INFIQC   05475
INSTITUTO DE INVESTIGACIONES EN FISICO- QUIMICA DE CORDOBA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Structural and conformational analysis of cis/trans ciclohexen-2-arylpyrimidin-
Autor/es:
WALTER J. PELÁEZ; SOL C. PARAJÓN PUENZO; FERENC FÜLÖP; GUSTAVO A. ARGÜELLO
Lugar:
Florianópolis - Brazil
Reunión:
Conferencia; 10th Latin American Conference on Physical Organic Chemistry - 10thCLAFQO; 2009
Resumen:
There is interest in structural, conformational and vibrational properties of ciclohexen-2-aryl-pyrimidin-4-ones, mainly due to their pharmacological uses and because its knowledge is relevant to study molecular processes in organic chemistry.1,2 Cis- (1a,c,d,f) and trans- (1b,e) ciclohexen-2-aryl-pyrimidin-4-ones have been selected as a case study aimed at finding out some correspondence between structural properties (ringfusion, tautomeric forms and conformers) and reactivity in the gas-phase(analyzed through NMR and UV spectra, and mass spectral fragmentation). The flash vacuum pyrolysis (fvp) of 1a–f is described in scheme 1. All compounds undergo ring fragmentation to the corresponding products (2,3) between 600 and 640 ºC. Mass spectrometry shows that all molecules give Retro Diels Alder (RDA) and heterocyclic fragmentation reactions. UV spectra show a strong conjugation (and correspondingly a batochromic shift) for the pCl-Ar compounds. NMR shows that cis compounds show two conformers (N-in, N-out) whilst trans counterparts show only one. Fvp reveals that 1a-c experiences RDA reaction with a slight substituent effect. For cis/trans counterparts 1b/e and 1c/f, the ring fusion produces a dramatic change in the reaction mechanism. The behavior of compound 1d does not follow the typical sequence as for the other cis compounds, since the double bond is not in the right position to afford a RDA reaction and either its relocalization or a stepped RDA reaction requires more energy than the fragmentation of the heterocyclic ring. This fact confirms, in turn that the RDA for 1a-c is a concerted mechanism and not a sequential one. As conclusions we can say: a) the mass spectra of cis compounds are simpler than the trans and its fragmentation is very selective giving only the RDA as confirmed in the fvp reaction. b) The stronger the conjugation shown by the UV spectra, the easier the fragmentation, irrespective of the compound being cis or trans. c) The more rigid trans compounds (like trans decalins) are more stable thermally because they have less strain ring energy.2        References: 1- Fülöp, F.; Bernáth, G.; Pihlaja, K.; Adv. Heterocycl. Chem. 1998, 69, 349-477. 2- Peláez, W. J.; Szakonyi, Z.; Fülöp, F. and Yranzo, G. I.; Tetrahedron, 2008, 64, 1049-1057.