Thermal Reaction of Highly Alkylated Azulenes with Dimethyl Acetylenedicarboxylate: HOMO(Azulene) vs. SHOMO(Azulene) Control in the Primary Thermal Addition Step

Article abstract of DOI:10.1002/hlca.19920750803

The reaction of highly alkylated azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin or tetralin at 180-200 deg C yields, beside the expected heptalene- and azulene-1,2-dicarboxylates, tetracyclic compounds of type "anti"-V and tricyclic compounds of type E (cf.Schemes 2-4 and 8-11).The compounds of type "anti"-V represent Diels-Alder adducts of the primary tricyclic intermediates A with ADM.In some cases, the tricyclic compounds of type E also underwent a consecutive Diels-Alder reaction with ADM to yield the tetracyclic compounds of type "anti"- or "syn"-VI (cf.Schemes 2 and 8-11).The tricyclic compounds of type E, namely 4 and 8, reversibly rearrange via <1,5>-C shifts to isomeric tricyclic structures (cf. 18 and 19, respectively, in Scheme 6) already at temperatures > 50 deg C.Photochemically 4 rearranges to a corresponding tetracyclic compound 20 via a di-?-methane reaction.The observed heptalene- and azulene-1,2-dicarboxylates as well as the tetracyclic compounds of type "anti"-V are formed from the primary tricyclic intermediate A via rearrangement (-> heptalenedicarboxylates), retro-Diels-Alder reaction (-> azulenedicarboxylates), and Diels-Alder reaction with ADM.The different reaction channels of A are dependent on the substituents.However, the main reaction channel of A is its retro-Diels-Alder reaction to the starting materials (azulene and ADM).The highly reversible Diels-Alder reaction of ADM to the five-membered ring of the azulenes is HOMO(azulene)/LUMO(ADM)-controlled, in contrast to that at 200 deg C irreversible ADM addition to the seven-membered ring of the azulenes to yield the Diels-Alder products of type E.This competing reaction must occur on grounds of orbital-symmetry conservation under SHOMO(azulene)/LUMO(ADM) control (cf.Schemes 20-22).Several X-ray diffraction analysis of the products were performed (cf.Chapt. 4.1).