The dual pathway in photocycloaddition of 1,3-diketonatoboron difluorides: excimer reactions

Abstract

. 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-

Aromatic Difluoroboron β-Diketonate Complexes: Effects of π-Conjugation and Media on Optical Properties

“…1,2,3,4 Difluoroboron β-diketonate complexes (BF 2 bdks) are classic fluorescent molecules with large molar absorptivities and often high quantum yields 5 that have been explored as photochemical reagents, 6,7,8,9,10,11 two-photon materials, 12,13 conjugated polymers, 14,15,16 semi-conductors, 17 photochromic materials, 18 near IR probes, 19 oxygen 20 and mechanical sensors. 21 Extensively investigated targets among these aromatic BF 2 bdks are difluoroboron dibenzoylmethane (BF 2 dbm) and its derivatives, given some of the parent diketones are commercial products, such as dbm 22 and avobenzone.…”

The photocycloaddition of dibenzoylmethanatoboron difluoride (DBMBF2) with conjugated enones and en-esters

“…The interaction with electron-rich conjugated dienes approaches the diffusioncontrolled rate (12) and generates olefin cation radicals that undergo the typical dimerization and rearrangement reported previously (14). On the other hand, those reactions with simple olefins are slower, leading to [2+2] cycloaddition to give 1, and subsequent rearrangement-hydrolysis to 1,5-diketones 2 in high quantum yields as shown in Scheme 1 (13). Mechanistic studies on these photocycloadditions were also described recently (13).…”

“…The photocycloadditioll of DBMBF, with en-esters 3a-3c and enones 3d-3f was carried out under similar conditions to those described for similar reactions with simple olefins (13). Generally, DBMBF, (0.05 M ) and a substrate (0.5 M) in acetonitrile were irradiated in a Rayonet photoreactor equipped with RPR 3500 lamps.…”

“…Using dibenzoylmethanatoboron difluoride(DBMBF,) as a model compound, we have shown that it interacts with various olefins from the lowest singlet excited state through exciplexes possessing different extents of electron transfer (12)(13)(14). The interaction with electron-rich conjugated dienes approaches the diffusioncontrolled rate (12) and generates olefin cation radicals that undergo the typical dimerization and rearrangement reported previously (14).…”

“…On the other hand, those reactions with simple olefins are slower, leading to [2+2] cycloaddition to give 1, and subsequent rearrangement-hydrolysis to 1,5-diketones 2 in high quantum yields as shown in Scheme 1 (13). Mechanistic studies on these photocycloadditions were also described recently (13).…”