Base-promoted eliminations within halogenated [n.m.1]propellene frameworks

Abstract

The reactions of halogenated [n.m.1]propellenes with potassium t-butoxide have been investigated. Propelladiene 9,9-dichloro-1,4,5,8-tetrahydro-4a,8a-methanonaphthalene affords 1,6-methano[10]-annulene, 4-methylazulene, and 5H-benzocycloheptene, while the related tetracyclic compound-1,1,8,8-tritrachloro-1a,2,3,6,7,7a-hexahydro-2a,6a-methano-1H-cyclopropa[b]naphthalene reacts to give 12-chlorotricyclo[5.4.1.0^3,5]dodeca-1,3(5),6,8,10-pentaene. Treatment of 8,8-dichloro-2,3,4,7-tetrahydro-3a,7a-methano-1H-indene with the same base produced a mixture of the chlorodiene 8-chloro-2,3-dihydro-3a,7a-methano-1H-indene, azulene, and 4-methylazulene. The structure of the chlorodiene was established by an X-ray crystallographic study of its Diels–Alder adduct with 4-phenyl-4H-1,2,4-triazole-3,5-dione. While reaction of tetracycle 1,1,syn-8-trichloro-1 a,2,3,6,7,7a-hexahydro-2a,6a-methano-1H-cyclopropa[b]naphthalene with base failed to produce any characterisable products, under the same conditions epimer 1,1,anti-8-trichloro-1a,2,3,6,7,7a-hexahydro-2a,6a-methano-1H-cyclopropa [b] naphthalene was converted into the cycloproparene tricyclo-[5.4.1.0^3,5]dodecs-1,3(5),6,8,10-pentaene. Mechanistic proposals which account for the observed conversions have been advanced and are supported by ¹³C-labelling studies. The initial steps in most of the reaction pathways of the substrates are probably 1,4-elimination processes involving abstraction of an allylic hydrogen, fragmentation of the strained propellene σ-bond, and ejection of the halogen in an endo-relationship to the abstracted proton. The primary product of these processes, bridgehead dienes such as bicyclo[4.4.1]undeca-1,3,6(11),8-tetraene, 4,4,12-trichloro-tricyclo[5.4.1.0^3.5]dodeca-1 (11), 7(12),9-triene. and 10-chlorobicyclo[4.3.1]deca-1,3,6(10)-triene, then undergo further reaction involving, amongst other things, 1,3-prototropic shifts. Attempts to probe the mode of formation of the C₁₁-4-methylazulene from the C₁₀-precursor 8,8-dichloro-2,3,4,7-tetrahydro-3a,7a-methano-1H-indene have uncovered a novel methylation reaction of azulene by the dimsyl anion.