Radical decarbonylation

The same is true for decarbonylation of acyl radicals. The rates of decarbonylation have been measured over a very wide range of structural types. There is a very strong dependence of the rate on the stability of the radical that results from decarbonylation. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s . ... 

The acyl radicals obtained by hydrogen abstraction from aldehydes easily attack protonated heteroaromatic bases. With secondary and tertiary acyl radicals decarbonylation competes with the aromatic acylation [Eq. (12)]. 

Phenols. Presumably they arise exclusively via dissociative Path A, subsequent radical diffusion from the solvent cage, and abstraction of a hydrogen from the solvent (65 -> 74). The yields are (1) increased with decreasing viscosity of the reaction medium (2) higher in nonpolar and lower in polar solvents (3) practically independent of the hydrogen-donating ability of the solvent and (4) increased if a radical counterpart of a phenoxy radical, i.e., an acyl radical, decarbonylates in the solvent cage for structural reasons. 

However, when the same radical was generated in solution under homogeneous conditions, either by the radical decarbonylation reaction of Winstein and Seubold [63] of aldehyde 62 (Scheme 25) or by thermal decomposition of the f-butyl perester 65 (Scheme 26), the result was different. 

Although not of much real preparative interest, radical decarbonylation has been observed with a large number of aldehydes and ketones at high temperatures and/or under ultraviolet irradiation or in the presence of peroxides. For example, carbon monoxide is eliminated in 90% yield when 3-methyl-3-phenylbutanal is heated for five hours at 130° in the presence of di-tert-butyl peroxide fractionation of the residue then affords 70% of a 1 1 mixture of terr-butyl- and isobutyl-benzene, so that the / ,/ -dimethyl-phenethyl radical formed on elimination of carbon monoxide from the C6H5C(CH3)2CH2CO— radical must be assumed to have partially rearranged.66... 

It has proved useful to carry out radical decarbonylation of aldehydes in the presence of small amounts of toluene-a>thiol, which apparently represses chain cleavage.68 For instance, it has been found that in the absence of toluene-a>thiol 2-ethylhexanal gives only 9% of heptane after ultraviolet irradiation for 20 hours at 140°, whereas in presence of the thiol it affords 55% of heptane in 7 hours under otherwise similar conditions. 

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]... 

Similar to the rates of a-cleavage reactions, the rates of acyl radical decarbonylation depend very strongly on the stability of the alkyl radicals formed. It was first suggested by Fischer and Paul that the rate... 

Although the reports by Quinkert and Turro suggested the feasibility of the decarbonylation reaction in crystals and demonstrated a very high chemo- and stereoselectivity, the factors that may facilitate or impede solid-state reactivity remained unexplained. Recognizing that the efficient a-cleavage and rapid acyl-radical decarbonylation of ketones with radical stabilizing a- and a -phenyl substituents may determine the reactivity of ketones 54 to 57 and 64 (Schemes 15 and 17), Choi et al. carried out the first systematic search for a correlation between the reactivity of solids and the stability of the radicals involved in the reaction (Scheme 18). 

The course of the reaction is controlled by the stability of the acyl radical intermediates. When less reactive acyl radicals are generated, C-C-bond formation could successfully compete with decarbonyla-tion, whereas, in the case of more reactive acyl radicals, decarbonylation preceded C-C-bond formation. 

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