Buttressing effect

In a study of the nitrosation of camphor-3-glyoxylic acid (89), Chorley and Lapworth isolated a compound whose structure (90) has recently been clarified by Hatfield and Huntsman. Decarboxylation and ring expansion occur and the reaction is rationalized in the sequence 89 90. The buttressing effect of a methyl group on... 

The Harmata group also found that certain ort/w-bromocinnamates underwent a Michael addition during the course of the Buchwald-Hartwig reaction. This one-pot process produced the same products as the two step process and with the same, complete stereoselectivity. For example, this was first observed with bromocinnamate 107, where the reaction with (7 )-77b afforded a 53% yield of sulfoximine 108 as well as a 36% yield of benzothiazine 95 under standard coupling conditions (Scheme 27). The cyclization was attributed to a buttressing effect of the ortho-methoxy in bromocinnamate 107. This presumably favored a conformation that placed the methyl group of its sulfoximine functionality near the p-carbon of the a,P-unsaturated ester, thus favoring cyclization. 

These effects are observed when the steric effect of the variable substituent is relayed by a constant substituent between it and the active site as in 12 where Y is the active site, Z is the constant substituent and X is the variable substituent. This is a buttressing effect. 

Further kinetic experiments with some stericaliy hindered phenylhydroxylamines gave results47 which suggest that under certain circumstances steric acceleration occurs, attributable to the buttressing effect of neighbouring 3-substituents. Thus the rate constants for the reactions of 41 and 42 are respectively greater than are those for 43 and 44. 

Buttressing effects are known to raise the barrier to rotation in the biphenyl series by preventing bond angle deformations of a substituent involved in direct interaction in the transition state. Similar effects were found in the 9-arylfluorene series (108). The barrier to rotation of 9-(3-bromo-6-methoxy-2,4-dimethyl-phenyl)fluorene (67, X = H) in chloroform-d at 56.3°C is 25.7 kcal/mol for... 

Substituent effects on the rates of isomerization of aryl-di-rerf-butylcarbinols have been studied. The results are shown in Table 17 (116). The substituent effect on the barrier is, as expected, small if the substituent is in either the 4-or the 3-position. However, a buttressing effect of the substituents in 2,3,4,5-tetramethylphenyl-di-rm-butylcarbinol is apparent. The barrier is raised by ca. 1 kcal/mol in this case. 

These results suggested examination of the buttressing effect in bromo compounds, so 1,2,3,4-tetrabromo- and l-bromo-9-(l,l-dimethyl-2-phenylethyl)-triptycenes were prepared. As expected, the tetrabromo compound had a relatively low barrier of 35.1 kcal/mol at 500 K. In contrast, the free energy of activation for rotation of the monobromo compound was 39.2 kcal/mol. The difference amounts to 4.1 kcal/mol (151). The reverse buttressing effect can be large if the substituents concerned are large. 

In order to see the effect of buttressing and the effect of size of the peri substituent on the barrier to rotation, 1-chloro- (112) and 1,3-di-tm-butyl-9-isopropyltriptycene (113) were prepared (161). The barrier for 112 was 22.9 kcal/mol. Comparing this to the tetrachloro compound 111 (all substituents Cl), AG = 25.5 kcal/mol, a normal buttressing effect seems to operate, in contrast to what is observed for the 9-rm-butyltriptycenes as discussed in the previous section. The barrier to rotation in 113 was 20.8 kcal/mol, which is very low... 

Z is the constant substituent and X is the variable substituent. This is a type of 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. 

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