ELECTRON-TRANSFER REACTIONS IN ORGANIC-CHEMISTRY .21. NEW CATALYSTS FOR THE DIELS-ALDER DIMERIZATION OF 1,3-CYCLOHEXADIENE - BRONSTED, LEWIS AND REDOX CATALYSTS

The dimerization of a model compound, 1,3-cyclohexadiene (CHD), to give the Diels-Alder dimer [(CHD)2] has been studied with the aim of finding other catalysts than the commonly used tris(4-bromophenyl)aminium ion (TBPA+.). Other radical cations with redox potentials (ArH+./ArH) greater-than-or-equal-to ca. 1.0 V vs. SCE worked equally well. Among inorganic species, iron(III) chloride was a good catalyst and iron(III)(phenanthroline) a slightly less efficient one. In contrast, the strong oxidants, hexachloroosmate(V), tungsten hexachloride and hexanitratocerate(IV) did not sustain DA dimerization but instead produced benzene in low yield. With trifluoroacetic acid as a catalyst, a maximum yield of 15 % of the DA dimer was obtained. There was evidence for formation of a coupling product between tris(4-methoxyphenyl)aminium ion and CHD. In the TBPA+.-catalyzed reaction, added 'inert' anions, like perchlorate and trifluoromethanesulfonate, almost eliminated DA dimerization. These solutions displayed spectacular color changes; the original blue color of TBPA+. decayed very fast (seconds) and was later replaced by a new blue color (minutes) which then slowly faded away (hours). A persistent radical species (triplet, showing hyperfine coupling to one nitrogen) was detected in the solutions after the TBPA+.-catalyzed reactions.