Metal-carbene complexes ligand substitution reactions

Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

JV-Heterocyclic carbene-metal complexes are powerful synthetic tools. The development of a catalyst can be problematic, and different methodologies to prepare such complexes have been reported. The most common methodology requires the generation of the free carbene followed by binding to a metal center via ligand substitution or dimer scission reaction. A direct and straightforward process via NHC transfer can be accomplished using silver salts. Indeed, the... 

The ligand substitution reactions of carbene complexes such as (CO)s-CrC(OCH3)CHj allow the synthesis of many phosphine- and phosphite-substituted carbene complexes. It is expected that these complexes will have modified reactivity and will provide a means of fine tuning reactions of carbene complexes. More importantly, the substitution reactions of carbene complexes proceed by a dissociative mechanism involving coordinatively unsaturated intermediates. Study of the ligand substitution reactions can give valuable information about these coordinatively unsaturated intermediates which are also involved in the important cyclopropanation, alkene scission, and thermolysis reactions of metal-carbene complexes. 

The reaction of metal-carbene complexes with electron-rich vinyl ethers occurs under milder conditions than the reaction with electron-poor unsaturated esters. The conditions are also milder than those required for ligand substitution of carbene complexes. The reaction products depend strongly on the external CO pressure with no added CO, alkene scission products predominate under 100 atm CO pressure, cyclopropanes are formed in 60% yield (Dotz and Fischer, 1972b). The ratio of isomeric cyclopropanes formed... 

Information about the stability and stereoselectivity of coordinately unsaturated metal carbene complexes is important for understanding a number of reactions, including olefin metathesis and cyclopropanation. A study of CO exchange in [(CO)5MC(OMe)(Ph)] (M = Cr, Mo, or W) shows that the carbene ligand is cis labilizing compared to CO. Ratedetermining CO loss is inferred from the similar rates of "CO exchange and cis-CO substitution by phosphines. Measurements of the ratio cis- CO/frans- CO as a function of percent CO incorporation imply that... 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

The reactivity displayed by the heteroatom-substituted Ru, Os, and Ir carbene complexes discussed in this section toward nucleophilic reagents contrasts sharply with that described for the Fischer compounds. The reactions of these Group 8 complexes are almost exclusively restricted to the metal-ligand framework, with only two related substituent substitution reactions being reported (44) ... 

We have already established that the carbene carbon is an electrophilic center and, hence, it should be very easily attacked by nucleophiles. In most reactions we believe that the first reaction step probably involves attachment of a nucleophile to the carbene carbon. In some cases, for instance with several phosphines (49) and tertiary amines (50), such addition products are isolable analytically pure under certain conditions (1 in Fig. 3). For the second step there exists the possibility that the nucleophilic agent may substitute a carbon monoxide in the complex with preservation of the carbene ligand (2 in Fig. 3). One can also very formally think of the carbene complex as an ester type of system [X=C(R )OR with X = M(CO)j instead of X = 0], because the oxygen atom as well as the metal atom in the M (CO) 6 residue are each missing 2 electrons for attainment of an inert gas configuration. So, it is not surprising that the... 

The aminocarbenes derived from reactions of coordinated isocyanides are treated in Section 14 and bridging alkyhdenes arising from C-H activation by Re atoms are presented in Section 9. The recent boom of metal complexes derived from simple ligand substitution with the stabilized N-heterocychc carbenes has produced only a very limited number of Re complexes like the luminescent (48). ... 

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