Unexpected Thermal Transformation of Aryl 3-Arylprop-2-ynoates: Formation of 3-(Diarylmethylidene)-2,3-dihydrofuran-2-ones

A number of aryl 3-arylprop-2-ynoates 3 has been prepared (cf. Table 1 and Schemes 3 – 5). In contrast to aryl prop-2-ynoates and but-2-ynoates, 3-arylprop-2-ynoates 3 (with the exception of 3b) do not undergo, by flash vacuum pyrolysis (FVP), rearrangement to corresponding cyclohepta[b]furan-2(2H)-ones 2 (cf. Schemes 1 and 2). On melting, however, or in solution at temperatures >150°, the compounds 3 are converted stereospecifically to the dimers 3-[(Z)-diarylmethylidene]-2,3-dihydrofuran-2-ones (Z)-11 and the cyclic anhydrides 12 of 1,4-diarylnaphthalene-2,3-dicarboxylic acids, which also represent dimers of 3, formed by loss of one molecule of the corresponding phenol from the aryloxy part (cf. Scheme 6). Small amounts of diaryl naphthalene-2,3-dicarboxylates 13 accompanied the product types (Z)-11 and 12, when the thermal transformation of 3 was performed in the molten state or at high concentration of 3 in solution (cf. Tables 2 and 4). The structure of the dihydrofuranone (Z)-11c was established by an X-ray crystal-structure analysis (Fig. 1). The structures of the dihydrofuranones 11 and the cyclic anhydrides 12 indicate that the 3-arylprop-2-ynoates 3, on heating, must undergo an aryl O→C(3) migration leading to a reactive intermediate, which attacks a second molecule of 3, finally under formation of (Z)-11 or 12. Formation of the diaryl dicarboxylates 13, on the other hand, are the result of the well-known thermal Diels-Alder-type dimerization of 3 without rearrangement (cf. Scheme 7). At low concentration of 3 in decalin, the decrease of 3 follows up to ca. 20% conversion first-order kinetics (cf. Table 5), which is in agreement with a monomolecular rearrangement of 3. Moreover, heating the highly reactive 2,4,6-trimethylphenyl 3-(4-nitrophenyl)prop-2-ynonate (3f) in the presence of a twofold molar amount of the much less reactive phenyl 3-(4-nitrophenyl)prop-2-ynonate (3g) led, beside (Z)-11f, to the cross products (Z)-11fg, and, due to subsequent thermal isomerization, (E)-11fg (cf. Scheme 10), the structures of which indicated that they were composed, as expected, of rearranged 3f and structurally unaltered 3g. Finally, thermal transposition of [17O]-3i with the 17O-label at the aryloxy group gave (Z)- and (E)-[17O2]-11i with the 17O-label of rearranged [17O]-3i specifically at the oxo group of the two isomeric dihydrofuranones (cf. Scheme 8), indicating a highly ordered cyclic transition state of the aryl O→C(3) migration (cf. Scheme 9).