Catalyst cross metathesis

The cross metathesis of acrylic amides [71] and the self metathesis of two-electron-deficient alkenes [72] is possible using the precatalyst 56d. The performance of the three second-generation catalysts 56c,d (Table 3) and 71a (Scheme 16) in a domino RCM/CM of enynes and acrylates was recently compared by Grimaud et al. [73]. Enyne metathesis of 81 in the presence of methyl acrylate gives the desired product 82 only with phosphine-free 71a as a pre-... 

The cross metathesis of vinylsilanes is catalyzed by the first-generation ruthenium catalyst 9. This transformation has been extensively investigated from both preparative and mechanistic points of view by Marciniec et al. [86]. Interestingly, the same vinylsilanes obtained from cross metathesis may also result from a ruthenium-hydride-catalyzed silylative coupling and there might be some interference of metathesis and nonmetathesis mechanisms [87]. 

Few reports describe the cross metathesis of allyl halides [88]. First-generation catalyst 9 does not seem to be sufficiently reactive to promote this reaction in preparatively useful yields and acceptable catalyst loadings, but second-generation catalyst 56d gives good results for allyl chloride. Cross-metathesis... 

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

The reversible nature of cross metathesis is of synthetic importance because, by the use of a sufficiently active metathesis catalyst, it generally ensures the preferential formation of the most thermodynamically stable product. This results in the transformation of terminal olefins into internal ones, and we have seen that undesired self-metathesis products can be recycled by exposing them to a second CM process. 

The significant potential of the ruthenium complex 65 was further underlined in the catalytic asymmetric ring-opening/cross metathesis of the cyclic alkene 70 (Scheme 44). This transformation is catalyzed by 5% mol of 65 at room temperature, in air, and with undistilled and nondegassed THF to deliver the corresponding diene 71 in 96% ee and 66% isolated yield. In standard conditions (distilled and degassed THF), the alkene 70 reacts in 75 min to give the diene in 95% ee and 76% yield, with only 0.5 mol % of catalyst. 

Note also that (1) d° Ta alkyhdene complexes are alkane metathesis catalyst precursors (2) the cross-metathesis products in the metathesis of propane on Ta are similar to those obtained in the metathesis of propene on Re they differ only by 2 protons and (3) their ratio is similar to that observed for the initiation products in the metathesis of propane on [(=SiO)Ta(= CHfBu)(CH2fBu)2]. Therefore, the key step in alkane metathesis could probably involve the same key step as in olefin metathesis (Scheme 27) [ 101 ]. 

Despite those challenges, both Johnson [161] and Grela [162] performed several cross metathesis reactions with vinylhalides using phosphine free catalysts. Turnover numbers (TON) above 20 were very few, while in many cases the TON stayed below ten. The diastereoselectivity of CMs with vinylhalides is shghtly in favour of the Z product which is similar to their acrolein-counterparts. 

For a thorough review of Ru-NHC-catalysts for metathesis, see Samojlowicz C, Bieniek M, Grela K (2009) Chem Rev 109 3708-3742 for ruthenium indenylidene-complexes in cross metathesis, see Boeda F, Bantreil X, Clavier H, Nolan SP (2008) Adv Synth Catal 350 2959-2966 For Hll-types systems, see Schrodi Y, Pederson RL (2007) Aldrichimica Acta 40 45-52... 

The synthesis and olefin metathesis activity in protic solvents of a phosphine-free ruthenium alkylidene bound to a hydrophilic solid support have been reported. This heterogeneous catalyst promotes relatively efficient ring-closing and cross-metathesis reactions in both methanol and water.200 The catalyst-catalyzed cross-metathesis of allyl alcohol in D20 gave 80% HOCH2CH=CHCH2OH. 

A similar strategy served to carry out the last step of an asymmetric synthesis of the alkaloid (—)-cryptopleurine 12. Compound 331, prepared from the known chiral starting material (l )-( )-4-(tributylstannyl)but-3-en-2-ol, underwent cross-metathesis to 332 in the presence of Grubbs second-generation catalyst. Catalytic hydrogenation of the double bond in 332 with simultaneous N-deprotection, followed by acetate saponification and cyclization under Mitsunobu conditions, gave the piperidine derivative 333, which was transformed into (—)-cryptopleurine by reaction with formaldehyde in the presence of acid (Scheme 73) <2004JOC3144>. 

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

Although the bulk of this review is concerned with well-defined metal carbene catalysts, it is important to note the contributions made to cross-metathesis chemistry by ill-defined or multicomponent catalysts. A brief discussion of the cross-metathesis reactions of functionalised alkenes using catalysts of this type will therefore be included here [1]. 

The use of ill-defined catalysts for the cross-metathesis of allyl- and vinylsi-lanes has also received considerable attention, particularly within the past decade. Using certain ruthenium catalysts, allylsilanes were found to isomerise to the corresponding propenylsilanes prior to metathesis [5]. Using rhenium- or tungsten-based catalysts, however, successful cross-metathesis of allylsilanes with a variety of simple alkenes was achieved [6,7] (an example typical of the results reported is shown in Eq. 3). 

In contrast, ruthenium catalysts gave the best results for the cross-metathesis reactions of vinylsilanes with a range of unfunctionalised alkenes [8] (a typical example is shown in Eq. 4). 

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. 

Prior to the first examples of the cross-metathesis of functionalised alkenes using these catalysts, however, was a report on the use of a lesser known tungsten complex 5 [18,19]. 

Although the application of tungsten catalyst 5 to the cross-metathesis reaction of other alkenes has not been reported, Basset has demonstrated that to-un-saturated esters [18] and glycosides [21], as well as allyl phosphines [22], are tolerated as self-metathesis substrates. 

A year later, Schrock confirmed that the cross-metathesis of two alkyl-substituted terminal alkenes could also be catalysed by his molybdenum catalyst [26] (Eq. 9). 

In 1995 Crowe and co-workers underlined the potential of the molybdenum alkylidene 3 as a catalyst for cross-metathesis when they reported the first examples of productive acrylonitrile metathesis [27] (for example Eq. 10). 

The success of the cross-metathesis reactions involving styrene and acrylonitrile led to an investigation into the reactivity of other Ji-substituted terminal alkenes [27]. Vinylboranes, enones, dienes, enynes and a,p-unsaturated esters were tested, but all of these substrates failed to undergo the desired cross-metathesis reaction using the molybdenum catalyst. 

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