The relative reactivities of ‘normal’ and angle strained cyclo-olefins in 1,3-dipolar cycloaddition reactions. Rotational barriers in nitrones

Abstract

Rate constants for addition of C-phenyl N-methyl nitrone, the ethylcyclohexyl nitrone (II), and dihydroisoquinoline N-oxide respectively to cyclohexene, norbornene, trans-cyclo-octene, and bicyclo[4,2,0]oct-7-ene have been determined. The barrier (ΔG^‡) to rotation about the formal double bond in the methylene nitrone (II) is estimated to be ca. 23 kcal mol^–1. It is concluded that the low reactivity of C-phenyl N-methyl nitrone and analogous ‘class II’ dipoles towards norbornene is due to steric factors.