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Rates of reduction by sodium borohydride in propan-2-ol have been measured for the 2-furyl and 2-thienyl ketones 2-(XC6H4CO)C4H2YE (E = O or S) for a range of X groups with Y = H, and of Y groups with X = H. The results permit direct comparisons to be made of transmission of substituent effects through the various aromatic rings to the same reaction centre, and it is concluded that the substituent effects are transmitted to virtually identical extents through the two heterocyclic rings, but markedly less effectively through the benzene ring. Charge distributions have been calculated by an ab initio method for furan, thiophen, and benzene and for the carbanions, 2-furylCH2–, 2-thienylCH2–, and PhCH2–. The change in the π-electron density at the 5-position on introduction of the 2-CH2– groups is smaller for furan than for thiophen, and the change in the latter case is slightly smaller than that induced at the 4-position on introduction of the 1-CH2– group for benzene, suggesting that transmission of conjugative effects between p-positions in the benzene ring is greater than that between the 5- and 2-positions in the heterocyclic compounds. The corresponding changes in total electron density, however, are effectively the same for all three rings. The electron density on the CH2– group is virtually identical in all three carbanions, suggesting that, contrary to the usual assumption, the π-systems of furan and thiophen rings do not interact more readily than those of benzene with a substituent capable of conjugation. Some of the data have been used to derive σ5,2 and σ4,2 constants for thiophen and furan systems.

Mono O-acylation of unsymmetrical diols via their dibutylstannyl derivatives takes place with high reversed chemoselectivity at the most substituted site, the reaction being complementary to standard methods.

Acylsilanes and C-stannylimines react with organic halides in the presence of KF–18-crown-6 ether to give ketones and ketimines, respectively.

Trimethylphenylthiosilane has been shown to catalyse the addition of benzenethiol to isocyanates, isothiocyanates, activated double and triple bond, and ketens. This silicon catalyst appears much more effective than the tin and lead counterparts, for most of the systems investigated.The slow formation at room temperature of the 1 : 1 adduct between the catalyst and the unsaturated system, and the low reactivity of the adduct itself towards benzenethiol exclude the intermediate formation of an insertion product in the catalytic process. A probable mechanism for these reactions is presented.

Earlier work on rates of nitration of the ions Ph[CH2]nX (X = NR3, SR2 PR3, etc.) is analysed in terms of the non-conjugative and conjugative interaction of [CH2]nX with the aromatic ring. Partial rate factors for meta- substitution are taken as a measure of the non-conjugative interaction. The variation of the deactivation at the meta-position with the distance of the pole from the ring is shown to be more consistent with the operation of a field effect than with that of an inductive effect. Apparent exceptions occur when the methylene chain is U shaped for the deactivation is then less than expected. From this and studies with bridged substrates, it is concluded that the transmission of the field effect occurs almost entirely through the molecular cavity but, unlike the inductive effect, is not restricted to the direction of chemical bonds with in that cavity. In the ions Ph[CH2]nX, there is no simple relationship between the orientation of substitution and the deactivation at the meta-position. The additional factor involved is identified as conjugative interaction and an approximate linear free energy relationship is derived relating the para : meta ratio to the σ°R value for the substituent. This relationship is used to provide information on the relative importance of conjugative and non-conjugative interaction, the comparison of nitrogen and phosphorus poles, and the possible effects of the conformation of the CH2NH3 substituent on the deactivation of the ring.

The insertion reactions of diethylaminotrimethylsilane (TMSDEA) and diethylaminotrimethylstannane (TMSnDEA) into oxirane, oxetane, and lactone rings are reported. Regioisomerically pure β- and γ-amino alcohols were obtained, with AlCl3 or Et2AlCl catalysis, from monoalkyloxiranes and monoalkyl-and aryl-oxetanes respectively, whereas, in the majority of cases, reactions involving polysubstituted oxiranes resulted in a loss of regioselectivity. Ring-opening reactions of the lactone rings with TMSDEA led, on the other hand, to β-amino acids or to ω-hydroxyamides depending upon the ring size.With TMSnDEA, spontaneous ring cleavage of the alkyloxiranes and of β-propiolactone occurred with reversed regioselectivity, while the oxetanes and γ- and δ-lactones were opened with the same regiochemistry as that found with TMSDEA for the same ring systems.

The reactivities, with respect to benzene, of certain open-chain ions (I; X = NMe3 or SMe2) and bridged ions (II; X = NMe2 or SMe) have been compared in nitration and bromination with special reference to the rate retardation caused by bridging [(I)→(II)]. This retardation decreases with an increase in the degree of methylation of the poles and in the length (m) of the methylene chain. For the compounds studied here, the retardation is greater when n= 2 than when n= 1 provided that the length of the bridge and the extent of alkylation of the poles are held constant. The retardation is attributed mainly to the change in the form of the molecular cavity with a further contribution, when n > 1 from the change in the distance of the poles from the aromatic ring.

