Olefin functional, metathesis

Catalytic ring-closing metathesis makes available a wide range of cyclic alkenes, thus rendering a number of stereoselective olefin functionalizations practical. The availability of effective metathesis catalysts has also spawned the development of a variety of methods that prepare specially-outfitted diene substrates that can undergo catalytic ring closure. The new metathesis catalysts have already played a pivotal role in a number of enantioselective total syntheses. 

McNaughton, B. R. Bucholtz, K. M. Camaano-Moure, A. Miller, B. L. Self-selection in olefin cross metathesis The effect of remote functionality. Org. Lett. 2005, 7, 733-736. 

Ring-closing metathesis seems particularly well suited to be combined with Passerini and Ugi reactions, due to the low reactivity of the needed additional olefin functions, which avoid any interference with the MCR reaction. However, some limitations are present. First of all, it is not easy to embed diversity into the two olefinic components, because this leads in most cases to chiral substrates whose obtainment in enantiomerically pure form may not be trivial. Second, some unsaturated substrates, such as enamines, acrolein and p,y-unsaturated aldehydes cannot be used as component for the IMCR, whereas a,p-unsaturated amides are not ideal for RCM processes. Finally, the introduction of the double bond into the isocyanide component is possible only if 9-membered or larger rings are to be synthesized (see below). The smallest ring that has been synthesized to date is the 6-membered one represented by dihydropyridones 167, obtained starting with allylamine and bute-noic acid [133] (Fig. 33). Note that, for the reasons explained earlier, compounds... 

Table A. Functionalized homoallylic alcohols from olefin cross metathesis and subsequent allylboration reactions with benzaldehyde...

The CM of olefins bearing electron-withdrawing functionalities, such as a,/ -unsaturated aldehydes, ketones, amides, and esters, allows for the direct installment of olefin functionality, which can either be retained or utilized as a synthetic handle for further elaboration. The poor nucleophilicity of electron-deficient olefins makes them challenging substrates for olefin CM. As a result, these substrates must generally be paired with more electron-rich crosspartners to proceed. In one of the initial reports in this area, Crowe and Goldberg found that acrylonitrile could participate in CM reactions with various terminal olefins using catalyst 1 (Equation (2))." Acrylonitrile was found not to be active in secondary metathesis isomerization, and no homodimer formation was observed, making it a type III olefin. In addition, as mentioned in Section 11.06.3.2, this reaction represents one of the few examples of Z-selectivity in CM. Subsequent to this report, ruthenium complexes 6 and 7a were also observed to function as competent catalysts for acrylonitrile... 

The ruthenium-catalyzed olefin cross-metathesis to the preparation of functionalized allyl boronates has resulted in a one-pot three-component coupling procedure for the synthesis of functionalized homoallylic alcohols.617,618 The utility of the protocol was demonstrated in asymmetric allylboration using a tartrate ester (Equation (152)).617... 

All the above cascade alkene metathesis reactions are based on the ROM of a cycloalkene moiety. Harrity and co-workers have described the synthesis of functionalized spiro cyclic systems by cascade selective olefin ringclosing metathesis reactions from an acyclic tetraalkene. The selectivity for five-membered ring closure over seven-membered ring closure would be the result of a kinetically favored cyclization process [42] (Scheme 20). The syn-... 

In 2005, Rowan, Nolte, and coworkers described an efficient and templated synthesis of porphyrin boxes using DCC and reversible metathesis reaction [54]. Cyclic tetramers were successfully prepared in good yields (62%) from an olefin-functionalized zinc porphyrin in the presence of first generation Grubb s catalyst and upon addition of a tetrapyridyl porphyrin (TPyP) serving as a template. While a mixture of linear and cyclic oligomers was obtained in the absence of template, addition of TPyP resulted in a reorganization of the DCL to favor the formation of the desired tetrameric box (Fig. 7a). 

It was recognized early that efficient olefin cross metathesis could provide new methods for the synthesis of complex molecules. However, neither (la) nor (2a) were very effective at intermolecular cross metathesis owing to poor reaction selectivity (cross vs. intramolecular metathesis) and low E. Z ratios see (E) (Z) Isomers) The advent of more active and functional group tolerant olefin metathesis catalysts recently made cross metathesis a viable route for constructing a large variety of fimctionalized acyclic alkenes. 

Grabbs reported the use of (4a) in the synthesis of symmetrical trisubstituted olefins, wherein isomer selectivity is not an issue. Isolated yields using neat isobutylene or 2-methyl-2-butene as both reactant and solvent and a variety of functionalized metathesis partners were good to excellent under low catalyst loadings, providing a viable alternative to the Wittig reaction (equation 21). 

A new procedure for GSL synthesis via olefin cross metathesis (164) is highly versatile in terms of the hydrophobic agly-cone. A protected 5 carbon amino alkene diol is the central building block to which the protected carbohydrate donor, long chain fatty acid, or, by olefin cross metathesis, the long alkenyl chain of the base can be coupled, in a variety of sequences. This atypical synthetic flexibility should allow a stmctural approach to dissecting the role of the lipid moiety in GSL receptor function and intracellular trafficking. 

Choi and co-workers [579] studied the reactivity of vinyl-terminated self-assembled monolayers (SAMs) of undec-lO-ene-1-thiol on gold (see Fig. 6.32) toward olefin cross-metathesis (CM). Vinyl groups on SAMs were successfully converted into a, P-unsaturated carbonyl groups by CM with acrylic acid, methyl acrylate, and acrylamide. Result shows that various useful functional groups can be introduced to SAMs on gold (and other solid surfaces) by olefin CM and suggests an alternative to the S3mthesis of desired molecules in solution [579]. 

