Dimerization competition with electron transfer

Both stereoisomers were formed, implying a loss of stereochemical integrity during the formation of the second carbon-carbon bond. When the reaction was conducted on ZnO, surface-related processes affected both the rate and stereochemistry. The effect of various quenchers could be explained as competitive adsorption at active sites, with or without interference with electron transfer. A reaction scheme involving formation of dimer, both in the adsorbed state and in solution, was proposed, the former route being the more important On CdS, the reaction could sometimes be induced in the dark as well because of the presence of acceptor-iike surface states. Neither particle size, surface area, nor crystal structure appeared to significantly influence the dimerization observations parallel to those found in the CdS photoinduced dimerization of N-vinylcarbazole... 

There is as yet insufficient evidence for proper evaluation of electron transfer reaction as the possible key process in the photohydration reactions (and perhaps in some of the dimerization processes). It has also been suggested that there is a distinct charge separation in the (overall) neutral excited pyrimidine molecule, and that the charge is sufficiently localized that reaction of the excited molecule with OH or H+ (or both successively) can become a competitive reaction pathway.116 Such a dipolar reactant species has also been specifically proposed by Wacker et al.60 (Chart 8). This is an electrophilic attack on water similar to that proposed above for uracil photohydration. 

As shown in Scheme 16 the triplet-excited state 3C o is quenched by electron transfer from [(BNA)2] generating the radical ion pair [(BNA)2+ and Qo] in competition with the decay to the ground-state. However, back-electron transfer is reduced by the fast cleavage of the C-C bond in the dimer [(BNA)2+], Finally, a second electron transfer from BNA" occurs leading to two molecules of C60. The... 

Br)]2, exclusively. Low concentrations of 1,2-bromochloroethane, however, yield the mixed halide metal dimers Pt2(pop)4(Br)(Cl) " and Ir2(p-pz)2(C0D)2(Br)(Cl). This result is predicted by the proposed mechanism (Figure 5). Photolysis results in formation of Pt2(pop)4 (Br)4 or Xr2(p-pz)2(C0D)2(Br) as intermeditaes. The intermediate can react with another bromochloroethane molecule, as it does when the latter species is in high concentration, to yield the dibromide dimer or it can react with the chloroethane radical to yield the mixed halide metal species. The latter pathway becomes competitive at low halocarbon concentrations. In general, the oxidative addition of halocarbons is typical of the photochemistry arising from electron transfer from d -d metal dimers with the final product being the stable d -d metal-metal bonded dimers (24-25). 

The photochemical Diels-Alder reactions of anthracene with fumarodinitrile and 1,4-benzoquinone have been studied in chloroform solution. Not surprisingly, the addition occurs in competition with dimerization of the arene and proceeds by way of electron transfer from anthracene to the dienophiles. The radical ion pair has been detected by transient absorption spectroscopy, and the resulting diradical precursor of adduct formation from the quinone was observed by ESR at 77 K. 2,7-Dibromotropone is reported to undergo (871+471) photoaddition to 9,10-dicyanoanthracene in benzene-methanol (9 1), giving (25) as the primary adduct which is then proposed to react with methanol and water (solvent contaminant) to yield the final product (26). In contrast, 2-bromotropone and the anthracene in CH2CI2 solution afford the substitution products (27) (62%) and (28) (25%). 

Taking into account these considerations and the appearance of the second wave at a potential where the dimer is not reduced (see following text), in addition to the high dimeri2ation rate (reaction 12 and Sch. 1), and cychc voltammetric patterns, Elving and coworkers concluded that the second electron transfer has to be faster than the dimerization and thus, NAD+ is directly reduced to the dihydropyridine at the second wave in an overall 2e process. The rationale for involvement of a proton in the overall NAD+-reduction process derives from the formation of enzymatically active NADH. Two different sequences were thus postulated by these authors (1) e , e , H+ or (2) e , H+, e . If protonation of the free radical formed on addition of the first electron is very rapid, as compared with its dimerization rate constant, either sequence could be competitive with the dimerization reaction in producing the dihydropyridine derivative otherwise, the first sequence would be more likely [75]. This uncertainty was clearly solved by the fact that the neutral radical NAD is not further reduced in aprotic media and thus, protonation... 

The reduction of benzaldehyde (Li) was investigated in [C4mim][N(Tf)2] and [C4mpyr][N(Tf)2] [45, 46]. Unlike those discussed previously, the voltammograms exhibit two reversible one-electron reduction processes in these two ILs. The first process is assigned to the primary reduction of Li to the radical anion, with an of — 1.75 V vs. Fc (Eq. 15.50). This radical anion is involved in two competitive pathways (1) a dimerization reaction, leading to the corresponding pinacol (Eq. 15.51) and (2) a second electron transfer of —2.33 V vs. Fc ) that reduces the radical anion to the dianionic species, which can lead to the formation of alcohol or alcoholate product (Eq. 15.50) [45,46]. The apparent second-order rate constant for the dimerization k was reported to be 1.0 x lO" and 1.4 x 10 L mol s in... 

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