**1. Introduction**

Olefin metathesis can be described as the redistribution of the two fragments obtained by breaking the double bond of an olefin (**Figure 1**). This reaction is of great interest not only for industry (for example, for the production of propene from ethylene and butene) but also for organic chemistry, mainly for the formation of cycles [1].

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

**Figure 1.** The three olefin metathesis reactions.

Historically, the reaction was not recognized as a metathesis reaction. At the end of the 1950s, there were many studies in industrial laboratories on the catalytic effects of systems containing transition metal ions on unsaturated hydrocarbons. These works had been initiated partly by the results of Ziegler and Natta in the field of ethylene and propene polymerization and partly by those obtained by Phillips and Standard Oil in ethylene polymerization by heterogeneous systems. Many observations were made which could not be explained by the reactions known at this period. Finally, it was really Calderon and Ofstead, at Goodyear, who obtained the first conclusive results, which led to the formulation of metathesis as a general principle of reversible scission and recombination of carbon-carbon double bonds.

This mechanism has been confirmed by the synthesis of homogeneous complexes containing a nucleophilic carbenic function and the formation of a metallacyclobutane by their reaction with an olefin [6, 7]. These species display a good activity in the olefin metathesis reaction, in agreement with a mechanism involving them. In addition, numerous complexes with metallacyclobutane intermediates were isolated and gave additional proofs to the Chauvin's mechanism [8, 9]. During last years, the development of highly active homogeneous and heterogeneous catalysts made the olefin metathesis reaction a powerful tool in numerous domains such as

Olefin Metathesis by Group VI (Mo, W) Metal Compounds

http://dx.doi.org/10.5772/intechopen.69320

69

petrochemistry, polymers synthesis, fine chemistry, and synthesis of natural products.

**2. Group VI complexes used in homogeneous catalysis**

**Figure 2.** Intermediates proposed for the olefin metathesis reaction.

**Figure 3.** Mechanism of the olefin metathesis reaction.

industrial systems.

LiAl(heptyl)<sup>4</sup>

Usually, olefin metathesis catalysts contain elements from groups 6 to 8, typically molybdenum, tungsten, rhenium, and ruthenium. While catalysts based on ruthenium are widely used in organic synthesis at the laboratory scale, molybdenum and tungsten are used industrially at a larger scale. Systems based on rhenium were developed but their use remains marginal. Due to the importance of catalysts based on group (VI) elements, this review will be limited to them, with the aim to have a better understanding of the nature of the active sites in the

The first homogeneous catalytic systems using group VI metals (W or Mo) were ill-defined Ziegler-Natta type compounds, formed *in situ* by contacting a precatalyst and a cocatalyst [10]. The first example of olefin metathesis was described in 1955 by Anderson and Merckling who observed the formation of polynorbornene during the reaction of norbornene and TiCl<sup>4</sup>

[11]. Two other catalytic systems based on quite the same elements (WCl6

/

/AlEt<sup>3</sup>

The olefin metathesis reaction can be divided into three different reactions (**Figure 1**): (i) the homo-metathesis and the cross-metathesis which involve the exchange of fragments of acyclic olefins; (ii) the ring opening metathesis polymerization (ROMP), which involves the opening of a cyclic olefin, and (iii) the ring closing metathesis (RCM), which corresponds to the formation of a cyclic olefin by reaction of a diene. Three other classes of olefin metathesis are (iv) ring-opening metathesis polymerization, (v) acyclic diene metathesis, and (vi) ethenolysis.

The mechanism of the olefin metathesis reaction remained unknown for several years and various intermediates were postulated. In 1968, Calderon proposed a cyclobutane coordinated to the metal as an intermediate species (**Figure 2**) [2]. Pettit proposed the formation of a tetramethylene complex [3], while Grubbs postulated the formation of a metallacyclopentane [4].

