Ru‐Catalyzed Heptalene Formation from Azulenes and Dimethyl Acetylenedicarboxylate

It is shown that azulenes react with dimethyl acetylenedicarboxylate (ADM) in solvents such as toluene, dioxan, or MeCN in the presence of 2 mol‐% [RuH 2 (PPh 3 ) 4 ] already at temperatures as low as 100° and lead to the formation of the corresponding heptalene‐1,2‐dicarboxylates in excellent yields ( Tables 1 and 2 ). The Ru‐catalyzed reaction of ADM with 1‐( tert ‐butyl)‐4,6,8‐trimethylazulene ( 31 ) takes place even at room temperature, yielding the primary tricyclic addition product 32 and its thermal retro ‐ Diels ‐ Alder product dimethyl 4,6,8‐trimethylazulene‐1,2‐dicarboxylate ( 21 ; Scheme 4 ). At 100° in MeCN, 32 yields 90% of 21 and only 10% of the corresponding heptalene. These observations demonstrate that [RuH 2 (PPh 3 ) 4 ] catalyzes the first step of the thermal formation of heptalenes from azulenes and ADM which occurs in apolar solvents such as tetralin or decalin at temperatures > 180° ( cf. Scheme 1 ).