Substitution buttressing effect

For both 104 and 105, values of AG (RI) 17 kcal/mol and AG (BS) 18 kcal/mol have been determined from NMR measurements (180). Analogous processes have been studied extensively in cyclooctatetraenes bearing other substituents (181). By way of comparison, ethoxycyclooctatetraene has AG (RI) = 12.5 kcal/mol and AGf(BS) = 16.0 kcal/mol (182). Values for other monosubstituted derivatives (118) differ by +2 kcal/mol and more heavily substituted compounds have substantially higher barriers for both processes, due to steric buttressing effects in the planar intermediate (181). Thus, while the transition metal substituent in 104 or 105 does increase the barriers for RI and BS relative to smaller substituents, this effect is not large enough to prevent observation of both these processes on the NMR time scale (180). Prior to our work, the effect of polyfluorination on the energetics of these isodynamic processes had not been evaluated. 

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. 

Attention has been drawn to the favourable buttressing effect of substituents in the thermal cyclisation of the 2-substituted 1 -allyloxybenzenes (95JCS(P1)2551). 

Molecular structures of4-tert-butylpyrazoles. Trofimenko and co-workers reported an interesting study of the buttressing effects observed for three tert-huiy substituted pyrazoles in both the solution and solid state that employed X-ray crystallography and measurements. The authors found that the major tautomers in solution were consistent with the X-ray single crystal tautomeric preferences for the three analogues. The shifts shown... 

Substituents are usually expected to exert a small steric effect on the 2-substituent, especially without 3-substituents. However a recent reestimation of the buttressing effects in methyl-substituted benzoic acid derivatives has revealed that this effect is not zero even in 2,5-dimethyl derivative, the interaction between more distant groups being neglected. [65]... 

This relationship was of interest for several reasons. Firstly it indicated that the biological assay was sufficiently precise to enable the QSAR approach to be used Secondly the observation that only 2dkyl substituents R affected the activity whereas alkyl substituents in the R position apparently had little influence. This parallels the observations in the 6-thioxanthine series where a similar relationship was derived for the bronchodilating activity ( ) Thirdly, bulky substituents in the Rp position had a beneficial effect. Benzyl-substituted conqiounds e.g. (v) were more active than equation 1 indicated, possibly because the usual value of Es for benzyl did not reflect the buttressing effect of the adjacent triazole ring as revealed by a study of space-filling models ( ). 

The ratio fa /p(Me)//0 /P(r-Bu) is 15,700 for the alkylbenzenes but only 69 for the 2-alkylthiophenes. Likewise the fa /m(Me)//0 /m(t-Bu) ratios are 44,700 (alkylbenzenes), 76.4 (2-alkylthiophenes), and 17.6 (3-alkylthi-ophenes). These results are free from any bond-order effects (which would equally affect the activation by both methyl and r-butyl substituents). They indicate that the 2-t-butyl group produces more hindrance than the 3-/-butyl group, possibly through buttressing between it and the sulfur d orbitals. This view is reinforced by results for electrophilic substitution of benzo[b]thiophene (Chapter 8, Section 2.A.b.i). 

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