Hydrogen Transfer from Solvents

Considerable interest remains in catalyzed hydrogen-transfer reactions using as donor solvents alcohols, glycols, aldehydes, amides, acids, ethers, cyclic amines, and even aromatic hydrocarbons such as alkylben- 

Mechanisms, exemplified by alcohol as donor (493, 496), usually invoke coordination of the substrate (olefins, saturated and unsaturated ketones, and aldehydes), then coordination of the alcohol and formation of a metal alkoxide, followed by /8-hydrogen transfer from the alkoxide and release of product via protonolysis  

Details of the various steps which will depend on the substrates and donors involved, are usually not well understood. Prior coordination of the donor followed by that of the substrate, equivalent to a hydride route (Section II,A), is also possible (494, 496). Formation of intermediate dihydrides from a donor (e.g., from an alcohol via oxidative addition to give a hydrido-alkoxide, and then /8-hydrogen transfer) has also been invoked (491, 492, 496, 499, 500) in mechanistic terms, the hydrogenations then become equivalent to using molecular hydrogen for the reductions. The /3-hydrogen transfer step is usually considered rate-determining (494, 496). 

Use of RuCl2(PPh3)3 with a glucofuranose derivative (61) as donor solvent leads to asymmetric hydrogenation of prochiral a,/3-unsaturated ketones to the saturated ketones, with up to 34% ee in the case of a cyclohexene-3-one (497). 

This appears to be the first asymmetric hydrogenation using a solvent as the hydrogen source. Closely related to this is the use of the ruthenium(II) catalysts Ru2C14(DIOP)3 (Section III,B) or RuCl2P3, where P is a chiral 

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The products shown for decomposition of the tetroxide of polystyrene are deduced from studies of the analogous decomposition of 1-phenylethylperoxy radicals 3531. Other reactions which should be considered in these oxidations include hydrogen transfer from solvent (Eq. (87))3S0) and reaction of the... 

Copper(I) can also be regenerated by electron transfer oxidation of radicals produced by hydrogen transfer from solvent by alkoxy radicals.101 Thus, reactions carried out in hydrocarbon solvents will produce alkyl radicals that are oxidized by Cu(II) at rates approaching diffusion control.95-100... 

The microautoclave solvent activity tests measure coal conversion in a small batch reactor under carefully controlled conditions. The tests are described as Kinetic, Equilibrium and SRT. The Kinetic and Equilibrium Tests measure coal conversion to tetrahydrofuran solubles at conditions where conversion should be monotonically related to hydrogen transfer. The Kinetic Test is performed at 399°C for 10 minutes at an 8 to 1 solvent to coal ratio. The combination of high solvent ratio and low time provide a measure of performance at essentially constant solvent composition. The measured conversion is thus related to the rate of hydrogen donation from solvent of roughly a single composition. In contrast, the Equilibrium Test is performed at 399°C for 30 minutes at a 2 to 1 solvent to coal ratio. At these conditions, hydrogen donors can be substantially depleted. Thus performance is related to hydrogen donor... 

West-Kentucky, and Itmann coals of three different ranks were sufficiently liquefied with hydropyrene under atmospheric pressure at 370°C regardless of their fusibility. The analyses of hydropyrene and the coal before and after the liquefaction clearly indicate the hydrogen transfer from the solvent to the coal substance. Lower rank coals look to show rather higher reactivity in such liquefaction, probably because their constituent molecule may have smaller condensed ring. 

The dialin donor solvents were also used directly in coal liquefaction studies. Inasmuch as details of coal structure are unknown, the present theory can only be tested in a qualitative way, as follows. First, if the liquefaction of coal occurs under kinetic control with hydrogen-transfer from the donor solvent involved in the rate-determining step, then we should expect the dialin donors to be more effective than the control solvent T.et-ralin (and also Dfecalin). This is suggested by the theory because the dialins possess higher energy HOMOs than Tetralin and... 

Mechanisms of the formation of the decal ins in the recovered solvent were based on their isotopic contents. The cis-Decalin-die had a greater protium content than the recovered Tetralin-di2-This suggested that hydrogen transfer from the coal to the tetralin is involved in the formation of cis-Decalin. 

Primary alcohols 121 undergo an efficient oxidative dimerization by [IrCl(coe)2]2 under air, without any solvent, to form esters 122 in fair to good yields (Equation 10.30) [54]. The reaction is initiated by the in situ generation of an Ir-hydride complex via hydrogen transfer from alcohols to afford aldehydes, followed by the dehydrogenation of hemiacetals derived from alcohols and aldehydes by action of the Ir-complex to afford esters. 

