Alkali metal complexes as efficient catalysts for hydroboration and cyanosilylation of carbonyl compounds

Abstract

We report here reactions between the N-adamantyliminopyrolyl ligand 2-(AdNCH)–C₄H₃NH (L–H) and alkali metal hexamethyldisilazides [MN(SiMe₃)₂] (M = Li, Na and K) to afford the dimeric [{2-(AdNCH)–C₄H₃NLi(THF)}₂] (1), [{2-(AdNCH)–C₄H₃N}{Na(THF)_1.5}₂] (2) and polymeric [{2-(AdNCH)–C₄H₃NK(THF)}_n] (3) complexes at ambient temperature. A one-pot reaction between L–H, [KN(SiMe₃)₂] and alkaline earth metal diiodide (AeI₂) in a 2 : 2 : 1 molar ratio, which resulted in the formation of a heteroleptic Ae metal complex [κ²-{2-(AdNCH)–C₄H₃N}₂Ae(THF)₂] [Ae = Mg (4), Ca (5)], is also reported. The solid-state structures of complexes 1, 3 and 4 were established through single-crystal X-ray diffraction analysis. The alkali and alkaline earth metal complexes 1–5 were utilised as precatalysts for the catalytic hydroboration of pinacolborane (HBpin) with aldehydes and ketones, and potassium complex 3 was identified as a competent catalyst under mild conditions. Additionally, cyanosilylation of carbonyl compounds was explored with trimethylsilyl cyanide and aldehydes/ketones, using the alkali metal precatalyst 3 under mild conditions. In both catalytic processes, the potassium catalyst 3 exhibited high tolerance towards a number of functional groups.