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The mechanism of the interaction of singlet excited dibenzoyl(methanato)boron difluoride (*DBMBF2 = *A) with olefins and substituted benzenes (SB) was investigated by the determination of effective quenching distances (R eff) from fluorescence monitors in acetonitrile as analyzed by the Smoluchowski−Collins−Kimball equation and by solvent effects on the dichotomy of the formation of cycloadducts and [4 + 2] dimers from 1,3-cyclohexadiene (CHD) and 1,3-cyclooctadiene (1,3-COD). The correlation of R eff with directly demonstrates that the reactants have to get closer to affect electron transfer from these substrates as the driving force become more positive and coalesce on exciplexes. Both photophysical and photochemical probes converge on the partition of encounter pairs to the SSRIP (the k et pathway which leads to radical ion reactions and nonfluorescent) and exciplexes (the k ex pathway which leads to cycloadducts and fluorescent species); the former pathway was promoted by low redox energies of donor−acceptor pairs and by high solvent polarity. The reaction with 1,3-cylcohexadiene gave high quantum yields of the [4 + 2] dimers in acetonitrile and of the cycloadduct in nonpolar solvents; the latter quantum yields were correlated with the solvent polarity parameter E T(30) in a smooth distribution regardless of pure non-SB or SB solvents or mixture solvents (including benzene), in spite of the fact that the reactive intermediates in the presence of SB are *DBMBF2−SB exciplexes. It is proposed that in the exciplex the locally excited state provides the driving force in the cycloaddition and the CT state regulates its regiospecificity. The quantum yield of the dimers from CHD was greatly enhanced, reaching their maximum at 12 mol % of p-xylene in acetonitrile; the enhancement was attenuated with the addition of toluene and benzene in that order. This shows that the p-xylene exciplex underwent extraordinary facile substitution with CHD by electron transfer in acetonitrile probably promoted by superexchange interactions. The *DBMBF2 excimer reacted with CHD and 1,3-COD to give higher yields of cycloadducts but not the dimers, which must mean that the excimer substitution with CHD proceeds to give the new exciplex but not the radical ion pair. The latter failure as well as that of the benzene exciplex was suggested to arise from the insignificant CT content of the excimer and exciplex. The frontier MO scheme was utilized to rationalize the enhanced reactivity of the exciplexes and excimer on the bases of increased electronic and orbital interactions, respectively.

YUAN L. CHOW, SHI-SEN WANG, and XIAN-EN CHENG. Can. J. Chem. 71, 846 (1993). Dibenzoylmethanatoboron difluoride (DBMBF,), the BF, complex of dibenzoylmethane, reacted from its singlet excited state with a,P-unsaturated ketones and esters to give 1,5-diketones by a [2+2] cycloaddition and ring-opening sequence in an analogous pathway to that observed in the photocycloaddition to olefins and dienes. The present photoreaction is unexpected since conjugated enones and en-esters are poor electron donors to comply with the previously proposed charge transfer requirement in DBMBF, photoreactions. The photocycloaddition to these substrates was highly regioselective and stereoselective, giving the head-to-head orientation, which could be enhanced by a-methyl substitution; P-methyl substitution increased the alternate head-to-tail orientation. The photolysis of a mixture of DBMBF, and a cyclic enone also caused the latter to dimerize to give head-to-head and head-to-tail dimers in significant yields. These photodimerizations were apparently caused by DBMBF, sensitization. The mechanism of the sensitization is discussed. [Traduit par la rCdaction1

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