Cyclic BF2 adducts 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 BF2 group provided an entry to alpha,alpha-branched beta-amino aldehydes or beta-amino acids. In addition, the stable form of an amino-substituted hydroxycarbene complex of chromium was characterized by X-ray diffraction.