Oxidative Imidation of Benzylic and Cycloalkane C(sp3)-H Bond Donors Using N-Aroyloxyquinuclidinium Salts and Nitriles under Photoredox Catalysis.

A series of N-aroyloxyquinuclidinium salts were prepared and used as reagents to perform efficient three-component Ritter-Mumm-type oxidative C-H imidation of donors of 1° and 2° benzylic C-H bonds used as limiting reagents with nitriles as a source of imide nitrogen under photocatalytic conditions; these reagents also exhibit somewhat lower reactivity toward cycloalkanes.

Oxidative Trifluoroacetoxylation of 1°, 2°, and 3° Benzylic C(sp3)-H Bond Donors Using N-Trifluoroacetoxyquinuclidinium Salts under Photoredox Catalysis.

N-Trifluoroacetoxyquinuclidinium trifluoroacetate was prepared in situ from quinuclidine N-oxide and (CF3CO)2O. Except for some electron-poor substrates, this reagent allows for the high-yielding oxidative trifluoroacetoxylation of 1°, 2°, and 3° benzylic C-H bonds under photocatalytic conditions. The trifluoroacetoxylation of an ibuprofen methyl ester allowed the selective functionalization of a 2° benzylic C-H bond. For alkylbenzenes, hydrogen-atom transfer from a benzylic C-H bond to a quinuclidine cation radical was proposed to be the reaction-product-determining step.

Spectroscopic studies of methyl paraoxon decomposition over mesoporous Ce-doped titanias for toxic chemical filtration.

The ever-constant threat of chemical warfare agents (CWA) motivates the design of materials to provide better protection to warfighters and civilians. Cerium and titanium oxide are known to react with organophosphorus compounds such Sarin and Soman. To study the decomposition of methyl paraoxon (CWA simulant) on such materials, we synthesized ordered mesoporous metal oxides (MMO) TiO2, CexTi1-xO2 (x = 0.005, 0.5, 0.10, 0.15) and CeO2. We fully characterized TiO2 and Ce-doped TiO2 and found phase-pure oxides with cylindrical hexagonally packed pores and high surface areas (176-252 m2/g). Methyl paraoxon decomposition was tracked through UV/Vis and found Ce0.15Ti0.85O2 to decompose the most methyl paraoxon, but CeO2 to be the most reactive when normalized to surface area. The surface area normalized rate constant (kSA) for CeO2 was 3-4.6 times larger than that of TiO2 and the CexTi1-xO2 series. While TiO2 and CexTi1-xO2 for 0.05 ≤ x ≤ 0.10 displayed no significant differences in the kinetics, the mostly amorphous Ce0.15Ti0.85O2 displayed a slight increase in reactivity. Our findings indicate that the nature of the cation, Ce4+ vs Ti4+, is less important to methyl paraoxon reactivity on these MMOs compared to other factors such as crystal structure type.

Reversible PtII-CH3 deuteration without methane loss: metal-ligand cooperation vs. ligand-assisted PtII-protonation.

Di(2-pyridyl)ketone dimethylplatinum(ii), (dpk)PtII(CH3)2, reacts with CD3OD at 25 °C to undergo complete deuteration of Pt-CH3 fragments in ∼5 h without loss of methane to form (dpk)PtII(CD3)2 in virtually quantitative yield. The deuteration can be reversed by dissolution in CH3OH or CD3OH. Kinetic analysis and isotope effects, together with support from density functional theory calculations indicate a metal-ligand cooperative mechanism wherein DPK enables Pt-CH3 deuteration by allowing non-rate-limiting protonation of PtII by CD3OD. In contrast, other model di(2-pyridyl) ligands enable rate-limiting protonation of PtII, resulting in non-rate-limiting C-H(D) reductive coupling. Owing to its electron-poor nature, following complete deuteration, DPK can be dissociated from the PtII-centre, furnishing [(CD3)2PtII(µ-SMe2)]2 as the perdeutero analogue of [(CH3)2PtII(µ-SMe2)]2, a commonly used PtII-precursor.

