Sekio Mitsui

Stereochemistry and mechanism of catalytic hydrogenation of substituted cyclohexanones

Abstract Substituted alkylcyclohexanones were hydrogenated over several transition metal catalysts. The hydrogenation with freshly prepared Raney Ni gave predominantly the axial alcohols but catalyst to which sodium hydroxide was added or aged catalyst increased the stereoselectivity. In contrast to the PtO2 hydrogenation in which the equatorial alcohols were favoured, more axial alcohols were obtained than the equatorial counterparts on Pt-black. On Pd catalyst the isomeric ratios of the cyclohexanols changed as the reaction progressed. This may be due to a decrease in the amount of hydrogen available caused by the strong adsorption of ketones on the catalyst. Rh catalyst gave more of the axial alcohols than any other catalyst used in the present study.

The deoxygenation in the catalytic hydrogenolysis of styrene oxides

Abstract In order to obtain information on deoxygenation in the hydrogenolysis of epoxides, a series of substituted styrene oxides were hydrogenolysed using Raney Ni, Pd and Pt as catalysts. Over Raney Ni, these styrene oxides gave ethylbenzenes (30–50%) as well as 2-phenylethanols. The ethyl-benzenes were proved to be producedvia the corresponding styrenes. These deoxygenations can be explained by the previously proposed mechanism involving the radical cleavage reaction. The hydrogenolyses over Pd gave selectively 2-phenylethanols. Over PtO2, the deoxygenations were less than 10% except 3,4-dichlorostyrene oxide (V) which gave 3,4-dichloroethylbenzene in 75% yield. However, the significant deoxygenations were observed in all hydrogenolyses over Pt-black. The ethylbenzenes were proved to be produced directly from the corresponding styrene oxides, but not from 1-phenylethanols as proposed by Park and Fuchs. These deoxygenations are considered to occurvia styrenes bytrans-β-elimination. The difference in behaviour over Pt catalysts can be ascribed to the alkaline substances contained in the catalyst. The significant deoxygenation in the hydrogenolysis of V over PtO2 can be explained by the effect of a trace of HCl produced from the substrate and/or products.

Stereospecific hydrogenolysis of benzyl-type alcohols

Abstract In order to obtain additional information regarding the stereospecificity of hydrogenolysis on Co and Cu as well as Ni and Pd, optically active methyl 3-hydroxy-3-phentybutyrate, 3-hydroxy-3-phenylvaleric acid, methyl 3-hydroxy-3-phenylvalerate and 3-phenylpentan-1,3-diol were hydrogenolysed. In the hydrogenolysis on Ni, Co and Cu, the configuration was retained whereas on Pd the inversion of configuration occurred as reported for other compounds.

Stereochemistry and mechanism of catalytic hydrogenation of substituted methylenecyclohexanes

Abstract Substituted alkylmethylenecyclohexanes were hydrogenated over several transition metal catalysts. The ratios of the epimeric products were almost unity over freshly prepared Raney Ni but the axial Me counterparts were favoured over aged catalyst. The axial Me products were also preferred on Pt or Rh catalysts. Pd catalysed hydrogenation gave predominantly the equatorial Me isomers at high catalyst ratio, while the axial Me counterparts were favoured at the early stage of the reaction at a catalyst-substrate ratio of 1:20. The change of the ratio of epimeric products depending on the amount of catalyst is due to the rapid migration of exo-cyclic double bond to the inside of the cyclohexane ring at high catalyst ratio. The mechanism of hydrogenation is discussed in terms of the modified Horiuti-Polanyi mechanism.

The catalytic hydrogenolysis of 1-phenylbicyclo[4.1.0]heptane and the corresponding azidirine and epoxide

