DESIGN AND SYNTHESIS OF CHIRAL N-HETEROCYCLIC CARBENES WITH APPLICATIONS TO ASYMMETRIC SYNTHESES AND SYNTHESIS OF DIBENZAZEPINONES BY PALLADIUM¨CCATALYZED INTRAMOLECULAR ARYLATION OF O-(2¡¯-BROMOPHENYL)ANILIDE ENOLATES

摘要

Chiral N-heterocyclic carbene (NHC) ligands were prepared and examined in three asymmetric syntheses (oxindole synthesis, pyrrolidine synthesis and cyanosilylation of aldehydes). The ligand generated from (4R,5R)-1,3-bis(ortho-cyclohexylphenyl)-4,5-diphenyl-4,5-dihydro-1H-imidazol-3-ium tetrafluoroborate provided the best asymmetric oxindole synthesis: yields of oxindoles up to 99% with enantioselectivities (ee) up to 80% were achieved in 3 h; catalyst loadings as low as 1 mol % could be used; the imidazolinium salt can be made efficiently in only two steps. A mechanism for this asymmetric oxindole synthesis was proposed. The rotation barrier of the N-aryl bond in the oxidative addition products was found crucial for the enantioselectivity.A new approach for the convenient synthesis of dibenzazepinones is reported. The key step is the formation of the seven-membered ring through palladium-catalyzed intramolecular arylation of an anilide enolate. The reactions were completed in 10 min at 100 ¡ãC with moderate to excellent yields. Aminodibenzazepinone, the core structure in secretase inhibitor LY411575, can be prepared in five steps from 2-bromophenylboronic acid and 2-iodoaniline in 60% overall yield. The synthesis reported here compares favorably with presently available approaches to this interesting ring system.