Metal-heteroatom multiple bonds complexes

As in the case of carbene complexes, 13C NMR spectroscopy is particularly useful in that the carbyne carbon typically resonates to low field (240 and 360 ppm), with heteroatom substituents shifting this to higher field. As noted above for carbene complexes, X-ray crystallography reveals that carbyne complexes have very short metal-carbon bonds, typically the shortest of any metal-carbon multiple bond, but lengthened if heteroatom substituents are present. 

The hydrosilylation of carbon-heteroatom multiple bonds had received little attention until it was found in 1972 that Rh(PPh3)3Cl is an extremely effective catalyst for the hydrosilylation of carbonyl compounds. This is a new and unique reduction method since the resulting silicon-oxygen bond can easily be hydrolyzed. Other transition metal complexes including platinum, ruthenium , and rhodium also have good catalytic activity in the selective and asymmetric hydrosilylation of carbonyl compounds "". 

Apart from the hardness and softness, two reactivity-related features need to be pointed out. First, iron salts (like most transition metal salts) can operate as bifunctional Lewis acids activating either (or both) carbon-carbon multiple bonds via 71-binding or (and) heteroatoms via a-complexes. However, a lower oxidation state of the catalyst increases the relative strength of coordination to the carbon-carbon multiple bonds (Scheme 1). 

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. 

Germanium-carbon multiple bonds, formation, 3, 709 Germanium-chalcogen bonds, reactivity, 3, 745 Germanium complexes with alkali metal bonds, 3, 748 with Isis // -arcnc chromium heteroatoms, 5, 340 with chromium carbonyls, 5, 208 coupling reactions, 3, 711 with CpMoCO, 5, 463... 

Hydride is added to the multiple bond (path AdN) as expected. Usually the metal cation complexes with the Y heteroatom nucleophilic attack on the polarized multiple bond will then yield a product that is stabilized by ion pairing. 

Organometallics react with this sink by addition to the multiple bond (path Ad r). The more covalent, less reactive organometallics, like R2Cd, react very slowly with almost all of these sinks, whereas organomagnesiums, RMgX, and organolithiums react quickly. Complexation of the metal ion to the Y heteroatom catalyzes this reaction. Organometallics react much faster as nucleophiles with polarized multiple bonds than as bases with the adjacent C-H bonds, (carbon-acid, carbon-base proton transfer is slow). C=Y example ... 

At the other extreme, metal carbenes that are electrophilic at carbon are called Fischer-type complexes, and they generally contain jt-donating heteroatom substituents [4], Fischer reported the first example in 1964 [5], In these cases, the metal-carbene interaction can be represented by three resonance structures, the first with a formal M=C double bond, the second with a M-C single bond and charge separation, and the third with additional multiple bond character between the carhon and the heteroatom substituent. 

A few final comments should be made on the insertions of substrates containing C-C multiple bonds into the bonds between a transition metal and an electronegative heteroatom. First, insertions of olefins into related thiolate and phosphide complexes are as rare as insertions into alkoxo and amido complexes. Reactions of acrylonitrile into the metal-phosphorus bonds of palladium- and platinum-phosphido complexes to give products from formal insertions have been observed, and one example is showm in Equation 9.90. However, these reactions are more likely to occur by direct attack of the phosphorus on the electrophilic carbon of acrylonitrile than by migratory insertion. Second, the insertions of alkynes into metal-oxygen or metal-nitrogen covalent bonds are rare, even though the C-C ir-bond in an alkyne is weaker than the ir-bond in an alkene. 

It should be noted that formation of trans-product can be achieved in an anti-addition reaction through the outer-sphere mechanism. Theoretical studies have demonstrated that syn-addition and anti-addition reactions may start from the same 7i-complex, and direction of the multiple bond activation depends on the polarity of solvent [17, 18]. Relative reactivity in the inner-sphere and outer-sphere mechanisms contributes to the overall -/Z- selectivity of the addition reaction to alkynes (stereoselectivity issue). In some cases it is possible to switch the direction of C-Het bond formation by finding a suitable ligand [19]. In case of alkenes syn-addition and a f -addition processes do not necessarily result in different stereochemistry (unrestricted rotation around the single C-C bond in the product). Occurrence of these mechanisms for the N [20, 21], P [22, 23], O [24-26], S, Se [27, 28] heteroatom groups and application of different metal catalysts are discussed in detail in the other chapters of this book. Stereochemical pathways of nucleometallation and development of enantioselective catalytic procedures were reviewed [29]. In this chapter we focus our attention on the mechanism of irmer-sphere insertion reaction involving double and triple carbon-carbon bonds. 

Spatial arrangement of heteroatom groups covered in this volume (N, P, O, S, Se, Te) gives rise to different isomers of transition metal complexes. For nitrogen and phosphorus (III) two isomers A and B may exist due to rotation around metal-heteroatom bond (Scheme 7). In the A-TS the lone pair of the heteroatom interacts with the multiple carbon-carbon bond, whereas in B-TS direct interaction is unlikely. Such different interactions may become a reason for changing relative stability of the transition states. 

A variety of different geometry orientations are accessible for other heteroatom groups as well (Scheme 8). Different isomers of the initial metal complex may initiate alternative pathways of multiple bond insertion in the same manner as described earlier (Scheme 7). 

Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (equations 1 and 2). Resonance form (2), considered as the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this carbon-heteroatom bond. This effect is responsible for the restricted rotation often observed for this bond in nuclear magnetic resonance (NMR) studies. For example cis and trans isomers (6) and (7) of methoxymethyl carbenes rapidly interconvert at room temperature, but can be frozen out in the proton NMR at -40 °C. By contrast, the M-C bond is close to single and often rotates freely. 

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