John R. Lindsay Smith

Oxidations by the system “hydrogen peroxide - manganese(IV) complex - acetic acid” — Part II. Hydroperoxidation and hydroxylation of alkanes in acetonitrile

Abstract Higher alkanes (cyclohexane, n -pentane, n -heptane, methylbutane, 2- and 3-methylpentanes, 3-methylhexane, cis - and trans -decalins) are oxidized at 20 °C by H 2 O 2 in air in acetonitrile (or nitromethane) solution in the presence of the manganese(IV) salt [L 2 Mn 2 O 3 ](PF 6 ) 2 (L = 1,4,7-trimethyl-1,4-7-triazacyclononane) as the catalyst. An obligatory component of the reaction mixture is acetic acid. Turnover numbers attain 3300 after 2 h, the yield of oxygenated products is 46% based on the alkane. The oxidation affords initially the corresponding alkyl hydroperoxide as the predominant product, however later these compounds decompose to produce the corresponding ketones and alcohols. Regio- and bond selectivities of the reaction are high: C(1) : C(2) : C(3) : C(4) ≈ 1 : 40 : 35 : 35 and 1° : 2° : 3° is 1 : (15–40) : (180–300). The reaction with both isomers of decalin gives (after treatment with PPh 3 ) alcohols hydroxylated in the tertiary positions with the cis/trans ratio of ∼ 2 in the case of cis -decalin, and of ∼ 30 in the case of trans -decalin (i.e. in the latter case the reaction is stereospecific). Light alkanes (methane, ethane, propane, normal butane and isobutane) can be also easily oxidized by the same reagent in acetonitrile solution, the conditions being very mild: low pressure (1–7 bar of the alkane) and low temperature (−22 to +27 °C). Catalyst turnover numbers attain 3100, the yield of oxygenated products is 22% based on the alkane. The yields of oxygenates are higher at low temperatures. The ratio of products formed (hydroperoxide: ketone: alcohol) depends very strongly on the conditions of the reaction and especially on the catalyst concentration (at higher catalyst concentration the ketone is predominantly produced).

Oxidation of alkanes and alkenes by iodosylbenzene and hydrogen peroxide catalysed by halogenated manganese porphyrins in homogeneous solution and covalently bound to silica

Abstract Manganese(III) 5-(pentafluorophenyl)-10,15,20-tri(2,6-dichlorophenyl)porphyrin, Mn(PFTDCPP), and manganese(II) 2,3,7,8,12,13,17,18-octachloro-5-(pentafluorophenyl)-10,15-20-tri(2,6-dichlorophenyl)porphyrin, Mn(PFTDCCl 8 PP), have been synthesised and used as catalysts in hydrocarbon oxidations by iodosylbenzene and hydrogen peroxide both in solution and covalently bound to aminopropylated silica. The former shows higher efficiency in the epoxidation of alkenes by iodosylbenzene, whereas the perchlorinated manganese porphyrin is more efficient in the hydroxylation of alkanes by this oxidant. The supported manganese(III) porphyrin show the same activity as its homogeneous analogue. With hydrogen peroxide as oxygen donor, Mn(PFTDCPP) is a stable and effective catalyst in the presence of imidazole. The perchlorinated analogue is a poor catalyst with this oxidant. The eight additional chlorine atoms on the porphyrin ring stabilise Mn(II) and unfavour the formation of the active species, Mn V O.

Efficient stereoselective oxygenation of alkanes by peroxyacetic acid or hydrogen peroxide and acetic acid catalysed by a manganese(IV) 1,4,7-trimethyl-1,4,7-triazacyclononane complex

Abstract The dinuclear manganese complex [LMn IV (O) 3 Mn IV L](PF 6 ) 2 , where L is 1,4,7-trimethyl-1,4,7-triazacyclononane, catalyses the oxygenation of alkanes by peroxyacetic acid or by H 2 O 2 in the presence of acetic acid to give alkanols, alkanones and alkyl hydroperoxides. The reactions can give large turnovers (up to 1350 after 1 h at 20 °C) and can occur with a high degree of retention of stereochemistry at tertiary carbon atoms.

Alkene epoxidation catalysed by ligand-bound supported metalloporphyrins

Abstract Iron(III) and manganese(III) tetra(2,6-dichlorophenyl)porphyrin bound to surface imidazole and pyridine groups on solid supports are efficient catalysts for the expoxidation of cyclooctene by iodosylbenzene.

Steric effects induced by ferrocenyl in tertiary organophosphines: crystal structure of trans-chloromethylbis(ferrocenyldiphenylphosphine)palladium(II) benzene disolvate

Abstract The structure determination of the title compound, trans -[PdMeCl(PPh 2 Fc) 2 ]·2C 6 H 6 , (PPh 2 Fc=C 22 H 19 FeP) shows the Pd(II) moiety to have a square planar geometry with the bulky phosphine ligands in a trans orientation. The compound crystallises in the triclinic space group P 1 with 1 mole per unit cell accompanied by two benzene solvent molecules. A 50% statistical disorder was observed in the methyl and chloride positions. Bond distances and angles of the coordination polyhedron are PdP=2.3328(10) A, PdCl=2.378(3) A, PdC(10)=2.108(10) A, PPdCl=92.70(9)° and PPdC(10)=86.8(4)°. The structure is compared with the isomorphous trans -[Pt(Cl) 2 (PPh 2 Fc) 2 ]·2C 6 H 6 and other relevant structures of Pt(II) and Pd(II) described in the literature. The steric demand of the PPh 2 Fc ligand was estimated from all known structures to be approximately 155° using the Tolman model.

