Syuhei Yamaguchi

Efficient Oxidative Alkyne Homocoupling Catalyzed by a Monomeric Dicopper‐Substituted Silicotungstate

The versatility and accessibility of polyoxometalates have led to various applications in the fields of analytical chemistry, medicine, electrochemistry, photochemistry, and catalysis, particularly in the field of oxidation catalysis. Interest in catalysis by metal-substituted polyoxometalates has grown significantly because of the unique reactivity that results from the composition and structure of their active sites. To date, various kinds of metal-substituted polyoxometalates have been synthesized and applied in selective oxidation reactions. 1,3-Diyne derivatives are very important materials in biological, polymer, and materials science because they can be converted into various structural entities, especially substituted heterocyclic compounds. Oxidative alkyne– alkyne coupling is a good candidate for the synthesis of 1,3diyne derivatives. Copper salts (stoichiometric amounts, Glaser conditions), copper salts with appropriate nitrogen bases and molecular oxygen (catalytic, Hay conditions), and a combination of copper and palladium salts (catalytic) have commonly been used to promote oxidative alkyne–alkyne coupling. However, most copper-catalyzed systems have shortcomings, especially their low turnover numbers, the formation of significant amounts of by-products, severe catalyst deactivation, narrow applicability to a limited number of alkynes, and/or the need for additives such as bases and co-catalysts. In 1964 Bohlmann and co-workers proposed that the copper(II)-catalyzed alkyne homocoupling reaction proceeds via the formation of the alkynyldicopper(II) intermediate {Cu2(m-C CR)2}, which would react further to give the 1,3diyne products directly (see the Supporting Information). This reaction mechanism has generally been accepted, although some detailed mechanistic work is still necessary. Thus, although it is expected that the homocoupling reaction should proceed efficiently in the presence of catalysts with a dicopper(II) core on the basis of this mechanism, an alkyne homocoupling reaction catalyzed by complexes with a dicopper(II) core is as yet unknown. Herein we report that the dicopper-substituted g-Keggin silicotungstate TBA4[g-H2SiW10O36Cu2(m-1,1-N3)2] (I, Figure 1; TBA = tetra-n-butylammonium) is an effective

Synthesis and catalysis of di- and tetranuclear metal sandwich-type silicotungstates [(gamma-SiW10O36)2M2(mu-OH)2]10- and [(gamma-SiW10O36)2M4(mu4-O)(mu-OH)6]8- (M = Zr or Hf).

