Cobaltacyclopentadiene 2-pyridones

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. 

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],... 

Yamazaki [51] has reviewed a number of stoichiometric cycloaddition reactions at the cobaltacyclopentadiene ring system that lead to a plethora of heterocycles. For example, five- and six-membered heterocycles containing nitrogen, sulfur, selenium, and phosphorus have been made accessible by this route (eq. (20)) [52]. The co-cyclization of substituted alkynes and isocyanates in the presence of a rhodium metallocycle gives 2-pyridones (eq. (21)) [53]. 

Cobaltacyclopentadiene polymers undergo thermal rearrangement to the more stable ( /" -cyclobutadiene)cobalt derivatives, and reaction with isocyanates affords new polymers containing 2-pyridone moieties in the polymer backbone (for details, see Chapter 4, Section 4,4.1) [98-100],... 

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].-... 

Chemical conversion of the organocobalt polymers (3) into functional polymers is attainable by the polymer reactions with appropriate reagents. Novel polymers having pyridone moieties in the mainchain (12) are produced from 3 by the reaction with isocyanates (scheme 8). This polymer reaction proceeds smoothly at 120°C, and the content to the 2-pyridone moieties reaches about 70% with respect to the starting cobaltacyclopentadiene units. In this case, the remaining 30% is not the starting cobaltacyclopentadiene unit but the r " -cyclobutadienecobalt moieties as a result of the rearrangement reaction. 

The metallacyclization of CpCo(PPh3)2 (140) with the diacetylene (144) resulted in the isolation of polymer (145), which was thermally rearranged to produce 146. It was established that the first step of the rearrangement was the loss of the triphenylphosphine ligands fi om cobalt. In the presence of isocyanate, the cobaltacyclopentadiene rings were converted to 2-pyridone rings as shown in Scheme 39. 

The synthetic methodology used by Tilley is similar to that used by Nishihara and Endo to prepare polymers witb cobaltacyclopentadiene groups in the back-bone. For example, Scheme 6 shows the reaction of 24 with 25 to produce the cobaltacyclopentadiene-containing polymer (26). Reaction of polymer 26 with an isocyanate (27) resulted in the formation of a 2-pyridone ring (28). 

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