mechanistic investigations have been performed using a series of stoichiometric experiments and successful isolation of the key intermediates was accomplished. The obtained results demonstrate the predominant Lewis acid activation mechanism over the pathways previously reported for covalent aluminum complexes catalyzed hydroboration of imines. The title cations form Lewis adducts with imines which are thoroughly characterized via multinuclear NMR measurements. For the hydroboration of alkynes, a detailed mechanistic study with the most efficient catalyst 2 supports the formation of a novel cationic aluminum alkenyl complex [LAl-C(Et)═CH(Et)]+[B(C6F5)4]- (7) via the hydroalumination reaction between the Al-H cation 2 and 3-hexyne. Similarly, hydroalumination of an unsymmetric internal alkyne 1-phenyl-1-propyne with 2 occurs regioselectively, leading to the formation of [LAl-C(Me)═CH(Ph)]+[B(C6F5)4]- (8). These unique cationic aluminum alkenyl complexes have been isolated and well characterized by multinuclear 1-D and 2-D NMR measurements. These alkenyl complexes further act as catalytically active species to carry forward the hydroboration reaction via the Lewis acid activation pathway.