Rate Studies and Mechanism of Ring-Closing Olefin Metathesis Catalyzed by Cationic Ruthenium Allenylidene Arene Complexes

Abstract

The ring-closing metathesis reaction of N,N-diallyltosylamide (2) catalyzed by [(η6-p-cymene)(PCy3)RuCl(CCCPh2)]X (X = OTf (CF3SO3) (1), PF6, BF4, SbF6) and by [(η6-p-cymene)(PCy3)RuCl(CCC(p-Y-C6H4)2]OTf (Y = MeO, Cl, F) complexes has been monitored in situ by 1H NMR, in benzene-d6 and in dichloromethane-d2, in the temperature range 33−58 °C. The reaction proceeds selectively to form N-tosyl-2,5-dihydropyrrole (3), in the case of complexes [(η6-p-cymene)(PCy3)RuCl(CCCPh2)]X (X = OTf, PF6), under thermal activation, while lower reactivity and selectivity are exhibited by the other complexes. Evidence is given for an activation step leading to the catalytic species. Under pseudo-first-order conditions, the metathesis reaction catalyzed by complex 1 is first-order in the diallylic substrate in benzene-d6 above 50 °C when the propagation step is slower than the activation of the catalytic species. The reaction is zero-order in substrate at lower temperatures when the activation of the ruthenium complex is slower than the ring-closing metathesis process and faster in benzene-d6 than in dichloromethane-d2. The presence of added p-cymene does not inhibit the reactivity, while inhibition occurs in the presence of added PCy3. In the latter case, the substrate is converted slowly into an isomeric product. When appropriate, the behavior of complex 1 as precatatyst is compared with that of other catalytic systems. 1H NMR, FT-IR, and UV−visible analyses indicate that the activation process of complex 1 is characterized by an intramolecular transformation of the ruthenium-allenylidene group into the corresponding ruthenium-phenylindenylidene moiety.