Schrock carbenes

Ziegler-Natta catalyst for polymerization of alkenes. Considerable attention has been directed to double-bonded Fischer carbenes of Cr and W, the Schrock carbenes of Ta and Ti, and cyclic polyene ligands of Fe, Co, Cr, and U. Carbonyls of transition metals from groups 6 to 10 of the periodic table include both the monomeric compounds such as Cr(CO)g, Fe(CO)5, Ni(CO)4 and those with two metal groups such as Mn2(CO)io and Co2(CO)s, which is used industrially for hydroformylation. Although their source has not been identified, it has been shown that volatile compounds from landfills contain carbonyls of Mo and W (Feldmann and Cullen 1997). 

Carbenes are both reactive intermediates and ligands in catalysis. They occur as intermediates in the alkene metathesis reaction (Chapter 16) and the cyclopropanation of alkenes. As intermediates they carry hydrogen and carbon substituents and belong therefore to the class of Schrock carbenes. As ligands they contain nitrogen substituents and are clearly Fischer carbenes. They have received a great deal of attention in the last decade as ligands in catalytic metal complexes [58], but the structural motive was already explored in the early seventies [59],... 

A prerequisite for the a-elimination is the absence of (3-hydrogen atoms in the alkyl groups and this was successfully achieved by using the neopentyl substituents at the metal centre. The nature of the double bond between the metal and carbon was established by its bond length and the occurrence of stereoisomers [13], Typical feature of the Schrock carbenes is that they contain an electrophilic, high-valent metal atom and an electron rich carbene carbon atom. The reverse is true for the older, Fischer carbene compounds, such as the one mentioned, (OC)5W=CPh2. 

It is well known that metal carbenes can be classified as Fisher and Schrock carbenes. The classification is mainly based on the n electron density distribution on the M = C moiety (Scheme 4.2). On the basis of the n electron density distribution, carbene complexes of the Fisher-type (E) are normally electrophilic at the carbene carbon while carbene complexes of the Schrock-type (F) are nucleophilic at the carbene carbon. Similarly, metal vinylidenes could also be classified into the two types Fisher-type (G) and Schrock-type (H). The majority of isolated metal vinylidenes belong to the Fisher-type. On the basis of the 7t electron density distribution shown in... 

For a long time metal carbenes have been either classified as Fischer- or Schrock carbenes, depending on the oxidation state of the metal. Since the introduction of N-heterocycHc carbene complexes this classification needs to be extended because of the very different electronic character of these ligands. Carbenes—molecules with a neutral dicoordinate carbon atom—play an important role in all fields of chemistry today. The first examples in the field of organic chemistry were published by Doering and Hoffmann in the 1950s [1], while Fischer and Maasbol introduced them to organometallic chemists about ten years later [2,3]. But it took another 25 years until the first carbenes could be isolated [4-8]. 

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). 

Mo and W alkylidene complexes 4, the so-called Schrock carbenes, have explosively evolved the polymerization chemistry of substituted acetylenes. Although the preparation of these catalysts is relatively difficult because of their low stability, in other words, high reactivity, they elegantly act as living polymerization catalysts for substituted... 

In many of their reactions, these carbenes behave like the familiar Wittig reagent, PhjPCH,. Schrock carbenes are important intermediates in olefin metathesis.68... 

They are called Schrock carbenes to distinguish them From the Fischer carbenes. One way to view these complexes is in terms ol two orbitals on the cat bene, each housing an unpaired electron (triplcl state) overlapping with two metal orbitals, each of which provides an electron/ ... 

The nucleophilic nature of a Schrock carbene is seen in its reaction wilh Me AI ... 

The reactivity of early transition metal silylenoid complexes is still emerging. An example of the chemistry that these complexes can participate in is the sila-Wittig reaction [equation (7.4)].53 In this transformation, a metathesis occurred between the chromium silylenoid 24 and the dimethyl carbonate to afford a new Schrock carbene, 25, and the trimerized product 26. This methodology allowed access a new carbene complex that eluded previous synthetic efforts. 

Transition metal carbene complexes can be divided into two classes electrophilic carbenes (Fischer carbene [69-71], Casey carbene [72,73]) and nucleophilic carbenes (Osborn carbene [74,75], Schrock carbene [76-79]) ... 

Osborn carbene Schrock carbene Schrock carbene... 

The cyclopolymerisation of 1,6-heptadiyne derivatives such as diethyl dipro-pargyl malonate by a Schrock carbene, [Me(CF3)2CO]2Mo(=NAr)=CHMe3, via a metathesis mechanism, proceeds in a living fashion to provide a conjugated polymer having both five-and six-membered rings, which is shown schematically below [154, 155] ... 

The living nature of the discussed cyclopolymerisation of the 1,6-heptadiyne derivative in the presence of Schrock carbene as the catalyst has been demonstrated by the synthesis of a block copolymer with 2,3,-dicarbomethoxynor-bornadiene [25]. 

Later Schrock showed that compounds in high oxidation states, for example, Cl2H(PR3)3Tav(CHCMe3), with fewer than 18 electrons could be made by a-hydro-gen loss from an alkyl group. These have been termed alkylidene complexes. Informally, the two types are often called Fischer carbenes and Schrock carbenes. The IUPAC recommends calling them all alkylidene complexes, with the term carbene being restricted to a free CR2 species. 

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]. 

Figure 1.1 Bonding in Fischer, Wanziick-Arduengo and Schrock carbene transition metal complexes.

The structures and reactivities of Fischer and Schrock carbenes can be explained by interactions of singlet and triplet carbenes with suitable metal d orbitals without any stabilisation from neighbouring nitrogen atoms at the carbene carbon atoms [14,128-132]. 

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

Fischer carbenes possess electrophilic carbene carbon centres (they react with nucleophiles) whereas Schrock carbenes show opposite reactivity, and have nucleophihc carbene centres (that react with electrophiles) [14,131],... 

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