Fast Olefin Metathesis: Synthesis of 2‐Aryloxy‐Substituted Hoveyda‐Type Complexes and Application in Ring‐Closing Metathesis

Four 1‐(4‐R‐phenoxy)‐2‐ethenylbenzenes (R=NMe 2 , H, Cl, NO 2 ) 4a , 4b , 4c and 4d were reacted with the ruthenium complexes [RuCl 2 (NHC)(3‐phenylindenylidene)(py)] in the presence of a protic resin to result in the formation of the respective Hoveyda‐type complexes 5a–d {NHC=SIMes [1,3‐bis(2,4,6‐trimethylphenylimidazolin)‐2‐ylidene]} and 6a–d {NHC=SIPr [1,3‐bis(2,6‐diisopropylphenylimidazolin)‐2‐ylidene]} in 66–84% yield. The lower steric bulk and the decreased donation of the diaryl ether oxygen atoms in complexes 5 and 6 led to rapidly initiating precatalysts. The Ru(II/III) redox potentials of complexes 6 were determined ( 6a–d: ΔE=0.89–1.08 V). In the crystal structure of 5b two independent molecules were observed in the unit cell, displaying RuO distances of 226.6(4) and 230.5(3) pm. The catalytic performance of complexes 5 and 6 in various ring‐closing metathesis (RCM) reactions was studied. Catalyst loadings of between 15–200 ppm are sufficient for the formation of >90% yield of the respective cyclic products. Complex 6b catalyzes the formation of N ‐protected 2,5‐dihydropyrroles with up to TON 64,000 and TOF 256,000 h −1 , of the N ‐protected 1,2,3,6‐ tetrahydropyridines with up to TON 18,200 and TOF 73,000 h −1 and of the N ‐protected 2,3,6,7‐tetrahydroazepines with up to TON 8,100 and TOF 32,000 h −1 with yields ranging between 77 and 96%.