Metals, activated hydrogen-metal 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]. 

Kubicka further reported that the specific activities of the metals for benzene hydrogenation fell in the sequence Ru > Pt > Tc Pd > Re. We note that, for the elements of the second transition series, the maximum activity was observed for the element of group VIIIi (group VIII2 was not studied). This should be compared with the results in Fig. 4 which show that the activities for the exchange reaction pass through a maximum at... 

In discussing the mechanism of the para hydrogen conversion or the hydrogen deuterium exchange reaction, Eley (117) suggested that it could occur at a single metal atom. Two orbitals are needed to bind the activated complex and these can be provided easily by the d orbitals of... 

Besides solid transition metals, certain soluble transition-metal complexes are active hydrogenation catalysts.4. The most commonly used example is tris(triphenylphosphine)-chlororhodium, which is known as Wilkinson s catalyst.5 This and related homogeneous catalysts usually minimize exchange and isomerization processes. Hydrogenation by homogeneous catalysts is believed to take place by initial formation of a rc-complex, followed by transfer of hydrogen from rhodium to carbon. 

Transition metals also catalyze isotopic exchange reactions. Platinum is the most active catalyst for most heterocycles. The mechanism may involve metallation, addition, o--addition and ir-complex formation. a-Hydrogen exchange in pyridine is favored over 3- and 7-positions, particularly by a cobalt catalyst whereas platinum is much less selective. In isoquinoline both the 1- and 3-position protons are exchanged at almost the same rates with very little exchange at any other position. In 3-substituted pyridines exchange is preferred at the 6-position, the more so as the size of the 3-substituent increases (73AHC(15)140). 

---