The di- and tetranuclear metal sandwich-type silicotungstates of Cs10[(gamma-SiW10O36)2{Zr(H2O)}2(mu-OH)2] x 18 H2O (Zr2, monoclinic, C2/c (No. 15), a = 25.3315(8) A, b = 22.6699(7) A, c = 18.5533(6) A, beta = 123.9000(12) degrees, V = 8843.3(5) A(3), Z = 4), Cs10[(gamma-SiW10O36)2{Hf(H2O)}2(mu-OH)2] x 17 H2O (Hf2, monoclinic, space group C2/c (No. 15), a = 25.3847(16) A, b = 22.6121(14) A, c = 18.8703(11) A, beta = 124.046(3) degrees, V = 8974.9(9) A(3), Z = 4), Cs8[(gamma-SiW10O36)2{Zr(H2O)}4(mu4-O)(mu-OH)6] x 26 H2O (Zr4, tetragonal, P4(1)2(1)2 (No. 92), a = 12.67370(10) A, c = 61.6213(8) A, V = 9897.78(17) A(3), Z = 4), and Cs8[(gamma-SiW10O36)2{Hf(H2O)}4(mu4-O)(mu-OH)6] x 23 H2O (Hf4, tetragonal, P4(1)2(1)2 (No. 92), a = 12.68130(10) A, c = 61.5483(9) A, V = 9897.91(18) A(3), Z = 4) were obtained as single crystals suitable for X-ray crystallographic analyses by the reaction of a dilacunary gamma-Keggin silicotungstate K8[gamma-SiW10O36] with ZrOCl2 x 8 H2O or HfOCl2 x 8 H2O. These dimeric polyoxometalates consisted of two [gamma-SiW10O36](8-) units sandwiching metal-oxygen clusters such as [M2(mu-OH)2](6+) and [M4(mu4-O)(mu-OH)6](8+) (M = Zr or Hf). The dinuclear zirconium and hafnium complexes Zr2 and Hf2 were isostructural. The equatorially placed two metal atoms in Zr2 and Hf2 were linked by two mu-OH ligands and each metal was bound to four oxygen atoms of two [gamma-SiW10O36](8-) units. The tertanuclear zirconium and hafnium complexes Zr4 and Hf4 were isostructural and consisted of the adamantanoid cages with a tetracoordinated oxygen atom in the middle, [M4(mu4-O)(mu-OH)6](8+) (M = Zr or Hf). Each metal atom in Zr4 and Hf4 was linked by three mu-OH ligands and bound to two oxygen atoms of the [gamma-SiW10O36](8-) unit. The tetra-nuclear zirconium and hafnium complexes showed catalytic activity for the intramolecular cyclization of (+)-citronellal to isopulegols without formation of byproducts resulting from etherification and dehydration. A lacunary silicotungstate [gamma-SiW10O34(H2O)2](4-) was inactive, and the isomer ratio of isopulegols in the presence of MOCl2 x 8 H2O (M = Zr or Hf) were much different from that in the presence of tetranuclear complexes, suggesting that the [M4(mu4-O)(mu-OH)6](8+) core incorporated into the POM frameworks acts as an active site for the present cyclization. On the other hand, the reaction hardly proceeded in the presence of dinuclear zirconium and hafnium complexes under the same conditions. The much less activity is possibly explained by the steric repulsion from the POM frameworks in the dinuclear complexes.

Synthesis of a dialuminum-substituted silicotungstate and the diastereoselective cyclization of citronellal derivatives.

A novel dialuminum-substituted silicotungstate TBA(3)H[gamma-SiW(10)O(36){Al(OH(2))}(2)(mu-OH)(2)] x 4 H(2)O (1, TBA = tetra-n-butylammonium) was synthesized by the reaction of the potassium salt of [gamma-SiW(10)O(36)](8-) (SiW10) with 2 equiv of Al(NO(3))(3) in an acidic aqueous medium. It was confirmed by the X-ray crystallographic analysis that compound 1 was a monomer of the gamma-Keggin dialuminum-substituted silicotungstate with the {Al(2)(mu-OH)(2)} diamond core. The cluster framework of 1 maintained the gamma-Keggin structure in the solution states. The reaction of 1 with pyridine yielded TBA(3)[(C(5)H(5)N)H][gamma-SiW(10)O(36){Al(C(5)H(5)N)}(2)(mu-OH)(2)] x 2 H(2)O (2), and the molecular structure was successfully determined by the X-ray crystallographic analysis. In compound 2, two of three pyridine molecules coordinated to the axial positions of aluminum centers and one of them existed as a pyridinium cation, showing that compound 1 has two Lewis acid sites and one Brønsted acid site. Compound 1 showed high catalytic activity for the intramolecular cyclization of citronellal derivatives such as (+)-citronellal (3) and 3-methylcitronellal (4) without formation of byproduct resulting from etherification and dehydration. For the 1-catalyzed cyclization of 3, the diastereoselectivity toward (-)-isopulegol (3a) reached ca. 90% and the value was the highest level among those with reported systems so far. The reaction rate for the 1-catalyzed cyclization of 3 decreased by the addition of pyridine, and the cyclization hardly proceeded in the presence of 2 equiv of pyridine with respect to 1. On the other hand, the reaction rate and diastereoselectivity to 3a in the presence of 2,6-lutidine were almost the same as those in the absence. Therefore, the present cyclization is mainly promoted by the Lewis acid sites (aluminum centers) in 1. DFT calculations showed that the formation of the transition state to produce 3a is sterically and electronically more favorable than the other three transition states for the present 1-catalyzed cyclization of 3.