Azo dye oxidation with hydrogen peroxide catalysed by manganese 1,4,7-triazacyclononane complexes in aqueous solution

A kinetic and mechanistic study is reported of the oxidation of a number of azonaphthol dyes with hydrogen peroxide in aqueous solution, catalysed by some mono and dinuclear manganese(IV) complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN). The results of UV-Vis investigations, augmented by EPR and ESI-MS studies, are described for a series of experiments in which concentrations, pH and ionic strength have been varied. The reactions are characterised by an induction period followed by a relatively rapid oxidation. For the dinuclear manganese complex 2, these are consistent with an initial perhydrolysis of the manganese complex involving both the dye anion and HO2-, to give mononuclear manganese species and the operation of a catalytic cycle incorporating MnIIIL(OH)3, O = MnVL(OH)2 and MnIVL(OH)3 (L = Me3TACN) (cf. the reactions of peroxidase enzymes). ESI-MS results provide evidence for the formation and reaction (with the dye) of MnIVL(OH)3. With the mononuclear manganese complex MnIVL(OMe)3, there is a short lag-phase attributed to perhydrolysis by HO2- followed by the same catalytic cycle.

Formation and reaction of OMnV species in the oxidation of phenolic substrates with H2O2 catalysed by the dinuclear manganese(IV) 1,4,7-trimethyl-1,4,7-triazacyclononane complex [MnIV2(μ-O)3(TMTACN)2](PF6)2

The oxidation of phenolic substrates with H2O2 catalysed by [MnIV2(mu-O)3(TMTACN)2](PF6)2 1, (TMTACN, 1,4,7-trimethyl-1,4,7-triazacyclononane) has been investigated by use of ESI mass spectrometry. The role of the phenols as one-electron reductants and as co-ligands in the stabilisation and reaction of an intermediate O=MnV species has been analysed and the presence of a variety of manganese species in solution has been explained. Our results lead to a proposed mechanism for the catalytic oxidation of phenols in this system.

A comparative mechanistic study of the oxidation of phenols in aqueous solution by oxomanganese(IV) and oxoiron(IV) 5,10,15,20-tetrakis(2-N-methylpyridyl) porphyrin

Abstract The kinetics of the reactions of phenol and six substituted phenols with oxomanganese(IV) tetra(2-N-methylpyridyl)porphyrin in aqueous solution (pH 7.7) have been studied. The reactions are shown to be first order in both phenol and the oxomanganese(IV) species. The second order rate constants have been measured and the influence of the substituents on these values have been analyzed using Hammett and modified Hammett equations. The magnitude of the ρ values obtained and the good correlation of the data with σ· and the dual parameter σ· + σ+ suggest that, like the reactions of the analogous oxoiron(IV) tetra(2-N-methylpyridyl)porphyrin, the rate-determining step in these oxidations involves hydrogen atom abstraction from the phenol by the oxomanganese(IV) species in which the transition state has partial charge separation.

Catalytic activity of halogenated iron porphyrins in alkene and alkane oxidations by iodosylbenzene and hydrogen peroxide

A poly-halogenated iron porphyrin, Fe(PCl8)Cl, has been synthesised and used as a catalyst in hydrocarbon oxidations by iodosylbenzene and hydrogen peroxide both in solution and covalently bound to aminopropylated silica. The poly-chlorinated iron porphyrin shows the same efficiency of the related Fe(P)Cl, in the epoxidation of alkenes but higher efficiency in the hydroxylation of alkanes by iodosylbenzene, with increased preference for the oxidation of secondary carbon in adamantane and primary carbon in the oxidation of pentane. These selectivities may reflect the steric constraints around the oxo-iron species or, alternatively, it may arise from the greater reactivity of the active oxidant from Fe(PCl8)Cl. The supported iron(III) porphyrin showed lower activity as compared with the homogeneous analogue and the related supported Fe(P)Cl. The poly-chlorinated iron porpyrin is a poor catalyst with hydrogen peroxide. Excessive substitutuion by electron withdrawing groups on the porphyrin periphery eventually prohibits the formation of the key intermediate in catalytic oxidations. The alternative oxidation mechanism could involve radical participation.

Photo-induced ligand substitution at a remote site via electron transfer in a porphyrin-appended rhenium carbonyl supermoleculeElectronic supplementary information (ESI) available: energy balance, synthesis and characterisation of 1+OTf– and fluorescence spectra of 1+, 2 and 5. See http://www.rsc.org/suppdata/cc/b2/b200127f/

The photochemical and electrochemical properties of a Zn-porphyrin appended rhenium(I) tricarbonyl bipyridine 3-Me-pyridine complex have been investigated; visible-light sensitisation of electron transfer results in ligand substitution at a site remote from the chromophore.