Radical hyperconjugation

The three highest occupied orbitals of sulfoxides are the lone pairs ns and n0, as well as the 7iso bond210. The 1,3-dithietane 1-oxide adds a lone-pair ionization and destabilizes the n0 and nso radical-cation states compared with thietane oxide. According to a hyperconjugative MO model, the ns+ combination in 1,3-dithietane is destabilized by about leV relative to the basis orbital energy a(ns) due to the combination with the... 

As with carbocations, the stability order of free radicals is tertiary > secondary > primary, explainable by hyperconjugation, analogous to that in carbocations... 

The anomeric configuration is set in the reductive lithiation step, which proceeds via a radical intermediate. Hyperconjugative stabilization favors axial disposition of the intermediate radical, which after another single electron reduction leads to a configurationally stable a-alkoxylithium intermediate. Protonation thus provides the j9-anomer. The authors were unable to determine the stereoselectivity of the alkylation step, due to difficulty with isolation. However, deuterium labeling studies pointed to the intervention of an equatorially disposed a-alkoxylithium 7 (thermodynamically favored due to the reverse anomeric effect) which undergoes alkylation with retention of configuration (Eq. 2). 

This reflects the relative ease with which the C—H bond in the alkane precursor will undergo homolytic fission, and more particularly, decreasing stabilisation, by hyperconjugation or other means, as the series is traversed. There will also be decreasing relief of strain (when R is large) on going from sp3 hybridised precursor to essentially sp2 hybridised radical, as the series is traversed. The relative difference in stability is, however, very much less than with the corresponding carbocations. 

The difference between the appearance potentials of the ions from methyl and ethyl radicals indicates that hyperconjugation is less effective in stabilizing an unpaired electron than a positive charge. 

Figure 3.54 shows the form of two singly occupied spin-NBOs in the vinylamine triplet excited state the 7tCN 1 half-bond of the (3 spin set (Fig. 3.54(a)) and the terminal nc radical orbital of the a spin set (Fig. 3.54(b)). Neither of these orbitals has a true counterpart in the MOs or NBOs of the ground-state molecule. These results emphasize that hyperconjugative reorganization is a strong feature of the excited-state orbitals, and the oversimplified HOMO-LUMO (Koopmans-like)... 

The corresponding hyperconjugative interactions in an open-shell singlet description of radical dissociation would similarly promote homolytic bond cleavage. 

Based on data from competition experiments, trapping of vinyl radicals occurs via a cr-type intermediate, which is lower in energy than the alternative jt-radical structure [55, 56], Stabilization of cr-radicals via hyperconjugation is small, which causes vinyl radicals to be more reactive than e.g. the methyl radical. /Z-Isomerization of a strained cr-vinyl radical proceeds with a rate constant k 3 x 108-1010 s-1 to provide the thermodynamically most favorable geometry [56],... 

For ethanol, the same procedure yields ER(EtO ) = -9.2 kJ mol-1 and s(EtO-H) = 449 kJ mol-1. This value is still some 48 kJ mol-1 higher than the value obtained for phenol, questioning our initial assumption that the bond strengths should be similar in both compounds. Yet we have forgotten two import issues the resonance stabilization of the phenoxyl radical and the hyperconjugation of the ethoxyl radical. 

Secondary 0-deuterium KIEs due to hyperconjugation in carbene and radical reactions 210... 

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