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Evidence for the presence of the CH–π‐interacted ap ‐conformers of benzyl formates
Author(s) -
Suezawa Hiroko,
Mori Akiyoshi,
Sato Masayuki,
Ehama Ritsuko,
Akai Ikuo,
Sakakibara Kazushisa,
Hirota Minoru,
Nishio Motohiro,
Kodama Yoshio
Publication year - 1993
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610060704
Subject(s) - conformational isomerism , chemistry , substituent , intramolecular force , proton , hydrogen bond , formate , stereochemistry , context (archaeology) , chemical shift , proton nmr , crystallography , photochemistry , molecule , organic chemistry , catalysis , paleontology , physics , quantum mechanics , biology
In order to examine the preferred conformations of benzyl, 1‐phenylethyl and cumyl formates, lanthanoid‐induced shifts (LIS) of 1 H and 13 C NMR and difference NOE spectra of these esters were measured. The measurements showed that a folded conformer capable of forming intramolecular CH–π 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–π 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, XC 6 H 4 CH 2 OCHO (X = CH 3 O, CH 3 , H, Cl, NO 2 ), 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 (α) proton signal remains constant irrespective of the nature of the substituent. This can be explained by assuming that the CH–π 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–π 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–π interaction.