Abstract The hydrogenolysisof 1-phenylbicyclo[4.1.0]heptane ( 1a ), cis -1-phenyl-2-methylbicyclo[4.1.0]heptane ( 1b ), 1-phenyl-7-azabicyclo[4.1.0]heptane ( 2 ) and 1-phenyl-7-oxabicyclo[4.1.0]heptane ( 3 ) was studied using Ni, Pd, Rh and Pt as catalysts. The hydrogenolysis of the C 1 C 7 bond of 1a and 1b led to the selective formation of trans -1-phenyl-2-methylcyclohexane ( 4a ) with retention of configuration. Compound 1a gave not only 4a but also phenylcycloheptane ( 6a ), which is the product of C 1 C 6 bond fission, and the ratio of 6a to 4a increased in the sequence: Ni ⪡ Pd, Rh 1 C 6 bond fission was observed in the hydrogenolysis of 1b . These results can be explained by a mechanism involving the formation of the π-benzyl complex. trans -2-Phenylcyclohexylamine ( 8 ) was obtained stereoselectively in the hydrogenolysis of 2 over Raney Ni. This selective formation can be ascribed to the competition of “SN i” and “radical” processes. The Pd catalysed hydrogenolysis gave cis -2-phenylcyclohexylamine ( 9 ) as the main product, while the presence of sodium hydroxide promoted the formation of 8 . Raney Ni catalysed hydrogenolysis of 3 yielded a mixture of phenylcyclohexane ( 13 ) and 2-phenylcyclohexanols ( 10 and 11 ). trans -2-Phenylcyclohexanol ( 10 ) was the dominant isomer; the hydrogenolysis resulted in the predominant configurational retention. Compound 13 was confirmed to be produced via 1-phenylcyclohexene ( 12 ). This deoxygenation may be explained by a mechanism involving the radical cleavage reaction of 3 . The presence of sodium hydroxide led to the formation of cis -2-phenylcyclohexanol ( 11 ). The Pd catalysed hydrogenolysis also gave mainly 11 . The difference in behaviour of cyclopropane, azidirine and epoxide we ascribe to the differences in the affinity for the catalyst and differences in the electronegativity between C, N and O atoms.

Stereochemistry and mechanism of the catalytic hydrogenation of methylcyclohexenols

Abstract Four methylcyclohexenols were hydrogenated over several transition metal catalysts. The isomeric distributions of the products obtained over Raney Ni were different from those obtained over Pd or Pt catalysts and the adsorption of the substrate on the catalyst is considered to be rate determining in the hydrogenation on this catalyst.

Stereochemistry and the mechanism of the hydrogenation of substituted cyclopentanones and methylenecyclopentanes

Abstract 2-Alkylmethylenecyclopentanes and 2- and 3-alkylcyclopentanones were hydrogenated over several transition metal catalysts. The stereochemistry of the reaction which is discussed in terms of the Horiuti-Polanyi mechanism shows the sharp contrast by the structure of the substrates, catalysts and the reaction conditions. In the platinum catalyst hydrogenation of cyclopentanones and methylenecyclopentanes the shape of the transition state of the hydrogen transfer from the catalyst to the adsorbed species which has olefin-like molecular geometry is considered to be product-like. This would support the previous postulate that the complex aluminium hydride reduction of cyclopentanone has essentially a product-like transition state in which the ring carbon C1 rehybridizes from sp2 to sp.3

Stereochemistry and mechanism of the catalytic hydrogenation of dimethylcyclohexenes

Dimethylcyclohexenes were hydrogenated over several transition metal catalysts. The Pd catalyzed hydrogenation gave always more stable of the two possible products, while the cis products were favored over Pt catalyst. In the hydrogenation over Raney Ni the amount of less stable of the two possible products increased with an increase of the aging period of the catalyst. The mechanism of the reaction is discussed in terms of the modified Horiuti-Polanyi mechanism.

Stereochemistry and mechanism of the hydrogenation of dialkyl cyclohexenes

Abstract In order to investigate the steric effect of the substituents to determine the product distribution, disubstituted cyclohexenes were hydrogenated over several transition metal catalysts. Some cyclohexenes which have two large substituents at the vicinal carbon atoms, at least one of which is trigonal, were not hydrogenated over Raney Ni at all but were hydrogenated over Pt catalyst under our experimental conditions. Presumably, the stereoselectivity depends on the competitive operation of the torsional angle strain and the catalyst hindrance with substituents. In the hydrogenation over Pd catalyst, thermodynamically more stable products were dominant at the standard condition but at high substrate to catalyst ratio the less stable products were slightly preferred. No appreciable stereoselectivity was observed in the hydrogenation of 1,4-disubstituted cyclohexenes.

Stereochemistry and mechanism of homogeneous hydrogenation of substituted 2-cyclohexenols with tris(triphenylphosphine)chlororhodium

Abstract Substituted 2-cyclohexenols were hydrogenated with tris(triphenylphosphine)chlororhodium in benzene or benzene: ethanol. Isomer distribution of the resulting cyclohexanols showed that the thermodynamically more stable of the two possible products was preferably obtained. In addition to the saturated products, 3-substituted cyclohexenols gave appreciable amounts of saturated ketones, derived by double bond migration, while no ketone was obtained from 2-substituted cyclohexenols. Results suggest that the rate and product determining step of the reaction is the intramolecular transfer of the second hydrogen in the σ-alkylrhodium species.