Fast Olefin Metathesis at Low Catalyst Loading

• Published in: Chemistry : a European journal 2012-10, Vol.18 (40), p.12845-12853
• Main Authors: Peeck, Lars H., Savka, Roman D., Plenio, Herbert
• Format: Article
• Language: English
• Subjects: alkenes; Catalysts; Chemistry; Conversion; Decomposition reactions; Formations; Ligands; Metathesis; precatalyst activation; RCM; ring-closing reactions; ruthenium; Toluene
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201201010

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 °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 %. The tortoise and the hare: The use of diphenylalkylamino‐based instead of phenyldialkylamino‐based styrenes (see figure) leads to rapidly initiating precatalysts that enable very fast ring‐closing metathesis reactions with turnover numbers of up to 58 000 and turnover frequencies of up to 232 000 h−1.