Studies of Some Strained Ring Systems

Abstract

Strained aromatic compounds, and in particular cycloproparenes, have been of considerable interest in the last twenty five years. The present study has been directed towards the use of bicyclo[4.4.1]undeca-1,3,5,7,9-pentaenes (1,6-methano[10]annulenes), e.g. 34, as precursors for the highly strained cycloproparenes. In addition, some cycloaddition chemistry of cycloproparenes and 9,9-dichloro-1,4-dihydro-4a,8a-methanonaphthalene 57 was scrutinised. A major synthetic route to cycloproparenes involves dehydrohalogenation using potassium t-butoxide in tetrahydrofuran. Regioselective additions of carbene and oxygen on the β-face and proximal to the bridge chlorine atom of 9-chloro-1,4,5,8-tetrahydro-4a,8a-methanonaphthalene 70 are highlighted and exploited in the stereoselective synthesis of chlorinated 1a,2,3,6,7,7a-hexahydro-2a,6a-methano-1H-cyclopropa[b]naphthalenes, e.g. 66 for the dehydrohalogenation study. Dehydrochlorination of such compounds provides a new route to tricyclo[5.4.1.03,5]dodeca-2,5,7,9-11-pentaenes (cyclopropa[c]methano[10]annulenes), e.g. 83 which has enhanced air stability over the parent compound. 9,9-Dichloro-1,4-dihydro-4a,8a-methanonaphthalene 57 contains both an isolated 'diene' and an 'ene' moiety and as such forms cycloadducts with both electron-poor dienes and dienophiles. The thermolysis of 57 is shown to be solvent dependent and in ethylene glycol disproportionation to 1,4-dihydronaphthalene 123 and dichlorocarbene occurs. The only known dicyloproparene is dicyclopropa[b,g]naphthalene 13. Increasing interest in synthesing dicyclopropabenzenes led to an investigation of the cycloaddition chemistry of tricyclo[5.4.1.0.3,5]dodeca-2,5,7,9,11-pentaenes as possible synthons for dicyclopropa[a,d]benzene 9. An approach directed towards cyclopropacycloheptatrienes involving (1aα,7bα,7aα,3aα)-1,1,8,8-tetrachloro-1a,7b-dihydro-3a,7a-methano-1H-cyclopropa[a]naphthalene 130, and the Diels-Alder chemistry of the latter was studied. The synthesis of 130 has been markedly improved upon. Cyclopropabenzene 1 has been separately reported to form a [4+2] and a [2+2] adduct with 1,4-diphenylisobenzofuran. The current study found that the reaction temperature controls the product formed and that the products arise from separate pathways. The first cycloadduct of cyclopropa[b]naphthalene 2 (with 4-phenyl-4H-1,2,4-triazoline-3,5-dione) is reported as is the thermal dimerisation of 2 to give rise to 6,13-dihydropentacene 157.