Cobaltacyclopentadienes synthesis

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). 

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],... 

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. 

A number of transition metal complexes has been developed for the [2 + 2 + 2]-cycloaddition reaction, among which cobalt-based catalysts are most widely used. In 1973, Yamazaki and Wakatsuki reported the first synthesis of pyridines using stoichiometric and later on, catalytic cobaltacyclopentadienes. ... 

Accordingly, this chapter deals the synthesis and the reactions of polymers containing reactive organometallics such as cobaltacyclopentadiene and titana-cyclopentadiene units in the mainchain. That is, cobaltacyclopentadiene-containing polymers have been prepared by the metallacyclization process of a low-valent cobalt... 

Synthesis of Substituted Heterocycles Transition-metal-mediated or transition-metal-catalyzed co-cycloaddition of two alkynes and one nitrile is one of the simplest synthetic pathways to construct pyridine framework. However, there is a critical problem in selectivity in the intermolecular coupling of two different alkynes and a nitrile resulting from the reaction mechanism via metalacyclopentadi-ene [17]. For example, in Co-mediated pyridine formation, cobaltacyclopentadiene 37 was first prepared from two different alkynes by sequential addition because aza-cobaltacyclopentadiene could not be formed via selective coupling of one alkyne and a nitrile. A mixture of two pyridine regioisomers was obtained in the final step due to the existence of two possible orientations of the nitrile toward cobaltacyclopentadiene intermediate 37 [Scheme 11.15, Eq. (1)] [17b,c]. To control the... 

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. 

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