Radical substitution, orientation

Finally attention must be drawn to the fact that the orienting effect of the nitro group in nucleophile and radical reactions usually differs from that in electrophilic reactions, and instead of meta orientation, ortho or para orientation takes place. The corresponding observations are referred to in chapters dealing with nucleophile and radical substitutions of nitro compounds (pp. 204, 207 and 212 respectively). 

Radical substitution reactions are less developed in azole chemistry than those involving electrophilic or nucleophilic reagents. In some reactions involving radicals, substituents have little orienting effect however, some rather selective radical reactions are now known. 

In radical substitution, an even AH is converted to an intermediate odd AH radical and in nucleophilic substitution to the corresponding anion. Since these differ from the corresponding electrophilic intermediate only in the number of nonbonding electrons, the energies of reaction should be the same in all three cases. The relative reactivities of different even AHs and the orientation of substitution in a given AH, should therefore be the same for all reagents. This is the case. Thus naphthalene substitutes mainly in the 1 position both with electrophiles (e.g., nitration) and with radicals (e.g., phenylation by phenyl radicals, as in the Gomberg reaction) and with nucleophiles (e.g., amination by sodamide, the Chichibabin reaction) i.e.. 

Halogenation. Liquid-phase monochlorination of ben2otrifluoride gives pronounced meta orientation (295) in contrast, vapor-phase halogenation favors para substitution (296). Sealed tube, photochemical, or dark chlorination (radical initiator) forms... 

Brown has also predicted, from localization energy calculations, that pyrrole and glyoxaline should react with radicals mainly at the 2-position, whereas pyrazole should be most reactive at the 3-position. Browm and Heffernan s calculation that the orientation in pyrimidine substitution should be 4 > 2 > 5 is in agreement with the results from the p-nitrophenylation of pyrimidine. ... 

The hydrostannation reaction can proceed either by a free-radical mechanism, or, with polar-substituted alkenes or alkynes, by a polar mechanism, respectively resulting in anti-Markownikoff or Markow-nikoff orientation. Both t3rpes of reaction are particularly suitable for preparing functionally substituted, organotin compounds. 

The most striking feature of these results is the orientation of the unique 13C hyperfine matrix axes, relative to those of the 57Fe hyperfine axes. This orientation led Fairhurst et al.41 to assign the spectrum to [Fe(CO)5] (2) and to describe the species as a substituted acyl radical. However, these authors did not discuss the orientation of the g-matrix axes. The y-axis, normal to the reflection plane, is common to all three matrices. The x- and z-axes of the g-matrix, however, are oriented about 21° away from the corresponding 57Fe hyperfine matrix axes. Since the iron d-orbital contribution to the SOMO appears to be nearly pure dz2, the 57Fe hyperfine matrix major axis must correspond to the local z-axis, assumed to be essentially the Fe-C bond. Thus we must ask Why are the g-matrix axes different The SOMO can be written ... 

Generated from diacetyl peroxide, methyl radicals attack 2-methylfuran at position 5 preferentially if both 2- and 5-positions are occupied as in 2,5-dimethylfuran there is still little or no attack at the 3(4)-position. If there is a choice of 2(5)-positions, as in 3-methylfuran, then that adjacent to the methyl substituent is selected.249 These orientation rules are very like those for electrophilic substitution, but are predicted for radical attack by calculations of superdelocalizability (Sr) by the simple HMO method. Radical bromination by IV-bromsuccinimide follows theory less closely, presumably because it does not occur through a pure radical-chain mechanism.249... 

Of course, any correlation between and the direction of, say, substitution does not prove that the reaction necessarily takes the ion-radical pathway. This means that the correlation may represent the relationship, for example, between the orientation and tendency of a substrate to locate a charge or between the electronic structure of the transition state and distribution of the spin density in the substrate ion-radical. Nevertheless, such correlation deserves to be considered it can serve as one, but not single and self-sufficient, proof in favor of the ion-radical pathway. 

The EPR spectra of the a-substituted 2-ethyl radical of 1,4-dioxane 60 (Scheme 24) were explained in terms of the presence of the radical group in both axial and equatorial orientations, the equilibrium being slow on the EPR timescale at room temperature (89MRC782). 

Taylor in 1925 demonstrated that hydrogen atoms generated by the mercury sensitized photodecomposition of hydrogen gas add to ethylene to form ethyl radicals, which were proposed to react with H2 to give the observed ethane and another hydrogen atom. Evidence that polymerization could occur by free radical reactions was found by Taylor and Jones in 1930, by the observation that ethyl radicals formed by the gas phase pyrolysis of diethylmercury or tetraethyllead initiated the polymerization of ethylene, and this process was extended to the solution phase by Cramer. The mechanism of equation (37) (with participation by a third body) was presented for the reaction, - which is in accord with current views, and the mechanism of equation (38) was shown for disproportionation. Staudinger in 1932 wrote a mechanism for free radical polymerization of styrene,but just as did Rice and Rice (equation 32), showed the radical attack on the most substituted carbon (anti-Markovnikov attack). The correct orientation was shown by Flory in 1937. In 1935, O.K. Rice and Sickman reported that ethylene polymerization was also induced by methyl radicals generated from thermolysis of azomethane. 

Since words sometimes hide meaning, we have to be careful that we don t substitute words wfiich don t mean much for ideas. I am very troubled at the moment over what Dr. Heck meant by acid base, how electron density in the olefin could lead to the orientation he mentions. I would like to ask whether or not he can explain this. Finally, if this proves difficult, has he thought about radical processes in this type of thing ... 

Perhaps it should be mentioned also the orientation of the Birch reduction which is strongly dependent on the nature of the aromatic substituents. Donor-substituted benzenes furnish predominantly 1-substituted 1,4-cyclohexadienes while acceptor-substituted analogues give 3-substituted 1,4-cyclohexadienes. The regioselectivities can be explained by the destabilizing d-d pairing in the intermediates from d-substi-tuted cyclohexadienyl radical anions leading to the 3-substituted products, and the... 

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