Stereoselective chromium- and molybdenum-mediated transformations of arenes

Abstract

Tricarbonylchromium-mediated dearomatization provides a rapid access to substituted cyclohexadienes. Efficient asymmetric routes to planar chiral arene complexes and to substituted cyclohexadienes have been developed. The article sums up the main features of this chemistry. Highly enantiomerically enriched ortho-substituted benzaldehyde complexes are accessible via asymmetric lithiation followed by trapping with electrophiles. In different solvents, the trimethylsilyl complex exhibits [α] values ranging from -174 to +108 for the same enantiomer. Details of two asymmetric syntheses of natural products are given: the alkaloid lasubine I starting from a highly enantiomerically enriched planar chiral arene complex and the marine furanosesquiterpene acetoxytubipofuran. The latter is assembled via asymmetric dearomatization of a benzaldehyde imine complex. Other key steps include an Eschenmoser-Claisen rearrangement and a regio- and diastereoselective Pd-catalyzed allylic substitution. The final section deals with labile arene metal complexes. For the first time, dearomatization reactions mediated by the Mo(CO)3 group have been realized. The reactions show strong analogies to the Cr(CO) 3-mediated reactions, but exhibit also marked differences: the arene-Mo bond is stronger, but more labile, and the sequential double additions show different selectivities compared to the chromium analogs.