A New Highly Efficient Ruthenium Metathesis Catalyst

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 %.

‘Pseudo-halide’ Derivatives of Grubbs- and Schrock-Type Catalysts for Olefin Metathesis

“…Grela and co-workers had previously shown that electron-donating methoxy groups on the aryl ring of the benzylidene ligand were very necessary to stabilize catalysts with methoxy-instead of propoxy-based alkoxy oxygens. [8][9][10] Therefore, Ru-based 'pseudo-halide' catalysts with CF 3 CO 2 -, CF 3 CF 2 CO 2 -, and CF 3 CF 2 CF 2 CO 2 -ligands were formulated from dichloride complexes with an a-asarone-derived ligand. 26 These three complexes possessed k p /k i values from 2 to 6 and facilitated living cyclopolymerizations of DEDPM which even rivaled the values obtained by Schrock catalysts; 26,86,87 however, in this series, the catalyst with the smallest fluorinated carboxylate (CF 3 CO 2 -) was the better initiator of the three examined.…”

Recent Developments in Olefin Cross‐Metathesis

“…Catalysts 4 b [91] and 4 c, [92] which incorporate steric bulk ortho to the chelating isopropoxy moiety, are powerful metathesis initiators. We found that the use of only trace amounts of these precatalysts (only 0.005 mol % in the case of 4 c) effects ROM-CM reactions between a range of norbornenes or oxanorbornenes and allyltrimethylsilane in near quantitative yields.…”

Synthesis, characterization, and catalytic activity of a ruthenium carbene complex coordinated with bidentate 2-pyridine-carboxylato ligands

“…After 90 min at room temperature a conversion of 94% and after 120 min complete conversion was achieved (Table 1). However, catalyst 7 is even after activation with HCl less active then catalyst 4 which at a concentration of only 1 mol% is capable to convert 99% of 9 within 90 min into 10 (Table 1, entry 6) [15].…”

“…Selected bond lengths (Å ) and bond angles (°): Ru-O31 2.173(3), Ru-O41 2.062(3), Ru-N3 2.078(3), Ru-N4 2.107(3), Ru-C1 1.876(5), Ru-C8 2.088(4), N1-C8 1.353(5), N2-C8 1.348(5); O31-Ru-O41 84.55(12), O31-Ru-N3 77.69(12), O31-Ru-N4 82.49(12), O31-Ru-C1 170.43(15), O31-Ru-C8 96.94(14), O41-Ru-N3 160.57(12), O41-Ru-N4 78.73(12), O41-Ru-C1 102.0(2), O41-Ru-C8 93.41(13), N3-Ru-N4 91.10(13), N3-Ru-C1 94.8(2), N3-Ru-C8 96.45(14), N4-Ru-C1 91.8(2), N4-Ru-C8 172.14(14), C1-Ru-C8 89.7(2), N1-C8-N2 106.1(4).…”