Reactivity of Carbene Complexes

Reactivity of Carbene Complexes 13.2.5.1. Reactivity of Fischer Carbene Complexes 

Much of the reaction chemistry of Fischer carbene complexes is closely related to the reaction chemistry of organic esters. Like esters, these carbene complexes are electrophilic 

however, carbene complexes covering a broad range of different reactivities have been prepared. Often it is no longer possible to predict whether a carbene complex will behave as an electrophile or as a nucleophile. Thus, a reactivity-based nomenclature would be difficult to apply consistently. For this reason in this book compounds with a carbon-metal double bond will be called carbene complexes or alkylidene complexes , terms not associated with any specific chemical behavior. 

The reactivity of carbenes is strongly influenced by the electronic properties of their substituents. If an atom with a lone pair (e.g. O, N, or S) is directly bound to the carbene carbon atom, the electronic deficit at the carbene will be compensated to some extent by electron delocalization, resulting in stabilization of the reactive species. If both substituents are capable of donating electrons into the empty p orbital of the carbene, isolable carbenes, as e.g. diaminocarbenes (Section 2.1.6), can result. The second way in which carbenes can be stabilized consists in complexation. The shape of the molecular orbitals of carbenes enable them to act towards transition metals as a-donors and 71-acceptors. The chemical properties of the resulting complexes will also depend on the electronic properties of the metallic fragment to which the carbene is bound. Particularly relevant for the reactivity of carbene complexes are the ability of the metal to accept a-electrons from the carbene, and its capacity for back-donation into the empty p orbital of the carbene. 

Four different types of metallic fragment can now be considered  

In situation (a) a strong carbon-metal bond results. To this group belong the typical Schrock-type carbenes [e.g. Np3Ta=CH(7Bu)], many of which are nucleophilic at carbon. Situation (b) should also lead to nucleophilic carbene complexes, albeit with a weaker carbon-metal bond. Typical reactions of nucleophilic carbene complexes include carbonyl olefination (Section 3.2.4) and olefin metathesis (Section 3.2.5). 

Metallic groups as in case (c) lead to electrophilic or even carbocation-like carbene complexes. Typical examples are Fischer-type carbene complexes [e.g. (CO)5Cr=C(Ph)OMe] and the highly reactive carbene complexes resulting from the reaction of rhodium(II) and palladium(II) carboxylates with diazoalkanes. Also platinum ylides [1,2], resulting from the reaction of diazoalkanes with platinum(Il) complexes, have a strong Pt-C o bond but only a weak Pt-C 7t bond. In situation (d) the interaction between the metal and the carbene is very weak, and highly reactive complexes showing carbene-like behavior result. Similar to uncomplexed carbenes. 

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Fig. 1.1. Reactivity of carbene complexes towards electrophiles (E+) and nucleophiles (Nu ).

Fig. 1.3. Reactivity of carbene complexes as a function of the electronic interaction between metal and carbene.

This reactivity pattern is certainly unexpected. Why should low-valent complexes react as electrophiles and highly oxidized complexes be nucleophilic Numerous calculations on model compounds have provided possible explanations for the observed chemical behavior of both Fischer-type [3-8] and Schrock-type [9-17] carbene complexes. In simplified terms, a rationalization of the reactivity of carbene complexes could be as follows. The reactivity of non-heteroatom-stabilized carbene complexes is mainly frontier-orbital-controlled. The energies of the HOMO and LUMO of carbene complexes, which are critical for the reactivity of a given complex, are determined by the amount of orbital overlap and by the energy-difference between the empty carbene 2p orbital and a d orbital (of suitable symmetry) of the group L M. 

The high reactivity of carbene complex 37 was successfully employed by K. B. Wagner for polymerizations of acyclic dienes, Eq. (35) [50]. Vinyl addition reactions could be avoided under these Lewis add free reaction conditions. 

The reactivity of metal-silylene complexes is more limited than the reactivity of carbene complexes. The cationic base-stabilized ruthenium-silylene complex in Equation 13.37 does not react with olefins or alkynes to undergo [2-1-2] addition reactions. However, a related complex did undergo [2-1-2] addition reactions with isocyanates, as shown in Equation 13.46. Other reactions of silylene complexes are distinct from those of carbene complexes or those of other conventional organometallic compounds. For example, the reaction of the silylene hydride with an acetylene generates a p-silylvinylarene complex... 

Reactivity of Carbene Complexes with N-Heterocyclic Silylenes... 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

Active Hgands, which may take an important role in the reactivity of the complexes/nanoparticles (e.g., hydrides, alkyl groups, carbenes, etc.)... 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

The chemistry of transition metal-carbyne complexes is rather less developed than the chemistry of carbene complexes. This is almost certainly because reactions which form new carbyne complexes are relatively rare when compared with those forming metal carbenes. The few theoretical studies of carbyne complexes which are available indicate that close parallels exist between the bonding in carbene and carbyne compounds. These parallels also extend to chemical reactivity, and studies of Group 8 complexes again prove instructive. 

