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Thermal Reaction of Highly Alkylated Azulenes with Dimethyl Acetylenedicarboxylate: HOMO(Azulene) vs. SHOMO(Azulene) Control in the Primary Thermal Addition Step
The reaction of highly alkylated azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin or tetralin at 180–200° 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°. Photochemically 4 rearranges to a corresponding tetracyclic compound 20 via a di‐π‐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 intermediates 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 the at 200° 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 analyses of the products were performed (cf. Chapt. 4.1).