Victor Gold

Reduction of carbonyl compounds by sodium borohydride (tetrahydridoborate) in water, dimethyl sulphoxide, and their mixtures as solvents: products and kinetics

Rate measurements are reported for the reduction of acetone pivalaldehyde, and benzaldehyde by sodium and tetramethylammonium tetrahydridoborates in dimethyl–sulphoxide–water systems, some containing a low concentration of sodium hydroxide to impede the hydrolysis of tetrahydridoborate. The reactions obey second-order kinetics. The rate constants decrease as the water content of the solvent is reduced but, with the most reactive substrate (benzaldehyde), the reduction is still detectable even in a solvent not containing any added water. Evidence has been obtained that the reduction product of benzaldehyde in dimethyl sulphoxide solution is sodium tetrakisbenzyloxyborate, which is readily hydrolysed to benzyl alcohol. A condensation product of benzaldehyde and dimethyl sulphoxide (2-methylsulphinyl-1-phenylethanol) has been isolated as a by-product. Attempts to trap borane during the reduction gave negative results.

Sodium borohydride as a reagent for nucleophilic aromatic substitution by hydrogen: the role of hydride Meisenheimer adducts as reaction intermediates

The reaction of o-bromo- and p-chloro-nitrobenzenes and of 1,4-dinitrobenzene with sodium borohydride in a dilute solution in dimethyl sulphoxide solution leads to some nitrobenzene. When tritiated borohydride is employed, aromatic hydrogen exchange occurs to a comparable extent.Under similar conditions 1-X-2,4-dinitrobenzenes mostly undergo either displacement of the 2-nitro-group by hydrogen or ring reduction. With these compounds it is possible to observe the n.m.r. spectra of the unstable cyclohexadienyl anions (hydride Meisenheimer adducts), formed by hydride addition to the 3- and/or the 5-position, which decompose into the final products.For 2,4-dinitroanisole it is shown by an isotope-labelling experiment that the displacement of the 2-nitro-group is accompanied by intramolecular hydrogen migration from the methylene group of the Meisenheimer adduct (at the 3-position) to the 2-position. This mechanism is the nucleophilic analogue of the mechanism of ipso-attack with migration, recognised as one of the reaction paths in electrophilic aromatic substitution. The term ‘vicinal attack’ is proposed for this general class of aromatic substitution reactions in which the entering group first attacks and attaches itself to a ring position adjacent to the position that it occupies in the reaction product and from which the leaving group is displaced.

Kinetics of hydrogen isotope exchange reactions. Part XXX. Steric course of γ-radiation-induced exchange between water and tartaric acids

γ-Irradiation of solutions of (+)-tartaric acid and of meso-tartaric acid in tritiated water at pH 7 leads to an exchange of carbon-bonded hydrogen with the solvent. For each substrate the exchange proceeds to an approximately equal extent with inversion and with retention of configuration. The results support the conclusions of a similar stereochemical study on cis- and trans-cyclohexane-1,2-diols.

Enthalpies of transfer involving ionic transition states : the redundance of extrathermodynamic assumptions

It is pointed out that the calculation of transition state enthalpies of transfer for reactions involving ions does not necessitate the estimation of thermodynamic quantities for single ions. The procedure is illustrated by data on the reaction between ethyl acetate and sodium hydroxide in water–dimethyl sulphoxide mixtures.

Meisenheimer adduct formation between 2,4-dinitrophenetole and ethanolic alkali ethoxide solutions: the course of the acidity function JE and thermodynamic solvent isotope effects

Spectrophotometric measurements of the equilibrium constant (Kapp) for formation of a Meisenheimer adduct from 2,4-dinitroanisole and ethanolic sodium and potassium ethoxide have been used to define an acidity function JE for alkoxide solutions in ethanol (analogous to the functions JM for methanol). Sodium and potassium ethoxide behave identically up to a concentration of ca. 0.3 mol dm–3; above this value solutions of sodium ethoxide are slightly less basic. The dependence of Kapp on base concentration is interpreted in terms of ion-pairing effects. The formation of a 1 : 2 adduct becomes significant at high base concentrations, and appears to involve cation-specific ion-pairing effects. The limiting value of log Kapp at low concentrations is -3.1. In EtOD solutions of the same alkoxide concentration, the equilibrium constant is greater than that in EtOH solutions by a factor of 2.5. This result and measurements in mixtures of EtOH and EtOD are consistent with the formulation of the ethoxide ion as an entity containing three hydrogen-bonded solvent molecules, and a deuterium fractionation factor of ca. 0.72–0.73 for the hydrogen-bonded positions.

