Peculiar structure of the HOOO(-) anion

The HOOO(-) anion (1) can adopt a triplet state (T-1) or a singlet state (S-1), where the former is 9.8 kcal/mol (DeltaH(298) = 10.3 kcal/mol) more stable than the latter. S-1 possesses a strong O-OOH bond with some double bond character and a weakly covalent OO-OH bond (1.80 A) according to CCSD(T)/6-311++G(3df,3pd) calculations (the longest O-O bond ever found for a peroxide). In aqueous solution, S-1 adopts a geometry closely related to that of HOOOH (OO(O), 1.388 A; (O)OO(H), 1.509 A; tau(OOOH), 78.3 degrees ), justifying that S-1 is considered the anion of HOOOH. Dissociation into HO anion and O(2)((1)Delta(g)) requires 15.4 (DeltaH(298) = 14.3; DeltaG(298) = 8.9) kcal/mol. Structure T-1 corresponds to a van der Waals complex between HO anion and O(2)((3)Sigma(g)(-)) having a binding energy of 2.7 (DeltaH(298) = 2.1) kcal/mol. Modes of generating S-1 in aqueous solution are discussed, and it is shown that S-1 represents an important intermediate in ozonation reactions.     

Evidence for the HOOO(-) anion in the ozonation of 1,3-dioxolanes: hemiortho esters as the primary products

Low-temperature ozonation (-78 degrees C) of 2-methyl-1,3-dioxolane (1a) in acetone-d6, methyl acetate, and tert-butyl methyl ether produced both the corresponding acetal hydrotrioxide (3a, ROOOH) and the hemiortho ester (2a, ROH) in molar ratio 1:5. Both intermediates were fully characterized by 1H, 13C, and 17O NMR spectroscopy, and they both decomposed to the corresponding hydroxy ester at higher temperatures. The mechanism involving the HOOO- anion formed by the abstraction of the hydride ion by ozone to form an ion pair, R+ -OOOH, with its subsequent collapse to either the corresponding hemiortho ester (ROH) or the acetal hydrotrioxide (ROOOH) was proposed. This mechanism is supported by the PISA/B3LYP/6-311++G(3df,3pd) calculations.     

Theoretical study on the mechanism of the gas-phase reaction of diborane(3) anion with carbon disulfide

The complex potential energy surface of the gas-phase reaction of HB(H)BH- with CS2 to give three low-lying products [B2H3S]- + CS, [BH2CS]- + HBS, and [BH3CS] + BS-, involving nine [B2H3CS2]- isomers and 12 transition states, has been investigated at the CCSD(T)/6-311++G(d,p)/B3LYP/6-311++G(d,p) level. Our calculations are in harmony with the recent experimental and theoretical results, and reveal some new bonding and kinetic features of this reaction system. Our theoretical results may help the further identification of the products [BH2CS]- + HBS and [BH3CS] + BS- and may provide useful information on the chemical behaviors of other electron-deficient boron hydride anions.     

Evidence for HOOO radicals in the formation of alkyl hydrotrioxides (ROOOH) and hydrogen trioxide (HOOOH) in the ozonation of C-H bonds in hydrocarbons.

Low-temperature ozonation of cumene (1a) in acetone, methyl acetate, and tert-butyl methyl ether at -70 degrees C produced the corresponding hydrotrioxide, C(6)H(5)C(CH(3))(2)OOOH (2a), along with hydrogen trioxide, HOOOH. Ozonation of triphenylmethane (1b), however, produced only triphenylmethyl hydrotrioxide, (C(6)H(5))(3)COOOH (2b). These observations, together with the previously reported experimental evidence, seem to support the "radical" mechanism for the first step of the ozonation of the C-H bonds in hydrocarbons, i.e., the formation of the caged radical pair (R(**)OOOH), which allows both (a) collapse of the radical pair to ROOOH and (b) the abstraction of the hydrogen atom from alkyl radical R(*) by HOOO(*) to form HOOOH. The B3LYP/6-311++G(d,p) (ZPE) calculations revealed that HOOO radicals are considerably stabilized by forming intermolecularly hydrogen-bonded complexes with acetone (BE = 8.55 kcal/mol) and dimethyl ether (7.04 kcal/mol). This type of interaction appears to be crucial for the relatively fast reactions (and the formation of the polyoxides in relatively high yields) in these solvents, as compared to the ozonations run in nonbasic solvents. However, HOOO radicals appear to be not stable enough to abstract hydrogen atoms outside the solvent cage, as indicated by the absence of HOOOH among the products in the ozonolysis of triphenylmethane. The decomposition of alkyl hydrotrioxides 2a and 2b involves a homolytic cleavage of the RO-OOH bond with subsequent "in cage" reactions of the corresponding radicals, while the decomposition of HOOOH is most likely predominantly a "pericyclic" process involving one or more molecules of water acting as a bifunctional catalyst to produce water and singlet oxygen (Delta(1)O(2)).     

