1
Industrial Milestones in Organometallic Chemistry

Ben M. Gardner1, Carin C.C. Johansson Seechurn2, and Thomas J. Colacot3

1Cambridge Display Technology Ltd, Unit 12 Cardinal Park, Cardinal Way, Godmanchester, PE29 2XG, UK

2Johnson Matthey, 28 Cambridge Science Park, Milton Road, Cambridge, CB4 0FP, UK

3Millipore Sigma (A Business of Merck KGAa Darmstadt, Germany), 6000N Teutonia Avenue, Milwaukee, WI 53209, USA

1.1 Definition of Organometallic and Metal-Organic Compounds

Organometallic compounds can be defined as compounds that contain at least one chemical bond between a carbon atom of an organic moiety and a metal. The metal can be alkaline, alkaline earth, transition metal, lanthanide, or a metalloid such as boron, silicon, and phosphorus. Therefore, metal-phosphine complexes are also often included in this category, although they do not contain a typical metal-carbon bond - they are more commonly referred to as "metal-organic compounds." For the purposes of this book, applications of both organometallic and metal-organic compounds are discussed on the basis of "organometallic chemistry."

1.1.1 Applications and Key Reactivity

The three major types of applications of organometallic compounds in industry are in the areas of electronics, polymers, and organic synthesis. In organic synthesis, the organometallic compounds are used as either catalysts or stoichiometric reagents.

1.1.1.1 Electronic Applications

For electronic applications typically, the organometallic complex is subjected to chemical vapor deposition (CVD) to form an appropriate thin layer or subjected to organometallic chemical vapor deposition (OMCVD) where the deposition ultimately occurs via a chemical reaction at the substrate surface to produce a high-quality material. The production of thin films of semiconductor materials is used, for example, for LED applications via metal-organic vapor-phase epitaxy (MOVPE) where volatile organometallic Me3E (E = Ga, In, Al, and Sb) compounds are used as precursors. They react with ultrapure gaseous hydrides in a specialized reactor to form the semiconducting product as a crystalline wafer [1-23].

1.1.1.2 Polymers

Another major application for organometallic complexes is in the polymer industry. Three common types of polymers produced via catalysis are particularly noteworthy. Polysiloxanes, also known as silicone, are polymers made up of repeating units of siloxane [4]. They have widespread application in a large number of different fields ranging from cookware to construction materials (e.g. GE silicone), medicine, and toys. Pt-based catalysts are commonly applied in the silicone industry for the production of a variety of products [5]. A milestone in the history of organometallic chemistry in the industry was the discovery of the Ziegler-Natta catalyst and its application in polymerization reactions [6]. Ziegler and Natta were awarded the Nobel Prize for their work in this field in 1963 [7]. Another area that has been recognized for its importance is olefin metathesis for which a Nobel Prize has been awarded to Grubbs, Schrock, and Chauvin. This has been applied to synthesize polymers via ROMP (ring-opening metathesis polymerization) [8].

1.1.1.3 Organic Synthesis

The focus of this book, however, is on the exploitation of organometallic compounds for organic synthesis, relevant to industry applications. One of the major applications in organic synthesis is catalysis.

In cases where the organometallic compound is used as a catalyst, for example in a process involving cross coupling, a precatalyst should be able to get activated to the active catalytic species to bind with the organic substrate(s), do the transformation, and release the product such that the active catalytic species returns to its original state in the catalytic cycle. During the organic transformation, the concentration of the catalyst can decrease with time because of poisoning. The efficacy and efficiency of the catalyst depend on how fast and how long it can retain its original activity. The turnover numbers (TONs) and turnover frequencies (TOF) are usually used to describe the activity of a catalyst. Organic chemists have started using organometallic compounds as catalysts to develop more efficient and practical processes [9-12].

