Metathesis functionalized olefins

Recently, additional catalyst systems which are effective for the metathesis of olefins bearing polar functional groups have been revealed. Nakamura and co-workers found that either WC16 or (C2H50)2MoCl3 in combination with triethylborane were capable of converting c/s-9-octa-decenyl acetate to 1,18-diacetoxy-9-octadecene and 9-octadecene at the rather high temperature of 178°C (89). 

In summary, the metathesis of olefins substituted with polar functional... 

Much more challenging is the targetted introduction of carbon substituents at terminal olefins by means of cross metathesis. Because of the mild reaction conditions under which alkene metathesis proceeds, cross metathesis could become an extremely valuable tool for the synthetic chemist if the critical parameters for productive cross metathesis between different, functionalized olefins were understood. 

Noteworthy is the surface-complex [ iORe( Bu)(=CH Bu)(CH 2Bu)], which displays catalytic activities much higher than both its homogeneous or heterogeneous homologues [68]. In fact, it is possible to achieve the metathesis of functional olefins such as methyl oleate with good activities (TOP and TON) (Table 3.6). 

The utility of Ru-catalyzed cross-metathesis in multicomponent coupling strategies has also been demonstrated. For instance, one-pot cross-metathesis/allylboration sequences have been reported by Miyaura [170] and by Goldberg and Grubbs [171]. Pinacol allyl boronate 174 was reacted with a series of functionalized olefins, which include symmetrically 1,2-disubstituted olefins as well as hindered olefins and styrenes, in the presence of catalyst 175 to produce intermediate allyl boro-nates (e.g. 176). The latter may then be reacted in situ with aldehydes to produce functionalized homoallylic alcohols with high levels of anti-selectivity (Scheme 8.80). 

Grubbs found that (4 a) ring-opened cyclic olefins, and then react with an acrylate to produce end functionalized linear olefins, giving a ring-opened cross metathesis product (ROCM) with two olefins with differing reactivity (equation 19). Key to the distribution of products was the relative rates of ring-opening and cross metathesis with the functionalized olefin. 

A significant breakthrough in cross metathesis was the discovery of general catalysts for reactions with directly functionalized olefins. The modification of the basic catalyst structure with N-heterocydic carbene ligands opened this area of research. 

Since tin-containing co-catalysts are essential for the metathesis of functionalized olefins [26], it was soon discovered that 1 supported on acidic metal oxides forms metathesis catalysts that are active without additives even for functionalized olefins [26]. Standard supports are Al203-Si02, or Nb205 and the activity is related to the surface acidity [2, 3, 26]. A high metathesis activity is observed when MTO is chemisorbed on the surface. No evidence for a surface carbene species was obtained, but there appears to be a correlation between the catalytic activity and the presence of an alkyl fragment on the surface [26a-c]. 

The classic ylid-based reactions used in the solution-phase synthesis of olefins have been applied successfully to the solid-phase. Cross-metathesis has been performed with a resin-bound and a dissolved olefin to provide more highly functionalized olefins. 

Olefin metathesis is being used increasingly in the specialty chemicals market. Olefin interconversion can be used to produce isomerically pure symmetrical internal olefins from a-olefins (Eq. 2 R H), and a-olefins can be produced from internal olefins via ethenolysis. Metathesis of olefins bearing heteroatom functional groups is also a very promising application of the metathesis reaction, which enables the synthesis, in only a few reaction steps, of many products that would otherwise be difficult to obtain. 

Because this book is devoted to fine chemical synthesis by heterogeneous catalysis, homogeneously catalyzed metathesis reactions will not be discussed. After a short overview of heterogeneous metathesis catalysts, the synthesis of fine chemicals from normal (unfunctionalized) olefins in the presence of these heterogeneous catalysts will be discussed. This is followed by a discussion of fine chemicals synthesized from functionalized olefins. 

Other heterogeneous catalyst systems have also been developed for the metathesis of unsaturated esters and other functionalized olefins. They include M0CI5/ Si02/R4Sn, where R is alkyl (e.g. Me or Et) [14], and CH3Re03/Si02-Al203... 

Many heterogeneous metathesis catalysts cannot catalyze the metathesis of functionalized olefins because of their intolerance to functional groups. Interference of the functional groups also reduces the activity of the catalysts that are active in this case. This increases the costs of the catalyst because much higher catalyst/substrate ratios must be used than are required for normal olefins. 

Table 1 shows some of the heterogeneous catalysts used for the metathesis of unsaturated carboxylic esters. These esters can be used as test substrates for functionalized olefin 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. 

Many other (cross-) metathesis reactions of functionalized olefins have been shown to be possible in the presence of rhenium-based catalysts, such as self-metathesis (or cross-metathesis with normal olefins) of allyl- and vinylsilanes, unsaturated nitriles, chlorides, bromides etc. The products of these reactions are not yet of use in fine chemistry, but this might be remedied by future developments in this area. 

Catalysts are now known that facilitate the metathesis of olefins containing all types of functional groups. Such reactions are described for acyclic olefins in Ch. 7, and for cyclic olefins in Ch. 12 and 13. Not only can allyl compounds be metathesized (Mol 1979), reaction (9), but even acrylonitrile will undergo crossmetathesis (Crowe 1995). 

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