Ru‐Catalyzed Heptalene Formation from Azulenes and Dimethyl Acetylenedicarboxylate

Abstract

Abstract 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 ).