Metal-stabilized carbene

As previously shown , transition-metal stabilized carbenes cannot be used as sources of free carbenes, due to their chemical inertness. However, selected reagents can operate specific transformations on these complexes (Scheme 11) ... 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

By applying the same tandem Michael addition carbocyclizalion to arylidene /3-keto-sulfones in the presence of 1 equiv of potassium t-butoxide, once again a metal-stabilized carbene was formed. Then internal trapping of this carbene by the carbonyl oxygen led to a variety of furo[3,4-c]furans and furo[3,4-c]pyrroles (Table 8). 

Although many transition metals form carbene complexes, only Group 6 (Cr, Mo, W) heteroatom-stabilized Fischer carbenes of the type... 

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 metal-carbene complexes are electrophilic in character. They can, in fact, be represented as metal-stabilized carbocations. 

The Lewis-like picture suggests20 that saturated metal alkylidenes (carbenes) H M=CH2 of special stability should correspond to duodectet-rule-conforming... 

Carbene complexes can be prepared by reaction of stabilized carbenes or carbenoids (e.g. a-haloorganolithium compounds) with transition metal complexes [610]. This method is particularly useful for the preparation of donor-substituted... 

The majority of Al-heterocyclic carbenes and their metal complexes are obtained from cyclic azolium derivatives (Fig. 8). Nevertheless, the first complex with a heteroatom stabilized carbene ligand, although it was not recognized as such, was prepared by Tschugajeff et al. as early as 1925 [157]. It was later identified as a heterocarbene complex [158], Tschugajeff reacted the nucleophihc proton base... 

Stable transition-metal complexes of this type are known and others have been recognized as likely intermediates in a number of reactions. Rightly or wrongly, they are called carbene-metal complexes, although they also can be regarded either as metal-stabilized carbocations or as metal-stabilized ylides (Section 16-4A). 

Two resonance-contributing structures (3a and 3b), in the formalism of ylide structures, can be used to describe metal carbene intermediates. The highly electrophilic character of those derived from Cu and Rh catalysts suggests that the contribution from the metal-stabilized carbocation 3b is important in the overall evaluation of the reactivities and selectivities of these metal carbene intermediates. Emphasis on the metal carbene structure 3a has led to the subsequently discounted proposal that cyclopropane formation from reactions with alkenes occurs through the intervention of a metallocyclobutane intermediate [18]. The metal-stabilized carbocation structure 3b is consistent with the cyclopropanation mechanism in which LnM dissociates from the carbene as bond-formation occurs between the carbene and the reacting alkene (Eq. 5.4) [7,15]. 

Diazocarbonyl compounds, especially diazo ketones and diazo esters [19], are the most suitable substrates for metal carbene transformations catalyzed by Cu or Rh compounds. Diazoalkanes are less useful owing to more pronounced carbene dimer formation that competes with, for example, cyclopropanation [7]. This competing reaction occurs by electrophilic addition of the metal-stabilized carbocation to the diazo compound followed by dinitrogen loss and formation of the alkene product that occurs with regeneration of the catalytically active metal complex (Eq. 5.5) [201. 

Carbenes, generated by several methods, are reactive intermediates and used for further reactions without isolation. Carbenes can also be stabilized by coordination to some transition metals and can be isolated as carbene complexes which have formal metal-to-carbon double bonds. They are classified, based on the reactivity of the carbene, as electrophilic heteroatom-stabilized carbenes (Fischer type), and nucleophilic methylene or alkylidene carbenes (Schrock type). 

The stability of metal alkylidene (carbene) complexes and the corresponding metallacycles can be dependent on various factors, but it is worth noting that the kind of metal, the metal oxidation state and the ligands surrounding the metal are considered to be of essential significance. Although stable metal carbene complexes are usually obtained from W and Mo compounds whereas metallacycles are obtained from Ti compounds, systems have been found in which both the metal alkylidene complex and its precursor metallacyclobutane can be detected at lowered temperature by NMR spectroscopy [45]. 

Heteroatom-stabilized carbene complexes of type 1, first discovered by E.O. Fischer in 1964 [1], nowadays belong to the best investigated classes of transition metal compounds. Such complexes are coordinatively saturated, intensely colored solids = 350-400 nm), which exhibit a sufficient stability for normal preparative use. Especially chromium carbene complexes (2) enjoy increasing importance in organic synthesis, and it must be added that thermal reactions such as benzannulations (i.e. the Ddtz reaction), cyclopropanations and additions to a,j8-unsatu-rated complexes clearly predominate [2J. 

Carbenes are formally divalent carbon species and as such are very reactive. Certain transition metals stabilize the carbene. The metal arbene complexes which result, some of which are stable, are called "carbenoids . since an actual carbene is no longer present. 

The main advantage of these coordinating anions is that they stabilize the active species. This is particularly obvious in the case of palladium complexes, whose tendency to decompose into black metal is well documented. Imidazolium-based ionic liquids can generate in situ formation of metal-imidazolylidene carbene complexes by a deprotonation of the imidazolium cation. The ease of deprotonation depends on the nucleophilicity of the anions. In this case, NAILs may act as both solvents and catalyst ligand precursors [13],... 

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