Kinetics and Mechanism of the Aminolysis of Methyl 4-Nitrophenyl, Methyl 2,4-Dinitrophenyl, and Phenyl 2,4-Dinitrophenyl Carbonates Enrique A. Castro,* Margarita Aliaga, Paola Campodo ´nico, and Jose ´ G. Santos* Facultad de Quı ´mica, Pontificia Universidad Cato ´ lica de Chile, Casilla 306, Santiago 22, Chile ecastro@puc.cl Received September 3, 2002 The reactions of methyl 4-nitrophenyl carbonate (MNPC) with a series of secondary alicyclic amines (SAA) and quinuclidines (QUIN), methyl 2,4-dinitrophenyl carbonate (MDNPC) with QUIN and 1-(2-hydroxyethyl)piperazinium ion (HPA), and phenyl 2,4-dinitrophenyl carbonate (PDNPC) with SAA are subjected to a kinetic investigation in aqueous solution, at 25.0 °C and an ionic strength of 0.2 M. By following spectrophotometrically the nucleofuge release (330-400 nm) under amine excess, pseudo-first-order rate coefficients (k obsd ) are obtained. Plots of k obsd vs [amine] at constant pH are linear, with the slope (k N ) being pH independent. The Bro ¨nsted-type plot (log k N vs amine pK a ) for the reactions of SAA with MNPC is biphasic with slopes  1 ) 0.3 (high pK a region) and  2 ) 1.0 (low pK a region) and a curvature center at pK a 0 ) 9.3. This plot is consistent with a stepwise mechanism through a zwitterionic tetrahedral intermediate (T ( ) and a change in the rate- determining step with SAA basicity. The Bro ¨nsted plot for the quinuclidinolysis of MNPC is linear with slope  N ) 0.86, in line with a stepwise process where breakdown of T ( to products is rate limiting. A previous work on the reactions of SAA with MDNPC was revised by including the reaction of HPA. The Bro ¨ nsted plots for the reactions of QUIN and SAA with MDNPC and SAA with PDNPC are linear with slopes  ) 0.51, 0.48, and 0.39, respectively, consistent with concerted mechanisms. Since quinuclidines are better leaving groups from T ( than isobasic SAA, yielding a less stable T ( , it seems doubtful that the quinuclidinolysis of PDNPC is stepwise, as reported. Introduction Although much attention has been drawn to the kinetics and mechanism of the aminolysis of aryl esters, 1 the aminolyses of alkyl aryl carbonates 2-4 and diaryl carbonates 5,6 have been less studied. The latter investiga- tions include reports on the reactions of alkyl aryl carbonates with pyridines, 2 secondary alicyclic amines (SAA), 3 and benzylamines 4 and the reactions of diaryl carbonates with quinuclidines (QUIN) 5 and SAA. 6 Most of these reactions were found to proceed through a zwitterionic tetrahedral addition intermediate (stepwise mechanism), 2-5 and others were found to occur in a single step with no intermediate (concerted mechanism). 6 In all these studies, the Bro ¨ nsted-type plot has been employed as an important tool for the diagnosis of the reaction mechanism. To shed more light on the mechanisms of the aminoly- sis of carbonates, we describe in this work a kinetic study of the reactions of SAA and QUIN with methyl 4-nitro- phenyl carbonate (MNPC), QUIN with methyl 2,4-dini- trophenyl carbonate (MDNPC), and SAA with phenyl 2,4- dinitrophenyl carbonate (PDNPC). We compare our results with those obtained for the same aminolysis of similar carbonates 3,5,6 to assess the effect of the nonleav- ing and leaving groups and the amine nature on the kinetics and mechanism of these reactions. In view of our kinetic results, in this work we also revise the kinetic data obtained for the reactions of SAA (1) (a) Johnson, S. L. Adv. Phys. Org. Chem. 1967, 5, 237. (b) Jencks, W. P.; Gilchrist, M. J. Am. Chem. Soc. 1968, 90, 2622. (c) Satterthwait, A. C.; Jencks, W. P. J. Am. Chem. Soc. 1974, 96, 7018, 7031. (d) Kirby, A. J. In Organic Reaction Mechanisms; Knipe, A. C., Watts, W. E., Eds.; Wiley and Sons: New York, 1980; p 29. (e) Jencks, W. P. Chem. Soc. 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A.; Andu ´ jar, M.; Campodo ´nico, P.; Santos, J. G. Int. J. Chem. Kinet. 2002, 34, 309. 10.1021/jo026390k CCC: $22.00 © 2002 American Chemical Society J. Org. Chem. 2002, 67, 8911-8916 8911 Published on Web 11/08/2002