Understanding Nickel-Catalyzed Hydroalumination: A Closer Look at Mechanisms

Hydroalumination reactions, particularly those involving nickel catalysts, have garnered significant attention in recent chemical research. A recent study explored the adduct formed between an amine-stabilized dimethylaluminum hydride and (cyclododecatriene)nickel, illustrating its potential as a model for understanding the mechanisms behind these reactions. The structure, confirmed through X-ray crystallography, showcased a unique configuration where the hydride bridges aluminum and a nickel center coordinated to an alkene.

Eisch's modified interpretation of this mechanism emphasizes how the regiochemistry of Al–H addition to various alkenes and alkynes differs dramatically, depending on the presence of a nickel catalyst. For instance, when 1,1-dimethylindene reacted with iBu2AlH in the absence of a nickel catalyst, the product was predominantly one specific isomer. In contrast, introducing Ni(acac)2 as a catalyst precursor resulted in a mixture of several products with varied ratios, revealing the complexity and nuances of catalytic effects on reaction pathways.

The study demonstrated that nickel-catalyzed hydroalumination proceeds via a syn addition mechanism, akin to certain thermal processes. The observed products indicate that the active hydrometallating agent formed in these reactions possesses distinct polar and steric properties compared to traditional aluminum alkyls. This is a key factor contributing to the observed regioselectivity—or lack thereof—when nickel is involved.

A proposed mechanism suggests that an aluminum-nickel hydride complex serves as a critical intermediate. This complex can arise through the oxidative addition of a dialkyl aluminum hydride or a trialkyl aluminum reagent, followed by β-hydride elimination. As the alkene coordinates with the nickel-hydride bond, an alkylnickel complex is formed, which can then undergo reductive elimination to yield the organoaluminum compound while simultaneously regenerating the nickel catalyst.

Recent studies using NMR spectroscopy provide further insights into the interactions between nickel complexes and aluminum hydrides. Notably, the signals observed in the NMR spectra affirm the oxidative insertion of Ni(0) into Al–H bonds. The nuances of these interactions highlight the importance of molecular structure and steric factors in determining the outcomes of hydroalumination reactions, paving the way for advances in synthetic methodologies.