Cyclic BF2Adducts of Functionalized Fischer Vinylcarbene Complexes: Preparation and Stereoselective Diels−Alder Reactions with 2-Amino 1,3-Dienes

A new type of cyclic amino-functionalized s-cis boroxyvinylcarbene complex of group 6 metals has been synthesized. These complexes underwent Diels−Alder-type reactions with 2-amino 1,3-dienes that proceeded with complete regioselectivity and high exo or endo diastereoselectivity, which was found to be highly dependent on the nature of the substituents on the diene. When chiral 2-amino-5-alkoxy dienes derived from ( S )-prolinol benzyl or methyl ether were used, an exclusive exo and highly diastereofacially selective [4 + 2] cycloaddition was achieved, affording spiro carbene complexes with three contiguous stereogenic centers and a high level of enantiomeric purity. Removal of the Cr(CO) 5 fragment and the BF 2 group provided an entry to α,α-branched β-amino aldehydes or β-amino acids. In addition, the stable form of an amino-substituted hydroxycarbene complex of chromium was characterized by X-ray diffraction.