Olefin metathesis utility

Despite the successful reactions mentioned above, olefin metathesis utilizing titanocene-methylidene is not necessarily regarded as a useful synthetic tool. Indeed, the steric interaction between the substituent at the carbon a to titanium and the bulky cyclopentadienyl ligand disfavors the formation of the titanocene-alkylidene 15. Hence, cleavage of the titanacycle affords only titanocene-methylidene and the starting olefin (Scheme 14.9). 

Tandem carbonyl olefmation—olefin metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefinic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefin metathesis between titano-cene methylidene and the ris-l,2-disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cydic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

An asymmetric version of olefin metathesis utilizes a Mo-carbene complex (2) with ( / ,/fl-l,2-bis(2-hydroxy-2,2-bistrifluoromethyl)ethylcyclopentane as a ligand. ... 

Table 8-5 indicates the wide variety of catalysts that can effect this type of disproportionation reaction, and Figure 8-7 is a flow diagram for the Phillips Co. triolefm process for the metathesis of propylene to produce 2-butene and ethylene. Anderson and Brown have discussed in depth this type of reaction and its general utilization. The utility with respect to propylene is to convert excess propylene to olefins of greater economic value. More discussion regarding olefin metathesis is noted in Chapter 9. 

The mechanistic investigations presented in this section have stimulated research directed to the development of advanced ruthenium precatalysts for olefin metathesis. It was pointed out by Grubbs et al. that the utility of a catalyst is determined by the ratio of catalysis to the rate of decomposition [31]. The decomposition of ruthenium methylidene complexes, which attribute to approximately 95% of the turnover, proceeds monomolecularly, which explains the commonly observed problem that slowly reacting substrates require high catalyst loadings [31]. This problem has been addressed by the development of a novel class of ruthenium precatalysts, the so-called second-generation catalysts. 

Another approach to synthetically useful olefin metathesis involves the utilization of higher homologues of titanium-methylidene 15, as shown in Scheme 14.11. If the resulting titanium carbene complex 20 is more stable than the starting alkylidene complex 15, this reaction can be employed for the generation of various titanocene-alkylidenes and as a method for the preparation of unsaturated compounds. 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

Catalytic olefin metathesis, in only a few years, has risen to be one of the most important and reliable processes in organic synthesis. Recently, several reports by Schrock and Hoveydallsbbond forming transformations efficiently and enan-tioselectively. A recent concise and enantioselective synthesis of exo-brevicomin by Burke utilizes chiral catalyst 91 (Scheme 13) to effect the desymmetriza-tion of 90 through a ring-closing metathesis.11531... 

The Trauner group utilized an olefin metathesis for the construction of the A-ring (Eq. 3) [131, 132] ... 

With the ready availability of 2-fluoro-allylic halides and a-fluoroacrylic acid derivatives, incorporation of a pendant fluorovinyl unit is easier than ever. The utility of these products is markedly enhanced by the reactivity of the fluorovinyl unit in olefin metathesis reactions. Some success has been found in cyclization reactions as shown below [83] (Scheme 36). 

Olefin metathesis chemistry has had a profound impact in several areas of chemical research, including organome-tallics, polymer chemistry, and small molecule synthesis,many of which have industrial applications. For example, CM is currently utilized in the commercial preparation of several agrochemicals, polymer and fuel additives, and pharmacophores. Unlike RCM reactions, which are typically conducted under dilute... 

Hoveyda s synthesis of fluvirucin-BI-aglycone (1) presented here is short and convergent It takes extensive advantage of modem synthetic methods, with transit on-metal catalysts or mediators utilized in nine of the fifteen steps. Important key steps worth noting include a zirconocene-catalyzed carbometallation and the olefin metathesis step. 

The cycloaddition of alkenes with metal alkylidene complexes remains the most common entry into the metallacyclobutane structural class. Consistent with metallacyclobutane intermediacy in the olefin metathesis reaction, the [2+2] cycloaddition is generally reversible a propensity for cycloreversion (Section 2.12.6.2.4), however, can significantly limit the utility of metallacyclobutane complexes as intermediates in other synthetic transformations. 

Olefin metathesis, in particular ring closing metathesis (RCM), remains a popular route to the synthesis of piperidines. This is exemplified in the following references which employed an RCM reaction utilizing Grubbs first generation catalyst (benzyl -... 

Green Chemistry Utilizing Olefin Metathesis Catalysts 35... 

RECENT METHODOLOGY DEVELOPED UTILIZING OLEFIN METATHESIS... 

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