TRICARBONYL(PYRIDINE)CHROMIUM COMPLEXES - CONVERSION INTO TRICARBONYL(DIHYDROPYRIDINE)CHROMIUM COMPLEXES VIA REGIOSELECTIVE AND STEREOSELECTIVE NUCLEOPHILIC-ADDITION REACTIONS

Tricarbonyl(eta-pyridine)chromium undergoes reduction in the presence of diisobutylaluminium hydride (DIBAL) to afford tricarbonyl(eta-1,2-dihydropyridine)chromium in essentially quantitative yield. Tricarbonyl(pyridine)chromium complexes undergo nucleophilic addition reactions with alkyl-lithium reagents, with subsequent methyl iodide quench, to generate tricarbonyl(eta-N-methyl-exo-2-alkyl-1,2-dihydropyridine)chromium complexes with complete stereocontrol. The reaction also occurs with the corresponding 2- and 4-methylpyridine complexes, no concomitant deprotonation of the methyl protons being observed. In the case of tricarbonyl(3-methylpyridine)chromium the addition takes place preferentially at the more hindered 2-position rather than the 6-position to give a 4.1 mixture of regioisomeric complexes. The regiochemistry of the reaction can, however, be controlled by the use of removable silyl groups to block the 2- and 6-positions by utilising tricarbonyl(2-tert-butyldimethylsilyl-3-methylpyridine)chromium or the corresponding 2,5-isomer. These addition reactions are shown to be quite general in the nature of the pyridine complex, the alkyllithium reagent and the electrophile employed since the methyl iodide can be replaced by a proton or other alkyl halides to give analogous tricarbonyl(dihydropyridine)chromium complexes.