Synthesis and Biological Evaluation of New Ozonides with Improved Plant Growth Regulatory Activity

The iron oxyallyl carbocation generated from 2,7-dibromocycloheptanone was induced to undergo [4 + 3] cycloaddition reactions with various furans, affording a series of 12-oxatricyclo-[4.4.1.1(2,5)]-dodec-3-en-11-one adducts. Similar methodology was used to prepare two additional cycloadducts using menthofuran and two homologous aliphatic dibromoketones. Dipolar cycloaddition of ozone to the adducts afforded the corresponding secondary ozonides (i.e., 1,2,4-trioxolanes) in variable yields. Ozonides were investigated by quantum mechanics at the B3LYP/6-31+G* level to study structural features including close contacts which may be responsible for enhancing ozonide stability. The effect of these ozonides and their corresponding precursor cycloadducts upon radicle growth of both Sorghum bicolor and Cucumis sativus was evaluated at 5.0 x 10(-4) mol L-1. The most active cycloadducts and ozonides were also evaluated against the weed species Ipomoea grandifolia and Brachiaria decumbens, and the results are discussed. Compared to ozonides previously synthesized in our laboratory, the new ozonides described herein present improved plant growth regulatory activity.

A comparison of the reactivity of germylene and dimethylgermylene with some methylgermanes. Direct kinetic and quantum chemical studies

Time-resolved studies of germylene, GeH2, and dimethygermylene, GeMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to try to obtain rate coefficients for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas-phase. GeH2 + Me2GeH2 was studied over the pressure range 1-100 Torr with SF6 as bath gas and at five temperatures in the range 296-553 K. Only slight pressure dependences were found (at 386, 447 and 553 K). RRKM modelling was carried out to fit these pressure dependences. The high pressure rate coefficients gave the Arrhenius parameters: log(A/cm(3) molecule(-1)s(-1)) = -10.99 +/- 0.07 and E-a = -(7.35 +/- 0.48) kJ mol(-1). No reaction could be found between GeMe2 + Me2GeH2 at any temperature up to 549 K, and upper limits of ca. 10(-14) cm(3) molecule(-1)s(-1) were set for the rate coefficients. A rate coefficient of (1.33 +/- 0.04) x 10(-11)cm(3) molecule(-1)s(-1) was also obtained for GeH2 + MeGeH3 at 296 K. No reaction was found between GeMe2 and MeGeH3. Rate coefficient comparisons showed, inter alia, that in the substrate germane Me-for-H substitution increased the magnitudes of rate coefficients significantly, while in the germylene Me-for-H substitution decreased the magnitudes of rate coefficients by at least four orders of magnitude. Quantum chemical calculations (G2(MP2,SVP)// B3LYP level) supported these findings and showed that the lack of reactivity of GeMe2 is caused by a positive energy barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper.

Surprisingly slow reaction of dimethylsilylene with dimethylgermane: time-resolved kinetic studies and related quantum chemical calculations

Time-resolved studies of silylene, SiH2, and dimethylsilylene, SiMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to obtain rate constants for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas phase. SiMe2 + Me2GeH2 was studied at five temperatures in the range 299-555 K. Problems of substrate UV absorption at 193 nm at temperatures above 400 K meant that only three temperatures could be used reliably for rate constant measurement. These rate constants gave the Arrhenius parameters log(A/cm(3) molecule(-1) s(-1)) = -13.25 +/- 0.16 and E-a = -(5.01 +/- 1.01) kJ mol(-1). Only room temperature studies of SiH2 were carried out. These gave values of (4.05 +/- 0.06) x 10(-10) cm(3) molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm(3) molecule(-1) s(-1) (SiH2 + MeGeH3 at 296 K). Rate constant comparisons show the surprising result that SiMe2 reacts 12.5 times slower with Me2GeH2 than with Me2SiH2. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) of the model Si-H and Ge-H insertion processes of SiMe2 with SiH4/MeSiH3 and GeH4/MeGeH3 support these findings and show that the lower reactivity of SiMe2 with Ge-H bonds is caused by a higher secondary barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper. Other, related, comparisons of silylene reactivity are also presented.

