Alkyl radical intermediates, evidence

Experimental evidence indicates that the alkyl radical intermediates from the anodic oxidation of carboxylates are generated in free solution and have no memory of the configuration of the carboxyl group that was eliminated. Where the carboxylic acid function is attached to an asymmetric carbon atom as in 15, the Kolbe coupling reaction leads to complete racemization [67]. Anodic oxidation of (+)-2-... 

Experimental evidence for this pathway is diverse. For example, the reactions occur with a 2 1 stoichiometry, but the rate law is second order (proportional to [M ] and [RX]). Products are generated with racemization at carbon, and the relative reactivity of alkyl halides follows the trend tertiary > secondary > primary > Me. These relative reactivities parallel the stabilities of R instead of the reactivity as an electrophile in a nucleophilic substitution process. Furthermore the alkyl radical intermediate in this mechanism has been trapped, has undergone rearrangements, and has been detected directly by EPR. 

Although Ce(IV) oxidation of carboxylic acids is slow and incomplete under similar reaction conditions , the rate is greatly enhanced on addition of perchloric acid. No kinetics were obtained but product analysis of the oxidations of -butyric, isobutyric, pivalic and acetic acids indicates an identical oxidative decarboxylation to take place. Photochemical decomposition of Ce(IV) carbo-xylates is highly efficient unity) and Cu(ll) diverts the course of reaction in the same way as in the thermal oxidation by Co(IIl). Direct spectroscopic evidence for the intermediate formation of alkyl radicals was obtained by Greatorex and Kemp ° who photoirradiated several Ce(IV) carboxylates in a degassed perchloric acid glass at 77 °K in the cavity of an electron spin resonance spectro-... 

One of the fundamental questions about the mechanism is whether the radical is really free in the sense of diffusing from the metal surface.7 For alkyl halides, there is considerable evidence that the radicals behave similarly to alkyl free radicals.8 One test for the involvement of radical intermediates is to determine whether cyclization occurs in the 6-hexenyl system, where radical cyclization is rapid (see Part A, Section 12.2.2). 

Since vinyl anions generally retain configuration 39> while isomeric vinyl radicals rapidly interconvert 40) these results constitute evidence that reductions of alkyl iodides do proceed via radical intermediates. Isomerization of stereoisomeric vinyl anions is ruled out by the lack of effect of phenol on the stereochemistry of the products (Scheme III). Since cis and trans-3-hexene are formed in differing proportions from the two halides, it may be concluded that the stereoisomeric vinyl radicals are being intercepted by electron trans-... 

The partition of the dimerization yields at 77 °K. into RPD and DD yields is shown in Table VIII. DD was obtained at 20°C. from the yields in the presence of added I2 and at 77 °K. by utilization of the efficiency factor equal to 2.3. Note the low yields at 20°C. for products of secondary radical combinations. We postulate that the intermediate excited primary radicals formed after neutralization in the UPD mechanism at 77 °K. may rearrange to secondary radicals before the final cross bond is formed. Evidence for this postulate is indicated in Table VIII by the observation that the n-C12 and 5-MeCu yields appear to be almost entirely due to the DD mechanism. Such isomerization of primary to secondary alkyl radicals in the solid phase at —196 °C. has recently been observed by electron spin resonance.62... 

It has been observed [23,91,92] that when an unsaturated hydrocarbon is reacted with (a) equilibrated and (b) non-equilibrated hydrogen—deuterium mixtures, the deuteroalkane distributions are identical. Such observations indicate that the direct addition of a hydrogen molecule across the olefinic bond does not occur, and provides strong evidence for the formation of a half-hydrogenated state , that is, an adsorbed alkyl radical, first suggested by Horiuti and Polanyi [81], as a relatively stable reaction intermediate. The process of hydrogenation may thus be represented as... 

According to Kerr and Webster (279), the radiolysis of TBP leads to alkyl radicals R and OP(OR)2OR . Investigations of the radical intermediates, by ESR examination or by use of electron scavengers, provided clear evidence regarding the formation of R radicals by dissociative electron capture (Equation 8.1) (294). 

Evidence of both types of potential intermediate in reduction by Sml2, the alkyl radical and the ketyl radical 27, has been provided by radical cyclisation reactions. Mechanism 4, which involves an Sjj2 substitution, has been eliminated because optically active halides are completely racemised. The rate of addition of alkyl radicals to ketones is very slow (<102 dm3 mol-1 s-1) the resulting alkoxy radicals (26) are very reactive and could not... 

It should be evident that radicals play a very important role within the realm of organofluorine chemistry. Fluorine substituents impart unique reactivity characteristics to free radical intermediates, and knowledge of how to generate and utilize such species is very important for those synthetic chemists who wish to incorporate fluorinated alkyl groups into organic substrates. It has been attempted in this review to provide a strategic overview of all aspects of organo-fluorine radical chemistry, with the hope that readers with an interest in the field will able to get their basic questions answered as well as be stimulated to dig deeper into specific aspects of the subject via the detailed references which have been provided. 

Several papers reporting new oxidative additions to [Pt(PPh3) ] have appeared. E.s.r. evidence has been presented which shows that addition of alkyl halide proceeds via two one-electron steps with a Pt1 intermediate.113 The rate-determining step (19) involves addition to [Pt(PPh3)2] with generation of alkyl radicals (R = Me, CD3, Et, PhCH2, or Ph3C) which have been trapped as the nitroxide Bu (R)NO. The... 

Mechanistic studies have been carried out on the oxidative addition of alkyl halides to Sn[CH(SiMe3)2]2 (79, 80). Radical intermediates were observed by spin trapping, and when an optically active alkyl halide was used racemization occurred. The evidence obtained led to the proposed mechanism shown in Eq. (40). 

There has been some controversy over the nature of intermediates in the Grignard reaction (54-56). Available evidence indicates that organic radicals form through cleavage of the carbon-halogen linkage everyone agrees on this. Controversy centers on the question of whether the alkyl radical remains adsorbed on the metal surface ( A model ) or whether it diffuses into solution ( D model ). Evidence appears to be more consistent with the D model (56), but the question cannot be considered as settled. 

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