Conformation mono-substituent effect

Accelerations (or decelerations) imposed by the cycloamyloses on the rate of an intramolecular reaction may be derived from a conformational effect. The rate of an intramolecular reaction depends not only on the proximity of the reactive groups but also on their relative orientation. For example, Bruice and Bradbury (1965) have shown that the rates of formation of cyclic anhydrides from mono esters of 3-substituted glutaric acids depend on the size of the substituent at the 3-position. This observation was interpreted as a change in the ground state population of reactive and non-reactive conformers as the 3-substituents are varied (Scheme IX). Reason-... 

The temperature dependence of the substituent proton resonances in 1,1, 3,3, 5,5, 7,7 -octamethyl-, 35, mono-t-butyl-, 32, and 1,1 -di-t-butyluranocene, 33, are all linear. Similarly, both the methylene and methyl protons of 1,1 -dineopentyluranocene 31 are linear. For this case, the results imply a relatively fixed conformation with the t-butyl group swung away from the central uranium (conformation A in Figure 12 R=t-OBu). The non-linearity of the methyl protons of 1,1 -diethyluranocene 28 is interpreted as an effect of temperature on the populations of different conformations having different pseudo-contact shifts. Conformation A in Figure 12 (R=CH3) predominates but other conformations also contribute. We have no simple interpretation of the non-linearity of 1,1 -dimethyluranocene, 27, at this time. Some of the results are summarized in Table X. 

A recent study indicated that the stereoelectronic effect of a curved aromatic stmcture dominates the stability of endo/exo-l24-R (Scheme 41) [142]. Steric repulsion is the main factor to facilitate the conformer cxo-124-SiMe3, whereas some mono-substituted sumanenes, such as methylsumanene 124-Me, hydroxy-sumanene 124-OH, and 2,6-xylylsumane 124-xyl, favor formation of the stable enrio-conformation. In methylsumanene and hydroxysumanene, the hyperconjugation between the benzylic proton and the sumanenyl convex face causes the endo conformer to be more stable than the exo conformer. In 124-xyl, the CH-jt interaction, which is caused by the methyl group in xylyl substituent and the concave face of sumanene, favors the cnrto-conformer. 

The conformational effects of mono-, di-, and trifluorination, as well as hydrmylation, on substituted azepanes have been investigated by Patel et al by the use of H NMR spectroscopy and computational modeling in chloroform for all studied compounds /hh couplings have been measured, and for those containing the fluorine substituent also /hf... 

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