Phenyl group contrasted with benzyl

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

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 3-benzyl-6-phenyl-l,4-diazepine-2,5-diones 21 and 22 exhibited complex NMR spectra indicative of limited conformational mobility in which the ring geometry is dictated by the two air-amide elements, which define individual planes <2003MI187>. Based on an analysis of the nuclear Overhauser effect (NOE) between protons on the ring, the preferred boat conformation in solution projects the 3-benzyl moiety pseudoequatorially with the 6-phenyl substituent disposed axially or equatorially, dependent upon the relative stereochemistry. This conformation is also observed in the solid state for the air-substituted isomer 21 in which the phenyl group is axial. In contrast, the bis-phenyl derivative 23 is conformationally mobile based on the H NMR spectmm where resonances were not resolved. 

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). 

It was envisaged that chiral PTC should play a crucial role in achieving an efficient chirality transfer hence, an examination was conducted of the alkylation of the dipeptide, Gly-L-Phe derivative 47 (Table 11.5). When a mixture of 47 and tetrabutylammonium bromide (TBAB 2mol%) in toluene was treated with 50% KOH aqueous solution and benzyl bromide at 0°C for 4h, the corresponding benzylation product 48 was obtained in 85% yield with the diastereomeric ratio (DL-48 LL-48) of 54 46 (8% diastereomeric excess (de)) (entry 1). In contrast, the reaction with chiral quaternary ammonium bromide (S,S)-9g possessing a 3,5-bis(3,5-di-tert-butylphenyl)phenyl group under similar conditions reahzed almost complete diastereocontrol (entries 4—6) [45]. 

In contrast to many furan syntheses, this method uses 2-(2-hydroxyalkyl)phenoIs containing a leaving group at the benzylic carbon atom. Thus, 2-(I-isopropyl-thio-2-hydroxy)phenol cyclizes on heating with concentrated hydrochloric acid, but when 2-(2-hydroxyethyl)phenol was similarly treated, cyclization did not occur. When R = H, 2-unsubstituted benzofurans are obtained in good yield minor modifications were made in the synthesis of the 2-phenyl derivatives, and... 

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). 

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. 

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