Divergent Syntheses of (Z)-3-Alkylideneisobenzofuran-1(3 H)-ones and 1 H-Isochromen-1-ones by Copper-Catalyzed Cycloisomerization of 2-Alkynylbenzoic Acids in Ionic Liquids

The cycloisomerization of readily available 2-alkynylbenzoic acids 1 in ionic liquids (ILs) as recyclable reaction media has been studied under the catalytic action of CuCl2. With substrates bearing an aryl group on the triple bond, a mixture of (Z)-3-alkylideneisobenzofuran-1(3H)-ones (from 5-exo-dig cyclization) and 1H-isochromen-1-ones (from 6-endo-dig cyclization) was observed in 1-ethyl-3-methylimidazolium ethyl sulfate (EmimEtSO4), while the reaction turned out to be selective toward the formation of the isobenzofuranone only using N-ethyl-N-methylmorpholinium dicyanamide [Mor1,2N(CN)2] as the solvent. The 5-membered product was also obtained selectively when the substrate bearing a terminal triple bond was employed, either in EmimEtSO4 or Mor1,2N(CN)2. On the other hand, 2-alkynylbenzoic acids bearing an alkyl or an alkenyl group on the triple bond selectively led, in EmimEtSO4, to 1H-isochromen-1-ones, while the formation of a regioisomeric mixture was observed in Mor1,2N(CN)2. In any case, the solvent/catalyst system could be easily recycled after extraction of the product from the reaction mixture with diethyl ether. DFT calculations have been carried out to clarify the reaction outcome depending on reaction conditions, and the structures of two representative products, which are (Z)-3-benzylideneisobenzofuran-1(3H)-one and (Z)-3-(4-methylphenylmethylidene)isobenzofuran-1(3H)-one, have been confirmed by X-ray diffraction analysis.