THE STRUCTURES OF DIOXOLAN-2-YLIUM CATIONS - CARBON-OXYGEN BOND DISTANCES IN ACETOXONIUM IONS

The X-ray structures of four dioxolan-2-ylium cations, 2-phenyl-1,3-di oxolan-2-ylium trifluoromethanesulfonate, 6, and 2-(4-methoxyphenyl)-1 ,3-dioxolan-2-ylium, 7, 2-(2,4-pentadienyl)-1,3-dioxolan-2-ylium, 8, a nd -methoxyphenyl)-4,4-dimethyl-1,3-dioxolan-2-ylium, 9, hexafluoroant imonates, have been determined. Crystals of 6 are monoclinic, C2/c, a = 28.934(5), b = 6.022(2), c = 14.831(3) angstrom, beta = 113.37(1)-de grees-, Z = 8; 1944 reflections were used to determine the structure; R = 0.0488, wR = 0.0477. Crystals of 7 are monoclinic, P2(1)/c, a = 11 .948(4), b = 10.686(3), c = 10.853(2) angstrom, beta = 106.31(2)-degre es-, Z = 4; 2260 reflections were used to determine the structure; R = 0.0551, wR = 0.0807. Crystals of 8 are monoclinic, P2(1)/c, a = 10.54 9(2), b = 10.457(3), c = 11.702(2) angstrom, beta = 105.69(1)-degrees, Z = 4; 2190 reflections were used to determine the structure; R = 0.0 337, wR = 0.0299. Crystals of 9 are orthorhombic, Pmnb, a = 8.297(4), b = 15.269(3), c = 12.638(2) angstrom, Z = 4; 1736 reflections were us ed to determine the structure; R = 0.0796, wR = 0.0726. Intensities we re measured on Syntex P2(1) or Nicolet P3 diffractometers at -65-degre es-C with use of Mo Kalpha radiation. These structures are discussed i n terms of the conjugation and charge distribution in these cations. I n each case the cations are almost planar with very small twist angles between the plane of the C(2) substituent and that of the dioxolanyli um ring. Cation 8 is the first reported example of the structure deter mination of an acyclic, substituted dienyl carbenium ion. The unsatura ted five-carbon chain in 8 has an all-trans conformation about all the C-C bonds, but, apart from the C(2)-C(1') distance, none of the other bond distances differ significantly from those found in corresponding neutral systems. The C(2)-C(1') distance in 7 is significantly shorte r than that of 6, suggesting enhanced conjugation of the aryl ring wit h the positive centre in the former cation. This suggestion is support ed by the significant quinonoid-type distortion that is observed in th e p-methoxyphenyl ring of 7. Systematic variations in the 0(1)-C(5)/O( 3)-C(4) bond distances are examined and it is shown that alkyl substit ution at C(4)/C(5) leads to a large increase in their lengths, suggest ing that the presence of a significant fraction of the positive charge is located on C(4) and (or) C(5). Support for this suggestion is foun d in a large variety of previously reported spectroscopic and calorime tric measurements.