Aryl C(sp2)-X Coupling (X = C, N, O, Cl) and Facile Control of N-Mono- and N,N-Diarylation of Primary Alkylamines at a Pt(IV) Center.

We present the first example of an unprecedented and fast aryl C(sp2)-X reductive elimination from a series of isolated Pt(IV) aryl complexes (Ar = p-FC6H4) LPtIVF(py)(Ar)X (X = CN, Cl, 4-OC6H4NO2) and LPtIVF2(Ar)(HX) (X = NHAlk; Alk = n-Bu, PhCH2, cyclo-C6H11, t-Bu, cyclopropylmethyl) bearing a bulky bidentate 2-[bis(adamant-1-yl)phosphino]phenoxide ligand (L). The C(sp2)-X reductive elimination reactions of all isolated Pt(IV) complexes follow first-order kinetics and were modeled using density functional theory (DFT) calculations. When a difluoro complex LPtIVF2(Ar)(py) is treated with TMS-X (TMS = trimethylsilyl; X= NMe2, SPh, OPh, CCPh) it also gives the corresponding products of the Ar-X coupling but without observable LPtIVF(py)(Ar)X intermediates. Remarkably, the LPtIVF2(Ar)(HX) complexes with alkylamine ligands (HX = NH2Alk) form selectively either mono- (ArNHAlk) or diarylated (Ar2NAlk) products in the presence or absence of an added Et3N, respectively. This method allows for a one-pot preparation of diarylalkylamine bearing different aryl groups. These findings were also applied in unprecedented mono- and di-N-arylation of amino acid derivatives (lysine and tryptophan) under very mild conditions.

An Electron-Poor Dioxa-[2.1.1]-(2,6)-pyridinophane Ligand and Its Application in Cu-Catalyzed Olefin Aziridination.

A novel macrocyclic 1,7-dioxa-[2.1.1]-(2,6)-pyridinophane ligand has been synthesized and crystallographically characterized. Two derived metal complexes, dichloropalladium(II) and chlorocopper(I), were prepared. In the palladium(II) complex LPdCl2, both in the solid state, according to its crystallographic characterization, and in CH2Cl2 solutions at -40 °C, according to 1H NMR spectroscopy, the ligand adapts a C1-symmetric κ2-N,N-coordination mode in which the metal atom binds to two nonequivalent pyridine fragments of the macrocycle. The complex is fluxional at 20 °C. In the crystalline copper(I) complex LCuCl, the macrocyclic ligand is also κ2-N,N-coordinated to the metal, but it utilizes two equivalent pyridine fragments for the binding. The copper(I) complex is fluxional in CH2Cl2 solutions in the temperature range between 20 and -70 °C and is proposed to be involved in a fast intermolecular macrocyclic ligand exchange which is slowed down below -40 °C. DFT calculations predict a lower thermodynamic stability of the dioxapyridinophane-derived complexes LPdCl2 and LCuCl, as compared to their [2.1.1]-(2,6)-pyridinophane analogs containing bridging CH2 groups instead of the oxygen atoms. The electron poor dioxapyridinophane chlorocopper(I) complex, in combination with NaBArF4 (BArF4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) in dichloromethane solutions, can serve as an efficient catalyst for aziridination of various olefins with PhINTs at 0-22 °C.

Reductive C(sp2)-N Elimination from Isolated Pd(IV) Amido Aryl Complexes Prepared Using H2O2 as Oxidant.

Di-2-pyridyl ketone (dpk)-supported amidoarylpallada(II)cycles derived from various 2-(N-R-amino)biphenyls (R = H, Me, CF3CO, MeSO2, CF3SO2) react with hydrogen peroxide in MeOH, THF, MeCN or AcOH to form the corresponding C-N coupled products, N-R-substituted carbazoles, in 82-98% yield. For R = MeSO2 and CF3SO2, the corresponding reaction intermediates, amidoaryl Pd(IV) complexes were isolated and characterized by single crystal X-ray diffraction and/or NMR spectroscopy. For the first time, the C(sp2)-N reductive elimination from isolated amidoaryl Pd(IV) complexes has been studied in detail.

