Radicals trigonal

The stereochemistiy of reactions involving substituted alkenyl free radicals indicates that radicals formed at trigonal centers rapidly undergo interconversion with the geometric isomer. Reactions proceeding through alkenyl radical intermediates usually give rise to the same mixture from both the E- and the Z-precursor ... 

A reasonable idea of the stability of the stereoisomeric trigonal vinyl cations can be gained from the behavior of vinyl anions and radicals. It is known that the interconversion between stereoisomeric vinyl anions is fairly slow, with an activation energy of the order of 18-24 kcal/mole (171). On the other hand, inversion of stereoisomeric vinyl radicals is reasonably rapid, even at fairly low temperatures, with an activation energy of the order of 2-8 kcal/mole (172). Hence, extrapolating from the electron-rich vinyl anion through the neutral vinyl radical to the electron-deficient vinyl cation, one would expect rapid interconversion between stereoisomeric vinyl cations and only a small amount (if any) of stereospecificity. To put it differently, the vinyl cation should be mostly linear with an empty p orbital and very little trigonal character. 

Crystal structure analysis of 43 revealed that the radical is perfectly trigonal planar, implying the sp -hybridization of the central Si atom and, hence, the localization of the unpaired electron on its 3pz-orbital. The remarkable planarity of the radical 43 was explained by the great steric bulk of the voluminous t-Bu2MeSi groups, which prefered to move away from each other as far as possible to avoid the steric repulsion between them. On the contrary, the significant electron donation of the positive silyl substituents... 

Undoubtedly, the uniform structural and spectral behavior of the trigonal-planar jt-radicals (t-Bu2MeSi)E (E = Si, Ge, Sn) 43-45 both in the solid state and in solution should be ascribed to the immediate impact of the bulky electropositive silyl substituents. In contrast, it is well known that simple alkyl and aryl substituents cause a highly pronounced pyramidalization at the radical centers, where the unpaired electron typically occupies the orbital with a high s-contribution (o-radicals)7 ... 

The pyramidal structure of the silyl radical provides the key to understanding that silicon is unlikely to appear in a trigonal planar form in silicon hydrides. 

Perhaps the most fruitful of these studies was the radiolysis of HCo(C0)4 in a Kr matrix (61,62). Free radicals detected in the irradiated material corresponded to processes of H-Co fission, electron capture, H-atom additions and clustering. Initial examination at 77 K or lower temperatures revealed the presence of two radicals, Co(C0)4 and HCo(C0)4 , having similar geometries (IV and V) and electronic structures. Both have practically all of the unpaired spin-density confined to nuclei located on the three-fold axis, in Co 3dz2, C 2s or H Is orbitals. Under certain conditions, a radical product of hydrogen-atom addition, H2Co(C0)3, was observed this species is believed to have a distorted trigonal bipyramidal structure in which the H-atoms occupy apical positions. 

The geometrical structure of most alkyl radicals is trigonal planar at the carbon having the unpaired electron. 

Abstraction of a hydrogen atom from C2 produces a trigonal planar radical that is achiral. This radical is achiral then reacts with chlorine at either face [by path (a) or path (b)]. Because the radical is achiral the probability of reaction by either path is the same therefore, the two enantiomers are produced in equal amounts, and a racemic form of 2-chloropentane results. 

A series of N-allyl sulfamates, phosphoramides, and phosphorimidates was prepared to explore the possibility of O- N rearrangements via the intermediacy of the contact alkene radical cation/anion pair, followed by 5-exo-trigonal radical cyclizations (Fig. 4) [142],... 

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