Metal hydride reduction of 9-anthryl ketones. Steric influence on reaction path

Abstract

Reduction of 9-anthryl styryl ketone and of anthracene-9,10-diyl bis(styryl ketone) by metal hydrides yields the corresponding saturated ketones, 9-anthryl phenethyl ketone and anthracene-9,10-diyl bis(phenethyl ketone), as sole products regardless of the metal hydride or the conditions used. The absence of the corresponding allylic and saturated alcohols normally associated with the reduction of αβ-unsaturated ketones by metal hydrides is attributed to the steric influence of the adjacent peri-hydrogen atoms of the anthracene ring. In the first case, steric hindrance to nucleophilic attack on the carbonyl carbon atom suppresses 1,2- in favour of 1,4-reduction and, in the second, a similar effect prevents further reduction of the enolate or enol precursors of the saturated ketones. Saturated 9-anthryl ketones may be reduced to the corresponding alcohols with lithium aluminium hydride and aluminium hydride, but not with the more selective reagents sodium borohydride and lithium tri-t-butoxyaluminium hydride.