Allyl radical substituent effects

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 transition state involves six partially delocalized electrons being transformed from one 1,5-diene system to another. The transition state could range in character from a 1,4-diradical to two nearly independent allyl radicals, depending on whether bond making or bond breaking is more advanced. The general framework for understanding the substituent effects is that the reactions are concerted with a relatively late transition state with well-developed C(l)—C(6) bonds. 

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... 

The experimental result seems to support this model. Table 11 lists values for rotational barriers in some allyl radicals (Sustmann, 1986). It includes the rotational barrier in the isomeric 1-cyano-l-methoxyallyl radicals [32]/ [33] (Korth et al., 1984). In order to see whether the magnitude of the rotational barriers discloses a special captodative effect it is necessary to compare the monocaptor and donor-substituted radicals with disubstituted analogues. As is expected on the basis of the general influence of substituents on radical centres, both captor and donor substituents lower the rotational barrier, the captor substituent to a greater extent. Disubstitution by the same substituent, i.e. dicaptor- and didonor-substituted systems, do not even show additivity in the reduction of the rotational barrier. This phenomenon appears to be a general one and has led to the conclusion that additivity of substituent effects is already a manifestation of a special behaviour, viz., of a captodative effect. The barrier in the 1-cyano-l-methoxyallyl radicals [32]/... 

An error-propagation analysis allows the conclusion that the captodative substituent effect on the rotational barrier in this allyl radical is at least additive and perhaps slightly greater. 

A considerable difference has been observed between the spectrum of cyclohexyl and that of the cyclopentyl radical, the former exhibiting a pronounced shoulder at 250 nm with e = 920 m -1 cm-1. Cyclohexenyl and cyclopentenyl radicals show a much stronger absorption with definite maxima at 240 nm. These are allyl type radicals and like the allyl radical itself they show extinction coefficients of 7000-9000 M -1 cm-1. The optical spectrum of the allyl radical is greatly affected by unsaturated substituents which conjugate with the allylic 1 and 3 positions. These positions bear all the spin density and their interaction with carboxyl groups, for example, shifts max to 270 nm with extinction coefficients of 20,000-40,000 M 1 cm 1 (Neta and Schuler, 1975). A carboxyl group attached to the central carbon of allyl has only a minimal effect on the absorption. 

With unsymmetrical alkenes, there are two regioisomeric meta adducts,1154 1160 1161 which are easily understood as the consequences of bond formation from the diradical intermediates 8.138 and 8.141, with the isolated radical centre forming a bond to either end of the allyl radical with little selectivity stemming from the presence of the substituent on the alkene. The low degree of selectivity is equally accommodated by a concerted reaction in which there is no intermediate. The main effect of having an electron-donating group, as in the reaction with ethyl vinyl ether 8.76, is that the ortho adduct 8.131 is the major product. The meta adducts 8.142-8.145 (R = OEt) are minor products (ortho.meta 65 35). With the rather less effective donor substituent, as in the reaction with vinyl acetate, the meta adducts 8.142-8.145 (R = OAc) are the major products (meta ortho 88 12). They show some selectivity in favour of the endo adducts (64 36 for R = OEt), and the major product 8.142... 

The role of cyano substituents has also been explored in detail. The symmetrical matched 2,5- and 1,3,4,6-systems and the unmatched (but still symmetrical) 1,3,5-cyano system were investigated. The effect of the substituents on TS energy was examined, as were the 1-6 and 3 bond distances. The energies of the TSs were evaluated relative to two allyl radicals by an isodesmic reaction. 

Table 11.1. Substituent Effects on the Stability of Allylic and Benzylic Radical from Calculation of Radical Stabilization Energy... 

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