Ruthenium carbene complex, with

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

The formation of cyclic Fischer carbene complexes by ruthenium-mediated activation of 3-butyn-l-ol and 4-pent5m-l-ol has been reported before (147,155,158-164). Correspondingly, a reaction of [Ru(bdmpza)Cl(PPh3)2] (24) with these terminal alkynols results... 

Ruthenium complexes B are stable in the presence of alcohols, amines, or water, even at 60 °C. Olefin metathesis can be realized even in water as solvent, either using ruthenium carbene complexes with water-soluble phosphine ligands [815], or in emulsions. These complexes are also stable in air [584]. No olefination of aldehydes, ketones, or derivatives of carboxylic acids has been observed [582]. During catalysis of olefin metathesis replacement of one phosphine ligand by an olefin can occur [598,809]. 

Figure 5 Steric repulsion between ruthenium-carbene complex with the silyloxy group in trans-substituents.

Olefin metathesis has proved to be a powerful synthetic tool in organic synthesis.5 The advent of well-defined metal carbene complexes with remarkable functional group tolerance has rendered metathesis as an efficient route to the synthesis of new C-C bonds. Examples of widely used ruthenium metathesis catalysts include [Ru-1],6 [Ru-2]7 and [Ru-3] 8 (Figure 1). 

Carbene and silylene complexes - Carbene complexes of ruthenium [312] and osmium [313] porphyrins were formed from the neutral dimers [M(P)]2 with diazoalkanes (Eq. 26). 

In 1998, Galardon et al. [58] reported the crystal structure of tetraphenyl-porphyrinate-ruthenium-(diethoxycarbonyl)carbene complex 25, which exhibited catalytic activity for cyclopropanation of EDA and styrene, giving 85% of the product with 93 7 trans-to-cis ratio. The Ru-C distance is 1.829 A and the carbon resonance is at <5C=271.3 ppm. In 2000, Bianchini and Lee [59] isolated the similar ruthenium carbene complexes 26 with tridentate imine ligands and EDA Ru=CH-, <5H=20.44 ppm (s), <5C=299.9 ppm [59]. Simonneaux et al. [60] isolated the phosphonate carbene complexes of ruthenium porphyrins. 

H. Werner, and J. Wolf, Synthesis of Rhodium and Ruthenium Carbene Complexes with a 16-Electron Count, in Handbook of Metathesis, Vol. 1 (Ed. R. H. Grubbs, Wiley-VCH, Weinheim, 2003, Chap. 1.8). 

Figure 3.66 Rhodlum(l) and ruthenium(ll) carbene complexes with methoxyalkyl functionalised NHC ligands.

Figure 3.157 Synthesis of a ruthenium(ll) pincer carbene complex with the Crubbs catalyst as starting point.

Figure 6.2 The reactivity of titanium-, molybdenum-, tungsten-, and ruthenium-carbene complexes with various functionalities.

In order to clarify the mechanism of cyclopropanation, several carbene-complexes of ruthenium have been isolated by reaction with diazocompounds. In the case of Pybox, the corresponding ruthenium-carbene complexes 38 were isolated and characterized using either NMR or X-ray analysis [32]. Similar ruthenium-carbene complexes, such as porphyrin-ruthenium carbene complex 39 [33] and pyridine-diimine-ruthenium complex 40 [34] were isolated and characterized (Chart 7.6). 

Furstner, A., Thiel, O. R., Kindler, N., Bartkowska, B. Total Syntheses of (S)-(-)-Zearalenone and Lasiodiplodin Reveal Superior Metathesis Activity of Ruthenium Carbene Complexes with lmidazol-2-ylidene Ligands. J. Org. Chem. 2000, 65, 7990-7995. 

