Construction of Quaternary Carbon Centers Using Organomolybdenum Chemistry

Article abstract of DOI:10.1021/ja00199a043

Two approaches are described for the construction of quaternary carbon centers using organomolybdenum complexes.The first involves carbon nucleophile addition to the methylated diene terminus of dicarbonyl(η⁵-indenyl)(1-4-η-1,4-dimethylcyclohexa-1,3-diene)molybdenum tetrafluoroborate (12).A wide range of nucleophiles add successfully to 12 giving gem-disubstituted cyclohexenylmolybdenum complex.This methodology, coupled with a lactonization/decomplexation reaction, has led to a short synthesis of 2-(2-hydroxy-1,4-dimethylcyclohex-3-enyl)propionic acid lactone(14), which has previously been employed as a key intermediate for trichothecene synthesis.The second method involves alkylation of cyano-stabilized carbanions generated from dicarbonyl(η⁵-cyclopentadienyl)(1-3-η-4-cyanocyclohexenyl)molybdenum and dicarbonyl(η⁵-cyclopentadienyl)(1-3-η-4-cyanocycloheptenyl)molybdenum.While a rather limited range of electrophiles could be used in this coupling reaction, it was found that stereochemical control is excellent and allows the construction of quaternary carbon centers with defined stereochemistry relative to other substituents on the cyclohexenyl or cycloheptenyl ligand.During the course of this study it was found that treatment of the exo-cyano-substituted ?-allylmolybdenum complexes with butyllithium led to novel decyanation reaction.This allowed the generation of nonstabilized carbanion species that could be alkylated regio- and stereospecifically.