Monosubstituted methyl radicals

Table 1 Radical stabilization energies (in kj/mol) of monosubstituted methyl radicals at 0 K according to Eq. 1... 

Radical stabilization energies calculated according to Equation 5.3 at 298.15 K have been compiled in Table 5.4 for a variety of monosubstituted methyl radicals at either G3(MP2)-RAD or higher level. In those cases in which results are available at more than one theoretical level, good agreement between these results is usually found. Overall, the mean absolute deviation of the G3(MP2)-RAD values from experiment is... 

TABLE 5.4 RSEs for Monosubstituted Methyl Radicals at 298.15 K Calculated According to Equation 5.3 Together with BDE(C-H) Energies for Selected Systems (All in kj/mol)... 

Methylation of aromatics. Aromatic substitutions effected by radicals are of more theoretical than preparative interest because the yields are low. Waters and co-workers" studied the methylation of monosubstituted benzenes with methyl radicals generated from di-f-butyl peroxide, separated the mixture of monomethyl... 

Some new reactions involving intermolecular reactions of fluoroaryl-substituted stabilized carbanions are summarized in Schemes 7—10. Studies on monosubstituted diethyl malonates have shown that the effect of a penta-fluorophenyl group on the acidity of the C—bond in this system is comparable to the effect of a 2- or 4-nitrophenyl group. Tris(polyfluoroaryl)methyl radicals have also (c/. Scheme 10) been generated by treatment with chromium(n) perchlorate of the corresponding cations generated from tris(polyfluoroaryl)carbinols and trifluoroacetic acid. ... 

Disubstitution products are obtained when dihalobenzenes (Cl, Br, I) react with aliphatic ketone enolate anions. Conversely, the reactions of o-iodohalobenzenes (X = I, Br, Cl) with the enolate anions of aromatic ketones, such as acetophenone, propiophenone and 2-naphthyl methyl ketone in DMSO yield mainly monosubstitution with the retention of one halogen (Scheme 10.7). The extent of dehalogenation is explained in terms of the energetics of the intramolecular ET from the ArCO-7t-system to the C—X bond in the monosubstituted radical anions proposed as intermediates [19]. 

The photostimulated reaction of 1,8-diiodonaphthalene with p-methyl-benzenethio-late ions in DMSO yields the substituted cyclized product 10-methyl-7-thia-benzo[de] anthracene (31) in moderate yield (Scheme 10.58) [54], The mechanism proposed to explain product 31 involves an intramolecular radical cyclization after monosubstitution in the propagation cycle of the SRN1 process. 

Divalent state stabilization energies are not easy to come by, as they require knowledge of both the first and second BDE, and the reactive intermediates MR2 are not trivially characterized. Quantum mechanical studies are certainly ahead of experiment in this area, and we can combine the results of two separate studies, one by Coolidge and Borden (109) and the other by Luke et al. (88), to assemble a small list of DSSEs for monosubstituted carbenes and silylenes. Specifically, Coolidge and Borden determined the effects of substituents, X, on the stability of methyl and silyl radicals through determination of the heat of reaction... 

The rates of radical-monomer reactions are also dependent on considerations of steric effects. It is observed that most common 1,1-disubstituted monomers — for example, isobutylene, methyl methacrylate and methacrylo-nitrile—react quite readily in both homo- and copolymerizations. On the other hand, 1,2-disubstituted vinyl monomers exhibit a reluctance to ho-mopolymerize, but they do, however, add quite readily to monosubstituted, and perhaps 1,1-disubstituted monomers. A well-known example is styrene (Ml) and maleic anhydride (M2), which copolymerize with r — 0.01 and T2 = 0 at 60°C, forming a 50/50 alternating copolymer over a wide range of monomer feed compositions. This behavior seems to be a consequence of steric hindrance. Calculation of A i2 values for the reactions of various chloroethylenes with radicals of monosubstituted monomers such as styrene, acrylonitrile, and vinyl acetate shows that the effect of a second substituent on monomer reactivity is approximately additive when both substituents are in the 1- or cr-position, but a second substituent when in the 2- or /3-position of the monomer results in a decrease in reactivity due to steric hindrance between it and the polymer radical to which it is adding. 

The copolymerizations of 2-methyl-1-propene (isobutylene) and acrylamides 13a f were studied in Lewis acid-promoted copolymerizations (Fig. 8). Although 13a-f can be homopolymerized in the presence of Lewis acids, poor conversions are obtained except with 13a. Presumably, complexation renders the radical and monomer too electron-deficient to react efficiently. This effect, however, should enhance the reactivity of the complexed radical toward more electron-rich alkenes and has been observed to increase the alternating character of copolymers of isobutylene and methyl acrylate [9], Isobutylene also is an ideal choice for a comonomer as it does not homopolymerize by radical pathways, and the analysis of the copolymer s tacticity is not complicated by additional stereocenters as would be the case with monosubstituted vinyl comonomers. 

Calix[n]arene derivatives with all the methylene bridges monosubstituted by bromine atoms (that will be referred as bromocalixarenes ) have been prepared via radical or photochemical reaction of the corresponding calix[/i]arene methyl ethers with NBS in a chlorinated solvent (usually CCI4, Eq. 4.19).For 11 [27], 27a [28], 27b [29,30], and the methyl-ether derivative of de-tert-butylated calix[4]arene [31], the main product is the all-cis isomer, as corroborated by X-ray crystallography. The photochemical bromination of 27c yields an achiral isomer with bilateral symmetry of the octabromo derivative as the kinetic product, that upon standing in chloroform isomerizes to an equilibrium mixture of isomers [32, 33]. From the equilibrium mixture, a single isomer was isolated via trituration with cold isopropanol [32]. 

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