Olefin Conversion via Metathesis:

An Overview of a very useful application of organic synthesis.

Abstract

Olefin metathesis reactions have been at the forefront of organic chemistry research since the early 1960s. These reactions were stumbled upon accidentally and immediately used to produce new cutting-edge products in industry. Included is a walkthrough of the many years of experimentation and research leading up to the acceptance of a widely accepted form of the mechanism for the reaction. Many of the most common forms of metathesis chemistry including ring-opening, ring-closing, ring-closing polymerization, cross metathesis, and acyclic diene polymerization are elaborated on. An assessment of the development of Grubbs, Schrock, and other catalysts and their contributions to the unveiling of the mechanism and chemistry of metathesis is included. Finally, included also is an overview of the many industrial applications of this type of reaction.

Introduction

An observation that would unveil the “reaction of the future” was stumbled upon by researchers Banks and Bailey at Phillips Pertroleum Co. close to 40 years ago. A pioneer with the process, Robert Grubbs, would later be quoted: “As with most catalytic processes, olefin metathesis was found by accident” [4]. It was an “outgrowth of the study of Ziegler polymerizations,” which was a study on the polymerization of 1-alkenes [4]. In the 1950s olefin metathesis was first studied and reported by industrial chemist, Herbert Eleuterio of DuPont petrochemicals, who observed that when you pass propylene feed over a molybdenum-on-titanium catalyst, the products yielded were propylene, ethylene, and 1-butene [1]. When replacing the propylene with cyclopentane he explained to Chemical & Engineering news that "the polymer I got looked like somebody took a pair of scissors, opened up cyclopentene, and neatly sewed it up again” [1]. This was the first recorded research on the topic of this paper, olefin metathesis reactions and would later become the topic of those awarded the 2005 Nobel Prize in Chemistry.

Surprisingly enough, researchers at other companies were also experiencing the unexplainable mechanism. In 1960 Standard Oil Co. had a patent on a process that would later be called metathesis [2]. Before long, chemical companies were delivering products that “could not be explained by the reactions of olefins known at the time” [2]. However, in 1967, Nissim Calderon at Goodyear Tire & Rubber finally figured out what was going on that allowed these products to be made. His conclusion was that “the unexpected products are due to cleavage and reformation of the olefins' double bonds. One carbon of the double bond of one olefin, along with everything attached to it, exchanges place with one carbon of the double bond of the other olefin, along with everything attached to it.” He then dubbed the term "olefin metathesis” [3].

At the point in time when this term was dubbed, much of the chemistry of the reaction was understood and used in multiple industrial applications. Man was able to harness the power of metathesis to a certain extent given all its pertinent knowledge. However, the mechanism of the reaction was unknown and it took decades of painstaking research and proposing new theories for a hypothesis to be actually accepted in the chemical world. Once a proposed mechanism was accepted, the real amazing part of the development of metathesis was beginning and industrial applications were sprouting more rapidly than ever.

Reaction Mechanism

In the late 1960’s chemists began working towards uncovering the mechanism of olefin metathesis. It began with Calderon’s idea that a cyclobutane intermediate was formed, however early work disproved his theory and allowed for further research. This is when Robert Grubbs began studying this kind of reaction and in 1972 he proposed that the redistribution of the groups around the double bonds was due to a redistribution of a metallocyclopentane intermediate [2]. These theories were all incorrect, and little did they know, two French chemists already published a paper suggesting otherwise in 1971.

Yves Chauvin and Jean-Louis Hérisson were the first to hypothesis the currently accepted mechanism of olefin metathesis. They suggested that the reaction was initiated with a metal carbene. The mechanism started with the “[2+2] cycloaddition of an alkene double bond to a transition metal alkylidene to form a metallocyclobutane intermediate. The metallocyclobutane produced can then cyclorevert to give either the original species or a new alkene and alkylidene,” and that “Interaction with the d-orbitals on the metal catalyst lowers the activation energy enough that the reaction can proceed rapidly at modest temperatures” [1]. However, much of Chauvin’s future efforts to help prove his theory were unsuccessful. And if it wasn’t for a simple observation by a chemistry professor at Columbia University, Thomas Katz, moving forward with the development of the olefin metathesis reactions would not be possible. Through experimentation, Katz found out that "what Chauvin did not recognize is that, when a metal carbene reacts with an olefin, two metal carbenes can result and the more stable one will be formed in larger amount” [1].

Further research eventually could not disprove Chauvin's theory, and eventually it was widely accepted at the mechinism. The general mechanism of the reaction is seen below.

