TRICARBONYLPYRIDINECHROMIUM COMPLEXES - STEREOSELECTIVE ALKYLATIONS AND ALDOL-TYPE REACTIONS INVOLVING ALPHA-CARBANIONS DERIVED FROM ETA-TRICARBONYL(2-ALKYL-PYRIDINE)CHROMIUM COMPLEXES

Reaction of tricarbonyl(eta-2-alkylpyridine)chromium(0) complexes with lithium diisopropylamide at -40-degrees-C generates the resonance stabilised carbanions at the alpha-position which undergo completely stereoselective electrophilic quench with alkyl halides to generate the corresponding alpha-branched tricarbonyl(eta-2-alkylpyridine)chromium(0) complexes. These complexes can then be subjected to nucleophilic additions with methyllithium followed by methyl iodide quench to afford the tricarbonyl(eta-exo-6-alkyl-1,2-dimethyl-1,2-dihydropyridine)chromium(0) complexes. Treatment of the resonance stabilised alpha-carbanion derived from tricarbonyl(eta-2-ethylpyridine)chromium(0) with the non-enolisable aldehydes benzaldehyde or pivalaldehyde affords the erythro-aldol-type products with complete control of stereochemistry at both the newly formed alpha- and carbinol centres. X-Ray crystal structures of SR(RS)-tricarbonyl[eta-exo-1,2-dimethyl-6-(2-pent-4-enyl)-1,2-dihydropyridine)-chromium(0) and RSS(SRR)-tricarbonyl[erythro-2-eta-(2-pyridyl)-1-phenylpropanol]chromium(0) are reported.