Carbenes metal complexes

Aumann and Fischer (7), as part of a larger project on carbene-metal complexes, have investigated the reaction of Cr(CO)5C(OCHj)CH3 and cyclohexyl isocyanide. They describe an initial 1 1 adduct of these reagents, to which they ascribe structure (XX) it is interesting to note that neither... 

In spite of the fact that silver(i) X-heterocyclic carbene complexes were widely employed as carbene-transfer reagents for the synthesis of other transition metal carbene complexes, their synthesis could also be achieved by the reaction of silver salts with relatively more labile carbene metal complexes, albeit rare. Complexes 71a-71c were reported to be synthesized from the reaction of the corresponding pentacarbonyl(carbene)chromium(i) complexes with silver(i) hexafluorophosphate in CDC13 under inert atmosphere (Scheme 17).117... 

The species responsible for alkyne polymerization, which is kinetically more facile than eyelotrimerization since only a small fraction of the added alkyne is converted to benzenes, is not yet known. Carbene-metal complexes, both mononuclear (54) and binuclear (y2-CR2) complexes (55,56), have been shown to act as alkyne polymerization initiators and several years ago it was shown that terminal alkynes and alcohols can react to give alkoxycarbene ligands (57) As yet, we have no evidence... 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

Fig. 9. Structural models of carbon- and nitrogen-anchored tris(carbene) metal complexes.

Downfield shifts due to electron deficiency are observed for carbene metal complexes [80 a] and for the sp carbons of metal carbonyls [80 b],... 

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). 

After a great deal of research on the mechanism of this reaction, it now appears likely that the crucial step is the formation of carbene metal complexes and that the products are formed by recombination of the carbenes with alkene in the various possible ways ... 

The reaction between a lithium amide and Cr(CO)s was first noted to give attack on a coordinated carbonyl and provide entry into amino-substituted carbene-metal complexes (equation 86).228,229... 

One of the earliest enantioselective carbon-carbon bond-forming processes catalyzed by chiral transition-metal complexes is asymmetric cyclopropanation discussed in Chapter 5, which can proceed via face-selective carbometallation of carbene-metal complexes. Some other more recently developed enantioselective carbon-carbon bond forming reactions, such as Pd-catalyzed enantioselective alkene-CO copolymerization (Chapter 7) and Pd-catalyzed enantioselective alkene cyclization (Chapter 8.7), are thought to involve face-selective carbometallation of acy 1-Pd and carbon-Pd bonds, respectively (Scheme 4.4). Similarly, the asymmetric Pauson-Khand reaction catalyzed by chiral Co complexes most likely involves face-selective cyclic carbometallation of chiral alkyne-Co complexes (Chapter 8,7). 

All these methods using carbenes, metal complexes of carbenes or carbenoids are stereospecific in that the geometry of the alkene is faithfully reproduced in the stereochemistry of the cyclopropane so trans-61 gives trans-68 specifically. They can also be stereoselective, particularly the Simmons-Smith on allylic alcohols thus the cyclopropane in 68 is on the same side of the alkene as the OH group in 67. We now come to a widely used method that is not stereospecific on the alkene. 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

N-Heterocyclic carbenes form intriguingly stable bonds with the majority of metals [12,21,29]. Whereas for saturated and unsaturated N-heterocyclic carbenes of comparable steric demand very similar bond dissociation energies have been observed, phosphines generally form much weaker bonds (Table 2) [21]. As a result, the equilibrium between the free carbene and the carbene metal complex lies far more on the side of the complex than... 

B. Cetinkaya, M. F. Lappert, G. M. McLaughlin, and K. Turner, Chloromethylene-ammonium Chlorides. Electron-Rich Carbenoids as Precursors to Secondary Carbene Metal Complexes Crystal and Molecular Structure of Trichloro(dimethylamino-methylene)bis(triethylphosphine)rhodium(III), J. Chem. Soc., Dalton Tram. 1974, 1591-1599. 

Grubbs [3] prepared high activity metathesis ruthenium metal carbene complexes, (IV), that were effective as depolymerization catalysts of unsaturated polymers and synthetic agents in preparing telechelic and alkene polymers. Other high activity metathesis ruthenium carbene metal complexes, (V), were prepared by Fogg [4]. 

