Transition metal-carbon double bonds

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... 

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.

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. 

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. 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

Understanding the role of orbital interactions can be beneficial from the practical perspective. For example, the symmetry of frontier molecular orbitals can explain why thermal [2+2] cycloaddition fails, whereas the analogous reaction of transition metal alkylidenes, compounds that can be described as having a metal-carbon double bond, proceeds efficiently under mild conditions (Figure 1.2). In this case, an extra orbital... 

The pi bond from the carbon-carbon double bond in the ring of the monomer reacts with the metal-carbon double bond in the transition metal catalyst (M=CHR) to first form a metaUacyclobutane complex. The metallacyclobutane complex then breaks up id form a new carbon-carbon double bond and a new metal-carbon double bond. The new carbon-carbon double bond is the first in the polymer chain, and the new metal-carbon double bond is available to react with additional monomers and thus grow the chain. As long as the metal from the catalyst is attached to the polymer via this metal-carbon double bond, the polymer is said to be living because it will continue to grow as more monomra- is added. The process we will consider is the polymerization of two cyclic alkenes (cyclobutene and 3-chlorocyclobutene) using ROMP with an appropriate transition metal catalyst (abbreviated as M=CHR) ... 

R.R. Schrock - Catalysis by Transition Metals Metal-Carbon Double Bonds and Triple Bonds, Science 219, 13,1983. 

The above chemistry suggests that a group property of transition metals is the ability to form metal-carbon double bonds and stabilize adjacent sp - hybridized carbon ligands. 

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Oxidative Cleavage of Carbon-Carbon Double Bonds 12.4.1. Transition Metal Oxidants... 

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