Acyl radicals decarbonylation rate constants

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. 

Hunsdiecker reaction does not work so well in aromatic carboxylic acids [75, 76]. The rate constants for decarbonylation of acyl radicals are lowered as shown in Table 1.18. 

Absolute and Relative Rate Information from Constant Intensity Irradiation of Aryl Esters in Which Acyl Radicals Do Decarbonylate Rapidly... 

Several acyl radical clocks have been calibrated, and these are collected in a recent excellent review of the general subject [44]. Examples of the two types of unim-olecular clock reactions, decarbonylations and cyclizations, are shown in Fig. 7, with rate constants for reactions at ambient temperature. Decarbonylations of acyl radicals, as shown for radical 16 [45], and the related decarboxylations of alkox-ycarbonyl radicals such as 17 [2] have log A terms of about 13 for cases where alkyl radical products are formed [46, 47]. The decarbonylation reactions involve a reduction in charge separation in the transition states, and the kinetics are sensitive to solvent polarity with decreases in rates as polarity increases [45]. Cyclization reactions, such as that shown for radical 18, are complicated. The 5-exo products shown are the predominant first-formed products, but they further rearrange to the thermodynamically favored 6-endo products by addition of the radical center to the carbonyl group to give a cyclopropyloxyl radical followed by ring opening [48]. 

The acyl radicals decayed either by decarbonylation or by radical-radical recombination. Decarbonylation reaction rates were found to decrease in more polar solvents, indicative of the more polar nature of acyl radicals relative to their decarbonylation products. Second-order rate constants for the reaction of a variety of acyl radicals with halogen atom donors, hydrogen atom donors, and oxygen were determined by monitoring the decay of the acyl radical, the growth of the product, or, when possible, both. The observed reactivity was roughly comparable with that of simple alkyl radicals. 

A considerable number of examples described below in which nucleic acid radicals are independently generated take advantage of the a-photo-cleavage (Norrish type I) of ketones (Scheme Most of the examples that wiU be cited involve photolysis of i-butyl or benzyl ketones (R ). This is consistent with the general quantum yield efficiency and rate constant for a-cleavage of the triplet excited state ketone. Consideration of the rate constant for decarbonylation of the ground state acyl radical is also relevant, with the efficiency of decarbonylation correlating with radical stability." ... 

First, methods of generation of acyl radicals and their kinetics were studied at 80 C (eqs 4 and 5) [38,39]. Then, rate constants for the reactions of acyl radicals with BusSnH and (TMS)3SiH were measured using the decarbonylation of primary acyl radicals as timing devices [40]. The values of 1.1 x 10 and 7.2 x 10 s l derived at 80°C for BusSnH and (TMS)3SiH, respectively, were used to obtain the rate constant of 3.7 x 10 S i for the decarbonylation of secondary acyl radicals. With these data in our hands we were able to design and carry out a series of experiments which allowed us to determine rate constants (at 80°C, temperature of the polymer carbonylation) for each elementary step of interest, as it is reported in Scheme 7 [41,42]. 

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