Ruthenium carbene complexes, syntheses

Poulsen, C. S., Madsen, R. Enyne metathesis catalyzed by ruthenium carbene complexes. Synthesis 2003, 1-18. 

Van der Schaaf et al. described a synthesis of the 14-electron complex [RuHCl(PPr13)2] (32) from [RuCl2(COD)]A.,PPr31,isopropanol,and abase.Compound 32 is a suitable precursor for ruthenium carbene complex 33, as outlined in Scheme 10. Although 32 was isolated and structurally characterized, it may also be generated in situ for the preparation of the carbene complex 33 [18]. 

Further improvements in activity of the ruthenium carbene complexes were achieved by incorporation of methyl groups in 3,4-position of imidazol-2-ylidene moiety. Introduction of sulfur in the trara-position to the N-heterocyclic carbene leads to increased stability of the resulting ruthenium complexes. The synthesis and the first applications of these new rathenium complexes are described herein. 

The ruthenium carbene complexes 1 discussed in the previous chapter have set the standards in the field of olefin metathesis and are widely appreciated tools for advanced organic synthesis [3]. A serious drawback, however, relates to the preparation of these compounds requiring either 3,3-diphenylcyclopropene or diazoalkanes, i.e. reagents which are rather difficult to make and/or fairly hazardous if used on a large scale [60]. Therefore, a need for metathesis catalysts persists that exhibit essentially the same activity and application profile as 1 but are significantly easier to make. 

Later Grubbs discovered ruthenium carbene complex and used it for a metathesis reaction to synthesize cyclic compounds 5a-d [Eqs. (6.4) and (6.5)]. In 1995, Grubbs found that ruthenium benzylidene carbene complex Ic," which is now commercially available, has the same reactivity as that of lb. Many researchers have therefore used this complex for olefin metathesis, and this reaction has been useful for the synthesis of carbo- and heterocyclic compounds and fused bicyclic compounds [Eq. [6.6)] °... 

On the other hand, it has been shown that ruthenium carbene complexes lb and Ic are also effective for enyne metathesis. Intramolecular reaction of enyne metathesis is now a useful method for synthesizing cyclic compounds having a diene moiety, and intermolecular enyne metathesis has provided a method for synthesis of 1,3-dienes. 

Scheme 1 Synthesis of heterocycles using a ruthenium carbene complex.

A concise total synthesis of dehydrohomoancepsenolide is achieved in an optically active form. The key steps are alkene metathesis and alkyne metathesis. A three-component coupling reaction affords dienyne 137, which undergoes ring-closing alkene metathesis in the presence of the first-generation ruthenium carbene complex to give 138,... 

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). 

Up to third-generation ruthenium-carbene complexed dendrimers (Fig. 6.2) prepared by Astruc et al. contain a chelating diphosphane, which is sufficiently stable for construction of the dendritic architecture while also sufficiently reactive to permit synthesis of the dendrimer depicted in Fig. 6.3 by ring-opening metathesis polymerisation (ROMP) [3]. 

Important progress has been made in the study of the process since the discovery and development of well-defined ruthenium carbene complexes. The interest in the reaction as a synthetic tool for organic synthesis has been growing, among others, because of its atom economy. For recent reviews on enyne metathesis see Refs. [91-93]. For a review on the application of enyne metathesis in organosilicon chemistry see Ref. [6]. 

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). 

Synthesis of a ruthenium carbene complex using caffeine as the carbene... 

This chapter is concerned specifically with olefin metathesis reactions catalyzed by ruthenium-carbene complexes, mainly because of their great success during recent years. We begin with an overview of these catalysts, and then focus on mechanistic considerations that are important for understanding the reactivity profiles of various catalyst derivatives. The second part of the chapter deals with applications of ruthenium-catalyzed olefin metathesis, especially RCM, CM, and combination processes in organic synthesis. 

Wakamatsu H, Saito Y, Masubuchi M, Fujita R (2008) Synthesis of imidazoUum-tagged ruthenium carbene complex remarkable activity and reusability in regard to olefin metathesis in ionic liquids. Synlett 1805-1808... 

Metathesis polymerization has become an important tool for polymer synthesis and has even attracted a Nobel Prize [46-49]. Acyclic diene molecules are capable of undergoing intramolecular and intermolecular reactions in the presence of appropriate transition metal catalysts, for example, molybdenum alkylidene and ruthenium carbene complexes. The intramolecular reaction, called ring-closing olefin metathesis (RCM), leads to cyclic compounds and the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, leads to oligomers and polymers. Altering the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Carbenes are useful in synthesis Ruthenium-carbene complexes undergo metathesis reactions ... 

Although numerous examples of successful RCM reactions have been demonstrated, a few limitations and/or side-reactions have been uncovered. Some cases where the RCM reaction proceeds with complications are depicted in Scheme 18. Sometimes RCM reactions are competitive with alkene isomerization. For example, the unexpected formation of 162 from precursor 160 was attributed to alkene isomerization (affording 161), followed by RCM to afford the ring-contracted compound 162. Later investigators went on to exploit this observation for the synthesis of cyclic enol ethers. Treatment of the allyl ether 164 with a ruthenium carbene complex catalyst affords the RCM... 

A very efficient group of catalysts are the 2-pyridylethanyl substituted ruthenium carbene complexes 9 and 10. Also this new class of catalyst can be easily prepared, either via the reaction of Grubbs benzylidene catalyst with a 2-(3-butenyl)pyridine or directly via a one-pot procedure for the synthesis of ruthenium carbenes starting from [RuCl2(l,5-cyclooctadiene)] via a ruthenium hydride species, see Scheme 5b. 

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