Cyclopentadienylcobalt complex

Polymercuration of (cyclobutadiene)cyclopentadienylcobalt complexes has also been reported. Upon reaction with Hg(OAc)2, complexes 88a-c afford a mixture of products containing one to five mercury atoms. Higher yields of the pentamercurials 89a-c were obtained when Hg(OCOCF3)2 was employed (Equation (34)). 

We have mentioned that the structural parameters of C2H4 bridged compounds can vary over a wide range. Whereas most examples reported do not have metal-metal bonds, there is one conspicuous exception. Theopold and Bergman succeeded in synthesizing the propane-1,3-d iyl cobalt derivative 125 from the radical anion [(t) ,-C5H5)Co(/z-CO)12 and 1,3-dibromopropane (98, 295) in 40 5 yield. This compound is best described as a dimetallacyclopentane, and its chemistry (thermolysis and reaction with CO and phosphines Scheme 34) supports this view. Formation of cyclopropane (100°C or I2/25°C) is probably the most remarkable feature of this cyclic system. Simple C—C bond formation has never been observed before in ligand-induced or thermal reactions of either mono- or binuclear cyclopentadienylcobalt complexes. The architectural details of... 

Two or sometimes three isomers of the rr-cyclopentadienone-TT-cyclopentadienylcobalt complexes have been isolated from reactions in xylene at 160°C of (w-C5H5)Co(CO)2 and the terminal alkynes, MeC=CH, CF3C=CH, tcrt-BuC=CH, and PhC CH 166). In these... 

The known complexes of formula (7r-C5H5)Co(cyclopentadienone) and (7T-C5H5)Co(cyclobutadiene), are listed in Table XIII. Tr-Cyclopenta-dienone- and 7r-cyclobutadiene- r-cyclopentadienylcobalt complexes have been formed in a variety of other reactions that do not involve alkynes. These reactions are summarized in Eqs. (32)-(37). 

Fritch, J. R., Vollhardt, K. P. C. Cyclobutadiene-metal complexes as potential intermediates of alkyne metathesis flash thermolysis of substituted 4-cyclobutadienyl- 5-cyclopentadienylcobalt complexes. Angew. Chem. 1979,91,439-440. 

F NMR parameters for a series of cyclopentadienylcobalt complexes containing C2F5 and CF(CF3)2 groups are shown in Table IX and with structures [159 and 160]. (83)... 

As phosphaalkynes are known to react similarly to alkynes, 338 and 339 were treated with the most easily available representative of this class of compounds, tert-butylphosphaethyne, under the reaction conditions applied for alkynes. Treatment of 338 in the presence of sodium amalgam at low temperature (—50 — —30 °C) with a 3-fold molar excess of tert-butylphosphaethyne gives unchelated diphosph-ete complex 350, which is in complete accord with the reaction of other cyclopentadienylcobalt complexes with tert-butylphosphaethyne (Scheme 64). 

The first step is dissociation of L(Ph3P), as addition of more triphenylphos-phine inhibits the catalysis. Other cyclopentadienylcobalt complexes CpCoL are also effective catalysts, (with = 1, 5-cyclooctadiene, 2C H or, less efficiently, 2CO). By replacing RC=CR with a nitrile RCN in the final step (i.e. by using a mixture of nitrile and alkyne), pyridines can be synthesized catalytically, although some benzene derivative is always obtained as well. These processes have been developed industrially for making pyridines and in the laboratory for effecting cycloadditions in natural product synthesis ... 

The cyclopentadienylcobalt complex 159 was shown to bear the CpCo moiety on the exo surface of the ligand system as shown by normalized one-dimensional... 

Among transition-metal complex catalyzed reactions of alkynes with carbon-heteroatom unsaturated compounds the most studied is co-cyclotrimer-ization of alkynes with nitriles to pyridines. For this process the same complexes can be used as for the cyclotrimerization of alkynes. The first report of a cyclopentadienylcobalt complex catalyzed co-cyclotrimerization of alkynes with nitriles appeared in 1973 [92] and was soon followed by other papers [93]. Co-cyclotrimerization of alkynes and nitriles with all its aspects has been recently reviewed [94] and because of that we will focus only on recent developments in this area. In this regard, advances have been made in simple co-cy-clotrimerization of ethyne with various nitriles [95], combinatorial synthesis of substituted pyridines [96], and co-cyclotrimerization of hydroxyalkynes with nitriles in aqueous media catalyzed by cobalt complex with hydrophobic chain attached to the cyclopentadienyl ring [97]. 

Cyclopentadienylcobalt complexes are also good for co-cyclotrimerization of alkynes with other unsaturated compounds containing the carbon-heteroatom double bonds, especially when they are part of the cumulene system such as isocyanates, diimides, and carbon dioxide. The reaction conditions are essentially the same as in the previously mentioned processes. However, the biggest problem remains the selectivity for the formation of heterocycles, because of the strong competition for the formation of benzene derivatives. Whereas co-cyclotrimerization of diimides and isocyanates results in the formation of reasonable yields of the corresponding heterocycles 170 and 171 (Scheme 75), in the case of carbon dioxide the yields are generally low [108, 109]. Recently, it has been shown that the ruthenium complex 106 is capable of efficient catalysis of co-cyclotrimerization of diynes and isocyanates [110] and isothiocyanates [111] under mild reaction conditions. 

The cyclopentadienylcobalt complexes CpCo(L)n are also good catalysts for co-cyclotrimerization of diynes with alkenes [79a]. Recently, it has been shown that the similar catalytic activity is exhibited by the ruthenium complex 106, which efficiently catalyzed cycloaddition of diynes with cyclic alkenes to the conjugated cyclohexadienes 174 (Scheme 77) [112]. 

---