The Formation and the Five-Fold Degenerate Rearrangement of Lewis Acid-Complexed-5-Acyl-1,2,3,4,5-pentamethylcyclopentadienes

Abstract

<p>This thesis describes the preparation of some bicyclo[3.1.0]-hexenyl cations with different substituents at C6 and a study of their degenerate rearrangements. The preparation of both isomers of 6-ethyl-1,2,3,4,5,6-hexamethylbicyclo[3.1.0]hexenyl cation was achieved by the photoisomerization of l-ethyl-1,2,3,4,5,6-hexamethylbenzenonium ion. A new cation, 1,1,2,3,4,5-hexamethylbenzenonium ion was prepared and characterized.</p> <p>The preparation of a series of 5-acyl-1,2,3,4,5-pentamethyl-cyclopentadienes has been developed in an attempt to generate bicyclo[3.1.0]hexenyl cations with the hydroxy group at C6. The protonation of 5-acyl-1,2,3,4,5-pentamethylcyclopentadienes results in the formation of dications which have been characterized by p.m.r. and uv spectroscopy. These dications are stable below 0º but at higher temperatures undergo a fragmentation reaction where protonated pentamethylcyclopentadiene is generated. This provides a convenient synthetic route for pentamethylcyclopentadiene.</p> <p>5-Acyl-1,2,3,4,5-pentamethylcylopentadienes react with various Lewis acids to generate σ-complexes in which the Lewis acid is bonded to the carbonyl group. The zwitterions have been characterized using infrared and nuclear magnetic resonance spectroscopy.</p> <p>The Lewis acid-complexed-5-acyl-1,2,3,4,5 pentamethylcyclopentadienes exhibit temperature dependent p.m.r. spectra. The averaging can best be explained in terms of a five-fold degenerate rearrangement in which the Lewis acid-complexed ketone group migrates around the periphery of the five-membered ring. The rates of circumambulation were determined by comparing the experimental and calculated p.m.r. spectra. The rate of automerization is dependent upon the nature of the acyl group as well as the Lewis acids used. A linear free energy relationship exists between the rates of circumambulation of AlCl3-complexed aromatic ketones and the para-substituent constant, the magnitude of the reaction constant ρ being 4.64 (correlation coefficient 0.999).</p> <p>These results have been discussed in terms of a mechanism which involves a bicyclic zwitterion as the transition state for the degenerate rearrangement. The possibility of a homoconjugative interaction between the electron-deficient C6 and the occupied orbitals of cyclopentadiene is discussed.</p>