Necessity to consider a three-water chain in modelling the hydration of ketene imines and carbodiimides

Abstract

A theoretical study of the hydration of a model ketene imine (R₂C[double bond, length half m-dash]C[double bond, length half m-dash]NH) and carbodiimide (RN[double bond, length half m-dash]C[double bond, length half m-dash]NR) has been undertaken. The detailed hydration mechanism of the simplest cumulenes by water and water clusters (HX[double bond, length half m-dash]C[double bond, length half m-dash]NH + n H₂O→H₂XCONH₂ + (n – 1) H₂O, n = 1, 2, 3 and X = CH, N) was modelled using high-level ab initio MO methods. Geometric and energetic parameters were determined for two possible reaction channels involving water attack across both C[double bond, length half m-dash]C and C[double bond, length half m-dash]N bonds of ketene imine. Using one and two actively participating water molecules to model the hydration, calculated results consistently show that the C[double bond, length half m-dash]N addition, giving first an amide enol, is favoured over the C[double bond, length half m-dash]C yielding immediately the amide product. A reverse situation occurs when a chain of three water molecules is used. Since attack in two different planes is possible in the latter case, reducing the unfavourable distortion of the methylene group, the C[double bond, length half m-dash]C addition becomes easier to perform than the C[double bond, length half m-dash]N, with an energy barrier of 48 kJ mol^–1 found at the CCSD(T)/6-31G(d,p) level, the lowest barrier of all the calculated water chain models. These findings are consistent with experimental evidence for direct formation of C[double bond, length half m-dash]C products in non-hindered ketene imines. Thus, water oligomers higher than the dimer seem to make a primordial contribution to the rate of the hydration and are really needed to perform a concerted reaction. These gas-phase results are confirmed when the effect of the solvent bulk is taken into account in PCM calculations. Hydration of the analogous carbodiimide, in which addition can only occur across the C[double bond, length half m-dash]N bond, was also studied. The C[double bond, length half m-dash]N addition with the aid of a three-water cluster is rate-determining followed by a tautomerization of the primary adduct leading to urea. Carbodiimide hydration turns out to be easier to achieve than ketenimine hydration.

References

1. H. Staudinger and E. Hauser, Helv. Chim. Acta., 1921, 4, 887.
2. M. W. Banker and W. E. McHenry, in The Chemistry of Ketenes, Alkenes and Related Compounds, ed. S. Patai, Wiley, New York, 1980, p. 701.
3. D. G. McCarthy and A. F. Hegarty, J. Chem. Soc., Perkin Trans. 2, 1980, 579.
4. T. K. Ha and M. T. Nguyen, J. Mol. Struct. (THEOCHEM), 1982, 87, 255.
5. J. Kaneti and M. T. Nguyen, J. Mol. Struct. (THEOCHEM), 1982, 87, 205.
6. M. T. Nguyen and A. F. Hegarty, J. Mol. Struct. (THEOCHEM), 1983, 93, 329.
7. M. T. Nguyen and A. F. Hegarty, J. Am. Chem. Soc., 1983, 105, 3811.
8. A. F. Hegarty, J. G. Kelly and C. M. Relihan, J. Chem. Soc., Perkin Trans. 2, 1997, 1175.
9. G. Raspoet, M. T. Nguyen, M. McGarraghy and A. F. Hegarty, J. Org. Chem., 1998, 63, 6867 and 6878.
10. GAUSSIAN94 Revision C.3; M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowki, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. H. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Comperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. DeFrees, J. Baker, J. J. P. Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian Inc., Pittsburgh, PA, 1995.
11. S. Miertus, E. Scrocco and J. Tomasi, Chem. Phys., 1981, 55, 117.
12. J. C. Guillemin, J. M. Denis, M. C. Lasne and J. L. Ripoll, J. Chem Soc., Chem. Commun., 1983, 238.
13. H. W. Kroto, G. Y. Matti, R. J. Suffolk, J. D. Watts, M. Rittby and R. J. Bartlett, J. Am. Chem. Soc., 1990, 112, 3779.
14. G. Bouchoux, F. Penaud-Berruyer and M. T. Nguyen, J. Am. Chem. Soc., 1993, 115, 9728.
15. A. Williams and I. T. Ibrahim, Chem. Rev., 1981, 81, 589.
16. M. Mikolajczyk and P. Kielbasinsky, Tetrahedron, 1981, 37, 233.
17. M. Birk and M. Winnewisser, Chem. Phys. Lett., 1986, 123, 386.
18. M. T. Nguyen and T. K. Ha, J. Chem. Soc., Perkin Trans. 2, 1983, 1297.
19. M. T. Nguyen, N. J. Riggs, L. Radom, M. Winnewisser, B. P. Winnewisser and M. Birk, Chem. Phys., 1988, 122, 305.
20. I. H. Williams, O. Spangler, D. A. Fermec, G. M. Maggiora and R. L. Schowen, J. Am. Chem. Soc., 1983, 105, 31.
21. O. N. Ventura, E. L. Coitino, A. Lledos and J. Bertran, J. Comput. Chem., 1992, 13, 1037.
22. M. T. Nguyen and A. F. Hegarty, J. Am. Chem. Soc., 1984, 106, 1552.
23. M. T. Nguyen, G. Raspoet, L. G. Vanquickenborne and Ph. T. Van Duijnen, J. Phys. Chem., 1997, 101, 7379.