Cobaltacyclopentadiene complexes

A direct synthesis of the thiophene nucleus has been achieved by allowing air-stable cobaltacyclopentadiene complexes (66) to react with sulfur the organometallic complexes are prepared in variable yields in a stepwise fashion from f75-cyclopentadienyIbis(triphenylphosphine)cobalt (65) (Scheme 82) 147,148 Reactions of the complexes 66 with selenium and nitrosobenzene give rise to selenophenes and pyrroles, respectively. 

Some properties of cobaltacyclopentadiene complexes, prepared in this way, are listed in Table II. The crystals are dark brown to orange brown in color and air stable. They can be stored in air. They are soluble in aromatic and polar organic solvents such as chloroform and THF, but not in aliphatic hydrocarbons. Their solutions are moderately stable to air. In general, these cobaltacyclopentadiene complexes are more stable when they contain more electronwithdrawing substituents. 

An important and extensively investigated application of CpCo complex chemistry is the catalyzed synthesis of pyridines from alkynes and nitriles. Dissociation of the L ligands liberates the CpCo fragment to react with alkyne to form a cobaltacyclopentadiene complex. The nitrile coordinates to this intermediate and subsequently undergoes insertion to form a seven-membered cobaltacycle. Reductive coupling gives pyridine and reforms CpCo. Asymmetric alkynes add to the carbon with the bulkier substituent adjacent to the nitrile carbon. 

The cobaltacyclopentadiene complex reacts with a variety of compounds containing multiple bonds to give cyclic compounds (Scheme 11.3) [21]. In some cases, the organic products remain coordinated to Co they can be removed by oxidation with Ce or Fe. ... 

The cobaltacyclopentadiene complex (58), which is easily obtained by reaction of 77 -cyclopentadienylbistriphenylphosphinecobalt with two moles of acetylene, reacts with isocyanates to give 2-oxo-l,2-dihydropyridines (59) in about 70%... 

Ph-C=C-A-C=C-Ph (A = / -phenylene, 4,4 -bipheny-lene, /J-C6H4CH2CH2-P-C6H4) [140]. This reaction gives polymers 68a-c, with = 9600, 5100 and 4200, respectively. These polymers can be used for chemical transformation based on the established chemistry of cobaltacyclopentadiene complexes [154]. Pyrolysis of 68a in THF in a sealed tube at 110°C gives a product with cyclobutadiene complex units, 69 [155] and reaction of isocyanates a product containing 2-pyridone units, 70 [156].-... 

Cobalt, Rhodium, and Iridium. —Cations of structure [Co(diene)(PRs)3]+ have been prepared (diene = butadiene or isoprene) they are fluxional in solution, but low temperature n.m.r. spectra indicate a square pyramidal structure with diene in the basal plane. The benzoquinone complex (87) has been prepared and undergoes successive mono- and di-protonation at oxygen methylation with Mel also occurs at oxygen to yield (88). Cobaltacyclopentadiene complexes Cp(PPh3)Co(CRi=CR CR =CR ) (RSR =Me, Ph, or CO Me) react with... 

Wakatsuki, Y., Kuramitsu, T. and Yamazaki, H. (1974) Cobaltacyclopentadiene complexes as starting materials in the synthesis of substituted benzenes, cyclo-hexadienes, thiophenes, selenophenes and pyrroles. Tetrahedron Letters, 15(51), 4549-4552. 

Finally, it should be mentioned that rearrangement of the cp-cobaltacyclopentadiene intermediate to the thermodynamically more stable [(T7 -cp)Co(i7 -cyclobutadiene)] complex (which is catalytically inactive) is a thermally forbidden process [Eq.(47)]. 

When disubstituted acetylenes are employed, the >/2-acetylene complex, [Co(i/5-C5H5)(PPh3)(RC=CR)], can be isolated. In such cases, one can construct a cobaltacyclopentadiene with two different acetylene units by addition of other acetylenes, or one can synthesize cobaltacyclopentene complexes6 by addition of olefins. 

Complex 294 with methyllithium in TFIF gives the lithium salt 295 and then, via the cobaltacyclic intermediate 296, is converted into the complex 297, or with trimethylphosphine to the cobaltacyclopentadiene 298 <2004JOC2516>. 

The main reaction modes and products that are formed from reaction of CpCoL2 and an alkyne are depicted in Scheme 27. If CpCo(PPh3)2 is treated with polar alkynes under ambient conditions, a monosubstitution product (27) can be isolated. Another isolable product is a cobaltacyclopentadiene (28) and, in certain instances, the dinuclear cobaltole complex (29) is formed, which has many analogs in the chemistry of Fe(CO)3 and other 14-electron fragments. The cyclobutadiene complex (30) is the direct product from collapse of (28) and is of high kinetic and thermodynamic stability. In catalytic cycles, (30) is inactive. If the ligand L in (28) is a third molecule of alkyne, insertion of the latter forms the arene (31). 

Supporting evidence for the above mechanistic patterns in the majority of metal systems was first established in elegant isotopic labeling studies, which showed clearly that no intermediate with the symmetry of a cyclobutadiene was involved.In one system, the reaction of a cobaltacyclopentadiene with MeC>2CC CC02Me, benzene formation does not involve direct complexation of the third alkyne to the metal. It has therefore been suggested that the conventional insertion process has been here replaced by a direct Diels-Alder reaction with the metallacycle, perhaps as a result of electronic factors (Scheme 25). o ... 

The 18-electron complex CpCo(CO)2 must lose at least one CO ligand before it is able to coordinate to PhC=CPh, another two-electron donor. One can draw two resonance structures for this complex, one of which is a cobaltacyclopropene, with Co(III). Loss of a second CO ligand (before or after coordination of PhC=CPh) and coordination of another equivalent of PhC=CPh is followed by an insertion reaction to give a cobaltacyclopentadiene. Coordination and insertion of another equivalent of PhC=CPh gives a cobaltacycloheptatriene, and reductive elimination gives the product and the 14-electron complex CpCo(I), which recoordinates PhC=CPh to reenter the catalytic cycle. 

The chemistry of monocyclopentadienyl complexes of cobalt has been extensiveb studied. Complexes of the type CoCpL. (L = PPh, CO, cod) are capable of catalyzing the cyclotrimerization of alkynes [19]. Tn this reaction, an q -acetylene complex formed first and the reaction with the second alkyne gives a cobaltacyclopentadiene... 

Figure 9.9 Cyclic voltammogram of a cobaltacyclopentadiene polymer complex, 30, at a glassy carbon electrode in 0.1 M NBU4CIO4-CH2CI2 at a scan rate of 0.1 Vs-1.

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