Luke A. Burke

A Ceric Ammonium Nitrate N-Dearylation of N-p-Anisylazoles Applied to Pyrazole, Triazole, Tetrazole, and Pentazole Rings: Release of Parent Azoles. Generation of Unstable Pentazole, HN5/N5-, in Solution

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.

First generation of pentazole (HN5, pentazolic acid), the final azole, and a zinc pentazolate salt in solution: A new N-dearylation of 1-(p-methoxyphenyl) pyrazoles, a 2-(p-methoxyphenyl) tetrazole and application of the methodology to 1-(p-methoxyphenyl) pentazoleElectronic supplementary information (ESI) available: experimental details. See http://www.rsc.org/suppdata/cc/b3/b301491f/

Ceric ammonium nitrate (CAN) in methanol-water gave a new N-dearylation of a series of substituted 1-(p-methoxyphenyl) pyrazoles and a 2-(p-methoxyphenyl)tetrazole producing p-benzoquinone and the parent azole in a mole for mole ratio. Application of this reaction to 1-(p-methoxyphenyl) pentazole at -40 degrees C produced p-benzoquinone. 15N NMR spectra suggest that pentazole, HN5, was also produced and held in solution as N5- with Zn2+ ion. The 15N signal from N5- was -10.0 +/- 2.0 ppm in agreement with calculated values.

Theoretical study of water clusters. II: Hexamer

Abstract Three water hexamer structures found with a 3–21G basis set molecular orbital optimization method were explored with the HF//6–311G* and HF/6–311G** basis sets. Thermodynamic quantities and IR and Raman spectra were calculated. With the larger basis set the differences in electronic energy, enthalpy, free energy and entropy between the chair form and the boat (prism) forms are 1.08 (−0.84), 0.96 (−0.84), 0.14 (3.67) kcal/mol, and 2.77 (−13.21) cal/mol K. Calculations were also carried out for the ring and isosceles bipyramid configurations of the pentamer using the HF/6–311G* and MP2/6–311G* methods.

Ion pair formation in water clusters: a theoretical study

Abstract Two hydrated ion pairs of the formula (H3O+)(H2O)3 (OH−) are studied using Hartree-Fock and MP2 wavefuctions with a 6-311 + + G∗∗ basis set. Two neutral water pentamers are also studied for comparison. Thermodynamics, IR spectra, dipole moments, rotational constants and spatial extent of the four clusters are examined in detail. Agreement of calculated thermodynamic quantities and IR spectra with experimental values is significant. The hydrated ion pairs are stablized more by electron correlation than the neutral species. This can be partially explained in terms of the strained configuration of these hydrated ion pairs. The hydrated ion pairs also have a suprisingly small calculated dipole moment.

6π-Cycloadditions of aryl and new heteroaryl N-sulphinylamines with 2,3-dimethylbuta-1,3-diene: synthesis, kinetics, substituent effects, theoretical calculations, and reaction mechanism

A range of new N-sulphinylamine derivatives of heteroaryl rings, including tetrazoles, oxadiazoles, thiadiazoles, thiazoles, and oxazoles is reported. The influence of the –NS + –O − group on the n.m.r. spectra of the ring to which it is bonded is discussed. The kinetics of the cycloaddition reactions of this group with 2,3-dimethylbuta-1,3-diene have been measured for a series of para-substituted phenyl-N-sulphinylamines and the rates correlated with MNDO calculated HOMO–LUMO separations and Hammett substituent constants.

Theoretical characterization of pentazole anion with metal counter ions. Calculated and experimental 15N shifts of aryldiazonium, -azide and -pentazole systems

Theoretical studies of proposed structures for NaN5, KN5, Mg(N5)2, Ca(N5)2, and Zn(N5)2 metal complexed pentazole anions have been carried out with the RHF, MP2, MCSCF, and DFT theoretical methods. Additional DFT calculations were performed on MgN5Cl, CaN5Cl, and ZnN5Cl 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 (Ea 5–10 kcal mol−1). N atom chemical shifts relative to NH3 and nitromethane were calculated for each species using the lowest energy configuration and the B3LYP//6-311++G(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 15N NMR shifts were within 20 ppm of experiment. Time dependent B3LYP/6-31G(d) and B3LYP/6-311+G(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.

Regioselectivity and endo/exo selectivity in the cycloadditions of the phthalazinium dicyanomethanide 1,3-dipole with unsymmetrical alkene and alkyne dipolarophiles. Unexpected reversals of regiochemistry: a combined experimental and DFT theoretical study

In the reactions of phthalazinium dicyanomethanide 1,3-dipole with electron-poor monosubstituted alkene and alkyne dipolarophiles the dicyanomethanide terminus bonds to the unsubstituted carbon giving 1-substituted pyrrolo[2,1-a]phthalazines. With electron-rich dipolarophiles the regiochemistry is reversed and the products are 2-substituted pyrrolo[2,1-a]phthalazines. An unexpected reversal of regiochemistry occurred with methyl methacrylate, due to a steric effect of the C–Me group, and the main product was 2-exo-methyl-2-endo-methoxycarbonyl-3,3-dicyano-1,2,3,10b-tetrahydropyrrolo[2,1-a]phthalazine, 17. endo Cycloadditions were strongly favoured with alkene dipolarophiles containing π bonds in substituents on the alkene unit. With N-substituted maleimides endo products were formed exclusively even when the N-substituent was tBu or adamantyl. Only with alkoxyvinyl ethers were exo-cycloadducts encountered. The mechanisms are discussed in conjunction with DFT calculations. An X-ray crystal structure is reported on endo-N-tert-butyl-3,3-dicyano-1,2,3,10b-tetrahydropyrrolo[2,1-a]phthalazine-1,2-dicarboximide, 25.

Theoretical study of the possible isomers and high-energy intermediates of HCNH2O complexes

Abstract All chemically significant structures and some possible intermediates involving unimolecular and bimolecular complexes of CH 3 NO were studied by using the 6–311G ∗∗ basis set at the Hartree-Fock, MP2 and density functional levels employing the three-parameter Becke functional (B3-LYP). Calculated structures and thermodynamic properties are presented. Formamide was found to be the most stable structure in all methods. Minor structural and energetic differences among the methods are discussed. Density functional computations were carried out on selected structures incorporating diffuse functions into the basis set (6–311++G ∗∗ ), and the results are compared with experimental values where available.

Computational investigation of rearrangements in huisgen cycloadducts of azolium N-dicyanomethanide 1,3-dipoles with alkynes: a mechanistic panoply.

The reaction surfaces leading to rearrangements and ring expansions of azapentalene cycloadducts of imidazolo- and triazolodicyanomethanide 1,3-dipoles with alkynes are studied with the B3LYP DFT method using the 6-31G(d) and 6-311+G(2d,p) basis sets. The surprisingly complex surface involves (1) consecutive but not combined pericyclic steps, a coarctate TS, and pseudopericyclic mechanisms, (2) anchimerically assisted H-atom transfer competing effectively with concerted symmetry-allowed sigmatropic steps, and (3) azolium methanide zwitterions and ketenimines as key intermediates. The azolium methanide is identified as the intermediate detected previously in a variable-temperature NMR experiment that converted the unstable cycloadduct to product imine.

THEORETICAL STUDY OF POSSIBLE PRODUCTS OF THE COMBINATION OF H2O AND HCN

Abstract Twenty-two structures with the empirical formula H 3 CNO are presented. These structures are examined since they have the same formula as the H 2 O…HCN complex. The H 2 O…HCN potential energy surface is of interest to chemists studying the water catalyzed polymerization of HCN. Structures, thermodynamics, and vibrational spectra are examined.