Poly(l,4-butadiene) segments prepared by the ruthenium-mediated ROMP of 1,5-cyclooctadiene can be incorporated into the ABA-type block copolymers with styrene (B-106) and MMA (B-107).397 The synthetic method is based on the copper-catalyzed radical polymerizations of styrene and MMA from the telechelic poly(butadiene) obtained by a bifunctional chain-transfer agent such as bis(allyl chloride) or bis-(2-bromopropionate) during the ROMP process. A more direct route to similar block copolymers is based on the use of a ruthenium carbene complex with a C—Br bond such as Ru-13 as described above.67 The complex induced simultaneous or tandem block copolymerizations of MMA and 1,5-cyclooctadiene to give B-108, which can be hydrogenated into B-109, in one pot, catalyzed by the ruthenium residue from Ru-13. 

Further modification in snpport of the ruthenium carbene complex with ionic liquid media was described by Wakamatsu et al. [54]. The RCM of diethyl diallyl-malonate using a new catalyst was carried out under an argon atmosphere in the presence of 5 mol% catalyst in CHjClj/[bmim]BF (9 1) at room temperature. The starting material was consumed after 0.5 h to provide a product with 94% yield (Scheme 17.8). Continuous fast conversion was observed, and reliable reusability was realized by the fifth cycle. 

The catalytic production of olefins, diethyl maleate and fumarate, from ethyl diazoacetate has been reported with osmium [ 149] and ruthenium [ 128] porphyrins. Despite the periodic relationship of ruthenium to iron and osmium and the syntheses of different carbene complexes of ruthenium porphyrins, developed by Collman et al. [125-127], it is only very recently that cyclopropanation [135,171] and ethyl diazoacetate insertion into heteroatom bond reactions [172] were observed using ruthenium porphyrins as catalysts. The details of the catalytic reaction of diazo esters with simple olefins catalyzed with ruthenium porphyrins have been reported [173]. Product yields. 

Fiirstner A, Thiel OR, Ackermann L, Schanz H-J, Nolan SP. Ruthenium Carbene Complexes with N,N -Bis(mesityl)imidazol-2-ylidene Ligands RCM Catalysts of Extended Scope. J Org Chem. 2000 65(7) 2204—2207. 

Blechert et al. successfully introduced a desymmetrization approach. An asymmetric ring-opening cross metathesis of norbornenedicarboxylic anhydride was rendered possible by a Hoveyda- Blechert type ruthenium- carbene complex with a chiral N-heterocyclic carbene (NHC) ligand (Scheme 5.6). The chiral iirforma-tion of the NHC backbone was translated into ruthenium s active coordination sphere by the steric repulsion of an isopropyl group with a skewed ort/ro-biphenyl substituent [17]. 

A complete Chauvin catalyst cycle of olefin metathesis at ruthenium-carbene complexes with different anionic ligands X leads to an inversion of configuration at the stereogenic metal center (Scheme 5.8). 

Scheme 5.10 Ruthenium(ll)-carbene complex with low catalytic activity R, R = Ph, Cy,...

Furstner A, Thiel OR, Kindler N et al (2000) Total syntheses of (s)-(-)-zearalenone and lasiodiplodin reveal superior metathesis activity of ruthenium carbene complexes with imidazol-2-ylidene ligands. J Org Chem 65 7990-7995... 

A. Fiirstner, O.R. Thiel, L. Ackermann, H.-J. Schanz, S.P. Nolan, Ruthenium carbene complexes with N,N -bis(mesityl)imldazol-2-ylidene Hgands RCM catalysts of extended scope, J. Org. Chem. 65 (2000) 2204—2207. 

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

As a final example in this section, a contribution by Grubbs et al. is discussed. The chloride-free ruthenium hydride complex [RuH2(H2)2(PCy3)2] (37) is believed to react, in the presence of alkenes, to form an unidentified ruthenium(O) species which undergoes oxidative additions with dihalo compounds, e.g., 38, to give the corresponding ruthenium carbene complex 9 (Eq. 4) [20]. 

Table 3 Ruthenium alkylidene complexes with JV-heterocyclic carbene ligands...

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