Radicals substituents

Despite some recent discoveries, free radical reactions are still very much less common in azole chemistry than those involving electrophilic or nucleophilic reagents. In some reactions involving free radicals, substituents have little orienting effect however, rather selective radical reactions are now known. 

A clear demonstration of the relative importance of steric and resonance factors in radical additions to carbon-carbon double bonds can be found by considering the effect of (non-polar) substituents on the rate of attack of (nonpolar) radicals. Substituents on the double bond strongly retard addition at the substituted carbon while leaving the rate of addition to the other end essentially unaffected (for example, Table 1.3). This is in keeping with expectation if steric factors determine the regiospeeificity of addition, but contrary to expectation if resonance factors are dominant. 

Free radical substituent effects have also been probed through the kinetics of rearrangement of 3-aryl-2,2-dimethylmethylenecyclopropanes 40 to 41252. The reaction... 

Fig. 10.12. Frontier orbital interpretation of radical substituent effects.

Similarly to the triphenylmethyl system, captodative-substituted 1,5-hexa-dienes, which can be cleaved thermally in solution into the corresponding substituted allyl radicals [15], dissociate more easily than dicaptor-substituted systems (Van Hoecke et al., 1986). Since ground-state and radical substituent effects cannot be separated cleanly, not only because of electronic but also because of steric effects, a conclusive answer cannot be provided. 

In a similar manner, p-bis(9-anthryl) phenylene gives a mono(anion-radical) or a mono(cation-radical) under reductive or oxidative conditions with spin delocalization around the whole molecular framework. In the case of m-bis(9-anthryl) phenylene, reduction or oxidation leads to the formation of dianion or dication-diradicals. Based on ESR experiments at cryogenic temperatures (6.5-85 K), these species contain two separated ion-radical moieties. They have parallel aligmnent of their spins (Tukada 1994). The work gives clear experimental evidence for the so-called ferromagnetic interaction between these ion-radical substituents. In some cases, release of the electron depends on temperature. See, for example, the anion-radical shown in Scheme 3.63. 

Method a. The bridge is considered as a normal divalent radical substituent, e.g., ethylene (—CH2—CH2—), o-phenylene (o-C6H4), retaining its own numbering system (but with primes and starting at the lower-numbered bridgehead), e.g., 36a and 37a. 

The out-of-plane (oop) bending angle is in this context defined as the deviation of one of the radical substituents from the plane defined by the radical center and the other two substituents. 

The latter compound and its substituted derivatives readily react with polyvinyl acetate radicals. Substituents in the aromatic ring were found to influence the reactivity of the hydroxylamine slightly p = — 0-16 0-04 (Simonyi et al., 1967a). In comparing this value with those of phenols, the decreased substituent effect can be rationalized by considering two factors. First, the 0—H bond is more remote from the aromatic ring in hydroxylamine than in phenol. Second, the stability of phenyl nitroxide radicals is higher than that of phenoxy radicals. It is... 

Relative rates of the aziridination of styrene 397 versus a series of ra-substituted styrenes 398 furnishing the respective aziridines 401 and 402 have been determined using Tp Cu(C2H4) (Tp =hydridotris(3,5-dimethyl-l-pyr-azolyl)borate) 400 as the copper precatalyst in combination with PhI=NTs 399 as the nitrene source. The experimental data of the aziridination reaction can be fit with a two-term equation of the type log(>fx/ H) = -t-p cr" a are Jackson s radical substituent constants) leading to the values =—0.28 0.06 (polar contribution) and p = 0.34... 

Radical electronic constants are substituent constants derived from free-radical reactions. The most popular are the radical parameter defined on the basis of the radical abstraction of a-hydrogens of substituted cumenes [Yamamoto and Otsu, 1967] and the ffa radical substituent constants defined by the benzylic a-hydrogen hyperfine coupling constants [Wayner and Arnold, 1984]. 

Treatment of the phosphino-iminium salts (169) with potassium hydroxide in THF affords the formylphosphines (170), which are remarkably stable in solution compared with the related phosphine oxides Phosphines bearing aminoyl radical substituents, e.g. (171), have also been prepared. The phospha[3]triangulane, (172), has been obtained from the reaction of bicyclo-propylidene with a metal complexed phenylphosphinidene. " The phosphino-trithiacyclophane (173) has been prepared by the base-promoted reaction of... 

CT radical substituent constants electronic substituent constants (0 field/resonance effect separation)... 

E. The Use of the AD Parameter as Radical Substituent Constant (ffrad) in Linear Free Energy Relationships, 235... 

The AD parameter is a spectroscopic measure of radical stabilization through spin delocalization by aryl substituents. A priori there is no reason to justify their use as radical substituent constants (ffrad) in linear free energy relationships, since for the latter the correlation of kinetic (rate constants) or thermodynamic (equilibrium constants) and not of spectroscopic data is of primary interest. Nevertheless, the linear correlation with the calculated RSE (Fig. 13) and other established substituent constants (cf. Section III.D) strongly suggests that the AD parameter may be appropriately employed to assess electronic effects in benzyl-type radicals. 

We shall demonstrate this for Arnold s EPR-spectroscopic scale (ax, Table 3) [36, 39], which relates the radical substituent constant ((7rad) to the benzylic a spin densities (p) according to Eq. (17), in which px and p refer to the substituted and unsubstituted benzyl radicals. Since the a hyperfine coupling constants (a) are linearily related to the spin densities by the McConnell equation [36,38], that is, ax oc px (cf. Section III.B), the coupling constants may be used instead of the spin densities [Eq. (18)]. According to Eq. (8) (pA = pB for the symmetrically substituted cases), the D value is proportional to the square of the spin density and a trAD scale may be defined by taking the square root of the ratio of the D values instead... 

Indeed, a plot of the AD [Eq. (9)] versus the cAD [Eq. (19)] values follows an excellent correlation (r2 > 0.999) with the expected slope of 0.1. A typical deviation amounts to 0.0001cm-1 and is, thus, within experimental error. Consequently, the initially defined AD parameter [Eq. (9)], like Arnold s values, serves as radical substituent constant. 

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