Captodative stabilization

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

The reaction energy for Eq. 5 amounts to -54.7 kj/mol at the ROMP2 level, indicating a substantial captodative stabilization for glycyl radical (9) even with this definition. 

Arnold s scale is derived for the action of a single substituent on the benzylic 7c-system. It cannot be used to estimate the influence of several substituents on the system under consideration. In this way it is, therefore, not possible to gain insight into the problem of captodative stabilization of a radical centre. The investigation of the spin-density distribution in benzylic radicals has been extended (Korth et al., 1987) to include multiple substitution patterns. Three types of benzylic radicals were considered a,p-disubsti-tuted a-methylbenzyl radicals [17], a-substituted p-methylbenzyl radicals [18] and a-substituted benzyl radicals [19]. In [17] and [18] the hyperfine coupling constants of the methyl hydrogens were used to determine the spin-density... 

In connection with the captodative effect, Riichardt (Zamkanei et al., 1983) has determined the BDE of the tertiary C—H bond in [20] and compared it with the tertiary bond in isobutane. He concludes that the stabilization of 12.8 kcal mol which he derives from this comparison falls 4kcal mol short of the value of 16.5 kcal mol which he calculates for the sum of the substituent effects for phenyl (9 kcal mol ), cyano- (5.5 kcal moP ) and methoxyl (1.5kcal mol ) groups. The latter values were derived from studies on C—C BDEs. Not even additivity of the substituent effects is observed. The existence of a captodative stabilization of radical [21] is denied (see, however, the studies on the thermolysis of [24]). 

Bordwell (Bordwell and Bausch, 1986) has developed a method to determine C—H BDEs from a combination of p ha values and oxidation potentials (E ) of the corresponding anions in dimethyl sulphoxide solution. These acidity-oxidation potentials (AOP) are taken as measures for BDEs and are related to the stabilization of the radicals formed. This procedure has been recently applied to the subject of captodative stabilization (Bord-well and Lynch, 1989). Values of ABZ) relative to the C—H BDE in methane are calculated according to (13). These values are set equal to the... 

A last example concerns the rotational barrier in phenoxyl radicals (Gilbert et ah, 1988). Compared to the parent phenols [37] and [39] the rotational barrier in [38] is increased by a factor of seven, whereas, with a captor substituent [40], the barrier increases only by a factor of 1.2. This could be interpreted in terms of a captodative stabilization in [38]. The captodative character of the radical [38] is represented by a resonance structure [41]. 

This anomeric stabilization of radicals is also observed using halonitrosugars such as 1-C-nitroglycosyl halides [22] 15. Captodative stabilization of the alcoxy nitro radicals explains the radical-chain substitution with mild nucleophiles such as ma-lonate or nitroalkane anions to form 16 (Scheme 8). 

Compared with other 1,1-disubstituted olefins, the captodative olefins do not seem to present abnormal behavior that could be related to the captodative stabilization of the transient growing radical. Both conversions and molecular weights can be rationalized by classical theories linking the polymerizability of these olefins to steric and dipole-dipole repulsions. 

The cycloaddition reactions of captodative olefins all are considered to proceed through the intermediacy of a 1,4-diradical, due to the captodative stabilization of the terminal radicals. In cross-cycloadditions captodative olefins easily give cyclobutanes when heated with fluoroolefins [141]. They also react with allenes to give methylenecyclobutanes [142], and with methylenecyclopropane to give spiro[2.3]hexanes [143]. 

The reaction with captodative stabilized radicals is also possible [82]. For instance, the preparation of 1-hydroxycyclopentanecarboxylic acid derivatives from a chiral glycolic acid equivalent is reported [83]. Cyclization and annula-tion procedures are depicted in Eqs. 32 and 33, respectively. 

The aforementioned analysis makes it evident that no special electronic effects (captodative stabilization, spin polarization, etc.) play a significant role in the localized triplet diradicals 9-11. Nevertheless, the question arises whether the electronic substituent effects in such triplet diradicals are additive, that is, whether the substituents on the two aryl groups act independently of one another. This feature was tested by comparison of the AD values of the unsymmetrical monosubstituted triplet diradicals 9 with those of the corresponding symmetrical disubstituted derivatives 10. Indeed, a linear correlation (Fig. 8) with a slope of nearly one-half (m = 0.558) unequivocally establishes that the electronic effects on the D parameter of localized triplet diradicals are additive [7], Thus, the localized triplet diradicals 9-11 may be considered to be electronically a composite of two independent (except for dipolar interactions) cumyl-type monoradicals, embedded in the molecular framework of the planar cyclopentane- 1,3-diyl ring system. This fact allows us to assess electronic substituent effects on the cumyl monoradicals 14 from the experimentally determined D parameter of the cyclopentane-1,3-diyl triplet diradicals 9-11 through the changes of the spin densities. 

Radical mechanisms are also known to be involved in 1,2-rearrangement reactions, such as the R alkyl group migration leading from ammonium ylids 16 to tertiary amines 19 (Scheme 3). Whilst the possible ion pair intermediate 17 is destabilized by the electron-withdrawing carbonyl group, the radical pair 18 is favored by the captodative stabilization of one of the radicals. 

In the cyclization reactions of homoallylic ethers and higher homologs, alkoxymethyl radicals play a prominent role [26]. Substrates like 28 supply more useful carbon-centered radicals enjoying captodative stabilization, and trany-2,3-disubstituted tet-rahydropyrans were prepared stereoselectively with judicious modification of the double bond in the substrate [27] (Scheme 12). The same radical species from the substrate 30 was reported to give the oxocane 31 via the xanthate transfer %-endo cyclization, from which lauthisan (32) was obtained [28] (Scheme 13). 

The Leroy analysis has been applied to captodative systems. Table 3.18 shows the radical RSE calculated for some captodative radicals. An interesting aspect of the data is the failure of cyano groups to provide captodative stabilization. A significant consequence of the captodative effect is that the C—H bond of a-amino derivatives of acids, esters, aldehydes, and ketones are expected to be very significantly weakened. 

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