Hydrogen abstraction stereoelectronic effect

The most direct evidence that stereoelectronic effects are also important in these reactions follows from the specificity observed in hydrogen atom abstraction from conformationally constrained compounds,18 60 C-H bonds adjacent to oxygen113"118 or nitrogen110 and which subtend a small dihedral angle with a lone pair orbital (<30°) are considerably activated in relation to those where the dihedral angle is or approaches 90°. Thus, the equatorial H in 20 is reported to be 12 times more reactive towards /-butoxy radicals than the axial 11 in 21.115... 

A further example of the importance of this type of stereoelectronic effect is seen in the reactions of /-butoxy radicals with spiro[2,n]alkanes (22) where it is found that hydrogens from the position a- to the cyclopropyl ring arc specifically abstracted. This can be attributed to the favorable overlap of the breaking C-H bond with the cyclopropyl cr bonds.120131 No such specificity is seen with bicyclo[n, 1,0]alkanes (23) where geometric constraints prevent overlap. 

Gilbert BC, Lindsay Smith JR, Taylor P, Ward S, Withwood AC (1999) The interplay of electronic, steric and stereoelectronic effects in hydrogen-atom abstraction reactions of S04, revealed by EPR spectroscopy. J Chem Soc Perkin Trans 2 1631-1637 Flasegawa K, Neta P (1978) Rate constants and mechanisms of reaction of Cl2 radicals. J Phys Chem 82 856-859... 

The susceptibility to hydrogen abstraction of C-H bonds a to ethereal oxygen, which often reflects thermochemical, polar, and stereoelectronic effects, enables selective functionalization of ethereal a-C-H bonds [1, 2]. The resulting a-alkoxy-alkyl radicals stabilized by conjugative electron delocalization between the oxygen... 

Further stereoelectronic effects on hydrogen abstraction reactions have been claimed by Malatesta and Ingold (1981) on the basis of their failure to observe the e.s.r. signal for the appropriate radical. Their necessary assumption, however, that rates of decomposition of product radical are the same as those of the reference radical is merely a pious hope. 

Stereodectronic Effects. The conformation of a ring effects the reactivity of cyclic ethers and we have already described the evidence for such processes in the literature 10). Here, however, we demonstrate that stereoelectronic effects may influence the reactivity of amines in free radical processes. Using a series of cyclic amines [15], radical addition to a fluorinated alkene may occur but then a choice is possible (a) the intermediate radical could react with amine to give the 1 1 adduct [16] (ki) or (b), a 1,5-shift of hydrogen can occur (k2) which ultimately leads to the 2 1 adduct [171. We anticipate that k2 would vary little with the amine but, of course, ki will vary with the ease of hydrogen—atom abstraction from the amine. Correspondingly, we find that the ratio of the 2 1 adduct [17] increases substantially in the series... 

Scheme 1. Polar and stereoelectronic effects in hydrogen abstractions...

Malatesta V, Ingold KU (1981) Kinetic application of electron paramagnetic resonance spectroscopy. 36. Stereoelectronic effects in hydrogen atom abstraction from ethers. J Am Chem Soc 103 609-614... 

Beckwith, A. L. J., Easton, C. J. (1981). Stereoelectronic Effects in Hydrogen Atom Abstraction from Substituted 1,3-Dioxanes. Journal of the American Chemical Society, 103, 615. 

The efficiency of hydrogen atom abstraction is often influenced by several factors involving thermodynamic, polar, stereoelectronic and steric effects. For instance, C-H bonds a to oxygen are not always susceptible to abstraction when nucleophilic radical initiators are used. P. Kaushal, P. L. H. Mok,... 

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