Captor substituent

The first indication of the existence of a captodative substituent effect by Dewar (1952) was based on 7t-molecular orbital theory. The combined action of the n-electrons of a donor and a captor substituent on the total Jt-electron energy of a free radical was derived by perturbation theory. Besides the formulation of this special stabilizing situation and the quotation of a literature example [5] (Goldschmidt, 1920, 1929) as experimental evidence, the elaboration of the phenomenon was not pursued further, neither theoretically nor experimentally. 

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]/... 

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]. 

Symbols d and c mean n donor and captor (substituents), respectively. 

In refluxing toluene the main reaction is the 1,3-dipolar isomerisation apparently via the interceptable CF3-dipole. In neat phase at higher temperatures reduction to the thioaminal and elimination of dimethylsulftde become the major reactions. Further results arise from trifluorothioamidium salts with varying N-substitution. Addition of cyanide furnished substituted thioaminals with two captor substituents and their thermal behaviour was studied (refs. 1, 13, Scheme 10). 

The concept of captodative substitution implies the simultaneous action of a captor (acceptor) and a donor substituent on a molecule. Furthermore, in the definition of Viehe et al. (1979), which was given for free radicals, both substituents are bonded to the same or to two vinylogous carbon atoms, i.e. 1,1- and 1,3-substitution, and so forth is considered. One might, however, also include 1,2-, 1,4-,. .. disubstitution, a situation which is more often referred to as push-pull substitution. Before discussing captodative substituent effects it might be helpful to analyse the terms capto and dative in more detail. 

At first glance the significance of these two terms seems to be obvious. A substituent which withdraws electron density is a captor and a substituent which donates electron density is a donor. However, both properties cannot be discussed independently from a partner from which they accept, or to which they donate, electrons. This raises the question of whether it is in principle possible to define a universal donor or acceptor character for a substituent. 

As a summary of these considerations we must conclude that on the basis of polar effects most substituents are captors and that it is the resonance effect which leads to the discrimination of two classes of substituents. 

Fig. 1 FMO diagram for the formation of a captodative-substituted radical c—C—d by successive interaction of (A) a carbon radical with a captor (c) and of (B) a captor-substituted carbon radical with a donor (d) substituent.

All these examples, and it would be possible to quote more, are a manifestation that captor and donor subtituents stabilize radicals. Judged by the temperature range where dissociation occurs it seems as if captodative substitution stabilizes better than dicaptor substitution (Stella et al., 1981). Mostly, however, these are qualitative or semiquantitative observations which do not allow one to evaluate the magnitude of stabilization in kcal mol". In particular, the question of a synergetic action of the captor and the donor substituent cannot be answered satisfactorily. In part, the observed effects might be related to steric interactions of the substituents. 

It is evident from the data in Table 6 that, with only one exception (entry 13), the combination of two captor or two donor substituents does not produce an additive effect, whereas, without exception, the captodative combinations display synergetic behaviour. Thus, the delocalization of the unpaired spin density in captodative radicals is markedly increased in comparison to pure additive superposition of capto and dative effects. This result is all the more significant since two identical substituents do not... 

Benzylic radicals offer themselves to a similar analysis. Some barriers to rotation have been determined (Conradi et ai, 1979). The barrier to rotation of 9.8 + 0.8 kcal mol for the a-cyano-a-methoxybenzyl radical [21] (Korth et al., 1985) could not be interpreted rigorously in terms of a captodative effect because estimates had to be made for the effect of a single captor or donor substituent on the rotational barrier. Within these limitations the barrier does not reflect more than an additive substituent effect. 

When BuSi(0SiMe20H)3 is heated, poly-condensation is immediately started, forming many different products. In order to synthesize low-molecular condensates, we have reacted BuSi(OSlMe2Cl)3 with one equivalent of the triol BuSi(0H)3 using triethylamine as HCl captor (compare also Eq. 2) the cage like BuSi(0SiMe20)3Si Bu is formed in 60% yield. In Fig. 3 the result of an X-ray structure analysis is depicted. Bicyclic molecules with the same skeleton of atoms but with different substituents are known [20,21]. 

The influence of substituents on CH3, 6h3, and CH3 is summarized in Table XLII, according to their type (captor, c donor, d). 

It is this fact which leads to the captodative effect, that is the stabilization of captodative radicals 1, bearing simultaneous captor and donor substituents. In turn, this means that compounds such as 2 and 3 tend to react by radical pathways [3], via radical abstraction (C-X homolysis of 2) or radical addition (to the n system of 3). 

---