Schrock, carbene complex

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

Early metal complexes often avoid the 18e configuration - Me3TaCl2 is ostensibly lOe, for example. This can provide coordinative unsaturation at the metal Schrock carbene complexes with <18e commonly have agostic C H bonds see Alkane Carbon-Hydrogen Bond Activation, Dihydrogen Complexes Related Sigma Complexes). When... 

Schrock carbene complexes may be identified by their alkyl substituents on the carbene carbon and an electropositive, early-transition metal. As shown in Figure 2, the Schrock complexes are considered a result of spin-coupling between the carbene triplet state and the two electrons on the metal [8]. The reactivity of the Schrock carbene carbon is nucleophilic [4]. The reaction involving bis(cyclopentadienyl)carbene titanium(IV) and the electrophile acyl chloride in Eq. (3) [9], for example, illustrates the nucleophilicity of... 

The three kinds of carbene complexes differ in several ways. As will be discussed in Section 10-3, Fischer carbene complexes tend to undergo attack at Ccarbene by nucleophiles and thus are termed electrophilic (equation 10.3), Schrock carbene complexes, on the other hand, undergo attack by electrophiles at Ccaibene and are considered nucleophilic species (equation 10.4). Because of their... 

When the metal is niobium, the two metal frontier orbitals, dy and dz2, are closer in energy and match the corresponding orbitals of triplet free carbene. Figure 10-4b provides a picture of the interactions involved (only the important orbital interactions are shown). The elections in the M-C bond are much more equally distributed between the metal and Ccarbene (because the energy levels of the metal and carbon orbitals are comparable) than with an electrophilic carbene complex. In resonance terminology, this means that structure 10 becomes an important contributor to the overall structure of this particular Schrock carbene complex.11... 

There exists a relevant analogy between Schrock carbene complexes and ylides. The term ylide typically refers to a dipolar compound where carbon is attached to a nonmetal such as N, P, or S. An ylide shows nucleophilic character at the carbon, with two resonance structures providing significant contribution to the overall structure of the molecule, (i.e., R Y=CR2 R Y+- CR2). For an introduction to the chemistry of ylides, see M. B. Smith,... 

Molecular Orbital Picture of a Schrock Carbene Complex... 

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. 

There are several parallels between the chemistries of the carbene and carbyne ligands. The classification of carbyne complexes into two major structural types—Fischer and Schrock—is perhaps the most obvious of these parallels. Complex 64 represents the prototypical Fischer carbyne complex Ccarbyne bonded to a low-valent metal with Ji-accepting CO ligands attached. Structure 65, on the other hand, is a classic example of a Schrock carbyne complex because a high-valent metal is present with electron-donating ligands attached. Atoms attached to Ccaibyne helpful in distinguishing between Fischer and Schrock carbene complexes (i.e., heteroatoms for the former and H and C for the latter), are less important in the case of carbyne complexes. It is convenient to classify carbyne complexes... 

The early development of Mechanism 3 was bold for its day because Fischer carbene complexes had just been discovered a few years earlier, and alkylidenes were not yet known. The carbene complexes prepared before 1971 also did not catalyze olefin metathesis. With the discovery of Schrock carbene complexes and the demonstration that some alkylidenes could promote metathesis, the non-pairwise mechanism became more plausible (Section 11-1-2). It was, however, the elegant work of Katz and co-workers that provided early substantial support for the Herisson-Chauvin mechanism. 

It can be argued that, if one considers Grubbs catalysts to be true alkylidenes (Schrock carbene complexes), the oxidation state of Ru is +4. 

Fig. 4 The a and Tt contribution to the metal-carbon bond of (a) a Fischer carbene complex, (b) a Schrock carbene complex and (c) a NHC-metal complex...

The and NMR chemical shifts of coordinated carbenes are distinctive. The " C chemical shifts of the carbene carbon in Fischer carbene complexes with oxygen donors on the carbene resonate between 290 and 365 ppm versus tetramethylsilane. Those with nitrogen donors resonate less far downfield, but generally give rise to signals between 185 and 280 ppm. The chemical shifts of the carbene carbons in Schrock carbene complexes are also far downfield. These carbons t5 ically resonate between 240 and 330 ppm. The H NMR chemical shifts of these species are also far downfield and are typically found between 10 and 20 ppm. The infrared vibrations are difficult to locate because the M=C bond vibrates at a low frequency, and these bands are generally not identified. 

As stated in Chapter 3, carbene complexes can be divided into the five classes illustrated in Figure 13.2. One class of carbene complex encompasses the Fischer carbenes that were first prepared in the laboratory of E. O. Fischer. These complexes were the first transition metal carbene complexes prepared, and they contain a ir-donating group on the carbene carbon. Complexes of these carbenes are typically electrophilic at the carbene carbon. A second class of carbene complex was first prepared by Richard Schrock. These complexes contain alkyl groups or hydrogens on the carbene carbon and are called alkylidene complexes or often "Schrock carbenes." Complexes of these carbenes are typically electrophilic at the metal and nucleophilic at the carbene carbon. 

In some cases, vinylidene complexes undergo [2-t-2] reactions that are characteristic of Fischer and Schrock carbene complexes. However, these [2+2] reactions involving vinylidene complexes can result from nucleophilic addition at the central carbon, rather than a concerted [2+2] process. For example, the reaction of an imine with the iron-vinylidene complex in Equation 13.28 leads to formation the product of a [2+2] reaction between the carbon-nitrogen double bond and the carbon-carbon double bond. ° - This reaction is believed to occur by nucleophilic attack of the nitrogen at the central carbon, followed by ring closure at the p-carbon, instead of a concerted [2+2] process. 

The two Schrock carbenes (complexes 2 and 3) are not stereoselective With trans-pent-2-ene the resulting olefin always has the thermodynamic cis/trans ratio while with a>pent-2-ene there is formation, at the beginning of the reaction, of cis and trans olefins in quite the same amounts. 

The frontier orbital theory also agrees with the observed experimental reactivity of Fischer and Schrock carbene complexes. Calculations show that the linear Cr=C —R group is more stable than the bent one. " Thus, the deviations of M = C —R fragments from linearity probably result from intermolecular interactions within the crystal lattice. 

The metal-carbene complexes are nowadays extensively used as reagents in organic synthesis. Fischer carbene complexes have become valuable building blocks in various stoichiometric reactions such as the aldol reaction, benzannu-lation, cycloaddition, cyclopropanation, and Michael reaction. In particular, Fischer alkenyl carbene complexes have been widely used in cycloadditions [2c,2f,5]. On the other hand, a typical reaction of Schrock carbene complexes is olefin metathesis (Figure 5.2) [6]. Schrock s olefin metathesis catalyst is a high reactivity, but air- and moisture-sensitive reagent. 

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