Effect of a-Methyl Substituents

An a-methyl substituent increases the rate toward 9-BBN over that of the parent 1-alkene due to hyperconjugative affect. However, opposite is true for disiamyl-borane. 

The ethyl substituent, on the other hand, increases the steric interactions with the 9-BBN moiety. 

The electronic factor thus controls in the case of 9-BBN, which has the relative openness of the boron atom. The inductive effect of the methyl group increases the availability of electron of the double bond. The transition state involves the development of electron deficiency at C-2, with partial hydridic character at the B-H moiety. 

The much greater steric congestions in disiamylborane must swamp these small electronic contributions, and the steric factor prevails. 

Addition of an a-methyl group in butene with internal C=C has little effect on the rate with 9-BBN, whereas the rate with disiamylborane is very low. In 2,3-dimethyl-2-butene there is drastic drop in rate with 9-BBN, and disiamylborane fails to react. 

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The separation of mixtures involving N-methyl-JLtetrahydropyridines into their pure components by means of gas-liquid chromatography was discussed in a report by Holik et al. (87). They found that, using tris(/3-cyanoethoxymethyl)-y-picoline as the stationary phase, the primary factors involved in the specific retention volumes of these enamines is the electronic effect of a methyl substituent and the nitrogen atom on the carbon-carbon double bond. It was observed that 1,3-dimethyl-Zl -tetrahydropyridine (141) has a smaller specific retention volume and, hence, is eluted before... 

The trends among the iodides are similar to those observed for the unimolecular dehydrohalogenation of chlorides and bromides. The effect of a-methyl substitu-... 

C—I bond fission and four-centre HI elimination/ The marked change in the branching fractions for primary, secondary, and tertiary iodides is mainly a consequence of the change in threshold energy for HI elimination the effect of a-methyl substituents on the threshold energy has been interpreted in terms of the avoided intersection between ionic and non-polar potential energy surfaces. 

A quantitative study has been made on the effect of a methyl group in the 2-position of five-membered heteroaromatic compounds on the reactivity of position 5 in the formylation and trifluoroacetylation reaction. The order of sensitivity to the activating effect of the substituent is furan > tellurophene >selenophene = thiophene (77AHC(2l)ll9). 

Glennon, R. A., Young, R., and Jacyno, J. M. (1983) Indolealkylamine and phenalkylamine hallucinogens Effect of a-methyl and N-methyl substituents on behavioral activity. Biochem. Pharmacol., 32 1267-1273. 

The identity of the substituent X (its chemical notation) is indicated in parentheses. Thus, the substituent effect of a methyl group is designated SCS(CH3). 

Oki and associates (163) further prepared 9-(l-methyl-2-propenyl) triptycenes (115) to see the effect of a vinyl substituent on the barrier to rotation. They were able either to isolate or to enrich one of the retainers in crystalline form, and they examined the barrier both by equilibration and by the dynamic NMR tech-... 

The effects caused by replacement of the geminal dimethyl grouping in the indole part of indolidan by a spiroalkane ring system have been investigated recently [84], It has been found that ring size and inotropic potency are inversely related and that potency within this series can be further increased by incorporation of a methyl substituent into the pyridazine nucleus. The most potent compound is represented by formula (26). It has an ED50 of 1.5 //g/kg (i.v. administration, anaesthetized dogs). 

The pATa values for the amines ammonia, methy-lamine, dimethylamine, and trimethylamine are 9.2, 10.6, 10.7, and 9.8 respectively. The electron-donating effect of the methyl substituents increases the basic strength of methylamine over ammonia by about 1.4 pATa units, i.e. by a factor of over 25 (10 " = 25.1). However, the introduction of a second methyl substituent has a relatively small effect, and the introduction of a third methyl group, as in trimethylamine, actually reduces the basic strength to nearer that of methylamine. 

Tris-(2-methylquinolin-8-olato)iron(III) has three mer N and three mer O-donor atoms the 0.2 A range of Fe— N bond distances is attributed to small but significant steric effects of the methyl substituents. Solubilities of tris-(quinolin-8-olato)iron(III) complexes in methanol-water mixtures are consistent with the expected more favorable solvation by methanol. ... 

The influence of substituents on the types of complex formed by variously methyl-substituted o-carboxyarylazopyrazolones (123) is, however, seen as being very significant. Thus the buttress effect of a methyl group in the 3- or 6-position in (123) would effectively shorten the distance between the two oxygen donor atoms in the ligand, thereby favouring the formation of a fac complex. No such effect is exerted by a methyl group in the 4- or 5-position in (123) and mer complexes are formed. 

The cyclopentane derivative 60b was obtained in both high yield (92%), and enantioselectivity (95% ee). Interestingly, lower yield (80%) and enantioselectivity (68% ee) were obtained when I-proline was used as a catalyst instead of (S)-61, showing the beneficial effect of the methyl-substituent at the a-position on catalytic efficiency. 

The simplest manifestation of substituent effects should be found by comparing the rates of addition of methyl radicals to branched olefins. As Table 1 shows, the effect of introducing methyl substituents adjacent to the double bond appears to be extremely small. The variation in rate is barely one power of ten compared with a variation of almost 106 for the heterolytic addition of bromine to ethylene and tetramethylethylene. 

The results reported for these systems, indicate that the subglass relaxation in P3M2NBM is indeed a broad and weak relaxational process [95], The low strength of the observed secondary processes in these systems, has been attributed to the bulkiness of the side chain. In one case, the low strength of the secondary relaxation prevents the analysis of this peak in terms of an empirical relaxation. The effect of the methyl substituent on the norbornyl ring is to lower the position of the value of the peak of the a -relaxation about 50 K. Another effect that can be observed in the low frequency side of the spectra in both polymers is the conductive one. 

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