Tungsten alkylidyne complex preparation

Although the structure of the complex arising from I52/CH2CI2 is not clear, this catalyst is excellent in terms of ease of preparation. The catalyst is very active for formation of cycloalkynes with ring sizes different from those of diynes (Table 6.5). In contrast to tungsten alkylidyne complex 150, catalyst 152/ CH2CI2 is sensitive toward an acidic proton such as amide proton and exhibited remarkable tolerance towards many polar functional groups (Table 6.5). 

This mechanism was later confirmed experimentally in 1981 by Schrock and others, who reported the first example of alkyne metathesis by tungsten(vi)-alkylidyne complex. They have prepared tungsten alkylidyne complex 120 (Equation (21)) and found that it reacts with diphenylacetylende to give tungsten alkylidyne complex 121 and another alkyne 122 (lequiv.) (Equation (22)). Furthermore, complex 121 works as a catalyst for the alkyne metathesis reaction. 

J., Hrib, C.G., Jones, P.G., and Tamm, M. (2008) Preparation of cyclophanes by room-temperature ring-closing alkyne metathesis with imidazolin-2-iminato tungsten alkylidyne complexes. Org. 

The preparation of high-valent, Schrock-type alkylidyne complexes containing poly(pyrazolyl)borate ligands may also be achieved by ligand substitution reactions. For example, the (dry) air-stable tungsten complexes Tp W( = CR)X2... 

Krouse, S.A., Schrock, R.R., and Cohen, R.E. (1989) Preparation of polycyclooctyne by ring-opening polymerization employing dO tungsten and molybdenum alkylidyne complexes. Macromolecules, 22, 2569-2576. 

The first monosUyl alkylidyne complex of tungsten without anionic r-bonding ligands Np2W(=CBu-t)(Si(SiMe3)3) was prepared recently upon metathesis of a neopentyl tungstenyl chloride derivative with the salt (MesSOsSiLi (equation 21). ... 

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 first examples of catalytic RCAM for the synthesis of functionalized macrocycles were reported by Ftirstner and Seidel [7]. Two catalyst systems were employed in this study the instant system prepared by reaction of catalytic amounts of Mo(CO)6 1 with stoichiometric quantities of phenols, and the well-defined tungsten-alkylidyne 2 first prepared by Schrock in 1981 [5, 8]. Subsequent applications of RCAM, particularly in natural product synthesis, have either utilized these two systems or a molybdenum system developed by Fiirstner that employs in situ reaction of trisamido molybdenum complex 3 [9] and CH2CI2 (Figure 7.1) [10]. 

The commercially available complex 2 shows higher activity and is metathesis active under mild conditions, typically ambient temperature up to 90 °C [3c, hj. RCAM reactions with 2 are often run at 80 °C in toluene or a related aromatic solvent However, rigorously inert (anhydrous and oxygen-free) conditions are required. The core structure is amenable to tuning by ligand modifications, reflected in a recent report which demonstrated that the effective RCAM of diyne 6 can be performed at room temperature to afford cyclic alkyne 8 in excellent yield when imidazolin-2-iminato tungsten alkylidyne catalyst 7 was employed (Scheme 7.5) [12], The same catalyst has also been employed in the preparation of cyclophanes [13]. 

A tungsten complex which possesses all three types of metal-carbon bonds, single (alkyl), double (alkylidene), and triple bonds (alkylidyne) has been prepared as shown in eq. (3) and structurally characterized [15]. 

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