Olefin-metathesis reaction, importance

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

The olefin metathesis reaction apparently has rarely been applied to allene synthesis so far, or at least the potential of this important process has not been exploited... 

The role of carbenes and metal carbene complexes in transition metal-catalyzed processes is suspected of being quite extensive (61). For example, the role of carbenes in the olefin metathesis reaction as described in the previous section is probably important (55, 60). It is quite possible that the o-v rearrangement is important in these reactions also, but this has not been investigated in detail. 

These results show that subtle changes in the nature of the metal carbene initiator or of the substrate can lead to important modifications in the relative energy levels of the three types of intermediates involved in catalytic olefin metathesis reactions. 

Castanospermine is a polyhydroxylated alkaloid found in the plant Castanospermum australe.1 Its ability to function as a selective inhibitor of a and p glycosidases has made it the focal point of much synthetic activity in recent years.2 One particularly elegant synthesis of ( + )-castanospermine is that of Pandit and Overkleeft.3 It features a remarkable intramolecular olefin metathesis reaction for indolizidine ring assembly. We now analyse this interesting route, which showcases many important reagents and reactions used in contemporary organic synthesis. 

The following important implications, which bear directly on the nature of the cycloolefin polymerization, have precipitated out of our understanding of the basic properties of the olefin metathesis reaction. 

A recent report by Miller and coworkers investigated the effects of remote functionality on the efficiency and stereochemical outcome of the olefin metathesis reaction [55]. Using a series of allyl- and homoallylamides, they demonstrated that both the yield of self-metathesis products and the ratio of cis- and trans-olefin isomers formed were strongly dependent on remote functionalities. Although it does not preclude the use of olefin metathesis in DCC experiments, it is an important factor that needs to be considered when designing olefin-based DCLs. Indeed, in an ideal scenario, one would expect the course of the reaction and product distribution in a DCL to be relatively insensitive to functionality remote from the reacting centers, which is unfortunately rarely the case. 

The ruthenium catalysed olefin metathesis reaction is one of the most important catalytic reactions [77-79] and one that is distinctly underdeveloped for asymmetric applications [80]. Only a few concepts have been brought forward [80,81], of which the combination of a NHC ligand with a 1,1-binaphlhyl scaffold carrying a hydroxyl anchor group is the most promising to date. 

This chapter is concerned specifically with olefin metathesis reactions catalyzed by ruthenium-carbene complexes, mainly because of their great success during recent years. We begin with an overview of these catalysts, and then focus on mechanistic considerations that are important for understanding the reactivity profiles of various catalyst derivatives. The second part of the chapter deals with applications of ruthenium-catalyzed olefin metathesis, especially RCM, CM, and combination processes in organic synthesis. 

Carbene complexes appear to be important intermediates in olefin metathesis reactions, which are of significant industrial interest these reactions are discussed in Chapter 14. 

The elucidation of the mechanism for olefin metathesis reactions has provided one of the most challenging problems in organometallic chemistry. In Volume 1 Rooney and Stewart concluded that the carbene chain mechanism is now generally accepted for olefin metathesis reactions, but much remains to be learned about the formation and reactivity of metal-carbene intermediates, metallocycles, and especially the mechanistic aspects of chain initiations. Since that report, systems have been designed that begin to reveal the important mechanistic features of olefin metathesis. 

Studies on these carbene complexes, especially those of the Schrock type, have attracted special interest in connection with the mechanism of catalytic olefin metathesis reactions. The formation of metallacyclobutane intermediate from the oxidative cycloaddition reaction between carbene complex and olefin was found to be an important key step in the catalytic cycle (eq. (5)). 

BTF can be used for some important transition metal catalyzed reactions, such as the Grubbs olefin-metathesis reaction [64] (11.1), the Petasis olefination [65]... 

Unstable metallacycloalkane intermediates play an important role in a variety of transition-metal-catalyzed isomerizations or rearrangements of strained carbocyclic systems and olefin metathesis reactions ... 

