A New Highly Efficient Ruthenium Metathesis Catalyst This work was supported by the Fonds der Chemischen Industrie.

Abstract

Catalytic olefin metathesis has recently become a powerful tool for carbon-carbon-bond formation in organic chemistry. [1] Over the last three years it has been demonstrated that ruthenium alkylidenes that bear N-heterocyclic carbene ligands, for example, 1 and similar analogues, exhibit extraordinary activity and stability. [2] Studies on the mechanism of olefin metathesis reactions have also been described. [2e,f] Phosphane-free catalyst 2 [3] has also recently been developed and studied. This catalyst possesses superior general reactivity toward electron-deficient olefins [4a,b,d] and is readily modified for attachment to solid supports, [4c,e] thus leading to enhanced recyclability [4c] and even to efficient metathesis in methanol and water. [4e] We have also prepared catalyst 3 with the aim of promoting asymmetric induction in metathesis reactions. [5] No asymmetric induction was found. However, activity studies [5] demonstrated a relative reactivity order of 3 > 1 > 2. BINOL-derived 3 also exhibited a similar shelf stability to 1 and 2. Herein we report the synthesis and catalytic activity of 4 (Scheme 1), a precatalyst with a markedly greater efficiency in metathesis processes than either 1, 2, or 3.At first, encouraged by the success of the BINOL-based catalyst 3, we were interested in determining which structural units in the ligand were responsible for the high initiation rates observed. Extensive studies indicated that the presence of steric bulk adjacent to the chelating isopropoxy moiety was critical. Therefore it seemed logical to synthesize complex 4 as shown in Scheme 1. 2-Hydroxybiphenyl-3-carbaldehyde (6) was synthesized from commercially available phenol 5, according to the literature procedure. [6] Ligand 8 was obtained by sequential alkylation and Wittig olefination of 6. The bright green complex 4 could be produced in good yield by the reaction of 1 with 8 (2 equiv), and was purified by flash chromatography. At this point, the potential of 4 in the ringclosing metathesis (RCM) of tosylamide 9 was tested. When these reactions were carried out at 0 8C in the presence of Scheme 1. Synthesis of catalyst 4. a) iPrBr, NaH, DMF, 50 8C, 82 %; b) Ph 3 P CH 3 Br À , tBuOK, Et 2 O, 0 8C, 88 %; c) 1 (1 equiv), 8 (2 equiv), CuCl (1 equiv), CH 2 Cl 2 , 408C, 71 %.

Origins of Initiation Rate Differences in Ruthenium Olefin Metathesis Catalysts Containing Chelating Benzylidenes

“…1 H NMR (500 MHz, CDCl 3 ) δ 10.53 (d, J = 0.8 Hz, 1H), 7.85 (dd, J 1 = 7.7 Hz, J 2 = 1.8 Hz, 1H), 7.55 (ddd, J 1 = 8.4 Hz, J 2 = 7.3 Hz, J 3 = 1.8 Hz, 1H), 7.09-7.04 (m, 1H), 6.98-6.95 (m, 1H), 4.36 (t, J = 5.7 Hz, 2H), 3 …”

“…Analytical data were in agreement with published data. 3 In our hands, it was found that the title compound decomposed rapidly in solution in solvents that were not rigorously dried and degassed. Thus, when using Method A, all manipulations were performed in the glovebox.…”

Ruthenium‐Catalyzed Ring‐Closing Metathesis: Recent Advances, Limitations and Opportunities

“…Initiation of C40 itself is slow, despite the low oxophilicity of Ru(II), owing to the high thermodynamic stability of the five-membered chelate ring. Improved turnon efficiency was achieved by steric (C41 [133,134]) or electronic (C42 [109,110,135]) destabilization of the chelate.…”

“…Substrates bearing terminal olefins were rapidly cyclized to five-to sevenmembered rings under mild conditions (0-23 ˚C, 1 mol % Ru) [109,110,[133][134][135]; slightly greater activity was generally manifested by C41b [110]. The substrates shown in Scheme 19 proved more challenging.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…A N-heterocyclic carbene ligand containing a binaphthyl substituent induces excellent enantioselectivity for some ring-opening/cross metathesis reactions in air, 167 and a functionalised alkylidene incorporating a hemilabile ether moiety has been investigated for ring closing metathesis. 168 A DFT study on the mechanism of olefin metathesis using ruthenium() N-heterocyclic carbene complexes has also been investigated. 169 In related chemistry a very rare example of a terminal carbide ligand has been isolated originating from a unique carbon atom transfer reaction as shown in Scheme 2.…”