Cyclic ketones, decarbonylation

Irradiation of cyclic ketones having perfluoroalkyl groups causes cleavage of a ring to yield acyclic products [I74 (equation 44) Similarly, perfluonnated ketones undergo decarbonylation when irradiated [775]. Gas-phase photolysis of perfluorodiazoketones, in the presence of a trapping agent, yields fluorinated furan as a major product [176] (equation 45)... 

The photolytic cleavage of cyclic ketones 14 leads to formation of a diradical species, that can undergo analogously the various reactions outlined above. The decarbonylation followed by intramolecular recombination yields a ring-contracted cycloalkane 15 ... 

The elimination of carbon monoxide can occur by a concerted process in some cyclic ketones. The elimination of carbon monoxide from bicyclo[2.2.1]heptadien-7-ones is very facile. In fact, generation of bicyclo[2.2.1]heptadien-7-ones is usually accompanied by spontaneous decarbonylation. 

Cyclic ketones undergo decarbonylation on irradiation in the vapour phase and in solution, provided they form stabilised radicals ... 

Five- or six-membered saturated cyclic ketones can also react by another pathway that does not involve decarbonylation. In these reactions, the biradical initially formed by a-cleavage undergoes internal disproportionation without loss of carbon monoxide, resulting in the formation of either an unsaturated aldehyde or a ketene. Methanol is usually added to convert the reactive ketene to a stable carboxylic-acid derivative (Scheme 9.2). 

Pyridinealdehyde yields pyridine and some dipyridyl, a reaction which proceeds without destruction of the heterocyclic ring 19). Ketones decarbon-ylate equally well. Benzophenone and dibenzoyl form diphenyl in high yields 2°). Several interesting eliminations have been found for cyclic and bi-cyclic ketones. In the case of cyclohexanone the decarbonylation accounts for only a small proportion of the reaction products. Benzoquinone decarbon-ylates to cyclopentadienone in good yields 20K Under the reaction conditions this compound dimerizes and decarbonylates again. 

Facile decarbonylation of aldehydes with the Rh complex (Wilkinson complex) is known [43,44], The reaction is explained by the oxidative addition of aldehyde to Rh, followed by decarbonylation and reductive elimination. However, the Rh-catalysed intramolecular reaction of some unsaturated aldehydes proceeds without the decarbonylation, and cyclic ketones are obtained. Treatment of unsaturated aldehyde... 

However, cyclic ketones can themselves be decarbony-lated by [RhCl(PPh3)3] in high-boiling solvents. There appears to be no correlation with ring size since both 3-hexadecylcyclobutanone and cyclododecanone can be decarbonylated. ... 

A pressure increase, brought about by an increase in the concentration of the ketone or by the addition of an inert gas, enhances the formation of the unsaturated aldehyde as compared to that of CO. The value of awehyde increases at the expense of 0co> thus, the ketone consumption yield is independent of pressure. This seems to be generally valid in the photolysis of the cyclic ketones it was confirmed, for instance, for cyclopentanone °, cyclohexanone 2-methyl cyclohexanone and 2,6-dimethyl cyclohexanone . An increase in wavelength also favours the formation of the aldehyde as compared to decarbonylation in the photolysis of cyclobutanone , cyclopentanone and cyclohexanone . At 3130 A, the decrease in temperature has a similar effect on the product distribution in the photolysis of cyclopentanone , cyclohexanone , 2-methyl cyclohexanone , and 2,6-dimethyl cyclohexanone to that caused by the increase in wavelength or pressure. However, at shorter wavelengths, the quantum yields seem to be independent of temperature . ... 

The reactions taking place in the vapour phase also occur in the condensed phase, and their mechanisms are probably similar. However, as may be expected on the basis of the results obtained for the gas phase photolysis, the formation of olefins, cycloparaffins, and CO is of less importance, while that of the saturated aldehydes is more important in the liquid phase or solution, where energy dissipation by collision is more efficient. The decarbonylation products were shown to be only of minor importance in the photolysis of liquid cyclopentanone and cyclohexanone . The unsaturated aldehyde was found to be the main product in the liquid-phase photolysis of cyclopentanone (methyl cyclohexanone . Unsaturated aldehydes were also identified in the photolysis products of other cyclic ketones in the liquid phase as well as in solution . ... 

The research team of D.F. Covey developed a synthetic route to convert 5p-methyl-3-ketosteroids into 7(S)-methyl substituted analogues of neuroactive benz[e]indenes. The synthesis began with 19-nortestosterone, in which the a,p-unsaturated cyclic ketone moiety was degraded to afford a tricyclic aldehyde. This aldehyde was unstable and could not be stored. For this reason it was immediately subjected to the Tsuji-Wllkinson decarbonylation to afford the decarbonylated product in high yield. 

The Norrish type I reaction of acyclic and cyclic ketones in solution typically results in recombination, decarbonylation and disproportionation (hydrogen abstraction) products.903 For example, irradiation of di-tert-butyl ketone (265) in hexane solution provides nearly exclusively decarbonylation products from both the singlet and triplet states (>90% chemical yield), whereas the carbonyl group-containing products are produced only in traces (Scheme 6.111).922 923... 

