Solution photodecarbonylation

Tetracarbonylfoctahydrotriborato(l-)] manganese is an air-sensitive liquid that decomposes slowly (about 5% in 4 days) at room temperature as a neat liquid in a vacuum. It is soluble in benzene, toluene, dichloromethane, diethyl ether, and tetrahydrofuran (THE), but decomposes upon heating at reflux in these solvents (especially in THF). Gas-phase thermal decarbonylation or solution photodecarbonylation of (CO)4Mn(B3H8) yields the novel and reactive compound (CO)3Mn(B3H8), in which the octahydrotriborate(l-) ligand is tridentate.3... 

Solution Photodecarbonylation of Ketones with Other (XrSubstituents... 

The possibility of predicting solid state reactivity from calculated thermochemical data was first addressed with ketodiesters 65a-e, which were substituted with methyl groups to vary the extent of the RSE in the radicals 65-BRl - 65-BR3 involved along the photodecarbonylation pathway (Scheme 7.19). " All ketones reacted in solution to give complex product mixtures from radical combination (66a-e) and disproportionation processes. Calculations revealed RSEs of 8.9 kcal/mol, 15.1 kcal/mol, and 19.8 kcal/mol for radicals 65-BRl (primary enol radical), 65-BR2 (secondary enol radical), and 65-BR3 (tertiary enol radical), respectively. In the... 

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

Ketone 8 and the naphthalene homologue can be generated in solution " from singlet oxygen addition to various alkylidenecycloproparenes (Section VLB.2). However, attempts to construct 87 from 5,7-dibromodibenzo[ ,c]cycloheptadienone were unsuccessful " and the only known condensed analogue is 88 that has been matrix isolated prior to photodecarbonylation". ... 

Mehta and Ravikrishna have described the ready formation of monosub-stituted semibullvalenes 11 by photodecarbonylation of the polycyclic ketones 12 in methanol solution, and by a similar procedure, meta and para [2.2] cyclophanes are obtained by bisdecarbonylation of 13 and 14 respectively (Isaji et al). 

Mehta and Ravikrishna have demonstrated that the monosubstituted semibullvalenes (155) can be readily prepared by photodecarbonylation of the polycyclic ketones (156). The reaction is best carried out in methanol solution. 

Higher conversions are precluded by the secondary photodecarbonylation of the cyclo-propanone to 2,3-dimethylbut-2-ene. Some dimethylketene was also found. Cooling the photolysate at — 78 °C allowed the removal of most of the unreacted cyclobutane-1,3-dione by recrystallization. Distillation under vacuum (10°C/20 Torr) of the supernatant liquid afforded a 10-15% solution of 2,2,3,3-tetramethylcyclopropane. 

Due to the cage effect in micelles, unsymmetrically substituted dibenzyl ketones such as 13 yield predominantly the unsymmetrical diphenyl-ethanes on photodecarbonylation, whereas in homogeneous solution all three possible products are formed in the statistical ratio 1 2 1 (Turro and Kraeutler, 1978). 

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

The expected correlation between photodecarbonylation efficiencies and the stability of the radicals formed can be illustrated with symmetric ketones giving rise to primary, secondary, and tertiary radicals in solution (Scheme 4). Results from irradiation of diethylketone (3-pentanone) in the gas and in solution are similar to those obtained with acetone. -"" In contrast, with two methyl groups on each a-carbon, the excited state of di-iso-propylketone 7 a-cleaves and loses CO to give decarbonylated products in 51% chemical yield. Finally, as expected from the presence of three radical-stabilizing methyl groups on each a-carbon, di-f-butylketone 8 was reported to decarbonylate with a quantum yield of = 0.7 from both singlet (3 = 0.4) states and a yield of CO formation of 90%. ... 

Other radical stabilizing substituents may also facilitate the photodecarbonylation reaction in solution, but very few have been systematically studied. Recent studies with the dodecanones 33a and 33b illustrate the enabling effect of a-hydroxyl groups on the decarbonylation reaction (Scheme 11). Although... 

Twelve years after the pioneering work of Quinkert et al., while the cage effect of several diphenyl propanones in several reaction media were being studied, the photodecarbonylation of crystaUine 1,3,4-triphenyl-2-pentanone 64 was reported by Turro and co-workers (Scheme 17). It was shown that irradiation of 64 in solution gave a complex mixture of products arising from random free-radical-radical reactions of 64A and 64B. In contrast, reactions carried out in crystals to very low conversion values gave... 

Robbins, W. K. and Eastman, R. H., Photodecarbonylation in Solution. I. Quantum Yields and Quenching Results with Dibenzyl Ketones, /. Am. Chem. Soc, 92, 6076, 1970. 

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