Chiral chromium(III) porphyrins as highly enantioselective catalysts for hetero-Diels-Alder reactions between aldehydes and dienes

Starting from enantiornerically pure 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octa-hydro-1,4:5,8-dimethanoanthracene-9-yl]porphyrin, treatment with CrCl2 and subsequent air oxidation afforded the corresponding Cr(III) complex, with chloride as counterion, in 96% yield. Anion exchange with AgBF4 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%.