Chiral Chromium(III) Porphyrins as Highly Enantioselective Catalysts for Hetero‐Diels–Alder Reactions Between Aldehydes and Dienes

Abstract

Abstract Starting from enantiomerically pure 5,10,15,20‐tetrakis[(1 S ,4 R ,5 R ,8 S )‐1,2,3,4,5,6,7,8‐octahydro‐1,4 : 5,8‐dimethanoanthracene‐9‐yl]porphyrin, treatment with CrCl 2 and subsequent air oxidation afforded the corresponding Cr(III) complex, with chloride as counterion, in 96% yield. Anion exchange with AgBF 4 gave the corresponding tetrafluoroborate. These hitherto unknown chiral chromium porphyrins are efficient and highly enantioselective catalysts for the hetero‐Diels–Alder reaction of aliphatic, aromatic, and heteroaromatic aldehydes with dienes of varying electron density. In the case of 1‐methoxy‐3‐(trimethylsilyloxy)butadiene (“Danishefsky's diene”), enantiomeric excesses >90% were achieved in a number of cases, with furfural affording the highest ee (97%). Metal‐coordinating aldehydes such as pyridine‐2‐carbaldehyde do not inactivate the Cr(III) porphyrin catalyst. The cycloaddition of less electron‐rich dienes (such as 1‐methoxybutadiene) is effected as well, affording diastereoselectivities up to >99 : 1, and enantiomeric excesses >80%.