Hydrogen transfer ability

Here we summarize some of our results obtained by exploiting the hydrogen transfer ability of heterogeneous copper catalysts and therefore their activity in the reduction of polyunsaturated compounds, racemization and dehydrogenation of unactivated secondary alcohols, and isomerization of allylic alcohols. 

So far, no example of reduction has been reported in the literature using a catalytic system relying on planar chirality. However, ansa-derivatives of Hantzsch esters were developed by Kanomata and Nagata [58] as NADH model compounds (Scheme 8.20). These reagents displayed excellent hydrogen transfer abilities-reduction of carbonyl compounds was achieved with almost perfect enantioselec-tivily. There is no doubt that, coupled with a proper metal-based hydride-transfer system, such NADH models could perform well in a catalytic version. 

The important criterion thus becomes the ability of the enzyme to distort and thereby reduce barrier width, and not stabilisation of the transition state with concomitant reduction in barrier height (activation energy). We now describe theoretical approaches to enzymatic catalysis that have led to the development of dynamic barrier (width) tunneUing theories for hydrogen transfer. Indeed, enzymatic hydrogen tunnelling can be treated conceptually in a similar way to the well-established quantum theories for electron transfer in proteins. 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

Selection of Solvents. The extraction yield of a low rank coal (Annesley) has been determined after digestion using a selection of solvents (Table II). The results show large variations in solvent power and, in particular, the high extraction yields obtained with hydrogen donor solvents. It is important to differentiate between the ability of a solvent to prevent polymerisation of the dissolved coal by hydrogen transfer, and its ability to retain the dissolved coal in solution. For example, Tetralin is frequently quoted as an... 

In the thermal decomposition of 45 in aniline and A -methylaniline, the carbene 54 showed electrophilic reactivity together with hydrogen abstraction ability (84JOC62). In fact, 48 [R = NHPh, N(Me)Ph], products of A -pyrrylation of anilines, together with 48 (R = H), a product of hydrogen transfer to 54, were obtained. In this case, aromatic substitution, typical of electron-rich benzene derivatives, was not observed. The formation of both A -pyrrylated anilines, evidence of involvement of 54s, and the... 

Chemoselective reduction of conjugated enones to allylic alcohols via hydrogen transfer from propan-2-ol over metal oxides is investigated in vapour phase conditions. The unique ability of Mgo to reduce exclusively carbonyl group is observed. However, because of the high basicity of MgO side reactions are present. It is shown that by doping the Mgo catalyst with HC1 a significant decrease of its basicity occurs and consequently side reactions are minimized. 

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