Yuzuru Takagi

Substituent effects in heterogeneous catalysis: I. Competitive hydrogenation of cyclohexanone and methylcyclohexanones over group VIII metals

Abstract Cyclohexanone and one of its methyl-substituted derivatives (2-, 3-, or 4-methylcyclohexanone) were hydrogenated competitively in pairs in cyclohexane solvent at 30 °C over group VIII metal catalysts. All of the methyl-substituted ketones were less reactive than cyclohexanone. The reactivity of a methylcyclohexanone relative to cyclohexanone was dependent upon the reaction mechanism and the atomic radius of the catalyst. INDO calculations, on the other hand, revealed that the carbonyl groups of the substituted and unsubstituted ketones are nearly identical in charge distribution and bond order, thus suggesting that the methyl substituent effect upon the hydrogenation rate is due primarily to steric hindrance to adsorption.

Catalytic deuteration of cyclohexanone and allied reactions over platinum metals

Abstract In order to gain some insight into the mechanism of ketone hydrogenation to alcohol, cyclohexanone was deuterated over platinum metal catalysts under elevated pressure at 80 °C. The straightforward addition of two deuterium atoms to the carbonyl linkage was predominant over the heavy triad (Os, Ir, Pt). In contrast, the deuteration over the light triad (Ru, Rh, Pd) was accompanied by considerable isotopic exchange at the C-2 or C-6 position, leading to substantial proportions of the polydeuterated alcohols. A mechanism for cyclohexanone hydrogenation is presented which consists of three interlinked hydrogenation paths each having a different half-hydrogenated intermediate. The deuterium incorporation into the C-2 or C-6 position is attributable to alternation between the 1-monoadsorbed and 1,2-di-adsorbed intermediates.

Potential bile acid metabolites. 24. An efficient synthesis of carboxyl-linked glucosides and their chemical properties.

A facile and efficient synthesis of the carboxyl-linked glucosides of bile acids is described. Direct esterification of unprotected bile acids with 2,3,4,6-tetra-O-benzyl-d-glucopyranose in pyridine in the presence of 2-chloro-1,3,5-trinitrobenzene as a coupling agent afforded a mixture of the α- and β-anomers (ca. 1∶3) of the 1-O-acyl-d-glucoside benzyl ethers of bile acids, which was separated effectively on a C18 reversedphase chromatography column (isolated yields of α- and β-anomers are 4–9% and 12–19%, respectively). Subsequent hydrogenolysis of the α- and β-acyl glucoside benzyl ethers on a 10% Pd−C catalyst in acetic acid/methanol/EtOAc (1∶2∶2, by vol) at 35°C under atmospheric pressure gave the corresponding free esters in good yields (79–89%). Chemical specificities such as facile hydrolysis and transesterification of the acyl glucosides in various solvents were also discussed.

Comparative studies of homogeneous and heterogeneous catalyses: Part III. The hydrogenation of disubstituted cyclohexenes over Rh and Rh(PPh3)3Cl

Abstract The homogeneous and heterogeneous hydrogenations of methyl 4-t-butyl -1-cyclohexenyl ether (I), methyl 4-t-butyl-1-cyclohexenyl ester (II) and methyl 4-t-butyl-1-methylcyclohexene (III) over Rh and Pd metals and the Rh metal complex catalyst, RhCl(PPh3)3, are reported. The isomer distributions of products formed from I, II and III are compared for the homogeneous and heterogeneous catalytic hydrogenations. The hydrogenations of I, II and III over Rh metal catalyst give mainly the cis isomer for I and II, whereas the trans isomer is formed in large excess over the Rh metal complex. The implications of these stereochemical results for the mechanism are examined on the basis of the exchange patterns obtained during the deuteration of the substrates, as well as on the basis of various kinetic factors. In addition, the effects of the solvent and the ligands of the Rh complex catalyst on the stereochemistry of hydrogenating II have been investigated. The trans isomer is obtained in 93% yield when o-cresol is used as the solvent. The Rh complex containing the dimethylaminophenylphosphine ligand gives the largest amounts of trans isomer (93%).

Comparative studies of homogeneous and heterogeneous catalyses. I. Geraniol hydrogenation catalyzed by Ru, Rh, Ru(PPh3)3Cl2, and Rh(PPh3)3Cl

Abstract Four catalysts (Ru, Rh, Ru(PPh3)3Cl2, and Rh(PPh3)3Cl) were compared using geraniol (A) as a substrate in hydrogenation at high pressures. With all the catalysts, A was first hydrogenated mainly to citronellol (B) and then further to 3,7-dimethyloctanol (C), accompanied by the formation of a few side products. Analysis of the kinetic data suggests that this seemingly consecutive reaction sequence A → B → C is mechanistically regarded as a simultaneous process B → A → C on heterogeneous catalysts although for homogeneous catalysts it is indeed consecutive. No hydrogenolysis accompanied the hydrogenation in homogeneous catalysis, whereas in heterogeneous catalysis, hydrogenolysis to 2,6-dimethyloctane was not negligible. The Rh complex was unique in its significant activity for hydrogen transfer to produce aldehydes, namely, citronellal and 3,7-dimethyloctanal. A mechanism is presented for each of the homogeneous and heterogeneous catalyses.

Deuteration of 4-t-butyl-1-cyclohexenyl methyl ether catalyzed by platinum metals: Evidence for staggered αβ-diadsorbed intermediates

Abstract Deuteration of 4-t-butyl-1-cyclohexenyl methyl ether was carried out at 80 °C in cyclohexane under pressure. Over Ru, Rh, and Pd, the reaction products consisted almost entirely of the cis and trans mixtures of the corresponding saturated ethers (cis- and trans-4-t-butyl-1-cyclohexyl methyl ether), whereas over Os, Ir, and Pt, hydrogenolysis to t-butylcyclohexane was not negligible. The isomeric mixtures were separated and analyzed for isotopic distributions by mass spectrometry (MS) and for deuterium positions within each molecule by NMR spectroscopy. With most catalysts, the MS-determined isotopic distribution pattern for the cis ether was quite different from that of the trans ether. Also the NMR-based deuterium distributions were markedly different between these isomers. These dissimilarities can be best explained by assuming a few staggered αβ-diadsorbed species as intermediates in enol ether hydrogenation.

Comparative studies of homogeneous and heterogeneous catalyses. II. Hydrogenation of olefinic double bonds by Ir and IrCl2H(Pph3)3: activation of the latter catalyst by H2O2

Abstract The reaction profiles of consecutive hydrogenation A → B → C (A. geraniol; B, citronellol; C, 3,7-dimethyloctanol) were compared for Ir and IrCl 2 H(Pph 3 ) 3 catalysts. The results suggest that on both catalysts this seemingy consecutive reaction is essentially simultaneous, B ← A → C, both B and C being formed from A through a common intermediate. The hydrogenation of A on IrCl 2 H(Pph 3 ) was accelerated by the addition of H 2 O 2 . Acceleration by H 2 O 2 addition was also noted for the hydrogenation of allyl alcohol and diethyl fumarate.

Application of nuclear magnetic resonance shift reagents to kinetic studies on catalytic deuteriation of 4-t-butylcyclohexanone

N.m.r. shift reagents have been applied to mechanistic studies on the deuteriation of 4-t-butylcyclohexanone; a possible mechanism is discussed.