Benzyl-type radical, structure

High energy radiation generates ionic and free radical intermediates in styrene, and both ionic and radical polymerizations take place simultaneously. Therefore, the observation of neutral radicals related to the radical polymerization had been expected. The absorption of radicals with a maximum at 320-330 nm was actually observed in the pulsed styrene and a-methylstyrene it decayed over a time interval of several hundred microseconds without being affected by the presence of water [12 14]. This absorption was attributed for the main part to a radical with a benzyl-type structure. The similar absorptions were observed in the cyclohexane solutions [21], Swallow suggested that this intermediate would be formed by an ionic process, probably by the protonation of a... 

Fig. 3. (a) The ESR spectra obtained upon UV irradiation of aqueous H2O2 in the presence of p-toluene sulfonic acid at pH 0.4 (al, identified as the benzyl radical), pH 4.8 (a2, cyclohexa-dienyl radical), and pH 11.5 (a3, a superposition of spectra from three phenoxyl type radicals). Note the different scales. Arrows marked d, t, and q indicate doublet, triplef and quartet spUtt-tings, respectively. The corresponding radical structures are shown in (b). The pH dependence of total signal amplitudes (obtained by integration of an isolated line and then scaled to represent the entire spectrum) are given in (c) for the benzyl radical ( ), the cyclohexadienyl radical (O), and the sum of the three phenoxyl radicals (A). (From Ref. 37 with permission.)... 

The same is true for decarbonylation of acyl radicals. The rates of decarbonylation have been measured over a very wide range of structural types. There is a very strong dependence of the rate on the stability of the radical that results from decarbonylation. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s . ... 

Trisubstituted carbon-centred radicals chemically appear planar as depicted in the TT-type structure 1. However, spectroscopic studies have shown that planarity holds only for methyl, which has a very shallow well for inversion with a planar energy minimum, and for delocalized radical centres like allyl or benzyl. Ethyl, isopropyl, tert-butyl and all the like have double minima for inversion but the barrier is only about 300-500 cal, so that inversion is very fast even at low temperatures. Moreover, carbon-centred radicals with electronegative substituents like alkoxyl or fluorine reinforce the non-planarity, the effect being accumulative for multi-substitutions. This is ascribed to no bonds between n electrons on the heteroatom and the bond to another substituent. The degree of bending is also increased by ring strain like in cyclopropyl and oxiranyl radicals, whereas the disubstituted carbon-centred species like vinyl or acyl are bent a radicals [21]. 

Generally, allylic and benzylic fluorides are more easily reduced than ordinary alkyl fluorides. The yields are also strongly dependent upon the method used, but some aliphatic structures give only low yields of hydrodefluorination products for several methods. In radical-type re-... 

Much of Janzen s criticism has been justified the spin distribution in benzyl radicals themselves shows little substituent dependence44 dimeth-ylanilinium radicals, by Walter s structural criterion of type S, have proved to be of type 045 the original data on triarylaminium and triarylmethyl radicals, used to exemplify type S, have proved ambiguous45-47 authenticated S type behavior has been found only in a small group of acyclic aminyl... 

It has been established [1, 44, 45] that the degradation of PPO based on phenol, o-cresol and 2,6-dimethylphenol depends on both the structure of the repeat unit and the molecular mass. The thermal degradation of these PPO under vacuum proceeds with the release of both gaseous (hydrogen, carbon monoxide, carbon dioxide, methane, methyl chloride and ethane) and non-volatile products are aromatic compounds of the type Ar-O-Ar. It has been suggested that the thermal degradation of PPO may proceed with the formation of benzyl radicals, namely ... 

The synthesis pathway started with the lithiation of ethylbenzene 121 at the benzylic position, followed by acylation of the toluate anirni intermediate at low temperature. It is noteworthy that a potentially competing orf/io-lithiati(Mi of the type championed by Snieckus 85) i.e. between the two stabilizing methoxyl radicals) was not reported under these conditions. Subsequent reduction of benzyUcetone 122 provided smooth access to the t/irco-dimethyl-substituted bicy-clic intermediate 123 via lactonization. DIBAL reduction (—> 124) and reductive debenzylation with palladium on charcoal gave the ring-opened alcohol 125, which was further demethylated to provide a 1,3-diphenol, and then carboxylated under buffered conditions to yield acid 117, also known as phenol B . This compound was formylated with trimethyl orthoformate and acid, then cyclized to give the quinone structure and natural product, 116 (Scheme 3.1). 

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