Free radicals decarbonylation

Extensive rearrangement of carbon skeletons is observed in the free-radical decarbonylation of some aldehydes such as tetralin-2-carboxaldehyde, /3-phenylisovaleraldehyde, and 3,3-dimethyl-4-phenylbutyraldehyde. On the other hand, no rearrangement occurred in the Pd-catalyzed decarbonylation of these aldehydes. " tert-Butylbenzene was obtained cleanly in 84% yield by the decarbonylation of /3-phenylisovaleraldehyde with Pd on carbon at 160 °C (Scheme 15).P61.[27]... 

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 ... 

Photochemical excitation results in a-cleavage to produce a primary, geminate radical pair, which may undergo radical combination reactions (1) in competition with decarbonylation or (2) to produce a secondary geminate radical pair. The latter may undergo radical combination (3) or produce a free-radical pair (4). The free radicals undergo radical combination reactions (5). 

Under free-radical conditions, the reaction of (TMS)3SiH with acid chlorides, RC(0)C1, gives the corresponding aldehydes and/or the decarbonylation products depending on the nature of substituent R [42]. The reduction of 1-adamantanecarbonyl chloride is given in Reaction (4.19). 

Pyridine-2-thione-A-oxycarbonyl (PTOC) derivatives of carboxylic esters 53 were developed by Barton et al. and serve as a convenient source of acyloxyl radicals, which upon decarboxylation provide specific routes to free radicals (equation 82). This process can also proceed by a radical addition (equation 83). Acyl selenides (54) are a convenient source of acyl radicals, which can undergo decarbonylation also giving specific free radicals (equation 84). ° ... 

The photoreactions of a-dicarbonyl compounds are quite different in the vapor and condensed phases. In the vapor phase, carbon-carbon bond cleavage is the preferred mode of reaction but in the condensed phase, many of the observed reactions can be rationalized by a mechanism involving hydrogen abstraction. Internal hydrogen abstraction, when possible, is generally preferred over abstraction from the solvent. With the exception of diethyl oxalate, which undergoes photoreactions typical of an ester, only those compounds that are reasonably strained or can yield reasonably stable free radicals give decarbonylation products. In the presence of suitable substrates, cycloaddition reactions have also been observed. 

For reviews of free-radical aldehyde decarbonylations, see Vinogradov Nikishin Russ. Chem. Rev. 1971, 40, 916-932 Schubert Kintner. in Patai, Ref. 189, pp. 711-735. 

Phenyl selenoesters have been reported to undergo reduction to the corresponding aldehydes and/or alkanes in the presence of (TMS)3SiH under free-radical conditions16. The decrease of aldehyde formation through the primary, secondary and tertiary substituted series, under the same conditions, indicated that a decarbonylation of acyl radicals takes place. Equation 11 shows an example of a tertiary substituted substrate. 

Free-radical cyclization of phenyl selenide 15 to indolizidinone 16 represented a key step in the total synthesis of (—)-slaframine (equation 52). The two pairs of diastereomers were first separated and then hydrolyzed to the corresponding alcohols in 76% overall yield77. (TMS)3SiH-mediated acyl radical reactions from phenylseleno esters 17 have recently been utilized for the stereoselective synthesis of cyclic ethers78. In fact, the experimental conditions reported in equation 53 are particularly good for both improving cis diastereoselectivity and suppressing decarbonylation. 

The key observation in the case of 152 is that photolysis in benzene conforms to the expected a-cleavage and decarbonylation reactions to form diphenylmethyl (A") and benzyl radicals (B"), which are free to diffuse apart. The statistical combination of free radicals A" and B gives a 1 2 1 mixture of products 154,155, and 156. In contrast, photochemical excitation in the crystalline phase led to the exclusive formation of 153 by combination of the geminate radical pair A B with a 100% cage effect. 

As indicated above, it is unlikely that free radical ions are formed in these CT quenching processes. If they were, protons more acidic than benzylic ones could be lost. For example, the radical cation of phenoxyacetic acid readily decarbonylates. Thus, the report that benzophenone can photosensitize the... 

CIDNP studies have proven to be a valuable tool in investigating the mechanisms of decarbonylation and disproportionation reactions in micelles27 29). Since the mechamisms involve the formation of triplet radical pairs, nuclear polarization of the protons near the radical centers occurs and results in the observation of emission or enhanced absorption in the NMR spectra of products of the radical pairs. For example, the photolysis of di-t-butyl ketone (11) in HDTCI yields both decarbonylation and disproportionation products (Scheme VII)27,29). The CIDNP spectra (Fig. 12) taken at various concentrations of copper chloride (free radical scavenger) illustrates that the intramicellar product is isobutylene (72), while 2,2,4,4-tetramethylbutane (13) and 2-methyl-propane (14) are the extramicellar products. 

Scheme 11.6 gives some examples of reactions in which free radical rearrangements have been observed. Entries 1 and 2 are phenyl group migrations in primary alkyl radicals generated by decarbonylation. The migration is competitive with the... 

The dominant photochemical reaction of ketones in the gas phase is cleavage of one of the carbonyl substituents, which is followed by decarbonylation and subsequent reactions of the free radicals that are formed. The initial cleavage occurs within 100 fs of excitation. There is an activation barrier for decarbonylation (see Table 11.2), so decarbonylation can be relatively slow with excitation at 270nm. At shorter wavelengths, there may be sufficient excess energy for rapid decarbonylation. This reaction is referred to as the Type-I or a-cleavage reaction of carbonyl compounds. 

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