Aluminum Amidinates: Insights into Alkyne Hydroboration

Abstract

The mechanism of the aluminum-mediated hydroboration of terminal alkynes was investigated using a series of novel aluminum amidinate hydride and alkyl complexes bearing symmetric and asymmetric ligands. The new aluminum complexes were fully characterized and found to facilitate the formation of the ( E )-vinylboronate hydroboration product, with rates and orders of reaction linked to complex size and stability. Kinetic analysis and stoichiometric reactions were used to elucidate the mechanism, which we propose to proceed via the initial formation of an Al-borane adduct. Additionally, the most unstable complex was found to promote decomposition of the pinacolborane substrate to borane (BH 3 ), which can then proceed to catalyze the reaction. This mechanism is in contrast to previously reported aluminum hydride-catalyzed hydroboration reactions, which are proposed to proceed via the initial formation of an aluminum acetylide, or by hydroalumination to form a vinylboronate ester as the first step in the catalytic cycle.

Recent Advances in Aluminum Compounds for Catalysis

“…They proposed the catalytic cycle starting with AlÀ H insertion in the alkyne CÀ C triple bond to form the intermediate (aluminum hydride), which undergoes transmetallation to regenerate the catalyst and (Z)-vinyl boronate ester. Additionally, C. Bakewell et al [42] investigated the aluminum-mediated hydroboration of aldehydes, ketones and terminal alkynes using a series of novel aluminum amidinate hydride and alkyl complexes bearing symmetric and asymmetric ligands. Besides, R. E. Mulvey et al [43] proposed bimetallic lithium aluminates and neutral aluminum counterparts are compared as catalysts in hydroboration reactions with aldehydes, ketones, imines and alkynes.…”

Aluminium complexes: next-generation catalysts for selective hydroboration

“…Very recently, Carmalt and Bakewell et al reported the hydroboration of terminal alkynes catalysed by a class of aluminium amidinate hydride and alkyl complexes. 65 Elucidation of the reaction mechanism led to the formation of an Al-borane adduct in the initial step which is different from the previously proposed ones involving aluminium acetylide or hydroalumination as the first step.…”

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

“…Similarly than hydroboration of CC bonds, the hydroboration of alkynes has been widely studied using aluminum catalysts (Scheme ). − In fact, the first application of aluminum catalysts was reported by Roesky in 2016, two years before the first magnesium complex …”