N. Sbarbati Nudelman

Electron transfer in the reactions of aryllithium compounds with carbon monoxide

Abstract The reactions of aryllithium compounds (Ar= phenyl, 1-naphthyl, xylyl, mesityl) with CO have been studied under several reaction conditions. A 13 C NMR method developed to determine radicals at preparative concentrations revealed the presence of radical anions in the reaction mixtures in concentrations comparable to those of the reagents. ESR spectroscopic studies, the effect of radical inhibitors, kinetic measurements and isolation of derivatives of some intermediates suggest a mechanism that involves electron transfer as the first and rate determining step of the reaction.

Mechanism of cyclization of substituted 2′-hydroxychalcones to flavanones

The conversion of 2′-hydroxy-(3), 2′,4′-dihydroxy-(4), 2′,3-dihydroxy-(5), and 2′,4-dihydroxy-chalcone (6) in aqueous alkali; and of (3) and 2′-hydroxy-4-nitrochalcone (7) in alkaline methanolic medium into the corresponding substituted flavanones has been studied kinetically and spectroscopically. It has been found that results from both methods are consistent with the existence of chalcones in the trans-s-cis conformation and indicate a mechanism that involves general acid attack to the ionized form of the 2′-hydroxychalcones, rotation through the CO–Cα bond and annelation to the flavanone.

The reaction of phenyllithium with carbon monoxide

Abstract Three main products are obtained from the reaction between phenyllithium and carbon monoxide, namely: benzophenone (I), benzoin (II) and α,α-diphenyl-acetophenone (III). Evidence is given for the existence of benzoyllithium as the first intermediate of the reaction and for the subsequent intermediates in the production of I, II and III. The basic sequences followed in the formation of those and of other minor products are outlined in the Scheme. Reaction conditions can be adjusted to obtain III in a high yield or to prevent further reaction of the first intermediate and obtain diarylalkylcarbinols, or substituted tetrahydrofurans.

Cyclization of substituted 2′-hydroxychalcones to flavanones. Solvent and isotope effects

The kinetic isotope effects on the cyclization of substituted 2′-hydroxychalcones (R = 3-hydroxy, 4′-hydroxy, 4,4′-dihydroxy, 4′-hydroxy-4-nitro, 4-nitro, or 5′-methyl) into the corresponding substituted flavanones have been measured in water and/or methanol. The pronounced isotope effects observed are consistent with the slow rupture of a bond to hydrogen. The effect of the medium permittivity on the rate of cyclization of 2′-hydroxy, 2′,4-dihydroxy, and 2′,4-dihydroxychalcones have been also studied in several mixed solvents and the observed dependence is consistent with a reaction between one ion and one dipole. These results confirm a mechanism that involves general acid attack of the ionized form of the 2′-hydroxychalcones, concerted rotation through the CO–Cα bond, and annellation to the flavanone.

Reaction of 2,6-dinitroanisole with cyclohexylamine in toluene–octanol binary solvents. Further support for the ‘dimer nucleophile mechanism’ in aromatic nucleophilic substitution

The reaction of 2, 6-dinitroanisole with cyclohexylamine in toluene, octanol and toluene–octanol mixtures has been studied at several amine concentrations. The reactions in toluene and in octanol–toluene mixtures up to 30% octanol show a third-order kinetic dependence on [amine]. Although the reaction rates in toluene and octanol are very similar, addition of small amounts of octanol to the toluene solution diminishes the rate of reaction, reaching a minimum value at 30–50% octanol, after which the rate increases almost linearly with the octanol content. These and other results, here described, are satisfactorily explained by the previously proposed ‘dimer nucleophile mechanism’. Previously reported similar observations in methanol–benzene mixtures were criticised as being due to the reversibility of the reaction. The present results confirm the previously reported mechanism and the irreversibility of the reaction under the reaction conditions.

Reactions of nitroanisoles. Part 6. Reaction of 2,6-dinitroanisole with cyclohexylamine in mixed solvents. Solvent evidence for the ‘dimer’ mechanism

The reaction of 2,6-dinitroanisole with cyclohexylamine in methanol and benzene–methanol was studied at several amine concentrations. Although the reaction rate in methanol is higher than in benzene, addition of small amounts of methanol to the benzene solution diminishes the rate of reaction (which is of third order in amine), reaching a minimum value at nearly 30% methanol, after which the rate increases almost linearly with the methanol content. These and other results, here described, are satisfactorily explained by a previously proposed reaction scheme, in which the dimer of the amine is operating.

