E/Z ISOMERISM IN HETEROCYCLIC TERTIARY BENZAMIDES: VARIABLE TEMPERATURE 1H NMR STUDY

JUAN A. BISCEGLIA; LILIANA R. ORELLI

Aveiro

Congreso; IV Iberoamerican NMR Meeting; 2012

Sociedad Química de Portugal

E/Z ISOMERISM IN HETEROCYCLIC TERTIARY BENZAMIDES: VARIABLE TEMPERATURE 1H NMR STUDY Juan Á. Bisceglia, Liliana R. Orelli a Departamento de Química Orgánica. Facultad de Farmacia y Bioquímica. CONICET. Junín 956, Buenos Aires, Argentina.lorelli@ffyb.uba.arAmides represent model compounds for the peptide bond,1 and their structural features of have been widely studied by NMR spectroscopy. In particular, the E/Z equilibrium in heterocyclic tertiary amides has been reviewed.2 The partial double bond character of the (O)C?N linkage in amides causes a substantial rotational barrier (15-23 Kcal mol-1). In unsymmetrically N,N-disubstituted amides, this entails the existence of non isolable E/Z diastereoisomers, which can be observed in the NMR spectra. In previous work we investigated the E/Z equilibrium in tertiary heterocyclic amides 1 (n=1,2).3,4 Compounds bearing a N-benzoyl group showed line broadening in their 1H and 13C NMR spectra, due to cross-conjugation of the carbonyl group with the phenyl ring. This prompted us to investigate the dynamic aspect of the E/Z equilibrium by variable temperature NMR. In order to assess the ring size influence, we synthesized benzamides 1a-c.5 The 1H and 13C NMR signals of the rotamers were unambiguously assigned and their E/Z ratios were determined. The 1H NMR spectra were then recorded at various temperatures. The corresponding E/Z interconversion barriers were calculated on the basis of the corresponding coalescence data. 1.S. Pedersoli, C.F. Tormena, R. Rittner, J. Mol. Struct., 2008, 875, 235-243 and references therein2. E. Kleinpeter, J. Mol. Struct., 1999, 380, 139-156.3. J.Á. Bisceglia, M.C. Mollo, L.R. Orelli, J. Mol. Struct., 2010, 966, 79-84.4. J.Á. Bisceglia, M.C. Mollo, L.R. Orelli. J. Mol. Struct., 2012, 1026, 65?70.5. J.Á. Bisceglia, J. E. Díaz, R. A. Torres, L. R. Orelli, Tetrahedron Lett., 2011, 5238-5240.This work has been carried out with financial aid of the University of Buenos Aires (20020100100935) and by CONICET (PIP 286).