Metal-catalyzed hydrogen exchange

Group VIII transition metal catalyzed hydrogen exchange reactions are important in providing useful information concerning the fundamental processes of bond rupture and bond formation on catalyst surfaces. The reactions are also a convenient source of deuterated and tritiated compounds for chemical and biological research (i). 

T. R. Cundari, S. O. Sommerer, and W. J. Stevens, Chem. Phys., 178, 235 (1993). Effective Core Potential Study of Transition and Lanthanide Metal Catalyzed Hydrogen Exchange. 

The tritiated version could be prepared from tritiated formic acid which we had prepared at high specific activity (2.5 Ci mmol-1) by a metal-catalyzed hydrogen-tritium exchange procedure using T2 gas. The material can be stored either as a solid or as a solution if the latter any release of tritium by back exchange can be easily monitored by 3H NMR spectroscopy. In our experience very little exchange occurs over several weeks of storage [51]. 

In pyrimidine, the most likely metallation site for lithiation is C-5 (cf. base-catalyzed hydrogen exchange, Section 2), but even two methoxy groups in the 4- and 6-positions failed to allow lithium-hydrogen exchange at -40°C in this system (65JCS6695). At lower temperature (<-70°C), however, 5-pyrimidinyllithium derivatives can be prepared by... 

These reversible reactions explain metal-catalyzed hydrogen-deuterium exchanges at the ortho positions of phosphites and phosphines. ... 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

On the basis of tt complex adsorption it is possible to formulate two new mechanisms for transition metal-catalyzed exchange reactions between aromatic molecules and water or hydrogen gas (2, 4). Similar to... 

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