Catalytic deuterium exchange reactions with organics. LXX. The deuteration of alkyl hydrogens in the short-chain alkylbenzenes using homogeneous platinum and heterogeneous platinum and nickel catalysts

Abstract

The mechanism of deuterium exchange in the short-chain alkylbenzenes has been studied using a homogeneous sodium tetrachloroplatinate catalyst in acetic acid solution. Carbon-14 labelled ethylbenzene has been synthesized and used to differentiate between previously proposed mechanisms. When deuteration of the side chain in ethylbenzene occurs, there is no scrambling of the α and β carbon atoms, thus eliminating the role of a symmetrically coordinated ethylene group previously proposed. This conclusion is confirmed by theoretical calculations of the exchange in ethylbenzene. The ethyl group in ethylbenzene derivatives deuterates with a low multiple character. These results from the homogeneous platinum system have been compared with corresponding data using heterogeneous platinum and nickel catalysts. Analogous intermediates are shown to be capable of existing in both homogeneous and heterogeneous systems, and are postulated to be general π-complexes for aromatic deuteration and π-allylic species for alkyl group exchange.