The formyloxyl radical: electrophilicity, C-H bond activation and anti-Markovnikov selectivity in the oxidation of aliphatic alkenes.

In the past the formyloxyl radical, HC(O)O˙, had only been rarely experimentally observed, and those studies were theoretical-spectroscopic in the context of electronic structure. The absence of a convenient method for the preparation of the formyloxyl radical has precluded investigations into its reactivity towards organic substrates. Very recently, we discovered that HC(O)O˙ is formed in the anodic electrochemical oxidation of formic acid/lithium formate.

Electrochemical Hydroxylation of Arenes Catalyzed by a Keggin Polyoxometalate with a Cobalt(IV) Heteroatom.

The sustainable, selective direct hydroxylation of arenes, such as benzene to phenol, is an important research challenge. An electrocatalytic transformation using formic acid to oxidize benzene and its halogenated derivatives to selectively yield aryl formates, which are easily hydrolyzed by water to yield the corresponding phenols, is presented. The formylation reaction occurs on a Pt anode in the presence of [Co W O ] as a catalyst and lithium formate as an electrolyte via formation of a formyloxyl radical as the reactive species, which was trapped by a BMPO spin trap and identified by EPR.

The kinetics and mechanism of oxidation of reduced phosphovanadomolybdates by molecular oxygen: theory and experiment in concert.

The reactivity of the HPVMoO polyoxometalate and its analogues as an electron transfer and electron transfer-oxygen transfer oxidant has been extensively studied in the past and has been shown to be useful in many transformations. One of the hallmarks of this oxidant is the possibility of its re-oxidation with molecular oxygen, thus enabling aerobic catalytic cycles. Although the re-oxidation reaction was known, the kinetics and mechanism of this reaction have not been studied in any detail.

Hydrogen-Atom Transfer Oxidation with HO Catalyzed by [FeII(1,2-bis(2,2'-bipyridyl-6-yl)ethane(HO)]: Likely Involvement of a (μ-Hydroxo)(μ-1,2-peroxo)diiron(III) Intermediate.

The iron(II) triflate complex () of 1,2-bis(2,2'-bipyridyl-6-yl)ethane, with two bipyridine moieties connected by an ethane bridge, was prepared. Addition of aqueous 30% HO to an acetonitrile solution of yielded , a green compound with λ=710 nm. Moessbauer measurements on showed a doublet with an isomer shift (δ) of 0.

Polyoxometalate-catalyzed insertion of oxygen from O(2) into tin-alkyl bonds.

The polyoxometalate H5PV2Mo10O40 mediates the insertion of an oxygen atom from H5PV2Mo10O40 into the tin-carbon bond of n-Bu4Sn through its activation by electron transfer to yield 1-butanol and (n-Bu3Sn)2O. The reaction is initiated by electron transfer from n-Bu4Sn to H5PV(V)2Mo10O40 to yield the ion pair n-Bu4Sn(•+)-H5PV(IV)V(V)Mo10O40. The H5PV(IV)V(V)Mo10O40 moiety was identified by UV-vis and EPR.

Electron transfer-oxygen transfer oxygenation of sulfides catalyzed by the H5PV2Mo10O40 polyoxometalate.

The oxygenation of sulfides to the corresponding sulfoxides catalyzed by H(5)PV(2)Mo(10)O(40) and other acidic vanadomolybdates has been shown to proceed by a low-temperature electron transfer-oxygen transfer (ET-OT) mechanism. First, a sulfide reacts with H(5)PV(2)Mo(10)O(40) to yield a cation radical-reduced polyoxometalate ion pair, R(2)(+*),H(5)PV(IV)V(V)Mo(10)O(40), that was identified by UV-vis spectroscopy (absorptions at 650 and 887 nm for PhSMe(+*) and H(5)PV(IV)V(V)Mo(10)O(40)) and EPR spectroscopy (quintet at g = 2.0079, A = 1.

Photochemical reduction of carbon dioxide catalyzed by a ruthenium-substituted polyoxometalate.

A polyoxometalate of the Keggin structure substituted with Ru(III), (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)] in which (6)Q=(C(6)H(13))(4)N(+), catalyzed the photoreduction of CO(2) to CO with tertiary amines, preferentially Et(3)N, as reducing agents. A study of the coordination of CO(2) to (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)] showed that 1) upon addition of CO(2) the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g(x)=2.146, g(y)=2.

Oxidative C-C bond cleavage of primary alcohols and vicinal diols catalyzed by H5PV2Mo10O40 by an electron transfer and oxygen transfer reaction mechanism.

Primary alcohols such as 1-butanol were oxidized by the H5PV2Mo10O40 polyoxometalate in an atypical manner. Instead of C-H bond activation leading to the formation of butanal and butanoic acid, C-C bond cleavage took place leading to the formation of propanal and formaldehyde as initial products. The latter reacted with the excess 1-butanol present to yield butylformate and butylpropanate in additional oxidative transformations.

Nitrosonium salts, NO+ X- (X = B(3,5-diCF3Ph)4(-) or PW12O40(3-)), as electrophilic catalysts for alkene activation in arene alkylation and dimerization reactions.

It has been found that in apolar reaction media the nitrosonium cation (NO+) activated alkenes under mild conditions toward electrophilic substitution of arene substrates to yield the alkylated arene with Markovnikov orientation. In the absence of arenes the alkenes react with themselves to yield a mixture of dimeric alkenes. The nitrosonium cation can be dissolved in the reaction medium by using the tetrakis-(bis-(3,5-trifluromethyl)phenyl) borate anion, where upon the reactions occur effectively at 30 degrees C.

