Metathesis, alkene olefin functional group

The order of reactivity of these three catalysts towards alkenes (but also towards oxygen) is 1 > 3 > 2. As illustrated by the examples in Table 3.18, these catalysts tolerate a broad spectrum of functional groups. Highly substituted and donor- or acceptor-substituted olefins can also be suitable substrates for RCM. It is indeed surprising that acceptor-substituted alkenes can be metathesized. As discussed in Section 3.2.2.3 such electron-poor alkenes can also be cyclopropanated by nucleophilic carbene complexes [34,678] or even quench metathesis reactions [34]. This seems, however, not to be true for catalysts 1 or 2. 

The main strategies used for the preparation of alkenes by cleavage from insoluble supports are (3-elimination and olefin metathesis (Figure 3.35). Because some of these strategies enable the preparation of pure alkenes, devoid of additional functional groups, the linkers are sometimes also called traceless linkers, although the C=C double bond reveals the original point of attachment to the support. 

Since the discovery of ruthenium and molybdenum carbene complexes that efficiently catalyze olefin metathesis under mild reaction conditions and that are compatible with a broad range of functional groups, olefin metathesis has increasingly been used for the preparation of alkenes on insoluble supports. In particular, the ruthenium complexes Cl2(PCy3)2Ru=CHR, developed by Grubbs, show sufficient catalytic activity even in the presence of air and water [781] and are well suited for solid-phase synthesis. 

With the discovery by Grubbs of ruthenium carbene complexes such as Cl2(PCy3)2Ru=CHR, which mediate olefin metathesis under mild reaction conditions and which are compatible with a broad range of functional groups [111], the application of olefin metathesis to solid-phase synthesis became a realistic approach for the preparation of alkenes. Both ring-closing metathesis and cross-metathesis of alkenes and alkynes bound to insoluble supports have been realized (Figure 5.12). 

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. 

More evidence has appeared showing that the olefin metathesis reaction can tolerate the presence of functional groups. The catalytic system Re207-Al203, promoted by a small amount of tetramethyltin, effects metathesis of olefins in fair yield (17—40%) in the presence of unsaturated ethers and ketones, alkenyl esters, and halogeno-alkenes. The reaction is performed in carbon tetrachloride as solvent at room temperature over 6 h. Electro-reduction of tungsten hexa-chloride with an aluminium anode in halogenated solvents appears to form a complex suitable for a clean metathesis, exemplified by the conversion of pent-2-ene into its equilibrium mixture with but-2-ene and hex-3-ene. ... 

Nonetheless, ADMET is a versatile technique that allows the incorporation of a wide variety of functional groups into the resultant polymers. Scheme 1.9 shows the catalytic cycle of ADMET, controlled by the metathesis catalyst, which can be either ruthenium- [76, 77] or molybdenum-based [78, 79]. While the kinetics are controlled by the catalyst (there is no reaction in its absence), it still follows the kinetic picture described in Section 1.3.2. This is because the catalyst is removed from the chain end after each successful alkene metathesis reaction (i.e., coupling) and the olefin with which it subsequently reacts is statistically random. 

The metathesis of olefins bearing functional groups provides potential routes to many valuable compounds. Metathesis catalysts, most of which contain a Lewis acid, are unfortunately poisoned by polar and basic compounds. As yet, only a few catalytic systems have proved to be active in metathesizing functionalized alkenes. In the homogeneous disproportionation of these substrates, the catalytic combination WCl —Sn(CH3)4 is still the best known [63]. In addition to its well known application in the conversion of unsaturated oxygen containing olefins, such as fatty acid esters and unsaturated ethers, its effectiveness for the homogeneous metathesis of unsaturated amines has also been described [64]. 

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