ABSTRACT
It has been established that an electron-deficient cyclopentadienyl rhodium(III) (CpE RhIII ) complex catalyzes the oxidative and decarboxylative [2+1+2+1] cycloaddition of benzoic acids with diynes through C≡C triple bond cleavage, leading to fused naphthalenes. This cyclotrimerization is initiated by directed ortho C-H bond cleavage of a benzoic acid, and the subsequent regioselective alkyne insertion and decarboxylation produce a five-membered rhodacycle. The electron-deficient nature of the CpE RhIII complex promotes reductive elimination giving a cyclobutadiene-rhodium(I) complex rather than the second intermolecular alkyne insertion. The oxidative addition of the thus generated cyclobutadiene to rhodium(I) (formal C≡C triple bond cleavage) followed by the second intramolecular alkyne insertion and reductive elimination give the corresponding [2+1+2+1] cycloaddition product. The synthetic utility of the present [2+1+2+1] cycloaddition was demonstrated in the facile synthesis of a donor-acceptor [5]helicene and a hemi-hexabenzocoronene by a combination with the chemoselective Scholl reaction.
ABSTRACT
It has been established that an electron-deficient (η5 -cyclopentadienyl)rhodium(III) [CpE RhIII ] complex is capable of catalyzing the decarboxylative and oxidative [2+2+2] annulation of benzoic acids with alkynes to produce substituted naphthalenes at room temperature. The appropriate choice of the additive and the solvent is crucial for this transformation. This catalyst system allowed use of oxygen as a terminal oxidant and broadened the substrate scope including both aromatic and aliphatic alkynes. In this catalysis, the electron deficient nature of the CpE RhIII catalyst would cause the strong rhodium-π interaction, which accelerates the decarboxylation as well as the C-H bond cleavage.