Olefin metathesis intermolecular

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

Olefin metathesis can also be used in intermolecular reactions.299 For example, a variety of functionally substituted side chains were introduced by exchange with the terminal double bond in 5.300 These reactions gave E Z mixtures. 

Grubbs has reported a similar tandem olefin metathesis-carbonyl olelination process for the preparation of cyclic olefins [31]. In this case, treatment of a keto-olefin with the molybdenum alkylidene 1 at 20°C generates an intermediate alkylidene complex. Under these conditions, competing intermolecular olelination does not occur. However, intramolecular carbonyl olelination of the initially formed alkylidene complex can occur and this results in the formation of a cyclic olefin. This tandem sequence is illustrated by the transformation of keto-olefins... 

Clark TD, Ghadiri MR. Supramolecular design by covalent capture. Design of a peptide cylinder via hydrogen-bond-promoted intermolecular olefin metathesis. J Am Chem Soc 1995 117 12364-12365. 

Intermolecular olefin metathesis starts to compete with traditional C=C-bond forming reactions such as the Wittig reaction and its modifications, as illustrated by the increasing use of electron-deficient conjugated alkenes for the ( )-selective construction of enals and enoates. 

Dienes can undergo olefin metathesis reactions of two types (i) intermolecular and (ii) intramolecular, as illustrated by the reactions of hexa-1,5-diene and octa-1,7-diene, equations 27 and 28, respectively. 

Frechet and Liang synthesised a fifth-generation benzyl ether dendrimer with 22 internal allyl groups. Subsequent olefin metathesis [1] led on the one hand to intramolecular linking, and on the other hand to creation of a covalent linkage site for olefin-functionalised guests (Fig. 6.1) [2]. In this way, intermolecular cross-linking - between G5 dendrimer molecules - could also be attained. 

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. 

Although intermolecular enyne metathesis is the simplest to envision, intramolecular enyne metathesis was the major focus of the initial work. Two representative intramolecular enyne meta theses are shown in Equations 21.44 and 21.45. The reaction in Equation 21.44 shows the value of this chemistry to form heterocycles. The reaction in Equation 21.45 shows how enyne metathesis can be used in combination with olefin metathesis to form bicyclic products. The initial enyne metathesis process in Equation 21.45 terminates in a ruthenium carbene complex. The carbene complex is then trapped by the remaining olefin in a [2+2] and retro-[2+2] cycloaddition sequence to generate the bicyclic organic product and a ruthenium carbene complex that re-enters the catalytic cycle by reaction witii the yne diene. 

Cross Metathesis. Of the three major types of olefin metathesis, cross metathesis (CM) has been the most challenging to selectively control. CM is an intermolecular reaction between two olefins that releases ethylene gas, among a statistical mixture of combinatorial products it is a thermodynamically controlled reaction, where impurities arising from homodimerization are common. In addition, unlike ROMP, which relieves ring strain, and RCM, which forms stable 5- and 6-membered rings, CM has no enthalpic driving force (71). 

A laboratory scale syndiesis was also performed utilizing intermolecular the Ti-Claisen condensation of methyl 10-decenoate followed by an intramolecular olefin metathesis using the Grabbs reagent afforded the 17-membered p-keto ester. The overall isolated yield is 74%, which is highest conqiared with hitherto reported syntheses. 

The olefin cross metathesis (CM) can be described as the intermolecular metathesis of alkylidene fragments between two different olefins [133]. It can be farther divided into three main subtypes cross metathesis, ring opening cross metathesis (ROCM) and enyne cross metathesis (ECM) (Scheme 3.9). 

In addition to transition metals, recent work has demonstrated that strong Lewis acids will catalyze the addition of silanes to alkynes in both an intra- and an intermolecular fashion.14,14a-14c The formation of vinylsilanes from alkynes is possible by other means as well, such as the synthetically important and useful silylcupration15,15a of alkynes followed by cuprate protonation to afford vinylsilanes. These reactions provide products which can be complementary in nature to direct hydrometallation. Alternatively, modern metathesis catalysts have made possible direct vinylsilane synthesis from terminal olefins.16,16a... 

Intermolecular Carbonyl Olefination and Subsequent Ring-Closing Metathesis... 

Intermolecular-enyne metathesis, if it is possible, is very unique because the double bond of the alkene is cleaved and each alkylidene part is then introduced onto each alkyne carbon, respectively, as shown in Scheme 9. If metathesis is carried out between alkene and alkyne, many olefins, dienes and polymers would be produced, because intermolecular enyne metathesis includes alkene metathesis, alkyne metathesis and enyne metathesis. The reaction course for intermolecular enyne metathesis between a symmetrical alkyne and an unsym-metrical alkene is shown in Scheme 9. The reaction course is very complicated, and it seems impossible to develop this reaction in synthetic organic chemistry. 

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