Abstract
A catalytic transfer-hydrogenation utilizing a well-defined Bi(I) complex as catalyst and ammonia-borane as transfer agent has been developed. This transformation represents a unique example of low-valent pnictogen catalysis cycling between oxidation states I and III, and proved useful for the hydrogenation of azoarenes and the partial reduction of nitroarenes. Interestingly, the bismuthinidene catalyst performs well in the presence of low-valent transition-metal sensitive functional groups and presents orthogonal reactivity compared to analogous phosphorus-based catalysis. Mechanistic investigations suggest the intermediacy of an elusive bismuthine species, which is proposed to be responsible for the hydrogenation and the formation of hydrogen.
Highlights
A catalytic transfer-hydrogenation utilizing a well-defined Bi(I) complex as catalyst and ammoniaborane as transfer agent has been developed
This reactivity is based on highly strained P(III) compounds, where the lone pair becomes oxidizable and more prone to nucleophilic attacks and formal oxidative additions.[8]. Such a groundbreaking approach opened the door to the possibility of performing catalytic redox processes beyond the transition metals (TM) block
With the aim of investigating unconventional catalytic redox processes of nontransition metals, we have recently started a program which focuses on the exploitation of the redox abilities of bismuth (Bi) to be applied in organic synthesis
Summary
Preliminary mechanistic investigations point out at a catalytic platform involving extremely reactive Bi(III) hydride intermediates (Figure 1C).[18]. When the reaction was performed with 1.0 equiv ND3BD3, a large KIE value of 7.05 was obtained These results suggest a mechanistic scenario in which both N−H and B−H bonds are broken in the ratedetermining step (RDS).[25] a competition experiment between two-electronically distinct azoarenes was performed (Scheme 2B). The same Bi(III) hydride species was detected when the transfer hydrogenation of 2a was performed under catalytic conditions and analyzed by HRMS (Scheme 3C) Preliminary mechanistic investigations suggest the intermediacy of highly reactive and elusive Bi(III) hydrides These results constitute a unique proof-of-concept of a pnictogen operating between oxidation states I and III to mimic transformations typically performed by TM-catalysts.
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