[32993-07-0] · C41H35CoP2 · Cyclopentadienylbis(triphenylphosphine)cobalt(I) · (MW 648.62)
(alkyne cyclotrimerization catalyst; co-cyclotrimerization catalyst for alkynes and nitriles, alkenes and heterocumulenes; reagent for synthesis of substituted thiophenes, selenophenes, and pyrroles)
Physical Data: mp 140-143 °C (benzene adduct).2a
Solubility: sol benzene, THF, toluene; insol hexane.
Form Supplied in: dark red, air-sensitive crystalline solid.
Preparative Method: can be prepared by addition of NaCp to Chlorotris(triphenylphosphine)cobalt in THF.2c
Purification: recrystallization from benzene/hexane.2c
Handling, Storage, and Precautions: store under an inert atmosphere.
The cyclotrimerization of alkynes and co-cyclotrimerization of alkynes with alkenes, nitriles, and heterocumulenes discussed in this section employ cyclopentadienyl(triphenylphosphine)cobaltacyclopentadiene complexes (2), either as stoichiometric reagents or as catalysts. They are obtained from the reaction of cyclopentadienylbis(triphenylphosphine)cobalt(I) (1) with 2 equiv of an alkyne (eq 1; Table 1). Either symmetrical or unsymmetrical alkynes may be employed but unsymmetrical alkynes often give modest regioselectivity.2c,2d The regioisomers are readily separated by chromatography.
Significantly, two different alkynes may be incorporated into the metallacycle by employing a stepwise addition of alkyne to the cobalt complex (eq 2; Table 2).2d Here too, regioselectivity is often modest with unsymmetrical alkynes.
The reaction will tolerate a variety of alkyne substituents including alkyl, aryl, alkoxycarbonyl, and chloro groups, but the presence of cyano or 2-pyridyl substituents leads to the formation of byproducts.2e,2f The use of diynes in this reaction in place of monoalkynes gives bicyclic cobaltacyclopentadiene complexes. The yields of the latter reaction depend upon the tether length as well as the presence of substituents on the alkyne or on the tether.2g,2h The cobaltacyclopentadiene complexes are generally air-stable materials which will react with a variety of unsaturated organic substrates in stoichiometric or catalytic reactions.
Substituted benzenes such as hexamethylbenzene can be prepared catalytically by treating an excess of the alkyne with the appropriate cobaltacyclopentadiene complex.3a The stoichiometric reaction of cobaltacyclopentadiene complexes with alkynes gives polysubstituted benzenes (eq 3; Table 3).2c,2g,3b,3c
Substituted pyridines have been prepared in similar manner by the stoichiometric reaction of cobaltacyclopentadiene complexes with nitriles.2h,4a More importantly, cobaltacyclopentadiene complexes will catalytically form pyridines from 2 equiv of an alkyne and 1 equiv of a nitrile.4b For example, acrylonitrile reacts with acetylene in the presence of cyclopentadienyl(triphenylphosphine)cobalta(tetraphenylcyclopentadiene) to form 2-vinylpyridine (eq 4). This catalyst may also be used to prepare substituted pyridines from substituted alkynes in good yield, but the use of unsymmetrical alkynes leads to mixtures of isomeric pyridines. Use of this catalyst, however, has been supplanted by more conveniently prepared catalysts.4c,4d
Likewise, the reaction of cobaltacyclopentadienes with heterocumulenes gives heterocycles (eq 5; Table 4).4a,5a,5b The reaction of alkynes with carbon disulfide, isothiocyanates, isocyanates, or carbodiimides using cobaltacyclopentadiene complexes affords dithiopyrones, thiopyridones, pyridones, or iminopyridones, respectively, in good yield. Pyridones and iminopyridones could also be prepared with catalytic cyclopentadienyl(triphenylphosphine)cobalta(tetraphenylcyclopentadiene), but unsymmetrical alkynes give mixtures of isomeric heterocycles.5b
Thiophenes and selenophenes have also been prepared by replacing the cobalt atom of the metallacycle with either sulfur or selenium.2c,3b Pyrroles may also be prepared using nitroso compounds, but the yields are low.3b
1,3-Cyclohexadienes or 1,3-cyclohexadiene-cobalt complexes (which render cyclohexadienes in good yield upon treatment with CeIV) are obtained in fair to good yield by reaction of the cobaltacyclopentadiene complexes with alkenes.2h,3b,6a Cis-alkenes give mainly the endo isomers similar to the Diels-Alder reaction. The catalytic cooligomerization of alkynes and alkenes gives either cyclic or linear products incorporating varying proportions of alkyne and alkene.6b,6c
Thomas E. Snead
Emory University, Atlanta, GA, USA