| Abstrakt: |
In an endeavor to utilize metal carbonyl complexes of acetylenic aldehydes and ketones to control the stereoselectivity of nucleophilic acyl addition, the dicobalt hexacarbonyl and dicobalt pentacarbonyl(triphenylphosphine) complexes of 3-phenylpropynal and 2-hexynal were conveniently prepared. Crotyl transfer from either (E)- or (Z)-crotyltributylstannane to Co2(CO)6-complexed 3-phenylpropynal and 2-hexynal produced 3,4-disubstituted-1,5-enynes with high syn diastereoselectivity. Allyl and 2-methylallyl transfer to Co2(CO)5PPh3-complexed aldehydes was also accomplished with high yields and diastereoselectivities. In all cases, decomplexation of the metal carbonyl moiety was effected, selectively, under very mild oxidative conditions. Exchange of a CO by PPh3 led to decreased overall reactivity. Two competing kinetic processes were observed: stereoselective allylation was observed at low temperature, but at higher temperatures, the first formed allylic alcohol preferentially underwent elimination leading to dienynes. |
