Structures of hydroxy/alkoxy substituted carbenium ion systems

Abstract

<p>The structures of two series of hydroxy/alkoxy substituted carbenium ion systems have been explored in this thesis. The structural effect of systematic substituent changes on cationic geometries has been probed and related to variable charge distributions within these cations. The structures of three 2-substituted 6-ethoxy-tetrahydropyrylium salts have been determined with use of x-ray crystallographic and NMR spectroscopic techniques. It has been shown that the conformations and structures of the tetrahydropyrylium ions observed in the crystalline state are similar, and that these conformations are also present in solution. The six-membered ring in each cation exists in a shallow half-chair conformation with the cationic centers C₆, C₅, O₁ and O₆ having an approximately planar arrangement. These cations each exhibit a Z-conformation about the O₆-C₆ bonds. Systematic analysis of the bond distances obtained has indicated severe C-O bond length distortions and has suggested variable degrees of positive charge delocalization to C₂ and C₇ in these systems. The conformations, details of C-O bonding and closest cation-anion contacts observed have been discussed in terms of the concept of stereoelectronic control. The crystal structures of three alkyl-substituted hydroxycyclopropylcarbinyl cations have also been determined. Each cation adopts, or approximately adopts, a bisected conformation. The systematic cyclopropyl bond length distortions observed, namely vicinal bond lengthening, distal bond shortening and C₁-C₂ bond contraction when compared to those observed for neutral cyclopropyl ketones, have been related to the degree of charge delocalization into the cyclopropyl ring in each cation and to variable substituent charge stabilizing ability. Semi-empirical and ab initio calculations have also been performed to further probe the nature of bonding within these systems. The optimized gas phase conformations and structures are very similar to those observed crystallographically. However, theory appears to underestimate the degree of structural distortion experimentally observed.</p>