Kinetics of the reduction of dialkyl peroxides. New insights into the dynamics of dissociative electron transfer

The concerted dissociative reduction of di-tert-butyl peroxide (DTBP), dicu myl peroxide (DCP), and di-n-butyl peroxide (DNBP) is evaluated by both het erogeneous and homogeneous electron transfer using electrochemical methods. Electrochemical and thermochemical determination of the O-O bond energies and the standard potentials of the alkoxyl radicals allow the standard pote ntials for dissociative reduction of the three peroxides in N,N-dimethylfor mamide and acetonitrile to be evaluated. These values allowed the kinetics of homogeneous ET reduction of DTBP and DCP by a variety of radical anion d onors to be evaluated as a function of overall driving force. Comparison of the heterogeneous ET kinetics of DTBP and DNBP as a function of driving fo rce for ET allowed the distance dependence on the reduction kinetics of the former to be estimated. Results indicate that the kinetics of ET to DTBP i s some 0.8 order of magnitude slower in reactivity than DNBP because of a s teric effect imposed by the bulky tert-butyl groups. Experimental activatio n parameters were measured for the homogeneous reduction of DTBP with five mediators, covering a range of 0.4 eV in driving force over the temperature range -30 to 50 OC in DMF. The temperature dependence of the kinetics lead s to unusually low preexponential factors for this series. The low preexpon ential factor is interpreted in terms of a nonadiabatic effect resulting fr om weak electronic coupling between the reactant and product surfaces. Fina lly, the data are discussed in the context of recent advances of dissociati ve electron transfer reported by Saveant and by German and Kuznestov. In to tal the results suggest that these peroxides undergo a nonadiabatic dissoci ative electron transfer and represent the first reported class of compounds where this effect is reported.