Site selectivity in the reactions of 1,3-dipoles with norbornadiene derivatives

Abstract

The cycloadditions of arylazides, benzonitrile oxides and diphenylnitrile imine to norbotnadiene derivatives 1a–c showed varying degree of site- and stereo-selectivity. With dipolarophile 1a arylazides and benzonitrile oxides attack, preferentially, the electron-poor tetrasubstituted double bond, while in the case of 1b and 1c is the substituted double bond which enters the cycloaddition. By contrast, 2-diazopropane and C-pnenyl-N-methyl-nitrone react with the sole tetrasubstituted double bond of 1a–c in stereo- and site-specific cycloadditions. The quantitative evaluation of the two possible reaction paths was performed by glc analysis. The compounds detected were those arising from Diels-Alder cycloreversions of the thermally labile intermediate adducts 2 and 3 (Scheme 1). The results were rationalized on the basis of a qualitative perturbation treatment which considers frontier orbital interactions only.