Trends in Labeling

While research continues particularly concerning heterogeneous catalysis and, to a lesser extent, the Wilzbach method to improve the 

Only preliminary studies of the homogeneous technique have been performed and the procedure has been used to label only relatively simple molecules. However, extrapolation to some heterocyclic systems has been achieved. In general, the orientation of isotope incorporated by both homogeneous and heterogeneous procedures is similar, ortho deactivation effects being observed for positions adjacent to bulky substituents such as alkyl groups. 

Two mechanisms have been proposed for the exchange of ring hydrogens in heterocycles,123-126 termed the homogeneous associative and 

Garnett, R. J. Hodges, and W. A. Sollich-Baumgartner, Proc. 4th Int. Congr. Catal., 1968, Paper 1, in press. 

Mechanistically, homogeneous metal-catalyzed exchange in heterocycles125 127 is complicated by the effect of the lone-pair electrons from the heteroatom and also the presence of acid in the exchange medium. In the present preliminary homogeneous studies (Table XIV) only a 

---