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 %.

The exchange of the PCy 3 ligand in complex 2 by an o-isopropyl-phenylether ligand leads to the extremely stable and highly selective initiator 3 for cross metathesis reactions. For the first time, Ru-catalysed cross coupling with acrylonitrile can be performed in good yields.Ru-complexes with sterically demanding N-heterocyclic carbene (NHC) ligands are presently of growing interest ( Figure). 1 Compound 2 exhibits a substantially higher lifetime and activity than the classical Grubbs catalyst 1, due to the bulkiness and the increased basicity of the NHC ligand compared to PCy 3 . Complex 2 has been employed for the synthesis of tri-and tetra-substituted double bonds in ring-closing metathesis (RCM) and cross metathesis (CM) reactions. 2 The ligand exchange also leads to a high tolerance towards functional groups. Thus, 2 in contrast to 1 can be used for a variety of selective CM reactions with a,b-unsaturated esters, ketones and aldehydes. Acrylonitriles, however, do not react. 3 Figure Ru-catalysts for olefin metathesisWe suppose that the activity in metathesis reactions strongly depends on the electronic properties of the Rucarbene complex. Therefore, we have studied olefin metathesis reactions of complex 3. As expected, 3 is extremely stable, does not decompose when exposed to water and/ or air, and can be purified by chromatography on silica gel without any special precautions. We found that the replacement of the second phosphine ligand leads to different reactivities and selectivities. Complex 3 turned out to be perfectly suitable for selective CM reactions with a,bunsaturated nitriles.The preparation of complex 3 was published recently. 4 We have developed two further syntheses. 5 SchemeThe known complex 4 6 was treated with imidazolidine 5 to afford the new complex 6. The second PPh 3 ligand was replaced by a conceptionally new ligand exchange by RCM with phenylether 7 affording 3 in 40% yield. The overall yield of these two steps is moderate (36%), but this pathway avoids the use of the rather expensive and sensitive PCy 3 and the phenyldiazomethane usually employed for the synthesis of 1 (and thus of 2). 7 An alternative synthesis starts with complex 8, 8 described by Hoveyda, in a PCy 3 -NHC exchange reaction. 9In systematic studies we have found that 3, like 2, is an excellent catalyst for selective cross coupling reactions between terminal olefins and a,b-unsaturated carbonyl compounds. 9 In addition, with this catalyst -for the first time in Ru-carbene complex chemistry -highly selective CM with acrylonitrile can be accomplished. 10 Hitherto, such reactions were only known to be catalysed by Mo-complexes of the Schrock type. 11 However, these complexes are very air-and moisture-sensitive and show a restricted tolerance of several heteroatom functionalities. Acids, reactive carbonyl groups and alcohols cause problems. 12 We now provide a methodology for CM reactions of acrylonitrile with a variety of substrates that could not be functionalised so far. The products 13 such as those shown in the Ta...

When methyl 5-(tert-butyldiphenylsilyl)oxy-2-pentenoate was refluxed in toluene in the presence of RuClH(CO)(PPh(3))(3) (5 mol %), double-bond migration took place to afford methyl 5-(tert-butyldiphenylsilyl)oxy-4-pentenoate in high yield. This means that the double bond conjugated with the ester moiety migrates to a deconjugated position by a ruthenium catalyst. We planned to prepare an enol ether from alpha,beta-unsaturated compounds having an ether moiety in a tether using ruthenium-catalyzed isomerization of the double bond. As a result, silyl or benzyl enol ether was obtained from the alpha,beta-unsaturated ester having alcohol protected by the silyl or benzyl group in a tether in high yield. In this reaction, double bond migration of alpha,beta-unsaturated ketone and alpha,beta-unsaturated amide took place to produce deconjugated compounds. Moreover, the double bond of alpha, beta-unsaturated ester having a triple or double bond in a molecule migrated to produce conjugated enyne and diene. On the other hand, treatment of a bis-metalated compound having an alpha, beta-unsaturated ester moiety or the double bond in a tether with RuClH(CO)(PPh(3))(3) gave allyl bis-metalated compound in good yield. These compounds are useful units in synthetic organic chemistry.

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 %.

A bulky binol‐based ligand is the key structural feature of the ruthenium complex 1, which displays an unprecedented metathesis activity combined with high stability. For a range of dienes, ring‐closing metathesis reactions catalyzed by 1 lead to near quantitative yields of cycloalkenes after a few minutes.