Molecular oxygen and oxidation catalysis by phosphovanadomolybdates.

The history of aerobic catalytic oxidation mediated by a subclass of polyoxometalates, the phosphovanadomolybdates of the Keggin structure, [PV(x)Mo(12-x)O40](3+x)-, is described. In the earlier research it was shown that phosphovanadomolybdates catalyze oxydehydrogenation reactions through an electron-transfer oxidation of a substrate by the polyoxometalate that is then reoxidized by oxygen. These aerobic oxidations are selective and synthetically useful in various transformations, notably diene aromatization, phenol dimerization and alcohol oxidation.

The high-valent iron-oxo species of polyoxometalate, if it can be made, will be a highly potent catalyst for C-H hydroxylation and double-bond epoxidation.

This study uses density functional theory (DFT) calculations to explore the reactivity of the putative high-valent iron-oxo reagent of the iron-substituted polyoxometalate (POM-FeO4-), derived from the Keggin species, PW12O40(3-). It is shown that POM-FeO4- is in principle capable of C-H hydroxylation and C=C epoxidation and that it should be a powerful oxidant, even more so than the Compound I species of cytochrome P450. The calculations indicate that in a solvent, the barriers, and especially those for epoxidation, become sufficiently small that one may expect an extremely fast reaction.

Selective ortho hydroxylation of nitrobenzene with molecular oxygen catalyzed by the H5PV2Mo10O40 polyoxometalate.

Nitrobenzene was regioselectively oxidized to 2-nitrophenol with oxygen in a reaction catalyzed by the H5PV2Mo10O40 polyoxometalate. The reaction was first order in oxygen and catalyst. 15N NMR showed the interaction between nitrobenzene and the polyoxometalate.

Mild, aqueous, aerobic, catalytic oxidation of methane to methanol and acetaldehyde catalyzed by a supported bipyrimidinylplatinum-polyoxometalate hybrid compound.

We have demonstrated that a bipyrimidinylplatinum-polyoxometalate, [Pt(Mebipym)Cl2]+[H4PV2Mo10O40]-, supported on silica is an active catalyst for the aerobic oxidation of methane to methanol in water under mild reaction conditions. Further oxidation of methanol yields acetaldehyde. The presence of the polyoxometalate is presumed to allow the facile oxidation of a Pt(II) intermediate to a Pt(IV) intermediate and to aid in the addition of methane to the Pt catalytic center.

Oxidation of alkylarenes by nitrate catalyzed by polyoxophosphomolybdates: synthetic applications and mechanistic insights.

Alkylarenes were catalytically and selectively oxidized to the corresponding benzylic acetates and carbonyl products by nitrate salts in acetic acid in the presence of Keggin type molybdenum-based heteropolyacids, H(3+)(x)()PV(x)()Mo(12)(-)(x)()O(40) (x = 0-2). H(5)PV(2)Mo(10)O(40) was especially effective. For methylarenes there was no over-oxidation to the carboxylic acid contrary to what was observed for nitric acid as oxidant.

Preparation and characterization of new ruthenium and osmium containing polyoxometalates, [M(DMSO)3Mo7O24](4-) (M = Ru(II), Os(II)), and their use as catalysts for the aerobic oxidation of alcohols.

A new heptamolybdate polyoxometalate structure containing ruthenium(II) or osmium(II) metal centers, [M(II)(DMSO)(3)Mo(7)O(24)](4-) (M = Ru, Os), was synthesized by reaction between (NH(4))(6)Mo(7)O(24) and cis-M(DMSO)(4)Cl(2). X-ray structure analysis revealed the complexes to contain a ruthenium/osmium center in a trigonal antiprismatic coordination mode bound to three DMSO moieties via the sulfur atom of DMSO and three oxygen atoms of the new heptamolybdate species. The heptamolybdate consists of seven condensed edge-sharing MoO(6) octahedra with C(2v) symmetry.

Oxygen transfer from sulfoxides: selective oxidation of alcohols catalyzed by polyoxomolybdates.

Benzylic, allylic, and aliphatic alcohols are oxidized to aldehydes and ketones in a reaction catalyzed by Keggin-type polyoxomolybdates, PV(x)Mo(12-x)O(40)(-(3+x)) (x = 0, 2), with DMSO as a solvent. The oxidation of benzylic alcohols is quantitative within hours and selective, whereas that of allylic alcohols is less selective. Oxidation of aliphatic alcohols is slower but selective.

Oxygen transfer from sulfoxides: oxidation of alkylarenes catalyzed by a polyoxomolybdate, [PMo12O40]3-.

The polyoxomolydate of the Keggin structure, PMo12O403-, catalyzes, under anaerobic conditions, oxygen transfer from sulfoxides to alkylarenes such as xanthene and diphenylmethane to yield xanthen-9-one and benzophenone, respectively. With use of 17O and 18O labeled phenylmethylsulfoxide it was shown that the sulfoxide is complexed by the polyoxometalate and the oxygen is transferred from the sulfoxide to the alkylarene. There is a good correlation between the reaction rate and the heterolytic benzylic C-H bond energy indicating a hydride transfer reaction from the alkylarene to the polyoxometalate-sulfoxide complex.