In scrutinizing the various proposed reaction sequences in Eq. (26), one may classify the behavior of carbene complexes toward olefins according to four intimately related considerations (a) relative reactivities of various types of olefins (b) the polar nature of the metal-carbene bond (c) the option of prior coordination of olefin to the transition metal, or direct interaction with the carbene carbon and (d) steric factors, including effects arising from ligands on the transition metal as well as substituents on the olefinic and carbene carbons. Information related to these various influences is by no means exhaustive at this point. Consequently, some apparent contradictions exist which seem to cast doubt on the relevance of various model compound studies to conventional catalysis of the metathesis reaction, a process which unfortunately involves species which elude direct structural determination. 

Steric effects were also found to be important for determining the reactivity of rhodium complexes containing N-heterocyclic carbene (NF1C) ligands [47] (Scheme 10), which have been the subject of intense in-... 

When the development of carbene-complex chemistry began in the mid seventies, two different patterns of reactivity emerged and led to a, maybe overemphasized, division of these compounds into (electrophilic) Fischer-type and (nucleophilic) Schrock-type carbene complexes (Figure 1.1). 

Because hydrogen, alkyl, or aryl groups can compensate only to a limited extent the electron deficit of the carbene carbon atom, it is mainly the metal and its ligands which provide stabilization in this type of carbene complex. For this reason the reactivity of these compounds depends mainly on the nature and oxidation state of the metal and on the electronic properties of the remaining ligands. 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

Non-heteroatom-substituted carbene complexes cover a broad spectrum of different reactivities, largely dependent on the electronic properties of the metal. In Chapter 1 the division of carbene complexes into Fischer-type and Schrock-type carbenes was discussed. This way of grouping carbene complexes, although difficult to apply... 

The oxygen as heteroatom in ethers or carbonyl compounds is weak to moderate Lewis base. Nevertheless, a highly reactive metal carbene complex can interact with the oxygen to generate oxygen ylide. The interaction between ether and metal carbene functional groups is believed to be rather weak as demonstrated by the facts that other metal carbene reactions, such as G-H insertion and cyclopropanation, can proceed in ethereal solvents." These experiments demonstrate that the formation of the metal ylide is much less favored in the equilibrium shown in Equation (1). ... 

Another attractive route to the synthesis of highly reactive tungsten carbene complexes involves alkylidene transfer from phosphoranes. Arylimido tungsten... 

The synthesis of metal complexes of type 213 can be performed by reacting metal-carbene complexes with selenium sources such as alkyneselenolates 203.430 Also the stability of unstable selenocarbonyl compounds such as selenoaldehydes can be enhanced by coordination to metal carbonyls and the reactivity of such complexes has been studied. Complex 216 can react with methylthiohexyne and the product is a different complex 217 with the selenium atom still coordinating to the metal carbonyl fragment (Scheme 66).431... 

They proceeded to synthesise the corresponding palladium(II) carbene complex and investigated the reactivity of this complex. It could be shown that the dimethyl and dibromo complexes could be synthesised and that from them cationic complexes featuring diverse ji-donor Ugands are accessible (see Figure 3.90). 

One of the synthetic procedures of metal complexes of type 53 is the reaction of metal-carbene complexes with selenium sources such as alkyneselenolates [109]. The stability of selenobenzaldehyde is enhanced by coordinating to metal carbonyls, and the reactivity of the complexes has been studied [110]. For example, the selenobenzaldehyde complexes reacts with methylthiohexyne even at - 30 °C to afford another type of complex where the selenium atom of the selenocarbonyl group is still coordinated to the metal (Eq. 29) [llOd]. 

Both of the examples of intramolecular Diels-Alder reactions of carbene complexes involve the 1,3-diene tethered through the heteroatom ancilliary substituent of the carbene carbon. - The example shown in Scheme 11 is the only example of a Diels-Alder reaction of an amino carbene complex. Al-kenylamino and alkynylamino complexes are inert to reaction under intermolecular conditions with very reactive dienophiles, such as cyclopentadiene and Danishefsky s diene. - The aminolysis of the meth-oxy complex (48b) with the amine (75) represents the most common method for the preparation of amino carbene complexes. - It is typical that two isomeric amino carbene complexes are obtained by this procedure, and, as is the case for the complexes (76) and (77), it is also typical that these isomers about the carbene-nitrogen bond are not interconvertable, even at elevated temperatures. The ( )-isomer (76) was separated and was found to undergo an intramolecular Diels-Alder reaction at 80 °C to give the interesting tricyclic caibene complex (78). 

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