Substituted Allyl Radicals

The allyl radical has been extensively studied by e.s.r. and the distribution of the unpaired electron over the 7r-orbital system has been the subject of experimental measurement and many theoretical calculations (Fessenden and Schuler, 1963 Kochi and Krusic, 1968 Heller and Cole, 1962). However, there is no direct evidence available on the effect of methyl substitution at the terminal positions on the distribution of the unpaired electron. 

Studies of the oxidation and pyrolysis of olefins show that the majority of the products are derived from reactions which involve the terminal carbon atom of the substituted allyl radical, R—CH=CH—CH2 (i.e. with the canonical form shown). It has been suggested that this is due to a higher spin density at this carbon atom than at the substituted end ofthe radical (Norrish and Porter, 1963 Bryce and Ruzicka, 1960). Thus the measurement of the spin density distribution in substituted allyl radicals would be of considerable interest. 

The hyperfine splittings in the e.s.r. spectra of radicals of the allylic type are considerably less than those of alkyl radicals, and for radicals trapped in their parent compounds the resolution is insufficient to determine all the hyperfine coupling constants. However, by use of the rotating cryostat, the unsubstituted radical and three methyl-substituted allyl radicals have been prepared in a matrix of adamantane and it has been possible to resolve all the hyperfine couplings. 

For the allyl radical (la) the complete set of hyperfine splittings were obtained including the two slightly different values for the terminal protons. The values (14-0, 15-0, 4-1 G) are identical within experimental 

Hyperfine Coupling Constants for Substituted Allyl Radicals 

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When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

If photolyzed with light of the intensity I, HBr adds to propadiene (la) in the gas phase with a rate given by v=kexp[HBr]I<). This transformation affords within the detection limit (GC) 2-bromo-l-propene (5a) as sole reaction product (Table 11.1). The conversion of methyl-substituted allenes, such as lc and If, under these conditions follows the same kinetic expression [37]. Results from competition experiments indicate that the reactivity of an allene towards HBr increases progressively with the number of methyl substituents from propadiene (la) (= 1.00) to 2,4-dimethylpenta-2,3-diene (If) (1.65). In all instances, Br addition occurs exclusively at Cp to furnish substituted allyl radicals, which were trapped in the rate determining step by HBr. 

The phenylselenyl radical adds irreversibly to the central carbon atom of 2-methylbuta-l, 2-diene (Id) with a rate constant of 3 x 106 M-1 s-1 (23 1 °C) (Scheme 11.7) [45], On a synthetic scale, PhSe addition to cumulated Jt-bonds has been investigated by oxidizing phenylselenol with air in the presence of mono-, 1,1-di- or 1,3-di-substituted allenes to provide products of selective fi-addition. Trapping of 2-phenyl -selenyl-substituted allyl radicals with 02 did not interfere with the hydrogen atom delivery from PhSeH (Scheme 11.7) [31]. 

The 5-dig-mode of cyclization has been applied in the synthesis of N-heterocycles. For example, treatment of the /i-allenyl dithiosemicarbazide 37 with Bu3SnH and AIBN in hot benzene furnishes the substituted 3H-pyrrole 38 in 41% yield and the isomeric heterocycle 39 in 30% yield (Scheme 11.13) [68], Iminyl radical 40 is formed via Bu3Sn addition to the thiocarbonyl group of the radical precursor 37 and fragmentation of the adduct (not shown). Nitrogen-centered radical 40 adds 5-dig-selectively to provide substituted allyl radical 41. The latter intermediate is trapped by Bu3SnH to furnish preferentially product 38 with an endocydic double bond. 

Similarly to the triphenylmethyl system, captodative-substituted 1,5-hexa-dienes, which can be cleaved thermally in solution into the corresponding substituted allyl radicals [15], dissociate more easily than dicaptor-substituted systems (Van Hoecke et al., 1986). Since ground-state and radical substituent effects cannot be separated cleanly, not only because of electronic but also because of steric effects, a conclusive answer cannot be provided. 

The study of substituted allyl radicals (Sustmann and Brandes, 1976 Sustmann and Trill, 1974 Sustmann et al., 1972, 1977), where pronounced substituent effects were found as compared to the barrier in the parent system (Korth et al., 1981), initiated a study of the rotational barrier in a captodative-substituted allyl radical [32]/[33] (Korth et al., 1984). The concept behind these studies is derived from the stabilization of free radicals by delocalization of the unpaired spin (see, for instance, Walton, 1984). The... 

According to experiments (Jonsson et al. 1996), the deprotonation of (Et2NH)+ takes place at the nitrogen rather than at the a-carbon. However, Costentin and Saveant (2004) showed that if a stabilized carboradical can be produced, some intramolecular reorganization takes place and a proton leaves the carbon, not the nitrogen. The formation of the amine-substituted allyl radical is just the case +NH2-CH2=CH-CH2H H+ -f NH2CH2=CH-CH2 . 

To illustrate the technique we will consider a few examples of free radicals which have been prepared in the rotating cryostat. In particular phenyl and acetyl radicals and methyl-substituted allyl radicals are of interest as they have not been trapped previously or identified with certainty. Since electron spin resonance has been used extensively to detect and identify the free radicals, account of the results will inevitably involve some description and analysis of their spectra, but we wish to focus the main discussion on the conclusions that can be drawn about structure and reactivity of the radicals. For information about the principles of e.s.r. and the interpretation of the spectra of free radicals the reader is referred to review articles and books on the subject (Symons, 1963 Norman and Gilbert, 1967 Maki, 1967 Horsfield, 1967 Carrington and McLachlan, 1967 Ayscough, 1967 Carrington and Luckhurst, 1968). 

The mono-substituted allyl radical, generated by adding a radical to a diene, usually reacts at the unsubstituted end of the allyl radical. We saw two examples of this on p. 188. This may simply be owing to product-development control, but it is also likely that the coefficient is larger at this site. 

A new and versatile method for the generation of radicals has been developed, based on the photolysis of alkyl 4-nitrobenzene-sulphenates the tertiary alkoxide radicals (97), formed on irradiation of the sulphenates (98), undergo /3-scission to afford the ethyl aryl sulphides (99) by the pathway outlined in Scheme 5. This approach should prove useful in the preparation of substituted allyl radicals. A mild and selective photosensitised hydrolysis of p-toluenesulphonyl esters has been described, using either 1,5-... 

In one study, various substituted allyl radicals were generated by sulfenate photolysis, and it was shown that coupling was controlled both by steric and by frontier molecular orbital considerations [153]. p-Scission is favored by a-substitution and the stability of the putative alkyl radical. A particularly clever device used in another study was the thermal equilibrium between allylic sulfoxides and sulfenates used to generate allyloxy and other C3H5O radicals [154]. 

CNDO/2 calculations of spin densities, discussion of conformation. i6o Tentative assignment. Radical described by [67Takl] to be 1-cyclopentene-methyI is probably a noncyclic substituted allyl radical. 1 Carbon atoms of (-butyl groups. ... 

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