Synthetic model of the phosphate binding protein: solid-state structure and solution-phase anion binding properties of a large oligopyrrolic macrocycle

The macrocyclic receptors 4-6 were synthesized via the anion-templated condensation of appropriately chosen dialdehyde and diamine building blocks. Whereas all three products could be obtained directly via the appropriate choice of reaction conditions, the larger [3+3] product, 6, which incorporates three of each precursor subunit, could also be obtained conveniently via an indirect procedure involving ring expansion of the smaller [2+2] macrocycle 4. As detailed earlier (Sessler, J. L.; Katayev, E. A.; Pantos, G. D.; Reshetova, M. D.; Khrustalev, V. N.; Lynch, V. M.; Ustynyuk, Y. A. Angew. Chem. 2005, 117, 7552-7556; Angew. Chem., Int. Ed. 2005, 44, 7386-7390), this ring expansion occurs under thermodynamic control in the presence of HSO4- and H2PO4- anions in acetonitrile solution and serves to effect the conversion of 4 to 6. An analysis of the X-ray crystal structure of complex 6H22+.HPO42- revealed a strong resemblance to the active site of the phosphate binding protein (PBP) with similar structural analogies being drawn between the active site of the sulfate binding protein (SBP) and the corresponding hydrogensulfate anion complex. In both cases, the anions are bound in a 1:1 fashion in the solid state through a complementary hydrogen bond network involving both the receptor 6 and the anions. UV-vis spectroscopic titrations provide support for the conclusion that macrocycle 6 binds the hydrogensulfate and dihydrogenphosphate anion (studied as the corresponding tetrabutylammonium salts) with high selectivity and affinity in acetonitrile (log Ka for the first binding interaction approaching 7), albeit with different receptor-to-anion binding stoichiometries (1:1 vs 1:3 for HSO4- and H2PO4-, respectively).     

Characterization of solution-phase and gas-phase reactions in on-line electrochemistry-thermospray tandem mass spectrometry

Electrochemistry was used on-line with high-performance liquid chromatography-thermospray tandem mass spectrometry to provide insight into the solution-phase decomposition reactions of electrochemically generated oxidation products. Products formed during electrooxidation were monitored as the electrode potential was varied. The solution reactions which follow the initial electron transfer at the electrode are affected by the vaporizer tip temperature of the thermospray probe and the composition of the thermospray buffer. Either hydrolysis or ammonolysis reactions of the initial electrochemical oxidation products can occur with pH 7 ammonium acetate buffer. Both the electrochemically generated and the synthesized disulfide of 6-thiopurine decompose under thermospray conditions to produce 6-thiopurine and purine-6-sulfinate. Solution-phase studies indicate that nucleophilic and electrophilic substitution reactions with purine-6-sulfinate result in the formation of purine, adenine, and hypoxanthine. Products were identified and characterized by tandem mass spectrometry. This work shows the first example of high-performance liquid chromatography used on-line with electrochemistry to separate stable oxidation products prior to analysis by thermospray tandem mass spectrometry. In addition, solution-phase and gas-phase studies with methylamine show that the site of the nucleophilic and electrophilic reactions is probably inside the thermospray probe. Most importantly, these results also show that the on-line combination of electrochemistry with thermospray tandem mass spectrometry provides valuable information about redox and associated chemical reactions of biological molecules such as the structures of intermediates or products as well as providing insight into reaction pathways.     

The taming of CN7(-): the azidotetrazolate 2-oxide anion

The highly sensitive 5-azidotetrazolate anion was oxidized to its corresponding N-oxide by aqueous oxidation in a buffered oxone solution to the azidotetrazolate 2-oxide anion. After acidic extraction and neutralization with ammonia, the ammonium salt was isolated. Several energetic salts of this novel anion were prepared from the ammonium salt, and in all cases were found to be of lower sensitivity than the corresponding 5-azidotetrazolate salt while still being highly sensitive towards mechanical stimuli. Explosive performances (detonation velocity, detonation pressure) of applicable salts were also found to be higher than the non-N-oxide variants. Preparation of the free acid 2-hydroxy-5-azidotetrazole was achieved by protonation of the anion and identified by NMR spectroscopy, whereas the majority of the azidotetrazolate 2-oxide salts have unequivocal crystallographic proof.     

