Structure captodative

With l,2-di(phenylthio)benzene (24), two successive two-electron steps [207] can be observed in aprotic DMF [E = —1.82 V and 2 =—2.11 V vs Ag/AgI 0.1 M / reference system). On the other hand, substitution by efficient electron-withdrawing groups might change dramatically the cathodic behavior of ArSR-type compounds. Apparently, the captodative character of substituents attached to the phenyl ring (see Structure 25) strongly... 

The foregoing discussion shows that the approach taken does not necessarily provide the organic chemist with an answer to the question of special effects on the radical centre in captodative-substituted radicals. Stabilization of the radical centre and stabilization of the complete radical structure must be considered separately. It is only the latter situation which can be dealt with by the approach of Leroy and coworkers. 

Katritzky (Katritzky et al., 1986) has recently advanced the idea that captodative-substituted radicals should be stabilized significantly by polar solvents. This hypothesis, which is qualitatively derived from the polar resonance structures for these radicals, was supported by semiempirical molecular orbital calculations. An experimental test was carried out by Beckhaus and Riichardt (1987). For the dissociation of [24] and [25] into the radicals [21] and [28], they were unable to confirm Katritzky s hypothesis. The rate of thermolysis of [24] and [25] is not affected by a change in solvent polarity. If the stabilization were of the order of Katritzky s prediction, it should, however, have become evident in the rate measurements. The experiments thus suggest that the contribution of polar resonance structures to the ground state of the radicals is not appreciable. See, however, the results obtained by Koch (1986) on the dl meso isomerization of [47]. 

Several attempts have been made to analyse the captodative effect through rotational barriers in free radicals. This approach seems to be well suited as it is concerned directly with the radical, i.e. peculiarities associated with bond-breaking processes do not apply. However, in these cases also one has to be aware that any influence of a substituent on the barrier height for rotation is the result of its action in the ground state of the molecule and in the transition structure for rotation. Stabilization as well as destabilization of the two states could be involved. Each case has to be looked at individually and it is clear that this will provide a trend analysis rather than an absolute determination of the magnitude of substituent effects. In this respect the analysis of rotational barriers bears similar drawbacks to all of the other methods. 

The rotational barrier about the C—O bond in the cyanomethoxymethyl radical, [35]/[36], constitutes a similar case, although the situation is somewhat more complicated (Beckwith and Brumby, 1987). As oxygen carries two lone pairs of electrons, the transition structure for rotation about the C—O bond can still be stabilized by conjugation. Compared to the methoxy-methyl radical, the barrier in the captodative-substituted radical is 1-2 kcal mol higher. 

A last example concerns the rotational barrier in phenoxyl radicals (Gilbert et ah, 1988). Compared to the parent phenols [37] and [39] the rotational barrier in [38] is increased by a factor of seven, whereas, with a captor substituent [40], the barrier increases only by a factor of 1.2. This could be interpreted in terms of a captodative stabilization in [38]. The captodative character of the radical [38] is represented by a resonance structure [41]. 

Structure of Lewis Acid-Coordinated Captodative Radicals... 

The stabilization of benzhydryl 31 and triphenylmethyl 2 is less than additive, as expected for the non-planar propeller-like structures of these radicals, which do not allow the development of full conjugation. The angle of twist is probably very similar in benzhydryl and trityl radicals 73) and one is tempted to attribute to each twisted phenyl an additive stabilization of 6 kcal mol"1. On the other hand two cyano groups in 32 likewise stabilize a radical less than additively. Phenyl and cyano (33) and phenyl and methoxy (34) show additive stabilization. For one cyclopropyl group in 35 a little more than 1 kcal mol-1 stabilization can be counted and additivity follows consequently for 36. The captodative radical 37 is stabilized according to additivity... 

Captodative radicals have both electron donor and electron acceptor substituents at the radical center. The separate stabilizing effects of these substituents appear to have the ability to be synergistic. A Linnett structure gives a similar representation. 

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