Schrock-type carbene complexes, transition metal

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. 

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). 

Some Schrock-type carbene complexes, i.e. high-valent, electron-deficient, nucleophilic complexes of early transition metals, can undergo C-H insertion reactions with simple alkanes or arenes. This reaction corresponds to the reversal of the formation of these carbene complexes by elimination of an alkane (Figure 3.36). 

As discussed in previous sections, high-valent carbene complexes of early transition metals have ylide-like, nucleophilic character. Some Schrock-type carbene complexes react with carbonyl compounds in the same manner as do phosphorus ylides, namely by converting the carbonyl group into an alkene. 

A number of bimetallic complexes which contain bridging alkylidene and vinylidene ligands semi-bonded to palladium are known, and as these also contain bridging carbonyl ligands, they are discussed in Section 8.04.1.8. In these dimers, the carbene ligand is more closely associated with the early transition metal, for which Schrock-type carbene complexes are well known. 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

Schrock-type carbene complexes of transition metals other than titanium are also utilized for carbonyl olefination, although their synthetic utility has not yet been fully investigated. In some cases, their reactions differ from those of titanium-carbene complexes in terms of stereo- and chemoselectivity and are complementary to olefination with titanium reagents. This section describes the use of carbene complexes of various transition metals in carbonyl olefination. 

Cyclopropylcarbene complexes of the type L M=C(XR )R2 (X = O, S R1 = alkyl, aryl R2 = cyclopropyl) having a stabilizing heteroalkyl (XR1) group on the electrophilic carbene ligand (Scheme 3) have found widespread application in organic synthesis. These so-called Fischer carbene complexes are best known via their group 6 transition metal carbonyl complexes (CO)5M=(OR )R2(M = Cr, Mo, W)132. Much less abundant are the Schrock-type cyclopropylcarbene complexes L M=CR R2 where no heteroatom is bound to the carbene carbon atom133. 

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 mechanism of the [2+2] reactions has been studied in detail with the Schrock-type and Grubbs-type ° carbene complexes. The results of studies of the mechanism of these [2+2] reactions are summarized in Scheme 13.15. In general, the olefin coordinates to the metal center prior to the [2+2] process. This olefin complex then proceeds through the transition state for the [2+2] reaction and generates a metallacyclobutane product. 

Phosphinidenes differ from carbenes because of the additional lone pair. This lone pair enables interactions with, e.g., a transition metal group for increased stability, while maintaining carbene-hke behavior. These terminal /] -complexed phosphinidenes differ from the p2-> fi3-> and p4-complexes, which are not part of this survey. Phosphinidenes that are stabilized by a transition metal group also relate to carbene complexes. A distinction in Fischer and Schrock-type complexes has been advanced to distinguish phosphinidene complexes with nucleophilic properties from those that are electrophiHc [ 13 ]. In this survey we address this topic in more detail. 

Electrophilic and nucleophilic phosphinidene complexes have been related to the corresponding carbene complexes of which the Fischer-type is usually considered as a singlet-singlet combination and the Schrock-type as a triplet-triplet combination. However, both the strongly preferred triplet state of R-P and the M=P bond analysis suggest this schematic interpretation to be less appropriate for transition metal complexed phosphinidenes. 

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). 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

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). 

Schrock type, but the high stability of NHCs has broken this rule affording new series of early transition metal complexes. For example, the NHC complexes shown in Scheme 13 were obtained from the reaction of the free NHCs and the corresponding TMEDA-, THF-, and py-metal adducts [54]. Direct reaction of the NHC with TiCU also provides the coordination of one carbene affording (NHQTiCU [55]. 

In particular, Schrock-type catalysts suffered from extreme moisture and air sensitivity because of the high oxidation state of the metal center, molybdenum. Due to the oxophilicity of the central atom, polar or protic functional groups coordinate to the metal center, poisoning the catalyst and rendering it inactive for metathesis. Since late transition metal complexes are typically more stable in the presence of a wide range of functionalities, research was focused on the creation of late transition metal carbene complexes for use as metathesis catalysts. 

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]. 

Two types of transition metal carbene compounds are traditionally referred to by the names of the scientists who first made them, namely E. O. Fischer [27] and R. R. Schrock [30]. The discovery of the first transition metal NHC complexes by Ofele [2] and WanzUck [3] falls in between the other two, but did not receive the same amount of recognition at the... 

Many transition metals can form carbene complexes, often generated indirectly due to the instability of the corresponding carbene. While some of these complexes are reactive and unstable intermediates, many are stable and some are even commercially available. Carbene complexes are divided into two types Fischer and Schrock carbenes. Fischer carbenes, such as chromium complex 8.6 (Figure 8.1), contain metals from groups VI to VII, have rr-acceptor ligands, especially carbon monoxide, and are electrophilic. A donor atom on the carbene carbon stabilizes the carbene. Schrock carbenes, such as tantalum complex 8.7, involve early transition metals, do not have rr-acceptor ligands and are nucleophilic. 

There are two types of transition metal carbene and carbyne complexes low-valent (so-called Fischer type)i" i" and high-valent (so-called Schrock type). The two classes of compounds are quite different in their chemical behavior. Such different chemical reactivity is sometimes rationalized on the basis that the metal-carbene and metal-carbyne bonds in Fischer-type complexes have donor-acceptor character, whereas the bonding in Schrock-type complexes is more typical for a normal multiple bond. 

S. F. Vyboishchikov and G. Frenking, /. Am. Chem. Soc., submitted for publication. Structure and Bonding of Low-Valent (Fischer-Type) and High-Valent (Schrock-Type) Transition Metal Carbene Complexes. 

One can distinguish the cationic or neutral complexes of electrophilic carbene of Pettit type, - - that tend to behave somewhat as carbocations coordinated to transition metals, and the neutral metal complexes of nucleophilic carbene complexes of Schrock type that tend to react as ylids. 

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