Benzhydrol, hydrogenation

Thionyl chloride Ammonia Thiourea Benzhydrol Hydrogen peroxide... 

A mixture of 26 g (0.1 mol) of 0 -(4-pyridyl)-benzhydrol, 1.5 g of platinum oxide, and 250 ml of glacial acetic acid is shaken at 50°-60°C under hydrogen at a pressure of 40-50 Ib/in. The hydrogenation is complete in 2 to 3 hours. The solution is filtered and the filtrate evap-rated under reduced pressure. The residue is dissolved in a mixture of equal parts of methanol and butanone and 0.1 mol of concentrated hydrochloric acid is added. The mixture is cooled and filtered to give about 30 g of 0 -(4-piperldyl)-benzhydrol hydrochloride, MP 283°-285°C, as a white, crystalline substance. 

Category 5. Hydrogen Atom Abstraction. When benzophenone is irradiated in isopropyl alcohol, the initially formed Si state crosses to the Ti state, which abstracts hydrogen from the solvent to give the radical 7. Radical 7 then abstracts another hydrogen to give benzhydrol (8) or dimerizes to benzpinacol (9) ... 

Benzophenone has also been found to be photoreduced in the presence of amines as hydrogen donors, although less efficiently than in the presence of benzhydrol or isopropyl alcohol. The photoreduction of ketones in aromatic amines is thought not to go by the same mechanism as the photoreduction in alcohols, for the following reasons ... 

These last two points are consistent with electron transfer from the tertiary amines to the triplet ketone in competition with hydrogen abstraction from benzhydrol ... 

When camphorquinone, which is known to scavenge all free ketyl radicals by hydrogen transfer,<74,76> was added to the photolysis mixture only the mixed pinacol and benzhydrol were produced, in the ratio of 1 to 0.8, respectively ... 

If the last step, the coupling of the ketyl radicals to form the product benz-pinacol, is fast, the rate of product formation will be determined by the rate of hydrogen abstraction from benzhydrol to produce the ketyl radicals. Therefore... 

We saw earlier that when benzophenone is photoreduced in the presence of optically active 2-butanol, the alcohol recovered from the reaction loses no optical activity/541 This was presented as evidence that there could be no appreciable reversibility of the initial hydrogen abstraction since this should lead to racemization of the unreacted alcohol. However, if one uses relabeled benzhydrol and examines the initially produced benzpinacol for the presence of the label, one finds that the product pinacol contains no 14C. This would indicate that there must be some type of rapid transfer of the hydrogen radical from the ketyl radical produced upon abstraction from benzhydrol,... 

During the hydrogenation of benzophenone to benzhydrol, the undesired side-product, diphenylmethane, is obtained by hydrogenolysis of the C-O bond (Scheme 4.25). 

An interesting example of hydrogenation with hydrogen in the absence of transition metal catalyst is reduction of benzophenone to benzhydrol with hydrogen in /er/-butyl alcohol containing potassium /er/-butoxide at 150-210° and 96-135 atm. Although the yields range from 47 to 98% the method is not practical because of its drastic conditions, and because of a cornucopia of more suitable reductions. 

The present work demonstrates that the oxidation of diphenylmeth-ane in basic solution follows a pattern similar to triphenylmethane and not to fluorene. At high concentrations of good electron acceptors it is possible to realize a situation wherein the rate of oxidation of fluorene is limited by and equal to the rate of ionization. The oxidations of benzhydrol and 9-fluorenol in basic solution are considered the difference in acidity of the methine hydrogens has a pronounced effect on the course of these oxidations. 

Oxidation of Potassium Peroxide. Determination of Potassium Superoxide. Potassium peroxide was prepared by the addition of a tert-butyl alcohol solution of 90% hydrogen peroxide to potassium tert-butoxide in DMSO or tert-butyl alcohol. Oxygen absorption was followed in the standard manner (20). Analysis of solid precipitates for potassium superoxide followed exactly the method of Seyb and Kleinberg (23). Potassium superoxide formed in the oxidation of benzhydrol was determined in a 15-ml. aliquot of the oxidation solution. To this aliquot 10 ml. of diethyl phthlate was added to prevent freezing of the solution. The mixture was cooled to 0°C., and 10 ml. of acetic acid-diethyl phthlate (4 to 1) added over a period of 30 minutes with stirring. The volume of the evolved oxygen was measured. 

