A Detailed Investigation of the Reaction of 5,9-Diphenylbenz[a]azulene with Dialkyl Acetylenedicarboxylates Leading to Dialkyl 8,12-Diphenylbenzo[a]heptalene-6,7-dicarboxylates

Abstract

The synthesis of 5,9-diphenylbenz[a]azulene (1) from 1,3-diphenylcyclopent[a]indene-2,8-dione (4) and cyclopropene has been re-investigated. The reduction of the decarbonylated cycloadduct 5 with LiAlH4/AlCl3 in Et2O leads not only to the expected 7,10-dihydrobenz[a]azulene 6, but also to small amounts of the cyclopropa[b]fluorenes exo-7 and endo-7 (cf. Scheme 2), the structures of which have been determined by X-ray crystal-structure analysis (cf. Fig. 1). The reaction of 1 with dialkyl acetylenedicarboxylates (ADR) in MeCN at 100° in the presence of 2 mol-% of catalysts such as [RuH2(PPh3)4] results mainly in the formation of the expected 8,12-diphenylbenzo[a]heptalene-6,7-dicarboxylates 3. A thorough investigation of the reaction mixture of 1 and dimethyl acetylenedicarboxylate (ADM) revealed the presence of a number of intermediates and side products (Scheme 5). Most important was the isolation and identification of the cyclobutene intermediate 9a (cf. Fig. 4), which is formed by a zwitterionic rearrangement of the primary adduct 2a of 1 and ADM and represents the direct precursor of the heptalene-diester 3a. Compounds of type 9a have so far only been postulated as necessary intermediates in the thermal reaction of azulenes and ADR to give corresponding heptalenedicarboxylates. Compound 9a is photochemically unstable and undergoes rearrangement even under the influence of normal laboratory light into a mixture of trans-10a and cis-10a (Scheme 8). Both diastereoisomers are also found in the original reaction mixture of 1 and ADM, but not when the reaction is performed under exclusion of light. On heating in MeCN at 100°, or better in DMF at 150°, trans-10a and cis-10a undergo rearrangement to the fluoranthene-1,2-dicarboxylate 11a (Scheme 9), which is also present in the original reaction mixture of 1 and ADM. The catalysts do not accelerate the reaction of 1 and ADR, but they lead to better yields of the benzo[a]heptalene-6,7-dicarboxylates 3, especially in the reaction of 1 with diisopropyl acetylenedicarboxylate (ADiP) (cf. Tables 1 and 2).