An investigation of the germylene addition reaction, GeH2+C2H2: Time-resolved gas-phase kinetic studies and quantum chemical calculations of the reaction energy surface

Time resolved studies of germylene, GeH2, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with acetylene, C2H2. The reaction was studied in the gas-phase over the pressure range 1-100 Tort, with SF6 as bath gas, at 5 temperatures in the range 297-553 K. The reaction showed a very slight pressure dependence at higher temperatures. The high pressure rate constants (obtained by extrapolation at the three higher temperatures) gave the Arrhenius equation: log(k(infinity)/cm(3) molecule(-1) s(-1)) (-10.94 +/- 0.05) + (6.10 +/- 0.36 kJ mol(-1))/RTln10. These Arrhenius parameters are consistent with a fast reaction occurring at approximately 30% of the collision rate at 298 K. Quantum chemical calculations (both DFT and ab initio G2//B3LYP and G2//QCISD) of the GeC2H4 potential energy surface (PES), show that GeH2 + C2H2 react initially to form germirene which can isomerise to vinylgermylene with a relatively low barrier. RRKM modelling, based on a loose association transition state, but assuming vinylgermylene is the end product (used in combination with a weak collisional deactivation model) predicts a strong pressure dependence using the calculated energies, in conflict with the experimental evidence. The detailed GeC2H4 PES shows considerable complexity with ten other accessible stable minima (B3LYP level), the three most stable of which are all germylenes. Routes through this complex surface were examined in detail. The only product combination which appears capable of satisfying the (P-3) + C2H4.C2H4 was confirmed as a product by GC observed lack of a strong pressure dependence is Ge(P-3) + C2H4. C2H4 was confirmed as a product by GC analysis. Although the formation of these products are shown to be possible by singlet-triplet curve crossing during dissociation of 1-germiranylidene (1-germacyclopropylidene), it seems more likely (on thermochernical grounds) that the triplet biradical, (GeCH2CH2.)-Ge-., is the immediate product precursor. Comparisons are made with the reaction of SiH2 with C2H2.

Effect of H-bonding on the ambivalence of SCN- towards copper(II)

Condensations of 2-(2-aminoethyl)pyridine with 4-methylimidazole-5-carboxaldehyde and 1-methyl-2-imidazolecarboxaldehyde generate the tridentate N donor ligands L and L' respectively. Reactions of Cu(NCS)(2) with L and L' yield respectively CuL(SCN)(NCS) (1) containing a CuN4S core and CuL'(NCS)(2) (2) having a CuN5 core. Both the cores are square pyramidal with SCN bound in 1 at the axial position through the S end. This differential behaviour of SCN in the two complexes despite the ligands being very similar, is investigated by DFT calculations at the B3LYP/TZV level. It is found that DFT calculations predict isolation of the Cu(ligand)(NCS)(2) species for both the ligands L and L'. Presence of an offsetting intermolecular H-bonding between the N atom of the thiocyanate and the N-H proton of the ligand L of an adjacent molecule makes the binding of SCN via the S end feasible in 1 resulting in the H-bonded-dimer Cu2L2(SCN)(2)(NCS)(2). The strength of the H-bond is estimated as 27.1 kJ mol (1) from the DFT calculations. The question of such H-bonding does not arise with L' as it lacks in a similar H atom. Dimeric 1 represents a case of two non-interacting spins. (C) 2008 Elsevier B. V. All rights reserved.

Solid state annular tautomerism in a molecule containing two imidazole moieties

The X-ray crystal structure of the 1:2 condensate (1) of hydrazine hydrate and 4-methyl-imidazole-5-carboxaldehyde has been determined. The molecule is centrosymmetric crystallising in the space group Fddd with cell dimensions: a = 10.557(14), b = 17.062(22), c = 24.759(27) angstrom. Fourier map shows that the NH hydrogen atom of each imidazole moiety has equal possibility of occupying any of its two ring N atoms. This poses the possibility of finding three tautomers in 1 in the solid state. Consideration of the H-bonding pattern observed in 1 and related B3LYP/6-311+G(2d, p) calculations show that only two tautomers are present in the solid state. The situation is compared with that in the structure of 4(5)-nitro-5(4)-methoxy-imidazole reported previously by Kubicki.

