Allan W. Timms

Sandmeyer reactions. Part 7. An investigation into the reduction steps of Sandmeyer hydroxylation and chlorination reactions

For Sandmeyer hydroxylation and chlorination in aqueous solution, the reduction steps have been investigated by means of correlation analyses of the effects of diazonium ion substitution on the rates of reduction. For simple hydroxylation, a change of behaviour between diazonium ions substituted by electron donor groups and those substituted by electron acceptor groups is interpreted as a change within an inner-sphere process from rate-determining electron transfer to rate-determining association of the reactants. By contrast, for citrate-promoted hydroxylation, a similar change in behaviour may be interpreted as a change between inner- and outer-sphere electron transfers. For chlorination, there is no mechanistic variation within the range of substituents examined but the pattern of behaviour is consistent with an inner-sphere mechanism. The various patterns of behaviour are rationalised in terms of the effects of diazonium ion substitution and catalyst ligation on the reduction potentials and self-exchange rates of the various reacting redox couples. Comparative correlation analyses of reductions and other electrophilic reactions of diazonium ions are used to support the arguments advanced in respect of Sandmeyer reduction steps. It is suggested that the CuI reductants react via a nucleophilic bridging ligand at the diazonium Nβ to give transient Z-adducts which are the precursor complexes and that activation for electron transfer involves rotation about the N–N bond.

Sandmeyer reactions. Part 6. A mechanistic investigation into the reduction and ligand transfer steps of Sandmeyer cyanation

For Sandmeyer cyanation at 298 K in 50% v/v aqueous acetonitrile buffered at pH 8, absolute rate constants have been obtained for the reduction of 4-methoxybenzenediazonium tetrafluoroborate by the cyanocuprate(I) anions CuI(CN)43−, CuI(NCMe)(CN)32− and CuI(NCMe)2(CN)2− as (0.50 ± 0.05), (0.12 ± 0.03) and 0.0 dm3 mol−1 s−1, respectively. The relative reactivity of the two reactive cyanocuprates reflects the estimated difference in their standard reduction potentials. Ligand transfer to the aryl radical from the cyanocuprate(II) anions produced in the reaction occurs within the solvent cage. By use of radical clocks, first order rate constants of the order of 1 × 108 s−1 for ligand transfer between the caged reactants can be evaluated although the transfer rate may vary from one aryl radical to another. No difference was discerned in ligand transferring reactivity between the two cyanocuprate(II) complexes involved.

Promotion of Sandmeyer hydroxylation (homolytic hydroxydediazoniation) and hydrodediazoniation by chelation of the copper catalyst: bidentate ligands.

Relative to the rate observed for the hexa-aqua ion, Cu(OH(2))(6)(2+), chelation of the copper catalyst by certain bidentate ligands enhances the rate of hydroxydediazoniation reaction (Sandmeyer hydroxylation); the ligands also provide a source of hydrogen in competitive hydrodediazoniation (H-transfer) reactions. By using the cyclisation of 2-benzoylphenyl radical as a radical clock, it has been possible to evaluate absolute rate constants for both processes effected by a variety of complexes involving one or two bidentate ligands (2-aminocarboxylate, 2-hydroxycarboxylate, 1,3-dicarboxylate, 1,2-diamine). The radical exhibits electrophilic character in both processes. The pattern of behaviour observed suggests the rate determining step in hydroxylation is reaction of the aryl radical at the metal centre to form an organocopper adduct which is rapidly converted into products. The relative reactivities of different complexes are explained qualitatively in terms of variations in the ligand field and Jahn-Teller distortion splittings of the copper d orbitals. Hydrodediazoniation is an S(H)2 H-abstraction process. Generally, coordination by Cu(2+) deactivates the first added ligand relative to its reactivity as a free species in the same state of protonation. For the majority of complexes studied, the relative reactivity as H-donors of 1 : 1 and 1 : 2 complexes is statistically determined but an additional electronic effect is discerned for doubly charged ions.

Sandmeyer reactions. Part 1. A comparative study of the transfer of halide and water ligands from complexes of CuII to aryl radicals

Rate constants have been estimated for the transfer to aryl radicals of Cl ligands from CuCl+ and CuCl2 relative to that for the transfer of water ligands from Cu2+: kOH:k1Cl:k2Cl= 1 : (59.3 ± 3.4): (205 ± 9) The rate data indicate that aryl radicals exhibit nucleophilic character during these ligand transfers and that, in the competition between chlorination and hydroxylation, the reactivity–selectivity principle is not obeyed. In competition between CuBr+ and CuCl+ in transferring halide ligands to 4-chlorophenyl radicals a value of k1Br/k1Cl= 2.3 is estimated; there is no selectivity between bromination and chlorination across a range of aryl radicals. The reactivities of the different copper complexes in ligand-transfer broadly parallel their reduction potentials. The nature of the transition state for ligand-transfer is discussed.

