Metal-carbon double bond

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 reaction of JV,iV-dimethylhydrazones (1-amino-1-azadienes) and alkenylcarbene complexes mainly produces [3C+2S] cyclopentene derivatives (see Sect. 2.6.4.5). However, a minor product in this reaction is a pyrrole derivative which can be considered as derived from a [4S+1C] cycloaddition process [75]. In this case, the reaction is initiated by the nucleophilic 1,2-addition of the nitrogen lone pair to the metal-carbon double bond followed by cyclisation and... 

The conclusion that there should not be a sharp distinction between the electrophilic and nucleophilic character of the metal-carbon double bond, but rather a spectrum of reactivity, also follows from the bonding model. There is some experimental evidence for this ... 

The osmium-carbyne carbon bond lengths for the three complexes do not differ significantly, and reference to Table IV indicates that these distances are distinctly shorter than the characterized metal-carbon double bonds of osmium carbene and carbonyl complexes. In both osmium alkylidene and carbyne complexes, then, the metal-carbon multiple bond lengths are largely insensitive to changes in the metal electron density (cf. Section IV,B). 

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

The 772-phosphinocarbene complexes of tungsten show ambiphilic behavior. With Lewis acids such as MeS+, electrophilic attack occurs at the metal-carbon double bond affording the dicationic tunstaphosphathiabi-cyclo[1.1.0]butane complexes 102.96,97 On the other hand, nucleophiles such as trialkyl phosphines or the cyclopentadienyl anion C5H5 add at the carbe-nic center, affording phosphoranylidene complexes 10397 or tungstaphos-... 

Many of the syntheses we have seen within this review depend on the carbonylation of a vinylcarbene complex for the generation of the vinylketene species. The ease of this carbonylation process is controlled, to some degree, by the identity of the metal. The electronic characteristics of the metal will clearly have a great effect on the strength of the metal-carbon double bond, and as such this could be a regulating factor in the carbene-ketene transformation. It is interesting to note the comparative reactivity of a (vinylcarbene)chromium species with its iron analogue The former is a fairly stable species, whereas the latter has been shown to carbonylate readily to form the appropriate (vinylketene)iron complex. 

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

The content of this book gathers in the same volume all aspects of vinylidene- and allenylidene-metal complexes, including the preparation of these organometallic carbon-rich systems with a metal-carbon double bond, their stoichiometric reactivity and theoretical aspects, and their applications in catalysis for the production of fine chemicals, mainly in the field of selective transformations of functional terminal alkynes. It provides essential general information on catalytic transformations of alkynes and their use in synthesis. 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

The pair of electrons in the sp1 orbital may be donated to a metal lo form a u bond, and an empty p. orbital is present to accept it electron density. Filled d orbitals of (he metal may donate electrons to the p. orbital, to give a metal-carbon double bond, and electrons from filled p orbitals of the oxygen atom may also be donated to form a carbon-oxygen double bond (Fig. 15.20). Resonance form 15.20b appears to be dominant and, although the M—C bond is shorter than expected fora single bond, it is too long for an M—C double bond, leading to the conclusion that the bond order is between one and two. 

A decade after the announcement of the metal-carbon double bond, Fischer s group reported the first complex containing a metal-carbon triple bond.711... 

Fig. 15.20 Resonance forms for a transition metal carbene complex. Form (a) shows metal-carbon double bond character which results from donation of metal d electron density to an empty p orbital of carbon. Form (b) shows oxygen-carbon double bond character which results from donation of oxygen p electron density to an empty p orbital of carbon Form (W provides the dominant contribution.

Complexes involving a formal metal-carbon double bond are referred to as either carbene or alkylidene complexes. For simplicity we shall consider the orbitals of singlet methylene, which consist of a filled sp2 orbital ( lone pair ) and an empty and highly electrophilic p orbital (Figure 1.15). 

The oxygen atom of acyl metallates may present nucleophilic character (Figures 3.25, 4.16). In 1964, E. O. Fischer showed that protonation of the benzoyl tungstate complex obtained from W(CO)6 and phenyllithium (Figure 3.20) provided the first characterized example of a metal-carbon double bond, viz. the hydroxycarbene complex (CO)5W=C(OH)Ph. This labile species decomposed to provide ben-zaldehyde however, it could be esterified with diazomethane to provide... 

Metal-Carbon Double Bond Cycloaddition and Further Reactions... 

The reaction mechanism shown in Eq. (6) is now generally accepted. That is, the active species is thought to be a complex having a metal—carbon double bond (C=M), which is called a metal carbene. Recently it has been disclosed that metal carbenes mediate various reactions70). When a cycloolefin is employed instead of an olefin, then a polymer is obtained (Eq. (7)), and a similar reaction mechanism is valid (Eq. (8)). 

Catalysts comprising a metal-carbon double bond (metallocarbenes, or metallocenes) are efficient. With these initiators, the polymerization mechanism appears to involve coordination of the C=C double bond in the cycio- or dicycloalkene at a vacant d orbital on the metal. The metallocyclobutane intermediate which is formed decomposes to produce a new metal carbene and a new C=C bond. Propagation consists of repealed insertions of cycloalkenes at the metal carbene. 

The mechanism for olefin metathesis is complex, and involves metal-carbene intermediates— intermediates that contain a metal-carbon double bond. The mechanism is drawn for the reaction of a terminal alkene (RCH=CH2) with Grubbs catalyst, abbreviated as Ru=CHPh, to form RCH = CHR and CH2 = CH2. To begin metathesis, Grubbs catalyst reacts with the alkene substrate to form two new metal-carbenes A and B by a two-step process addition of Ru=CHPh to the alkene to yield two different metallocyclobutanes (Step [1]), followed by elimination to form A and B (Steps [2a] and [2b]). The alkene by-products formed in this process (RCH=CHPh and PhCH=CH2) are present in only a small amount since Grubbs reagent is used catalytically. 

IUPAC has recommended that the term alkylidene be used to describe all eomplexes containing metal-carbon double bonds and that carbene be restricted to free =CR2. For a detailed description of the distinction between these two terms (and between carbyne and alkylidyne, discussed later in this chapter), see W. A, Nugent and J. M. Mayer, Metal-Ligand Multiple Bonds, Wiley-Interseience, New York, 1988, pp. 11-16. O. Fiseher and A. Maasbol, Angew. Chem., Int. Ed., 1964, 3, 580. 

Transition metal carbene complexes are described by the general formula L M=CR,R2, where the carbene ligand (=CRiR2) is bonded to the metal by a metal-carbon double bond. The first transition metal carbene complex was reported by Fischer and Maasbol in 1964 [2]. Subsequently, many other carbene complexes have been synthesized by the classic route of Fischer or by new synthetic methods. 

The name methylene for CH2 can only be used in connection with a bridging bonding mode (p-methylene), whereas a CH2 ligand bonding to one metal only has a metal-carbon double bond and should be named as methylidene (see Section IR-10.2.4). 

Carbene complexes contain metal-carbon double bonds 7 they have the general structure shown below (X, Y = alkyl, aryl, H, or highly electronegative atoms such as O, N, S, or halogens). First synthesized in 1964 by Fischer and Maasbol,8 carbene complexes are now known for the majority of transition metals and for a wide range of ligands, including the prototype carbene CH2. 

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