S_N2 or Electron Transfer?? A new technique discriminates the mechanisms of oxidative addition of alkyl halides to corrinato- and porphyrinatocobalt(I).

Zhou, De-Ling; Walder, Peristera; Scheffold, Rolf; Walder, Lorenz

A fast electrochemical technique for the discrimination of one- and two-electron mechanisms in the oxidative addition of alkylating agents (RX) to corrinato- and porphyrinatocobalt(I) ([CoIL]) is described. It is based on single-scan voltammograms of [CoIIL] in the presence of RX and variable amounts of the radical trap acrylonitrile. In the first part of the voltammogram, [CoIIL] is reduced, and fast oxidative addition of RX to [CoIL] is triggered. If the reaction proceeds via a two-electron mechanism, [RCoIIIL] is formed independently of acrylonitrile concentration, but if a transient free radical R is involved, R is competitively trapped by acrylonitrile and [CoIIL] to yield, at high enough acrylonitrile concentration, exclusively the olefin-inserted [RCNCoIIIL]. [RCNCoIIIL] is reducible in the intermediate potential range, [RCoIIIL] at the negative end of the single-scan voltammogram. Hence, from the appearance of the reduction waves due to [RCNCoIIIL] and [RCoIIIL], the mechanism of oxidative addition of RX to [CoIL] is easily deduced. The method is applied to the study of the mechanistic borderline of oxidative addition using a series of 15 RX and 4 [CoL]'s, i.e. cobalamin (Cbl), heptamethyl cobyrinate ('Cby'), (tetraphenylporphyrinato) cobalt ([Co(tpp])), and (octaethylporphyrinato) cobalt ([Co(oep)]). All non-activated primary alkyl iodides and bromides exhibit, at room temperature, pure two-electron mechanisms with all [CoIL]'s, except neopentyliodide with Cb1I and 'Cby'I. All secondary alkyl iodides involve free radicals with Cb1I and 'Cby'I, but a pure two-electron mechanism or a mixed one-electron two-electron mechanism with [CoI(tpp)] and [CoI(oep)]. The mechanistic switch from a two-electron to a one-electron mechanism for increasingly sterically demanding RX's occurs earlier with the supernucleophilic Cb1I and 'Cby'I than with [CoI(tapp)] and [CoI(oep)].

Helvetica Chimica Acta, 1992, Vol 75, Issue 4, p995

0018-019X

Academic Journal

10.1002/hlca.19920750403