Another example that the successfiil discovery of new reactions may effect fine chemical synthesis is the selective cross-metathesis of acrylonitrile with terminal olefins to give substituted acrylonitriles. This is the first time that an olefin functionalized directly at the double bond undergoes cross-metathesis. ... 

Linker 1 is immobilized through the carboxy function. Linker 2 is generated on solid support by a Wittig reaction. A second alkene is generated during or at the end of the synthesis to give the second olefin for metathesis. 

Methyl oleate (methyl c -q-octadecenoate) is an attractive functionalized olefin for metathesis because of its ready availability and the utility of the metathesis products. An early example is the proposed route to civetone by metathesis of methyl oleate followed by cyclocondensation (Eqs. lla,b). Civetone is a seven-teen-membered unsaturated macrocyclic ketone (d5-9-cycloheptadecen-l-one) identical with the natural compound (civet cat). It has an intense musk odour, and is therefore an attractive perfume component. 

Metathesis of olefins containing functional groups leads to synthesis of many valuable compounds. However, such olefins undergo metathesis with difficulty. Therefore, examples of metathesis of functionalized olefins are scarce. This is due to the following reasons (1) functional groups may react with the catalyst in some cases, they may cause its decomposition and (2) competitive complexation of functional groups rather than C = C bonds may take place. 

In contrast to the preparative methods described above, a functionalized allyl-boronate can be created from a simpler allylboronate by olefin cross-metathesis [81, 82]. Here, treatment of pinacol allylboronate (2) with various olefin partners, exemplified with styrene in Equation (33), in the presence of ruthenium catalyst 58 smoothly furnishes a more elaborate 3-substituted allylboronate, the cross product 38 [81]. These reactions are noteworthy for their exceptional functional group tolerance allylboronates bearing primary halides can be directly synthesized using this method. Unfortunately, the E/Z selectivity in the formation of the 3-substituted allylboronates is variable. This metathesis approach to allylboronates was employed as the beginning of a tandem cross-metathesis/carbonyl allylation process [82] (discussed in more detail in Section 6.4.1.3). 

This concept was refined by the research group of Li, who employed the thiol-yne reaction instead of the olefin cross-metathesis reaction as key step [60]. In this case, the carboxylic acid component served as anchor, whereby terminal alkynes were introduced by the remaining components (5-hexyn-l-al and propargyl isocyanoacetamide). Interestingly, thiol-yne addition of 3-mercaptopropionic acid to the pendant alkynes enabled not only the incorporation of further carboxylic acids, but also resulted in additional branching. Therefore, the second generation dendrimer, synthesized in three steps, exhibited 16 peripheral triple bonds. Moreover, this concept offers the opportunity to introduce structural diversity into the dendrimer architecture because the use of only one alkyne-functionalized compound in the Passerini-3CR still results in branching due to the thiol-yne reaction. Here, a structural sequence of employed phenylacetaldehyde and 2-nitrobenzaldehyde was demonstrated. 

Morrill C, Grubbs RH. Synthesis of functionalized vinyl bor-onates via ruthenium-catalyzed olefin cross-metathesis and subsequent conversion to vinyl halides. J. Org. Chem. 2003 68 6031-6034. 

Olefin metatheses are equilibrium reactions among the two-reactant and two-product olefin molecules. If chemists design the reaction so that one product is ethylene, for example, they can shift the equilibrium by removing it from the reaction medium. Because of the statistical nature of the metathesis reaction, the equilibrium is essentially a function of the ratio of the reactants and the temperature. For an equimolar mixture of ethylene and 2-butene at 350°C, the maximum conversion to propylene is 63%. Higher conversions require recycling unreacted butenes after fractionation. This reaction was first used to produce 2-butene and ethylene from propylene (Chapter 8). The reverse reaction is used to prepare polymer-grade propylene form 2-butene and ethylene ... 

Hexacarbonyldicobalt complexes of alkynes have served as substrates in a variety of olefin metathesis reactions. There are several reasons for complex-ing an alkyne functionality prior to the metathesis step [ 125] (a) the alkyne may chelate the ruthenium center, leading to inhibition of the catalytically active species [125d] (b) the alkyne may participate in the metathesis reaction, giving undesired enyne metathesis products [125f] (c) the linear structure of the alkyne may prevent cyclization reactions due to steric reasons [125a-d] and (d) the hexacarbonylcobalt moiety can be used for further transformations [125c,f]. 

Olefin metathesis, an expression coined by Calderon in 1967,1 has been accurately described in Ivin and Mol s seminal text Olefin Metathesis and Metathesis Polymerization as the (apparent) interchange of carbon atoms between a pair of double bonds (ref. 2, p. 1). This remarkable conversion can be divided into three types of reactions, as illustrated in Fig. 8.1. These reactions have been used extensively in the synthesis of a broad range of both macromolecules and small molecules3 this chapter focuses on acyclic diene metathesis (ADMET) polymerization as a versatile route for the production of a wide range of functionalized polymers. 

Diol-functionalized telechelic polymers have been desired for the synthesis of polyurethanes however, utilizing alcohol-functionalized a-olefins degrades both 14 and 23. Consequently, in order for alcohols to be useful in metathesis depolymerization, the functionality must be protected and the oxygen atom must not be /3 to the olefin or only cyclic species will be formed. Protection is accomplished using a/-butyldimcthylsiloxy group, and once protected, successful depolymerization to telechelics occurs readily. 

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