The cleavage mechanism of 1,4-dithiaspiro(4.4)nonane by gas-phase pyrolysis

The initial product formed in the pyrolysis of 1,4-dithiaspiro(4.4)nonane (I) is the thioketone (7). During pyrolysis at 600 °C, this material has been isolated in absolute yields as high as 4.85%. The increase in yield of benzene (1), toluene (3), and naphthalene (14) as the temperature is increased from 600 to 800 °C could be anticipated from dehydrogenation under pyrolysis conditions.The other products isolated, with the exception of the rearrangement products and toluene, appear to arise via further pyrolytic reaction of thioketone (7). These saturated products are dehydrogenated by sulfur to generate the aromatic compounds.     

Dual role of (-)-sparteine in the palladium-catalyzed aerobic oxidative kinetic resolution of secondary alcohols

The mechanistic details of the palladium-catalyzed aerobic oxidative kinetic resolution of secondary alcohols have been elucidated. (-)-Sparteine was found to have a dual role as a chiral ligand and an exogenous base. Saturation kinetics were observed for the dependence on (-)-sparteine concentration. A first-order dependence on [alcohol] and [catalyst] as well as inhibition by addition of (-)-sparteine HCl were observed. These results are consistent with rate-limiting deprotonation under low (-)-sparteine concentrations and rate-limiting beta-hydride elimination using saturating (-)-sparteine concentrations. This conclusion is further supported by a kinetic isotope effect of 1.31 +/- 0.04 under saturation. The enantioselectivity events are also controlled by addition of (-)-sparteine in which high concentrations afford a more selective kinetic resolution.     

The (SO2)2N3- anion

The recently proposed (SO2)2N3- anion was structurally characterized by single-crystal X-ray diffraction of the [Cs][(SO2)2N3] salt (P2(1)/c, a = 8.945(2) A, b = 10.454(2) A, c = 8.152(2) A, beta = 109.166(3) degrees, Z = 4, and R1 = 0.0329 at 130 K). In the (SO2)2N3- anion, both SO2 ligands are coordinated to one terminal nitrogen atom of the N3- anion.     

The self-assembly mechanism of the Lindqvist anion [W(6)O(19)](2-) in aqueous solution: a density functional theory study

The formation mechanism is always a fundamental and confused issue for polyoxometalate chemistry. Two formation mechanisms (M1 and M2) of the Lindqvist anion [W(6)O(19)](2-) have been adopted to investigate it's self-assembly reaction pathways at a density functional theory (DFT) level. The potential energy surfaces reveal that both the mechanisms are thermodynamically favorable and overall barrierless at room temperature, but M2 is slightly dominant to M1. The formation of the pentanuclear species [W(5)O(16)](2-) and [W(5)O(15)(OH)](-) are recognized as the rate-determining steps in the whole assembly polymerization processes. These two steps involve the highest energy barriers with 30.48 kcal mol(-1) and 28.90 kcal mol(-1), respectively, for M1 and M2. [W(4)O(13)](2-) and [W(4)O(12)(OH)](-) are proved to be the most stable building blocks. In addition, DFT results reveal that the formation of [W(3)O(10)](2-) experiences a lower barrier along the chain channel.     

Parallel solution-phase synthesis of (Z)-3-(arylamino)-2,3-dehydroalanine derivatives and solid-phase synthesis of fused pyrimidones

N-Protected (Z)-3-(arylamino)-2,3-dehydroalanine esters 5 and 10 were prepared in one step from methyl (Z)-2-acylamino-3-(dimethylamino)prop-2-enoates 3 and 9 and anilines 4 employing a parallel solution-phase synthetic approach. In most cases, analytically pure products 5 and 10 were obtained. On the other hand, a three-step parallel solid-phase synthesis of 2-acetylamino-4H-azino[1,2-x]pyrimidin-4-ones 15 via the polymer-bound methyl (Z)-2-acetylamino-3-(dimethylamino)prop-2-enoate (12) was also developed.     

Gas-phase and solution-phase polymerization of epoxides by Cr(salen) complexes: evidence for a dinuclear cationic mechanism

The gas-phase reactions of a series of mass-selected mononuclear and dinuclear Cr(salen) complexes with propylene oxide suggest that the enhanced reactivity of the dinuclear complexes in gas-phase and in solution may derive from a dicationic mechanism in which the alkoxide chain is mu(2)-coordinated to two Lewis acidic metal centers. The double coordination is proposed to suppress backbiting, and hence chain-transfer in the gas-phase homopolymerization of epoxides.     

