Carbene complexes bonding

Does a Silylene-Complex exist This rhetorical question is the title of a theoretical paper published in 1983 [84], As a result of an ab-initio calculation, the authors came to the conclusion that a moderately positive answer can be given. However, silylene complexes are thermodynamically less stable than carbene complexes (the MSi bond energy for the hypothetical complex (OC)5Cr = Si(OH)H is 29.6 kcal/mol, the bond energy of the MC bond in (OC)5Cr = C(OH)H is 44.4 kcal/mol) [85], and therefore silylene complexes should be difficult to isolate. 

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... 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

The ability of Fischer carbene complexes to transfer their carbene ligand to an electron-deficient olefin was discovered by Fischer and Dotz in 1970 [5]. Further studies have demonstrated the generality of this thermal process, which occurs between (alkyl)-, (aryl)-, and (alkenyl)(alkoxy)carbene complexes and different electron-withdrawing substituted alkenes [6] (Scheme 1). For certain substrates, a common side reaction in these processes is the insertion of the carbene ligand into an olefinic C-H bond [6, 7]. In addition, it has been ob-... 

Electron-deficient 1,3-dienes are known to react when heated with metho-xy(aryl)- or methoxy(alkyl)carbene complexes to afford vinylcyclopropane derivatives with high regioselectivity and diastereoselectivity [8a, 24]. Cyclo-propanation of the double bond not bearing the acceptor functional group and... 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

In a similar process, tertiary enaminones react with alkynylcarbene complexes to give the corresponding pyranylidene complexes following a reaction pathway analogous to that described above. First, a [2+2] cycloaddition reaction between the alkynyl moiety of the carbene complex and the C=C double bond of the enamine generates a cyclobutene intermediate, which evolves by a conrotatory cyclobutene ring opening followed by a cyclisation process [94] (Scheme 49). 

Carbene complexes containing either the dienophile or the diene functionality bonded directly to the carbene carbon undergo intramolecular [4+2] cycloadditions under mild conditions [108] (Scheme 60). 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

J-Oxygen-functionalised sp3 organolithium compounds react with alkenyl-carbene complexes to generate the corresponding cyclic carbene complexes in a formal [3+3] process (see Sect. 2.8.1). In those cases where the organolithium derivative contains a double bond in an appropriate position, tricyclic ether derivatives are the only products isolated. These compounds derive from an intramolecular cyclopropanation of the corresponding cyclic carbene complex intermediate [89] (Scheme 83). 

Alkynylcarbene complexes react with strained and hindered olefins yielding products that incorporate up to four different components by the formation of five new carbon-carbon bonds [15b]. This remarkable transformation is explained by an initial [2+2] cycloaddition followed by CO insertion. The resulting intermediate suffers a well precedented [1,3]-migration of the metal fragment to generate a non-heteroatom-stabilised carbene complex intermediate which reacts with a new molecule of the olefin through a cyclopropana-tion reaction (Scheme 85). 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

The selectivity for two-alkyne annulation can be increased by involving an intramolecular tethering of the carbene complex to both alkynes. This was accomplished by the synthesis of aryl-diynecarbene complexes 115 and 116 from the triynylcarbene complexes 113 and 114, respectively, and Danishefsky s diene in a Diels-Alder reaction [70a]. The diene adds chemoselectively to the triple bond next to the electrophilic carbene carbon. The thermally induced two-alkyne annulation of the complexes 115 and 116 was performed in benzene and yielded the steroid ring systems 117 and 118 (Scheme 51). This tandem Diels-Alder/two-alkyne annulation, which could also be applied in a one-pot procedure, offers new strategies for steroid synthesis in the class O—>ABCD. 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

The thermal benzannulation of Group 6 carbene complexes with alkynes (the Dotz reaction) is highly developed and has been used extensively in synthesis [90,91]. It is thought to proceed through a chromium vinylketene intermediate generated by sequential insertion of the alkyne followed by carbon monoxide into the chromium-carbene-carbon double bond [92]. The realization that photodriven CO insertion into Z-dienylcarbene complexes should generate the same vinylketene intermediate led to the development of a photochemical variant of the Dotz reaction (Table 14). 

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... 

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