The industrially important olefin metathesis reaction is a reaction that involves the simultaneous cleavage of two olefin double bonds followed by the formation of the alternate double bonds [Eq. (4)] ... 

The most important advance over the past 15 years has been the preparation of numerous well-defined metal carbene complexes which can act directly as initiators of all types of olefin metathesis reaction. These second-generation catalysts allow much closer control and better understanding of the mechanism of the olefin metathesis reaction. The initiating and propagating species can be closely monitored and in some cases the intermediate metallacyclobutane complexes can also be observed. Well-defined metallacyclobutane complexes also can sometimes be used as initiators. 

Cyclopropanes and oxiranes can play an important role in initiation of olefin metathesis reactions via the formation of four-membered metallacycles and their subsequent cleavage sequences (13a, 13b). Conversely, the reverse of such reactions may sometimes be responsible for terminating a metathesis chain reaction. 

Many kinds of metals have been studied by evaporation-cocondensation (120y 124-126) but thus far, little recoil chemistry has been carried out. The mechanistic interest and commercial importance of olefin metathesis reactions (127) suggests... 

Initially, the most obvious mechanism for olefin metathesis was a land of molecular square dance in which two different olefin molecules join to form a cyclobutane ring and then change partners to form two new olefin molecules. While this thermal reaction is Woodward-Hoffmann forbidden, transition metals were initially perceived to allow violations of these rules. However, no cyclobutanes were detected in olefin metathesis reactions, nor did cyclobutanes produce olefins when placed into metathesis reaction mixtures. The breakthrough came in 1971 when Yves Chauvin (1930- ), at the French Petroleum Institute, made the concepmal link between the Phillips Petroleum reaction discovered in 1964 and metallocarbenes isolated in the same year by Ernest Otto Fischer (see chapter 7). Other important discoveries were made by Michael F. Lappert (1928- ) at Sussex, Charles P. Casey (1942- ) at Wisconsin, and especially Thomas J. Katz (1936- ) at Columbia. The mechanism involves formation of a metallocyclobutane (see the accompanying figure), from reaction of a metallocarbene ( M=CR 2 ) with an olefin (R2C=CR2), that splits into a new olefin (R 2C=CR2) and a new metallocarbene ( M=CR2 ). 

Scheme 1. The olefin metathesis reaction generalized process (a) and mechanism (b). Note although not specifically defined on this scheme, it is important to recognize that each of these steps is reversibie.

By 1992, Grubbs and co-workers had discovered an alternative catalyst that overcame many of these shortcomings. Indeed, although ruthenium alkylidene 12 (Scheme 5) displays a lower metathesis activity than Schrock s molybdenum systems, it importantly demonstrated air stability and the ability to initiate metathesis in the presence of alcohols, water, and carboxylic acids. Thus, 12 represents the first true catalyst for general bench top olefin metathesis reactions, and over time has been optimized to 13 (Scheme 6), which has proven far easier to prepare than the parent structure 12 and constitutes the current gold standard with which all new catalyst systems are compared. Without question, this... 

Fatty acid esters are generally obtained from the transesterification of fats and oils with a lower alcohol, e.g. methanol, along with glycerol. More than 90% of all oleochemical reactions (conversion into fatty alcohols and fatty amines) of fatty acid esters (or acids) are carried out at the carboxy functionality. However, transformation of unsaturated fatty acid esters by reactions of the carbon-carbon double bond, such as hydrogenation, epoxidation, ozonolysis, and dimerization, are becoming increasingly of industrial importance. Here we will discuss another catalytic reaction of the carbon-carbon double bond, viz. the olefin metathesis reaction, in which olefins are converted into new products via the rupture and reformation of carbon-carbon double bonds [2]. Metathesis of unsaturated fatty acid esters provides a convenient route to various chemical products in only a few reaction steps. 

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