Ru 3 PCy

Guo et al. [70,71,73] recently attempted to hydrogenate NBR in emulsion form using Ru-PCy complexes. However, successful hydrogenation can only be obtained when the emulsion is dissolved in a ketone solvent (2-butanone). A variety of Ru-phosphine complexes have been studied. Crosslinking of the polymer could not be avoided during the reaction. The use of carboxylic acids or first row transition metal salts as additives minimized the gel formation. The reactions under these conditions require a very high catalyst concentration for a desirable rate of hydrogenation. 

FlCi. 8, Molecular stmclure of Cp Ru(PCy ,lCI (9). Hydrogen aloms are omitted ftn claritv. 

Total synthesis of baconipyrone C, summarized in Scheme 12.5 [10], is the first and only application of Ru-catalyzed enantioselective olefin metathesis to natural product synthesis. Treatment of oxabicycle 11 with styrene and 2mol% chiral Ru complex [Ru]-XV [11] leads to the formation of pyran 12 in 62% yield and with an enantiomeric ratio of 94 6. Ru-carbene [Ru]-XV is generated in situ by subjecting the corresponding Ag-based N-heterocyclic carbene (NHC) to an achiral Ru-PCys complex and Nal. It is also worthy of note that the diketone fragment of baconipyrone C was synthesized through a tandem double-allylic alkylation process promoted by a chiral NHC-Cu complex that is structurally related to carbene [Ru]-XV. 

The versatile starting material lCp RuCI 4 (1) reacts rapidly with sterically demanding phosphines (PCy and P Pr ) as well as with the nucleophilic carbene ligands (L) to give deep blue, coordinatively unsaturated Cp Ru(L)CI complexes 2-8 (L= l,.Tbis(2,4,6-lrimethylphenyl) (IMes. 2) 1,3-R2-imidazol-2-ylidene = cyclohcxyl (ICy, 3) 4-methylphenyl (ITol, 4) 4-chlorophenyl (IPCl, 5) adamanlyl (lAd, 6) 4..5-dichloro-1,3-bis(2.4,6-trimethylphenyl) (IMesCI, 7) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidenc (IPr. 8) in high yields according to Eq. (4). 

Ru]= [RuCI(Ti6-p-cymene)(PR3)] PR3= PCys, P/Pr3, PPh3 Scheme 2.4 Acid-promoted allenylidene to indenylidene rearrangement. 

The most straightforward application of the Grubbs reaction is to effect homologation of a terminal vinyl group. One concern with the use of the second generation Grubbs catalyst 3 is the cost, about 100/mmol. In conjunction with a total synthesis of the macrolide RK-397, Frank McDonald of Emory reported (J. Am. Chem. Soc. 126 2495, 2004) that the conversion of 1 to 2 required 10 mol % of 3, but that it proceeded efficiently with just 2 mol % of the Hoveyda Ru catalyst 4 (J. Am. Chem. Soc. 122 8168,2002). The alkene so prepared was cleanly trans. An advantage of 4 is that it avoids the use of the expensive PCy,. 

Note The second generation Grubbs catalyst [Ru(PCyJ(IMes)(=CHPh)ClJ [117] employs a phosphane and a NHC and is activated by loss of the PCy ligand. The Herrmann variant [Ru(IMes)J=CHPh)ClJ [116] is inferior simply because it would have to lose one of the two NHC ligands, a process that takes longer because NHC bind stronger than phosphanes. 

---