Rate-limiting proton-transfer in the σ-adduct forming reactions of 1,3,5-trinitrobenzene and 4-nitrobenzofuroxan with substituted anilines in dimethyl sulfoxide

Kinetic and equilibrium results are compared for the reactions in dimethyl sulfoxide of 1,3,5-trinitrobenzene, 1, and 4-nitrobenzofuroxan, 4, with a series of substituted anilines in the presence of Dabco or in, some cases, quinuclidine. pKa values for the corresponding anilinium ions are reported. The reactions of 1 and 4 are likely to proceed through nucleophilic attack by the aniline to yield zwitterionic intermediates which may transfer an acidic proton to the bases present to yield the anionic adducts 9 or 12 respectively. In the formation of 9 from 1 the proton transfer step is rate-limiting; however, the slower interconversion of 4 and its zwitterion leads to only partial rate-limiting proton transfer in the formation of 12. Results with substituted anilines including 2-substituted and N-methyl aniline indicate that steric effects are not a major factor in determining rates of proton-transfer in these systems. Contrary to previous reports no evidence was found for a strong interaction between 1 and Dabco in DMSO

Carbanion reactivity, kinetic and equilibrium studies of σ-adduct formation and elimination in the reactions of 4-nitrobenzofurazan derivatives with nitroalkane anions

1H NMR studies are reported of the reactions in [2H6]-DMSO of 4-nitrobenzofurazan, 2a, and its 7-chloro- and 7-methoxy-derivatives, 2b and 2c respectively, with anions derived from nitromethane, 3, nitroethane, 4, and 2-nitropropane, 5. The initial reactions result in -adduct formation by carbanion attack at the 5-position of 2a–c and in the case of reaction of 2a with 5 the adduct at the 7-position is also observed. These reactions may be followed by base catalysed elimination of nitrous acid to yield anionic alkene derivatives. Kinetic and equilibrium measurements of these reactions were made spectrophotometrically in methanol. The carbon nucleophilicities of the carbanions decrease in the order 3 > 4 > 5, as also found in their reactions with benzhydrylium cations, and are much lower than the nucleophilicities of some cyano-substituted carbanions. Comparison with corresponding -adduct forming reactions of 1,3,5-trinitrobenzene, TNB, show that here 2 and TNB have similar electrophilicity, although the value of the intrinsic rate coefficient ko = 0.05, for reaction of 2 is rather lower than that, ko = 0.20, for the TNB reactions. Literature data suggest that for reaction with a variety of nucleophiles 2 and TNB show similar electrophilicities. Measurements of the rates of elimination of nitrous acid from some 5-adducts in methanol catalysed by methoxide ions are reported. Values of rate constants may be influenced both by steric requirements at the reaction centre and by the electronic effects of the 7-substituent

sigma-adduct formation and oxidative substitution in the reactions of 4-nitrobenzofurazan and some derivatives with hydroxide ions in water

The reactions of hydroxide ions with 4-nitrobenzofurazan, 1a, 4-nitrobenzofuroxan, 1b, and with three 4-nitro-7-substituted benzofurazans have been examined using H-1 NMR and UV-visible spectroscopies. In each case initial reaction is at the 5-position to give an anionic sigma-adduct. Kinetic and equilibrium results are reported. NMR spectra show that in the case of 1a oxidation of the anionic adduct yields 4-nitro-5-hydroxybenzofurazan. In the case of 1b rearrangement of the 5-hydroxy adduct to the thermodynamically more stable 7-hydroxy adduct, possibly by a Boulton-Katritzky mechanism, precedes oxidation. When the 7-substituent in the 4-nitrobenzofurazan is Cl, OMe or OPh the eventual product is 7-hydroxy-4-nitrobenzofurazan produced by nucleophilic displacement of the substituent

The Strecker reaction: kinetic and equilibrium studies of cyanide addition to iminium ions

Kinetics studies are reported of the reactions of benzylidene benzylamine, 4a, and of benzylidene allylamine, 4b, with cyanide in aqueous buffers to give the corresponding a-aminonitriles. The results allow the calculation of values of rate and equilibrium constants for reaction of the iminium ions formed from 4a and 4b with cyanide ions. These values are compared with those, obtained from the hydrolysis reactions, for reaction of the iminium ions with hydroxide ions and with water. Comparison with some other iminium ions reveals that those formed from 4a and 4b are relatively unreactive due to the possibilities of charge delocalisation

Catalytic, asymmetric Strecker reactions catalysed by titanium(IV) and vanadium(V)(salen) complexes.

VanadiumV(salen) complex 3 has been found to be an effective catalyst for the asymmetric addition of hydrogen cyanide (generated in situ from trimethylsilyl cyanide) to imines. The best results (up to 81% enantiomeric excess) were obtained for aromatic imines in which the nitrogen atom is protected with a benzyl group and in which the imine bond is not sterically encumbered

Effects of ortho- and para-ring activation on the kinetics of S_NAr reactions of 1-chloro-2-nitro and 1-phenoxy-2-nitrobenzenes with aliphatic amines in acetonitrile

Rate constants are reported for reaction of 4-substituted 1-chloro-2,6-dinitrobenzenes 1, 6-substituted 1-chloro-2,4-dinitrobenzenes 2, and some of the corresponding 1-phenoxy derivatives, 3 and 4, with n-butylamine, pyrrolidine and piperidine in acetonitrile as solvent. Values of k1, the rate constant for nucleophilic attack at the 1-position, increase with increasing ring-activation but may be reduced by steric repulsion at the reaction centre which increases in the order Cl pyrrolidine > piperidine

The uppermost Middle and Upper Albian succession at the Col de Palluel, Hautes-Alpes, France: an integrated study (ammonites, inoceramid bivalves, planktonic foraminifera, nannofossils, geochemistry, stable oxygen and carbon isotopes, cyclostratigraphy)

An integrated study of the ammonites, inoceramid bivalves, planktonic foraminifera, calcareous nannofossils, geochemistry, stable carbon isotopes, and cyclostratigraphy is provided for the upper Middle to upper Upper Albian succession exposed in the Col de Palluel section east of Rosans in Hautes-Alpes, France.The Albian-Cenomanian boundary interval described by Gale et al. at Mont Risou is re-examined, a total thickness of 370 m of the Marnes Bleues Formation. Zonal schemes based on ammonites, inoceramid bivalves, planktonic foraminifera, and calcareous nannofossils are integrated with the stable carbon isotope curve and key lithostratigraphic markers to provide a sequence of more than 70 events in the uppermost Middle Albian to basal Cenomanian interval. Time series analysis of the Al2O3 content of the 500 m Albian sequence present in the Col de Palluel and Risou sections reveals the presence of the 20 kyr precession, 40 kyr tilt, 100 kyr short eccentricity, and 406 kyr long eccentricity cycles. Correlation using planktonic foraminiferan and nannofossil data provide a link between the Col de Palluel and Risou sections and the Italian sequence at Gubbio, and in the Piobbico core. This provides a basis for the extension of the orbital time scale of Grippo et al. to the sequence. It reveals a major break in the Col de Palluel succession at the top of the distinctive marker bed known as the Petite Vérole that may represent as much as 2 Ma. It also provides a basis for the estimation of the length of the Albian Stage at 4.12 Ma, 0.8Ma for the early Albian, 2.84 Ma for the Middle Albian, and 3.68 Ma for the late Albian substages