Analogous decarbonylations have been observed for ketones under ultraviolet irradiation at elevated temperatures. From a preparative point of view the behavior of diphenyl-substituted and a,a-divinyl-substituted ketones is interesting, as they are decarbonylated when their benzene solutions are subjected to ultraviolet irradiation.69 This favorable effect of phenyl sub-stitutents on decarbonylation has been noted also for cyclic ketones for example, under suitable conditions l-phenyl-2-indanone eliminates carbon monoxide and yields 90% of 5,6,ll,12-tetrahydro-5,6-diphenyldibenzo[a,e]-cyclooctene as a mixture of cis- and trans-forms,10 whereas 2-indanone itself is hardly decarbonylated at all in benzene solution. That in other cases decarbonylation is usually more profitable in the gas than in the liquid phase is shown by studies of the photolysis of tetramethyl-l,3-cyclobutanedione which in the gas phase gives 2 equivalents of carbon monoxide and a quantitative yield of 2,3-dimethyl-2-butene.71... 

The Norrish Type I fragmentation of the cyclic ketone (178) has been used in a synthesis of grandisol (179), a major component of the male boll weevil pheromone.125 The cleavage reaction gave the aldehyde (180) which was subsequently decarbonylated to give the desired product. 

Irradiation of 2-[N-(pentafluorophenyI)amino]-3-phenylcyclopropenone promotes decarbonylation to give N-(pentafluorophenyl)phenylethynamine and 2-phenyl-3-[N-(pentafluorophenyl)amino]acrylic acid by a process for which there is no known precedent,and the photoextrusion of carbon monoxide from l,3-bis(ethylenedioxy)indan-2-one has been used as the first step in a new synthesis of 1,2-dioxobenzocyclobutene. This represents an unusual example of the decarbonylation of a five-membered cyclic ketone in the preparation of a highly strained and functionalised cyclobutane derivative. The photolysis of a-naphthaleneacetic acid in aqueous solution proceeds by decarboxylation and oxidation of the aromatic ring, and has been carried out at a variety of different wavelengths. The primary step occurs by pseudo-first order kinetics and the optimum photolysis rate has been observed using Ti02 as photocatalyst. Within the cavity of P-cyclodextrin, naproxene (129) has been photodecarboxylated to... 

C-C cleavage of strained rings and ketones has been used to develop useful catalytic reactions. For example, vinylcyclopropanes and vinylcyclobutanes react with alkynes (Equation 6.66) to generate products from 5+2 and 6+2 addition processes that form seven- and eight-membered ring products by overall transformations that are homologs of the Diels-Alder reaction. " The mechanism of these catalytic reactions continues to be studied, but these reactions most likely occur by coordination of the olefin to rhodium and insertion of the metal into the cyclopropene or cyclobutane. Decarbonylation of dialkyl ketones, including relatively unstrained cyclic ketones, has been reported and most likely occurs by oxidative addition into the acyl-alkyl C-C bond, subsequent de-insertion of CO, and C-C reductive elimination. 

This reaction is referred to as the type-I or a-cleavage reaction of carbonyl compounds. This type of reaction is not so common in solution, although some cyclic ketones do undergo decarbonylation ... 

Cyclic ketones 8-10 also undergo similar a-cleavage, decarbonylation and hydrogen abstraction reactions to give products [6, 8]. 

The high decarbonylation efficiency of di-t-butyl ketone 8 impHes that the acyl and alkyl fragments of the primary pair can separate and remain separated as free radicals long enough for the pivaloyl radical to lose CO with a time constant of -1.2 to 1.5 ps." " In contrast, the decarbonylation efficiency of the cyclic ketone analogs in Scheme 5 is remarkably reduced by the average proximity of the two radical termini, which facilitates faster radical-radical reactions. 

Photo- and thermal decarbonylation of cyclic unsaturated ketones leads to the formation of cyclic 1,3-dienes. Such decarbonylations are commonly observed in 7-ketonorbomenes and related bridged bicyclic systems to give cyclohexadienes (equation 84)132. 

The first reports on the photochemical decarbonylation of ketones in the crystalline state were documented during the early 1960s by Quinkert et al., while exploring the effects of viscosity on the recombination of radical pairs. Examples included acyclic structures such 1,1,3-tiphenylpropanone 152 (Scheme 2.37) [67] and cyclic structures such cis- and trcms-diphenylindanoncs 157 and 158 (Scheme 2.38) [68]. 

Ordinary cyclic mono-ketones were also decarbonylated by the action of a stoichiometric amount of RhCl(PPh3)3 [46, 47]. Treatment of cyclobutanone with (Ph3P)3RhCl results in decarbonylation to afford the corresponding cyclopropane together with the rhodium carbonyl complex 28. Insertion of rhodium... 

The methodology was also extended for the synthesis of cyclic ethers [38]. Reaction of the selenoester 142 with BuaSnH and AIBN furnished a mixture of 1-endo and 6-exo cyclized products 143 and 144. Interestingly the selenoester 145 furnished the 1-endo trig cyclized product 146 and the perhydrobenzofuran 147 obtained via decarbonylation of the initial radical followed by a 5-exo trig cyclization. Once again the presence of two sp carbons in the chain, e.g. benzannulated system 148, completely retarded the 1-endo mode and only 6-exo cyclization was observed leading to the formation of ketones 149. 

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