Cyclopentadienyl cobalt

In the context of pyridin synthesis from alkynes and nitriles homogeneously catalyzed by (cyclopentadienyl)cobalt complexes (Eq. 13), it was found that electron-withdrawing groups on the cyclopentadienyl ring significantly increase the activity [63]. During the screening of various cobalt half-sandwich complexes... 

Yong et al. developed a cobalt-catalyzed [2+2+2] cyclotrimerization of terminal alkynes in good yields in aqueous media (80/20 mixture of water and ethanol) at room temperature. A cyclopentadienyl cobalt complex bearing a pendant phosphine ligand was used as a catalyst (Eq. 4.59). The cyclotrimerization of internal alkynes resulted in lower yields and required an elevated temperature, most likely due to steric interactions. For example, cyclotrimerization of 2,5-dimethyl-3-hexyne gave hexaisopropylbenzene in 51% yield and the reaction of diphenylethyne resulted in a 47% yield of hexaphenylbenzene.112... 

Hydrocarbon Formation on Polymer-Supported -Cyclopentadienyl Cobalt... 

Weiss has also carried out the analogous insertion reaction with dicar-bonyl(cyclopentadienyl)cobalt(I), which yielded a mixture of products (see Section V.C)94. 

Commercial development of a range of cycloalkene-cobalt homogeneous catalysts has prompted their application in the synthesis of pyridine and 2-substituted pyridines. Thus, bis(cyclopentadienyl)cobalt catalyzes the reaction of acetylene with hydrogen cyanide, acetonitrile or acrylonitrile to yield pyridine, 2-methylpyridine and 2-vinylpyridine respectively (Scheme 4 R = H, Me or CH=CH2) (76S26, 78AG(E)505, 75BEP846350). The high cost of the catalyst has so far limited full commercial realization of this route. Acrylonitrile... 

C. (18-CROWN-6)POTASSIUM TRICYANO(CYCLOPENTADIENYL) COBALTATE(III), [K(18-CROWN-6)][CpCo(CN)3]... 

D. DODECACYANOTETRAKIS CYCLOPENTADIENYL) COBALT(III) TETRAKIS PENTAMETHYL CYCLOPENTADIENYLRHODIUM(III) TETRAKIS (HEXAFLUOROPHOSPHATE), [(CpCo(CN)3)4(Cp Rh)4](PF6)2... 

Intramolecular [2 + 2 + 2] cycloadditions belong to the same type of valence isomerization as those of 1,5-unsaturated compounds to cyclobutanes.70 The cobalt-mediated cyclization of l-en-5-ynes stereoselectively converts enediynes directly to bicyclo[4.2.0]hexa-l,3-dienes, as single diastereomers, when a stoichiometric amount of dicarbonyl(cyclopentadienyl)cobalt is used. This cyclization90 has a high efficiency (92%). An example is the synthesis of the protoilludane framework 23 from the enediyne precursor 22.71... 

In a similar manner, silacyclopentadienes react with cyclopentadienyl-cobalt dicarbonyl to give an - -complex (49, 138) ... 

Dicarbonyl( 75-cyclopentadienyl)cobalt may be synthesized from Co2(CO)8 (Ref. 5) or purchased from Strem or Pressure Chemicals. Diphenyl-acetylene (Farchan, commercial grade) can be used without purification. 

More recently, the parent mono- and di-methylenecyclopropane cyclopentadienyl-cobalt -complexes were prepared by ligand exchange reactions583. Reaction of methyl-enecyclopropane (and its tetramethyl derivative) with CpCo(ethylene)2 in pentane affords... 

Much recent work has been done on the synthesis of pyridines from alkynes and nitriles over cobalt catalysts. For example, 2-vtnylpyndine has heen obtained in good yield from acetylene and acrylonitrile using a cyclopentadienyl-cobalt catalyst. Pyridine has also been obtained from cyclopentadiene and ammonia over a sihca/alumina catalyst. 

C. Cyclopentadienyl Cobalt Complexes of OFCOT and the Perfluorobicyclo[3.3.0]octadienediyl Ligand... 

Dicarbomethoxynorbomene, 578 Dicarbonyl(cyclopentadienyl)cobalt, 161-162 Di-[x-carbonylhexacarbonyldicobalt, 162-163 Dichloroacetylene, 552 Dichlorobis(cyclopentadienyl)titanium, 163—... 

At the present time the preparation of the trifluoromethylated derivatives of low valent transition metals by ligand-exchange reactions appears to be quite general. However, as exemplified by the nickel reaction above, the utility of the method is obviously subject to the inherent stability of the desired product. In many cases, such as the preparation of the trifluoromethyl derivatives of the cyclopentadienyl cobalt system, (CF3)Co(Cp)(CO)I and (CF3)2Co(Cp)(CO), the reaction of the dihalide with (CF3)2Cd glyme represents the simplest reaction... 

The involvement of the a-elimination reaction in this cycle has been in question following experiments on cyclopentadienyl cobalt complexes, where evidence for olefin insertion for Ziegler-Natta polymerization catalysis has been obtained by labelling experiments, using C2H4 with a deuterated cobalt complex (70) ... 

Cyclobutadiene(cyclopentadienyl)cobalt has been prepared by Amiet and Pettit (4) from dichlorocyclobutene and sodium tetracarbonyl cobaltate, while Rosenblum and North (177) prepared it from photo-a-pyrene and C5H5Co(CO)2. The mass spectrum shows that fragmentation of the cyclobutadiene ring occurs before loss of the cyclopentadienyl ring, and the major ions in the spectrum correspond to M+ m/e 176, [M — C2H2]+ m/e 150, and [M — C4H4]+ m/e 124 (4, 177). Metastable peaks have been reported by Rosenblum and North for the transitions 176 150,... 

Cyclopentadienyl cobalt and rhodium complexes with 1,2-benzenedithiolate or S2C2R2 type ligands readily add unsaturated carbon molecules to form tridentate ligands exhibiting S2C donor sets. When nitrenes are added, dithio-lene derived ligands with S2N donor sets result. Scheme 3 shows examples of... 

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