Hydrogen transfer/exchange

Ribonucleotide reductase differs from the other 5 -deoxyadenosyl-cobalamin requiring enzymes in a number of respects. Hydrogen is transferred from coenzyme to the C2-position of the ribose moiety without inversion of configuration. Also since lipoic acid functions in hydrogen transfer, exchange with solvent protons takes place. Furthermore, exchange between free and bound 5 -deoxyadenosylcobalamin occurs rapidly during catalysis. Evidence for a Co(I)-corrin as an intermediate for this reduction is presented in our section on electron spin resonance. 

T-bond metathesis and hydrogen transfer/exchange processes can also occur where SISHAs interactions reduce the barriers to such processes much as for H/H2 exchanges above. Chaudret has studied tr-ligand substitution mechan-... 

For exchange of non-labile organic hydrogen atoms, acid-base catalysis (or some other catalytic hydrogen-transfer agent such as palladium or platinum) is required. The method routinely gives tritiated products having a specific activity almost 1000 times that obtained by the Wilzbach method shorter times are required (2-12h) and subsequent purification is easier. 

A rare-earth-exchanged zeolite increases hydrogen transfer reactions. In simple terms, rare earth forms bridges between two to three acid sites in the catalyst framework. In doing so, the rare earth protects... 

In another experiment, naphthalene-d8 was used to investigate the chemistry of hydrogen transfer between coal and nondonor solvent at 380°C. An analysis of the recovered naphthalene-d8 showed that approximately 4% of the hydrogen in the coal and in the naph-thalene-d8 exchanged. Most of the protium incorporated in the naphthalene-d8 was found in the a-position. The coal products contained approximately 2 wt % chemically-bound napththalene-d8. 

The introduction of an impurity into a specimen (accompanied by a change in tv and es ) will transfer us from one point to another in Fig. 9. Suppose that when a donor impurity is introduced into the specimen (decrease in v and e8 ), we are transferred from the point A to the point B. This involves a decrease in K, as can be seen from Fig. 9. Such a decrease in the photocatalytic effect caused by the addition of donor impurities has been observed by Kohn and Taylor (40) who studied the photoreaction of hydrogen-deuterium exchange on zinc oxide exposed to y radiation. 

In the presence of strong alkali, the rhodium analog of 62, or RhCl(C8H,2)PPh3, hydrogenates aliphatic ketones at 1 atm and 20°C, and after treatment with borohydride the systems similarly reduce aromatic ketones to the alcohols (526). Deuterium exchange data for acetone reduction were interpreted in terms of hydrogen transfer within a mononuclear hydroxy complex containing substrate bound in the enol form (63). 

The kinetics of hydrogenation transfer is covered by the use of an exchange superoperator assuming a pseudo first-order reaction. Thereby, competing hydrogenations of the substrate to more than one product can also be accommodated. In addition, the consequences of relaxation effects or NOEs can be included into the simulations if desired. Furthermore, it is possible to simulate the consequences of different types of pulse sequences, such as PH-INEPT or INEPT+, which have previously been developed for the transfer of polarization from the parahydrogen-derived protons to heteronuclei such as 13C or 15N. The... 

Harrison and McDowell 138) observed that while neither zinc oxide nor a, o-dip enyl-/3-picryl hydrazyl (a solid free radical) alone, catalyze hydrogen-deuterium exchange measurably at 77° K, a mixture of the two solids possesses considerable catalytic activity. They suggested that the effective catalyst in this mixture is the zinc oxide and that its catalytic activity is enhanced by electron transfer to the a,a-diphenyl-/3-picryl hydrazyl. It should be noted, however, that this implies a dependence of catalytic activity on electron concentration which is opposite to that observed by Molinari and Parravano 134). To bring the results for this system into line with those obtained by the latter workers, it would be necessary to postulate electron transfer in the opposite direction, i.e., from the a, a-diphenyl-/8-picryl hydrazyl to the zinc oxide. ... 

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