<i>S</i>_N2 or Electron Transfer?? A new technique discriminates the mechanisms of oxidative addition of alkyl halides to corrinato‐ and porphyrinatocobalt(I)

Abstract

Abstract 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) ([Co I L]) is described. It is based on single‐scan voltammograms of [Co II L] in the presence of RX and variable amounts of the radical trap acrylonitrile. In the first part of the voltammogram, [Co II L] is reduced, and fast oxidative addition of RX to [Co I L] is triggered. If the reaction proceeds via a two‐electron mechanism, [RCo III L] is formed independently of acrylonitrile concentration, but if a transient free radical R is involved, R is competitively trapped by acrylonitrile and [Co II L] to yield, at high enough acrylonitrile concentration, exclusively the olefin‐inserted [R CN Co III L]. [R CN Co III L] is reducible in the intermediate potential range, [RCo III L] at the negative end of the single‐scan voltammogram. Hence, from the appearance of the reduction waves due to [R CN Co III L] and [RCo III L], the mechanism of oxidative addition of RX to [Co I L] 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 [Co I L]'s, except neopentyliodide with Cb1 I and ‘Cby’ I . All secondary alkyl iodides involve free radicals with Cb1 I and ‘Cby’ I , but a pure two‐electron mechanism or a mixed one‐electron two‐electron mechanism with [Co I (tpp)] and [Co I (oep)]. The mechanistic switch from a two‐electron to a one‐electron mechanism for increasingly sterically demanding RX's occurs earlier with the supernucleophilic Cb1 I and ‘Cby’ I than with [Co I (tapp)] and [Co I (oep)].