The stereochemistry of the catalytic hydrogenation of compounds containing an asymmetric carbon atom.

4-Phenylpent-3-en-2-one has been prepared by the reaction of dimethyl cadmium with beta-methylcinnamoyl chloride, and its structure confirmed by a haloform degradation to trans-beta-methylcinnamic acid. Reduction of the ketone with aluminium isopropoxide gave (+/-)-4-Phenylpent-3-en-2-ol, characterised by the preparation of the N- a-naphthyl and N-4-diphenylyl-carbamates. On catalytic hydrogenation (+/-)-4-phenylpent-3-en-2-ol gave a mixture of the two diastereoisomeric racemates of (+/-)-4-phenylpentan-2-ol in which a new centre of asymmetry has been generated at C[4]. On vapour-phase chromatography the two racemates separated; from the chromatogram the ratio in which they are present has been determined, and hence the degree of asymmetric synthesis calculated and found to be: (i) 5%, (ii) 57%. By oxidation the original centre of asymmetry at C[2] has been removed to yield (+/-)-4-phenylpentan-2-one. The stereochemistry of the hydrogenation is discussed. (+)-alpha-Pinene has been hydrogenated over a Raney nickel catalyst to give almost exclusively (+)-cis-pinane, and the stereochemistry of this hydrogenation is also discussed. The possibility of purifying (+)-alpha-pinene by the technique of progressive freezing has been examined. (+/-)-beta-Naphthylmethylcarbinol has been hydrogenated over a platinum oxide catalyst, and over active and stabilized Raney nickel catalysts. In all cases a mixture of isomers of 1,2,3,4-tetrahydro-2-ethylnaphthalene and 1,2,3,4-tetrahydro-6-ethylnaphthalene was the exclusive, or predominating, product. In the hydrogenation with a stabilized catalyst a small quantity of a tetrahydro-beta-naphthylmethylcarbinol was isolated, and the presence of two isomeric 2-ethyldecalins was also detected. The hydrogenation of beta-naphthylmethylcarbinyl acetate, and of the methyl ether, produced in each case, exclusively, a mixture of the two isomeric ethyltetralins. p-Cymene has been acetylated to give a mixture of two acetyl cymenes in which 2-acetyl-p-cymene predominates. Aluminium isopropoxide reduction gave a mixture of two cymyl alcohols, the hydrogenation of which, under various conditions, gave in all cases an isomeric mixture of ethylcymenes, generally as the major product. With a stabilized Raney nickel catalyst a small quantity of the menthyl alcohols was obtained, and vapour-phase chromatography showed that hydrogenation had produced a mixture of four diasterio-isomeric racemates of the menthyl alcohols, which were present in unequal amounts, indicating that an asymmetric hydrogenation had occurred. The stereochemistry of the hydrogenation is discussed