Influence of a halide ion on ribbons of water pentamer

Reactions of CuF2, CuCl2 center dot 2H(2)O and CuBr2 with 2,2'-dipyridylamine (HDPA) in water at room temperature using Cu: HDPA = 2: 1 mol yield [Cu(HDPA) (H2O)(2)F]F center dot 3H(2)O (1), Cu(HDPA) Cl-2 (2) and [Cu(HDPA) Br-2 (3) respectively. The structures of 2 and 3 are isostructural in spacegroup C-2 with cell dimensions; for 2, a = 14.702(8), b = 7.726(2), c = 4.829(6) angstrom, beta = 96.68(8)degrees and for 3, a = 14.2934(8), b = 7.9057(6), c = 5.1982(5) angstrom, beta = 94.049(7)degrees. In the X-ray crystal structure, the complex 1 is found to contain tapes of water pentamers. Our DFT calculations at the B3LYP/LanL2DZ level show that the reaction Cu(HDPA)X-2 + 2H(2)O = [Cu(HDPA)(H2O)(2)X]X is most exothermic in the gas phase when X- = F-, i.e., the tendency of water uptake is maximum for Cu(HDPA) F-2. It seems that the exothermicities of the aquations of Cu(HDPA) Cl-2 and Cu(HDPA) Br-2 are not sufficient to stabilise the type of ribbons of water observed in 1 and consequently water is eschewed when X- = Cl- or Br-.

Reactions of artemisinin and arteether with acid: Implications for stability and mode of antimalarial action

The currently accepted mechanism of trioxane antimalarial action involves generation of free radicals within or near susceptible sites probably arising from the production of distonic radical anions. An alternative mechanistic proposal involving the ionic scission of the peroxide group and consequent generation of a carbocation at C-4 has been suggested to account for antimalarial activity. We have investigated this latter mechanism using DFT (B3LYP/6-31+G* level) and established the preferred Lewis acid protonation sites (artemisinin O5a >> O4a approximate to O3a > O2a > O1a; arteether O4a >= O3a > O5b >> O2a > O1a; Figure 3) and the consequent decomposition pathways and hydrolysis sites. In neither molecule is protonation likely to occur on the peroxide bond O1-O2 and therefore lead to scission. Therefore, the alternative radical pathway remains the likeliest explanation for antimalarial action.

Design and theoretical study of nickel catalysts for syndiotactic polyolefins

A nickel catalyst was modeled with ligand L-2, [ NH = CH-CH = CH-O](-), which should have potential use as a syndiotactic polyolefin catalyst, and the reaction mechanism was studied by theoretical calculations using the density functional method at the B3LYP/ LANL2MB level. The mechanism involves the formation of the intermediate [(NiLMe)-Me-2](+), in which the metal occuples a T-shaped geometry. - This intermediate has two possible structures with the methyl group trans either to the oxygen or to the nitrogen atom of L-2. The results show that both structures can lead to the desired product via similar reaction paths, A and B. Thus, the polymerization could be considered as taking place either with the alkyl group occupying the position trans to the Ni-O or trans to the Ni-N bond in the catalyst. The polymerization process thus favors the catalysis of syndiotactic polyolefins. The syndiotactic synthesis effects could also be enhanced by variations in the ligand substituents. From energy considerations, we can conclude that it is more favorable for the methyl group to occupy the trans-O position to form a complex than to occupy the trans-N position. From bond length considerations, it is also more favoured for ethene to occupy the trans-O position than to occupy the trans-N position.

