Decarbonylation of acetone

Figure 12. Comparison of experimental kinetic energy release distribution to phase-space calculations for decarbonylation of acetone by Co+. Data from reference 38.

Fig. 12.23. Energetics for a-cleavage and decarbonylation of acetone showing die effect of radical stabilization. Reproduced from J. Org. Chem., 67, 3749 (2002), by permission of the American Chemical Society.

TABLE 48.6 Calculated Bond Dissociation Energies (BDE in kcal/mol) for the First and Second Homolytic Dissociations in the Decarbonylation of Acetone and Ketodiesters 92a-f... 

FIGURE 48.9 Calculated heats of reaction (B3LYP/6-31G ) correlating the experimental results of the photochemical decarbonylation of acetone and acetone dicarboxylates 92a to 92f. Compound 92c has not been studied experimentally. 

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.

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. 

Upon acylation of some benzyl carbonyl compounds (25, R = H, Me 51, R = OH) dibenzo[a,tropylium salts 65 have been isolated in low yields (5-15 %) along with the major products, 2-benzopyrylium salts. Veratryl acetone 25 (R = Me) as well as homoveratric aldehyde 25 (R = H) (or carboxonium ions 31 which are formed from them) may undergo an oxidative a-cleavage, resulting in the benzyl cation 64. The formation of the same cation from homoveratric acid 51 is the result of decarbonylation of the acylium ion 63. Further interaction of the benzyl cation 64 with the substrate, followed by cyclization and oxidation, results in the polycyclic tropylium salts 65 (82ZOR589). 

Unlike acetone, diethyl ketone cleaves well even in cumene 22>. 2-Pentanone triplet undergoes considerable a-cleavage in competition with rapid intramolecular hydrogen abstraction, as judged by the buildup of an efficient triplet quencher 23>. Biacetyl is the only likely candidate for that quencher and is the major product of a-cleavage of methyl ketones at temperatures low enough that decarbonylation of the acetyl radical is slow. Isopropyl, tert-butyl, and benzyl ketones all cleave quite efficiently and various reports have appeared on the CIDNP spectra of products derived from the radicals 24>. 

Soderquist has reported a slightly more effective method not subject to the losses due to decarbonylation of intermediates. Acylation of a-methoxyvinyltin (100) under palladium(0) catalysis afforded a good yield of the a-methoxyenone (101). Hydrolysis in acetone/aqueous acid releases the diketo functionality (equation 80). Only the unsubstituted vinyl system has been employed thus far. 

There is a good correlation between the bond dissociation energies (Dc co) and the corresponding rates of a-cleavage and subsequent decarbonylation, so that the quantum yield of the reaction is directly related to the stability of the radicals formed. For example, the excitation energies of acetone ( s 373 kJ mol Ej 332 kJ mol J) and acetophenone (Es 330 kJ mol 1, If 310 kJ mol 1) are usually sufficient for exothermic release of stabilized benzyl or tert-butyl radicals (Figure 6.6, Table 6.9), whereas formation of methyl or phenyl radicals is inefficient. 

The present discussion is by no means exhaustive. It is designed to provide a summary of the most significant and reliable kinetic data, at least those that appear so to the author. There is a variety of methods for producing alkyl radicals, and, naturally, there will be certain restrictions on experimental conditions depending on the method chosen. Some of the common methods for generation of methyl radicals, for example, include photolysis of acetone, pyrolysis of di-ferf-butyl peroxide, photolysis of biacetyl, photolysis of azomethane and decarbonylation of acetaldehyde. In the majority of cases discussed here, the reactions were followed by product determinations, employing gas chromatography. 

The acetone-sensitized decarbonylation of -(lS),2(S) (125a) has been studied. The principal reaction is the formation of the 2(S),3(R)-cyclopropane (126a). Other products (127a), (128a) and (129a) are also formed in low yield. The reaction arises from the triplet state and this was confirmed by using Michler s ketone as the sensitizer and by quenching experiments. A similar selectivity is... 

The photochemical decarbonylation of ketones can be traced back to 1910 when acetone was photolysed in the gas phase to yield ethane and carbon monoxide. A radical process involving a-cleavage (Norrish type I reaction) and decarbonylation as two separate steps was proposed a few years later by Norrish and Appleyard (Scheme 1). Each of the two cleavage reactions has been the subject of numerous theoretical and mechanistic studies that have been covered in several reviews. 

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 results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

The success of the phase space theory in fitting kinetic energy release distributions for exothermic reactions which involve no barrier for the reverse reaction have led to the use of this analysis as a tool for deriving invaluable thermochemical data from endothermic reactions. This is an important addition to the studies of endothermic reactions described above. As an example of these studies, consider the decarbonylation reaction 11 of Co+ with acetone which leads to the formation of the... 

---