Derek Pletcher

The catalysis of the electrochemical reduction of alkyl bromides by nickel complexes: The formation of carboncarbon bonds

Abstract The square planar, macrocyclic nickel complex, N, N′-ethylenebis(salicylideneiminato)nickel(II), is shown to be an effective catalyst for the electrochemical reduction of substituted alkyl bromides; this indirect cathodic reduction can lead to a good yield of dimeric products. The reduction of alkyl bromides in the presence of an activated olefin is shown to lead to mixtures of products compatible with radical addition to the double bond. The mechanism of the reaction of nickel(I) complexes with alkyl bromides is discussed in the light of these results.

The synthesis and electrochemistry of CpTiCl2(OR) (R = alkyl, aryl) complexes

The syntheses of several new CpTiCl2(OR) (R = alkyl, aryl) complexes are described. It was possible to isolate pure product when the R group is substituted such as to cause steric crowding at the metal centre; for example, particularly good yields of the phenolate complexes were obtained when there were isopropyl substituents in the 2 and 6 positions of the phenolate. Electrochemical studies of the complexes in dry THF show that the TiIII complexes are relatively stable, but only a diol complex could be reduced further to a TiII species. In general, the TiIV complexes undergo a reversible 1e− reduction reaction. The chemistry is more complex if the electrolyte contains added water: both the TiIV and TiIII complexes can react with water, the OR group being replaced by OH. The reaction is particularly rapid for the TiIII alkoxide complexes.

Reaction of electrogenerated square-planar nickel(I) complexes with alkyl halides

The electrochemical reduction of the complexes, [NN′-ethylenebis(salicylideneiminato)]nickel(II), [NiL2], and (5,5,7,12,12,14-hexamethyl-1,4,8,11 -tetra-azacyclotetradecane)nickel(II), [NiL1]2+, to the corresponding nickel(I) complexes at a platinum cathode in acetonitrile has been studied. The nickel(I) complexes react rapidly with alkyl bromides and alkyl iodides and, with the exception of the [NiL1]2+-primary alkyl bromide systems, the reaction leads to an alkyl radical and regeneration of the nickel(II) complex. Mence in most cases the electrolysis of alkyl halides in the presence of catalytic quantities of the nickel complexes leads to their complete reduction. It is proposed that the chemical reaction involves an organonickel species and that the overall behaviour of the systems is determined by the lifetime of the nickel–carbon bond.

The electrochemical reduction of some substituted π-cyclopentadienylnickel(II) complexes

The electrochemical reduction of the complexes [CpNi(PR3)2]+, where R = C2H5, C3H7, C4H9 or [C6Ni,(diphos)]+ and [CpNi(diars)+ in acetonitrile is described and the data are compared with those for the complexes Cp2Ni, [Ni(PR3)4]2+ and Ni(diphos)2+2.

The electrochemical reduction of CP2TiCl2 in the presence of trimethylphosphine

The electrochemistry of the complexes Cp2TiClx(PMe3)2-x in anhydrous THF has been studied. The most stable complexes at the three oxidation states of titanium are Cp2TiCl2, Cp2TiCl(PMe3) and Cp2Ti(PMe3)2, and each of these species is readily formed by electrolysis. It has also been demonstrated that oxidation/reduction of these species is followed by facile and rapid ligand exchange to form the preferred species in the new oxidation state provided a stoichiometric concentration of the required ligand is present. The consequences of this redox and ligand exchange chemistry for the synthetic reactions catalyzed by lower oxidation states of Ti are discussed. Finally, the voltammetry of a titanocycle is reported, and it is shown that the corresponding TiIII metallocycle is stable.

The oxidation of alcohols and aromatic hydrocarbons by dichromate in two phase systems

A rapid and selective method for the oxidation of primary alcohols to aldehydes and polynuclear aromatic hydrocarbons to quinones is described. In the recommended procedure the substrate in an organic solvent is shaken with aqueous acidic dichromate and a phase-transfer agent and the effect of the reaction conditions on the selectivity and rate of the reactions is discussed.

The electrochemical behaviour of alkanes in fluorosulphonic acid

The anodic oxidation of several alkanes in fluorosulphonic acid containing various carboxylic acids has been studied, the product in each case being an αβ-unsaturated ketone. These species can undergo further oxidation and electroanalytical techniques have been used to define the optimum conditions for the formation of the αβ-unsaturated ketones. The oxidation potential for an alkane in fluorosulphonic acid is shown to depend on the proton donating ability of the medium and the nature of this ‘pH’ dependence shows that there is a rapid protonation equilibrium prior to the electron transfer step and that it is the protonated alkane which undergoes oxidation. In addition, in more acid solutions, the alkanes are shown to undergo spontaneous chemical reactions which yield similar products. Possible mechanisms for the chemical and anodic reactions are discussed and it is shown that electroanalytical techniques may be used to investigate the chemistry of alkanes in fluorosulphonic acid. The application of these reactions to synthesis is considered.

Electrochemical reduction of Group 6 metal hexacarbonyls in aprotic solvents

The mechanism of the reduction of [Cr(CO)6] to [Cr2(CO)10]2– at a platinum electrode in acetonitrile has been investigated by cyclic voltammetry, and it shown that [Cr(CO)5][graphic omitted] is a species stable on the time scale of a few seconds. The mechanisms of reduction of [Mo(CO)6] and [W(CO)6] are apparently similar. Intermediates in the reductions of these Group 6 metal carbonyls react with alkyl halides and carbon dioxide.

Remote anodic oxidation of carboxylic acids in fluorosulphuric acid: a synthesis of lactones and unsaturated cyclic ketones

The oxidation of octanoic, heptanoic, 4-methylpentanoic, and 5-methylhexanoic acids at a smooth platinum anode in fluorosulphuric acid containing potassium fluorosulphate (1M) at 273 K is described. Oxidation leads to cleavage of a C–H bond and formation of a cationic centre at a place in the carbon skeleton remote from the functional group. The final products isolated are 5- and 6-membered ring compounds, lactones, and unsaturated cyclic ketones, and the ratio of products depends on the length of time the anolyte solution is left prior to work-up. A mechanism which explains the formation of all products is suggested.

The partial anodic oxidation of n-alkanes in acetonitrile and trifluoroacetic acid

The partial anodic oxidation of several n-alkanes at a smooth platinum electrode in acetonitrile or trifluoroacetic acid containing a tetra-alkylammonium tetrafluoroborate as the electrolyte have been studied. In both media these hydrocarbons give well-formed oxidation waves and a detailed investigation of the oxidation of n-octane in acetonitrile showed that at high potentials the electrode reaction is a diffusion-controlled two electron process while on the rising portion of the i–E curve, electron transfer is rate determining. Preparative electrolyses showed that in both acetonitrile and trifluoroacetic acid, the initial products of the electrode reaction are secondary carbonium ions which then react rapidly with the solvents to give mixtures of isomeric N-s-alkylacetamides (total yield 35–40%) and isomeric s-alkyl trifluoroacetates (total yields 50–80%) respectively.