Captodative effect method

As no clear-cut conclusion can be drawn from the analysis of these ab initio calculations it is not surprising that further attempts have been made to answer the question of the existence of a special captodative effect by improving the calculational methods. Clark (Clark, 1988) has carried out extensive calculations for the cyanomethyl-, amlnomethyl- and aminocyano-methyl-radical system by perturbational methods including different amounts of correlation. The results using the isodesmic reactions (5)-(8) are shown in Table 5. 

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. 

In a similar process, the a-chloro-a-thioacetamide 37 leads to the pyrrolizidines 38 and 39 upon chlorine atom-transfer cyclization initiated by catalytic ruthenium chloride (Eq. 2). The high efficiency of this method, which was applied to alkaloid synthesis, was attributed to the captodative effect [30],... 

Investigators using experimental tools like ESR [2, 30-39] or kinetics methods [40-48], which are able to quantitatively investigate small stabilization effects more accurately than thermochemical methods, always find a synergistic effect in accordance with the captodative theory. Accordingly there is little doubt, at the present time, of the validity of the captodative theory. 

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