Alcohols ionic hydrogenation

A considerable extension of the synthetic utility of the hypoiodite reaction is achieved if the steroid hypoiodite (2) is generated from the alcohol and acetyl hypoiodite and then decomposed in a nonpolar solvent. In this case ionic hydrogen iodide elimination in the 1,5-iodohydrin intermediate (3) is slow, thereby allowing (3) to be converted into an iodo hypoiodite (5). 

Another iron porphyrin complex with 5,10,15,20-tetrakis(2, 6 -dichloro-3 -sulfonatophenyl)porphyrin was applied in ionic liquids and oxidized veratryl alcohol (3,4-dimethoxybenzyl alcohol) with hydrogen peroxide in yields up to 83% to the aldehyde as the major product [145]. In addition, TEMPO was incorporated via... 

The conversion of alcohols directly into the structurally related hydrocarbons by ionic hydrogenation can provide a means of synthesis for compounds that would be extremely difficult or impossible to obtain by other methods. A good example is the synthesis of 2-terr-butyladamantane (12, R = Me). This interesting, highly strained compound may be synthesized in moderate overall yield by a conventional multiple-step route.149 Alternatively, it is obtained in 90% isolated yield upon treatment of a dichloromethane solution of the readily available 2-/c/7-bulyI -2-adamantanoI (11, R = Me)150 and one equivalent of either tri-n-hexylsilane151152 or triethylsilane153 with trifluoroacetic acid at room temperature (Eq. 16). 

The cationic tungsten dihydride [Cp(CO)2(PMe3)W(H)2]+ hydrogenates the C=0 bond of propionaldehyde within minutes at 22 °C, leading to the formation of cis and truns isomers of Cp(CO)3W(l IO"Pr) Oif (Eq. (28)) [42]. The cis isomer of the alcohol complex released the free alcohol faster than the trans isomer. A similar stoichiometric ionic hydrogenation of acetone was also observed using [Cp(CO)2(PMe3)W(H)2]+. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

Scheme 17.8 Concerted ionic hydrogenation mechanisms for the hydrogenation of (a) ketones and (b) C02. The acidic ligand is shown as an alcohol in Scheme 17.7 b, but could equally well be water, a secondary amine, or a carboxylic acid.

Bisagni and co-workers (74) have synthesized the 1-chloroellipticine 126 in unique fashion (Scheme 20). Condensation of the readily available aldehyde 122 with lithiated chloropyridine 124 gave alcohol 125. Ionic hydrogenation and cyclization afforded 126 in 35% yield from aldehyde 122. This route is much shorter than an 11-step procedure reported earlier (75). 

Direct reductive elimination of the hydroxyl group can be achieved readily only for tertiary and benzylic alcohols, which are especially liable to form carbocationic intermediates in acidic media. These intermediates are able to abstract hydrogen from such hydride donors as triethylsilane. This procedure, ionic hydrogenation, was elaborated into a preparatively useful protocol ... 

Although hydrosilanes reduce ketones, in trifluoroacetic acid, to the corresponding methylene compounds or dimeric ethers via ionic hydrogenation, the reduction of a-amino and a-oxy ketones and p-keto acid derivatives with hydrosilanes, particularly PhMe2SiH, under these conditions proceeded with high anti selectivity to the alcohols. No racemization was observed at the carbon a to the carbonyl group. Intramolecular hydrosilylation catalyzed by Lewis acids provided a highly stereoselective route to anti-1,3-diols from p-hydroxy ketones (Section 1.1.3. ). ... 

Also, oxidation of primary and secondary alcohols with hydrogen peroxide in the presence of catalytic amounts of MTO and NaBr in [bmim][BF ] has been reported by Sain and coworkers [4]. The results obtained from ionic liquid as solvent were better than those obtained from some organic solvents (Scheme 14.4). 

Chhikara BS, Chandra R, Tandon V (2005) Oxidation of alcohols with hydrogen peroxide catalyzed by a new imidazolium ion based phosphotungastate complex in ionic Uquid. J Catal... 

In 1992 the Bullock and Norton groups reported the reduction of aldehydes and ketones under similar conditions, and isolated an isopropyl alcohol complex formed thereby (eq 44) [75] similar results have recently been reported with H2Re(NO)(CO)(PR3)2/CFjC02H by Bakhmutov and co-workers [76]. These reactions resemble the ionic hydrogenations long known with silanes as hydride donors [77],... 

The same steps involved in ionic hydrogenation — and H transfer to the substrate ketone — are involved in catalysis of hydrogen transfer from isopropyl alcohol to a prochiral ketone (eq 3). Traditionally, ketone coordination to the catalyst was invoked to rationalise the achievement of enantioselectivity. While transfer hydrogenation is usually done under basic conditions, the base is required more for the generation of the active catalyst than for the reaction itself the dihydride HjRuLj can catalyse the transfer of hydrogen from isopropyl alcohol to various ketones by itself [88, 89]. 

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