Oxidation of a Monomethylpalladium(II) Complex with O2 in Water: Tuning Reaction Selectivity to Form Ethane, Methanol, or Methylhydroperoxide.

Photochemical aerobic oxidation of n-Pr4N[(dpms)Pd(II)Me(OH)] (5) and (dpms)Pd(II)Me(OH2) (8) (dpms = di(2-pyridyl)methanesulfonate) in water in the pH range of 6-14 at 21 °C was studied and found to produce, in combined high yield, a mixture of MeOH, C2H6, and MeOOH along with water-soluble n-Pr4N[(dpms)Pd(II)(OH)2] (9). By changing the reaction pH and concentration of the substrate, the oxidation reaction can be directed toward selective production of ethane (up to 94% selectivity) or methanol (up to 54% selective); the yield of MeOOH can be varied in the range of 0-40%. The source of ethane was found to be an unstable dimethyl Pd(IV) complex (dpms)Pd(IV)Me2(OH) (7), which could be generated from 5 and MeI. For shedding light on the role of MeOOH in the aerobic reaction, oxidation of 5 and 8 with a range of hydroperoxo compounds, including MeOOH, t-BuOOH, and H2O2, was carried out. The proposed mechanism of aerobic oxidation of 5 or 8 involves predominant direct reaction of excited methylpalladium(II) species with O2 to produce a highly electrophilic monomethyl Pd(IV) transient that is involved in subsequent transfer of its methyl group to 5 or 8, H2O, and other nucleophilic components of the reaction mixture.

Selective aryl-fluoride reductive elimination from a platinum(IV) complex.

A difluoro(mesityl)platinum(IV) complex underwent highly selective reductive elimination of 2-fluoromesitylene upon heating in toluene. Kinetic analysis and DFT calculations suggest that the CF coupling involves a five-coordinate Pt(IV) transient intermediate resulting from the rate-limiting dissociation of the pyridine ligand.

Mechanistic study of the oxidation of a methyl platinum(II) complex with O(2) in water: Pt(II)Me-to-Pt(IV)Me and Pt(II)Me-to-Pt(IV)Me(2) reactivity.

The mechanism of oxidation by O2 of (dpms)Pt(II)Me(OH2) (1) and (dpms)Pt(II)Me(OH)(-) (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4-14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤ 8, the reaction leads to a C1-symmetric monomethyl Pt(IV) complex (dpms)Pt(IV)Me(OH)2 (5) with high selectivity ≥97%; the reaction rate is first-order in [Pt(II)Me] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O2 activation at the Pt(II) center to form a Pt(IV) hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O2 than neutral complex 1 (pKa = 8.15 ± 0.02). At pH ≥ 10, the oxidation is inhibited by OH(-) ions; the reaction order in [Pt(II)Me] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by Pt(IV) hydroperoxide 4. At pH ≥ 12, formation of a C1-symmetric dimethyl complex 6, (dpms)Pt(IV)Me2(OH), along with [(dpms)Pt(II)(OH)2](-) (7) becomes the dominant reaction pathway (50-70% selectivity). This change in the product distribution is explained by the formation of a Cs-symmetric intermediate (dpms)Pt(IV)Me(OH)2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; kH/kD = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A SN2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.

Mechanism of O2 activation and methanol production by (di(2-pyridyl)methanesulfonate)Pt(II)Me(OH(n))((2-n)-) complex from theory with validation from experiment.

