Indazolone formation

Azo compounds are reactive dienophiles. Indeed, one of the very first Diels-Alder reactions was the addition of diethyl azodicarboxylate to cyclopentadiene (equation 134)128 129. Other early examples of the reaction are the formation of tetrahydropyridazines from indazolone 252 and phthalazinedione 253 (equations 135 and 136)130. 

Facile insertion of CO takes place, and the reaction offers a synthetic method of orf/to-substituted benzoic acid derivatives. The 2-aryl-3-indazolone 421 was obtained in high yield by carbonylation of 4-methylazobenzene complex 420 in alcohol or water [176]. Cobalt-catalyzed carbonylation of 421 and subsequent hydrolysis afforded aniline and 5-methylanthranilic acid (422). The results show that the cr-bond formation is an electrophilic substitution of PdCl2 on the benzene ring [176,177]. 

The reaction of the indazolones by CO insertion may be regarded as analogous to the formation of N,N -dibenzylurea from hydrazobenzene and carbon monoxide [697]. 

Indazolones can be prepared by a related route utilizing a key intramolecular C—N bond formation [88,89], o-Halobenzohydrazides such as 40 were shown to undergo cyclization via the action of an L-proline-derived copper catalyst, yielding indazolone products such as 41 in excellent yield (Scheme 24.18) [88],... 

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