Phenyl, contrasted with benzyl

The rearrangement of furfuryl benzenesulfinate (1) appears of special interest. In contrast with the corresponding benzyl ester, this sulfinate was found to undergo a facile rearrangement to sulfone. Furthermore, in nonhydroxylic solvents a mixture of furfuryl phenyl sulfone (2) and 2-methyl-3-furyl phenyl sulfone (3) is obtained (equation 6)40. 

Steric factors play a determining role in directing the course of ring closure. They can be invoked to rationalize cyclialkylation of the 30 arylalkene177 [Eq. (5.39)]. The less severe 1,4 interaction between the benzyl and methyl groups during cycli-zation contrasted with the 1,3 interaction between the ethyl and phenyl groups ensures the predominant formation of 31 ... 

The 4 -monophosphate was also prepared from the same compound as above (Figure 5). Although the 4 -hydroxyl group in 12 has rather low reactivity, its phosphorylation could be performed with phenyl phosphate and dicyclohexylcarbodiimide (DCC) in pyridine. The reaction product was converted into the benzyl phenyl ester ( ) to facilitate purification. In contrast to the above dibenzyl ester of the glycosyl phosphate, the benzyl phenyl ester of the 4 -phosphate was stable and could be purified by silica gel column chromatography after removal of the propenyl group without decomposition. Hydrogenolytic deprotection of (first with Pd-black then with PtC>2) afforded the 4 -monophosphate 17. 

The molecular structure of decabenzylferrocene also helped to explain the rather low solubility of this compound in toluene or benzene, in sharp contrast with the previously found excellent solubility properties of other decabenzylmetallocenes and the pentabenzyl-Cp thallium and indium complexes in these solvents. The steric crowding of the benzyl groups in the ferrocene and their limited mobility leave little space for the solvent arene molecules to interact and subsequently to dissolve the molecule. This is quite similar to the situation encountered in the pentaphenylcyclopenta-dienyl derivatives with their closely packed phenyl rings (see Fig. 3). 

The mechanism of the photochemically induced conversion of vinylazides into 2H-azirines has been examined using ab initio MO calculations. A new approach to the elusive thionitrosobenzene system has been reported. Direct irradiation of the 3-azido-2,l-benzisothiazole (88) affords a transient 2-cyanothionitrosobenzene (89) which can be trapped as the cycloadduct (90) with cyclopentadiene. Nitrene cyclization is the preferred pathway on irradiation of the 5-formyl- or 5-benzoyl-6-azidouracils (91) and yields the isoxazolo[3,4-d]pyrimidines (92). In contrast, 5-phenyl- and 5-benzyl-6-azidouracils were converted into pyrimido[4,5-b]indoles and pyrimido[4,5-ti Jquinolines, respectively, by a pathway involving photochemically induced loss of nitrogen and intramolecular nitrene insertion. 

As shown in Figure 11.11, there is a strong correlation between the ratio and radical stability, as measure by the difference in the enthalpy of formation. The outlying value for neopentyl can be improved by a correction for steric strain. On the other hand, the rate of decomposition of the peroxy ester is nearly independent of the nature of R, even for the stabilized benzyl or destabilized phenyl cases.This is in marked contrast to a strong dependence on the structure of the acyl group (see above) and indicates that the fragmentation of the alkoxy radical is not concerted with the peroxy bond cleavage, but must be a separate step. 

If, in silane, one hydrogen is replaced by a phenyl group, the silicon-hydrogen bond dissociation enthalpy is only reduced by about 9 7 kJ/mol (19). This is in sharp contrast with the decrease observed for the carbon analogues, where the large stabilization energy of benzyl radical makes D(Me-H)-D(PhCH -H) = 71 4 kJ/mol. The effect of a second phenyl group in the hydrocarbon family is also known, D(Ph CH-H) = 340 8 kJ/mol (27,28) and there is a recent estimate for D(Ph C-H), 338 13 kJ/mol (28). 

Thus, in contrast to an ionization process from a neutral substrate, which initially generates an intimate ion pair, deamination reactions generate a cation which does not have an anion closely associated with it. The stereochemistry of substitution is shown for four representative systems in Table 5.15. Displacement on the primary 1-butyl system is much less stereospecific than the 100% inversion observed on acetolysis of the corresponding brosylate (entry 1, Table 5.14). Similarly, the 2-butyl diazonium ion affords 2-butyl acetate with only 28% net inversion of configuration. Small net retention is seen in the deamination of 1-phenylethylamine. The tertiary benzylic amine 2-phenyl-2-butylamine reacts with 24% net retention. These results indicate that the lifetime of the carbocation is so short that a symmetrically solvated state is not reached. Instead, the composition of the product is determined by a nonselective collapse of the solvent shell. 

By contrast with PMMA, poly(methyl acrylate), PMA, and several other aliphatic polyacrylates were found by Shultz and Bovey (240) to imdergo cross-linking and gel formation on irradiation with 1-MeV electron beams. They reported G(S) = 0.15 and G(X) = 0.52 for PMA. Graham (241,242) reported that the phenyl, benzyl, and 2-phenyl ethyl acrylate polymers also undergo cross-linking on y radiolysis imder vacuum. There was evidence of side-chain scission also, and according to Fox and co-workers (243,244) the major volatile products were similar to those observed for PMMA. 

The first example of the ruthenium-catalyzed synthesis of amides from alcohols and amines was reported by Murahashi et al. in 1991 [82aj. The contrast results were obtained from the RuH2(PPh3)4-catalyzed reaction of 5-aminopentanol. Thus, piperidine was obtained in 79% yield, while similar treatment in the presence of a hydrogen acceptor of l-phenyl-l-buten-3-one gave piperidone in 65% yield (Eq. (7.36)). Recently, Williams reported the intermolecular amidation reaction of benzyl alcohols with amines in the presence of [Ru(p-cymene)Cl2]2 and 3-methyl-2-butanone [82bj. 

In contrast, butyraldazine 446 reacts with phenyl or benzyl isocyanate, in the presence of picric acid, to give the pyrazoline derivatives 447 and 448. The isocyanate attack is... 

Nitrosamino-3-phenyl-l,2,4-thiodiazole and the corresponding 3-methyl derivative (57, R = Ph,Me) give with diazomethane, 5-(mcthylnitrosamino) derivatives (58) From 5-nitrosamino-3-benzyl-l,2,4-thiodiazole (57, R = PI1CH2), by contrast, some 5-nitrosimino-4-methyl-3-benzyl-l,2,4-thiodiazoline (59) is also pro-... 

In contrast, A, JV-diethyl-3-(methylsulfanyl)-5-phenyl-3//-azepin-2-amine (22) on treatment with methyl or benzyl halides and potassium amide in liquid ammonia undergoes alkylation solely at the masked benzylic 5-position to yield 4,4-disubstituted 4//-azepines, e.g. 23.224... 

In contrast, when chromium atoms were reacted with benzyl ether, complexation to only one phenyl group of the ether was observed. 

If one of the species is anionic and we need to transport it to the organic phase, then a phase-transfer catalyst may be employed. Consider the example of benzyl penicillin where the reaction between phenyl acetic acid and the penicillin carboxylate ion, with penicillin amidase as a catalyst, is relevant, and which at pH 4.5 - 5.0 is shifted in the desired direction. Here a catalyst like tetrabutylammonium halide works, and with chloroform as a solvent 60% yield can be realized in contrast to a yield of only 5 - 10 % in water. 

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