Studies of Some Strained Ring Systems

Abstract

The present study has been directed towards investigating two facets of small, strained-ring chemistry. The first part of the work describes the attempts made at introducing aziridine functions into a series of appropriately substituted bicyclo[2.2.1]hept-2-enes and bicyclo[2.2.1]hepta-2,5-dienes, primarily through decomposition of the corresponding Δ2-triazoline cycloadducts. The thermal and photochemical modes of nitrogen elimination from these Δ2-triazolines have been examined. From both modes of decomposition, the adducts derived from bicycloheptenes have been shown to lead stereospecifically to the corresponding 3-azatricyclo[3.2.1.02,4]octanes. The photochemically-induced deazetation of unsaturated adducts derived from bicycloheptadiene also proceeds stereospecifically and provides a convenient and simple synthesis of 3-azatricyclo[3.2.1.02,4]oct-6-ene derivatives. However, the thermal decomposition of these same adducts proceeds by retrodiene fragmentation rather than via the elimination of molecular nitrogen. The stereochemistries of the fused aziridines and the Δ2-triazoline precursors have been determined by nmr spectroscopy. The second part of this work has involved a study of substituted arocyclopropenes. The addition of secondary diazoalkanes to 1,4-quinone-N,N'-dibenzenesulphonylimines, previously reported to yield gem-diarylarocyclopropenes, has been reinvestigated and the addition extended to other 1,4-quinonoid systems. The products obtained are reassigned as simple fused cyclopropane derivatives and not arocyclopropenes. Attempts to obtain gem-dihalogenobenzocyclopropenes by base-induced dehydrohalogenation of appropriate bicyclo[4.1.0]hept-3-enes have been examined, and the systematic chemistry of 7,7-dichloro-2,5-diphenylbenzocyclopropene studied. In particular, a series of replacement reactions leading to gem-dialkyl and diaryl benzocyclopropenes and, in several instances, to products resulting from subsequent intramolecular rearrangement have been developed. Substantial evidence is presented for the existence of the benzocyclopropenyl cation.