STEREOSPECIFIC FUNCTIONALIZATION OF (PI-ALLYL)MOLYBDENUM COMPLEXES DERIVED FROM CYCLOPENTENONE - TOWARD THE STEREOCONTROLLED CONSTRUCTION OF SUBSTITUTED 2-CYCLOPENTENONES

(Eta5-cyclopentadienyl) [(2,3,4-eta)-1-oxo-2-cyclopenten-4-yl]dicarbonylmolybdenum on treatment with NOPF6 in 1,2-dimethoxyethane at 0-degrees-C evolves CO and formed carbonyl(eta5-cyclopentadienyl)[2,3,4-eta)-1-oxo-2-cyclopenten-4-yl]nitrosylmolybdenum hexafluorophosphate in a highly diastereoselective manner. This electrophilic species reacted with nucleophiles (higher-order cuprates, malonates, enolates, and cyanoborodeuteride) to form (eta2-alkene)carbonyl(eta5-cyclopentadienyl)nitrosylmolybdenum complexes of 4-substituted 2-cyclopentenones in good to excellent yields and with high regioselectivity. High yield oxidative demetallation with ceric ammonium nitrate provided the 4-substituted cyclopentenones. The product resulting from reaction of carbonyl)(eta5-cyclopentadienyl)[(2,3,4-eta)-1-oxo-2-cyclopenten-4-yl]nitrosylmolybdenum hexafluorophosphate with Me2Cu(CN)Li2 was characterized by X-ray diffraction, proving that the methyl group was delivered anti to the molybdenum atom and to the 4-position of the cyclopentenone ligand. In the crystal structure, the metal carbonyl (Mo-CO) moiety is situated parallel to the metal olefin unit while the metal nitrosyl (Mo-NO) bisects the olefin. The stereochemistry at molybdenum relative to the 4-methylcyclopentenone ligand (2S,3R,4S,MoR/2R,3S,4R,MoS) demonstrates that either the nucleophilic attack has occurred trans to NO in the exo conformer of the eta3-allyl or cis to the NO in the endo conformer. This stands in direct contrast to earlier studies of unbiased cationic (pi-allyl)molybdenum nitrosyl complexes where nucleophilic addition occurred trans to the NO in the endo conformer and cis to the NO in the exo conformer. A parallel series of reactions (CO --> NO+ conversion then nucleophilic addition) was conducted on (eta5-cyclopentadienyl)dicarbonyl[(2,3,4-eta)-1-oxo-5-methyl-2-cyclopenten-4-yl)] molybdenum. Small, sterically unbiased nucleophiles (NaCH(COOMe)2 and deuteride) reacted with high selectivity to produce cis-4,5-disubstituted cyclopentenones eta2-coordinated to molybdenum, while larger nucleophiles [Me2Cu(CN)Li2 and NaC(Me)(COOEt)2] gave predominantly the 2,5-disubstituted cyclopentenone eta2-complex.