The reaction of cerium(IV) ammonium nitrate (CAN) with a range of N-(p-anisyl)azoles in acetonitrile or methanol solvents leads to N-dearylation releasing the parent NH-azole and p-benzoquinone in comparable yields. The scope and limitations of the reaction are explored. It was successful with 1-(p-anisyl)pyrazoles, 2-(p-anisyl)-1,2,3-triazoles, 2-(p-anisyl)-2H-tetrazoles, and 1-(p-anisyl)pentazole. The dearylation renders the p-anisyl group as a potentially useful N-protecting group in azole chemistry. The azole released in solution from 1-(p-anisyl)pentazole is unstable HN5, the long-sought parent pentazolic acid. p-Anisylpentazole samples were synthesized with combinations of one, two, and three 15N atoms at all positions of the pentazole ring. The unstable HN5/N5- produced at -40 degrees C did not build up in the solution but degraded to azide ion and nitrogen gas with a short lifetime. The 15N-labeling of the N3- ion obtained from all samples proved unequivocally that it came from the degradation of HN5 (tautomeric forms) and/or its anion N5- in the solution.

Theoretical studies of proposed structures for NaN 5 , KN 5 , Mg(N 5 ) 2 , Ca(N 5 ) 2 , and Zn(N 5 ) 2 metal complexed pentazole anions have been carried out with the RHF, MP2, MCSCF, and DFT theoretical methods. Additional DFT calculations were performed on MgN 5 Cl, CaN 5 Cl, and ZnN 5 Cl pentazoles. The structures considered are unidentate I, bidentate II, and metallocene-like III. For Mg, Na, K, and Ca pentazoles at every level of theory, II is the most energetically favoured, followed by I, then III. Complex I is preferred with Zn complexes due to favourable d orbital interactions. For double ring complexes only II (I for Zn) with perpendicular rings has all positive vibrational frequencies. For single ring complexes, both II (I for Zn) and III have all positive vibrations. Structure I (II for Zn) is a transition state structure for metal ion rotation around the ring (E a 5-10 kcal mol Ϫ1 ). N atom chemical shifts relative to NH 3 and nitromethane were calculated for each species using the lowest energy configuration and the B3LYP//6-311ϩϩG(2d,p) method on the B3LYP//6-31G(d) optimised geometry. Additional calculations were done for 1-arylpentazoles, 1-arylpentazene, aryl azides, and aryldiazonium ions. Calculated 15 N NMR shifts were within 20 ppm of experiment. Time dependent B3LYP/6-31G(d) and B3LYP/6-311ϩG(d) calculations were performed on all stable species. All 1 (π,π) transitions were calculated to be below 180 nm, while the 1 (n,π) transitions were below 210 nm. The lowest energy transitions are from the lone pairs to the empty metal s orbital. For Mg and Zn these transitions are at ∼220 nm. For Na, Ca and K the transitions are considerably lower in energy, ∼250 nm.

The nature of the rate enhancements caused by gradually increasing the mole fraction of water in the solvent (from 0 to 1) for the cycloaddition reactions of pyridazinium-dicyanomethanide 1,3-dipole, 2, with the dipolarophiles ethyl vinyl ketone (a water-super dipolarophile) and methyl acrylate (a water-normal dipolarophile) in the organic solvents acetonitrile, acetone, methanol, ethanol, and tert-butyl alcohol at 37 degrees C are explored. In each case as the mole fraction of water surpassed ca. 0.9, exponential rate enhancements were triggered. When methanol replaced water, the rate enhancements were smaller, and no triggering effect was observed. The dramatic rate enhancement triggered for the water-super dipolarophile was significantly reduced as the temperature was raised in the range 29-64 degrees C. The results suggest that a dominant hydrogen-bonding effect operates in water-induced rate enhancements of 1,3-dipolar cycloaddition reactions with water-super dipolarophiles as well as the hydrophobic effect. The hydrogen-bonding effect involves secondary bridging hydrogen bonding from the primary water-solvation shell of the transition state and the growth of structured water clusters. Theoretical calculations strongly support these conclusions.