Carbon monoxide from radical decarbonylation

Acyl radicals undergo dccarbonylation. For aliphatic acyl radicals the rate constant for decarbonylation appears to be correlated with the stability of the alkyl radical formed. Values of the dccarbonylation rate constant range from 4 s for CH3C 0) to 1.5x10s s l [lor (CH.,)2C(Ph)C( )0] at 298 °C.3M The loss of carbon monoxide from phenacyl radicals is endothermic and the rate constant is extremely low (ca 10 8 s 1 at 298 nC).388 Consequently, the reaction is not observed during polymerization experiments. 

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... 

That resonance stabilization of intermediate biradicals is important in determining the efficiency of decarbonylation follows from the following examples yielding benzyl radicals upon loss of carbon monoxide(57) ... 

The fate of the free acyl radical 68 and radical 74 is not known. Most probably it is a constituent of polymer deposits on the wall of the irradiation vessel which hitherto have not been identified more definitely.29 Moreover, the identification of methane and carbon monoxide among the gaseous products of the photolysis of 4-methylphenyl acetate (55) provides evidence for the existence of the acetyl fragment. This intermediate is expected to decarbonylate to give carbon monoxide and a methyl radical, which in turn abstracts hydrogen from the solvent.34... 

Reaction of all of the radical pairs within their initial cages does not preclude one or both of the radicals from undergoing a structural change (N. B., step [6] in Scheme 13.1). In the case of photo-Fries reactions, the most commonly encountered structural change is loss of carbon monoxide (CO) from the acyl radical, leading to formation of an alkyl radical. The rates of decarbonylation of acyl radicals have been measured for a wide variety of acyl structures as a function of the medium viscosity... 

Diamagnetic RunOEP—CO is stable against air oxidation [E° = 1.21 V for the Run/Rum couple and 0.64 V for the -cation radical formation] and undergoes rapid dimerization when the carbon monoxide is split off by light. Preliminary conclusions from an X-ray study of Run(C02) TPP which implied bent R—C—O bonds (154°) [Cullen (42)] have been shown to be in error The compound is in fact RuIl(CO) (EtOH)—TPP and the Ru—C—O bond is linear (176°) [Bonnett (8a)]. Decarbonylated... 

This fragmentation reaction occurs very readily. Acyl radicals also have a tendency to undergo fragmentation with elimination of carbon monoxide, but decarbonylation is a function of the stability of the radical formed by elimination of CO, and products derived from the acyl radical and its decarbonylation product are often formed competitively. For example, when isobutyraldehyde reacts in carbon tetrachloride with initiation by t-butyl peroxide, both isopropyl chloride and isobutyryl chloride are formed ... 

Several observations indicate that this is reasonable. When a mixture of benzaldehyde and wo-butyryl chloride is treated with the organotin hydride, benzyl iro-butyrate is formed, but iro-butyl benzoate is not. Similarly when a mixture of benzoyl chloride and wo-butyraldehyde is reduced wo-butyl benzoate is formed, but benzyl iso-butyrate is not. Ethyl chloro-formate is reduced very slowly at 80° C by tri-n-butyltin hydride, but the addition of 2 mole-% of azobisisobutyronitrile leads to facile reduction. When triphenylacetyl chloride is reduced at 100° C with the same hydride the products include triphenylacetaldehyde, triphenylmethane, and carbon monoxide, but little, if any, j8,j8,j3-triphenylethyl triphenylacetate. The formation of carbon monoxide and triphenylmethane can be attributed to decarbonylation of the intermediate acyl radical to the more stable trityl radical, Eq. (61), which subsequently abstracts a hydrogen atom from the... 

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