Transition metal alkylidenes

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. 

Kinetic studies using 1,9-decadiene and 1,5-hexadiene in comparison widi catalyst 14 and catalyst 12 demonstrate an order-of-magnitude difference in their rates of polymerization, widi 14 being the faster of the two.12 Furdier, this study shows diat different products are produced when die two catalysts are reacted widi 1,5-hexadiene. Catalyst 14 generates principally lineal" polymer with the small amount of cyclics normally observed in step condensation chemistry, while 12 produces only small amounts of linear oligomers widi die major product being cyclics such as 1,5-cyclooctadiene.12 Catalyst 12, a late transition metal benzylidene (carbene), has vastly different steric and electronic factors compared to catalyst 14, an early transition metal alkylidene. Since die results were observed after extended reaction time periods and no catalyst quenching or kinetic product isolation was performed, this anomaly is attributed to mechanistic differences between diese two catalysts under identical reaction conditions. 

The reaction between an early transition metal-alkylidene and a ketone to yield a metal-oxo group and an olefin was first noted by Schrock Schrock, R. R. J. Am. Chem. Soc. 1976, 98, 5399. 

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. 

Figure 4.14 Optimized geometries of saturated group 6-10 third-series transition-metal alkylidenes H M=CH2 (M = W-Pt).

Table 4.10. Optimized bond lengths Rue and Rmh and angles hmc and hmh of transition-metal alkylidenes H ,M=CH2 in Fig. 4.14. (The subscripts i and "o distinguish H—M—H angles that are respectively in plane and out of plane with respect to the methylene group.)...

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. 

M]=CH2 = Transition metal alkylidene Q = Polymer Scheme 10. Solid-phase synthesis using RCM... 

The metathesis reaction is a powerful strategy in synthetic organic chemistry [1], and it is generally accepted that this reaction is catalyzed by highly efficient transition metal alkylidenes [2, 3]. Intermolecular diene metathesis produces... 

The polymerization of cyclic, strained olefins by transition metal alkylidenes of general formula L M = CRR (L = ligand, R, R = H, alkyl, aryl) yields polymers formed via ring-opening that contain unsaturated double bonds within each repetitive unit. Since the mechanism is based on repetitive metathesis steps, this polymerization reaction is known as ring-opening metathesis polymerization (ROMP) (Scheme 1). 

Figure 6.4 Schematic presentation of metal-carbon bonding in (a) a transition metal carbene electrophilic complex and (b) a transition metal alkylidene nucleophilic complex...

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. 

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. 

The cycloaddition of free carbenes to alkenes to give cyclopropanes is well known (see equation 10.2). Transition metal-carbene complexes, acting either stoichiometrically or catalytically, promote cyclopropanation, sometimes in a synthetically-useful manner. Several mechanistic pathways have been observed, some of which involve mid-transition metal alkylidenes. Two of the most common of these are outlined in Scheme 10.6.67... 

Though this book focuses on late transition metals, tlie few well-defined Group IVA- or VA-based systems that exist shall be outlined briefly. Among the first transition metal alkylidenes that were observed in ROMP was the bimetallic compound (Cp2TiCH2ClAlMe2) [29, 30]. Upon reaction with norborn-2-ene (bicyclo[2.2.1] hept-2-ene, NBE) at ambient temperature it forms the rnetallacycle shown in Fig. 5.1 which opens at 65 °C under ring-opening [31]. 

M. R. Buchmeiser, Homogenous metathesis polymerization by well-defined group VI and group VIII transition metal alkylidenes fundamentals and applications in the preparation of advanced materials, Chem. Rev., 100 (2000) 1565-1604. 

Olefin metathesis is a catalytic process in which alkenes are converted into new products via the rupture and reformation of carbon-carbon double bonds. The key step in this process is the 2 + 2 reaction between an olefin and a transition metal alkylidene (carbene) complex, generating an unstable metallacyclobutane intermediate. This intermediate can either revert to the starting material, or open productively to regenerate a metal carbene and produce a new olefin (Eq. 1). 

Homogeneous Metathesis Polymerization by Well-Defined Group VI and Group VIII Transition-Metal Alkylidenes Fundamentals and Applications in... 

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