James H. P. Utley

Electro-organic reactions. Part 41. Diels-Alder reactions of o-quinodimethanes from the cathodic reduction of α, α′-dibromo-1,2-dialkylbenzenes

Abstract Cathodic reduction of α, α′-dibromo-1,2-dialkylbenzenes in DMF containing Et4N+ Br− as supporting electrolyte, in the presence of dienophiles (e.g. maleic anhydride derivatives), yielded Diels-Alder adducts of o-quinodimethanes. Selective reduction of the benzylic dibromides was often possible even when their irreversible cyclic voltammetric reduction peak potentials were more negative than the Eo values for the anhydrides. This might be a consequence of redox catalysis, with the anhydrides having a dual role as mediators and dienophiles.

Electroorganic reactions. Part 56: Anodic oxidation of 2-methyl- and 2-benzylnaphthalenes: factors influencing competing pathways ☆

Abstract A systematic investigation of the anodic oxidation in nucleophilic media of 2-methyl and 2-benzylnaphthalenes, substituted at the 6-position in the naphthalene nucleus and at the 4-phenyl position of the benzylic side chain, has been carried out to identify factors favouring side-chain substitution. Cyclic voltammetry confirms that 6-substitution has a profound effect on the oxidation potentials of the naphthalene nucleus and 13C chemical shifts indicate polar effects at the benzylic carbon. However, little side-chain anodic oxidation is observed under any conditions tried; the radical–cations of electron-rich substrates preferentially dimerise and a strongly electron-withdrawing substituent at the 6-position (EtOSO2) promotes nuclear substitution. In contrast, oxidation with DDQ in aqueous acetic acid gives efficient side-chain oxidation for electron rich substrates, consistent with hydride transfer, possibly intramolecularly via a charge transfer complex.

An electrochemical route to poly(p-phenylenevinylene) polymers (PPVs) and copolymers via cathodically generated quinodimethanes: preparative and structural aspects

An electrochemical route to poly(p-phenylenevinylenes)(PPVs) is described which involves cathodic cleavage of bis(dibromomethyl)arenes at a mercury pool cathode. Insoluble and organosoly PPVs are obtained in good yields and are characterised as regular and linear. Copolymers are obtained by co-electrolysis of different bis(dibromomethyl)arenes. The method is versatile and much functionality is tolerated. The polymers can be made conducting by doping with BF3 or SO3. The effect of substituents on conductivity correlates, roughly but significantly, with Hammett σ-values. Poly(p-phenylenevinylene), the parent PPV, forms conducting blends with poly(vinyl alcohol).

Electrochemical synthesis of poly(p-xylylenes)(PPXs) and poly(p-phenylenevinylenes)(PPVs) and the study of xylylene (quinodimethane) intermediates; an underrated approach

The electrosynthesis of poly(p-xylylenes) (PPXs) and poly(p-phenylenevinylenes) (PPVs) by the direct and mediated cathodic reduction of (halomethyl)arenes is reviewed and, where appropriate, comparisons made with more conventional chemical routes. Attention is drawn to the advantages of mild (non-thermal) conditions and to the consequent toleration of a wide range of substituents in the (halomethyl)arene precursors. Copolymers are obtained by co-electrolyses and aprotic and aqueous conditions may be used. Several polymers that are inaccessible by the Wessling and related routes may be prepared electrochemically. The properties, including lifetimes, of the key quinodimethane (xylylene) intermediates have been examined by cyclic voltammetry and the results are consistent with a radical-chain mechanism for polymerisation, initiated by electrogenerated radical-anion. The physical properties of electrochemically derived polymers are comparable re molecular weight and photoelectrical properties to those prepared chemically.

Anodic cleavage of lignin model dimers in methanol

Abstract The anodic oxidation in methanol of some lignin model compounds, the C17 – C18 compounds ((1) – (5), has been studied to ascertain factors influencing cleavage of Cα  Cβ bonds cf. substitution. Cyclic voltammetric oxidation peak potentials were influenced by structure but not by pH. Preparative-scale electrolyses gave 3,4-dimethoxybenzaldehyde as the major product from compounds (1) – (3), corresponding to cleavage of th Cα  Cβ bonds. In contrast compounds (4) and (5) were not cleaved with significant efficiency. The results are convincingly rationalised in terms of rapid cleavage of the Cα  Cβ bonds in the first-formed radical cations.

