Stereochemistry conjugate reduction, catalytic hydrogenation

Double bonds conjugated with benzene rings are reduced electrolytically [344] (p. 23). Where applicable, stereochemistry can be influenced by using either catalytic hydrogenation or dissolving metal reduction [401] (p. 24). Indene was converted to indane by sodium in liquid ammonia in 85% yield [402] and acenaphthylene to acenaphthene in 85% yield by reduction with lithium aluminum hydride in carbitol at 100° [403], Since the benzene ring is not inert toward alkali metals, nuclear reduction may accompany reduction of the double bond. Styrene treated with lithium in methylamine afforded 25% of 1-ethylcyclohexene and 18% of ethylcyclohexane [404]. 

The hydrogenation of alkenes conjugated to aromatic systems is usually much less difficult than partial reduction of polyenes, as the aromatic ring is less susceptible to reduction. Many catalysts of Group 10 metals have been used for this transformation, including Raney nickel, Pd adsorbed on carbon, Pd adsorbed on calcium carbonate, Pd on barium sulfate, metallic palladium, PtOa, chloroplatinic acid or platinum metal. Normally, the stereochemistry of the reduced product arises from syn addition of a hydrogen molecule to the less-hindered face of the double bond, as exemplified in the catalytic hydrogenation of a steroidal styrene over metallic palladium (Scheme 93). ... 

Reduction of alkenes. Siegel et al have discussed the stereochemistry of diimide reduction of alkenes as compared with catalytic hydrogenation. The same laboratory investigated the reduction of conjugated dienes to enes no evidence for 1,4-reduction was obtained. 

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