Second generation ruthenium carbenes

Second-generation ruthenium-carbene complex, (tricyclohexylphosphine-[l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene]ruthenium (IV) dichloride),4e was purchased from Strem Chemicals. 

Figure 3 Typical second-generation ruthenium-carbene complexes.

Scheme 3 Enyne metathesis using second-generation ruthenium carbene compiexes.

The opposite sequence is described with alkynyl silyloxy-tethered enynes. A cascade CM-RCM reaction proceeds in the presence of a second generation ruthenium carbene to give cyclic siloxanes. The initial CM is directed to occur on the alkyne by employing sterically hindered substituted alkenes tethered to the alkyne via a silyl ether group [24] (Scheme 11). 

Figure 9.1 Second generation ruthenium carbene complexes for metathesis reactions.

RCM of an ene-ynamide substrate has been reported by Mori and Sato, using a second-generation ruthenium carbene catalyst. The process generates cyclic dienamides with a preferential Z configuration of the non-cyclic double bond (Scheme 88) [182]. 

Second generation ruthenium catalysts N-heterocyclic carbene ligands... 

Second-Generation Ruthenium Indenylidene Catalysts Bearing a N-Heterocyclic Carbene or a Chelating NHC-Phosphine Ligand... 

The possibility of phosphine or NHC ligand dissociation from the respective flrst-and second-generation hemilabile carbenes should not be excluded, because it is generally assumed that ruthenium-catalyzed metathesis reactions proceed through 14-electron intermediates. Conseqnently, we have postulated a mechanism for the first- and second-generation hemilabile-catalyzed 1-octene metathesis reaction (Scheme 12.3) whereby the 0,N-ligand remains attached to the Ru-center. 

For the last 2 decades ruthenium carbene complexes (Grubbs catalyst first generation 109 or second generation 110, Fig. 5.1) have been largely employed and studied in metathesis type reactions (see Chapter 3) [31]. However, in recent years, the benefits of NHC-Ru complexes as catalysts (or pre-catalysts) have expanded to the area of non-metathetical transformations such as cycloisomerisation. 

Gmbbs has described a chiral ruthenium-based catalyst and has illustrated its application to enantioselective desymmetrization. The catalyst is a derivative of Grubbs second-generation dihydroimidazolinylidene-containing catalyst with the chirality derived from diphenylethanediamine in the carbene backbone. Substrate 90 undergoes ringclosing metathesis in 90% ee on treatment with this catalyst and Nal ... 

The same reaction (RCM) was used as the key step for the formation of a family of potent herbicidal 10-membered lactones. An important aspect from the preparative point of view is the control of stereochemical outcome of the RCM by the choice of catalyst. Thus, the use of the ruthenium indenylidene complex IX always leads to the corresponding ( )-alkenes, whereas the second generation of Grubbs catalyst bearing a N-heterocyclic carbene ligand affords the isomeric (Z)-olefin with good selectivity (Scheme 8.19) [64]. 

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