Determination of TBT in water and sediment samples along the Argentine Atlantic coast

Cases of imposex have been reported for some organisms living in areas of the Argentine Atlantic coast. Since this is one of the known effects of the anti‐fouling agent tributyltin (TBT), quantitative determinations of organotins in samples of water and sediments collected from sites along the Argentine coast were carried out. Severe cases of imposex were first reported for two gastropod species living in the Mar del Plata area, and determinations of TBT in samples collected from this site gave extremely high values and showed a close correlation between the degree of imposex and TBT concentration. Recent investigations in the area have shown a significant decrease. Surveys were also conducted in sites that exhibit highly irregular coastal profiles to examine the relevance of physical environments. Alarming concentrations of TBT were determined in most of the sites where heavy boat traffic and/or marine activities occur, demonstrating the urgent need for regulations to avoid further input of TBT. Reports from other sites in South America reveal that this should be a subject of regional concern in order to avoid severe damage to the biodiversity of regional marine organisms.

Aggregation effects in the reactions of 2,4-dinitrochlorobenzene with aniline in aprotic solvents

The kinetics of the reaction of 2,4-dinitrochlorobenzene with aniline have been studied in toluene and in toluene–methanol binary solvents and compared with the reactions in benzene and chloroform. The reactions in aprotic solvents exhibit a rate dependence that is third-order in amine consistent with aggregates of the aniline acting as the nucleophile. Molecular complexes between the aniline and the substrate were also detected spectrophotometrically. Additionally, an inhibitory effect of toluene complexes was observed, which is similar to the previously reported effect of benzene in the reaction of the same substrate with n-butylamine. The reaction in the presence of pyridine shows an amine dependence which indicates formation of ‘mixed aggregates’ between aniline and pyridine. All the above results, as well as recently reported studies of other reactions different from SNAr, are fully interpreted within the ‘dimer nucleophile’ mechanism.

KINETIC STUDY OF THE REACTIONS OF2‐AMINO‐5‐CHLOROBENZOPHENONE WITH HCl in MeOH–H2O

The reaction of 2-amino-5-chlorobenzophenone (1) with 0·5–2 M HCl was studied in 1:1 (v/v) MeOH–H2O at 60 and 80 °C. Products that were isolated were characterized as 2-(N-methylamino-5-chlorobenzophenone) (2), 2- amino-3,5-dichlorobenzophenone (3), 2-N-methylamino-3,5-dichlorobenzophenone (4), 2-(N,N)-dimethylamino-5- chlorobenzophenone (5), 2,4-dichloro-10-methyl-9,10-acridinone (6) and 2,4-dichloro-9,10-acridinone (7). The rates of reaction of 1 and the rates of formation of 2–5 were measured at several HCl concentrations. The methyl transfers, the chlorination and the cyclization reactions that give rise to 2–7 were unexpected under the present reaction conditions. A set of differential equations was proposed in order to calculate the rate constants for each step of this complex reaction. The proposed reaction scheme also takes into account the reaction 21 and permits the calculation of the rate constants for this reversible reaction. The experimental values of the rate constants for reaction of 1 were compared with those for 2 under the same reaction conditions, in order to evaluate the importance of the methyl group on the methyl transfer reactions; it was found that the methyl group is not required for the unexpected reaction to occur.

Theoretical calculations of chemical interactions. Part 5. 1,1-Disubstituted 2,4,6-trinitrocyclohexadienylide (‘Meisenheimer’) complexes

All-valence-electron MNDO SCF-MO calculations with almost complete geometry optimization have been performed on 1,1-dimethoxy-2,4,6-trinitrocyclohexadienylide. The computed geometrical parameters have been compared with X-ray data. Charge distributions have been calculated for this complex and for 2,4,6-trinitroanisole, and the differences in electron densities for each atom compared with the 1H and 13C n.m.r. chemical shifts for both compounds. Similar calculations have been performed for 1,1-difluoro-2,4,6-trinitrocyclohexadienylide and for picryl fluoride. The results have been compared with available 1H n.m.r. data. The reasonable good agreement between calculated and experimental data indicates that for these systems the MNDO method can be safely used to calculate properties of species not yet amenable to experiment.