Finally, Chauvin proposed the now admitted and experimentally proved mechanism of the olefin metathesis reaction and obtained the Nobel Prize in 2005 with Grubbs and Schrock for this discovery [5]. This mechanism necessitates the presence of a metallocarbenic species which can coordinate an olefin, leading to the formation of a metallacyclobutane. Upon rearrangement this cycle will lead to the formation of a new olefin and restore the metal carbene species (**Figure 3**).

This mechanism implies that the reactions are equilibrated and the metallacyclobutane can lead to new products (productive metathesis) or to the starting olefins (degenerative metathesis).

Olefin Metathesis by Group VI (Mo, W) Metal Compounds http://dx.doi.org/10.5772/intechopen.69320 69

**Figure 2.** Intermediates proposed for the olefin metathesis reaction.

**Figure 3.** Mechanism of the olefin metathesis reaction.

Historically, the reaction was not recognized as a metathesis reaction. At the end of the 1950s, there were many studies in industrial laboratories on the catalytic effects of systems containing transition metal ions on unsaturated hydrocarbons. These works had been initiated partly by the results of Ziegler and Natta in the field of ethylene and propene polymerization and partly by those obtained by Phillips and Standard Oil in ethylene polymerization by heterogeneous systems. Many observations were made which could not be explained by the reactions known at this period. Finally, it was really Calderon and Ofstead, at Goodyear, who obtained the first conclusive results, which led to the formulation of metathesis as a general principle

The olefin metathesis reaction can be divided into three different reactions (**Figure 1**): (i) the homo-metathesis and the cross-metathesis which involve the exchange of fragments of acyclic olefins; (ii) the ring opening metathesis polymerization (ROMP), which involves the opening of a cyclic olefin, and (iii) the ring closing metathesis (RCM), which corresponds to the formation of a cyclic olefin by reaction of a diene. Three other classes of olefin metathesis are (iv) ring-opening metathesis polymerization, (v) acyclic diene metathesis, and (vi) ethenolysis.

The mechanism of the olefin metathesis reaction remained unknown for several years and various intermediates were postulated. In 1968, Calderon proposed a cyclobutane coordinated to the metal as an intermediate species (**Figure 2**) [2]. Pettit proposed the formation of a tetramethylene complex [3], while Grubbs postulated the formation of a metallacyclopentane [4]. Finally, Chauvin proposed the now admitted and experimentally proved mechanism of the olefin metathesis reaction and obtained the Nobel Prize in 2005 with Grubbs and Schrock for this discovery [5]. This mechanism necessitates the presence of a metallocarbenic species which can coordinate an olefin, leading to the formation of a metallacyclobutane. Upon rearrangement this cycle will lead to the formation of a new olefin and restore the metal carbene species (**Figure 3**). This mechanism implies that the reactions are equilibrated and the metallacyclobutane can lead to new products (productive metathesis) or to the starting olefins (degenerative metathesis).

of reversible scission and recombination of carbon-carbon double bonds.

**Figure 1.** The three olefin metathesis reactions.

68 Alkenes

This mechanism has been confirmed by the synthesis of homogeneous complexes containing a nucleophilic carbenic function and the formation of a metallacyclobutane by their reaction with an olefin [6, 7]. These species display a good activity in the olefin metathesis reaction, in agreement with a mechanism involving them. In addition, numerous complexes with metallacyclobutane intermediates were isolated and gave additional proofs to the Chauvin's mechanism [8, 9]. During last years, the development of highly active homogeneous and heterogeneous catalysts made the olefin metathesis reaction a powerful tool in numerous domains such as petrochemistry, polymers synthesis, fine chemistry, and synthesis of natural products.

Usually, olefin metathesis catalysts contain elements from groups 6 to 8, typically molybdenum, tungsten, rhenium, and ruthenium. While catalysts based on ruthenium are widely used in organic synthesis at the laboratory scale, molybdenum and tungsten are used industrially at a larger scale. Systems based on rhenium were developed but their use remains marginal.

Due to the importance of catalysts based on group (VI) elements, this review will be limited to them, with the aim to have a better understanding of the nature of the active sites in the industrial systems.