The mechanism of the (dpms)Pt(II)Me(OH(n))((2-n)-) oxidation in water to form (dpms)Pt(IV)Me(OH)2 and (dpms)Pt(IV)Me2(OH) complexes was analyzed using DFT calculations. At pH < 10, (dpms)Pt(II)Me(OH(n))((2-n)-) reacts with O2 to form a methyl Pt(IV)-OOH species with the methyl group trans to the pyridine nitrogen, which then reacts with (dpms)Pt(II)Me(OH(n))((2-n)-) to form 2 equiv of (dpms)Pt(IV)Me(OH)2, the major oxidation product. Both the O2 activation and the O-O bond cleavage are pH dependent. At higher pH, O-O cleavage is inhibited whereas the Pt-to-Pt methyl transfer is not slowed down, so making the latter reaction predominant at pH > 12. The pH-independent Pt-to-Pt methyl transfer involves the isomeric methyl Pt(IV)-OOH species with the methyl group trans to the sulfonate. This methyl Pt(IV)-OOH complex is more stable and more reactive in the Pt-to-Pt methyl-transfer reaction as compared to its isomer with the methyl group trans to the pyridine nitrogen. A similar structure-reactivity relationship is also observed for the S(N)2 functionalization to form methanol by two isomeric (dpms)Pt(IV)Me(OH)2 complexes, one featuring the methyl ligand trans to the sulfonate group and another with the methyl trans to the pyridine nitrogen. The barrier to functionalize the former isomer with the CH3 group trans to the sulfonate group is 2-9 kcal/mol lower. The possibility of the involvement of Pt(III) species in the reactions studied was found to correspond to high-barrier reactions and is hence not viable. It is concluded that the dpms ligand facilitates Pt(II) oxidation both enthalpically and entropically.

Unprecedented 1,3-migration of the aryl ligand in metallacyclic aryl α-naphthyl Pt(IV) difluorides to produce ß-arylnaphthyl Pt(II) complexes.

Electrophilic fluorination of aryl α-naphthyl Pt(II) complexes leads to an unprecedented 1,3-migration of the aryl ligand to the ß-position of the naphthyl group. The reaction proceeds via the initial oxidative addition of two fluoro ligands to the Pt center followed by C(sp(2))-C(sp(2)) coupling and aryl migration.

Electrophilic fluorination of organoplatinum(II) iodides: iodine and platinum atoms as competing fluorination sites.

A series of diphosphine Pt(II) aryl iodo complexes were reacted with XeF(2) to cleanly produce the corresponding Pt(II) difluoro complexes and free iodoarenes. However, when aryl ligands bearing fluoro substituents in the ortho positions were used, the formation of the corresponding Pt(II) aryl fluoro complexes was observed in the reaction with XeF(2). In the case of the Pt-C(6)F(5) complex, the products of the fluoride-for-iodide exchange were the only products observed by means of (31)P and (19)F NMR spectroscopy. The experimental and theoretical studies suggest that the formation of iodine-fluorine bond may accompany this transformation. The plausible "I-F" species could be trapped by electron-richer organoplatinum complexes to give a Pt(IV) transient which subsequently eliminates the corresponding aryl iodide. Hence, in some cases a pathway involving an attack of XeF(2) at the iodo ligand of Pt(II) aryl iodo complexes to generate I-F species can be operative in addition to or instead of the XeF(2) attack at the metal center. Our DFT studies demonstrate that the electrophilic attacks of XeF(2) at both sites, platinum and iodide, can be competitive.

Surprising acid/base and ion-sequestration chemistry of Sn9(4-): HSn9(3-), Ni@HSn9(3-), and the Sn9(3-) ion revisited.

K(4)Sn(9) dissolves in ethylenediamine (en) to give equilibrium mixtures of the diamagnetic HSn(9)(3-) ion along with K(x)Sn(9)((4-x)-) ion pairs, where x = 0, 1, 2, 3. The HSn(9)(3-) cluster is formed from the deprotonation of the en solvent and is the conjugate acid of Sn(9)(4-). DFT studies show that the structure is quite similar to the known isoelectronic RSn(9)(3-) ions (e.g., R = i-Pr). The hydrogen atom of HSn(9)(3-) (δ = 6.18 ppm) rapidly migrates among all nine Sn atoms in an intramolecular fashion; the Sn(9) core is also highly dynamic on the NMR time scale. The HSn(9)(3-) cluster reacts with Ni(cod)(2) to give the Ni@HSn(9)(3-) ion containing a hydridic hydrogen (δ = -28.3 ppm) that also scrambles across the Sn(9) cluster. The Sn(9)(4-) ion competes effectively with 2,2,2-crypt for binding K(+) in en solutions, and the pK(a) of HSn(9)(3-) is similar to that of en (i.e., Sn(9)(4-) is a very strong Brønsted base with a pK(b) comparable to that of the NH(2)CH(2)CH(2)NH(-) anion). Competition studies show that the HSn(9)(3-) ⇄ Sn(9)(4-) + H(+) equilibrium is fully reversible. The HSn(9)(3-) anion is present in significant concentrations in en solutions containing 2,2,2-crypt, yet it has gone undetected for over 30 years.

