Michael–type reactions of β-nitrostyrenes with piperidine: effect of solvents on reactivity and reaction mechanism

Abstract

Present report deals with the effect of solvent on the kinetics and mechanism of Michael-type reaction of ƒÒ-nitrostyrene with piperidine. Three different types of solvents are used for the study namely, nonpolar (chloroform, toluene, hexane), dipolar aprotic (acetonitrile, dimethylsulfoxide, ethylacetate) and polar protic (methanol, ethanol, hexanol, octanol) solvents. The upward curve for the plots of kobs vs. [pipiridine] in all the cases (except octanol) indicates that the reactions proceed through both catalyzed and uncatalyzed path. Analyzing observed rate constants and correlating with various solvent polarity parameters, the following conclusions are drawn. The reactions proceed through both catalysed and unanalysed routes for polar aprotic and aprotic solvents whereas in nonpolar solvents only catalysed route is present. The rate of reaction is higher in polar aprotic solvents as compared to polar protic solvents as compared to other solvents. A stepwise mechanism is proposed for polar protic and aprotic solvents whereas in nonpolar solvents it proposed to proceed through concerted process. The difference in mechanism and reactivity is based on the differential solvation of nucleophile and the zwitterionic intermediates due to difference in polarity, hydrophobicity, hydrogen bond donor and acceptor capabilities of the solvents.