Photodecarbonylation reactions

An example of a reaction cavity is illustrated by the van der Waals surface plots derived from the X-ray structures of di-2,6-dihydroxy-2,6-diphenylcyclohexanone 51 and its photodecarbonylation products, the cis- and tra i-2,6-diphenylcyclopen-tane-l,2-diols 52 and 53 (Scheme While the two products are formed in... 

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 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. 

Some phenyl esters give rise to photodecarbonylation as a side reaction [37], This is also the case for certain phenylpropionamides, which implies cleavage of the CO—C bond in the acyl radical, after the breaking of the N—CO bond [99], If CO—C cleavage occurs prior to N—CO cleavage, then an isocyanate is formed (Scheme 38) and PFR does not occur. 

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 photodecarbonylation reaction of thiophenones 118a-c leading to the formation of alkyl-substituted thietes 119a-c in high yields turned out to be a useful method for their synthesis (Table 12) <1997HCA510>. 

Synthetic applications of the reaction are somewhat limited as the highly reactive biradicals and radical pairs tend to undergo reactions that compete with C—C bond formation. As in previous cases, the reaction may have synthetic value for the synthesis of strained structures involving small rings. For example, the preparation of the simplest [2]-ladderane 55 by photodecarbonylation of bicyclo[3.2.0]heptan-3-one 54 gave the bicyclic structure in 5% yield with a ring-opened 1,5-heptadiene being the dominant product (Scheme 2.14) [41]. 

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]. 

Ng, D., Yang, Z., and Garcia-Garibay, M.A. (2002) Engineering reactions in crystals gem-dialkoxy substitution enables the photodecarbonylation of crystalline 2-indanone. Tetrahedron Letters, 43, 7063-7066. 

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. 

In addition to serving as substrates with which to further study the effects of various substituents on the photochemical transformations of cyclobutanones, 3-methylenecyclobutanones are capable of generating the theoretically interesting trimethylenemethane biradical species (32) by way of the photodecarbonylation reaction. Indeed, Dowd and Sachdev (33) observed a triplet ESR spectrum upon photolysis of 3-methylenecyclobutanone [69]... 

The parent azaferrocene is obtained in two steps by the reaction of (6c) with pyrrole to give CpFe-NC4lLi, followed by photodecarbonylation. Preparation of the Cp analog... 

A study of the photochemistry of the diketones (246) has been reported. The double photodecarbonylation of the diketones (247), yielding the aryl compounds (248), has been reported a reaction of this type was first reported in 1968. ° ... 

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. 

Photo-decarbonylation of the cyclohexanone (32a) is efficient with a quantum yield of 0.9. The reaction yields the two products (33) and (34) in a ratio of 1 2. The cyclopentanone (32b) also decarbonylates photochemically but is less efilcient with a quantum yield of 0.5. A laser flash study has been carried out on these systems and has identified the biradicals produced by the Norrish type I process. The lifetime of the biradicals (35a) and (35b) are O.Ojxs and 0.5 xs respectively. In a related study the photodecarbonylation of cIs- and frans-2,6-diphenylcyclo-hexanone has been shown to yield a mixture of cis- and 2-diphenyl-cyclopentane and cIs- and... 

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. 

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

Good yields of the cyclopentenes are obtained from the photodecarbonylation of the cyclopentene aldehydes (100).ei Kinetic analysis of the reaction indicates that decarbonylation arises mainly from the singlet state, although the triplet state does show some reactivity. 

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). ... 

Photodecarbonylation is a well-established reaction and its recent applications involve synthesis of the tetra-t-butyltetrahedrane compound (equation 127) and [4.5]-coronane (equation 128) . Three examples of carbon dioxide elimination are exhibited in equations 129-131 to furnish corresponding alkanes . 

The carbene complexes M(CO)5 =C(OMe)C6H4feCPh (M = Cr, W) undergo photodecarbonylation and alkyne coordination to form complex 56. The corresponding molybdenum complex undergoes the same reaction in the dark. The chromium species rearranges thermally to form an organic chrysene derivative. s... 

The synthesis of 1,4,8,11-tetraphenylpentacene and l,4,8,ll-tetra(2 -thienyl)pentacene has been achieved via photodecarbonylation of the corresponding a-diketone precursors (62) (Strating-Zwanenburg reaction). 

Mondal R, Okhrimenko AN, Shah BK, Neckers DC (2008) Photodecarbonylation of a-diketones a mechanistic study of reactions leading to acenes. J Phys Chem 112 11-15... 

KUZ 09] Kuzmanich G., Gard M.N., Garcia-Garibay M.A., Photonic Amplification by a Singlet-State Quantum Chain Reaction in the Photodecarbonylation of Crystalline Diarylcyclopropenones , Journal of the American Chemical Society, vol. 131, pp. 11606-11614, 2009. 

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