An investigation of the reduction in aqueous acetonitrile of 4-methoxybenzenediazonium ion by the tetrakis(acetonitrile)Cu(I) cation catalysed by hydrogenphosphate dianion.

In aqueous acetonitrile containing a phosphate buffer, 4-methoxybenzenediazonium ion is reduced by one or more of the partially aquated cations derived from tetrakis(acetonitrile)Cu(I) cation in this medium. Investigation of the reaction mechanism indicates the rate determining step to be the association of the diazonium ion with the hydrogenphosphate dianion to give an adduct which then undergoes reduction by Cu(I). The reaction gives a range of products which have been identified and quantified by GC. One of these, 4-methoxyphenol was unexpected in the reducing conditions; its presence could be explained by the disproportionation of a 4-methoxyphenylcopper(II) complex giving bis(4-methoxyphenyl)copper(III) which reacts with water to produce the phenol and an equivalent amount of methoxybenzene. A scheme is proposed which accounts for all the observed products and computer modelling gives a satisfactory description of the distributions of the five major products as functions of the relative proportions of the reactants for dilute conditions and those where the reductant is in excess. When the diazonium ion is in excess, the behaviour of the model and the experimental reactant accountability suggest the occurrence of additional reactions which give products unobserved by GC.

Sandmeyer reactions. Part 2. Estimation of absolute rate constants for some hydrogen-transfer reactions and for the transfer of water ligands on CuII to aryl radicals by use of a Pschorr radical clock

The rate constant for the homolytic Pschorr cyclization of 2-benzoylphenyl radical has been estimated for ambient temperature (20 °C) by competing the cyclization against intermolecular hydrogen-abstractions from alcohols and assuming the latter to occur at rates equal to those measured by Schuler for 4-carboxyphenyl radical. The cyclization was then used as a radical clock for estimating rate constants for other reductions of aryl radicals and for the transfer of water ligands to aryl radicals from CuII. It is found that sulfate ion inhibits the transfer of water and an explanatory mechanism is proposed.

EPR investigation into the kinetics and mechanism of the one-electron reduction of benzenediazonium ions by FeII–EDTA and other iron(II) complexes

An EPR approach has been developed and applied to the determination of the rate constants for the decomposition of some arenediazonium ions by FeII complexes to give aryl radicals. In continuousflow experiments, steady-state concentrations have been measured both for the aryl radical-adducts to the fumarate anion and for ˙CH2CO2–(obtained by reaction of Ar˙ with the iodoacetate anion): analysis of the dependence of these upon [FeII] and [ArN2+] leads to values of the rate constants for electron transfer.For the reaction between FeII–EDTA and a series of diazonium ions, for which values of k are in the range 2 × 104–4 × 105 dm3 mol–1 s–1, it is shown that there is a good dual-parameter Hammett–Taft correlation (involving σI and σR+, and with ρ= 1.74), consistent with the occurrence of an outer-sphere electron-transfer process.

Electron paramagnetic resonance studies of electron attachment to thiophene, bi(thienyl) and some halogen-substituted thiophenes

Exposure of dilute solutions of 2, 2′-bithienyl in 2-methyltetrahydrofuran glasses at 77 K to ionizing radiation gave the corresponding radical-anion, as indicated by the EPR spectra. Similar treatment of thiophene solutions failed to give the expected it radical-anion, although electron capture clearly occurred: for CD3OH and CD3OD solutions the primary products were identified as the H- and D-atom-addition products, formed by protonation of the anions.Under similar conditions, 2-bromothiophene gave clear EPR signals with features characteristic of the C(2)–˙Br–σ* radical-anion (with a spin density of ∼ 37% on bromine). A σ* anion, with the unpaired electron localized in one of the C–Cl σ* orbitals, was detected following electron-addition to 2,5-dichlorothiophene, whereas for 2-chlorothiophene the corresponding σ* radical-anion, if formed, evidently decomposes rapidly to the 2-thienyl radical, whose EPR spectrum is reported. Only for 2,2′-bithienyl was the radical-anions EPR spectrum detected in fluid solution following electron attachment.These findings and the results of MO calculations are discussed in terms of the structures of the radicalanions and the alternative routes available for reaction following electron-addition.