A DFT study of propylene polymerization using neutral salicyladiminato nickel(II) and palladium(II) as catalysts

Propylene polymerization using salicyladiminato metal catalalysts has been studied using density functional theory at the B3LYP/LANL2DZ level. In particular, the effects on the reaction mechanisms of changing the metal from Pd(II) to Ni(II) have been investigated. While the reaction mechanisms involving the salicyladiminato Ni(II) catalyst have been found to be similar to those established previously for the salicyladiminato Pd(II) catalyst, the nickel catalyst was found to differentiate the trans-O intermediate from the trans-.N intermediate with an energy difference of 46.63 U mol(-1) significantly more than the palladium catalyst for which the energy difference was calculated as 35.82 kJ mol(-1). The energy difference between the trans-O configuration and the trans-N configuration is decreased significantly when combining a molecule of propylene with the catalyst. For the Ni catalyst, the trans-O isomer is more stable than the trans-N isomer to a greater extent than for Pd, so that the insertion of propylene from 20 is relatively less favoured for Ni than for Pd. It is predicted that the mechanism of isomerization from 20 to 2N through a rotational transition state TS2O2N is more appropriate for the Ni catalyst system. The palladium system shows a larger preference for pi-coordination than its nickel counterpart, although the latter possesses a lower reaction barrier. It was found that the occupation of the trans-O position in the asymmetric salicyladiminato catalyst is also more favored by the alkene as it is by the alkyl so that insertion of the alkene may always start from a particular configuration so that specific products are obtained. (c) 2005 Elsevier B.V. All rights reserved.

Chalcogenide-halides of niobium (V). 1. Gas-phase structures of NbOBr3, NbSBr3, and NbSCl3. 2. Matrix infrared spectra and vibrational force fields of NbOBr3, NbSBr3, NbSCl3, and NbOCl3

The molecular structures of NbOBr3, NbSCl3, and NbSBr3 have been determined by gas-phase electron diffraction (GED) at nozzle-tip temperatures of 250 degreesC, taking into account the possible presence of NbOCl3 as a contaminant in the NbSCl3 sample and NbOBr3 in the NbSBr3 sample. The experimental data are consistent with trigonal-pyramidal molecules having C-3v symmetry. Infrared spectra of molecules trapped in argon or nitrogen matrices were recorded and exhibit the characteristic fundamental stretching modes for C-3v species. Well resolved isotopic fine structure (Cl-35 and Cl-37) was observed for NbSCl3, and for NbOCl3 which occurred as an impurity in the NbSCl3 spectra. Quantum mechanical calculations of the structures and vibrational frequencies of the four YNbX3 molecules (Y = O, S; X = Cl, Br) were carried out at several levels of theory, most importantly B3LYP DFT with either the Stuttgart RSC ECP or Hay-Wadt (n + 1) ECP VDZ basis set for Nb and the 6-311 G* basis set for the nonmetal atoms. Theoretical values for the bond lengths are 0.01-0.04 Angstrom longer than the experimental ones of type r(a), in accord with general experience, but the bond angles with theoretical minus experimental differences of only 1.0-1.5degrees are notably accurate. Symmetrized force fields were also calculated. The experimental bond lengths (r(g)/Angstrom) and angles (angle(alpha)/deg) with estimated 2sigma uncertainties from GED are as follows. NbOBr3: r(Nb=O) = 1.694(7), r(Nb-Br) = 2.429(2), angle(O=Nb-Br) = 107.3(5), angle(Br-Nb-Br) = 111.5(5). NbSBr3: r(Nb=S) = 2.134(10), r(Nb-Br) = 2.408(4), angle(S=Nb-Br) = 106.6(7), angle(Br-Nb-Br) = 112.2(6). NbSCl3: Nb=S) = 2.120(10), r(Nb-Cl) = 2.271(6), angle(S=Nb-Cl) = 107.8(12), angle(Cl-Nb-Cl) = 111.1(11).

