Chapter 1
Fundamental Reactions to Cleave Carbon-Carbon s-Bonds with Transition Metal Complexes

Masahiro Murakami and Naoki Ishida

1.1 Introduction

Transition metal-catalyzed reactions proceed through multiple elementary steps in general and, consequently, the mechanisms are often complicated, especially when backbone structures are reconstructed through a sequence of cleavage and formation of C-C bonds. A step-by-step understanding of elementary steps would be valuable to understand such catalytic transformations. This chapter focuses on elementary steps during which carbon-carbon s-bonds are cleaved by means of organometallic complexes.

An elementary step to cleave C-C bonds is a reverse process of a C-C bond forming process. Oxidative addition of a C-C bond to a low-valent transition metal complex is a reverse process of reductive elimination, which occurs with a high-valent diorganometal, forming a C-C bond. ß-Carbon elimination is a reverse process of insertion of an unsaturated bond into a carbon-metal bond, that is, carbometallation, or 1,2-addition of an organometal across a double bond. Such fundamental reactions are described along with typical examples. Besides this chapter, there are some excellent reviews on C-C bond cleavage available [1].

1.2 Oxidative Addition

Oxidative addition is insertion of a metal into a covalent bond. It involves formal two-electron oxidation of the metal center or one-electron oxidation of two metal centers (Scheme 1.1).

Scheme 1.1

Oxidative addition offers a direct method to cleave a covalent bond. Although a wide variety of bonds, such as C-I and C-Br, are known to facilely undergo oxidative addition reactions to low-valent transition metal complexes, examples of oxidative addition of C-C single bonds are far more rare. The scarcity is in part associated with the thermodynamic stability of C-C bonds. Whereas oxidative addition of C-Br and C-I bonds to low-valent metals is thermodynamically favored in general, that of a C-C single bond is often thermodynamically disfavored.

The kinetic reason for the difficulty in breaking C-C single bonds is the constrained directionality of their s-orbital. Figure 1.1 shows the interaction of a metal orbital with a C-C single bond. The interactions with C-C double bonds and C-H single bonds are also depicted for comparison. The p-orbital of a C-C double bond is oriented sideways, and thus it interacts with a metal orbital without significant strain and severe steric repulsion. The s-orbital connecting hydrogen and carbon atoms lies along the bond axis and the directionality is less matched with the metal orbital. However, the constituent 1s orbital of the hydrogen atom is spherical, and can interact with a metal orbital from any direction without distortion. The hydrogen atom has no other substituents except the bonded carbon, thus sterically rendering the direct approach of the metal center less cumbersome. On the other hand, the s-orbital of a C-C single bond possesses high directionality along the bond axis. Moreover, there are several substituents on both ends, which sterically prevent the approach of metal orbitals. Thus, interaction of such a C-C s-orbital with a metal orbital is much more difficult than those of a C-C double bond and a C-H bond. Not only the thermodynamic stability, but also this kinetic barrier renders C-C s-bonds considerably inert.

Figure 1.1 Orbital interactions of a metal with C=C, C-H, C-C bonds.

Despite the intrinsic difficulties mentioned above, a number of strategies have been devised to realize oxidative addition of C-C s-bonds. For example, release of ring strain of a substrate molecule affords both kinetic and thermodynamic drive for oxidative addition. A chelating effect also assists both kinetically and thermodynamically. Aromatization is also exploited as the driving force for oxidative addition of a C-C bond. Each case is exemplified in the following sections.

1.2.1 Oxidative Addition Utilizing Ring Strain

The orbitals of cyclopropane C-C bonds form "banana bonds", which protrude away from the bond axis between the two carbon atoms (Figure 1.2). Consequently a metal center can interact with them similarly, to some extent, to the case of a metal-olefin interaction. This interaction lowers the kinetic barrier of the C-C oxidative addition. In addition, the enlargement of the three-membered cyclopropane ring to a four-membered metallacyclobutane relieves the structural strain owing to its constrained bond angles. Thus, the use of cyclopropanes as substrates for oxidative addition of C-C bonds is advantageous both kinetically and thermodynamically.

