Abstract
The past 20 years has seen significant advances in main group chemistry and their use in catalysis. This Minireview showcases the recent emergence of phosphorus and arsenic containing heterocycles as catalysts. With that, we discuss how the Group 15 compounds diazaphospholenes, diazaarsolenes, and their cationic counterparts have proven to be highly effective catalysts for a wide range of reduction transformations. This Minireview highlights how the initial discovery by Gudat of the hydridic nature of the P−H bond in these systems led to these compounds being used as catalysts and discusses the wide range of examples currently present in the literature.
Highlights
During the course of this century, there has been a growing surge in using main-group compounds to replicate the roles of transition metals.[1,2] This is driven in part by the ever growing need to find more economically viable and environmentally sustainable alternatives to these metals, and by scientific curiosity
Comparisons can be made between NHPs and the familiar Arduengo N-heterocyclic carbenes (NHCs), NHPs have inverse electronic properties (Figure 1)
Given our groups previous interest in arsenic chemistry,[33,34] we looked to determine whether arsenic could mimic this reactivity by performing hydroboration of aldehydes with hydroboration of carbonyl derivatives with pinacolborane (HBpin)
Summary
During the course of this century, there has been a growing surge in using main-group compounds to replicate the roles of transition metals.[1,2] This is driven in part by the ever growing need to find more economically viable and environmentally sustainable alternatives to these metals, and by scientific curiosity. The formation of the active catalyst 1 from 4 occurs, which is able to deliver a hydride and reduce the imine substrate.[38] This reduction of imines was speedily followed up by the report of the first example of enantioselective reduction using a chiral diazaphospholene. In a further attempt to develop stable main group catalysts, Speed employed air and water stable phosphine(V) oxide precatalysts in the reduction of imines In these systems the precatalyst will be reduced into the catalytically active diazaphospholenes upon addition with HBpin. Phosphine oxide pre-catalyst 9 could enable conjugate reduction, where chalcone was smoothly reduced using 1 mol % of 9 and 1.1 equivalents of HBpin (Scheme 16). Bottom: secondary amines with CO2 using 5 mol % 1 as a catalyst
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