Rates and mechanism of rhodium-catalyzed [2+2+2] cycloaddition of bisalkynes and a monoalkyne

The mechanism of RhCl(PPh₃)₃-catalyzed [2+2+2] cycloaddition of alkynes is investigated in the case of the reaction of symmetrical diynes la and lb with the monoalkyne 3 (HOCH₂- Cdeg;₂OH), leading to highly substituted benzene derivatives in dichloromethane at 25 °C. The two main steps of the catalytic cycle are characterized. The intermediate rhodacyclopentadiene Rh¹¹¹ complexes 2a and 2b (formed by oxidative coupling after the coordination of the diynes la and lb to RhCl(PPh₃)₂) are characterized by cyclic voltammetry, conductivity measurements, ³¹P NMR, and ESI-MS. The formation of complexes 2a and 2b (step A) and their further reactions with the monoalkyne 3, which deliver RhCl(PPh₃)₃ and the final product (step B), are followed by means of electrochemical techniques that deliver kinetic data for the two successive separately investigated steps. From the relative values of the half-reaction times of step A (t_A1/2 = 650 and 75 s for la and lb, respectively) and step B (t_B1/2 = 130 and 680 s for 2a and 2b, respectively) determined under stoichiometric conditions, it emerges that step A (coordination of the two C°C bonds of 1, followed by oxidative coupling) is rate-determining in the reaction involving 2a, whereas step B (reaction of the intermediate complexes 2 with the monoalkyne 3) is rate-determining in the reaction involving lb. Kinetic data for the catalytic cycle of a RhCl(PPh₃)₃-catalyzed [2+2+2] cycloaddition of alkynes are thus presented for the first time.