Heteroatom-Substituted Carbene Complexes

The subject of this book has been organized in three main sections preparation and applications of heteroatom-substituted carbene complexes (Fischer-type carbenes), non-heteroatom-substituted carbene complexes, and acceptor-substituted carbene complexes. In each section the different types of reaction have been ordered either according to the mechanism or according to the type of product. In addition to a selection of illustrative examples, several experimental procedures have been included. These were chosen taking into account safety, availability of starting materials, relevance of the products, and general interest. 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Finally, the possibility of building the M=C bond into an unsaturated metallacycle where there is the possibility for electron delocalization has been realized for the first time with the characterization of osmabenzene derivatives. For these reasons then, it seemed worthwhile to review the carbene and carbyne chemistry of these Group 8 elements, and for completeness we have included discussion of other heteroatom-substituted carbene complexes as well. We begin by general consideration of the bonding in molecules with multiple metal-carbon bonds. 

The development of the chemistry of carbene complexes of the Group 8a metals, Ru, Os, and Ir, parallels chemistry realized initially with transition metals from Groups 6 and 7. The pioneering studies of E. O. Fischer and co-workers have led to the characterization of many hundreds of carbene complexes in which the heteroatoms N, O, and S are bonded to the carbene carbon atoms. The first carbene ligands coordinated to Ru, Os, and Ir centers also contained substituents based on these heteroatoms, and in this section the preparation and properties of N-, O-, S-, and Se-substituted carbene complexes of these metals are detailed. 

Halide displacement from the carbene ligands of Ru, Os, and Ir halocarbene complexes by N-, O-, and S-based nucleophiles frequently leads to the formation of new heteroatom-substituted carbene complexes. This important class of reactivity will be discussed in more detail in Section V,D, but it is appropriate here to illustrate the scope of this method with several examples ... 

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

Heteroatom-Substituted Carbene Complexes Some Structural Parameters"... 

The reactivity displayed by the heteroatom-substituted Ru, Os, and Ir carbene complexes discussed in this section toward nucleophilic reagents contrasts sharply with that described for the Fischer compounds. The reactions of these Group 8 complexes are almost exclusively restricted to the metal-ligand framework, with only two related substituent substitution reactions being reported (44) ... 

Table 1.1. Chemical shifts for carbon atoms (C ) and protons (H ) in representative heteroatom-substituted carbene complexes L M=Cot(R)H(j.

In this section the preparation and uses of heteroatom-substituted carbene complexes L M=C(Xn)R,2 n) (n = 1, 2 X NRj, OR, SR) will be discussed. In these complexes the electron deficit at the carbene carbon atom is compensated both by electron-donation from the lone pairs on the heteroatom and by d-electron backbonding from the metal (Figure 2.1). 

Because of n-electron donation by the heteroatom, these carbene complexes are generally less electrophilic at C than the corresponding non-heteroatom-substituted complexes (Chapter 3). This effect is even more pronounced in bis-heteroatom-substituted carbenes, which are very weak Tt-acceptors and towards low-valent transition metals show binding properties similar to those of phosphines or pyridine. Alkoxycarbenes, on the other hand, have electronic properties similar to those of carbon monoxide, and stable heteroatom-monosubstituted carbene complexes are also usually formed from metals which form stable carbonyl complexes. 

Structural parameters and other data have been calculated for several models of heteroatom-substituted carbene complexes [3-5,8]. 

Particularly stable are coordinatively saturated, 18-electron carbene complexes of the type (CO)5M=C(X)R (M W, Cr X OR, NR2 R H, alkyl, aryl). These complexes are often referred to as Fischer-type carbene complexes, in honor of E. O. Fischer, who prepared these compounds for the first time in 1964 [61]. Since then these compounds have attracted broad interest, and many hundreds of heteroatom-substituted carbene complexes have been synthesized. Thereby valuable new insights were gained into the nature of the carbon-metal double bond. These complexes are also becoming increasingly important for organic synthesis, both as reagents and as catalysts. 

In Figure 2.2 the most important synthetic approaches to alkoxy- or (acy-loxy)carbene complexes from non-carbene precursors are sketched. Some of these strategies can also be used to prepare amino- and thiocarbene complexes. These procedures will be discussed in detail in the following sections. In addition to the methods sketched in Figure 2.2, many complexes of this type have been prepared by chemical transformation of other heteroatom-substituted carbene complexes. Because of the high stability of most of these compounds, many different reactions can be used to modify the substituents at C without degrading the carbon-metal double bond. The generation of heteroatom-substituted carbene complexes from other carbene complexes will be discussed in Section 2.2. 

Table 2.1. Heteroatom-substituted carbene complexes prepared from carbonyl complexes and carbon nucleophiles.

Carbonyl complexes also react with non-carbon nucleophiles. The resulting carbonic acid derivatives can serve as starting material for the preparation of bis-heteroatom-substituted carbene complexes [93]. Heterocyclic carbene complexes can be obtained from nucleophiles with a leaving group in -position (Table 2.2). 

Haloiminium salts can react with metallates or similarly nucleophilic transition metal complexes to yield heteroatom-substituted carbene complexes (Figure 2.7) [120]. This reaction is closely related to the acylation of metallates with derivatives of carboxylic acids (Section 2.1.1.2). Examples are given in Table 2.5. 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

Table 2.7. Formation of heteroatom-substituted carbene complexes from alkynes, vinylidene complexes, and alkynyl complexes.

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

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