ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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)].
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19920750403
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