Electron transfer reactions in organic chemistry. XX. Fate of tris(4-bromophenyl)ammoniumyl hexachloroantimonate during catalysis of the Diels-Alder dimerization of 1,3-cyclohexadiene

Abstract

The Diels-Alder (DA) dimerization of 1,3-cyclohexadiene (CHD) by the radical cation salt, tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA+ SbCl6-), has been studied by kinetic methods and product identification. The main goal was to elucidate the pathways by which TBPA+ is consumed. These can be summarized as follows. In the absence of any added base, DA dimers are formed in high yield and the radical cation is consumed in an essentially quantitative oxidative addition reaction between CHD+ and a chloride ion source, most likely hexachloroantimonate ion. In the presence of a hindered pyridine base, DA dimerization is almost blocked and CHD+ is deprotonated by the base ultimately to give benzene (up to 40 % yield) and oligomers of CHD. Thus in both cases the role of TBPA+ is to abstract an electron from CHD, the radical cation of which is presumably the product-forming species. The mechanism is complex, but it could be shown that DA dimerization is second-order in [CHD] under the hypothetical conditions that [TBPA+]/[TBPA] is constant. It is assumed (and supported by Marcus-type calculations) that the reduction of the dimer radical cation, (CHD)2+ occurs via reaction with TBPA, thus making the mechanism of rather conventional catalytic type. TBPA is formed to an extent of > 50% in the very early part of the run. The measured rate constant, κ(CHD)obs, is then proportional to [TBPA+]0/[TBPA]∞. The disappearance of the radical cation via the oxidative addition process was second order in [TBPA+], indicative of the so-called complexation mechanism for radical cation/nucleophile reactions.