Electroorganic reactions. Part 57.† DDQ mediated anodic oxidation of 2-methyl- and 2-benzylnaphthalenes

Electron-rich 2-methyl and 2-benzylnaphthalenes are efficiently converted into the corresponding carboxaldehyde or ketone by mediated electrolysis in the presence of about 25 mol % of DDQ. The reaction conditions are simple (constant current, undivided cell of 500 ml volume, graphite electrodes, aqueous acetic acid at 80 °C) and the construction of a reaction profile (products vs charge) shows the intermediate formation of side-chain acetoxy and hydroxy derivatives and the parallel formation of a byproduct involving side-chain substitution by DDQ.

Electro-organic reactions. Part 46. Diels-Alder trapping of o-quinodimethane generated by redox-mediated cathodic reduction of α,α′-dibromo-o-xylene in the presence of hindered dienophiles☆

Abstract Hindered, electron-deficient alkenes 5–11 have been prepared, including analogues of maleic anhydride, N-phenylmaleimide and benzoquinone. Cyclic voltammetry showed that most of these compounds can undergo reduction to form relatively persistent radical-anions, which can mediate the reductive cleavage of α,α′-dibromo-o-xylene. Preparative electrolytic reduction of α,α′-dibromo-o-xylene was performed in the presence of the alkenes 5–11; in several cases the Diels-Alder adducts of the alkenes and o-quinodimethane could be isolated, with particularly high yields of cycloadducts being obtained from the dienophiles 5a, 5b and 6a.

Stereoselectivity and mechanism in the electrohydrodimerisation of esters of cinnamic acid

Rate constants (kobs) and reaction orders have been determined for the cathodic reduction in DMF solution of 11 cinnamic acid esters including some derived from chiral alcohols and a dicinnamate derived from trans-cyclohexane-1,2-diol. The cinnamic acid esters typically reduce with high stereoselectivity to all-trans 3,4-diphenylcyclopentanone-2-carboxylates. The enhancement of rates of reaction by addition of water was studied for selected substrates and low energies of activation were found. Changes in the alkoxy or aryloxy groups also caused significant changes in rate and log kobs correlated linearly with E° values. The results from kinetic experiments were complemented by product studies of reactions aimed at probing reversibility of key reaction steps. The combined evidence is interpreted as unambiguous support for radical anion–radical anion coupling as the key step with complexation with water, prior to coupling, being crucial.The relative stereochemistry at C-3 and C-4 is fixed, irreversibly, at the coupling stage and there is strong evidence to suggest that templating in the complex between two radical anions and water determines the stereochemical outcome.

Electrochemical route to xylylene polymers and copolymers via cathodically generated quinodimethanes: preparative and structural aspects

Electrochemical reductive elimination of bis(α-bromoalkyl)arenes gives quinodimethane intermediates that polymerize, mostly in high yield, to linear polymers of the poly(p-xylylene) type. At a stirred mercury cathode reaction proceeds to completion and the polymers precipitate. A considerable variety of structural variation in the precursors is tolerated. Co-electrolysis of different precursors gives copolymers. In some cases organo-sole fractions are formed. Key structural features of the polymers and copolymers are obtained by NMR spectroscopy [1H and 13C (solution and solid-state)] and by direct pyrolysis mass spectrometry. The high reactivity of the p-quinodimethanes is evident from the observed random polymerization and co-polymerization.

Electro-organic reactions—14. The intramolecular cathodic cyclization of abscisic acid methyl ester

Abscisic acid methyl ester (1) is reduced smoothly at controlled potential at a mercury cathode in a 2 F mol−1 reaction which gives the bicyclic ester (5) in good yield. Cyclic voltammetry of (1), and several related model compounds, reveals that the C-6 hydroxyl group precludes conjugation throughout the molecule and thereby allows selective electron transfer to the side chain. The radical-anion so formed isomerises and undergoes rapid 6-exo-trig cyclization.