EVIDENCE FOR THE PRESENCE OF CH-PI-INTERACTED AP-CONFORMERS OF BENZYL FORMATES

In order to examine the preferred conformations of benzyl, 1-phenylethyl and cumyl formates, lanthanoid-induced shifts (LIS) of H-1 and C-13 NMR and difference NOE spectra of these esters were measured. The measurements showed that a folded conformer capable of forming intramolecular CH-pi interaction are predominant with all three series of formate esters. The LIS and other experiments suggested the coexistence of considerable amounts of extended conformers, however. As the ester group is enforced to take supposedly unfavourable ap-conformation in the CH-pi contiguous folded conformer, the predominance of the conformer was a surprise and needed explanation. The NOE experiments on a series of para-substituted benzyl formates, XC6H4CH2OCHO (X = CH3O, CH3, H, Cl, NO2), showed that the enhancement of aromatic proton signals (ortho and meta) induced by the irradiation of formyl proton increases gradually as the substituent becomes more electron donating, whereas the enhancement of benzylic (alpha) proton signal remains constant irrespective of the nature of the substituent. This can be explained by assuming that the CH-pi contiguous folded conformer is in equilibrium with the extended conformer. The trend of the substituent effect supports the hydrogen bond-like nature of the CH-pi interaction. A similar trend was also observed with substituted 1-phenylethyl formates. Hence the unexpected stability of the ap-conformer of benzyl formates could be ascribed to the stabilization due to the CH-pi interaction.