Oxidation of dimethyldi(2-pyridyl)borato Pt(II)Me(n) complexes, n = 1, 0, with H2O2: tandem B-to-Pt methyl group migration and formation of C-O bond.

New dimethyldi(2-pyridyl)borato (dmdpb) platinum(II) complexes, (dmdpb)Pt(II)Me(SMe(2)) (1), (dmdpb)Pt(II)(L)(SMe(2))(+), L = MeOH (2), MeCN (3), supported by dimethylsulfide ligand and featuring one (1) or no hydrocarbyls at the metal (2, 3) were prepared and their oxidation with hydrogen peroxide was studied. Both complex 1 bearing the formal charge of +1 on the metal and the methanol complex 2 capable of losing the proton of the methanol ligand to form the methoxide derivative 4 charged similarly to 1, are reactive towards H(2)O(2). However, the cationic complex 3 with a formal charge of +2 on the metal does not react with H(2)O(2). The oxidation of the monomethyl platinum(II) complex 1 leads to the B-to-Pt methyl transfer and formation of a robust dimethyl Pt(IV) species 5 which does not undergo C-O reductive elimination up to 100 °C. By contrast, oxidation of 2 in methanol-d(4) leads to quantitative formation of dimethyl ether-d(3), CD(3)OCH(3). It was presumed that the latter reaction involves the B-to-Pt methyl transfer and formation of a highly reactive cationic monomethyl Pt(IV) species whose methyl group carbon atom can accept nucleophilic attack by the methanol-d(4) solvent to form dimethyl ether-d(3).

Direct functionalization of M-C (M = Pt(II), Pd(II)) bonds using environmentally benign oxidants, O2 and H2O2.

Atom economy and the use of "green" reagents in organic oxidation, including oxidation of hydrocarbons, remain challenges for organic synthesis. Solutions to this problem would lead to a more sustainable economy because of improved access to energy resources such as natural gas. Although natural gas is still abundant, about a third of methane extracted in distant oil fields currently cannot be used as a chemical feedstock because of a dearth of economically and ecologically viable methodologies for partial methane oxidation. Two readily available "atom-economical" "green" oxidants are dioxygen and hydrogen peroxide, but few methodologies have utilized these oxidants effectively in selective organic transformations. Hydrocarbon oxidation and C-H functionalization reactions rely on Pd(II) and Pt(II) complexes. These reagents have practical advantages because they can tolerate moisture and atmospheric oxygen. But this tolerance for atmospheric oxygen also makes it challenging to develop novel organometallic palladium and platinum-catalyzed C-H oxidation reactions utilizing O(2) or H(2)O(2). This Account focuses on these challenges: the development of M-C bond (M = Pt(II), Pd(II)) functionalization and related selective hydrocarbon C-H oxidations with O(2) or H(2)O(2). Reactions discussed in this Account do not involve mediators, since the latter can impart low reaction selectivity and catalyst instability. As an efficient solution to the problem of direct M-C oxidation and functionalization with O(2) and H(2)O(2), this Account introduces the use of facially chelating semilabile ligands such as di(2-pyridyl)methanesulfonate and the hydrated form of di(2-pyridyl)ketone that enable selective and facile M(II)-C(sp(n)) bond functionalization with O(2) (M = Pt, n = 3; M = Pd, n = 3 (benzylic)) or H(2)O(2) (M = Pd, n = 2). The reactions proceed efficiently in protic solvents such as water, methanol, or acetic acid. With the exception of benzylic Pd(II) complexes, the organometallic substrates studied form isolable high-valent Pt(IV) or Pd(IV) intermediates as a result of an oxidant attack at the M(II) atom. The resulting high-valent M(IV) intermediates undergo C-O reductive elimination, leading to products in high yields. Guidelines for the synthesis of products containing other C-X bonds (X = OAc, Cl, Br) while using O(2) or H(2)O(2) as oxidants are also discussed. Although the M(II)-C bond functionalization reactions including high-valent intermediates are well understood, the mechanism for the aerobic functionalization of benzylic Pd(II) complexes will require a more detailed exploration. Importantly, further optimization of the systems suitable for stoichiometric M(II)-C bond functionalization led to the development of catalytic reactions, including selective acetoxylation of benzylic C-H bonds with O(2) as the oxidant and hydroxylation of aromatic C-H bonds with H(2)O(2) in acetic acid solutions. Both reactions proceed efficiently with substrates that contain a directing heteroatom. This Account also describes catalytic methods for ethylene dioxygenation with H(2)O(2) using M(II) complexes supported by facially chelating ligands. Mechanistic studies of these new oxidation reactions point to important ways to improve their substrate scope and to develop "green" CH functionalization chemistry.