Carbene metal complexes are the reactive species in these catalyzed processes and arc known as carbenoids . Copper carbenoids are the most widely used catalysts in the mediation of carbene reactions. During the last decade, however, the use of other metal salts has been particularly beneficial (e.g., Rh.Pd). 

Gradient-corrected (BP86) density functional calculations have been carried out on various P-heterocyclic carbene metal complexes, including where the carbene was derived from a 1,3-azaphosphole <2005JOM(690)6068> and quantum-mechanical calculations at the DFT level were used to analyze metal-ligand interactions in mixed iron(ll) metallocenes where, in one instance, one of the ligands was a 1,3-diphospholide anion <20040M5308>. 

Semmelhack, M. F., Tamura, R., Schnatter, W., Park, J., Steigerwald, M., Ho, S. Carbene-metal complexes. New processes and applications in organic synthesis. Stud. Org. Chem. (Amsterdam) 1986, 25, 21-42. 

Arylcyclopropanes are also obtained in good yields by decomposition of aryldiazomethanes with various metals and salts in the presence of an alkene. The reactions do not involve free carbenes, but carbene-metal complexes, and this is probably the reason why the cisjtrans ratios for the arylcyclopropanes vary when the catalyst is changed. Thus, when 4-tolylcarbene, generated by decomposition of 4-tolyldiazomethane with various salts, was allowed to react with (Z)-but-2-ene the cisjtrans ratio of the product, cw-2,3-dimethyl-l-(4-tolyl)cyclopropane (1), varied considerably. When the decomposition is facilitated by trifluoroacetic acid, however, 1 is obtained with a cisjtrans ratio of 1 1. ... 

Intermolecular reactions of carbenic metal complexes are also known - e.g. formation of 11. ... 

The subject of carbene-metal complexes reappears in Section IIIB,... 

As a consequence of the high reactivity of metal alkyls, there are few examples of olefin insertions in which the simple insertion product (with the metal still a bonded to the alkyl group) can be isolated. This lack of clear examples has led to alternative explanations involving formation of a carbene metal complex from the alkyl, which then forms a metallocyclobutane with the olefinA final hydrogen shift with ring opening yields the product ... 

Carbodiimides. Palladium(II) chloride, primary amines, and isonitriles form a carbene—metal complex (1), which on treatment with silver oxide is converted into a carbodiimide (2) in yields of 75-95%. Actually, the complex... 

The reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

A third mechanism involving a chain reaction having a carbene-metal complex as the active catalyst (Fig. 36) has recently been investigated using 1,7-octadiene and l,7-octadiene-l,l,8,8-c/4 as the olefin in three different catalyst systems (92). 

The first route to 2H-azaphosphirene complexes XIII, published by Streubel et al. in 1994 [22], used a triethylamine-induced condensation-rearrangement cascade starting from amino(aryl)carbene metal complexes VIII and [bis-(trimethylsilyl)methylene]halophosphanes IX (X=Cl,Br) (Schemes) the yields are generally good (50 - 85%) [19,23]. Experimental details and scope of this method as well as progress in this field were summarized quite recently [24]. 

If ILs are to be used in metal-catalyzed reactions, imidazoHum-based salts may be critical due to the possible formation and involvement of heterocyclic imidazo-lylidene carbenes [Eqs. (2)-(4)]. The direct formation of carbene-metal complexes from imidazolium ILs has already been demonstrated for palladium-catalyzed C-C reactions [40, 41]. Different pathways for the formation of metal carbenes from imidazolium salts are possible either by direct oxidative addition of imidazolium to the metal center in a low oxidative state [Eq. (2)] or by deprotonation of the imidazolium cation in presence of a base [Eq. (3)]. It is worth mentioning here that deprotonation can also occur on the 4-position of the imidazolium [Eq. (4)]. The in-situ formation of a metal carbene can have a beneficial effect on catalytic performances in stabilizing the metal-catalyst complex (it can avoid formation of palladium black, for example). However, given the remarkable stability of this imidazolylidene-metal bond with respect to dissociation, the formation of such a complex may also lead to deactivation of the catalyst This is probably what happens in the telomerization of butadiene with methanol catalyzed by palladium-phosphine complexes in [BMIMj-based ILs [42]. The substitution of the acidic hydrogen in the 2-position of the imidazolium by a methyl group or the use of pyridinium-based salts makes it possible to overcome this problem. Phosphonium-based ILs can also bring advantages in this case. 

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