Photocycloadditions and photosensitizations promoted by electron transfer: β-diketonatoboron difluorides as electron acceptors

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The boron difluoride complexes of 2-acetylcyclohexanone, 2-acetylcyclopentanone, and acetylacetone (abbreviated as ACHBF2, ACPBF2, and AABF2) were irradiated in the presence of benzene to give the 1:1 adducts as the primary photoadducts; for certain BF2 complexes, toluene, chlorobenzene, benzonitrile, and methyl benzoate were also used as substrates. The 1,5-diketone photoadducts were assumed to form by a [2+2] photocycloaddition followed by cyclobutane opening and hydrolysis to give 1,2 adducts. They undergo a variety of secondary thermal reactions, probably acid catalyzed, to give enol ethers, enol acetates, acetophenones, and ketonylacetophenones. The efficiency of these secondary reactions determines the final products. Photoaddition with a monosubstituted benzene preferentially occurs at the 3,4 bond without regioselectivity. Under oxygen, ACHBF2 photolytically reacts with benzene to give a secondary oxidation product of a 10-membered cyclic alkylphe-none, which is proven by X-ray crystallographic analysis to have the benzene ring and carbonyl group in orthogonal orientation. It is shown that the singlet excited state ACHBF2 initiates the photoaddition, probably through the formation of the benzene exciplex, which could be detected by its emission. While the Stern–Volmer rates are small, the quantum yield of photoaddition products is as high as 0.12–0.19 under limiting conditions. Key words: [2+2] photocycloaddition, non-planar alkanophenone, macrocyclic alkanophenone, boron difluoride complexes, photoaddition to benzenes.

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confidence: 99%

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confidence: 99%

The photoaddition of 1,3-diketonatoboron difluorides with benzene derivatives

Chow¹,

Ouyang²

1991

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“…The photocycloaddition of DBMBF2 to 1,3-pentadiene was an exception since similar irradiation processes in the presence of other electron-rich dienes (cyclohexadiene and 2,4-dimethyl-l,3-pentadiene) caused sensitization without cycloaddition (1). In this reaction, four 1:l photoaddition products were obtained, as shown by GC-MS; two of them were in trace amounts (<3%, by GC) and probably regioisomeric to the other two.…”

Section: Photocycloadditionsmentioning

confidence: 82%

“…In 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- [Traduit par la redaction] is a very efficient methodology to construct carbon skeletons, it suffers from low quantum efficiencies.…”

Section: Introductionmentioning

confidence: 99%

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

Chow¹,

Xiang²

1991

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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-

“…W e have reported that dibenzoylmethanatoboron difluoride (DBMBF,) reacts from its singlet excited state with simple olefins to form cycloaddition products (1,2), and that 2-acetylcyclohexanonatoboron difluoride (ACHBF,) and acetylacetonatoboron difluoride (AABF,) similarly undergo photocycloaddition with a number of benzene derivatives (3). These primary photocycloadducts generally undergo hydrolysis at a certain stage to eliminate the BF, moiety and rearrange by acid catalysis to give final products under the photolysis conditions.…”

Section: Introduciionmentioning

confidence: 99%

1,3-Diketonatoboron difluoride sensitized cation radical reactions

Chow¹,

Xiang²

1991

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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?.