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 (BH3), 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.
Highlights
The application of main-group metals in catalysis has flourished over recent years, largely driven by the need to alleviate global demand on conventional precious metal systems and find more sustainable alternatives.[1]
We have reported a series of structurally varied aluminum hydride and alkyl complexes, including the first example of an unsupported monomeric aluminum amindinate dihydride
The aluminum hydrides 1−4 and aluminum alkyls 5 and 6 were all found to mediate the hydroboration of phenylacetylene, with rates of reaction linked to structure, size, and stability
Summary
The application of main-group metals in catalysis has flourished over recent years, largely driven by the need to alleviate global demand on conventional precious metal systems and find more sustainable alternatives.[1] Main-group compounds have been widely shown to mimic transition metal behavior and, as they usually react via different mechanistic pathways, can offer divergent reactivity. Numerous examples of structurally diverse aluminum catalysts have been reported for the hydroboration of terminal alkynes, nitriles, and alkenes such as conjugated bis-guanidinate-supported aluminum dihydrides (B), reported by Nembenna and co-workers
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