A series of benzhydryl azides have been reported to be prepared from the benzhydrols in trichloroacetic acid using hydrogen azide. 

One of the oldest linkers for amides is the (4-methylbenzhydryl)amine linker (MBHA Entry 1, Table 3.11). In contrast to the corresponding benzhydrol linker (which is cleavable by 5% TFA in DCM, 5 min [45]), acidolysis of the benzylic C-N bond of the MBHA linker requires treatment with hydrogen fluoride or a similar acid. As for A-benzylamides, the acid-lability of Al-(diarylmethyl)amides increases with the number of electron-donating substituents on the aryl groups. 

Alcohols such as methanol, 2-propanol, and benzhydrol are cleanly oxidized to the corresponding carbonyl compounds upon photoexcitation with Na3PWi2O40 in water or with (n-P NfoPW C in CH3CN (406). The quantum yields appear to be governed by the oxidation potential of the alcohol, the availability of a-hydrogens, and the tightness of complexation with the photocatalyst The reactivity order is primary alcohol > secondary alcohol > tertiary alcohol. 

T. Yokozawa, and R. Noyori, Selective hydrogenation of benzophenones to benzhydrols. Asymmetric synthesis of unsymmetrical diarylmethanols, Org. Lett. 2000a, 2, 659-662. 

A recent kinetic study (Wender, Greenfield, Metlin, Markby and Orchin, 21) with benzhydrol as substrate indicates that the rate of the hydrogenation to diphenylmethane increases with increasing concentration of dicobalt octacarbonyl catalyst and is first order with respect to the concentration of substrate. The rate also increases slightly with hydrogen pressure and the correct rate equation probably includes the concentration of hydrogen to a fractional exponent (probably %). 

A kinetic study of the hydrogenation of benzhydrol to diphenyl-methane (Wender, Greenfield, Metlin, Markby and Orchin, 21) in different solvents showed that the rate decreased in the following order ethanol > benzene > cyclohexane. This is the solvent effect to be expected in a reaction with an ionic intermediate. Furthermore, the reduction is completely inhibited with pyridine as a solvent, a fact consistent with acid catalysis. 

An additional interesting fact uncovered by the kinetic study of the hydrogenation of benzhydrol (Wender, Greenfield, Metlin, Markby and Orchin, 21) is the accelerating effect of small quantities of metallic copper. The addition of copper to the reaction mixture increased the rate of reduction about tenfold. The exact significance of this fact which probably involves the ability of copper to act as an electron donor, Cu°—> Cu+ + 1(e), is yet to be explained. 

In a patent, lead-poisoned palladium catalyst was claimed to be effective for hydrogenation of benzophenone to benzhydrol at 115°C and 0.34 MPa H2.115 Kumbhar and Rajadhyaksha hydrogenated benzophenone to benzhydrol in 98.4% selectivity at 88% conversion over Ni-Fe (75 25) on Ti02 using methanol-10% water as solvent and NaOH (0.1 wt% of benzophenone) as additive at 135°C and 5.9 MPa H2.116... 

Another route to cyclic peroxides from carbonyl compounds is illustrated by the reaction of dichlorodiphenylmethane with hydrogen peroxide to form the dimeric benzophenone peroxide (77),62 which reacts with zinc in acetic acid to form benzopinacolone (Ph3C—CO— Ph), and with aluminum amalgam to form benzhydrol. On fusion (183-225°), 77 decomposes to form benzophenone. 

Sandner et al (21) observed that addition of small amounts of tri-ethylamine (0.02 M) greatly enhanced the jAoto-induced polymerization of methyl acrylate (1.0 M in tert-butanol, nitrogen-flushed) in the presence of benzophenone (0.02 M). Photoinitiation was not effected by triethyl-amine alone, nor by triphenylamine, isopropanol, or benzhydrol (all 0.02 M) in the presence of benzojAenone. Quantum yidds measured for benzophenone disappearance indicated that methyl acrylate itself acted as a quencher of photoexcited benzophenone, effectively suppressing hydrogen abstraction from terf-butanol, and benzhydrol. How-... 

The reaction of benzophenone (34) with benzhydrol (35) is a representative example of hydrogen abstraction ... 

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