Metal-alkylidene complex

The presence of a heteroatom at the alkylidene carbon atom moves properties of metal-alkylidene complexes to the borderline of Fischer carbene chemistry.69 The... 

Although transition metal alkylidene complexes, i.e., carbene complexes containing only hydrogen or carbon-based substituents, were first recognized over 15 years ago, it is only relatively recently that Ru, Os, and Ir alkylidene complexes have been characterized. Neutral and cationic complexes of these Group 8 metals are known for both metal electron configurations d8 and d6. The synthesis, structural properties, and reactivity of these compounds are discussed in this section. 

Olefin metathesis is a catalytic process whose key step is a reaction between an olefin and a transition metal alkylidene complex, usually M=CHR (Eq. 1) or M= CH2, in a 2+2 fashion to give an unstable intermediate metalacyclobutane ring [1]. All possible reactions of this general type are reversible, possibly nonproductive, and in competition with one another, so the overall result depends heavily on relative rates, and in the case of formation of volatile or insoluble products, displacement of equilibria as those products form. 

Metal alkyl cocatalysts, 20 153 Metal-alkylidene catalysts, 26 948 Metal-alkylidene complexes, 26 927 discovery of, 26 925 Metal amalgams, amines by reduction, 2 492 93... 

Metal alkylidene complexes find application in the metathesis of alkenes, the cvclopropanation of alkenes (Grubbs, Schrock), Wittig type reactions, and the McMurry reaction. In suitable complexes a-elimination can occur twice yielding alkylidync complexes. See Figure 2.21 for an example with tungsten. 

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

The cycloaddition of alkenes with metal alkylidene complexes remains the most common entry into the metallacyclobutane structural class. Consistent with metallacyclobutane intermediacy in the olefin metathesis reaction, the [2+2] cycloaddition is generally reversible a propensity for cycloreversion (Section 2.12.6.2.4), however, can significantly limit the utility of metallacyclobutane complexes as intermediates in other synthetic transformations. 

Nucleophilic metal alkylidene complexes are more useful for promoting the metathesis polymerisation of cycloolefins than electrophilic metal carbenes. For instance, Br2(Me3CCH20)2W=CHCMe3 is a moderately active catalyst [75,89] that can be further activated by the addition of Lewis acids such as GaBr3 to... 

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]. 

Figure 2.5 Some important reactions and interactions of metal-alkyl and metal-alkylidene complexes.

The mechanism of all metathesis reactions consists of three basic steps. The first step is the formation of metal-alkylidene complexes. The second step is the formation of metallocyclobutanes. The third step is the opening of the metallocyclobutane rings, which leads to product formation. The catalytic cycle... 

The evidence for the proposed mechanism and reactions 7.11 to 7.13 come from a variety of observations. First of all cleavage of the alkenes only at the double bonds, that is, generation of species such as 7.38 and 7.40, is indicated by isotope-labeling studies. A mixture of but-2-ene and perdeuterated but-2-ene on exposure to metathesis catalysts shows that the product but-2-ene is duterated only at the 1,2 positions. Second, fully characterized metal -alkylidene complexes such as 7.43 and 7.44 have been shown to be active metathesis catalysts. 

Athough transition metal alkylidene complexes are successfully used for the alkenation of carbonyl compounds, various 1,1-bimetalloalkanes, often prepared by the hydrometal-lation of alkenyl organometallics, are also useful reagents for the alkenation of carbonyl compounds. 

The mode of action of alkene metathesis is still under investigation, but Chauvin in 1971 was the first to postulate the now accepted catalytic cycle. In the catalytic cycle, first a metal alkylidene complex 6 reacts formally in a [2+2]-cycloaddition with one alkene 7 to form a metalla-cyclobutane 8, which undergoes a formal [2+2]-cycloreversion in the next step to form a new metal alkylidene complex 9 and one product alkene 10. The newly formed metal alkylindene complex 9 then reacts with the second substrate alkene 11 to the metalla-cyclobutane 12 which gives upon formal [2+2]-cycloreversion the second product alkene 13 and again the metal alkylidene 6 to start a new catalytic cycle. 

The first metal alkylidene complexes for alkene metathesis with a defined molecular structure were the molybdenum-based complex 14 prepared by Schrock in 1990 and the ruthenium-based complex 15 synthesized by Grubbs in 1995. 

Although early transition metal-alkylidene complexes have rather well-defined chemistry involving the nucleophilicity of Ccarbene, mid- to late transition metal alkylidenes and Fischer carbene complexes sometimes have a different chemistry associated with them. In this section, we will look at some of this chemistry, some of which has significant impact on modern organic synthesis methodology. We will compare the chemistry of these complexes with that of traditional Fischer carbene complexes. 

While this compound would have to be classified as a carbene rather than an alkylidene complex, the synthesis involves an intramolecular H atom transfer, which is a recurring phenomenon in metal-alkylidene complex synthesis. 

Metal alkylidene complexes see Schrock type Carbene Complexes have been proposed as intermediates in many catalytic reactions, including alkene metathesis see Organic Synthesis Using Metal-mediated Metathesis Reaction, ... 

There was a long controversy regarding the mechanism of the olebn metathesis, but now the process has been established to proceed through involvement of a transibon metal alkylidene species that forms a metallacyclobutane reversibly on interacbon with an olebn [124]. Elementary processes in typical catalybc processes involving metal alkylidene complexes are illusbated in Scheme 1.41. 

Interacbon of a metal alkylidene complex with an olebn forms a metallacyclobutane. Cleavage of the metallacyclobutane complex at bonds different from the original ones leads to bberabon of a new olebn and formation of a new metal carbene complex (Scheme 1.41a). This type of bansformabon is operabve in a catalybc cycle of olebn metathesis as shown in Scheme 1.42 [125]. 

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