Ruthenium complexes cross-metathesis

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

An alternative approach to phosphine-free ruthenium precatalysts is based on pyridine complex 70 [48], which has been established by Grubbs et al. as a valuable precursor for other mixed NHC-phosphine complexes (cf. Scheme 15). Complex 70 is only moderately active in the cross metathesis of allylbenzene... 

The significant potential of the ruthenium complex 65 was further underlined in the catalytic asymmetric ring-opening/cross metathesis of the cyclic alkene 70 (Scheme 44). This transformation is catalyzed by 5% mol of 65 at room temperature, in air, and with undistilled and nondegassed THF to deliver the corresponding diene 71 in 96% ee and 66% isolated yield. In standard conditions (distilled and degassed THF), the alkene 70 reacts in 75 min to give the diene in 95% ee and 76% yield, with only 0.5 mol % of catalyst. 

For a thorough review of Ru-NHC-catalysts for metathesis, see Samojlowicz C, Bieniek M, Grela K (2009) Chem Rev 109 3708-3742 for ruthenium indenylidene-complexes in cross metathesis, see Boeda F, Bantreil X, Clavier H, Nolan SP (2008) Adv Synth Catal 350 2959-2966 For Hll-types systems, see Schrodi Y, Pederson RL (2007) Aldrichimica Acta 40 45-52... 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

For a review of asymmetric Mo-catalyzed metathesis, see Catalytic Asymmetric Olefin Metathesis, A. H. Hoveyda, R. R. ScHROCK, Chem. Eur. J. 2001, 7, 945-950 for reports on chiral Ru-based complexes, see (b) Enantioselective Ruthenium-Catalyzed Ring-Qosing Metathesis, T.J. Sei-DERS, D.W. Ward, R.H. Grubbs, Org. Lett. 2001, 3, 3225-3228 (c) A Recyclable Chiral Ru Catalyst for Enantioselective Olefin Metathesis. Efficient Catalytic Asymmetric Ring-Opening/Cross Metathesis In Air, J. J. Van Veldhuizen, S. B. 

Cross-metathesis of terminal alkyne 142 and cyclopentene gives cyclic compound 143 having a diene moiety [Eq. (6.114)]. ° Terminal ruthenium carbene generated from an alkyne and methylidene ruthenium carbene complex reacts with cyclopentene to afford two-carbon elongated cycloheptadiene 143 ... 

With the discovery by Grubbs of ruthenium carbene complexes such as Cl2(PCy3)2Ru=CHR, which mediate olefin metathesis under mild reaction conditions and which are compatible with a broad range of functional groups [111], the application of olefin metathesis to solid-phase synthesis became a realistic approach for the preparation of alkenes. Both ring-closing metathesis and cross-metathesis of alkenes and alkynes bound to insoluble supports have been realized (Figure 5.12). 

Organosubstituted octasilsesquioxanes (Fig. lb) have also been prepared by cross-metathesis (CM) and silylative coupling of vinylsilsesquioxane with olefins in the presence of the ruthenium carbene complex Cl2(PCy3)2Ru(=CHPh) (Grubbs catalyst) and Ru-H (Ru-Si) complexes, for example, RuHCl(CO)(PCy3)2, respectively [57]. 

Enynes containing a cycloalkene moiety can lead, in the presence of another olefin and a catalytic amount of ruthenium carbene complex, to ringopening and ring-closing metathesis (ROM-RCM) followed by cross metathesis (CM) to produce trienes (Scheme 7). 

Enynes without the cycloalkene moiety can also react with electron-deficient alkenes by a cascade ring-closing metathesis-cross metathesis (RCM-CM) process [23] (Scheme 10). The use of Hoveyda s catalyst is necessary, not to stop the reaction at the RCM step, but to perform the subsequent CM step. Indeed, the organic product arising from the RCM is first formed and then reacts with the alkene in the presence of the ruthenium complex to give the CM reaction. 

Figure 6.7 Two ruthenium-carbene complexes that are active for cross-metathesis reactions.

Substituted vinylphosphonates (195) and allylphosphonates (196) with E-olefin stereochemistry have been prepared for the first time via intermolecular olefin cross-metathesis (CM) using ruthenium alkylidene complex (197) in good yield. A variety of terminal olefins, styrenes and geminally substituted olefins has been successfully employed in these reactions (Scheme 49). ... 

Cross-metathesis of trialkoxy- and trisiloxy-substituted vinylsilanes [21] as well as octavinylsilsesquioxane [15] with vinyl sulfides proceeds efficiently but only in the presence of the 2nd generation Grubbs catalysts (IV) to offer a new and very attractive route for syntheses of [alkyl(aiyl)]sulfide-substituted vinylsilanes and vinyl-silsesquioxane with high preference for the -isomer, as illustrated by exclusive isolation of such isomers. The Fischer-type ruthenium carbene complex Ru(=CHSPh)Cl2(PCy3)2 has recently been reported as an effective catalyst in the ring opening/cross-metathesis of norbomene derivatives with vinyl sulfide [22], suggesting that these carbenes can be reactive in the cross-metathesis. 

The two reactions catalyzed by ruthenium complexes, i.e. silylative coupling (SC) (trans-silylation) catalyzed by I, II, V, and VI and cross-metathesis (CM) (catalysts III and IV) of vinyl- and allyl-substituted hetero(N,S,B)organic compounds with commercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxanes provide a universal route toward the synthesis of well-defmed molecular compounds with vinylsilicon functionality. 

Metathesis has been applied in oleochemistry for many years, but only fairly recently technical realization comes within reach [33, 34]. As typical catalysts, ruthenium carbene complexes of the Grubbs type are applied because of their very high activity (turnover numbers up to 200 000). In principle, oleochemical metathesis can be divided into two different types in self-metathesis the same fatty substrate reacts with itself and in cross-metathesis a fatty substrate reacts with, for example, a petrochemical alkene. The simplest case, the self-metathesis of methyl oleate forms 9-octadecene and dimethyl 9-octadecenedioate. The resulting diester can be used along with diols for the production of special, comparatively hydrophobic, polyesters. An interesting example of cross-metathesis is the reaction of methyl oleate with an excess of ethene, so-called ethenolysis. This provides two produds, each with a terminal double bond, 1-decene and methyl 9-decenoate (Scheme 3.3). 

The development of stable and active well-defined ruthenium carbene complex Cl2(PCy3)2Ru = CHPh and its second-generation successors (la) and (lb) (Figure 9.1) has meant that olefin metathesis reactions have become valuable tools in synthetic organic chemistry.111 Among them the ring-closing (RCM), en-yne and cross-metathesis (CM) reactions have received much attention as they... 

A pyrene derivative was also used as an anchor to immobilize a ruthenium alkylidene complex onto SWCNTs [94]. The immobilization of the Ru complex was performed by two different paths (1) adsorption of pyrene derivative precursor onto the sidewall of nanotubes by ti-ti interactions, followed by cross-metathesis with the ruthenium alkylidene complex, and (2) adsorption onto the sidewall of SWCNTs of the pyrene-substituted ruthenium alkylidene prepared previously. 

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