Studies of Some Strained Organic Molecules
The characterisation of rare examples of C1-substituted cyclopropanaphthalenes has been achieved with silanes (104) and (112) by employing the C1 anion (106). With toluene, N,N-dimethylacetamide, and cyclopropanaphthalene (58) this same anion gives the novel 6-methyl-7H-dibenzo[b,g]fluorene (179), a formal dimer of cycloproparene (58). Hydrocarbon (179) is the sole dibenzo[b,g]hydrocarbon characterised and this has required extensive spectroscopic study with confirmation from X-ray analysis. A possible new route to alkylidenecyclopropanaphthalenes (114) employing lithiate (170) and either cycloproparene (58) or its disilyl analogue (105) was found to offer no advantage over known procedures. Application of the protocols embodied in this procedure to brominated synthons (114o) and (114p) has afforded novel pi-extended methylidene compounds (197a) and (199) in low yield. Cyclopentadienylidene (197a) has also been prepared in better yield from benzophenone-containing methylidenecycloproparene (200). Initial attempts to obtain (200) from anion (193) and N,N-dimethylbenzamide were unsuccessful and gave instead the new phenol (114q). The first acylcycloproparenes (189) and (202) have been obtained in modest yield from anion (103) and N,N-dimethyl-acetamide, and -benzamide. With N,N-dimethyl-carbamoyl chloride anion (103) gives the bis-amide (205). With hydrochloric acid these acylcycloproparenes give rise to 2,3-disubstituted naphthalenes rather than 2-substituted naphthalenes that typically arise from protonation at the aromatic ring. Thermolysis leads to ring expansion and naphthofuran formation. Enolate formation from the 1-acyl-cyclopropanaphthalenes (189) and (202) and anion capture at oxygen affords the first cyclopropanaphthalenylidene enol ethers (219) and (220). 1H-Cyclopropa[b]naphthalene-3,6-dione (154) adds buta-1,3-diene across the enedione Pi-bond to give the tetrahydrocyclopropanthraquinone (160). Enolisation of (160) provides phenolate (234) that can be diverted to ether (229) or oxidised to the dihydroanthraquinone (230). Dehydrogenation of (229) is readily achieved and gives the first anthraquinone of the cycloproparene series 1H-cyclopropa[b]anthracene-3,8-dione (162); quinone (162) is only the second cyclopropaquinone to have been characterised. Alternative routes to quinone (162) and its 3,8-dimethoxy analogue (163) have been examined with a view to providing the first alkylidenecyclopropanthracenes. The first examples of cross-conjugated dithiole-containing cycloproparenes, (169) and (267), have been prepared from cyclopropanthraquinone (162) but they are unstable solids. The pi-extended dithiole-containing methylidene compound (273) has been prepared in good yield from Wittig-Horner olefination of the benzoylmethylidene compound (200). Evidence was obtained to support the formation of a charge-transfer complex from it. Ketones already carrying a conjugated dithiole moiety participate in the Peterson olefination with the alpha-silyl anion (106) and give the new pi-extended methylidenecyclopropanaphthalenes (274) and (277) of limited stability.