Tungsten complexes vinylidenes

Etherification using a metal vinylidene has also been combined with G-G bond formation through the reaction of an alkynyl tungsten complex with benzaldehyde (Scheme 14). The addition of an internal alcohol to the incipient /3,/Udialkylvinylidene that is generated leads to dehydration and the formation of a Fischer-type alkylidene complex. Further reactions of this carbene with a range of nucleophiles have provided access to various furan derivatives.374,375... 

Mechanistic studies (108) have suggested that the formation of the binu-clear complex (71) on acidification of the product of the reaction between methyllithium and W(CO)s[C(OEt)Me] proceeds via the intermediate formation of dimethylcarbene- and vinylidene-tungsten complexes. These then cycloadd and rearrange as shown ... 

Tungsten(O) pentacarbonyl-methylene chloride complex and alkynes form vinylidene complexes by photo-irradiation, which react with an imine or a dialkylcarbodimide to afford /3-lactams after decomplexation of the metal.233... 

In 1979, Rudler ef al. reported another example of the presence of a vinylidene complex during the reaction of pentacarbonyl[methoxy(methyl)carbene] tungsten 5 with MeLi followed by acidification with TFA [4]. It was proposed that the vinylidene complex 7 was generated by deprotonation of the a-proton of the carbene complex followed by elimination of methoxide and reaction with the dimethylcarbene complex 8, the addition-elimination product of MeLi with the starting carbene complex, to give the dinuclear complex 6 (Scheme 5.2). 

Furthermore, Fischer rendered this chemistry more practical by generating vinylidene complexes of pentacarbonylchromium and tungsten directly in situ from terminal alkynes [9]. For example, treatment ofterminal alkynes with WCO)5(CH2Cl2), generated by photolysis of W(CO)6 in CH2CI2, gave thermo-labile tt-alkyne W(CO)5... 

An important contribution that developed into the catalytic use of the vinylidene complexes for the construction of carbon frameworks was reported by two research groups independently for the preparation of Fischer-type carbene complexes by the reaction of terminal alkynes with pentacarbonylchromium or tungsten species in the presence of oxygen nucleophiles. 

In the latter case, the red anionic vinylidene complex is formed at — 78°C quenching with DzO gives a 1 1 mixture of the mono- and dideuterocar-byne complexes, as a result of a primary kinetic isotope effect by which the anion selectively abstracts a proton from the monodeuterocarbyne complex before quenching is complete. Similar observations were made in the tungsten series (55). 

Catalysis by alkali metal ions has recently been reported as an alternative route. In an argon matrix, acetylene forms a n complex with the metal. On irradiation, it isomerizes to the vinylidene form, M C=CH2. When complexed with metals, vinylidene is much more stable, in the same way that metal carbenoids are generally much more stable than carbenes, and rearrangement of a tungsten alkyne complex to a tungsten vinylidene complex has been reported. ... 

The tungsten carbene complex [(CO)5W=CHAr] (Ar = Ph, p-tol) reacts with 2-butyne to give stilbenes and [(CO)5W(MeC=CMe)]. Terminal alkynes, such as Bu C=CH, are however polymerized. Vinylidene complexes are proposed as intermediates since the acetylene-vinylidene rearrangement has ample precedence in stoichiometric reactions (Scheme 26). 

The above shown polymer photocrosslinks when irradiated in the presence of small amounts of W(CO)6 using a light beam at 320-390 nm. UV photolysis of W(CO)6 in the presence of an alkyne results in the formation of a ri -alkyne tungsten pentacarbonyl complex, [(r -RC=CR )W(CO)5], that rearranges to a vinylidene derivative [R(H)C=C=W(CO)5] and is an intermediate in the initiation step in the polymerization of alkynes. 

When trans-halo-tetracarbonylcarbyne complexes of molybdenum and tungsten, X(CO)4M=CR, are treated with sodium cyclopentadienyl, not only the displacement of the halide, but an additional elimination of two carbonyl ligands is observed, affording dicarbonyl(rj -cyclopenta-dienyl)carbyne complexes [2,3]. A different synthetic approach converts vinylidene ligands into carbyne ligands to yield comparable bisdimethylphosphite substituted complexes of molybdenum [4]. 

For example, the cycloaromatization of tungsten-vinylidene complexes, generated from 2-ethynylstyienes, proceeded to give the corresponding substituted naphthalenes (Scheme 21.53) [60]. 

Tungsten carbyne complexes are synthesised from alkynide complexes by double addition of electrophiles. Thus addition of FS03Me or Et30 to [W(CCBu )C0)5] leads to the vinylidene W(=C=CButR)(C0)5 (R = Me or Et). Addition of CF3S03H/Me4NI to this vinylidene or directly to the alkynide allows the high yield isolation of t ran -WI (CO ) [ C (CHBu R ) ]. ... 

Once generated, one way to trap a vinylidene complex could be in an electrocyclic reaction. Indeed, treatment of an alkyne 8.143, having a dienyl unit attached, with a ruthenium catalyst yields a tricyclic product 8.144 (Scheme 8.40). Deuterium-labeling experiments are consistent with the alkyne-vinylidene isomerization. A tungsten catalyst, W(CO)5.THF, may also be used. ... 

Another class of transition metal complexes studied by our group consists of alkyne and vinylidene complexes of tungsten and molybdenum in high oxidation states. Alkyne complexes are interesting from a theoretical point of view because they may be considered to be either side-on coordinated ligand complexes or metallacyclopropenes. They are also important as possible intermediates in the cyclooligomerization and polymerization of alkynes. o " Scheme 3 shows the two types of alkyne complex that were studied. 

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