Tungsten, terminal alkyne

The symmetrical dienyne 58a was converted to a fused bicyclo [4.3.0] ring in 95% yield [17] (Eq. 27). With substrate 58c containing an unsymmetrical diene tether, two different products, 59c and 59c, were obtained in a ratio of 1 to 1 (Eq. 28). The reaction course in the formation of the different bicyclic rings is shown in Scheme 8. This dienyne metathesis is also catalyzed by tungsten or molybdenum complex 62 or 63 (Fig. 1), and a dienyne bearing terminal alkyne 58b could be cyclized to give 59b in 97% yield. 

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. 

Although classic ternary systems based on molybdenum, tungsten and rhodium have been widely used for these purposes [25-52], terminal alkynes are still best polymerized by well-defined Schrock-type metathesis initiators in... 

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 polymerization kinetics of terminal alkynes have been determined. The reaction is initiated by tungsten carbene complexes (10) the chelating olefin is displaced by acetylene as the first step in the reaction, followed by formation of... 

F., and Mortreux, A. (2007) Selective terminal alkyne metathesis synthesis and use of a unique triple bonded dinuclear tungsten alkoxy complex containing a hemilabile ligand. Adv. Synth. Catal, 349, 2259-2263. 

The NMR characterisation of fBA adducts formed in the reaction of tungsten and molybdenum seven-coordinate compounds leaves little doubt concerning their formulation. However, we were unable to separate such compounds and to prepare a crystalline sample. To obtain a more stable terminal alkyne compound, we introduced two phosphine ligands into the coordination sphere of tungsten(II). In this way a cationic complex of tungsten(II), [WCl(CO)( BuC=CH)(NCMe)(PPh3)2]2[SnCl6], was prepared [45]. The crystal structure of this compound consists of two independent cations and a... 

The seven-coordinate compounds react rapidly at room temperature with an excess of terminal alkyne, such as PA or r-BA, to give an unsaturated polymer in high yield. All binuclear compounds of tungsten(II) polymerise PA at comparable rates that are qualitatively very fast (Table 1). 

It is now apparent that seven-coordinate tungsten(II) and molybdenum(II) complexes of the type [(CO)4M( Li-Cl)3M(M Cl3)(CO)3] and [MCl(M Cl3)(CO)3(NCMe)2] (M = W, Mo M = Sn, Ge) can be synthesised and isolated in crystalline forms. These complexes readily react with alkenes and alkynes in mild conditions to give crystalline products. In the presence of an excess of terminal alkyne or cyclic olefins such compounds are catalysts of the polymerisation reaction. 

Katz and Sivavec, who used a Fischer tungsten carbene complex as a catalyst. After the report of a chromium-catalyzed reaction by Mori et al., they disclosed the utility of ruthenium-carbene complexes for the enyne metathesis (Scheme 24.44). The Ru-carbene-catalyzed reaction under the ethylene atmosphere brought about good results in the case of terminal alkyne substrates such as 164b. ... 

When alkynes are treated with catalytic amounts of a carbene complex, polymerization instead of metathesis can occur (Figure 3.44) [565,595,597,752-754]. The use of carbene complexes to catalyze alkyne polymerization enables much better control of the reaction than with heterogeneous or multi-component catalysts. Pure acetylene oligomers (n = 3-9) with terminal fcrf-butyl groups have been prepared with the aid of a tungsten carbene complex [755]. 

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