Redox vs. Lewis acid catalysis. The chemistry of 1,2-diarylethanes in antimony trichloride-rich molten salt media

Abstract

The product selectivity for the reactions of 1,2-diarylethanes, ArCH/sub 2/CH/sub 2/Ar (Ar = phenyl (DPE), 1-naphthyl (DNE), and 9-anthryl (DAE), in SbCL/sub 3/-rich molten salt media at 80-150/sup 0/C is found to be highly dependent on the nature of the aryl group. This chemistry, which has been examined primarily by in situ /sup 1/H NMR and by quench and separation techniques, is interpreted in terms of a change in mechanism from Lewis acid catalysis to redox catalysis as the aryl group becomes more oxidizable. DPE is activated solely by the Lewis acid function of the melt, which generates a benzylic carbenium ion, ArC/sup +/HCH/sub 2/Ar, as a reactive intermediate by hydride abstraction by SbCl/sub 2//sup +/ x nSbCl/sub 3/. This intermediate leads to transalkylation products resulting from the selective cleavage of the benzylic (sp/sup 2/-sp/sup 3/) carbon-carbon bond. DAE, on the other hand, is activated by the redox function of the melt, which generates a radical cation intermediate, ArCH/sub 2/CH/sub 2/Ar/sup +/, by oxidation by Sb/sup 3 +/. At 80 /sup 0/C, the primary reaction pathway for the DAE radial cation is an intramolecular aryl-aryl coupling with internal hydrogen transfer to produce an unusual spiro isomer of DAE in highmore » yield. At higher temperatures (150/sup 0/C) cleavage of the ethylenic (sp/sup 3/-sp/sup 3/) carbon-carbon bond in the radical cation becomes an important process in competition with the intramolecular coupling. For DNE, combined Lewis acid catalysis (positional isomerization and transalkylation) and redox catalysis (intramolecular coupling) generates a complex product mixture. The reactions observed for DAE/sup +/ and DNE/sup +/ in these highly purified anhydrous SbCl/sub 3/-rich melts are discussed in relation to reaction pathways for alkylbenzene and diarylethane radical cations in other media. 54 references, 1 figure.« less