Ketones decarbonylation

Ketone decarbonylation reactions have been useful for the preparation of highly reactive molecules. A few examples include the preparation of ynols, yneamines, and strained alkynes by in situ photolysis of cyclopropenones 51 to 53 (Scheme 14). These high-energy compounds are short-hved transients that must be either detected by fast kinetic methods or kineticaUy stabilized in cryogenic matrices to facihtate their detection. Phenanthrocyclopropenone 53 was generated in situ by photolysis of a his(diazo)ketone or by photodecarboxylation of dicarboxyphenathrene anhydride. ... 

Various organotin reagents react with acyl and aroyl halides under mild conditions without decarbonylation to give carbonyl compounds[390,39l]. Alkyl- or alkenyltin reagents react with acyl and aroyl chlorides to give ketones[548.733,734]. One example is the preparation of the a,/3-dnsaturated 7-keto esters 860 and 861, carried out under a CO atmosphere[735]. The reaction has been applied intramolecularly to the synthesis of the macrocyclic keto... 

The cyano ketone 89 is converted into the nitrile 90 by heating at 140 C with Pd(Ph3P)4[73,74]. The a-lcetophosphonate 91 is decarbonylated with PdMe2(PMePh2)2 complex to give the phosphonate 92[75]. 

Ketones such as 2,2,5,5-tetraphenylcyclopentanone and 2,2,6,6-tetraphenylcyclohex-anone decarbonylate rapidly because of the stabilization afforded by the phenyl groups. The products result from recombination, disproportionation, or fragmentation of the diradical intermediate. ... 

Ketones in which the double bond is located in the p,y position are likely candidates for a-cleavage because of the stability of the allyl radical that is formed. This is an important process on direct irradiation. Products then arise by recombination of the radicals or by recombination after 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 ... 

At elevated temperatures, methylene carbons cleave from aromatic rings to form radicals (Fig. 7.44). Further fragmentation decomposes xylenol to cresols and methane (Fig. 7.44a). Alternatively, auto-oxidation occurs (Fig. 1.44b ). Aldehydes and ketones are intermediates before decarboxylation or decarbonylation takes place to generate cresols and carbon dioxide. These oxidative reactions are possible even in inert atmospheres due to the presence of hydroxyl radicals and water.5... 

Scheme 9.13 Selective decarbonylation of a cyclobutanone to form the corresponding ketone with PR ligands or the aldehyde with NHC ligands...

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

Figure 7.24. Solid-state photochemical decarbonylation model for ketones. The dashed path corresponds to the experimentally determined energies of acetone (in kcal/mol). The effects of substituents with radical stabilizing energies (RSEs) are illustrated by the solid line in the reaction coordinate. See color insert.

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 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. 

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. 

We have seen that the observation of mixed radical combination products from mixtures of 2CHCOCH< 2 and CH2COCH2 upon photolysis indicates the decarbonylation to be stepwise rather than concerted, at least for these molecules. Further evidence in support of a stepwise process was reported by Robbins and Eastman in a study of p-methoxybenzyl ketone/64 Three products are isolated from this reaction ... 

Acyclic ketones containing radical-stabilizing groups similarly undergo decarbonylation. In this case photolysis of mixtures of ketones results in products arising from mixed radical combinations/67 This result is direct evidence for the free radical nature of these decarbonylations ... 

The decarbonylation of dibenzyl ketone has been shown to result from the carbonyl triplet state by its ability to be quenched by 1,3-cyclohexadiene or l,3-pentadiene.<66) Using 1,3-cyclohexadiene as quencher, photodimers of the cyclohexadiene were obtained. Since these are formed only by triplet sensitization,<66) the quenching of ketone triplet states, rather than their excited singlets, was assured. Further evidence for a triplet reaction follows from the fact that decarbonylation could be sensitized by acetone under conditions where the sensitizer absorbed 93% of the light. 

As can be seen from Figure 3, the ratio of the isopropyl ether to isobutyrate is about 1 1. It is clear that after a-cleavage of the ketone the two radicals primarily formed are captured directly by nitroxide. This takes place without decarbonylation of the acyl radical (reaction (7)) ... 

Phenolic compounds were confirmed to be very stable against thermal treatment. Diphenyl methanol and benzophenone were stable against decomposition but hydrogenated to form diphenyl -methane quantitatively. Phenyl benzyl ketone was found to be partially hydrogenated or decarbonylated to form diphenyl alkanes. 

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