Elba I. Buján

Reactivity of the insecticide fenitrothion toward O and N nucleophiles.

The reactivity of Fenitrothion (1) toward several O- and N-based nucleophiles, including ambident and alpha-nucleophiles, was investigated in basic media at 25 degrees C in water containing 2% 1,4-dioxane. In the reactions with HO(-) and HOO(-) quantitative formation of 3-methyl-4-nitrophenoxide (2) was observed indicating a S(N)2(P) pathway. In the reactions with NH(2)OH, NH(2)O(-), and BuNH(2), demethylfenitrothion (4) was formed along with 2, indicating competition between the S(N)2(P) and S(N)2(C) pathways; no evidence of a S(N)Ar pathway was observed in any case. The observed rate constants were dissected into the values corresponding to the S(N)2(P) and S(N)2(C) pathways. The yield of 4 depends on the nucleophile and on the pH of the reaction, being the main product in the case of BuNH(2). With HOO(-), NH(2)OH, and NH(2)O(-) a significant alpha-effect was observed, confirming the participation of the nucleophile in the rate-limiting step of the reaction.

Investigation of the inclusion of the herbicide metobromuron in native cyclodextrins by powder X-ray diffraction and isothermal titration calorimetry

We report the formation of inclusion complexes between the phenylurea herbicide metobromuron [3-(p-bromophenyl)-1-methoxy-1-methylurea] and beta- and gamma-cyclodextrin in the solid state. Formation of crystalline inclusion complexes by the kneading method was confirmed by powder X-ray diffraction and further structural characterization using the principles of isostructurality followed. In addition, DeltaH degrees , DeltaS degrees , DeltaG degrees and the association constants (K) at 298K were determined for complex formation in solution using isothermal titration calorimetry. The magnitudes of K for the formation of 1:1 complexes between metobromuron and alpha-, beta- and gamma-CD were estimated as 598, 310 and 114, respectively.

Effect of cyclodextrins on the reactivity of fenitrothion

The hydrolysis reaction of fenitrothion was studied in water containing 2% dioxane and in the presence of native cyclodextrins (α-, β- and γ-CD) and two commercially available modified derivatives, namely, permethylated β- and α-cyclodextrin (TRIMEB and TRIMEA, respectively). The kinetics of the reaction in the presence of TRIMEA could not be measured because the complex formed is insoluble and precipitated even at low concentration. On the other hand, the reaction is only weakly affected by the presence of α-CD. The hydrolysis reaction is inhibited by all the other cyclodextrins. From the kinetic data the association equilibrium constants for the formation of the 1:1 inclusion complexes were determined as 417, 511 and 99M(-1) for β-CD, TRIMEB and γ-CD, respectively. Despite the differences in the association constants for β- and γ-CD, the observed inhibition effect is about the same and this is due to the fact that the rate of hydrolysis in the cavity of γ-CD is smaller than that in the cavity of β-CD. The strongest inhibitor is TRIMEB and this result is consistent with the known structure of the complex in the solid state.

Solid-state structures and thermal properties of inclusion complexes of the organophosphate insecticide fenitrothion with permethylated cyclodextrins

The X-ray crystal structures and thermal stabilities of the inclusion complexes formed between the organophosphate insecticide fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate] and the host compounds TRIMEA and TRIMEB (permethylated alpha- and beta-cyclodextrins, respectively) are reported. In the complex (TRIMEA)(2).fenitrothion 1, the guest phosphate ester group is disordered and the molecule is fully encapsulated within a novel TRIMEA dimer in which the secondary rims of the two host molecules are in close contact. In contrast, the complex TRIMEB.fenitrothion 2 is monomeric and the guest molecule is statistically disordered over two positions, with the phosphate group inserted in the host cavity in both cases. Thermal analysis indicated gradual and partial loss of the guest in 1 during heating between 130 degrees C and the melting point of the complex (approximately 200 degrees C), whereas complex 2 displayed significant mass loss only after fusion of the complex at 161 degrees C.

