Radicals conformational equilibrium

The Coleman-Fox two state model describes the situation where there is restricted rotation about the bond to the preceding unit (Scheme 4.3). If this is slow with respect to the rate of addition, then at least two conformations of the propagating radical need to be considered each of which may react independently with monomer. The rale constants associated with the conformational equilibrium and two values of Pirn) are required to characterize the process. 

Conformational effects have also been invoked to explain some inverted temperature dependence of radical reactions. Liining has examined the stereoselectivity of the radical addition of A-bromophthalimide to cyclohexene (Scheme 7) [15]. Higher trans selectivities were observed at elevated temperatures. The existence of two radical conformers in equilibrium explains this outcome. At low temperature, the reaction occurs exclusively through the most stable equatorial conformer 7 however, this conformer does not react with very high selectivity. The pathway via the less stable axial conformer 8 becomes aceessible at higher temperature and is highly anti selective. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

A typical example of steric control over spin delocalization has been described for the cation-radical of 3,4-bis(thioisopropyl)-2,5-dimethyl-l-phenylpyrrole (Domingo et al. 2001). Scheme 3.15 depicts this sitnation. In this cation-radical, one thioisopropyl group is almost coplanar with the pyrrole ring, whereas the other one occupies an orthogonal position. Accordingly, the ESR spectra established an eqnilibrinm between the symmetrical and asymmetrical conformations of the cation-radical. This equi-librinm is shifted toward the asymmetrical form at low temperatmes. The main feature of the equilibrium is the widening of spin delocalization, which includes not only the pyrrole ring but also one donor sulfur atom at the expense of the other sulfur atom. The steric control predetermines the discrimination of the other sulfur atom in the spin-delocalization process. 

As noted, the twisted conformer, which has a lower IP, rapidly scavenges the chair form of the cation-radical. Being endothermic, the backward transfer is relatively slow, and equilibrium is reached in 20-30 min. Thus, the electron transfer can be described as a series of periods of very fast hole migration between the chair forms and intermittent migration with the participation of the twist forms. 

Becanse of the high concentration of isomeric molecules (>0.1 mol dm ), this equilibrium is established instantaneously. The IP of trans-AtcaYm is 0.02 eV lower than the IP of cw-decalin (9.24 eV versus 9.26 eV). Therefore, the electron-transfer eqnilibrinm is shifted slightly to the left side. Thns, in terms of charge-transfer kinetics, the two ions behave as a single species. It shonld be worth noting that decalin has only two isomeric forms, cis and trans. On the contrary, n-nonane exists in the multitude of conformations. The rate constant of electron exchange between parent nentral molecules of nonane and its cation-radicals is much lower, namely, 2 orders lower than the diffnsion-controlled limit (Borovkov et al. 2007). 

As established, 4,5,9,10-tetrahydropyrene yields the cation and anion-radicals in the form of an equilibrium mixture of conformers (Iwaizumi and Isobe 1975 Scheme 6.25). 

A pentopyranosyl radical is much more flexible than a hexopyranosyl radical. Because the alkyl-anchor at C-5 is absent, the radical is now so flexible that several species of similar energy can coexist. According to ESR spectroscopic data, the arabinopyranosyl radical 9 exists as an equilibrium between the 4C19a and the B03 9b conformation, which both realize a coplanar arrangement of the C-O bond and the SOMO [9] (Scheme 6). Reactions with alkenes are unselective. However, the arabinofuranosyl radical 10 reacts with high stereoselectivity [9]. This is due to its 2E conformation in which the si-side of the radical center is sterically hindered by the large benzoyl group. 

It is important to note that the above presentation, justifying the reactivity-selectivity principle, is based on a number of fundamental assumptions. First, it is assumed that the Leffler-Hammond postulate is valid, which in turn implies that the reaction under consideration obeys a rate-equilibrium relationship [eqn (2)]. This assumption often cannot be verified since for reactions of highly active species such as carbenes, free radicals, carbonium ions, etc., equilibrium constants are generally not measurable. However it follows that for reactions which do not conform to a rate-equilibrium relationship, no reactivity-selectivity relationship is expected. Also, in Fig. 4, the difference in the free energy of the... 

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