Fischer ligand effect

Low-valent, 18-electron carbene complexes (Fischer-type) are electronically and coordinatively saturated and must usually be activated to become efficient metathesis catalysts. This activation can be effected by thermal or photolytic [781,782] removal of one ligand, e.g. carbon monoxide, or by treatment with a Lewis acid (Table 3.15) [783]. 

Equation 10.52 shows the chemistry used to produce 64 and similar complexes this not only is the original procedure employed by Fischer, but also constitutes perhaps the most versatile method for the preparation of Fischer carbyne complexes. The reaction begins with an electrophilic abstraction of the alkoxy group from starting carbene 69 to give the cationic carbyne 70. Carbyne ligands are such powerful n acceptors that they exert a strong trans effect (Section 7-1-1). If... 

A comprehensive treatment of the benzannulation of Fischer carbene complexes with alkynes is not possible in this review, and thus instead the material presented here will hopefully serve to give the reader an overview of its scope and limitations. The first report of this reaction was in 1975 by Dotz in which he describes the formation of the naphthol chromium tricarbonyl complex (236) from the reaction of the phenyl chromium complex (la) with diphenylacetylene. In the intervening years over 100 papers have been published describing various aspects of this reaction.The reaction of the generic cartene complex (233 Scheme 34) with alkynes will serve to focus the organization of the scope and limitations of the benzaimulation reaction. The issues to be considered are (i) the regioselectivity with unsymmetri-cal alkynes (ii) possible mechanisms (iii) applications in natural product syntheses (iv) the effect of substitution on the aryl or alkenyl substituent of the carbene carbon (v) functionality on the alkyne (vi) effects of the solvent and the concentration of the alkyne (vii) tandem applications with other reactions of carbene complexes (viii) reactions where aromatization is blocked (cyclohexadienone annulation) (ix) annulation of aryl versus alkenyl carbene complexes (x) the effect of the ligands L on the metal (xi) the effect of the ancilliary substituent RX and (xii) reactions with —C X functionality. 

In this Scheme the ligand is often bipyridyl if the metal is Cul and, in early ATRP, ruthenium compounds were used. The nature of the ligand controls the effectiveness of the catalyst. The system is living since the concentration of the transition metal in the higher oxidation state is sufficient to ensure that deactivation to the dormant state is much faster than any other polymer radical termination steps, including bimolecular recombination. Because of the absence of a Trommsdorff effect the reaction may be carried out in the bulk (Matyjaszewski, 1999). The detailed mechanism is complex since the reaction rate depends on the metal, the organic halide, the ligand and the counter-ion (Fischer, 2001). 

Bagnall and Fischer (24) have studied the effect of ligand, metal and substitution on Cp on the formation of MCp XY complexes. With the large counter cation AsPh, [MCp CNCS) ] was isolated for M = U, Np, Pu. It was also found that U(MeCp) o(NCS) (MeCN), dissociates more readily than the Cp analogue. The investigation of the effect of an additional anion Y on the formation of [UCP3XY]" (X and/or Y = F", oh", CN", NCBH3 , NCO") showed that only NCS" and NCO are... 

If L is CO, then the halide ion (Cl, Br, or I) displaces the CO trans to the carbyne in the intermediate cationic complex this shows the high trans effect of the carbyne. On the other hand, if L is PMc3, then the cationic species is the final product. Oxidation states for Fischer carbynes are normally assigned by considering CR as an LX ligand, CR that is, X(CO)4M=CMe is M(II). 

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... 

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