Hydrogen atom transfer reactivity

The hydrogen atom-transfer reactivity of the 3(da pa) excited state has been generalized to other complexes. Among these metal com-... 

There are few addition reactions to a,/J-disubstituted enoyl systems 151 that proceed in good yield and are able to control the absolute and relative stereochemistry of both new stereocenters. This is a consequence of problematic A1,3 interactions in either rotamer when traditional templates such as oxazolidinone are used to relieve A1,3 strain the C - C bond of the enoyl group twists, breaking conjugation which results in diminished reactivity and selectivity [111-124], Sibi et al. recently demonstrated that intermolecular radical addition to a,/J-disubstituted substrates followed by hydrogen atom transfer proceeds with high diastereo- and enantioselectivity (151 -> 152 or 153, Scheme 40). 

A number of approaches based on calculations have been applied in order to obtain information on the factors that influence the reactivity of radical reactions. In particular, for the hydrogen atom transfer [Eq. (15)] the applied methodologies have spanned from high-level ab initio to empirical calculations. 

Electron transfer does not follow the simple rule of order of reactivity observed for n-7T, tt-tt and CT states for hydrogen atom transfer. The 77—77 singlet excited state (S of 9-cyanoanthracene for instance also undergoes electron transfer with amines at a diffusion-controlled rate. 

Both of the above reaction pathways are suppressed if 443-D and perdeuteriated solvents are used simultaneously in cyclization reaction 265. Mainly oligomer and less than 5% of alkane 450 were then produced. This has been explained by different reactivities of radical 445 formed directly from 444 or formed by abstraction of hydrogen atom from 443 through complex 453. The more energetic radicals formed by hydrogen atom transfer from 443, add intermolecularly to an alkene and initiate preferentially oligomerization512, while... 

Reid DL, Armstrong DA, Rauk A, Nese C, Schuchmann MN, Westhoff U, von Sonntag C (2003) H-atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates. Phys Chem Chem Phys 5 3278-3288 Roberts BP (1996) Understanding the rates of hydrogen abstraction reactions empirical, semi-em-pirical and ab initio approaches. J Chem Soc Perkin Trans 2 2719-2725 Russell GA (1973) Reactivity, selectivity, and polar effects in hydrogen atom transfer reactions. In Kochi JK (ed) Free radicals. Wiley, New York, pp 275-331 Russo-Caia C, Steenken S (2002) Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution. Phys Chem Chem Phys 4 1478-1485... 

Russell, G. A. Reactivity, Selectivity and Polar Effects in Hydrogen Atom Transfer Reactions. In Free Radicals Koch, J. K., Ed. Wiley New York, 1973 Vol. I, pp. 275-331. 

Scheme 1.1. Some characteristic reactions of radicals. A and B are any two types of reactive radical. A = B reaction [1]+ is a dimerisation. The reverse [ 1 ] is bond homolysis and may be induced thermally or photochemically. Reaction [2] represents addition to an alkene derivative. Reaction [3], hydrogen atom transfer, is one of the most important displacement reactions of radicals.

An exciting aspect of atom-transfer reactivity is the possibility of photocatalytic processes. The metal dihydrides formed in the initial photochemical hydrogen atom-transfer reaction may be turned over to... 

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