Alkene metathesis components

Table 3.14. Examples of multi-component catalysts for homogeneous-phase alkene metathesis.

A concise total synthesis of dehydrohomoancepsenolide is achieved in an optically active form. The key steps are alkene metathesis and alkyne metathesis. A three-component coupling reaction affords dienyne 137, which undergoes ring-closing alkene metathesis in the presence of the first-generation ruthenium carbene complex to give 138,... 

Recently, highly efficient catalysts have been developed that are one component systems and contain a preformed alkylidene or metallacycle as the catalyst initiatorSome of these complexes have shown excellent utility in the synthesis of organic molecules and will be the major topic of Ae last part of this chapter. These complexes will not only change many of the approaches in simple alkene metathesis but will also have a major influence on the future applications in organic and polymer synthesis. 

The metathesis reaction between carbon-carbon double bonds (alkene metathesis) is well established in commercial scale synthesis. It is a key component of some polymerization processes and is the route to nonfunctionalized alkenes which find applications in fine chemical synthesis. The development of well-defined, functional group tolerant catalysts will lead to a much greater role for alkene metathesis in synthesis. 

Combination of WCl6 and EtAlCb (or Et3Al) is also effective for acetylene polymerization . This catalyst shows high activity towards linear and cyclic alkene metathesis and a metathesis propagation step is to be invoked also in this case. However, this combination is a typical Ziegler-Natta catalyst system, and it may be probable that, in this particular case, the components are acting as a simple coordinated anionic catalyst system. 

Alkene metathesis has grown from a niche technique to a common component of the synthetic organic chemistry toolbox, driven in part by the development of more active catalyst systems, or those optimized for particular purposes. While the range of synthetic chemistry achieved has been exciting, the effects of structure on reactivity have not always been particularly clear, and rarely quantified. Understanding these relationships is important when designing new catalysts, reactions, and syntheses. Here, we examine what is known about the effect of structure on reactivity from two perspectives the catalyst, and the substrate. The initiation of the precatalyst determines the rate at which active catalyst enters the catalytic cycle the rate and selectivity of the alkene metathesis reaction is dependent on how the substrate and active catalyst Interact. The tools deployed in modern studies of mechanism and structure/activity relationships in alkene metathesis are discussed. 

UV/visible spectroscopy is useful for the monitoring of organometaUic species, but is not useful for monitoring the organic component of typical metathesis reactions. Ruthenium species relevant to alkene metathesis are typically very highly colored (red or green) and have molar absorptivities... 

The most thoroughly studied reactions are the metathesis of propene to ethene and 2-butene, and the metathesis of 2-pentene to 2-butene and 3-hexene. Generally, the thermodynamic equilibrium ratio of the trans and cis components of the products is obtained. The reacting alkene molecules need not be identical, two different alkenes react with each other in the same way. 

Another intramolecular ene-yne metathesis followed by an intermolecular metathesis with an alkene to give a butadiene which is intercepted by a Diels-Alder reaction was used for the synthesis of condensed tricyclic compounds, as described by Lee and coworkers [266]. However, as mentioned above, the dienophile had to be added after the domino metathesis reaction was completed otherwise, the main product was the cycloadduct from the primarily formed diene. Keeping this in mind, the three-component one-pot reaction of ene-yne 6/3-94, alkene 6/3-95 and N-phenylmaleimide 6/3-96 in the presence of the Grubbs II catalyst 6/3-15 gave the tricyclic products 6/3-97 in high yield (Scheme 6/3.28). 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

Blechert reported a skillful method of cross-enyne metathesis. Solid-supported alkyne 139 is reacted with alkene in the presence of Ic to give 140. For cleavage of 1,3-diene from solid-supported product 140 having an allyl acetate moiety, palladium-catalyzed allylic substitution is used. Thus, 140 is treated with Pd(PPh3)4 in the presence of methyl malonate to afford three-component coupling product 141 in good yield ... 

Construction of organic nanotubes starting from porphyrin dendrimers with core/shell architecture is also feasible. Figure 8.29 also shows how covalent nanotubes can be produced by removal of the dendritic component of the molecule. A coordination polymer is first synthesised from a dendritic metallopor-phyrin with alkene end groups. This is subjected to intramolecular and intermo-lecular crosslinking by ring-closing metathesis at the periphery. 

The most likely scenario for enyne metathesis is an intramolecular combination of the alkene and alkyne moieties to form a ring, which is really a variation of RCM. Examples of intermolecular enyne metathesis (CM) have been successful if they are run in an atmosphere of ethene, using it as the alkene component. Even for some intramolecular enyne metatheses, preequilibration of the catalyst with ethene caused vastly improved yields.74 Equations 11.2475 and 11.2576 show... 

Cross-metathesis will then also afford ethene, which will escape the reaction because of its volatility. Alternatively, a 1,2-disubstituted (Z)-alkene bearing the same substituent in position 1 and 2 may be used as the solution component. 

In contrast to the reliable, high-yielding, and selective intramolecular, n-enyne metathesis reaction, intermolecular enyne metathesis (enyne cross-metathesis) has seen less use in the synthesis of complex molecules due to limited selectivity, despite its potential in fragment-coupling processes (404). The most common use of intermolecular enyne metathesis employs ethylene as the alkene component, providing a particularly convenient method for the production of... 

Since that discovery there has been much interest in this photochemical reaction, which produces a catalyst working in mild conditions without an organometallic component [1-23]. The same catalyst induces the metathesis of acyclic alkenes [1-17] and the ringopening metathesis polymerisation (ROMP) of cyclic olefins studied recently by Sundararajan et al. [18, 22, 23], as well as the polymerisation of various substituted acetylenes investigated by Masuda et ai [13, 19]. I was first of all intrigued to know what intermediates are formed in the above photochemical reaction and which one is the true catalyst in metathesis and related reactions. 

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