Ketones photodecarbonylation

Keywords dicumyl ketone, photodecarbonylation, crystal-to-crystal reaction... 

While the examples in Scheme 7.16 hinted at the practicality of the solid state photodecarbonylation of ketones, the factors controlling this reaction remained unknown until very recently. As a starting point to understand and predict the photochemical behavior of ketones in terms of their molecular structures, we recall that most of the thermal (kinetic) energy of crystals is in the form of lattice vibrations. 

As it pertains to the solid state photodecarbonylation reaction, the model assumes that most aliphatic ketones have similar excitation energies, that reactions are more likely along the longer-lived triplet excited state, and that each reaction step must be thermoneutral or exothermic to be viable in the solid state. " Using acetone and its decarbonylation intermediates as a reference reaction (dashed lines in Fig. 7.24), we can analyze the energetic requirements to predict the effects of substituents on the stability of the radical intermediates. The a-cleavage reaction of triplet acetone generates an acetyl-methyl radical pair in a process that is 3.5 kcal/mol endothermic and the further loss of CO from acetyl radical is endothermic by 11.0... 

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... 

The photodecarbonylation of a series of dibenzyl ketones was studied by Robbins and Eastman/63 The results of this study are presented in Table 4.5. The data in Table 4.5 indicate that the presence of a p-methyl or a p-methoxy group has little effect on the quantum yield for this reaction. p-Cyano groups, on the other hand, essentially totally eliminated the decarbonylation. Since the reaction could also be quenched (inefficiently) by benzonitrile or biphenyl, it was concluded that the decarbonylation occurs from a short-lived triplet state. The effect of the p-cyano groups then could result from internal triplet quenching. 

Photodecarbonylation of p-tolyl benzyl ketone Photolysis of dibenzyl ketones with CuCl2 scavenger... 

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... 

The utility of the solid-state photodecarbonylation of crystalline ketones was recently demonstrated in the total syntheses of two natural products, where the key step is the solid-state reaction. The first example involves the synthesis of the sesquiterpene ( )-herbertenolide [80] by the solid-state decarbonylation of cyclohexanone 189, followed by cyclization of the photoproduct 190 (Scheme 2.46). With precursor 189 obtained by simple methods and a solid-state reaction carried out to 76% conversion, herbertenolide was obtained in good overall yield in a record number of steps from commercial starting materials. With a similar synthetic strategy, samples of the natural product (i)-a-cuparenone were obtained in about 60% overall yield from 191 by a very succinct procedure that included four simple steps and a solid-state reaction at — 20 °C [81]. 

The enantioselective syntheses of (R)-oc-cuparenone and (S)-a-cuparenone, both of which are natural products from different sources, were also completed using the solid-state photodecarbonylation of diasteromerically pure difluorodioxaborinane ketones 192 and 194 (Scheme 2.47). The latter were prepared in two steps from 191, and irradiated as nanocrystalline suspensions to optimize the chemical yields of the transformation. The photoreaction of the optically pure ketones was 100% stereoselective with an isolated yield of 80%. The two natural products were obtained by simple acid removal of the chiral auxiliary. 

As is true for all chemical processes, the photodecarbonylation of ketones is limited by structural effects known to result in fast competing pathways, and by structural attributes that give rise to quenching interactions. Whilst one might have predicted, based on the RS E values of the a-substituents, that the three ketones in Scheme 2.48 should have reacted in the solid state by loss of CO and radical combination, the... 

Mortko, C.J. and Garcia-Garibay, M.A. (2006) Engineering stereospecific reactions in crystals synthesis of compounds with adjacent stereogenic quaternary centers by photodecarbonylation of crystalline ketones, in Topics in Stereochemistry, Vol. 25 (eds S.E. Denmark and J.S. Siegel), John Wiley Sons, Hoboken, NJ, pp. 205—253. 

Pioneering work on the photochemical diastereocontrol in zeolite supercages was reported by Turro and coworkers in 1991 [48]. They investigated the diastereoselective photodecarbonylation of 2,4-diphenyl-3-pentanone (DPP) adsorbed in various cation-exchanged X and Y zeolites to find that the diastereo-selectivity of d9l- over mestf-2,3-diphenylbutane increases in the order LiX NaX < LiY NaY < KY. In 1996, Ramamurthy and coworkers reported the first example of photochemical asymmetric induction in chirally modified zeolites [49], where they employed the Norrish/Yang type II reaction of cis-4-tert-butyl-cyclohexyl aryl ketones to the corresponding cyclobutanols. Since then, a variety of asymmetric photoreactions in zeolite supercages have been reported as reviewed below. 

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. 

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%. ... 

Solution Photodecarbonylation of Ketones with Other (XrSubstituents... 

Remarkably, the photodecarbonylation of ketones 39, 40, and 41 proceeds in >90% chemical yield whether occurring via free radical for 39 or biradical intermediates for 40 and 41. Tetramethyldivinyl... 

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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