Fischer-carbene type ligands

In order to explore the effect of the heteroatom in Fischer-carbene type ligands on the reactivity and thermal stability of ruthenium complexes, Grubbs et al. prepared and characterized a series of well-defined bis-phosphine 4-8, NHC imidazole (IMes) 9-12, and NHC imidazolidine (HglMes) 13 complexes (Figure 12.3) [17]. The exceptionally high stability of 8 at 55 C (20 days before half the complex was decomposed) could be explained by the chelation ofthe amide carbonyl to the ruthenium center. When tested in ROMP reactions, all Fischer carbene complexes demonstrated rapid and quantitative polymerization of norbornene (NBE) derivatives at room temperature, although the polymerization... 

Finally, the coupling reaction of vinyl and alkylidene (see Alkylidene) ligands in complexes of the type (166) results in the formation of (jj -allyl)FeCp(CO) complexes (equation 38). The vinyl substituent in these reactions is most often an enolate, whereas the alkyhdene unit is formally of the Fischer carbene type. The ultimate product depends upon the starting material and additional reagents. In the case of (ferra-/3-diketonato)difluoroboron complexes (167), the resulting anion is isolated as tetramethylammonium salt (168). In the case of Fp(acyl) complexes as starting materials, benzoylation of the resultant anions gives complexes (163 ... 

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. 

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]. 

An aminophosphinidene complex Cp Os(CO)2 PCl(N-Tr2) was prepared by reaction of [Cp Os(CO)2] with Tr2NPCl2. As with Fischer carbenes (see Fischer-type Carbene Complexes), the nitrogen group stabilizes the electron-deficient phosphorus atom of the phosphinidene ligand. Based on a crystal structure of the ruthenium analog, there is likely... 

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

This general type of metal carbenes is termed Schrock carbenes . These carbenes differ from Fischer carbene complexes (those with o-hetero-substituents) in a number of ways. Schrock carbenes are viewed as X2 ligands with -i-2 charge, whereas Fischer carbenes are considered neutral L ligands. Fischer carbenes are electrophilic at the a-carbon and Schrock carbenes are nucleophilic. Thus complexes 1 and 2 are formally W(VI) and Mo(VI) complexes [66]. 

Fischer carbenes are found with low oxidation state metals, middle and late transition metals Fe(0), Mo(0), Cr(0), Ji-electron acceptor metal ligands, and Jt-donor substituents on the carbene atom such as alkoxy and alkylated amino groups. The chemical bonding (Figure 5.10) is based on a-type electron donation of the filled lone pair orbital of the carbene atom to an empty metal d-orbital, and Ji electron back-bonding of a filled metal d-orbital to the empty p-orbital on carbon. 

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. 

Nucleophilic additions to carbenes are restricted to cationic Fischer carbene complexes or Fischer carbene complexes that possess ancillary ligands that stabilize negative charge. The high-valent, early metal "Schrock-type" carbene complexes are generally... 

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