The rates of nitration of the open-chain ions (I) in aqueous sulphuric acid are compared with those of the corresponding bridged ions (II). The comparison is used to provide evidence on how the substituent effects of the groups –[CH2]n˙+NR3 depend on the conformation of the carbon chain. The results indicate that for n 2, the deactivation produced by the nitrogen poles depends more directly on the distance of the poles from the centre of the ring than on the number of carbon atoms in the carbon chain. This is interpreted in terms of the field effect of the pole. For n= 1, there is evidence for additional modes of deactivation but the field effect still appears to be the major factor.

The transalkylation between some ring-nitrogen activated alkyl heterocyclic ethers and thiophenol has been studied quantitatively for alkoxy-1-methylbenzimidazoles. The influence of structural changes of the alkoxy-moiety and of the benzene nucleus on the basicity and reactivity of the system has been investigated. The results are consistent with a reaction mechanism where a rapid acid–base equilibrium between thiol and ether was established, followed by an SN2 attack by ArS– at the ether saturated carbon atom. 1-Methyl-2-methylthiobenzimidazole did not react with thiophenol: this fact was interpreted in terms of the low electron-donating ability of the sulphur atom of the sulphide.

The kinetics of hydrogen isotope exchange between deuteriosulphuric acid and the aromatic protons of diquaternary ions belonging to the series (I; n= 1–3) and (II; n= 2 or 3) have been studied by n.m.r. spectroscopy. A logarithmic plot of the rates of hydrogen isotope exchange against the rates of the corresponding nitration reactions is linear with a slope of 1·5. This relationship supports the view that steric hindrance is not a major factor in determining the deactivation of the bridged ions (II) relative to the open-chain ions (I) for a given value of n. This deactivation is attributed to the greater field effect of the nitrogen poles in the bridged ions.

The kinetics of the hydrolysis of 2-methoxy-1-methylbenzimidazole and of the corresponding 5-nitro-derivative have been investigated as a function of pH. The results, together with some evidence from 18O exchange, indicate the presence of several reaction paths which vary in importance with medium acidity.

Partial rate factors have been determined for molecular chlorination in nitromethane and protodesilylation in H2SO4–AcOH mixtures of methyl phenyl and diphenyl sulphoxide. The electron-activating effect observed in molecular halogenation is interpreted in terms of polarization of the sulphoxide group in the transition state, induced by the electrophilic reagent. In H2SO4–AcOH mixtures Krel for protodesilylation at the para-position of methyl phenyl sulphoxide is very low, possibly because of hydrogen bonding between the sulphinyl group and the acidic medium. For methyl phenyl sulphide, sulphoxide, and some other compounds the ground state electron densities at the para-position have been evaluated from their 13C n.m.r. spectra.

International audience; Chiral amines can be synthesized with high yields and enantioselectivities from abundant organic substrates using transition metal catalytic systems containing most of the time an appropriate chiral bidentate ligand. Reductive amination starts from ketones, hydroamination from alkenes, or alkynes in an inter- or intramolecular mode. In the hydroaminoalkylation reaction, a C–H function in α-position with regard to the nitrogen is activated and forms a new C–C bond. In the hydroaminomethylation, a one-pot carbonylation reaction, the olefin reacts with CO/H2 and amine. Complexes chiral-only-at-metal begin to be efficient to perform the photocatalytic chiral amination under mild conditions. Many recent trends are focused onto first-line transition metals. The major difficulties lie in the synthesis of chiral ligands and the design of a suitable coordination sphere.

Un elemento dell’atmosfera culturale che ci circonda è costituito da un rinnovato interesse per il Negativo, legato alla possibilità di individuare, tra le sue pieghe, la trama di una valenza costruttiva. Il negativo, cioè, non sarebbe, come intuitivamente appare, semplicemente l’opposto del positivo, ma qualcosa che gioca un ruolo più articolato e promettente, non solo qualcosa da cui difendersi, ma qualcosa che può essere utilizzato. Nessuna disciplina può rivendicare, a buon diritto, la paternità di questo clima. È più esatto dire che ci sono, a monte, più padri, i quali, hanno contribuito a crearlo, mantenendo viva la speranza, antica quanto il mondo, di dominare, in qualche modo, quella componente “negativa” senza la quale molti fenomeni della vita, compresa la sua distruzione, rimangono insufficientemente spiegabili. La matematica, la fisica, la psicoanalisi, la fotografia, la pittura, la fisiologia e la chimica sono i vertici attraverso i quali sette autori hanno considerato le relazioni tra "positivo" e “negativo" con l’augurio che il lettore possa respirare "positivamente" l’atmosfera del negativo, facendone propri, a suo modo, gli elementi più stimolanti.