Methyl substituents, effect

In fact, a similar intramolecular cyclisation was studied by Reusch [11] and he found a remarkable methyl substituent effect on the aldol equilibrium. Starting from the cis-decalones 25 (easily prepared from the Wieland-Miescher ketone), in which the angular methyl group prevents isomerisation to the more stable trans-decalone, it was found that other methyl groups may exert profound but less... 

The same group studied the lithium cation basicities of a series of compounds of the general formula R R R PO, i.e. phosphine oxides, phosphinates, phosphonates and phosphates, by using Fourier Transform Ion Cyclotron Resonance (FTTCR) mass spectrometry. A summary of their results is shown in Figure 4. The effect of methyl substitution on LCA as well as the correlation between LCA and PA was also investigated by Taft, Yanez and coworkers on a series of methyldiazoles with an FTICR mass spectrometer. They showed that methyl substituent effects on Li binding energies are practically additive. 

The H-NMR methyl substituent effect is very different on the various ring protons of TP from this Makisumi et al. (64CPB204) concluded the ring system possesses only a small aromaticity. The simple HMO method cannot be used for calculations of tautomerism of isomeric oxotriazolopyr-... 

Table 4.6. Methyl Substituent Effects on 13C Chemical Shifts of Cyclopentane and Methylcyclopentane Resonances [210]...

The 13C NMR spectroscopic data of the average cationic center in 495 and 496 were found to be at 813C 205 and 197 (/C H 65 Hz), respectively, indicating their regular trivalent carbenium nature. From studies of methyl substituents effects, Olah and Donovan95 reached the same conclusions and these are supported by laser Raman and... 

Substituent effects do not appear to be reliable probes for hyperconjugation, however. Even in the 2-norbornyl cation, where considerable C-C bond delocalization is generally considered to be present16S), substituents fail to indicate any electron deficiency at the 6-position of the developing 2-norbornyl cation 162,166>16 ). Methyl substituent effects may also not be expected to provide a reliable test for C-C hyperconjugation in the 1-adamantyl system. 

Methyl substituent effects in indoles (ppm relative to indole)... 

The H NMR spectra of magellanine and of paniculatine are summarized in [582] (341) and [583]. (342) The 13C NMR spectrum of the latter alkaloid [584] provides an example of the use of model compounds in aiding signal assignment. For example, shifts were calculated for the model compound [585] from the shifts in [586], [587], and [588] (3) together with the methyl substituent effects shown in Table III. 

The methyl substituent effects are approximately additive, confirming that (as in benzene) steric effects toward hydrogen exchange are effectively absent. 

The effect of the high reactivity of indolizine also shows up in substituent effects that are very small, as they should be according to the reactivity-selectivity principle. The exchange-rate coefficients for deuteriation in DzO/dioxan at 50°C are given in Table 8.12 (71T4171), and from these data the methyl substituent effects may be calculated. Notable features are [87JCS(P2)591] summarized below. 

Rate Coefficients and Methyl Substituent Effects for Deuteration of Indolizine"... 

Data have also been obtained for deuteriation of fl-hydroxy derivatives of borazathienopyridines (which react as the anhydrides) (8.90 and 8.91) and their 5- and 7-methyl, 4- and 6-methyl derivatives, respectively (77JHC893). Rate coefficients were obtained at 57, 65.2, and 72.9°C for exchange at the 2- and 3-position the data for the 3-position at 65.2°C and the average ki/k2 rate ratios at 65.2 and 72.9°C are given in Table 8.16. The 3-positions are in each case more reactive than the 2-positions, but by only a relatively small factor. Both 8.90 and 8.91 have similar reactivities at the 3-positions, but the 2-positions are more reactive in compounds 8.91 (R = H, Me). This is difficult to understand, because electron withdrawal from the 2-position in 8.91 as in 8.92 should make this position relatively less reactive. Methyl-substituent effects are small and rather... 

In a i C-nmr study of [10] Olah and White (1969c) compared the chemical shift (197 ppm, ref. TMS) and coupling constant (y H = Hz) for the central carbons with reference data. Their results confirmed their previous conclusions about the nature of [10]. From studies of methyl substituent effects Olah and Donovan (1977) reached the same conclusion which is also supported by laser Raman investigations (Olah et al., 1971a). 

Gassman and coworkers have observed small cumulative methyl substituent effects (accelerations of 2.2 and 4.7 for the first and second methyls, respectively) in the tricyclo[3.2.1.0 ]octyl system 120 which are consistent with a highly delocalized ion. 

Doucet, J.P., Panaye, A. and Dubois, J.-E. (1983). Topological Correlations of Carbon-13 Chemical Shifts by Perturbation on a Focus DARC-PULFO Method. Attenuation and Inversion of a-Methyl Substituent Effects. J.Org.Chem., 48, 3174-3182. 

Substitution of the methyl group in the r-butyl cation by hydrogen thus causes an upfield shift of 10.4 ppm. Although the CMR shift of the carbocation center of the r-butyl cation is more deshielded than that of the isopropyl cation (by about 10 ppm), this can be explained by the methyl substituent effect, which may amount to 22 ppm. The tertiary butyl cation thus is more delocalized and stable than the secondary isopropyl. 

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