Carbene complexes general

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... 

The porphyrin ligands in the diamagnetic ruthenium and osmium carbene complexes generally exhibit four-fold symmetry by NMR, indicating that the barrier to rotation about the M=C bond is low. The carbenoid protons appear shifted down-field in the H NMR spectra, for example appearing for Ru(TTP)=CHC02Et and Ru(TTP)=CHSiMc3 at 13.43 and 19.44 ppm, respectively, and for the osmium... 

Caution All operations should be conducted in a well-ventilated hood with breathing protection. The chromium carbene complex generally is contaminated with the very volatile and toxic chromium hexacarbonyl, which is also generated as a by-product of the reaction. 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

The intermolecular reaction of imines with acceptor-substituted carbene complexes generally leads to the formation of azomethine ylides. These can undergo several types of transformation, such as ring closure to aziridines [1242-1245], 1,3-dipolar cycloadditions [1133,1243,1246-1248], or different types of rearrangement (Figure 4.9). 

Addition Reactions to Carbene Complexes General Procedure65 ... 

There are several methods to release the carbene ligand from the metal, but in no case the intermediacy of free carbenes has been established. The thermal decomposition of carbene complexes generally results in a dimerization of the carbene ligand to give aUcenes. The replacement of the carbene ligand by carbon monoxide, tertiary phosphines or amines is a less drastic way to cleave the metal-carbene bond and, in principle, allows for a recovery of the low-valent metal species. 

The carbene complexes [Os(P Bu2Me)2 =G(OR)Me HGl] form rapidly at low temperatures upon addition of vinyl ethers to [Os(P Bu2Me)2H3Gl], via the intermediacy of 77 -alkene complexes. While the carbene complexes generally decompose upon warming to form a mixture of products, changing the phosphine to P Pt3 allows the clean formation of the carbyne complex [Os(P Pr3)2(=GMe)HGl] (R = Ph) from H2G=GH(OPh), but the vinylidene [Os(P Pr3)2(=G=GH2)HGl] with H2G=GH(OEt). This difference in reactivity arises from the better stability of PhO compared to EtO as a free nucleophile, and the Bronsted basicity of the ethoxide anion that allows it to deprotonate a Os=GMe to afford the vinylidene product. ... 

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. 

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-... 

The Diels-Alder reaction of simple alkoxy alkenylcarbene complexes leads to mixtures of endo and exo cycloadducts, with the endo isomer generally being the major one [96,97]. Asymmetric examples of endo Diels-Alder reactions have also been reported by the use of chiral auxiliaries both on the carbene complex and the diene. Thus, the reaction of cyclopentadiene with chiral alkenylcarbene complexes derived from (-)-menthol proceeds to afford a 4 1... 

In attempts to understand the photochemical reactions of Fischer carbene complexes, several matrix isolation and flash photolysis studies have been conducted using both Cr and W (but not Mo) complexes [5-11]. Although the complexes studied and conditions used varied, several general conclusions were drawn ... 

The generally accepted mechanism is a chain mechanism, involving the intervention of a metal-carbene complex (126) and a four-membered ring containing... 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

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. 

Charge. The small amount of charge distribution data for carbyne complexes (based on Mulliken population analyses) indicates that the metal-carbon bond is generally polarized Ms+—C5- and that the carbyne carbon is always more negative than adjacent carbonyl carbons (28,30). These conclusions are directly analogous to those derived for carbene complexes. 

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

Imidazolines are also formed in silver cyanide-catalyzed cyclization of alkyl isocyanides with aliphatic diamines (Scheme 103).169 This simple synthesis can be applied in a general way with difunctional nucleophiles and has been used to prepare benzimidazoles, oxazoles, thiazoles, and oxazines.169 It is suggested that transient carbene complexes are formed in these reactions (cf. 87 in Scheme 103) but further work is required to ascertain the mechanism and scope of these processes. 

Reports on the coordination chemistry of A-heterocyclic carbene-containing metal complexes started to appear as long ago as 1968,50,51 while metal-free carbenes have only been isolated very recently.52 In view of the fact that the general chemistry and applications of organic carbenes and related metal complexes in chemical synthesis have been reviewed several times recently,53-57 examples limited only to those carbene complexes with silver(i) have been discussed. Nevertheless, it is worth mentioning that the developments in silver(i) A-heterocyclic carbenes have also been reviewed recently by Lin.5... 

In contrast to the Simmons-Smith reagent and similar carbenoids, which are reactive and therefore difficult to characterize, adducts of the fV-heterocyclic l,3-diorganylimidazol-2-ylidenes are remarkably stable. The first iV-heterocyclic carbene complex of zinc, namely the l,3-di(l-adamantyl)imidazol-2-ylidene diethylzinc complex 48 (Figure 22), was reported by Arduengo et al. in 1993.98 Because of the general utility of these iV-heterocyclic... 

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