Hydrogenolysis of the Nitrogen-Oxygen Bond

Both benzyl-oxygen and nitrogen-oxygen bonds undergo hydrogenolysis readily in the presence of a benzyloxy-nitrogen bond but it appears that the benzyl-oxygen bond can be made to cleave selec-... 

Pre-treatment of the cathode produces a finely divided copper surface and this catalyses the hydrogenolysis of phenylhydroxylamine to aniline which becomes the principal reaction product. The reaction probably involves promotion of nitrogen-oxygen bond cleavage in the chemisorbed phenylhydroxylamine intermediate. 

The initial step in the reduction of protonated oximes has been shown [146] to be a hydrogenolysis of the oxygen-nitrogen bond in cyclic derivatives of oximes the same occurs. In 4-(4 -methoxyphenyl)-2,3-benzoxazin-l-one (XLV) the reduction proceeds in two steps, and it is possible to isolate the rather stable intermediate ketimine [146] ... 

It is widely accepted that a benzyl-nitrogen bond is not as readily cleaved as a benzyl-oxygen bond. The presence of nitrogen can, however, influence the hydrogenolysis of C-O bonds. It has been reported that butylamine completely prevented the hydrogenolysis of the benzylic C-O bond [62]. The same effect was observed for molecules containing both benzylic C-O and C-N bonds [62,63]. In a mixture of A-benzylcyclohexylamine and benzyl cyclohexyl ether only the C-N bond was hydrogenolyzed on Pd/C, whereas in the presence of HCl the... 

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]. 

Hydrogenolysis is analogous to hydrolysis and ammonolysis, which involve the cleavage of a bond induced by the action of water and ammonia, respectively. Chemical bonds that are broken by hydrogenolysis reactions include carbon—carbon, carbon—oxygen, carbon—sulfur, and carbon—nitrogen bonds. An example of hydrogenolysis is the hydrodealkylation of toluene to form benzene and methane ... 

Because of the need for initial coordination with the catalyst, only benzylic or allylic C-X bonds can be reduced, but the X can be oxygen as well as nitrogen. We will come back to benzyl groups, and their hydrogenolysis, as a means for temporary protection of amines and alcohols later in the chapter. For the moment, though, we should take a broader look at catalytic hydrogenation as our second (after hydride reduction) important class of reductions. 

Borohydride-reduced palladium. Reduction of palladium chloride in methanol with sodium borohydride until evolution of a gas ceases leads to a black material, which is not particularly sensitive to air and is not pyrophoric. The material is useful for selective hydrogenations, It catalyzes rapid hydrogenation of bonds of the type C=C, N=N, and N=0, but not the type C=N and C=0. No hydrogenolysis of nitrogen or oxygen functions is observed in alcohols, amines, amides, esters, ethers, or lactones. Epoxides are opened to alcohols very slowly. 

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