Enamine formation from cyclic ketones

Abstract

The compound 3,4,5,6,7,8-hexahydro-2(4aH)-naphthalenone (hereafter referred to as octalone) was synthesized by condensation of 2-formylcyclohexanone and diethylaminobutanone methiodide. Conjugate additions of vinyland isopropenylmagnesium bromides, respectively, to octalone (catalyzed by Cu++) were carried out. Attempts to purify the conjugate adducts are described. During the synthesis of octalone an intermediate ketol was isolated and identified as trans-2-decalon-9-ol(i) by infrared and mass spectral studies. The research initially planned was to study the acid-catalyzed rearrangement of pure vinyl adduct. The vinyl adduct obtained during the conjugate addition of vinyl-bromide to the octalone catalyzed by Cu++ was found to be contaminated with the starting material (octalone). It was decided to examine enamines as a possible means of separation of adduct (saturated ketones) from the starting material. Hence this research was undertaken to determine the kinetics of enamine formation, knowledge of which is necessary for the separation of products. In the present study cyclohexanone and octalone were used as model compounds for saturated and a,B-unsaturated cyclic ketones, respectively. The secondary amines used were pyrrolidine, morpholine and piperidine. There is a considerable difference in the rates of enamine formation for the two ketones. The results obtained were rationalized in terms of the steric environments of the ketones. The rate of formation of cyclohexanone enamines, in the case of morpholine, was found to be about ten times faster than the formation of piperidine enamine. In the case of octalone the rate of formation of morpholine enamine is about thirty times faster than that of piperidine enamine. The basicity of the secondary amines was also considered in discussion on the difference in rates. Rate determinations were successfully achieved using gas chromatographic analysis. The reaction followed secondorder kinetics.