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

Abstract

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.

Unusually strong emission from an exciplex formed between benzenoid solvents and dibenzoylmethanatoboron difluoride. Formation of a triplex

“…This emission increases and is steadily red-shifted as the concentration of the benzene derivative is increased, in agreement with an exciplex formation. The quenching of the singlet excited state of DBMBF2 by 1,3-cyclohexadiene (E,, = 1.58 VISCE (22)) was reported earlier (23). The transformation of the latter into its radical cation by electron transfer to DBMBF2 has already been reported (23) although no exciplex fluorescence could be ob~erved.~ Quenching of the fluorescence of DBMBF2 by 1,4-cyclohexadiene (E,, = 1.90 VISCE (24)), 1,3-cyclooctadiene (E,, = 1.89 VISCE (25)), and 1-methylcyclohexene (E,, = 1.77 VISCE (26)) is also observed but, again, no exciplex emission could be detected.…”

1,3-Diketonatoboron difluoride sensitized cation radical reactions

“…The instrumentation and conditions were the same as those described the case of l-cyanonaphthalene sensitization (16, (2,3). The authentic sample of the syn-3 dimer was '(AABF'--NBD+) + 3 (~~~F~--N~D f 1, which can effec-prepared by direct photolysis of 1 through a Pyrex filter (9), that of the tively retard the generation of triplet state NBD.…”

“…Bauld and co-workers (5,6) demonstrated that the aminium radical initiated dimerization of trans-anethole (1) in CH2C12 (EgK = 1.11 V) follows formally concerted [ r 2 s + r l s ] cycloadditions with retention of configuration to give the head-to-head trans, anti,trans-cyclobutane 2 and the head-to-head trans,syn, traizscyclobutane 3 in about 1: 1 ratio at -35°C; at room temperature only the anti isomer 2, the more stable of the two, is obtained (eqs. [2] and [3]). The reaction is nicely demonstrated to be a reversible cation radical chain reaction of 1+' arising from thermal electron transfer to tris(p-bromopheny1)aminium hexachloroantimonate, and its activation parameters are determined through kinetic studies (6).…”

“…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.…”

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

“…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).…”