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

Article abstract of DOI:10.1002/hlca.19940770723

The thermal reaction of 1-<(E)-styryl>azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin at 190-200 deg does not lead to the formation of the corresponding heptalene 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 -catalized reaction of 4,6,8-trimethyl-1-<(E)-4-R-styryl>azulenes (R=H, MeO, Cl; Scheme 4) with ADM in MeCN at 110 deg 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 (3-5percent; Scheme 4).The benzanellated azulenes trans-10a and trans-10b are also found in small amounts (2-3percent) 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 (1-3percent).Similar results are obtained in the -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 (29percent).That the substituent 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 (10percent).Again, the main product is the corresponding azulene-1,2-dicarboxylate 25 (20percent).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 (28percent) as well as Ru-catalyzed in MeCN (40percent).Whereas only small amounts of the azulene-1,2-dicarboxylate 8 (1 and 5percent, respectively) are formed, the corresponding benzanellated azulene trans-29 is found to be the second main product 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 deg in decalin and at 110 deg in MeCN by catalysis.The yields (30percent) are much better in the catalized reaction.The formation of by-products (e.g. 39-41; Scheme 14) in small amounts (0.5-5percent) in the Ru-catalized reactions allows to understand better the reactivity of zwitterions (e.g. 42 and their tricyclic follow up products (e.g. 43) built from azulenes and ADM (cf.Scheme 15).