Ziegler-Hafner synthesis

A prototype for the new cyclization reaction is the Ziegler-Hafner synthesis of azulene (J)4 by heating the fulvenoid decapentaene (/). Woodward28 was the first to argue that a thermal, electrocyclic [rr 10 s] ring-closure could be involved in this thermolytic process. Of course it seems likely that the nonisolable intermediate (2) may be formed by an intramolecular electrophilic attack of atom 10 to the electron-rich position 1 in compound (1). Indeed a distinction between the two different reaction modes is mechanistically irrelevant in this case. But only its interpretation as an electrocyclic process allows the generalization to [ir 6 s] cyclizations too. 

A. Ziegler-Hafner s Synthesis as a Prototype for an Electrocyclic Ring-... 

Pyridinium ions with acceptor substituents, for example, N-(2,4-dinitrophenyl) pyridinium salts 231 (cf p. 356), are converted to pentamethine cyanines 232 (Konig s salts) by ring-opening with secondary amines. Cyanines 232 condense with Na-cyclopentadienide to yield vinylogous aminofulvenes 233, which on cyclization afford azulene (Ziegler/Hafner azulene synthesis, cf p. 304) [137] ... 

Fulvene derivatives can serve as building blocks for construction of the polycyclic conjugated systems. Polycyclic conjugated systems involving a fulvene moiety such as azulene 4, pentalene 5, and indacene 6 have also received much attention from the viewpoint of aromaticity (Figure 6.2), and their synthesis has been developed on the basis of fulvene chemistry. In the well-known Ziegler-Hafner azulene synthesis, a fulvene derivative 7 is employed as an important intermediate (Scheme 6.1) [2]. 

Ring fission of pyridinium ions with acceptor substituents, e.g. 7V-2,4-dinitrophenylpyridinium salts 221 (see p 280) leads, on treatment with secondary amines, to pentamethine cyanines 222 (Konig s salts). They condense with sodium cyclopentadienide to yield vinylogous aminofulvenes 223, which on cyclization produce azulene (azulene synthesis according to Ziegler and Hafner, see p 229 [73]) ... 

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