Radical chain racemization

Rate constants for the reaction of thiyl radicals with the t-BuMePhSiH were also extracted from the kinetic analysis of the thiol-catalysed radical-chain racemization of enantiomerically pure (S)-isomer [34]. Scheme 3.2 shows the reaction mechanism that involves the rapid inversion of silyl radicals together with reactions of interest. The values in cyclohexane solvent at 60 °C are collected in the last column of Table 3.5. 

Scheme 3.2 Thiol-catalysed radical chain racemization...

The relative ease with which aryl benzyl sulfoxides undergo homolytic dissociation (Rayner et al., 1966) as compared to aryl benzyl sulfides or sulfones is supportive of this idea that ArSO radicals are easier to form than ArS or ArS02 radicals. Another interesting set of observations is the following. Booms and Cram (1972) found that optically active arene-sulfinamides ArS(0)NRPh (R = H or CH3) racemize thermally very readily at room temperature and that this racemization is the result of a free radical chain reaction (160) that is initiated by the dissociation of some of the sulfinamide into an ArSO and a PhNR radical (159). While the length of the inhibition... 

Co-oxidation of indene and thiophenol takes place readily if the reactants in benzene solution are shaken with oxygen at temperatures in the range 20° to 40°C. (7). The major primary product has been shown to be frans-2-phenylmercapto-1 -indanyl hydroperoxide, I, which rearranges spontaneously to the two racemes of frans-2-phenylsulfinyl-l-indanol, II (8), and a tentative reaction scheme involving a three-step radical chain based on the suggestion of Kharasch, Nudenberg, and Mantell (11) was proposed for the formation of I. These three products accounted for 86% of the oxygen absorbed. 

New methods for the preparation of germanes and stannanes reported since 1995 are dealt with in Section n. In Section III, radical chain chemistry involving trialkyltin hydrides is examined. In particular, the synthetic utility of tributyltin hydride will be reviewed, as well as that of other stannanes. Recent advances in the area of asymmetric radical chemistry involving chiral non-racemic stannanes are also included. Section IV details a limited number of examples of non-radical stannane chemistry, while Section V covers recent advances in germane and plumbane chemistry. While we have restricted ourselves largely to the literature since the beginning of 1996, some salient features of earlier work are included when relevant to the discussion. 

Another complication with gas phase pyrolyses is that many possible nonconcerted reaction pathways are possible. Alkyl halides xmdergo elimination in the gas phase, and some compounds, such as ethyl chloride, appear to undergo unimolecular elimination. Their unimolecular decompositions may involve transition structures with significant carbocation character. For example, p5u-olysis of (-l- )-2-chlorooctane in the gas phase at 325-385°C was found to produce racemization of the starting material as well as elimination of HCl. Some compounds appear to react by radical chain mechanisms, and heterogeneous radical reactions often complicate studies that are not carried out in "well-seasoned" (i.e., coated with a layer of organic material) vessels. Furthermore, there appears to be a significant radical (but not radical chain) component to the pyrolysis of sulfoxides. These complications mean that many control studies are necessary to clarify the mechanism of gas phase elimination reactions. 

The mechanism of the metathesis reaction of the new vanadium hydride [V(C5H5)H(C0)3] with alkyl halides is a free-radical chain process involving [V(CsH5XCO)3] based on normally applied criteria such as the similar rates for primary, secondary, and tertiary R and the formation of completely racemic... 

Other Reactions of Allenes.— Radical-chain additions to allenes are presumed to involve radical intermediates of allylic structure by initial attack at the central carbon atom or of vinylic structure by attack at the terminal carbons. The extent to which these structures are preferred depends upon structure of starting allene, nature of attacking radicals, and reaction conditions. Addition of toluene-/ -sulphonyl iodide to chiral allenes (cyclonona-1,2-diene and penta-2,3-diene) results in racemic products (376), and hence attack on... 

Racemization of sulphinamides in solution in an inert solvent involves a radical-chain mechanism. Rotation barriers concerning the S—bond in halogenosulphinamides and rapid halogen exchange in the case of the Cl and Br compounds have been estimated by variable-temperature n.m.r. methods. ... 

An optically active polymethacrylate (2) having a binaphthol moiety in the side chain was synthesized by radical polymerization. This polymer coated on silica gel resolved several racemates.50 However, no data on the influence of the stereoregularity of the main chain on resolution have been reported. The chiral recognition by this polymer may simply arise from the binaphthyl group. 

Refluxing or photolytic treatment of O-acyl esters (2) in the presence of a hydrogen donor such as Bu3SnH or ter -BuSH, provides the corresponding reduction products via alkyl radicals. This reaction can be applied to primary-, secondary-, and tertiary-alkyl chained carboxylic acids, and can also be used for steroids, sugars, and peptides as shown in eq. 8.4 [9-14]. Racemization does not occur at other chiral centers. 

FIGURE 14.3 Nuclear spin symmetry relationships for main-chain radical la from PMMA, as a function of polymer tacticity. The two radicals shown have mirror plane ( meso ) or C2 axis ( racemic ) symmetry elements establishing magnetic equivalence of each set of 3-methylene protons. 

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