Figure 1.2 Orbital interactions of a metal with a C-C bond of cyclopropane.

In fact, PtCl2 reacted with cyclopropane to form platinacyclobutanes (Scheme 1.2) [2]. Cyclopropanes substituted with more electron-donating groups reacted faster and cyano and keto-substituted cyclopropanes remained intact [3].

Scheme 1.2

It is often observed that C-H activation precedes C-C activation. For instance, photoirradiation of Cp*Rh(PMe3)(H2) generated coordinatively unsaturated Cp*Rh(PMe3) with liberation of dihydrogen (Scheme 1.3) [4]. The rhodium complex reacted with cyclopropane at -60 °C to furnish a C-H oxidative addition product. No cleavage of a C-C bond was observed at this low temperature. Upon raising the temperature to 0-10 °C, the cyclopropylrhodium rearranged to a rhodacyclobutane. This result indicates that oxidative addition of a C-H bond is kinetically favored and oxidative addition of a C-C bond is thermodynamically favored in this case. The kinetic preference for the oxidative addition of the C-H bond demonstrates the greater steric accessibility of the C-H bond compared with the C-C bond. The analogous rearrangement of a (cyclobutyl)(hydride)rhodium complex into rhodacyclopentane has also been reported [5].

Scheme 1.3

Oxidative addition would proceed via coordination of the s-bond to the metal (agostic interaction). A rhodium complex with an agostic interaction between a cyclopropane C-C s-bond and a rhodium center has been reported (Figure 1.3) [6]. The bond lengths of Rh-C3 and Rh-C4 are 2.352 and 2.369 Å, longer than typical Rh-C single bonds, but within the sum of the van der Waals radii of Rh and C. The C3-C4 bond (1.6 Å) is longer than typical cyclopropane C-C bonds (about 1.5 Å), but again within the sum of the van der Waals radii of two carbons. The bonding between Rh and C3-C4 indicates that it might be the precursory structure for oxidative addition of cyclopropane C-C bonds.

Figure 1.3 A rhodium complex with an agostic interaction with a C-C bond.

Biphenylenes undergo oxidative addition to various low-valent metals to form the corresponding dibenzometalloles [7]. The reaction with Cp*Rh(PMe3) involved C-H activation prior to C-C activation, as with the case of C-C activation of cyclopropane [7]c. On the other hand, the reaction with [Rh(Pi-Pr3)2]+ initially formed the ?6-arene complex, which led to the dibenzorhodacycle [7]g. Density functional theory (DFT) calculation suggested the C-C activation proceeds via the rhodium ?4-cyclobutadiene intermediate (Scheme 1.4).

Scheme 1.4

A wide variety of strained C-C bonds, such as methylenecyclopropanes [8], vinylcyclopropanes [9], perfluorocyclopropenes [10], and cubane [11], also undergo oxidative addition. The oxidative addition of small ring molecules serves for initiation of their catalytic transformations (Chapters 2 and 3).

1.2.2 Chelation-Assisted Oxidative Addition

The Lewis basic functionalities, like phosphines, facilely coordinate to a metal, whereby the metal center is brought into proximity to a specific C-C bond. The coordination facilitates insertion of the metal into a C-C bond both kinetically and thermodynamically. For example, a system consisting of [RhCl(olefin)2]2 and a bulky, pincer diphosphine ligand led to site-selective metal insertion into an aryl-methyl bond at room temperature (Scheme 1.5) [12]. Initially, the simultaneous formation of a C-H activated complex and a C-C activated complex was observed. The C-H activated complex was gradually converted to the C-C activated complex at room temperature, demonstrating that the C-C activated complex is thermodynamically more stable than the C-H activated product. Furthermore, a kinetic study revealed that, if the numbers of bonds available for activation are taken into account, metal insertion into the C-C bond is also kinetically preferred over the competing insertion into the C-H bond in this case. Electronic perturbation of the aromatic ring by introduction of a methoxy group...