Chiral 1,3,2‐Diazaphospholenes as Catalytic Molecular Hydrides for Enantioselective Conjugate Reductions

Abstract

Secondary 1,3,2-diazaphospholenes have a polarized P-H bond and are emerging as molecular hydrides. Herein, a class of chiral, conformationally restricted methoxy-1,3,2-diazaphospholene catalysts is reported. We demonstrate their catalytic potential in asymmetric 1,4-reductions of α,β-unsaturated carbonyl derivatives, including enones, acyl pyrroles, and amides, which proceeded in enantioselectivities of up to 95.5:4.5 e.r.

Arsenic Catalysis: Hydroboration of Aldehydes Using a Benzo‐Fused Diaza‐benzyloxy‐arsole

“…The catalytic 1,2‐ hydroboration of carbonyls and imines as well as the catalytic 1,4‐hydroboration of α,β‐unsaturated carbonyl derivatives were then subsequently established by Kinjo and Speed. In the later part of 2017 and 2018, interest in this area led to the rapid development of chiral‐diazaphospholenes for the asymmetric variant of 1,2‐ and 1,4‐reduction reactions by Speed and Cramer respectively. In addition, diazaphospholenes have also been shown to be active in the reduction of pyridines and in other transformations, including the transfer hydrogenation using NH 3 BH 3 as well as hydrosilylation of CO 2 .…”

“…Although the hydroboration reaction has been rigorously explored using transition metal catalysts, metal‐free alternatives are seldom reported . However, the use of metal‐free hydroboration catalysts such as those derived from diazaphospholenes or borane Lewis acids has gathered momentum …”

A 1,3,2‐Diazaphospholene‐Catalyzed Reductive Claisen Rearrangement

“…Recent reports from our group and Speed et al. independently developed chiral DAP catalysts for asymmetric reductions …”

Structure–property-reactivity studies on dithiaphospholes

“…17 The Cramer group has also used 1,3,2diazaphospholenes for chiral reductive chemistry. 18 The related phosphenium cations have also been used as pre-catalysts in the reduction of pyridines and imines with pinacolborane or silanes. 19,20 The ability to tailor the reaction chemistry through application of the isolobal analogy gives rise to a large number of heterocycles closely related to the diazaphosphole unit.…”