Disubstituted methyl radicals, RSEs

In a recent investigation, the photostimulated reaction of methyl 2,5-dichlorobenzoate 46 (R=H) or methyl 3,6-dichloro-2-methoxybenzoate 71 (R = OMe) with PhS ions in excess in liquid ammonia was studied (Eq. 10.27). Compound 46 afforded mainly the product 72 (R=H, 88%) with retention of the chlorine, whereas 71 yielded the disubstitution product 73 (R=OMe) with 80% yield. The different reactivity observed was explained on the basis of the activation energy necessary for the from the tr to the a of the C—Cl bond and the fragmentation of the radical anion intermediates [65] ... 

Steric effects similar to those in radical copolymerization are also operative in cationic copolymerizations. Table 6-9 shows the effect of methyl substituents in the a- and 11-positions of styrene. Reactivity is increased by the a-methyl substituent because of its electron-donating power. The decreased reactivity of P-methylstyrene relative to styrene indicates that the steric effect of the P-substituent outweighs its polar effect of increasing the electron density on the double bond. Furthermore, the tranx-fl-methylstyrene appears to be more reactive than the cis isomer, although the difference is much less than in radical copolymerization (Sec. 6-3b-2). It is worth noting that 1,2-disubstituted alkenes have finite r values in cationic copolymerization compared to the values of zero in radical copolymerization (Table 6-2). There is a tendency for 1,2-disubstituted alkenes to self-propagate in cationic copolymerization, although this tendency is low in the radical reaction. 

Since radical polymerizations are generally carried out at moderately high temperatures, most of the resulting polymers are highly atactic. This does not mean that there is a complete absence of syndiotacticity. There is a considerable difference in the extent of syndiotacticity from one polymer to another. Thus, methyl methacrylate has a much greater tendency toward syndiotactic placement than vinyl chloride. Whereas the poly(vinyl chloride) produced at the usual commerical polymerization temperature ( 60°C) is essential completely atactic, that is, (r) (m) 0.5, this is not the case for poly(methyl methacrylate). The polymerization of MMA, usually carried out at temperatures up to 100°C, yields polymers with appreciable syndiotacticity—(r) is 0.73 at 100°C. The difference is a consequence of the fact that MMA is a 1,1-disubstituted ethylene, leading to greater repulsions between substituents in adjacent monomer units. 

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. 

In contrast to most other oxygen-centered radicaLs e.g. benzoyloxy (3.4.2.2.1), hydroxy (3.4.2.3)1, t-butoxy radicals and other r-alkoxy radicals (3.4.2.1.2) show relatively high regiospecificity in reactions with carbon-carbon double bonds (Table 3.8). Nonetheless, significant amounts of head addition are observed with the halo-olcfins, simple alkcncs, vinyl acetate and methyl acrylate. " Head addition is generally not observed with 1,1-disubstituted monomers. The exception is vinylidene niioride" where head addition predominates (Section... 

The absolute configuration, i.e. S,S), of a tetrahydropyran (17) that is present in civet Viverra civetta) has been determined, using a chiral shift reagent [Eu(hfc)3] and 360 MHz n.m.r. spectroscopy in comparison with a synthetic sample of (+)-(5,5)-(17) and its methyl ester.X-Ray analysis was applied to the determination of the conformation of the violet form of cunaniol acetate (18), which was shown to have an undistorted chair form with both substituents equatorial. An e.p.r. spectral study has shown that the radical which is formed from several stereoisomeric 2,4-disubstituted tetrahydropyrans is the same, namely the cis-2-alkoxy-4-methyltetrahydropyran-2-yl radical. 

Attack of the 1,2,3-triazine system by an organic radical is used in the introduction of a carboxamide function at C-5 by the treatment of 4,6-disubstituted l,2,3-triazin-2-ium 2-dicyanomethylides 34 (R = H R, R = Me, Ph, Et) with formamide in the presence of ammonium persulfate at 80 °C. Under these conditions, the 1,2,3-triazine is restored by loss of dicyanomethyl radical. The corresponding 2-methyl-l,2,3-triazin-2-ium iodides gave the demethylated... 

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