Cyansäureester, XXI Kinetik und Mechanismus der Isomerisierung von äthylcyanat

Farewell to the HSAB treatment of ambident reactivity

The concept of hard and soft acids and bases (HSAB) proved to be useful for rationalizing stability constants of metal complexes. Its application to organic reactions, particularly ambident reactivity, has led to exotic blossoms. By attempting to rationalize all the observed regioselectivities by favorable soft-soft and hard-hard as well as unfavorable hard-soft interactions, older treatments of ambident reactivity, which correctly differentiated between thermodynamic and kinetic control as well as between different coordination states of ionic substrates, have been replaced. By ignoring conflicting experimental results and even referring to untraceable experimental data, the HSAB treatment of ambident reactivity has gained undeserved popularity. In this Review we demonstrate that the HSAB as well as the related Klopman-Salem model do not even correctly predict the behavior of the prototypes of ambident nucleophiles and, therefore, are rather misleading instead of useful guides. An alternative treatment of ambident reactivity based on Marcus theory will be presented.

Photochemistry and vibrational spectra of matrix-isolated 5-ethoxy-1-phenyl-1H-tetrazole

A combined matrix isolation FT-IR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the molecular structure and photochemistry of 5-ethoxy-1-phenyl-1H-tetrazole (5EPT) was performed. A new method of synthesis of the compound is described. Calculations show three minima, very close in energy and separated by low-energy barriers (less than 4 kJ mol-1), in the ground-state potential energy profile of the molecule. The method of matrix isolation enabled the reduction of the number of populated conformational states in the experiment at low temperature due to the effect known as conformational cooling. As a result, the spectrum of the as-deposited matrix of 5EPT closely matches that of the most stable conformer predicted theoretically, pointing to the existence of only this conformer in the low-temperature matrixes. In this structure, the dihedral angle between the two rings, phenyl and tetrazole, is ca. 30 degrees, whereas the ethyl group stays nearly in the plane of the tetrazole ring and is as far as possible from the phenyl group. In situ UV irradiation (lambda > 235 nm) of the matrix-isolated 5EPT induced unimolecular decomposition, which led mainly to production of ethylcyanate and phenylazide, this later compound further reacting to yield, as final product, 1-aza-1,2,4,6-cycloheptatetraene. Anti-aromatic 3-ethoxy-1-phenyl-1H-diazirene was also observed experimentally as minor photoproduct, resulting from direct extrusion of molecular nitrogen from 5EPT. This species has not been described before and is now characterized by infrared spectroscopy for the first time.