Olefin during metathesis

High loading can, however, be an inconvenience if resin-bound substrates are able to react with themselves (e.g. during olefin cross-metathesis or macrocyclizations), or if bi- or polyfunctional, unprotected reagents are to be used in solid-phase synthesis. 

Olefin metathesis. During metathesis, the alkylidene groups of the reactant olefins trade partners and rearrange to give new combinations of alkenes in the products. 

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. 

The products separate during the reaction, forming a biphasic liquid/liquid system decantation may be an option. This type of separation has been applied in reactions that produce apolar products such as hydrocarbons. Examples are given wdth olefin oligomerization, metathesis, or hydrogenation. The tunable dissolving power of ILs may provide opportunities to couple biphasic reaction and selective separation. The primary products separate as they are formed before subsequent reactions to unwanted byproducts can take place (see Section 5.2.2.1). 

These examples illustrate the occurrence of metal contamination during mech-anochemical treatment of organic molecules without affecting the reactivity and yields, however wear in organic mechanosynthesis can also lead to incompatibility in a chemical sense, which especially holds for metal-catalyzed reactions. In a recently pubhshed demonstration of olefin cross-metathesis reaction conducted under ball-milhng conditions [33], the authors reported on an observation that the model metathesis reaction of styrene to produce stilbene proceeded with diminished... 

Further investigation on the metathesis of 1-decene (an expected olefin during the alkane metathesis of -decane) and the hydrogenation of the products formed clearly demonstrated that the distribution resulting from alkane and olefin metathesis completely differs with the same catalyst. If there is no double-bond migration, 9-octadecene and ethylene are expected to be the major primary products. Indeed, these primary products are observed, as the temperature reaches 150°C. However, after just 15 min, Cj to C12 and C13 to C20 olefins are also observed, clearly indicating that some isomerization of double bond occurs leading to several competitive metatheses (Fig. 5). 

A variety of additives have been developed to suppress the migration or isomerization of olefins during an olefin metathesis process. In Ru-catalyzed olefin metathesis, it has been proposed that ruthenium hydride species, formed in situ during a metathesis reaction due to catalyst decomposition, are largely responsible for the isomerization process. For example, Grubbs and coworkers [57] have isolated the decomposition product 100, and have shown that it can promote olefin isomerization (Scheme 12.30). 

Interest in the olefin disproportionation (metathesis) reaction catalysed by transition-metal compounds has increased markedly during the present period and although a number of features of such reactions remain unexplained or are topics of controversy, sufficient evidence has now been obtained to suggest that carbene intermediates are involved in most catalytic systems. The exact nature of the catalytic sites remains obscure but the recent discovery of one-component metathesis catalysts may enable more rapid advances to be made in this direction. 

In order to avoid side reactions, particularly isomerization, alkaline ions can be added during the impregnation stage of catalyst preparation. By this method, 1-butene production can be greatly reduced during metathesis of propylene and thus avoid the synthesis of other olefins by cross disproportionation between the normal metathesis products and the isomerized products. 

In this process, which has been jointly developed by Institute Francais du Petrole and Chinese Petroleum Corp., the C4 feed is mainly composed of 2-butene (1-butene does not favor this reaction but reacts differently with olefins, producing metathetic by-products). The reaction between 1-butene and 2-butene, for example, produces 2-pentene and propylene. The amount of 2-pentene depends on the ratio of 1-butene in the feedstock. 3-Hexene is also a by-product from the reaction of two butene molecules (ethylene is also formed during this reaction). The properties of the feed to metathesis are shown in Table 9-1. Table 9-2 illustrates the results from the metatheses reaction at two different conversions. The main by-product was 2-pentene. Olefins in the range of Ce-Cg and higher were present, but to a much lower extent than C5. 

An illustrative example of an alternative strategy (cf Fig. 11c) involving the use of a novel traceless linker is found in the multistep synthesis of 6-epi-dysidiolide (363) and several dysidiolide-derived phosphatase inhibitors by Waldmann and coworkers [153], outlined in Scheme 70. During the synthesis, the growing skeleton of 363 remained attached to a robust dienic linker. After completion of intermediate 362, the terminal olefin in 363 was liberated from the solid support by the final metathesis process with concomitant formation of a polymer-bound cyclopentene 364. Notably, during the synthesis it turned out that polymer-bound intermediate 365a, in contrast to soluble benzoate 365b, produced diene 367 only in low yield. After introduction of an additional linker (cf intermediate 366), diene 367 was released in distinctly improved yield by RCM. 

During the past decade, NHCs have been coordinated to virtually all transition metals (TM) and studied in numerous catalytic transformations, pushing back the frontiers of catalysis. In this regard, the most salient examples are found in olefin metathesis and cross coupling reactions, and more recently in organocatalysis. 

The application of olefin metathesis to the synthesis of piperidines continues to be widely employed. The use of ring closing metathesis (RCM) in the synthesis of fluorovinyl-containing a,P-unsaturated lactams 148 and cyclic amino acid derivatives 149 is shown below. A key improvement in these reactions is the addition of the Grubbs 2nd generation catalyst (G2) in small portions during the reaction to compensate for catalyst decomposition that occurs at elevated reaction temperatures <06EJOl 166>. 

Cyclopropanes in low yield were first noted in 1964 by Banks and Bailey (12) during the disproportionation of ethylene, but little significance was attached to that observation until recently, because such products had no obvious relevance to early mechanistic concepts based on pairwise rearrangements of bisolefin complexes. However, the subsequent adoption of carbenelike species as metathesis intermediates (4) provided a foundation for later development of cyclopropanation concepts. The notable results of Casey and Burkhardt (5) made an impact which seemed rather neatly to unify mechanistically the interconversion of cyclopropanes and metathesis olefins, although the reactions which they observed were stoichiometric rather than catalytic [see Eq. (4)]. Nevertheless, their work indicated a net redistribution of =CPh2 and =CH2 from (CO)5W=CPh2 and isobutylene, respectively, to form CH2=CPh2. Dissociation and transfer of CO yielded W(CO)6. Unfortunately, the fate of the isopropylidene moiety remained unknown. In 1976,... 

Ruthenium complexes B are stable in the presence of alcohols, amines, or water, even at 60 °C. Olefin metathesis can be realized even in water as solvent, either using ruthenium carbene complexes with water-soluble phosphine ligands [815], or in emulsions. These complexes are also stable in air [584]. No olefination of aldehydes, ketones, or derivatives of carboxylic acids has been observed [582]. During catalysis of olefin metathesis replacement of one phosphine ligand by an olefin can occur [598,809]. 

Evidence for Ru Release and Return During Olefin Metathesis... 

As shown in Tab. 11.7 for the RCM of diethyl diallylmalonate, the original catalyst (10) suffered complete loss of metathesis activity after only two runs (entry 1). On the other hand, the second generation catalyst (55) retained modest activity in the third consecutive run (entry 2). As in the prior study [18], the addition of a terminal olefin additive was required to extend the catalyst lifetime past four runs (entry 3). Use of triphenylphosphine as a second additive just ten minutes before filtration led to further improvements in activity during catalyst reuse. However, as mentioned previously, the use of additives is somewhat wasteful and exclusive, since... 

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