beta-Ketonitriles: Mass Spectrometry Study of Tautomeric Equilibria

DANILA RUIZ; JUAN GIUSSI; MERCEDES SCHIAVONI; JORGE FURLONG; PATRICIA ALLEGRETTI

Los Cocos

Conferencia; 9th Latin American Conference on Physical Organic Chemistry; 2007

Beta-Ketonitriles are an important class of difunctional intermediates for the synthesis of many heterocycles, including dihydropyrans and dihydrothiopyrans, pyrazoles, pyrimidines, pyridones , and quinolines. Recent interest in beta-ketonitriles has been focused on their bioreduction and parallel kinetic resolution for the preparation of enantiopure ketones and alcohols containing a quaternary stereocenter in line with the growing importance of optically active beta-hydroxy nitriles as intermediates in the preparation of gamma-amino alcohols (like the antidepressant fluoxetine). In this respect, the diversity of the beta-ketonitriles available significantly impacts the range of structures which can be accessed. Selected beta-ketonitriles and their sulfur analogues were synthesized according to literature procedures. Their structures were confirmed by 1H y 13C n.m.r. Their electron impact mass spectra were studied to evaluate the simoultaneous occurrence of two tautomerization equilibria yoelding both the enol structure and the ketenimine structure. After careful examination of the spectra it can be concluded that; 1)The keto-enol equilibrium is predominant. 2)The sulfur analogues exhibit a noticeable increase in the enol content, what is consistent with previous studies for other family of compounds in our laboratory. 3)Steric and electronic effects on the equilibrium were also considered. In addition, the experimental results are consistent with semiempirical calculations (PM3 single point).