A Comparative Study of Thermal- and Radiation-Induced Single Electron Transfer in Reactions of 1,4-Diaza-1,3-butadienes with Dialkylzinc Compounds

Abstract

The reaction of R2Zn with t-BuDAB (t-BUN-CH-CH=N-t-Bu) results in the quantitative formation of the red 1:l coordination charge transfer complexes RzZn-t-BuDAB (R = Me (la), CH2SiMe3 (5a)). Upon heating (T > 35 C), both complexes undergo an inner-sphere single electron transfer (SET) leading to the radical pair [(RZn-t-BuDAB)*(R.)], that collapses, depending on R., to several alkylation products, i.e. the N-alkylated product MeZn(tBuN-CH=CH-N(Me)-t-Bu) (lb) and the exocyclic enamines RZn(t-BuN=CH-C(=CHR')Nt-Bu) (R = Me (If) , CH2SiMe3 (5f/5g); R' = H, SiMes). Alternatively, the alkyl radical R. escapes from the organozinc radical (RZn-t-BuDAB)., that subsequently dimerizes to the dinuclear species (RZn-t-BuDAB)z (R = Me (le), CH2SiMes (58)). The charge transfer complex Me2Zn-t-BuDAB was irradiated in order to induce a SET reaction within the complex. The irradiation reaction resulted in the formation of the symmetric dinuclear species le (30%) and the unsymmetric dinuclear species [MeZn{t-Bu(H)NCH(Me)CH(N-t-Bu1-j {-CH(N-t-Bu)CMe=Nt-BujZnMel (lh) (70% ). Product l h was independently prepared from MeZn(t-BUN-CH(Me)-CH-N-t-Bu) (IC). The latter complex is in a tautomeric equilibrium with its enamine MeZn(t-BUN-CH(Me)=CH-N(H)-t-Bu), that reacts via a condensation reaction with a parent molecule IC to give lh.