Phenyl factor

The widespread occurrence of these ligands is not fortuitous, but reflects the folklore of laboratories undertaking synthesis, where it is appreciated that compounds with phenyl substituents and aromatic peripheries often have lesser solubilities in synthetic reaction mixtures, and therefore crystallize. It is also common but undocumented knowledge that compounds with phenyl substituents frequently form high-quality crystals. The prevalence of phenylated ligands and aromatic peripheries and the influences of specific phenyl-phenyl interactions in supramolecular chemistry are dubbed the phenyl factor. 

The paradigm is clear in molecules subject to the phenyl factor, the crystal field energy is large and dominant. The classic organic example where the crystal field dominates molecular structure is biphenyl, where the isolated molecule has its rings twisted by 42 but the crystal lattice imposes coplanarity it is estimated that the intramolecular strain in the coplanar molecule is 8.3kJmol", while the crystal field is 19.4kJ mol" [52c]. 

Specifically directional attractive interactions commonly occur between phenyl groups and other unsubstituted aromatic (and heterocyclic) groups. These interactions, dubbed the phenyl factor, are ubiquitous and significant in coordination compounds and organometallic molecules. 

The 7t-7i and n-cj interactions, which are also called phenyl factors in some systems, occurs between aromatic n-systems. The interaction is the result of a dispersion interaction and a columbic polarization. In energy terms,... 

The nitration of nitro- and dinitro-biphenyls has been examined by several workers. i - As would be expected, nitration of the nitro-biphenyls occurs in the phenyl ring. Like a phenyl group, a nitrophenyl group is 0 -directing, but like certain substituents of the type CH CHA ( 9.1.6) it is, except in the case of w-nitrophenyl, deactivating. Partial rate factors for the nitration at o °C of biphenyl and the nitro-biphenyls with solutions prepared from nitric acid and acetic anhydride are given below. The high o p-v2X o found for nitration of biphenyl... 

The nitration of phenylpyridines and related compounds has attracted attention for a long time, and measurements of isomer proportions have been made for several compounds of this type. Nitration occurs in the phenyl ring. For 2-phenylpyridine and 2-phenylpyridine i-oxide measurements of the dependence of rate of nitration upon acidity in 75-81 % sulphuric acid at 25 °C show that both compounds are nitrated as their cations (table 8.1). The isomer distribution did not depend significantly upon the acidity, and by comparison with the kinetic data for quinolinium ( 10.4.2) the partial rate factors illustrated below were obtained.They should be compared with those for the nitration of 2-nitrobiphenyl ( 10.1). The protonated heterocyclic groups are much... 

Acrylonitrile reacts with the sodium salt of 4.5-dimethvl-A-4-thiazoline-2-thione (73J (R4 = R5 = Me) to yield 3-(2-cyanoethyl)-4.5-dimethyl-A-4-thiazoline-2-thione (74) (R4 = R, = Me) (Scheme 35 (160). Humphlett s studies of this reaction showed that the size of the R4 substituent is a determinant factor for the S versus N ratio (161. 162). If R4 == H, 100% of the N-substituted product (74) is obtained this drops to 50% when R4 = methyl, and only the S-substituted product (75) is obtained when R4 = phenyl. The same trend is observed with various CH2 = CH-X (X = C00CH3. COCH3) reagents (149). The S/N ratio also depends on the electrophilic center for CH2 = CH-X systems thus S-reaction occurs predominantly with acrylonitrile, whereas N-substitution predominates with methvlvinvlketone (149). 

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).

Fig. 1-21. Partial rate factors for the phenylation and the thiazol-2-ylation of aromatic substrates (414).

The search for a system with less decomposition and a higher separation factor has been summarized (27—29). The most promising system is the BE —anisole system, in which BE (g) exchanges with the anisole [100-66-3] (methyl phenyl ether) -BF3 complex (1) (30) ... 

Biological activities also may correlate with electronic substituent factors alone, eg, the inhibition of acetylcholinesterase by six diethyl phenyl phosphates (36) gave r = 0.95 for... 

The most widely used method of analysis for methyl chloride is gas chromatography. A capillary column medium that does a very good job in separating most chlorinated hydrocarbons is methyl siUcone or methyl (5% phenyl) siUcone. The detector of choice is a flame ionisation detector. Typical molar response factors for the chlorinated methanes are methyl chloride, 2.05 methylene chloride, 2.2 chloroform, 2.8 carbon tetrachloride, 3.1, where methane is defined as having a molar response factor of 2.00. Most two-carbon chlorinated hydrocarbons have a molar response factor of about 1.0 on the same basis. 

Phenyl radicals attack azoles unselectively to form a mixture of phenylated products. Relative rates and partial rate factors are given in Table 7. The phenyl radicals may be prepared from the usual precursors PhN(NO)COMe, Pb(OCOPh)4, (PhC02)2 or PhI(OCOPh)2. Substituted phenyl radicals react similarly. 

Table 7 Relative Rates and Partial Rate Factors for the Homolytic Phenylation of Five-membered...

---