Grubbs, carbene complex

Exposure of a monosubstituted allene to a Grubbs carbene complex allows the formation of a 1,3-disubstituted allene accompanied with polymers (Scheme 16.93) [99]. The product distribution depends considerably on the alkyl substituents on the allene moiety and particularly the reaction of a series of phenylallenes undergoes complete conversion to polymers. 

RCM was also employed as a key design element for the synthesis of tricolorin A (106). This novel strategy demonstrated the efficiency of macrocyclizations using the standard ruthenimum catalysis either by the classical Grubbs carbene complex or the recently developed cationic ruthenium aUenylidene complex as the... 

Another comparative study of the CM reaction catalyzed by a second generation Grubbs carbene complex (without imidazolium tag) has been recently presented by Tang and coworkers [264]. They compared the catalytic transformation of styrene in [BMIM][BF4] and [BMIM PFs] with the same reaction in CH2Q2 (3 h, 45 °C) and obtained comparable yields of the CM product in all solvents in the first cycle. In the case of the two ionic liquids the product was extracted with diethyl ether and the remaining ionic catalyst solution could be recycled four times with only a small drop in catalytic activity (product yield 75% in the fourth cycle compared to 85% in the first cycle in [BMIMjfPFs]). The scope of the study also included the successful... 

When 1,6-diene 46 was catalyzed by Grubbs carbene complex/trimethylsilyl vinyl ether or NiBr2(PPh3)2/Et2AlCl, exo-methylene cyclic compound 47 was obtained as the major product of the cycloisomerization (Scheme 27) (64,65). 

An efficient approach to a new family of highly functionalised phosphorus analogues of a-trifluoromethyl-substituted phenylalanine (492) and its homologues via Ru-catalysed intermolecular ene-yne metathesis has been developed. Thus, cross-metathesis of a-allynyl-a-trifluoromethyl-ot-aminophosphonates (489) with alkenes (490) catalysed by a second-generation Grubbs carbene complex (493) afforded corresponding aminophosphonates (491) with 1,3-diene backbone. Finally, one-pot Diels-Alder reaction-aromatisation step gave desired products (492) (Scheme 146). ... 

Conceptually, the productive metatheses of 1-octene in the presence of the hemilabile Grubbs carbene complexes are illustrated in Schemes 12.3 and 12.4. The mechanism... 

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]. 

Fig. 3.28 ROMP initiators, first, second and third generation Grubbs catalysts (complexes 71, 72, and 73, respectively) 74a-c 3-phenyl-indenylidene replaces the benzylidene carbene...

Given the success of the Grubbs-type NHC-Ru catalysts in metathesis polymerisation (Chapter 3), it is somewhat surprising that more research has not been done on mid-transition metal carbene complexes for coordination-insertion polymerisation. At this stage however, there are only a few reported attempts with the metals Co, Fe and Ir. 

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. 

Fig. 6 Grubbs nickel complex bearing sterically encumbered nucleophilic heterocyclic carbene ligand...

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. 

Initiating a remarkable series of papers in 2008-2009, Grubbs and Whited reported the formation of an Ir(I) carbene complex, 44, from the reaction of methyl f-butyl ether (MTBE) with Ozerov s (PNP)lrH2 (PNP = [N(2-P Pr2-4-Me-C6H3)2] ) and NBE [121-127]. C-H addition to (PNP)lr presumably occurs followed by... 

Ring-Opening Metathesis Polymerization (ROMP) of Norbornene by a Group VIII Carbene Complex in Protic Media, S.T. Nguyen, L.K. Johnson, R.H. Grubbs, et al, J. Am. Chem. Soc. 1992, 114, 3974-3975. 

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)] °... 

Grubbs synthesized ruthenium carbene complex 104a, and a high enantiomeric excess (up to 90%) was observed in the desymmetrization of prochiral trienes 80c ... 

Better control of the reaction is achieved by using stable, isolable metal-carbene complexes, such as the Schrock initiators based on molybdenum and tungsten (LXIV, Mt = W, Mo) and the Grubbs initiators based on mthenium (LXV). The Schrock initiators... 

In 1999, Herrmann, Nolan, and Grubbs independently synthesized novel ruthenium carbene complexes Id-lg (Figure j) 5 5 5b,6,6a,7,7a complexes have an iV-heterocyclic carbene ligand in common and are called second-... 

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