Cobalt, complexes cyclopentadienone

Stable, isolable metallacycles are also obtained from reaction of complexes that serve as sources of the CpCo fragment (e.g. CpCo(PPh3)2) and alkynes. Upon carbonylation diese typically give high yields of cobalt-complexed cyclopentadienones. Direct reaction of CpCo(CO)2 with alkynes is similarly useful. The cycloaddition of di(t-butoxy)acetylene upon photolysis with CpCo(CO)2 is an example (Scheme 5). In all these systems the final complexes lack coordinated CO, and therefore amine oxides are not suitable reagents for liberating the stable cyclopentadienones. Tetra(t-butoxy)cyclopentadienone is accessible on a preparative scale via controlled electrochemical oxidation. Other oxidants such as Cr have been used as well in other systems. 

C. Possible Intermediates in the Formation of Tr-Cyclopentadienone-and TT-Cyclobutadiene-cobalt Complexes... 

The polarographic reduction of i/4-diene complexes was first noted for [M(i/4-l,4-quinone)Cp] (M = Rh or Ir, quinone = duroquinone or 2,6-di(t-butyl)quinone), which show one two-electron wave (M = Ir), or two one-electron waves (M = Rh) at potentials more negative than those of the free quinone (575). The cyclopentadienone compounds [M(i/4-C5R40)Cp] (M = CoorRh, R = Ph or C6F5) undergo reversible one-electron reduction to radical anions (e.g., M = Co, R = Ph, E° = — 1.46 V in thf). The cobalt complexes, more stable than those of rhodium, are generated by alkali metal reduction at 100 K and show ESR spectra characteristic of metal-based d9 species (374). 

Related superphane molecules 279 and 280 having cyclopentadienone rings stabilized by CoCp units have been obtained in analogous reactions in the presence of CO-containing cobalt complexes (Scheme 47) 84-386... 

A number of examples exist involving reactions of CpCo(CO)2 and silylated alkynes. Cycloaddition of MesSiC CH gives a 3 1 mixture of 2,5- and 2,4-bis(trimethylsilyl)cyclopentadienone complexes. The use instead of [C5Me5]Co(CO)2 reverses the regioselectivity to 1 3.7, presumably due to the increased steric crowding about cobalt (equation 9). This effect has yet to be exploited in synthesis. 

Other aromatic transition metal complexes for which electronic spectra and assignments have been reported are (CsHs TiXb [X=C1, Br, I] 96), (C6He)2Cr 59), [(C6H6)2Cr]+I- 3io.42i.526), (C5H5)2Ni 17,422), and tetramethyl- and tetraphenyl-cyclopentadienone cobalt cyclopenta-... 

A problem is that the Pauson-Khand reaction uses two equivalents of cobalt. More efficient versions, many of them catalytic, using other metals have been developed. These include carbonyl complexes of titanium, molybdenum, tungsten (Scheme 7.15), rhodium and ruthenium (Scheme 7.16). Rhodium, iridium and iron (Scheme 7.17) have also been used with two alkynes to give cyclopentadienones, often as complexes 7.59. A version of the Pauson-Khand reaction employing a nickel catalyst and an isonitrile in place of CO has been developed. The product is an imine, which can be hydrolysed to a cyclopentenone. 

Interactions of alkynes with CpCo(CO)2 led to cyclobutadiene or cyclopentadienone complexes or benzene derivatives. Despite the fact that the mono-adduct of cobalt is most likely to be the first intermediate to exist in this alkyne di-or trimerization, such an intermediate has only seldom been isolated. The use of an alkyne substituted by strong electron-withdrawing groups such as BUSO2- allowed Gleiter to isolate this key compound 220 (Figure 50). ... 

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