Radical phenylative

The radical phenylation of a large number of mono- and dialkyl-thiazoles has been investigated (393,395,396,399-405, for a general review cf. 398) and analyzed in terms of partial rate factors. As in other instances the alkyl groups slightly activate the substrate in certain positions toward phenyl radicals, but they also induce some steric hindrance to the approach of the aryl radical from the onho positions (Fig. 1-19). 

Fig, H8. (a) Partial rale factors of free radical phenylation, relative to benzene (397). (b) Free valence calculated by HMO method (117). (c) Radical localization energy (in units) calculated by HMO method (117). 

Fig. 1-19. Partial rate factors of free radical phenylation relative to benzene (398).

Radical phenylation of iV-substituted pyrazoles has been studied by Lynch et al. (b-76MI40402). The most significant results are shown in Figure 24. 

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

The first step of a free radical aromatic substitution, the formation of the a-com-plex, is also an addition step. The o,m,p-product ratio therefore also responds to steric effects. This is shown for the free radical phenylation and dimethylamination of toluene and r.-butylbenzene in Table 8. The larger the substituent on the aromatic system and the bulkier the attacking radical, the more p-substitution product is obtained at the expense of o-substitution. In the phenylation reaction the yield of m-product also increases in contrast to the dimethylamination reaction. The substitution pattern of this latter reaction is, in addition to the steric effect, governed heavily by polar effects because a radical cation is the attacking species113. ... 

Another important ESR method of NO detection is based on the interaction of NO with the stable radical phenyl-4,4,5,5-tetramethylimidazoline-l-oxyl (PTIO) and its derivatives carboxy-PTIO or trimethylammonio-PTIO [100]. It is interesting that in this assay NO reduces stable free radical PTIO to another stable free radical PTI and the N02 radical, and the reaction can be monitored by both a decrease in the ESR PTIO spectrum and an increase in the ESR PTI spectrum [101]. 

As for radical substitutions in compounds XV, XVII, XXV, and some other compounds, the F values (hence also Ar and Sr values, cf. section V, A) correctly predict the experimental reactivity order. The calculated and experimental orders disagree in the case of compounds XXI and, particularly, XVI the latter case (radical phenylation of quinoline) represents a serious failure of the theory, for the experimental study was very thorough.160 It is worth noting that in the compounds which have no meso-position the center of radical reactivity is the position adjacent to the nitrogen atom (with the exception of the just mentioned phenylation of quinoline). 

The free radical phenylation experiments indicated that the order of reactivity is (3) > thiophene > (7). Calculations on all the three classical thienothiophenes, however, had predicted a different order of reactivity, viz. (8) > thiophene > (3) > (7). The most reactive positions were also expected to be the a-positions (76AHC(19)123). 

Our kinetic work (10) showed that the small molecule radical produced by chain transfer with monomer had to be a stable radical. This was confirmed in the present paper by analysis of the isotope effect on the bulk polymerization rates. The isotope effect on molecular weights and rates unequivocally showed that almost 100% of the chain transfer involved the vinyl hydrogen. There is some evidence in the literature to support the idea of a stable vinyl radical. Phenyl acetylene acts as a retarder when copolymerized with styrene or methyl methacrylate (25). Thus the phenyl vinyl radical is very stable compared to the growing styryl or methacrylyl radical. 

For a disscussion of free radical phenylation of 1-phenyl pyrazole by benzoyl peroxide, see Lynch el al. 6430... 

Partial rate factors relative to benzene for free radical phenylation Free valence calculated by HMO method ... 

Aryl radicals. Phenyl radicals attack azoles unselectively to form a mixture of phenylated products. Relative rates and partial rate factors are given in Table 8. The phenyl radicals can be prepared from PhN(NO) COMe, Pb(OCOPh)2, (PhC02)2, or PhI(OCOPh)2. 

Vinyltins were used for synthetic purposes in radical addition-elimination sequences. The main limitation comes from the necessity to functionalize the olefin by groups such as esters able to stabilize the transient carbon-centred radical . Phenyl-substituted systems proved to be reactive as well, whereas methyl- and cyclohexenyl-substituted ones failed to react . An intramolecular version was developed giving access to methylene cyclopentane units (equation 44). ... 

This amino acid, as its name indicates, is the phenyl derivative of alanine. The radical phenyl, (CeHo—), is from the hydrocarbon benzene, CeHe. 

Another interesting feature related to the a nature of the cyclopropyl and vinyl radicals is their reactivity. In general, a radicals are more reactive and less selective than n radicals. Phenyl, vinyl, and cyclopropyl a radicals will abstract hydrogen atoms from saturated hydrocarbons at 77 K, conditions under which k radicals are unreactive [23]. In Ruchardt s [24] radical reactivity classification, based on the reaction of a series of o and n radicals with BrCClj and CCl, the cyclopropyl radical, an inverting a radical in an sp -hybridized orbital, closely resembled the noninverting phenyl radical. Further support for this finding... 

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