Alkylidenes Fischer-type

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. 

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

Etherification using a metal vinylidene has also been combined with G-G bond formation through the reaction of an alkynyl tungsten complex with benzaldehyde (Scheme 14). The addition of an internal alcohol to the incipient /3,/Udialkylvinylidene that is generated leads to dehydration and the formation of a Fischer-type alkylidene complex. Further reactions of this carbene with a range of nucleophiles have provided access to various furan derivatives.374,375... 

Molybdenum dinitrosyl complexes with the general formula Mo(NO)2(CHR) (0R )2(A1C12)2 have been found to be active in a variety of metathesis reactions [110]. New alkylidenes could be identified. Variations such as Mo(NO)2(CHMe) (RC02)2 also are known [111]. Complexes of this type are believed to be more reduced than typical d° species discussed here, although they appear to be much more active as metathesis catalysts than typical Fischer-type carbene complexes. 

The decisive difference between, e.g., [Cp(CO)2Fe]" and H4Ta- is the smaller amount of orbital overlap of the former with the carbene 2 p orbital, resulting in less efficient transfer of electron density from the metal to C . Although Fischer-type carbene complexes are formally low valent, backbonding to the carbene is less effective than in tantalum alkylidene complexes. 

Carbenes, generated by several methods, are reactive intermediates and used for further reactions without isolation. Carbenes can also be stabilized by coordination to some transition metals and can be isolated as carbene complexes which have formal metal-to-carbon double bonds. They are classified, based on the reactivity of the carbene, as electrophilic heteroatom-stabilized carbenes (Fischer type), and nucleophilic methylene or alkylidene carbenes (Schrock type). 

The reactions of carbyne complexes will be discussed, albeit more briefly, along the lines developed for alkylidene complexes, to emphasize the parallels. Once again, as with alkylidenes, early generalizations suggested two classes of complexes, Fischer-type and Schrock-type, which have subsequently been recognized as extremes in a tunable reactivity continuum. Furthermore, within the chemistry of tungsten, Fischer-type... 

The first carbene compound to be well characterized was prepared in 1966 and was one of many Fischer-Type Carbene Complexes to be reported (see equation 7). Fischer carbenes are characterized by heteroatom substituents at the carbene carbon, stabilization by a low-valent metal center, and a partial positive charge at the carbene carbon. In contrast, Schrock-Type Carbene Complexes, or alkylidenes," that have alkyl substituents, are found on metal centers in higher oxidation states, and are nucleophihc at carbon. Many Fischer carbenes are known for chromium, whereas chromium alkylidenes are much less common. Monohalocarbenes of chromium, for example, (OC)5Cr=C(F)NEt2, have also been extensively investigated." Two carbene reactions of note for their application to organic synthesis are the cycloaddition of alkenes with carbene complexes and the reaction of aromatic carbenes with aUcynes to yield complexed naphthols (the Dotz reaction ). ... 

Alkylidenes have been prepared by reduction of alkyli-dynes, by C H oxidative addition from alkyls, and by treatment of unsaturated metal clusters with diazoalkanes. In most instances, the alkylidene adopts a /r2-h coordination mode. However, alkylidenes with heteroatom substituents may also be found in terminal coordination modes. The latter are typically prepared by the Fischer-type carbene route (see Fischer-type Carbene Complexes) (sequential addition of nucleophilic and electrophilic alkylating agents to carbonyl or isocyanide ligands), by condensation of metal fragments with mono- or dimetallic carbene complexes, or by C-H activation of alkylamines. These heteroatom substituted carbenes may also bind in a p3-ri mode, as in (12). 

Complexes of type (120) are examples of an amphiphilic alkylidene, that is, the CHR ligand undergoes both electrophilic addition, typical of Schrock-type carbenes, and nucleophilic addition, typical of Fischer-type carbenes, as exemplified in Scheme 25. ... 

The chemistry of transition metal carbene complexes has been examined with an eye to applications in organic synthesis ever since their discovery by Fischer in 1964, and the growth in the number of useful applications has been exponential with tirne. " There are two types of transition metal carbene complexes those which have electrophilic carbene carbons and which are typified by the pentacarbonylchro-mium complex (1), and those which have nucleophilic carbene carbons and which are typified by the biscyclopentadienyltitanium complex (2). Complexes (1) and (2) are often referred to as carbene and alkylidene complexes, respectively. This review will be limited to the chemistry of electrophilic carbene complexes of the Fischer type. The chemistry of the nucleophilic alkylidene complexes will be covered in Chapter 9.3, this volume. ... 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

It is interesting that the Ru alkylidenes that Grubbs discovered are both alkylidenes and complexes with the metal in a relatively low oxidation state (+2).35 As Grubbs has mentioned in his writings and conversations, there is a whole spectrum of reactivity in metal-carbene complexes, and it is difficult to classify the reactivity of many of these as being either Fischer type or Schrock type.36... 

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. 

With experimental support for the metal-carbene-mediated mechanism of olehn metathesis, a number of groups initiated studies with isolated metal-carbene and metallacyclobutane complexes. Early work by Chauvin and Katz on the polymerization of strained olefins using Fischer-type carbenes demonstrated the success of such an approach [56], The introduction of high oxidation state alkylidene complexes led to well-defined catalyst in which the propagating species could be observed and studied, such as the tungsten-based systems developed by Osborn, Schrock, and Basset [59,60], The best-studied and useful of these have been the Schrock arylimido alkylidene complexes, and we will return to these later in this chapter. 

A similar dichotomy of bonding models exists for other classes ofTM compounds where the chemical bond can be discussed either in terms of electron-sharing interactions TM-R or as donor-acceptor bond TM-L. Examples are TM compounds with carbenes CR2 or carbynes CR as ligands, which can be considered as either Fischer-type complexes or as Schrock-type alkylidenes and alkylidynes [31, 32). We want to emphasize that the two bonding models should be considered as sketches of two extreme situations, whereas the electronic situation of real molecules has components of both forms. The value of such dichotomic models lies in the fact that they establish an ordering scheme, which is very useful for describing the physical and chemical properties of molecules. 

Some particular stoichiometric reactions of stable carbenes complexes (Fischer type) of ylides and of alkylidene complexes are also considered. 

In Fischer-type alkylidene complexes, the carbene carbon atom very readily undergoes nucleophilic attack. Unlike the acyl complexes such as Mn(COMe)(CO)5, in pen-tacarbonylcarbene complexes nucleophilic attack occurs at the carbene carbon atom and not at carbonyl ligands. This fact cannot be explained on the basis of positive... 

Unlike Fischer-type compounds, alkylidene derivatives of niobium and tantalum have nucleophilic carbene carbon atoms. Therefore, they react with Lewis acids. 

Fischer-type complexes react only with selected activated olefins. More reactive carbene complexes which do not contain heteroatoms in the alkylidene ligand react with a greater number of olefins. 

Fischer-type and Casey carbenes are 18-electron complexes and are too stable to work as catalysts. Photoirradiation and/or addition of Lewis acids promotes the generation of active species. Schrocktype electron-deficient Mo and W alkylidene... 

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