The competing mechanisms of substitution, hydrolysis, and cyclization of 1-chloro- and 1,4-dichloro-2,3-diazabuta-1,3-dienes

Abstract

In aqueous dioxan 1-chloro- and 1,4-dichloro-1,4-diaryl-2,3-diazabuta-1,3-dienes (2) and (3) are smoothly converted to arylidenearenecarbohydrazides (5) and 2,5-diaryl-1,3,4-oxadiazoles (6) respectively. Substituent effects in Ar¹(ρ=–2·3) and Ar²(ρ=–1·2) on the rates of hydrolysis have been measured for both series of compounds and indicate a similar slow step in both reactions, viz. unimolecular C–Cl bond cleavage to form a stabilized azocarbonium ion in which there is efficient delocalization of charge towards the remote aryl ring. This is in agreement with the observed variation of rate with solvent (m= 0·97) and the large common ion effect. In acidic solution azomethine bond hydrolysis (with formation of arenecarbaldehyde and substituted benzohydrazide) competes with C–Cl bond cleavage for the monochlorides (2). The dichlorides (3) do not undergo CN bond cleavage under these conditions but an intermediate in the cyclization of (3) to (6)(N-benzoylbenzohydrazonyl chloride) is sufficiently long lived to permit its spectrophotometric detection in acid. In basic solution both series of compounds (2) and (3) undergo S_N2 attack by HO^– but only if electron-withdrawing substituents are present; otherwise the rate is depressed at high [HO^–]. This change-over in mechanism results in markedly non-linear free energy relationships for hydrolysis in this region.