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.