Tricarbonylpyridinechromium complexes: stereoselective alkylations and aldol-type reactions involving α-carbanions derived from η-tricarbonyl(2-alkylpyridine)chromium complexes
Reaction of tricarbonyl(η-2-alkylpyridine)chromium(0) complexes with lithium diisopropylamide at –40 °C generates the resonance stabilised carbanions at the α-position which undergo completely stereoselective electrophilic quench with alkyl halides to generate the corresponding α-branched tricarbonyl(η-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(η-exo-6-alkyl-1,2-dimethyl-1,2-dihydropyridine)chromium (0) complexes. Treatment of the resonance stabilised α-carbanion derived from tricarbonyl(η-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 α-and carbinol centres. X-Ray crystal structures of SR(RS)-tricarbonyl[η-exo-1,2-dimethyl-6-(2-pent-4-enyl)-1,2-dihydropyridine)-chromium (0) and RSS(SRR)-tricarbonyl[erythro-2-η-(2-pyridyl)-1-phenylpropanol]chromium (0) are reported.