Conformations of 2-substituted 2,3-dihydro-1,4-benzodioxines

Abstract

X-Ray crystallographic studies have shown that the imidazolinium group in 2-(2-methoxy-2,3-dihydro-1,4-benzodioxin-2-yl)-4,5-dihydroimidazolium bromide (4), (+)-2-(2-methoxy-2,3-dihydro-1,4-benzodioxin-2-yl)-4,5-dihydroimidazolium hydrogen dibenzoyltartrate (5), and 2-(2,3-dihydro-1-benzopyran-2-yl)-4,5-dihydroimidazolium chloride (7) occupies the equatorial position on the dihydroxine ring of compounds (4) and (5) and the dihydropyran ring of compound (7). The imidazolium group in 2-(2,3-dihydro-1,4-benzodioxin-2-yl)-4,5-dihydroimidazolium chloride (2), (+)-2-(2,3-dihydro-1,4-benzodioxin-2-yl)-4,5-dihydroimidazolium hydrogen dibenzoyltartrate (3), and 2-(2-isopropyl-2,3-dihydro-1,4-benzodioxin-2-yl)-4,5-dihydroimidazolium chloride (6) occupies the axial position on the dihydrodioxine ring; the axial choice in compound (6) is attributed to the equatorial preference of the bulky isopropyl group, and the conformational anomaly in compounds (2) and (3) appears to be caused by an interaction between the axial imidazolium group and a lone-pair orbital on the oxygen atom O(4) of the dihydrodioxine ring.