1.JPG

Figure 1: The general mechanism of an olefin metathesis reaction

Chemistry

There are multiple different uses of metathesis that have popularized including, ring opening metathesis, ring-closing metathesis, ring opening metathesis polymerization, cross-metathesis, and Acyclic diene metathesis. Applications of these reactions will be explained further on.

Ring-opening metathesis (ROM) is a process where strained rings in a molecule are split by a ruthenium carbene catalyst and an example can be seen in Figure 2.


2.JPG
Figure 2: An example of ROM.

Ring-opening metathesis polymerization (ROMP) happens when the absence of a second reaction partner occurs and the rings polymerize in a second step when they polymerize which can be seen below in Figure 3. These are widely used “versatile reaction[s] to synthesize macromolecular materials” [14].

3.JPG
Figure 3: Polymerization of olefins post-ROM.


Ring-closing metathesis (RCM) is a highly valuable reaction in organic chemistry due to the ability to synthesize between 5 and 30 membered cyclic alkenes. Ruthenium carbene catalyst promote the reaction and which is driven by the removal of ethene from the mixture [18]. “Ring closing metathesis has proved [to be] a spectacularly useful tool in the hands of chemists synthesising natural products and their analogues” [11].


4.JPG
Figure 4: An example of RCM.


Cross-metathesis (CM) is usually what is meant by those when using the term metathesis alone. It can also be described as the transalkylidation of two terminal alkenes under the release of ethene and can be seen below.


5.JPG
Figure 5: The general form of CM.


The last and more recently researched (and progressed) type of olefin metathesis reaction is Acyclic diene metathesis (ADMET). This is another form of metathesis that is an extremely “versatile tool for both polymerization and depolymerization” [10] as Michael Buchmeiser explains in his overview of ADMET. The driving force of ADMET is the formation of ethylene that is then removed from the mixture. Buchmeiser also stated that “a large variety of homo- and copolymers may be prepared according to the general ADMET scheme” [10] seen below in Figure 6.


6.JPG
Figure 6: The general reaction scheme for ADMET polymerization [10]


Catalysts

The possibility of harnessing the power of olefin metathesis reactions can only exist with the ability to catalyze these normally reduced reactivity reactions. The process of producing these well-defined catalysts has been part of the journey of Olefin metathesis reactions and integral in uncovering the mechanisms and chemistry of the reactions. Grubbs could not have said it better that “each improvement in catalysts reactivity and selectivity has led to a variety of new applications and synthetic strategies” [4].

Although “trialkoxide compounds were found to be highly active for alkyne metathesis and were the first well-defined catalysts for this reaction,” [8] current advances specify that there are four main elements used as a basis for the formation of catalyst for Olefin metathesis reactions, but they are not limited to only these four. The current catalyst basis elements that have been extensively studied are tungsten based, ruthenium based (Grubbs catalyst), molybdenum based (Schrock catalyst), and just emerging are the NHC type ruthenium based.

The first studies of promoting olefin metathesis were with tungsten based catalysts that had very low selectivity and reactivity. It wasn’t until years later that Huerterio at DuPont petrochemicals accidentally found that molybdenum had higher selectivity for metathesis reactions [16]. This led to the start of what some say is the next revolution in organic chemistry.

Early catalyst research by chemist Richard Schrock gave rise to the first molybdenum based catalysts that were specifically designed to carry out metathesis reactions. Even earlier, many other researchers discovered that molybdenum helped catalyze olefin metathesis reactions, but it wasn’t until Schrock completed his research before they had well-defined catalysts for these reactions. At this point in the 1970’s, metathesis was only possible with hydrocarbons without functional groups. Schrock was the first to realize that, by creating molybdenum complexes, the ability to run the reactions with other functional groups present was possible; however the catalysts were still not friendly to most functional groups. The molybdenum based catalyst processes are highly reactive yielding relatively high olefin conversion. However the catalysts sensitivity to air, water, and many functional groups limit their applications [4].

As Schrock was focusing on molybdenum, Grubbs began research on the topic of catalysts based on ruthenium, which “hoped would be less oxygen-sensitive and therefore more functional group tolerant” [2]. A breakthrough in the 1980’s led to the discovery of ruthenium based catalyst that not only increased olefin metathesis reaction rates, but also polymerized olefins and synthesized high molecular weight polymers. Grubbs explains in his paper Olefin Metathesis that organic chemist Bruce Novak “demonstrated that a strained olefin and ruthenium(II) were the keys to the formation of an active catalyst” for ROMP reactions [4]. Grubbs also explains that without these important observations from Novak, the possibility of making a strong metathesis catalyst in the future would not have been possible. The ruthenium catalysts are easy to use and are not sensitive to air, water and most functional groups widening the range of possible applications for metathesis. However, they are much less reactive than others and therefore limit their applicability [20].