Hydrogen isotope exchange in the aldehyde group during the reduction of benzaldehyde by tritiated sodium borohydride (tetrahydridoborate)

The reduction of benzaldehyde by tritiated sodium tetrahydridoborate in dimethyl sulphoxide and dimethyl sulphoxide–water mixtures as solvents is accompanied by the incorporation of tritium into the aldehyde group of unchanged benzaldehyde. The occurrence of the exchange implies that the hydride-transfer step of the reduction is reversible. As it is improbable that there is reversibility after the formation of a stable boron–oxygen bond, it is concluded that the hydride-transfer step precedes the step in which the boron–oxygen bond is formed. However, attempts to detect the implied intermediate borane in dimethyl sulphoxide solution by trapping with amines had proved negative. It follows that borane is destroyed by reaction with the other product of the first reaction step (the benzyl oxide ion) more effectively than it can react with an amine. Since the reaction of borane with a tertiary alkylamine is likely to be very rapid, the reactions between borane and benzyl oxide ion (leading either back to the starting materials with hydrogen exchange or, predominantly, to an alkoxyhydridoborate species) are considered to occur competitively as cage reactions before the primary reaction products have diffused apart.The exchange reaction exhibits kinetic complications which are attributed to a more rapid hydrogen exchange between benzaldehyde and the reaction product sodium tetrakisbenzyloxyborate (which is shown to take place) and to the instability of this product in the media employed. It is suggested that the high basicity of the reaction solutions is due to the presence of some sodium benzyl oxide in equilibrium with sodium tetrakisbenzyloxyborate, leading to the formation of dimsyl ions.

Kinetics of hydrogen isotope exchange reactions. Part XXVII. Catalysis of aromatic tritium exchange between benzyl alcohol and solvent water by potassium tetrachloroplatinate(II)

Tritium uptake into the aromatic positions of benzyl alcohol from tritiated solvent water (containing some acid and chloride ions, at 50°) is catalysed by tetrachloroplatinate(II). The reaction shows a somewhat erratic induction period [due to platinum (IV) impurity] which is greatly increased by addition of potassium hexachloroplatinate(IV). Almost no exchange occurs during the induction period, so that for several days the reaction appears to be totally suppressed by PtIV. The results are interpreted in terms of the rate-limiting formation of a common intermediate (an adduct between benzyl alcohol and PtCl3–) which is oxidised by PtIV more rapidly than it can proceed to exchange hydrogen with the solvent. The rate of the exchange reaction (after the induction period) is approximately proportional to the concentration of PtCl42– and inversely proportional to the concentration of chloride ion. The dependence of the exchange rate on hydrogen-ion concentration is consistent with a protolytic equilibrium involving the alcohol group in a benzyl alcohol–PtCl3– complex (pK 3·6), with the basic form being more reactive than its conjugate acid.

Radiolytic displacement of substituent groups in benzene by hydrogen (tritium)

The experimental work examines the extent to which β-radiation-induced hydrogen exchange of aromatic substrates in aqueous solution with the solvent is accompanied by displacement of a substituent group in the aromatic compound. Radiation-induced substituent displacement is observed to any significant extent only for the halogenobenzenes (Phl > PhBr > PhCl). A dual function is ascribed to copper(II) ions, which act as inhibitor of both exchange and substituent displacement: they act as oxidising agent towards hydrogen atoms and substituted cyclohexadienyl radicals with attendant formation of the corresponding cations. This model explains why, in the case of bromobenzene, copper(II) ions inhibit the substituent displacement reaction more than the tritium exchange.