Preparation and C-X reductive elimination reactivity of monoaryl Pd(IV)-X complexes in water (X = OH, OH2, Cl, Br).

Monohydrocarbyl palladium(IV) complexes bearing OH, OH(2), Br, and Cl ligands at the metal and supported by facially chelating 1-hydroxy-1,1-bis(2-pyridyl)methoxide were readily prepared in water at 0 °C. These complexes reductively eliminate Ar-X (X = OH, Br, Cl) in water at room temperature in high yield, and the corresponding first-order rate constants k(OH), k(Cl), and k(Br) are on the same order of magnitude.

Aryl-bromide reductive elimination from an isolated Pt(IV) complex.

A Pt(IV) complex bearing two aryl and two bromo ligands, which undergoes selective elimination of a bromoarene molecule has been prepared and fully-characterized. The mechanistic studies of this reaction are presented.

Homogeneous catalytic transfer dehydrogenation of alkanes with a group 10 metal center.

Unambiguous catalytic homogeneous alkane transfer dehydrogenation was observed with a group 10 metal complex catalyst, LPt(II)(cyclo-C6H10)H, supported by a lipophilic dimethyl-di(4-tert-butyl-2-pyridyl)borate anionic ligand and tert-butylethene as the sacrificial hydrogen acceptor.

Ligand-enabled Pt(II)-C(sp3) bond functionalization with dioxygen as a direct oxidant.

This feature article summarizes the progress achieved in the field of aerobic functionalization of Pt(II) monoalkyl complexes in protic solvents, water and alcohols. These functionalization reactions are possible in the presence of tripodal semi-labile ligands such as di(2-pyridyl)methanesulfonate (dpms). The reactions include two subsequent transformations: (i) direct (mediatorless) oxidation of a Pt(II) monoalkyl or a Pt(II)-oxetane to produce a Pt(IV)(OH) alkyl or a Pt(IV)(OH)-oxetane, respectively, and (ii) reductive elimination from the Pt(IV) center of an oxygenated organic derivative with a new C-O bond such as alcohols (MeOH, HOC2H4OH), ethers (Me2O), or olefin (ethylene, cis-cyclooctene, norbornene) oxides. All the reactions are highly chemo- and stereoselective. Mechanisms of dioxygen activation with (dpms)Pt(II) alkyls and C-O reductive eliminations from a Pt(IV) center are discussed, including the direct C-O elimination of epoxides from Pt(IV) oxetanes previously undocumented for this metal. Modification of the "slow" Pt(II)-based systems led to the discovery of a "fast" dioxygenase-like acetoxylation of benzylic C-H bonds catalyzed by homogeneous Pd complexes with O2 as the sole oxidant.