Treatment of 2-cyclopropylazobenzenes with concentrated sulfuric acid affords the intramolecular N-and C-alkylation products. Treatment of azobenzenes with trifluoroacetic acid results in quantitative rearrangement to arylindazoles via the intermediate formation of acid-stable indazolium ions. It is suggested that the formation of indazolium ions in trifluoroacetic acid results from the synchronous opening of the cyclopropane ring and stabilization of the developing carbocation by an internal nucleophile (the azo-group).
We have previously shown [i] that 2-cyclopropylazobenzene (I) on treatment with concentrated sulfuric acid is converted into the N-arylaminoindole (III) and the azoalcohols (IV) and (V), the percentages and ratios of which remain constant with time. We were able to show that the aminoindole (III) is formed, not from the arylcyclopropane (I), but from its isomerization product, ortho-propenylazobenzene (II). *(See scheme on top of next page.)
The key step in the conversion of compound (I) is the formation of the bemzyl carbocation (A). Azobenzenes containing in the orthoposition a substituent which is capable under the reaction conditions of giving rise to the corresponding carbocation are known to be able to undergo intramolecular reactions to give the indazoles [2,3].* *It should be pointed out that the synthesis of indazoles from the appropriate orthosubstituted azobenzenes is of limited usefulness as a result of the relative inaccessibility of the starting materials.