Recent research “has significantly extended the scope of olefin metathesis reactions as valuable synthetic tools in organic synthesis” [13]. Recent developments in metathesis catalyst research have brought about a new generation of N-heterocyclic carbene ligand incorporated ruthenium catalysts [19]. These catalysts increase metathesis activity and also maintain the same functional group acceptance of the previous ruthenium catalysts. Some of these NHC catalysts have the ability to “not only render the catalyst soluble in water and some organic solvents” and also make it insoluble in diethyl ether making separation from organic products much easier (e.g. precipitation from diethyl ether) [12].

Applications

Once a good deal of work was put into creating catalyst for metathesis, industrial applications were possible. On top of this large amount of work, a common, relatively inexpensive material, ruthenium chloride, is able to form ruthenium based catalysts at about 90% yield, and according to Grubbs, “The commercial availability of the ruthenium catalyst made its widespread use possible” [4]. The applications of this process range from making polymer composites to synthesizing pharmaceutical intermediates.

The Phillips triolefin process was developed by Phillips Petroleum Co over 40 years ago and was implemented in one of their plants from 1966 to 1972 [5]. Researchers working for Phillips were trying to find a replacement for and HF acid catalyst that would convert olefins into high octane gasoline. They noticed that replacing the HF acid catalyst with a molybdenum one, the olefin molecules were split and that propene could be reacted to yield ethene and 2-butene. Due to the low demand for propene at the time, the process was implemented in the plant to convert the unwanted propene into the more economically favorable ethene and 2-butene components [5]. Current economics warrant the use of the reverse reaction in order to produce large amounts of propene, which can be seen in many newer and upcoming refineries around the globe.

Currently at the Institut Francias du Petrole researchers are working on a high conversion metathesis process named Meta-4 for the production of propene from ethene and 2-butene in the liquid phase in the presence of a rhenium based catalyst [6]. The process reaches a conversion of over 60%. The reason this has not commercialized however, is due to the high cost of catalyst. New catalyst formulations are being studied to overcome this problem.

The Shell Higher Olefins Process was explained by Mol in Industrial Applications of Olefin Metathesis as "a large-scale industrial process incorporating olefin metathesis .. for producing linear higher olefins from ethene" [5]. In the process, ethene reacts to form C6-C18 1-alkenes which are then separated and can be used for the synthesis of polyethene. Polyethene is a highly used chemical in a wide variety of applications including synthetic lubricants and fatty acids, as well as plasticizer and detergent alcohols. In these applications, “the highly active ruthenium complexes are very attractive [catalysts], due to their robustness to air, water and oxygenates, high reaction rates and selectivities” [5].

The ring-opening metathesis polymerization of cycloalkenes is a widely used method for the creation of linear polymers when using low quality monomers [5]. An example of a use of this method is called Polyoctenamer and produces the product Vestenamer 8012, which is used as an additive for rubberized asphalt.

Another application of ring-rearranging metathesis polymerization/depolymerization is in the production of neohexene, which is an important species in the synthesis of Tonalide (synthetic perfume) and Terbinafine (anti-fungal) [5]. The reverse reaction is the polymerization and used for the synthesis of polynorbornene, which was the first of all of the metathesis polymers to be commercially used. Polynorbornene is marketed as Norsorex and is used for oil spill recovery and sound dampening.

Other chemicals synthesized using ring-opening metathesis polymerization include poly-dicyclopentadiene, Zeonex, and Zeonor. Polydicyclopentadiene, which is a tough, rigid, thermoset polymer of excellent impact strength,” [5] is used for heavy-vehicle applications. Zeonex and Zeonar are colorless and transparent polymers used for optical and electronic applications.

Conclusion

The reactions discussed in this document are relatively cutting-edge in organic synthesis and more recently are recognized as “green chemistry” reactions due to their ability to create large amounts of desired products while keeping costs at a minimum by reducing the use of catalyst, raw materials, and utilities. They also reduce the amount of synthetic steps as well as reducing the contaminant by-products commonly produces [7].

While many industrial applications of this process exist, new developments in catalyst technology are regularly producing new possibilities for applications.


References

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