Carbene complexes of tungsten

The first [3S+2C] cycloaddition reaction using a Fischer carbene complex was accomplished by Fischer et al. in 1973 when they reported the reaction of the pentacarbonyl(ethoxy)(phenylethynyl)carbene complex of tungsten and diazomethane to give a pyrazole derivative [45]. But it was 13 years later when Chan and Wulff demonstrated that in fact this was the first example of a 1,3-dipolar cycloaddition reaction [46,47a]. The introduction of a bulky trime-thylsilyl group on the diazomethane in order to prevent carbene-carbon olefi-nation leads to the corresponding pyrazole carbene complexes in better yields (Scheme 15). 

The imino carbene complexes of tungsten and chromium (e.g., 81) also serve as nitrile ylide synthons (40). The mngsten complexes gave higher yields of the adducts and were strongly regioselective for product 82. For example, for 81 (M=W R = Me, Ph R = Pr, Ph R" = H), the pyrrole 82 was produced in yields of 65-75% with <1% of 83. This route to pyrroles thus has clear advantages over... 

Dimidcai carbene complexes of tungsten have also been prepared and they have been used to catalyze the metathetical polymerization of alkenes and alkynes 143],... 

Electrophilic carbene complexes of tungsten or iron smoothly transfer their phenylcarbene ligand to electron rich olefins. Pentacarbonyl(phcny]carbcnc)tungsten(0) (4) is generated in situ by acid treatment of the precursor salt 36, while the cationic iron complex 5 is a stable, isolable compound7-10. 

The catalyst for this and many other ring-opening metathesis polymerizations is a carbene complex of tungsten (W). 

Ring-opening metathesis polymerization (Section 14.15) The double bonds of strained cyclic alkenes are cleaved by certain carbene complexes of tungsten and, in the process, undergo polymerization. 

Catalyst. Two types of catalysts were used in the present study. The one is the so-called Ziegler type catalyst composed of a tungsten, molybdenum or rhenium compound (A component) and an organometallic compound (B component). The other type of catalyst is composed of a carbonyl-carbene complex of tungsten and a Lewis acid(24). 

Carbene complexes of tungsten, molybdenum or rhenium initiate the ring-opening polymerization of cycloolefins to polyalkenamers. Thus, Ph2C= W(CO)5 (1) polymerizes cyclobutene, 1-methylcyclo-butene, cyclopentene, cycloheptene, cyclooctene and norbornene while Ph(MeO)C=W(CO)5 (2) polymerizes cyclobutene and norbornene to highly stereospecific polyalkenamers [28]. 

A colloidal palladium nanoparticle prepared from a Fischer carbene complex of tungsten with K2PdCl4 as the reductant and PEG as the capping agent, efficiently catalyzed the Hiyama cross-coupling reactions in air. ... 

Heteroatom-substituted (Fischer-type) carbene complexes are mostly used as stoichiometric reagents. For this reason only carbene complexes of reasonably cheap metals, such as chromium, molybdenum, tungsten, or iron have found broad application in organic synthesis. 

Cycloadditions of alkynyl carbenes. Alkynyl carbene complexes of chromium and tungsten undergo [2 + 2]cycloaddition at room temperature with a wide range of enol ethers. 

Similarly, thermolysis of cyclopropyl(5-phenyl-4-pentynoxy)carbene complexes of molybdenum and tungsten gave via intramolecular coupling the corresponding 3,4,5,7-tetrahydro-5-phenylcyclohepta[ ]pyran-6(2//)-one (equation inter-... 

When solutions of pentacarbonyl-alkoxy(organyl)carbene complexes of chromium, molybdenum or tungsten are treated at low temperatures with BCI3, BBr3 or BI3, the... 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

Like the chromium- and molybdenum-carbene complexes of the type illustrated in Scheme 7-3, the corresponding octahedral tungsten complexes exhibit only a one-electron oxidation, which is thought to be centered on both the ferrocenyl and the tungsten fragments. The redox potentials are summarized in Table 7-10. 

Chiral binuclear carbene complexes of the Fischer type were synthesized utilizing (/ ,/ )-l-(l-dimethylaminoethyl)-2-lithioferrocene (296). The lithiated precursor reacts with tungsten hexacarbonyl to give a bimetallic acylate, CpFe 7j -2-C5H3 C(Me)HNMe2 C(OLi)W(CO)5 (109t),... 

The reactions are stereospecific. The ratio of stereospecific cyclopropanes formed in the reactions of pentacarbonyl[phenyl(methoxy)carbene] complexes of chromium(O), mo-lybdenum(O) or tungsten(O) with methyl ( )-but-2-enoate, diethyl (Z)-but-2-enedioate and ethoxyethene depends on the type of transition metal, which is consistent with a carbenoid transition state. 

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