Radicals pyramidal configuration

In the course of the reaction, the nitrite ion leaves the primary anion-radical. This produces the cyclohexyl radical in the pyramidal configuration. The vicinal methyl group sterically hinders the conversion of the pyramidal radical into the planar one. With a high concentration of the nucleophile, the rate of addition exceeds the rate of conversion, that is, Then the entering PhS group... 

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

Reductive decyanations of 2-cyanotetrahydropyran derivatives with sodium in ammonia yield predominantly axially protonated products. The observations are consistent with the reductive decyanation proceeding via the pyramidal, axial radical which accepts a second electron to give a configurationally stable carbanion, which in turn abstracts a proton from ammonia with retention of configuration (Rych-novsky, S. D. Powers, J. P. LePage, T. J., J. Am. Chem. Soc., 1992, 114, 8375-8384). Provide an explanation for the axial preference of the intermediate free radical on the basis of orbital interactions. Hint The title of the paper by Rychnovsky et al. is Conformation and Reactivity of Anomeric Radicals. ... 

A radical is paramagnetic and so can be observed by electron spin resonance (esr) spectroscopy. Radicals are planar in configuration, but the energy difference between pyramidal and planar forms is very small. 

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