The rhodium-catalyzed cyclization of a series of 2,2-diarylalkanoic acids in the presence of copper acetate as an oxidant smoothly proceeded through double C-H bond cleavages and subsequent decarboxylation to produce the corresponding fluorene derivs. E.g., in presence of [CpERhCl2]2 [CpE = 1,3-bis(ethoxycarbonyl)-2,4,5-trimethylcyclopentadienyl] and Cu(OAc)2, dehydrogenative cyclization of 2,2-diphenylpropanoic acid gave 65% 9-methylfluorene. The direct cyclization of triarylmethanols also took place efficiently by using an iridium catalyst in place of the rhodium, while the hydroxy function was still intact. [on SciFinder(R)]
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