2,2-Dialkyl-2H-benzimidazoles, the high energy tautomers of the corresponding 1,2-dialkyl-1H-benzimidazoles. Syntheses and their complexes with Cu(I) and Ag(I)

Reaction of Cu(1,2-phenylenediamine)(2)(ClO4)(2) with neat RR'=O (R = methyl and/or ethyl) (lives Cu(2,2-dialkyl-2H-benzimidazole)ClO4. demetallation of which by the action of aqueous ammonia yields Pure 2,2-dialkyl-2H-benzimidazoles. These are characterised by NMR. hi the X-ray crystal Structure, Ag(2,2-methyl-2H-benzimi-dazolc)NO3 is Found to be a spiral 1D coordination polymer where the 2H-benzimidazole acts as an N,N bridge between two Ag(I) centus. Although 2H-benzimidazoles are very unstable in the free state, they are quite stable in their Cu(I)(1) and Ag(I) complexes. The 1,2-tautomerisation in imidazole and benzimidazole have been Studied by means of transition state calculations at B3LYP/6-3 11 +G(2d,p)* level.

A new tetrameric Cu-II cluster with square topology exhibiting ferro- and antiferromagnetic magnetic pathways: which is which?

[Cu4L2(bpy)(4)(H2O)(3)](ClO4)(4).2.5H(2)O, 1, a new tetranuclear Cu-II cluster showing square planar geometry, formed with aspartate bridging ligand (L) has been synthesized. The global magnetic coupling is ferromagnetic but theoretical DFT/B3LYP calculations are necessary to assign which Cu-L-Cu side is ferro or antiferromagnetically coupled.

Synthesis and structural characterization of two nostoclide analogues

The vinylogous aldol reaction between appropriate aldehydes and furan-based silyloxy diene synthon generated from 3-benzyl-5H-furan-2-one (3) afforded two truncated lactone analogues [compounds (4) and (5)] of nostoclides (2). The compounds were fully characterized by IR, NMR (H-1 and C-13), 2D NMR spectroscopy experiments (HMBC, HSQC and NOESY), MS spectrometry and X-ray crystallography. Compounds (4) and (5) crystallized in the space group P2(1)2(1)2(1) and P2(1)/c, respectively. Although expected correlations between hydrogen atoms in spatial close proximity were not observed for compound (5) using NMR, the stereochemistry of the exocyclic double bond of both (4) and (5) was unambiguously determined to be Z and E, respectively, using X-ray crystallography. The packing of both compounds within the crystal are stabilized by non-classical inter-molecular hydrogen bonds. DFT calculations (B3LYP/6-31+G* level) confirmed that the crystal structures possessed the lowest energies in the gas phase when compared to their geometric isomers. (c) 2006 Elsevier B.V. All rights reserved.

Isomeric olefin tetracarbonyl complexes of tungsten(I): an infrared spectroelectrochemical study at low temperatures

In situ electrolysis within an optically transparent thin-layer electrochemical (OTTLE) cell was applied at 293-243 K in combination with FTIR spectroscopy to monitor spectral changes in the carbonyl stretching region accompanying oxidation of four tetracarbonyl olefin complexes of tungsten(0), viz., trans-[W(CO)(4)(eta(2)-ethene)(2)], trans-[W(CO)(4)(eta(2)-norbornene)(2)], [W(CO)(4)(eta(4)-cycloocta-1,5-diene)], and [W(CO)(4)(eta(4)-norbornadiene)]. In all cases, the one-electron-oxidized radical cations (17-electron complexes) have been identified by their characteristic nu(CO) patterns. For the bidentate diene ligands, the cis stereochemistry is essentially fixed in both the 18- and 17-electron complexes. The radical cation of the trans-bis(ethene) complex was observed only at 243 K, while at room temperature it isomerized rapidly to the corresponding cis-isomer. The thermal stability of the three studied radical cations in the cis configuration correlates with the relative strength of the W-CO bonds in the positions trans to the olefin ligand, which are more affected by the oxidation than the axial W-CO bonds. For the bulky norbornene ligands, their trans configuration in the bis(norbornene) complex remains preserved after the oxidation in the whole temperature range studied. The limited thermal stability of the radical cations of the trans-bis(alkene) complexes is ascribed to dissociation of the alkene ligands. The spectroelectrochemical results are in very good agreement with data obtained earlier by DFT (B3LYP) calculations.