The last observation concerns the oxidation of the lsoprene carbanionic dimers.— We do not observe the monomer formation, but the presence of monoalcohols with a high yield. In "Inverse oxidation we obtain 52% of monoalcohol and 80% In "direct oxidation . These monoalcohols have been Isolated and characterized (Fig. 1). They come from the transfer reaction from the primary radical to the solvent. The high yield of monoalcohol shows that the hydrogen transfer from the solvent Is a competitive reaction towards functionalization. This fact Is confirmed by the similar values of the two rate constants ( 108 M-i.S-1) . . ... 

Interestingly, the metallocene-activated C-F activation is not limited to tertiary C F bonds (in contrast to the Na/benzophenone system). Perfluorocyclohexane has been defluorinated in the presence of bis(r/5-cyclopentadienyl)zirconium(IV) chloride or bis(t/5-cyclopentadienyl)-titanium(IV) chloride. This reaction is accompanied by hydrogen transfer from the solvent onto the substrate yielding 1,2,4,5-tetrafluorobenzene as product.210... 

In coal liquefaction, highly dispersed, disposable, catalysts are needed because maximal contact between coal and catalysts is essential. It is assumed that one of the roles of the coal liquefaction catalyst is to assist in the rehydrogenation of the donor solvent (e.g. tetralin) by facilitating the hydrogen transfer from the gas phase.36,37... 

In a competing and novel type of photoreaction, aroyl diphenyl phosphines 1d - k yield diphenylphosphinous acid (4, diphenylphos-phine oxide) as the photoproduct of a complex transfer of the oxygen from the carbonyl carbon onto the phosphorus atom followed by C-P-bond cleavage and hydrogen abstraction from solvent. The mass spectrometric product analysis of an u.v. irradiation experi-... 

In this section, we discuss the photoinduced hydrogen transfer from phenol to water and ammonia in phenol-water and phenol-ammonia clusters, respectively, as a representative model of excited-state chromophore-to-solvent hydrogen transfer reactions. 

Yang and Burton studied reductive radical additions of iododifluoroacetate 37 to olefins 38 and dienes catalyzed by 6-17 mol% of a catalyst generated from NiCl2 and stoichiometric amounts of zinc in the presence of water (Fig. 8) [90, 91]. Olefins gave the reductive addition products 40a in 60-83% yield, while 1,5-hexadiene or 1,8-nonadiene provided double addition products exclusively in 55% and 73% yield. 1,7-Hexadiene gave an inseparable mixture of the expected acyclic double addition product and a tandem addition/cyclization product, in which the former dominated. The radical nature of the addition is supported by inhibition of the reaction by para-dinitrobenzene. The reaction proceeds probably via initially formed atom transfer product 39, which is subsequently reduced by nickel(0) and zinc. This is supported by deuterium incorporation, when D20 was used instead of water. No deuterium incorporation was observed with THF-dg, thus ruling out hydrogen transfer from the solvent. 

When part of a delocalized aromatic molecule, the anion, after excited state electron transfer, gives rise to a not too reactive radical so that if one plays with the sequence of hydrogen abstraction from solvent followed by proton abstraction from the base, the initial anion may be restored and the solvent usefully replaces the sacrificial donor. This is the case with naphtholate as anion, certainly an interesting anionic sensitizer to test in other systems. Since the solvent is by definition at high concentration, its recovering role is played with a maximum efficiency reaction between the solvent and one of the radicals arising from the electron transfer is a good way to circumvent the back electron transfer. 

The products from these hydrogenations were separated into gases (analyzed by G.C.), water (analysed by azeotropic distillation), insolubles (CH2CI2 insolubles), asphaltene (CH2CI2 soluble/X4 insoluble) (Shell X4 40-60 C b.p. light petroleum), oils (CH2CI2 soluble/X4 soluble). Hydrogen transferred from the donor solvent was determined by G.L.C. analysis of the ratio of tetralin to naphthalene in the total hydrocarbon liquid product. 

Cyclopropyl-substituted ketones are suitable substrates for generating distonic radical anions from ketyl radical anions. A series of cycloalkanone substrates with unsaturated side-chains, to trap the primary radical formed after cyclopropylcar-binyl ring opening, has been investigated (Scheme 31) [118, 119]. For the first electron-transfer step triethylamine is used as electron donor. The reaction sequence is terminated by proton or hydrogen transfer from the solvent or the a-amino radical formed after deprotonation of the amine radical cation. 

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