Theoretical studies of palladium-catalyzed cycloaddition of alkynyl aryl ethers and alkynes.

Density functional theory (DFT) was used to investigate palladium(0)-catalyzed cycloaddition of alkynyl aryl ethers and alkynes to generate 2-methylidene-2H-chromenes. Calculations indicated that the cycloaddition had five possible reaction pathways: I, II, III, IV, and V. In the palladium(0)-alkynyl aryl ether complex IM1, the oxidative addition of the Caryl-H bond occurred prior to the dissociation of a ligand PMe3. The dissociation of a ligand PMe3 from the five-coordinated complex IM2 was much easier to achieve than the hydrogen transfer reaction and the substitution reaction of alkynes. In the palladium(0)-hydride complex IM4, the hydrogen migration of H1 from palladium to carbon C1 was much easier to achieve than migration to carbon C2. In the four-coordinated aryl-palladium-alkyne complexes IM6a and IM6b, the alkyne insertion reaction into the Pd-Caryl bond occurred prior to that into the Pd-Calkenyl bond. The reaction channel IM1 → TS1 → IM2 → IM4 → TS3a → IM5a → IM6a → TS4a1 → IM7a1 → TS5a1 → IM8a was the most favorable among the catalytic reaction pathways of the cycloaddition of alkynyl aryl ethers and 2-butynes catalyzed by the palladium(0)/PMe3 complex. Moreover, hydrogen migration was the rate-determining step for this channel. The dominant product was 2-methylidene-2H-chromenes P1, which is in agreement with experimental studies.