Similarity ruthenium, Grubbs

Similarly, a Grubbs type ruthenium catalyst on a phosphine-derivatized polystyrene [56] can be illustrated as follows ... 

Further detailed investigations towards new chiral ruthenium catalysts that could enhance enantioselectivity and expand the substrate scope in asymmetric RCM were reported by Grubbs and co-workers in 2006 [70] (Fig. 3.24). Catalysts 59 and 61, which are close derivatives of 56 incorporating additional substituents on the aryl ring para to the ort/to-isopropyl group, maintained similar enantioselectivity than 56b. However, incorporation of an isopropyl group on the side chain ortho to the ortho-isopropyl group 60 led to an increase in enantioselectivity for a number of substrates. 

A Grubbs-type ruthenium complex and a Hoveyda ruthenium complex were compared under similar conditions for recycled activity. Both the reference catalysts showed a large drop in metathesis activity in the subsequent tests. For example, a Grubbs-type ruthenium alkylidene catalyst showed a drop of nearly 50% conversion in the second run. 

Plumet et al. described domino metathesis of propargyl (2-endo-7-oxanorborn-5-enyl) ethers 62a-62c with allyl acetate in the presence of Grubbs ruthenium catalyst Ic (Scheme 22). The reaction proceeds stereoselectively to produce substituted m-fused bicyclic ethers 63a-63c. In a similar manner, indolizidinone derivative 64 is obtained from compound 62d instead of pyrrolizidine derivative 63d. ... 

Thanks to the development of the Grubbs benzylidene catalyst (2) and other related ruthenium complexes, olefin metathesis has experienced spectacular advances over the past 10 years. The various incarnations of the reaction (acyclic diene metathesis, ring-closing metathesis, ring-opening metathesis polymerization, etc.) have now acquired first rank importance in synthesis. Clearly, the emergence of a similar, generic, efficient catalytic system for con-... 

Ring closing metathesis using Grubbs ruthenium catalyst has also been used in a novel preparation of the Fischer-type chromium carbene complex 10 from the precursor 9 in >98% yield [01SL757], Similarly, the TV-substituted tetrahydroazepine 12 could be accessed in near quantitative yield from 11 [01JOC3564],... 

Even with immobilized catalysts being developed, removal of ruthenium by-products remains an important challenge. Georg and coworkers found that addition of 50 equiv (relative to ruthenium) of dimethyl sulfoxide or triphenylphosphine oxide brought ruthenium levels in reaction mixtures down from 50 to l-2qgmg-. The ruthenium levels in purified products are similar to those reported by Grubbs, where the metal was removed as trishydroxymethylphosphine complexes, " and those from the Pb(OAc)4 oxidation of ruthenium reported by Paquette. 

Irrespective of the reaction conditions used (/. e. ultrasound, microwave, changing reaction times, temperature and solvents), the maximum turnover number (TON) that was achieved was 75. In principle, second generation Grubbs-type initiators immobilized on non-porous silica should behave similar to those immobilized on monolithic supports[16]. In fact, catalysts immobilized onto monolithic supports give similar maximum TONs (< 65) in the absence of any chain transfer agent (CTA). Ruthenium measurements by means of ICP-OES revealed quantitative retention of the original amount of ruthenium at the support within experimental error ( 5%), thus otfering access to metal free products. 

Transformation of aldehydes 8 and ent-% into conduritols and inositols was achieved in a few steps via dienes 34 and 35. Conduritols B and F were easily obtained, after separation, by a RCM with the first-generation Grubbs ruthenium catalyst [46]. Syntheses of myo-inositol and c/ifro-inositol were achieved in a few further steps in good yields. Similarly, both enantiomers of cyclophellitol were synthesized from 2,3,4-tri-O-benzyl-D-xylopyranose according to the same strategy... 

Grubbs examined the effect that a C2 symmetric imidazolin-ium ligand would have on dichloro-, dibromo-, and diiodo-ruthenium alkylidene catalysts (112a-f) (Scheme 18). The new complexes had activities and stabilities similar to the parent complex (4a) and were thus tested for asymmetric induction in enantioselective desymmetrization reactions. The A rei using these new complexes were low, leading to very low... 

Lamaty reported the synthesis of several PEG-bound ruthenium catalysts similar to 60 [98]. The most active one was the catalyst 61 that used an JV-het-erocyclic carbene ligand in place of the tricyclohexylphosphine ligand in 60. Some decrease in activity in reuse of 61 was noted. This work included extensive characterization data for 61 along with studies of ligand exchange of 61 and related species with low molecular weight Grubbs catalysts. 

A significant development for the selective synthesis of alkenes makes use of alkene metathesis. Metathesis, as applied to two alkenes, refers to the transposition of the alkene carbon atoms, such that two new alkenes are formed (2.110). The reaction is catalysed by various transition-metal alkylidene (carbene) complexes, particularly those based on ruthenium or molybdenum. The ruthenium catalyst 84, developed by Grubbs, is the most popular, being more stable and more tolerant of many functional groups (although less reactive) than the Schrock molybdenum catalyst 85. More recently, ruthenium complexes such as 86, which have similar stability and resistance to oxygen and moisture as complex 84, have been found to be highly active metathesis catalysts. 

---