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. Can. J. Chem. 69, 1575 (1991).The lowest singlet excited state of dibenzoylmethanatoboron difluoride DBMBF2, a model compound of the BF2 complexes of 1.3-diketones, reacted with various simple olefins to give regiospecific and stereospecific photocycloadducts of 1,5-diketones similar to those from the de Mayo type reaction. DBMBFz in acetonitrile exhibited two discrete fluorescences at 398 and 416 nm for the monomer and at 522 nm for the excimer; they were both quenched, but in different proportions, by a simple olefin. An "oxygen test" showed that the excimer of DBMBF2 is formed irreversibly in acetonitrile. The quantum yields of the photocycloaddition were shown to be proportional not only to olefin concentrations but also to DBMBF2 concentrations. Kinetic analysis has established that the total quantum yield is the sum of those arising from the interactions of the singlet excited DBMBF2 and its excimer, respectively, with an olefin, i.e., the sum of the quantum yields of exciplex and triplex pathways. The contributions from the two pathways are determined by the type of olefins and the range of DBMBFz concentrations. For endocyclic olefins, the triplex pathway is more important and the corresponding photocycloaddition becomes very efficient as soon as the excimer starts to form in [DBMBF2] > 0.001 M. For the monosubstituted olefins, on the contrary, the exciplex pathway is always more important than the triplex pathway; they react primarily from the singlet excited state of DBMBF2.Key words: singlet state photocycloaddition, irreversible excimer formation, excimer cycloaddition, triplex and exciplex reactions. IntroductionIn a preliminary communication (1) we summarized various types of photoreactions displayed by dibenzoylmethanatoboron difluoride (DBMBF2), a representative of the BF2 complexes of 1,3-diketones; they involve photocycloaddition with olefins, sensitization of electron-rich olefins by electron transfer to form cation radicals, exciplex~xcimer formations, and subsequent reactions therefrom. It was proposed that these pho-

The lowest singlet excited state of dibenzoylmethanatoboron difluoride interacts with alkenes by an electron-transfer mechanism to give cycloaddition or alkene dimerization products.The reaction of P-diketones with BF3 affords boron difluoride complexes,1-3 such as acetylacetonatoboron difluoride ( aaBF2) , benzoylacetonatoboron difluoride (baBF2), and dibenzoylmethanatoboron difluoride ( dmbBF2), that possess reduction potentials2.4 lower than that of the parent P-diketones by ca. 0.5 V. Similar BF2 complexes have been used as electron acceptors in the design of photoconducting materials.4.5 We report that these BF2 complexes in their singlet excited state are a new class of electron acceptors in reactions with alkenes.6 In this paper dbmBF2 is used as a model compound to describe their photocycloadditions and photosensitization.In contrast to the lack of fluorescence emission of dbm

Y. L. CHOW and XIANEN CHENG. Can. J. Chem. 69, 133 1 (1991). Dibenzoylmethanatoboron difluoride (DBMBF?) and allied BF2 complexes interact from their singlet excited state with trans-anethole (t-A), quadricyclene (QC), and norbornadiene (NBD) by electron transfer to generate the corresponding cation radicals, which undergo the reported reactions. By sensitization, t-A undergoes dimerization to form the anti head-to-head and syn head-to-head dimers with retention of stereochemistry. The formation is reversible under sensitization conditions, leading to accumulation of the more stable anti isomer. However, irradiation of the absorption band of the DBMBF, -t-A ground state complex did not lead to dimerization of t-A. By DBMBF2 sensitization, QC is cleanly converted to NBD while NBD is not affected. The calculation shows QC+' possesses higher energy than NBD+' by 7.5 kcal/mol, hence an irreversible rearrangement. Other sensitizers (e.g., cyanoaromatics and tetrachlorobenzoquinone) also promote these cation radical reactions but not as cleanly as DBMBF?.

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