Radical carbon-centred

However, deuteration at carbons 9, 10, 12 and 13 did change the spectrum to one consisting of only a nitrogen triplet, indicating that the radical trapped had a deuterium attached to the carbon centre. This would imply that the carbon-centred radical lies at one of these four positions. 

From these data, and the similarity of the data for the other radicals contained in Table 1, it therefore appears that, unlike carbon-centred radicals, the tricoordinate trialkyl radicals of Group IV elements have the tetrahedral structure 1. 

As the temperature is raised above 77 K, the anion radical IV disappears at approximately 190 K and at room temperature only the carbon centred radicals V - VII are observed. 

Irradiation at 77 K in trichlorofluoromethane of cyclic tertiary amines also affords radical cations that can be trapped indefinitely. In these systems there apparently is no reaction between the radical cations and free amine. The EPR spectra of the radical cations were recorded. The cations produced under these conditions can be trapped indefinitely and do not undergo proton loss to give the corresponding carbon-centred radical. Several systems (20-24) were examined in this way and all were found to be stable. In the bis amine (22, n = 1) evidence was obtained from the EPR study that there was weak N-N interaction4. The influence of silicon in 25 was also examined16. 

Figure 15.13. Carbon-centred radical producing catalysts...

Thiols catalyse radical-chain addition of primary aliphatic aldehydes (R CH2CH0) to terminal alkenes (H2C=CR R ) to give ketones, R CH2C0CH2CHR R. The thiol acts as an umpolung catalyst to promote the transfer of the aldehydic hydrogen to the carbon-centred radical formed when an acyl radical adds to the alkene. 

Glycine anion (230) is decarboxylated when exposed to hydroxyl radicals. The major initial product is an amino radical cation (231), which suffers rapid 100 ns) fragmentation into CO2 and a carbon-centred radical (232). Oxidative decarboxylation... 

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

The reactions of sodium dimethyl and diisopropyl phosphite with 4-nitrobenzyl chloride, 9-chlorofluorene, and diphenylchloromethane provided information that supported the proposed reaction mechanism. The RaPO anion acts towards an arylmethyl chloride as a base and abstracts a proton to form a carbanion, which can then participate in single-electron transfer processes to produce carbon-centred radicals. ... 

An inner-sphere electron reduction has been proposed as a possible mechanism for the Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) (75) and (76). Benzoic acid was found to be the major product. The nucleophilic Ee(II) species attack the ozonide from the less hindered side of the electrophilic 0-0 a orbital to generate exclusively the Ee(III) oxy-complexed radical (inner-sphere electron transfer). After selective scission of the C-C bond, the resulting carbon-centred radical produced the observed product. The substituent effect determine the regioselective generation of one of the two possible Fe(III)-complexed oxy radicals. The bond scission shown will occur if R is bulkier than R. ... 

Trisubstituted carbon-centred radicals chemically appear planar as depicted in the TT-type structure 1. However, spectroscopic studies have shown that planarity holds only for methyl, which has a very shallow well for inversion with a planar energy minimum, and for delocalized radical centres like allyl or benzyl. Ethyl, isopropyl, tert-butyl and all the like have double minima for inversion but the barrier is only about 300-500 cal, so that inversion is very fast even at low temperatures. Moreover, carbon-centred radicals with electronegative substituents like alkoxyl or fluorine reinforce the non-planarity, the effect being accumulative for multi-substitutions. This is ascribed to no bonds between n electrons on the heteroatom and the bond to another substituent. The degree of bending is also increased by ring strain like in cyclopropyl and oxiranyl radicals, whereas the disubstituted carbon-centred species like vinyl or acyl are bent a radicals [21]. 

For a long time, this knowledge on carbon-centred radicals has driven the analysis of spectroscopic data obtained for silicon-centred (or silyl) radicals, often erroneously. The principal difference between carbon-centred and silyl radicals arises from the fact that the former can use only 2s and 2p atomic orbitals to accommodate the valence electrons, whereas silyl radicals can use 3s, 3p and 3d. The topic of this section deals mainly with the shape of silyl radicals, which are normally considered to be strongly bent out of the plane (a-type structure 2) [1]. In recent years, it has been shown that a-substituents have had a profound influence on the geometry of silyl radicals and the rationalization of the experimental data is not at all an extrapolation of the knowledge on alkyl radicals. Structural information may be deduced by using chemical, physical or theoretical methods. For better comprehension, this section is divided in subsections describing the results of these methods. 

Persistent and stable silyl radicals have attracted considerable attention [42]. Bulky aryl or alkyl groups that generally make carbon-centred radicals persistent [43,44] have a much weaker effect on the silyl radicals. The high reactivity of the Ph3Si radical contrary to the stable Ph3C radical is mentioned above. The decay of the trimesitylsilyl radical at 63°C follows a first-order kinetics with a half-life of 20 s [37]. Tri-tert-butylsilyl radical is also not markedly persistent showing the modest tendency of tert-h Ay groups to decrease pyramidalization... 

In Table 3.2 are collected the rate constants of some silicon hydrides with a variety of radicals like the ir-type secondary or tertiary alkyl radicals, the o-type phenyl or acyl radicals, and the halogenated carbon-centred radicals. A few Arrhenius parameters are also available and reported here below. 

The kinetic data for halogenated carbon-centred radicals with silicon hydrides are also numerous and a few examples are shown in Table 3.2. The kinetic data for perfluoroalkyl radicals were obtained by competition of the appropriate silane with the addition to an olefin [16-18]. The kinetic deuterium isotope effects (/ h/ d) on the attack of on the Si—D bond of... 

The kinetics data on the reactions of silyl radicals with carbon-centred radicals are also available. The rate constant for the cross-combination of CHs with MesSi was measured to be 6.6 x 10 M s in the gas phase [19]. Studies on the steady-state and the pulse radiolysis of EtsSiH in methanol showed that the cross-combination of Et3Si with CH30 andHOCH2 occurs with rate constants of 1.1 x 10 and 0.7 x 10 M s , respectively [20]. 

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