Complex, Fischer-type metal-carbene

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. 

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

In contrast to transition metal carbene complexes generated catalytically, those of the early transition metals are generally stable (Fischer-type carbenes) and undergo... 

Among the first 18-electron (18e) Fischer-type metal carbene complexes to be used as part of an olefin metathesis catalyst system were W[=C(OMe)Et](CO)5 with BU4NCI (for pent-l-ene)79, and W[=C(OEt)Bu](CO)5 with TiCLt (for cyclopentene)80. These complexes may also be activated thermally, e.g. for the polymerization of alkynes81, or photochemically, e.g. for the ROMP of cycloocta-1,5-diene82. The essential requirement is that a vacancy be created at the metal centre to allow the substrate to enter the coordination sphere. Occasionally the substrate may itself be able to displace one of the CO ligands. 

The structure and bonding of metal silylene complexes varied from those of their carbon analogs. While Fischer-type metal carbene complexes without solvent adducts have been extensively characterized,48 most metal silylenoid complexes contain a bound solvent molecule or counterion on the silicon atom. The bond energy for donor silicon complex 22 was determined to be between 15 and 20kcal/mol 49,50... 

Heteroatom-stabilized carbene complexes of type 1, first discovered by E.O. Fischer in 1964 [1], nowadays belong to the best investigated classes of transition metal compounds. Such complexes are coordinatively saturated, intensely colored solids = 350-400 nm), which exhibit a sufficient stability for normal preparative use. Especially chromium carbene complexes (2) enjoy increasing importance in organic synthesis, and it must be added that thermal reactions such as benzannulations (i.e. the Ddtz reaction), cyclopropanations and additions to a,j8-unsatu-rated complexes clearly predominate [2J. 

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]. 

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. ... 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

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... 

From a methodological point of view, it should be pointed out the formation of 51, which is a result of the addition of acetone to an allenylidene ligand. Heteroatom-containing cyclic metal-carbene complexes [24] have been conveniently prepared via metal co-haloacyl, carbamoyl, alkoxycarbonyl, or imido intermediates [25], opening of epoxides by deprotonated Fischer-type carbene complexes [26], and activation of homopropargylic alcohols with low-valent d complexes [27], including ruthenium(II) derivatives [28]. In general, the preparation of unsaturated cyclic carbene complexes requires the previous preparation of functional carbenes to react with P-dicarbonyl derivatives, acrylates, and enol ethers [29]. 

Actual metal carbene complex catalysts can be divided into two broad classes, Fischer-type and Schrock-type . The Fischer-type carbene complexes are low-valent and generally characterized by the presence of one or two heteroatoms (O, N, or S) bonded to the carbene carbon. Such complexes do not normally initiate the chain metathesis of olefins, since they are both coordinatively and electronically (18e) saturated. However, they can sometimes be activated for metathesis by heating, or by reaction with a cocatalyst, or photochemically. Some examples are listed in Table 2.1. 

It was first observed by Woon (1974) and Farona (1974) that acetylenes could be polymerized by catalysts of the type Mo(CO)3(toluene). This was followed by the discovery that conventional metathesis catalysts such as M0CI5 (Masuda 1974) and WCls (Navarro 1976 Masuda 1976), with or without a cocatalyst, could also bring about polymerization of acetylenes. At first there was some doubt as to whether these polymerizations were being propagated by the metathesis mechanism (Scheme 10.2) or whether a Ziegler-Natta mechanism was operating. However, the observation that metal carbene complexes could react with acetylenic molecules to form simple adducts as in reaction (20) (Fischer, H. 1980), and the fact that such complexes could initiate the polymerization of acetylenes, albeit somewhat slowly, but cleanly and in fair yield, soon allayed these doubts. 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

The bonding interactions of a carb)me ligand are essentially those of a metal carbene complex, but with an additional ir-bond (Figure 2.16). One orbital of cr-symmetry and two of iT-symmetry overlap with three metal orbitals of appropriate symmetry. When considered trianionic, all three orbitals of the ligand fragment contain two electrons. Theoretical studies of heteroatom-substituted or "Fischer-type" carbyne complexes - indicate that the HOMO predominantly consists of the metal fragment, and the LUMO consists of one of the TT -orbitals of the metal-carbon bond. This result explains the tendency of nucleophiles to attack the carbyne carbon in carbyne complexes, just as they attack the carbene carbon in Fischer carbene complexes. 

In 1964 Fischer prepared the first stable metal carbene complexes [11], such as W[=C(OMe)Me](CO)5. Six years later Herisson and Chauvin proposed that olefin metathesis reactions were initiated and propagated by complexes of this type in a chain reaction involving the intermediate formation of metallacyclobutane complexes, eq. (4) where [Mt] is a transition-metal atom surrounded by various ligands [12]. 

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