Published on Web 10/11/2005
3/2-Order Chain Kinetics Involving a Postulated Dicationic Intermediate in the Isomerization of a P-Isocyano to a P-Cyano Azaphosphatrane Monocation Jesudoss V. Kingston, John G. Verkade,* and James H. Espenson* Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011 Received June 23, 2005; E-mail:
[email protected] The isomerization of organic isocyanides to cyanides has sporadically piqued interest over the past 45 years.1 In the gas phase, evidence has been put forth for a unimolecular sigmatropic pathway,2 whereas in solution, sigmatropic, free radical, and cationic chain mechanisms can occur, depending on the conditions.1a,3 Silyl cyanides have been shown to exist in equilibrium with the isocyanide form, exchanging intermolecularly in the liquid phase and intramolecularly in dilute solution.4 Because of severe instability, the few compounds known in which isocyanide is bonded to a heteroatom5 (O, S, N, P) have not been candidates for isomerization studies. We recently reported the stable isomers 1 and 2.6 Here we report a detailed kinetic study of the isomerization of 1 in which we postulate an unusual dicationic intermediate in a chain mechanism.
Scheme 1. Postulated Reaction Scheme for Isomerization of 1
With the steady-state approximation for the concentrations of the P2+ and CN- intermediates, these expressions result:
d[P2+] d[CN-] ) ) 0 ) k1[1] - k4[P2+][CN-] dt dt ∴ [P2+]ss ) [CN-]ss ) Isomerization of 1 to its more stable isomer 2 takes place slowly in acetonitrile (90% conversion in 96 h at 100 °C), but not in benzene (80 h reached completion within 2 h. Moreover, the kinetic pattern was entirely different. The data from each experiment fit precisely to first-order kinetics, V ) 1k[1], with the NMR data showing that the concentration of 3 remained constant at the added value. In a series of such experiments, values of 1k were determined over the range 0.05 < [3] < 0.25 mol L-1. A plot of log 1k against log [3] was constructed to determine the order with respect to [3] (Figure 3). The least-squares line through the rather scattered points has a slope of 1.25 ( 0.26. (The scatter may be due to the relatively high temperature sensitivity of the reaction; from one experiment each at 35 and 40 °C, Ea is ca. 80 kJ mol-1.)
Acknowledgment. Support was provided by the U.S. National Science Foundation from Grants CHE-020409 (J.H.E.) and CHE9905354 (J.G.V.). References (1) (a) Ru¨chardt, C.; Meier, M.; Haaf, K.; Pakusch, J.; Wolber, E. K. A.; Muller, B. Angew. Chem., Int. Ed. Engl. 1991, 30, 893-1050. (b) Sung, K. J. Org. Chem. 1999, 64, 8984-8989. (c) Ugi, I. Isocyanide Chemistry; Academic Press: New York, 1971. (d) Chan, S. C.; Rabinovitch, B. S.; Bryant, J. T.; Spicer, L. D.; Fujimoto, T.; Lin, Y. N.; Pavlou, S. P. J. Phys. Chem. 1970, 74, 3160-3176. (e) Kohlmaier, G.; Rabinovitch, B. S. J. Phys. Chem. 1959, 63, 1793-1794. (f) Shen, D.; Pritchard, H. O. J. Phys. Chem. 1994, 98, 1743-1745. (g) Casanova, J., Jr.; Werner, N. D.; Schuster, R. E. J. Org. Chem. 1966, 31, 3473-3482. (h) Meier, M.; Muller, B.; Ru¨chardt, C. J. Org. Chem. 1987, 52, 648-652. (i) Saxe, P.; Yamaguchi, Y.; Pulay, P.; Schaeffer, H. F., III. J. Am. Chem. Soc. 1980, 102, 3718-3723. (2) (a) Maloney, K. M.; Rabinovitch, B. S. Isocyanide Chemistry; Academic Press: New York, 1971; p 41. (b) Glionna, M. T.; Pritchard, H. O. Can. J. Chem. 1979, 57, 2482-2483. (c) Schneider, F. W.; Rabinovitch, B. S. J. Am. Chem. Soc. 1963, 85, 2365-2370. (d) Schneider, F. W.; Rabinovitch, B. S. J. Am. Chem. Soc. 1962, 84, 4215-4230. (3) (a) Meier, M.; Ru¨chardt, C. Tetrahedron Lett. 1983, 24, 4671-4674. (b) Meier, M.; Ru¨chardt, C. Chimia 1986, 40, 238-239. (4) Seckar, J. A.; Thayer, J. S. Inorg. Chem. 1976, 15, 501-504. (5) (a) Buschmann, J.; Lentz, D.; Luger, P.; Perpetuo, G.; Preugschat, D.; Thrasher, J. S.; Willner, H.; Wolk, H.-J. Z. Anorg. Allg. Chem. 2004, 630, 1136-1142. (b) Suginome, M.; Ito, Y. Sci. Synth. 2004, 19, 445530. (6) Kingston, J. V.; Verkade, J. G. Angew. Chem., Int. Ed. 2005, 44, 49604963. (7) Taira, Z.; Osaki, K. Inorg. Nucl. Chem. Lett. 1971, 7, 509-512. (8) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.; McGraw-Hill: New York, 1995; p 29.
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