Metallacyclic carbene complexes

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

Donor-substituted alkynes can insert into the C-M double bond of alkoxycarbene complexes, yielding donor-substituted vinylcarbene complexes [191,192]. In addition to this, photolysis or thermolysis of a-alkoxycyclopropyl carbonyl complexes or a-alkoxycyclobutanoyl complexes can lead to rearrangement to metallacyclic carbene complexes (Table 2.11). This methodology has not been used as extensively for the preparation of carbene complexes as the other methods described above. 

Other metallacyclic carbene complexes result when 2-lithio-l,3-dithiane is added to Cr(CO)6 in ether at 0°C and the reaction mixture, after removal of the solvent in vacuo, is treated with [Et30][BF4] in CH2CI2 ... 

A second series of similar metallacycle carbene complexes was obtained from treatment of an iron allyl alcohol complex with Bu Li at — 20°C, followed by methylation of the anion with McsO BF4, as shown in Equation (2). 

Finally, the possibility of building the M=C bond into an unsaturated metallacycle where there is the possibility for electron delocalization has been realized for the first time with the characterization of osmabenzene derivatives. For these reasons then, it seemed worthwhile to review the carbene and carbyne chemistry of these Group 8 elements, and for completeness we have included discussion of other heteroatom-substituted carbene complexes as well. We begin by general consideration of the bonding in molecules with multiple metal-carbon bonds. 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

Fig. 3.31. Generation of carbene complexes by [2 -i- 2] cycloreversion of four-membered metallacycles [26,27,605,607-609].

Transformations to the cyclotrimeric boiazines and cyclotetrameric tetraza-2,4,6,8,l,3,5,7-tetraboracanes also occur. The rate of dimerization for amino iminoboranes has been shown to be stabilized by bulky substituents (76,79,83). This stabilization through dimerization is essentially a [2 + 2] cycloaddition. There are a number of examples of these compounds forming cycloadducts with other unsaturated polar molecules (78). Iminoboranes can add to electron-deficient carbene complexes of titanium such as (C5H5)2Ti(CH2) [84601-70-7] by [2 + 2] cyclo addition, yielding the metallacycle shown in equation 26 (84). 

Butadiene)zirconocene reacts with metal carbonyls in a similar way as with ketones. Carbon-carbon coupling between a butadiene terminus and the carbonyl carbon atom occurs readily to yield a metallacyclic product, only that generally a metallacyclic n-allyl metallocene system is obtained from the reactions with metal carbonyls.8 Sometimes, O-zirconated Fischer-type carbene complexes are readily formed in this way that cannot be easily obtained by other routes. A typical example is the... 

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]. 

Metallacycles, formation from carbene complexes of transition metals 91UK169. 

Another route to heteroatom stabilized carbenes involves first the interaction of a terminal alkyne with an iridium complex to yield an iridium vinylidene compound. The vinylidene complex then reacts with nucleophiles, typically alcohols, to form oxa stabilized carbene complexes. O Connor extended this strategy to form a carbene ligand on the same iridium center as a metallacycle (76) and was also able to... 

Although there was considerable controversy over the mechanism of this unique reaction, it is now generally accepted that the reaction proceeds through a series of metallacycle and carbene complexes as shown in equation (3). Although the relative stabilities of the carbenes and metailacycles can change with reaction conditions, catalyst composition and alkene substitution, the fundamental mechanism appears to be the same for all catalysts. ... 

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

Schrock carbene complexes undergo reaction with multiple bonds via four-center metallacyclic intermediates (51). Chapter 11 will consider what occurs when alkylidenes react with alkenes, a reaction known as alkene metathesis. Below are examples of Schrock carbenes reacting with polar multiple bonds such as C-N and C=0. 

Insertion of isocyanides into the Mn—N bond of bis(iminoacyl) complexes (35) gave the aza-manganacycles (36) as major products (Equation (1)) <890M2589>. X-ray studies suggest that these species are carbene complexes (as shown in (36)) rather than true heterocycles. Analogous insertion of isocyanides into the Pt—C bond has led to Pt-S metallacycles <90JCS(D)78i > (see Section 3.20.9.5) and into the Ir—N or Ir—O bonds has afforded Ir-N or Ir-O heterocycles <88JA3704> (see Section... 

Metallacycles from Insertion of Monoolefins into Metal-Carbene Complexes. 

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