Formation and Thermal Rearrangement of Dimethyl Tricyclo[6.2.2.01,7]dodeca‐2,4,6,9,11‐pentaene‐9,10‐dicarboxylates

Abstract

AbstractThe high‐pressure reaction of 1‐methylazulenes 1 with excess of dimethyl acetylenedicarboxylate (ADM) in hexane at 30° and at pressures up to 7 kbar affords the tricyclic compounds 2 in reasonable‐to‐good yields (cf. Table 1). The crystalline compounds 2 decompose on melting into the starting materials and undergo rearrangement to the corresponding heptalene‐1,2‐dicarboxylates 6. The X‐ray crystal‐structure analyses of 2f and 2g (cf. Fig. 1) reveal the presence of a perfectly planar seven‐membered ring and comparably long C(1)–C(10) as well as C(1)–C(11) bonds (cf. Tables 2 and 3). The thermolysis of 2g in different solvents leads in aprotic media to the formation of the starting azulene 1g and, depending upon the polarity of the solvents, to varying amounts of the corresponding heptalene‐1,2‐dicarboxylate 6f (cf. Table 11). The formed amounts of 1g depends linearly on the ET values of the solvents (cf. Fig. 4). The same is valid for the thermolysis of 2g in protic media (cf. Table 10 and Fig.3). However, in these cases instead of the heptalene‐1,2‐dicarboxylate 6g, the corresponding (E)‐ and (Z)‐isomers of the 1‐(azulen‐1‐yl)ethene‐1,2‐dicarboxylates 7g are formed. The other tricyclic compounds 2 exhibit the same behavior on thermolysis in MeCN and BuOH (cf. Tables 8 and 9, resp.). The results show that the tricyclic compounds 2 undergo at temperatures up to 110° two competing reactions, namely heterolysis of the C(1)–C(10) bond, leading to the formation of heptalenes 6 in polar aprotic media, and the (E)‐ and (Z)‐ethene‐1,2‐dicarboxylates 7 in polar protic media, and concerted homolysis of the C(1)–C(10) and C(8)–C(9) bonds in the sense of a retro‐Diels‐Alder reaction in apolar media, yielding the starting azulenes and ADM, the amount of which decrease with increasing polarity of the solvent. The kinetic and activation parameters measured for 2g and the other tricyclic compounds 2 are collected in Tables 12–15. The tricyclic compounds 2a and 2b show in polar aprotic media (MeCN) a different behavior in that they form, instead of heptalenes, the corresponding 3,4‐dihydrocyclopent[cd]azulene‐1,2‐dicarboxylates 16a and 16b, respectively (Scheme 4). Experiments with [8‐2H]‐2a showed that these compounds are not formed via intramolecular H‐shifts (cf. Schemes 8 and 9).