Photodecarbonylation solid state

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... 

K.K., Veerman, M., Mortko, C.J., and Garcia-Garibay, M.A. (2008) Solid state photodecarbonylation of diphenyl-cyclopropenone a quantum chain process made possible by ultrafast energy transfer. Journal of the American Chemical Society, 130, 1140-1141. 

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... 

Photodecarbonylation in the solid state allows to maintain stereochemical control. Thus, this highly stereospecific process can be exploited for the synthesis of enantiomerically pure natural products. As an example, the photodecarbonylation of an aqueous suspension of the diastereomeric difluoro-dioxaborinane complexes of p-keto-(5)-a-methylbenzylamide (1 and 2) was exploited for the synthesis of both enantiomers of a-cuparenone (3 and 4) with 100% stereoselectivity and in 80% yield (Scheme 2.2). ... 

Notably, in some cases photocycloaddition has been observed in the solid state in preference to other processes that are usually very fast. As an example, ketosulfone 36 did not undergo the expected photodecarbonylation reaction (see Section 2.2.1) but a very clean solid-to-solid intramolecular [2 + 2] cyclization to form the tetracyclic compound 37 as the only product in quantitative yield (Scheme 2.21). Small-scale reactions ca. 50 mg) could be completed also with sunlight within 2h and with no apparent changes in the aspect of the crystalline specimens. 

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