Allyl carbonates hydrogenolysis

Pd(0)-catalyzed hydrogenolysis of vinylepoxides offers an attractive regio- and dia-stereoselective route to homoallylic alcohols (Scheme 9.36) [104, 155, 156]. Thus, hydrogenolysis of ( ) olefin 88 affords syn isomer 89 with inversion of configuration at the allylic carbon, while subjection of (Z) isomer 90 to identical reaction conditions results in the anti isomer 91. The outcomes of these reactions are ex-... 

Benzyl and Allyl Carbon Atoms Hydrogenolysis of a carbon-heteroatom bond is extremely fast if the heteroatom is connected to allylic or benzylic carbon atoms. The benzylic compounds are used very frequently in organic chemistry as protecting groups. Because of its great importance the benzylic compounds are discussed separately in the next section. 

In the dry Pauson-Khand reaction system oxygen is necessary in order to prevent hydrogenolysis of the allylic carbon-oxygen bond in the product. This process is presumably effected by hydridocobalt... 

Asymmetric hydrogenolysis of allylic carbonates was realized by the use of chiral ligand, (R)-MOP-Phen, with l,8-bis(dimethylamino)naphthalene (NRj) as a base to afford the optically active tenninal olefins (Scheme 7). 

The driving force for the hydrogenolysis is the aromatization in cooperation with the particular nature of the side chain stabilizing the intermediate (crypto) allylic or benzylic cation. The reaction starts with a reversible proton addition to the carbonyl function and proceeds via addition of a hydride ion to an allylic carbon atom. Concurrently or subsequently the double bond in the side chain is hydrogenated (Fig. 12). 

Asymmetric hydrogenolysis of allylic esters with formic acid with satisfactory ee was observed[387], Geranyl methyl carbonate (594) was reduced to 570 with formic acid using l,8-bis(dimethylamino)naphthalene as a base and MOP-Phen as the best chiral ligand, achieving 85% ee. 

Furthermore, the regioselective hydrogenolysis can be extended to internal allylic systems. In this case, clean differentiation of a tertiary carbon from a secondary carbon in an allylic system is a problem. The regioselectivity in the hydrogenolysis of unsymmetrically substituted internal allylic compounds depends on the nature and size of the substituents. The less substituted alkene 596 was obtained from 595 as the main product, but the selectivity was only... 

Highly regio- and stereoselective 4a-deuteration in steroids is possible by the hydrogenolysis of the cyclic allylic /3-carbonate 628 with NaBD4. the extent of 6a-deuteration is only 3%[393],... 

A benzyl carbonate was prepared in 83% yield from the sodium alkoxide of glycerol and benzyl chloroformate (20°, 24 h). It was also prepared by a lipase-catalyzed ester exchange with allyl benzyl carbonate. It is cleaved by hydrogenolysis (H2/Pd-C, EtOH, 20°, 2 h, 2 atm, 76% yield) and electrolytic reduction (—2.7 V, R4N X, DMF, 70% yield). A benzyl carbonate was used to protect the hydroxyl group in lactic acid during a peptide synthesis." ... 

In discussing the reaction pathways, we believe that the general evidence leads to the conclusion that hydrogenolysis proceeds via adsorbed hydrocarbon species formed by the loss of more than one hydrogen atom from from the parent molecule, and that in these adsorbed species more than one carbon atom is, in some way, involved in bonding to the catalyst surface. In the case of ethane, this adsorption criterion is met via a 1-2 mode or a v-olefin mode. Mechanistically it is difficult to see how the latter could be involved in C—C bond rupture in ethane. With molecules larger than ethane, other reaction paths are possible One is via adsorption into the 1-3 mode, and another involves adsorption as a ir-allylic species. 

Unsaturated epoxides are reduced preferentially at the double bonds by catalytic hydrogenation. The rate of hydrogenolysis of the epoxides is much lower than that of the addition of hydrogen across the carbon-carbon double bond. In a, -unsaturated epoxides borane attacks the conjugated double bond at -carbon in a cis direction with respect to the epoxide ring and gives allylic alcohols [660], Similar complex reduction of epoxides occurs in a-keto epoxides (p. 126). 

Hydrogenolysis of allylic acetates.1 This reaction can be effected in high yield by hydrogenolysis catalyzed by Pd(OH)2 on carbon and with cyclohexene as the... 

Partial reduction of phenols affords mixtures of allylic and vinylic alcohols. From the generality derived for aliphatic systems, the most hydrogenolysis of this mixture is expected with platinum, palladium, and iridium catalysts, and much less with rhodium and ruthenium, an expectation substantiated in practice. For example, hydrogenation of resorcinol in neutral medium affords 20, 19, and 70% cyclohexanediol over palladium-, platinum-, and rhodium-on-carbon, respectively (29). Many examples attest to the value of rhodium and ruthenium at elevated pressure in avoiding hydrogenolysis. 

Hydrogenolysis of allylic acetates. Allylic acetates (or carbonates or chlorides) are converted into terminal alkenes by ammonium formate using a Pd(ll) catalyst com-plexed with a trialkylphosphine, particularly P(n-Bu),. 

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