Methylene-group selectivity

Fio. 26. Methylene group selectivity, ocn,i of several hydroorganic mobile phases when octadecyl silica stationary phase is used. The selectivity is the ratio of the retention factor of a member of a homologous series to that of another member which differs in having one less methylene group. The solvents shown here are (A) acetone, (B) acetonitrile, and (C) methanol. The dau were taken at ambient temperature and the selectivity values are plotted on a logarithmic scale. Reprinted with permission ftom Kaiger et al. (/4S).. ... 

Equation (10.6) explains why the methylene-group selectivity is a function of only the mobile phase and the stationary phase, but not the nature of the analyte the last term in Equation (10.6) contains only the composition of the mobile phase and the stationary phase. Furthermore, Equation (10.6) gives a reason for the commonly encountered observation that plots of the logarithm of the retention factor versus the number of methylene groups in a homologous series exhibit a common intersection point, when the mobile-phase composition or the temperature are varied (Fig. 10.2). 

If the stationary-phase composition of different columns is similar, one should also expect, on the basis of Equation (10.6), that the methylene-group selectivity of (Afferent columns is similar. This has been found indeed (6). Also, the same assumption implies a collinear relationship of retention factors between similar columns. In some cases, this has bem observed. But also exoq>tions have been found, which can be explained on the basis of differences in the stationary phase composition, due to the influence of silanols. Thus the partitioning-displaoenient model presents an intuitive description of the retention process that results in an accurate description the experimental observations. 

The hydrophobic selectivity is the methylene group selectivity of the stationary phase, that is, the selectivity of two compounds differing by a methylene group, which are retained predominantly due to their hydrophobicity. This selectivity is to be used, for example, in the case of neutral, hydrophobic analytes for the comparison of columns and assessment of their similarity, and has a much higher significance than the hydrophobicity treated in Section 4.4.1, and later sections. 

Figure 4.5 Hydrophobicity and Methylen group selectivity. Reproduced from Kromidas [6], with permission of John Wiley Sons.

In the case of pure RP interactions (e.g., a difference of only an additional CH2 group, methylene group selectivity ), the stationary phase is largely unimportant. The choice of the column is of secondary importance. Therefore, one obtains only a small difference in the separation factors between Zorbax Bonus and Zorbax Extend, which are two completely different phases. If the analyte has polar groups, as in the case of hydroxybenzoates, a differentiation of columns is more easily possible and the separation factors cover a wider range. In the case of triphenylene/o-terphenyl (additional steric effect), the range is even wider. If the polar phase can display its capacity for polar interactions, a better selectivity is observed in all cases. If the difference is only a shorter alkyl chain, as in the case of Zorbax SB Cg, then no distinct positive influences are observed. 

Retention Mode and Temperature Dependence on Selectivity for Ordered Phases It is known that conventional ODS or alkyl phases can recognize the hydrophobicity of solutes in HPLC and this hydrophobicity is measured by the methylene group selectivity of the stationary phases. This reflects the possibility of the phase being able to separate two molecules that differ only in methylene groups, e.g., amylbenzene and butylbenzene or ethylbenzene or toluene. The retention mode as well as the extent of hydrophobic interaction among the solutes and the packing materials in HPLC can be determined by retention studies... 

Various terminal allylic compounds are converted into l-alkenes at room temperature[362]. Regioselective hydrogenolysis with formate is used for the formation of an exo-methylene group from cyclic allylic compounds by the formal anti thermodynamic isomerization of internal double bonds to the exocyclic position[380]. Selective conversion of myrtenyl formate (579) into /9-pinene is an example. The allylic sulfone 580 and the allylic nitro compound... 

Halogen-substituted succinimides are a class of products with important appHcations. /V-Bromosuccinimide [128-08-5] mp 176—177°C, is the most important product ia this group, and is prepared by addition of bromine to a cold aqueous solution of succinimide (110,111) or by reaction of succinimide with NaBr02 iu the presence of HBr (112). It is used as a bromination and oxidation agent ia the synthesis of cortisone and other hormones. By its use it is possible to obtain selective bromine substitution at methylene groups adjacent to double bonds without addition reactions to the double bond (113). 

PPL and Hpase from Pseudomonas sp. catalyze enantioselective hydrolysis of sulfinylalkanoates. For example, methyl sulfinylacetate (46) was resolved by Pseudomonas sp. Hpase in good yield and excellent selectivity (62). This procedure was suitable for the preparation of sulfinylalkanoates where the ester and sulfoxide groups are separated by one or two methylene units. Compounds with three methylene groups were not substrates for the Hpase (65). 

SELECTIVE INTRODUCTION OF ALKYL AND METHYLENE GROUPS INTO THE STEROID SYSTEM... 

Dehydrofluorination by primary and secondary aliphatic amines occurs at room temperature and is the basis of diamine cross linkmg, which occurs by dehydrofluonnation and subsequent nucleophihc substitution of the double bond The locus of dehydrofluonnation is a VDF unit flanked by two perfluoroolefin units This selectively base-sensitive methylene group also undergoes elimination as the first step in phase-transfer-catalyzed cross-hnking with quaternary ammo mum or phosphomum salts, bisphenols, and morganic oxides and hydroxides as HF acceptors [31, 32]... 

The tetrasubstituted isomer of the morpholine enamine of 2-methyl-cyclohexanone (20) because cf the diminished electronic overlap should be expected to exhibit lower degree of enamine-type reactivity toward electrophilic agents than the trisubstituted isomer. This was demonstrated to be the case when the treatment of the enamine with dilute acetic acid at room temperature resulted in the completely selective hydrolysis of the trisubstituted isomer within 5 min. The tetrasubstituted isomer was rather slow to react and was 96% hydrolyzed after 22 hr (77). The slowness might also be due to the intermediacy of quaternary iminium ion 23, which suffers from a severe. 4< strain 7,7a) between the equatorial C-2 methyl group and the methylene group adjacent to the nitrogen atom, 23 being formed by the stereoelectronically controlled axial protonation of 20. 

The selective bromination of a ketone in the presence of another susceptible functional group was achieved in a diterpene synthesis 240). A competing bromination of an anisole ring could be avoided here through the use of a pyrrolidine enamine derivative for activation of the methylene group adjacent to the carbonyl function. 

Irradiation of 4-(3-benzoylpropionyl)-1,4-morpholine (267) yielded an epimeric mixture of 9-hydroxy-9-phenylperhydropyrido[2,l-c][l,4]oxazin-6-ones 268 and 269 via hydrogen abstraction from the position 3 of the morpholine moiety of 267 (98T2529). It was assumed that the steric hinderance between the phenyl group and the hydrogen atoms of 5-methylene group of 267 in the biradicals contributed to the observed selectivity. 

Fig. 32. 2H spectra of a polymer model membrane, cf. Fig. 27b), selectively deuterated at the a-methylene group of the hydrophobic chain. The spectra are compared for the monomer as well as the polymer lamellar phases at the same temperatures, respectively... 

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