The reactivity of organometallic complexes toward various reagents is the reason behind the widespread use of organometallic compounds as catalysts for a variety of organic transformations. The most important types of organometallic reactions are oxidative addition, reductive elimination, carbometalation, hydrometalation, ß-hydride elimination, organometallic substitution reaction, carbon-hydrogen bond activation, cyclometalation, migratory insertion, nucleophilic abstraction, and electron transfer. In the following paragraphs, we will provide a brief overview of the basic theory with some selected applications.

Oxidative addition involves the breakage of a bond between two atoms X-Y. Splitting of H2 with the formation of two new metal-H bonds is an example of an oxidative addition process (Scheme 1.1). Reductive elimination is the reverse of this process. In an oxidative addition process, the oxidation state of the metal is increased by 2, whereas in reductive elimination, oxidation state of the metal is decreased by 2. Both steps are crucial for metal-catalyzed cross-coupling reactions, as the first and the last steps of the catalytic cycle. Several factors can affect these two steps. The structure of the ligand (phosphine or other molecules coordinated with the metal), the coordination number of the metal in the complex, and the way in which the complex is activated to the catalytic species in the catalytic cycle, etc., can be modified and tailored to get the best outcome for a particular reaction [13]. The oxidative addition of H2 onto Vaska's complex (Scheme 1.1) is a crucial step in metal-catalyzed hydrogenation reactions. The application of this methodology to industrially relevant molecules is further discussed in Section 1.3.3.

Scheme 1.1 Oxidative addition and reductive elimination.

Carbometalation involves, as the name suggests, the simultaneous formation of a carbon-metal and a CC bond. This is most commonly used to form a stoichiometric metal-containing reagent, such as the reaction between ethyllithium and bis-phenylacetylene in the synthesis of TamoxifenTM, a breast cancer drug (Scheme 1.2) [14].

Scheme 1.2 Carbometalation as a key step toward the synthesis of TamoxifenTM.

Hydrometalation is similar to carbometalation, where, instead of a CC bond, a CH bond is formed alongside the carbon-metal bond. One such example is hydroalumination, where DIBAL (i-Bu2AlH) is added across an alkyne (Scheme 1.3) [15]. This, similar to carbometalation, is most commonly a stoichiometric transformation with the aim of preparing an organometallic reagent that can be used as a reactant for subsequent desired transformations.

Scheme 1.3 Hydroalumination of alkynes.

ß-Hydrogen elimination, technically the reverse of hydrometalation, can in some cases result in the formation of undesired side products. In other cases, it is a "blessing" as the preferred reaction pathway. In Shell higher olefin process (SHOP), for the oligomerization to occur, a final ß-hydrogen elimination reaction is performed to release the substrate from the catalyst (Scheme 1.4a) [16]. In the cross-coupling reaction between an aryl halide and an organometallic reagent containing ß-hydrogens, this reaction can form the undesired alkene side products, hence detrimental. This is the reason why sp2-sp3 coupling and sp3-sp3 coupling become very challenging even today. However, a few success stories of these types of cross-coupling reactions have been reported, such as sp2-sp3 Negishi reaction for the synthesis of LX2761, a diabetes drug by Lexicon Pharmaceuticals (Scheme 1.4b) [17].

Scheme 1.4 a) ß-hydride elimination is exploited in the Shell higher olefin process (SHOP). b) sp2-sp3 cross-coupling in the synthesis of a diabetes drug.

Organometallic substitution reactions can occur either via an associative or a dissociative substitution mechanism. This can be compared to SN1 and SN2 substitution mechanisms in organic chemistry. The overall outcome in either case is an exchange of a ligand on the organometallic complex. Scheme 1.5 illustrates an associative substitution mechanism to exchange Cl for X on Vaska's complex. This complex does not have any significant references to being employed in industry as a catalyst, but studies of its reactivity has been vital in providing the conceptual framework for homogeneous catalysis [18].

Scheme 1.5 Organometallic substitution reaction exemplified by Vaska's complex.

Source: Wilkins 1991 [24]. Reproduced with permission of John Wiley &...