Carbon-oxygen bonds benzyl hydrogenolysis

Clive, D.L.J. and Wang, J. (2004) Synthesis of ( )-hamigeran B, (-)-hamigeran B, and ( )-l-epi-hamigeran B. Use of bulky silyl groups to protect a benzylic carbon-oxygen bond from hydrogenolysis. J. Org. Chem., 69, 2773-2784. 

Other esters are sometimes used to protect carboxylic acids, especially when there is a desire to deprotect the acid by using different conditions from those available for methyl esters. Benzyl esters are prepared in the usual manner but can be cleaved by reaction with hydrogen and a catalyst. Again it is the benzylic carbon-oxygen bond that is broken in the hydrogenolysis reaction ... 

The same was found for nitrogen-containing compounds protected by benzyl-oxymethyl groups. These compounds have two reactive bonds. The regioselectivity of the hydrogenolysis depends on the catalyst used. The carbon-oxygen bond was ruptured by use of Pd/C as catalyst whereas the rupture of the C-N bond occurred when hydrogenolysis was performed with Pd(OH)2 in MeOH (Scheme 8) [65]. 

A few hydrogenolyses have been studied under sonication (Fig. 14). The first one mentioned in the literature without any detail reports the cleavage of a benzylic carbon-oxygen bond during the elaboration of a P-lactamic antibiotic. The hydrogenolysis of hydrazines was studied in some detail, and sonication was shown to provide many advantages to a reaction otherwise limited in scope. s... 

Certain types of functionality can be removed and replaced by hydrogen under catalytic hydrogenation conditions. This is called hydrogenolysis. For example, aromatic halogen substituents are usually removed by reduction over metal catalysts. Aliphatic halides are less reactive but hydrogenolysis is promoted by base. Carbon oxygen bonds at benzyl and allyl positions are cleaved by catalytic hydrogenation. ... 

Exposure of benzylic alcohols, ethers, or esters to hydrogen in the presence of metal catalysts results in rupture of the reactive benzylic carbon-oxygen bond. This transformation is an example of hydrogenolysis, cleavage of a o- bond by catalytically activated hydrogen. 

Because the hydrogenolysis of the phenylmethyl (benzyl) ether in the final step occurs under neutral conditions, the tertiary alcohol function survives untouched. A tertiary butyl ether would have been a worse choice as a protecting group, because cleavage of its carbon-oxygen bond would have required acid (Section 9-8), which may cause dehydration (Section 9-2). 

A more expected difference between platinum oxide and palladium-on-carbon was found in the hydrogenolysis of 5-phenyI-2-(3,4-dimethoxybenzyI)-2-oxazoline. Cleavage occurred at the benzyl-oxygen bond over both catalysts, but over platinum, the less substituted phenyl group was saturated as well (78). 

The removal of a carbobenzyloxy group can be separated into two steps (Figure 1). The first step comprises the hydrogenolysis of the benzyl oxygen bond of the Cbz-protected amino acid 1 to form a carbamic acid intermediate 2 and toluene 3. The carbamic acid intermediate decaiboxylates to give the deprotected amino acid 4 and one equivalent of carbon dioxide 5. 

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