Thermal and Ru‐catalyzed Reactions of Styryl‐Substituted Azulenes with Dimethyl Acetylenedicarboxylate

The thermal reaction of 1‐[( E )‐styrl]azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin at 190–200° does not lead to the formation fo the corresponding heptalene‐1,2‐dicarboxylates ( Scheme 2 ). Main products are the corresponding azulene‐1,2‐dicarboxylates (see 4 and 9 ), accompanied by the benzanellated azulenes trans ‐ 10a and trans ‐ 11 , respectively. The latter compounds are formed by a Diels ‐ Alder reaction of the starting azulenes and ADM, followed by an ene reaction with ADM ( cf. Scheme 3 ). The [RuH 2 (PPh 3 ) 4 ]‐catalyzed reaction of 4,6,8‐trimethyl‐1‐[( E )‐4‐R‐styryl]azulenes (R=H, MeO, Cl; Scheme 4 ) with ADM in MeCN at 110° yields again the azulene‐1,2‐dicarboxylates as main products. However, in this case, the corresponding heptalene‐1,2‐dicarboxylates are also formed in small amounts (3–5%; Scheme 4 ). The benzanellated azulenes trans ‐ 10a and trans ‐ 10b are also found in small amounts (2–3%) in the reaction mixture. ADM Addition products at C(3) of the azulene ring as well as at C(2) of the styryl moiety are also observed in minor amounts (1–3%). Similar results are obtained in the [RuH 2 (PPh 3 ) 4 ]‐catalyzed reaction of 3‐[( E )‐styryl]guaiazulene (( E )‐ 8 ; Scheme 5 ) with ADM in MeCN. However, in this case, no heptalene formation is observed, and the amount of the ADM‐addition products at C(2) of the styryl group is remarkably increased (29%). That the substitutent pattern at the seven‐membered ring of ( E )‐ 8 is not responsible for the failure of heptalene formation is demonstrated by the Ru‐catalyzed reaction of 7‐isopropyl‐4‐methyl‐1‐[( E )‐styryl]azulene (( E )‐ 23 ; Scheme 11 ) with ADM in MeCN, yielding the corresponding heptalene‐1,2‐dicarboxylate ( E )‐ 26 (10%). Again, the main product is the corresponding azulene‐1,2‐dicarboxylate 25 (20%). Reaction of 4,6,8‐trimethyl‐2‐[( E )‐styryl]azulene (( E )‐ 27 ; Scheme 12 ) and ADM yields the heptalene‐dicarboxylates ( E )‐ 30A / B , purely thermally in decalin (28%) as well as Ru‐catalyzed in MeCN (40%). Whereas only small amounts of the azulene‐1,2‐dicarboxylate 8 (1 and 5%, respectively) are formed, the corresponding benzanellated azulene trans ‐ 29 ist found to be the second main product (21 and 10%, respectively) under both reaction conditions. The thermal reaction yields also the benzanellated azulene 28 which is not found in the catalyzed variant of the reaction. Heptalene‐1,2‐dicarboxylates are also formed from 4‐[( E )‐styryl]azulenes ( e.g. ( E )‐ 33 and ( E )‐ 34 ; Scheme 14 ) and ADM at 180–190° in decalin and at 110° in MeCN by [RuH 2 (PPh 3 ) 4 ] catalysis. The yields (30%) are much better in the catalyzed reaction. The formation of by‐products ( e.g. 39–41 ; Scheme 14 ) in small amounts (0.5–5%) in the Ru‐catalyzed reactions allows to understand better the reactivity of zwitterions ( e.g. 42 ) and their triyclic follow‐up products ( e.g. 43 ) built from azulenes and ADM ( cf. Scheme 15 ).