Time evolution and competing pathways in photodegradation of trifluralin and three of its major degradation products

The herbicide trifluralin (I)(N,N-di-n-propyl-2,6-dinitro-4-trifluoromethylaniline) decomposes, by the action of UV-Vis light (lambda > 300 nm), to several products, the most important (because they give subsequent photochemical reactions) being N-n-propyl-2,6-dinitro-4-trifluoromethylaniline (VI), 2-ethyl-7-nitro-5-trifluoromethyl-1H-benzimidazole 3-oxide (VII) and 2,6-dinitro-4-trifluoromethylaniline (XII). The time evolution of degradation of trifluralin (I) and the aforementioned three main photoproducts was studied in water and acetonitrile as solvents. The pseudo-first order rate constants allow one to calculate the branching ratios for some of the reactions involved. The preference for either N-dealkylation or cyclization depends on the solvent employed. Dissolved oxygen accelerates the photodegradation, especially the dealkylation.

Structural investigation of 1-amino-2,4,6-trinitrobenzenes in the solid state and in solution

The crystal structure of 1-pyrrolidino-2,4,6-trinitrobenzene3, 1-morpholino-2,4,6-trinitrobenzene4 and 1-piperidino-2,4,6-trinitrobenzene5 have been determined by single-crystal X-ray diffraction data and the structure in solution was investigated by UV-visible spectrophotometry and13C nmr. The three compounds are monoclinic, space groupP2i/n. Unit cell dimensions area=6.6660(1),b=8.612(1),c=20.696(3) Å, β=90.73(1)o, andDc=1.58 g cm−3 for compound3;a=7.090(2),b=21.493(9),c=8.298(3) Å, β=101.27(1)o, andDc 1.60 g cm−3 for compound4 anda=10.426(1),b=10.038(1),c=12.291(1) Å, β=90.04(1)o withDc=1.53 g cm−3 for compound5 forZ=4. In the solid state, differences regarding the planarity of the aromatic ring in the three substrates were found. Rotation of theortho-nitro groups and of the amino group out of the aromatic plane was observed both in the solid state and in the solution. Greater coplanarity of the two rings was found in3 than in4 and5.

Inclusion of the insecticide fenitrothion in dimethylated-β-cyclodextrin: unusual guest disorder in the solid state and efficient retardation of the hydrolysis rate of the complexed guest in alkaline solution

Summary An anhydrous 1:1 crystalline inclusion complex between the organophosphorus insecticide fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl)phosphorothioate] and the host compound heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB) was prepared and its structure elucidated by single-crystal X-ray diffraction. This revealed two independent host molecules in the asymmetric unit. In one of these, the cavity is occupied by two disordered guest components (distinguishable as rotamers with respect to the P–OAr bond) while in the other, three distinct guest components with site-occupancies 0.44, 0.29 and 0.27 appear, the last having a reversed orientation relative to all the other components. Kinetic studies of the alkaline hydrolysis of fenitrothion in the presence of DIMEB showed a remarkable reduction of 84% in the rate of this reaction relative to that for the free substrate, a value exceeding those previously attained with the native hosts, β- and γ-cyclodextrin, and fully methylated β-cyclodextrin.

Preparation of benzimidazole N-oxides by a two-step continuous flow process

A continuous flow process for the synthesis of nitrobenzimidazole N-oxides from 2,6-dinitrochlorobenzene and amines or amino acids is reported. The process, performed in a two-step sequence, is faster than previously reported batch processes and avoids some of the isolation and purification steps.

Permethylated β-cyclodextrin/pesticide complexes: X-ray structures and thermogravimetric assessment of kinetic parameters for complex dissociation

The X-ray crystal structures of the inclusion complexes formed between three pesticides (two organophosphorus insecticides and one chloroacetanilide herbicide) and permethylated β-cyclodextrin (TRIMEB) are reported. The complexes TRIMEB–fenitrothion (1), TRIMEB–fenthion (2) and TRIMEB–acetochlor (3) are members of a commonly occurring isostructural series. The mode of inclusion of the two organophosphate insecticides is very similar, while the acetochlor molecule, which is structurally quite distinct from the two insecticide molecules, adopts a somewhat different position within the TRIMEB cavity. In addition to the structural elucidation of these complexes, their thermal behaviour was investigated using isothermal and non-isothermal thermogravimetry. The isothermal results showed that the dissociation of the guest molecules from the TRIMEB cavities can best be described by two mechanisms, namely a first-order reaction model and a three-dimensional diffusion model. Both the isothermal and non-isothermal methods allowed the determination of the activation energies of the guest loss process for each complex.