Solvent- and reagent-induced change of rate-limiting step in acid-catalysed α-halogenation of amidines with tetrahalogenomethanes

Abstract

The acid-catalysed α-halogenation of amidines with tetrahalogenomethanes involves the intermediate formation of the ketene aminal tautomer, which then reacts with the tetrahalogenomethane. The kinetics of the acid-catalysed reaction of 2,10-diazabicyclo[4.4.0]dec-1-ene (1) with CCl₄ in benzene, methylene dichloride, and DMF, and with CBrCl₃ in DMF, respectively, have been investigated. The kinetics of the reaction of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)(5) with CCl₄ in DMF and DMSO has also been studied. The reaction of amidine (1) with CCl₄ in DMF is first-order in [(1)], close to zero-order in [CCl₄], and close to pseudo-first-order in [acid], indicating rate-limiting formation of the ketene aminal (2). In contrast, in the non-polar solvents the rate of reaction is found to be limited by the reaction of the tautomer (2) with CCl₄, which is much slower than in DMF. This change of rate-limiting step is also observed for the reaction of (1) in benzene when CBrCl₃ is exchanged for CCl₄, and is most likely induced by a much faster reaction of tautomer (2) with CBrCl₃ than with CCl₄. The present results support a non-chain one-electron-transfer mechanism for the halogenation step, with formation of an intermediate radical-ion pair. A pK_a difference of 2.9 between amidine (1) and the α-chlorinated product (4) has also been determined using an appropriate ¹³C n.m.r. method.