Structure of the Be(OH)4(2-) anion

Sr[Be(OH)(4)] has been synthesised and its structure determination contains the first definitive characterisation of the discrete Be(OH)(4)(2-) anion.     

NMR investigation of the dihydrogen-bonding and proton-transfer equilibria between the hydrido carbonyl anion [HRe2(CO)9]- and fluorinated alcohols

The interaction of fluorinated alcohols with the anionic hydrido complex [HRe2(CO)9]- (1) has been investigated by NMR spectroscopy. According to the acidic strength of the alcohols, the interaction may result not only in the formation of dihydrogen-bonded ROH...[HRe2(CO)9]- adducts 2, but also in proton transfer to give the neutral species [H2Re2(CO)9] (3). With the weaker acid trifluoroethanol (TFE) evidence for the occurrence of the dihydrogen-bonding equilibrium was obtained by 2D 1H NOESY. The dependence of the hydride chemical shift on TFE concentration at different temperatures provided values for the constants of this equilibrium, from which the thermodynamic parameters were evaluated as deltaH(degrees) = -2.6(2) kcal mol(-1), deltaS(degrees) = -9.3(2) cal mol(-1) K(-1). This corresponds to a rather low basicity factor (E(j) = 0.64). Variable-temperature T1 measurements allowed the proton-hydride distance in adduct 2 a to be estimated (1.80 angstroms). In the presence of hexafluoroisopropyl alcohol (HFIP) simultaneous occurrence of both dihydrogen-bonding and proton-transfer equilibria was observed, and the equilibria shifted versus the protonated product 3 with increasing HFIP concentration and decreasing temperature. Reversible proton transfer between the alcohol and the hydrido complex occurs on the NMR timescale, as revealed by a 2D 1H EXSY experiment at 240 K. For the more acidic perfluoro-tert-butyl alcohol (PFTB) the protonation equilibrium was further shifted to the right. Thermal instability of 3 prevented the acquisition of accurate thermodynamic data for these equilibria. The occurrence of the proton-transfer processes (in spite of the unfavorable pK(a) values) can be explained by the formation of homoconjugated RO...HOR- pairs which stabilize the alcoholate anions.     

Parallel solution-phase synthesis of substituted 2-(1,2,4-triazol-3-yl)benzimidazoles

A solution-phase synthesis for the preparation of substituted 2-(1,2,4-triazol-3-yl)benzimidazoles from triazole aldehydes and ortho-phenylenediamines has been developed for the purpose of producing diverse lead generation libraries. Crude products were obtained and further purified by mass-guided preparative HPLC.     

Kinetics and mechanism of the gas-phase reaction of O(3P) atoms with acetone

The kinetics of the reaction of O(³P) atoms with acetone were investigated using fast flow methods. The reaction was studied over a temperature range of 298 to 478°K. The specific rate constant obtained was (1.9 ± 0.4) × 10¹² exp(—5040 ± 180/1.987 T) cm³/mol·sec. The observation of a sizable primary H/D kinetic isotope effect in comparing rates of CH₃COCH₃ and CD₃COCD₃ led to the conclusion that the major reaction channel involves H atom abstraction, namely, The rather low Arrhenius preexponential factor obtained in this reaction is compared and contrasted with those reported for other reactions of O(³P) with low molecular weight compounds.     

Correlation between computed gas-phase and experimentally determined solution-phase infrared spectra: models of the iron-iron hydrogenase enzyme active site

Gas-phase density functional theory calculations (B3LYP, double zeta plus polarization basis sets) are used to predict the solution-phase infrared spectra for a series of CO- and CN-containing iron complexes. It is shown that simple linear scaling of the computed C--O and C--N stretching frequencies yields accurate predictions of the the experimentally determined nu(CO) and nu(CN) values for a variety of complexes of different charges and in solvents of varying polarity. As examples of the technique, the resulting correlation is used to assign structures to spectroscopically observed but structurally ambiguous species in two different systems. For the (mu-SCH2CH2CH2S)[Fe(CO)3]2 complex in tetrahydrofuran solution, our calculations show that the initial electrochemical reduction process leads to a simple one-electron reduced product with a structure very similar to the (mu-SCH2CH2CH2S)[Fe(CO)3]2 parent complex. For the iron-iron hydrogenase enzyme active site, our computations show that the absence or presence of a water molecule near the distal iron center (the iron center further from the [4Fe4S] cluster and protein backbone) has very little effect on the predicted infrared spectra.