Cobaltacyclopentadienes

Cobalt, cyclopentadienyl(ij -tolane)-(triphenylphosphine)-reaction with phenyl isocyanide, 1, 666 Cobaltacycles selenophenes from, 4, 968 Cobaltacyclopentadienes reactions, I, 671 Cobester... 

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. 

A closely related route to a monocyclic dithiole derivative is shown in Scheme 122.186 The easy replacement of cobalt by sulfur in this type of process has analogy in the synthesis of a variety of condensed heterocycles from rhodacyclopentadienes (see Scheme 127 in Section IV,H,1) and cobaltacyclopentadienes (see Scheme 82 in Section IV,C,1). 

Direct pyridine syntheses by the cocyclooligomerization of one molecule of a nitrile and 2 molecules of an acetylene can be achieved catalytically by cobalt(I) species prepared in situ198 or by the cobaltacyclopentadiene derivative 109. The latter compound is a good catalyst but can be trouble-... 

Heteroatom transfer in metallacyclopentadienes was first developed in the context of cobalt chemistry in the mid-1970s [27]. Cobaltacyclopentadienes were converted into various five-membered heterocyclic compounds such as pyrrole and thiophene, and into six-mem-bered heterocyclic compounds such as pyridine and pyridone derivatives. In the case of zirconacydopentadienes, the heteroatom compound must bear at least two halide substituents, since the Cp2Zr moiety is re-converted to the stable Cp2ZrX2. Indeed, this is the driving force behind the heteroatom transfer of zirconacydopentadienes. 

Further cycloadditions at the cobaltacyclopentadiene ring that lead to heterocycles have been comprehensively reviewed by Yamazaki (81 Mil 87MI8) [Eq.(38)l,... 

According to Vollhardt (82CC953), the cp-cobaltacyclopentadiene intermediate has a greater affinity for nitriles than for alkynes, and as a result, the reaction proceeds preferentially to give pyridine rather than benzene. 

Wakatsuki et al. (83JA1907) studied the transformation of cpCo(bisal-kyne) into cp-cobaltacyclopentadiene by ab initio self-consistent field-molecular orbital (SCF-MO) calculations. For the cpCo(bisalkyne), they... 

Fig. 4. The reaction of a nitrile with the cobaltacyclopentadiene intermediate a, Diels-Alder addition b, insertion into the Co—C bond.

In some cases, it is possible to promote the Diels-Alder-type reaction. We have found that in the presence of polar auxiliary ligands (stabilizer), the Co-mediated reaction of alkynes with activated C,C or C,0 double bonds takes place (87MI6). These findings are only understandable if there is a direct interaction between the cp-cobaltacyclopentadiene and the polar stabilizer, which may act by blocking free coordination sites at the Co [Eq.(45)]. 

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. 

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. 

For general properties of cobaltacyclopentadiene, see Section D. Yield, melting point, and NMR data of the cobaltacyclopentadienes, prepared from two different acetylenes, are listed in Table III. The regioselectivity of substituents has been explained by their bulkiness.12... 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

Although some degree of regioselectivity is observed in the formation of cobaltacyclopentadienes 112 from unsymmetrically substituted alkynes, the reaction has little synthetic interest because the cyclization of terminal alkynes and unsymmetric internal alkynes gives a mixture of all possible regioisomers of... 

Control of the regioselectivity in Co-catalysed cross-cyclization has been solved in an ingenious way utilizing 1,5-hexadiyne (116) as one component and bis(trimethyl-silyl)acetylene (118) as the other. Although bulky bis(trimethylsilyl)acetylene (118) itself cannot cyclotrimerize to hexasilylbenzene due to steric hindrance, it reacts with the cobaltacyclopentadiene 117 formed from the less bulky diyne 116 to produce the... 

Pyridones are prepared by insertion at the N=C bond of isocyanate using cobaltacyclopentadiene as a catalyst [74], The reaction was applied to the synthesis of the intermediate 182 of camptotecin from the isocyanate 180 and the alkyne 181 [75], Insertion at the C=0 bond of aldehydes also occurs. The Ni(0)-catalysed reaction of 3,9-dodecadiyne (183) with butanal affords the a-pyran 184 [76],... 

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>. 

Estrones. An A —> BCD approach to estrones involves a cobalt catalyzed intramolecular 2 + 2 -y 2] cycloaddition of the enediyne 1 to form the B, C. and D rings in one step. The high stereoselectivity suggests that the cyclization involves a Dicls-Alder-type reaction of the vinyl group with a cobaltacyclopentadiene formed by coupling of the two alkyne units. The homoannular diene obtained on demetalation is isomcrized easily to the diene 3. 

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). 

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. 

More recently, Yasufuku et al. obtained >/ -coordinated phosphole oxides by reaction of a cobaltacyclopentadiene ring with phosphites ... 

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 ... 

Cobaltacyclopentadienes react with CS2 to form thiopyran-2-thiones, and with methyl isothiocyanate to form pyridine-2-thiones, respectively, in two little-explored analogs of the processes described above. 

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