Fast Olefin Metathesis at Low Catalyst Loading

Reactions of the Grubbs 3rd generation complexes [RuCl2(NHC)-(Ind)(Py)] (N-heterocyclic carbene (NHC) = 1,3-bis(2,4,6-trimethylphenylimidazolin)-2-ylidene (SIMes), 1,3-bis(2,6-diisopropylphenylimidazolin)-2-ylidene (SIPr), or 1,3-bis(2,6-diisopropylphenylimidazol)-2-ylidene (IPr); Ind = 3-phenylindenylid-1-ene, Py = pyridine) with 2-ethenyl-N-alkylaniline (alkyl = Me, Et) result in the formation of the new N-Grubbs-Hoveyda-type complexes 5 (NHC = SIMes, alkyl = Me), 6 (SIMes, Et), 7 (IPr, Me), 8 (SIPr, Me), and 9 (SIPr, Et) with N-chelating benzylidene ligands in yields of 50-75%. Compared to their respective, conventional, O-Grubbs-Hoveyda complexes, the new complexes are characterized by fast catalyst activation, which translates into fast and efficient ring-closing metathesis (RCM) reactivity. Catalyst loadings of 15-150 ppm (0.0015-0.015 mol%) are sufficient for the conversion of a wide range of diolefinic substrates into the respective RCM products after 15 min at 50 degrees C in toluene; compounds 8 and 9 are the most catalytically active complexes. The use of complex 8 in RCM reactions enables the formation of N-protected 2,5-dihydropyrroles with turnover numbers (TONs) of up to 58 000 and turnover frequencies (TOFs) of up to 232 000 h(-1); the use of the N-protected 1,2,3,6-tetrahydropyridines proceeds with TONs of up to 37 000 and TOFs of up to 147 000 h(-1); and the use of the N-protected 2,3,6,7-tetrahydroazepines proceeds with TONs of up to 19 000 and TOFs of up to 76 000 h(-1), with yields for these reactions ranging from 83-92%.