Mononuclear copper(II)–hydroperoxo complex derived from reaction of copper(I) complex with dioxygen as a model of DβM and PHM

A mononuclear copper(II)-hydroperoxo species has been generated by the reaction of Cu(I)-H2BPPA complex with dioxygen, which illustrates the enzymatic reaction process of the CuB site in the DbetaM and PHM.

Basic approach to development of environment-friendly oxidation catalyst materials. Mononuclear hydroperoxo copper(II) complexes

Abstract In this review, basic studies on the binding and activation of dioxygen species by copper complexes with originally designed ligands are described as the initial step for development of environment-friendly oxidation catalyst. In order to examine the stability/reactivity of such a mononuclear copper(II) complex, some copper complexes with hydroperoxide ion havebeen constructed using the ligands that have been prepared on the basis of the active center structures of metalloenzymes,and the effects of (i) hydrogen bond, (ii) hydrophobic sphere, (iii) coordination structure around metal, and (iv) coordinating atoms have been investigated systematically, from the point of view of synthetic, spectroscopic, structural, kinetic, and theoretical chemistries. It has also been found out that the decomposition rate constants and the O–O bond strengths of hydroperoxo copper(II) complexes are strongly correlated.

H-atom abstraction reaction for organic substrates via mononuclear copper(II)-superoxo species as a model for DβM and PHM

Hydrogen atom abstraction reactions have been implicated in oxygenation reactions catalyzed by copper monooxygenases such as peptidylglycine alpha-hydroxylating monooxygenase (PHM) and dopamine beta-monooxygenase (DbetaM). We have investigated mononuclear copper(I) and copper(II) complexes with bis[(6-neopentylamino-2-pyridyl)methyl][(2-pyridyl)methyl]amine (BNPA) as functional models for these enzymes. The reaction of [Cu(II)(bnpa)]2+ with H2O2, affords a quasi-stable mononuclear copper(II)-hydroperoxo complex, [Cu(II)(bnpa)(OOH)]+ (4) which is stabilized by hydrophobic interactions and hydrogen bonds in the vicinity of the copper(II) ion. On the other hand, the reaction of [Cu(I)(bnpa)]+ (1) with O2 generates a trans-mu-1,2-peroxo dicopper(II) complex [Cu(II)2(bnpa)2(O2(2-]2+ (2). Interestingly, the same reactions carried out in the presence of exogenous substrates such as TEMPO-H, produce a mononuclear copper(II)-hydroperoxo complex 4. Under these conditions, the H-atom abstraction reaction proceeds via the mononuclear copper(II)-superoxo intermediate [Cu(II)(bnpa)(O2-)]+ (3), as confirmed from indirect observations using a spin trap reagent. Reactions with several substrates having different bond dissociation energies (BDE) indicate that, under our experimental conditions the H-atom abstraction reaction proceeds for substrates with a weak X-H bond (BDE < 72.6 kcal mol(-1)). These investigations indicate that the copper(II)-hydroperoxo complex is a useful tool for elucidation of H-atom abstraction reaction mechanisms for exogenous substrates. The useful functionality of the complex has been achieved via careful control of experimental conditions and the choice of appropriate ligands for the complex.

Construction of a square-planar hydroperoxo-copper(II) complex inducing a higher catalytic reactivity

A novel hydroperoxo-copper(II) complex with a square-planar geometry has been prepared, which has exhibited a higher selectivity and catalytic reactivity for dimethyl sulfide, in contrast to that with a trigonal-bipyramidal one.

Electronic structures of protonic conductors SrTiO3 and SrCeO3 by O 1s X-ray absorption spectroscopy

The electronic structures of protonic conductors SrTi 1-x Sc x O 3 and SrCe 1-x Yb x O 3 have been studied by O 1s X-ray absorption spectroscopy (XAS). In SrTi 1-x Sc x O 3 , hole state and acceptor-induced level are observed in the band gap energy region. Such a structure is also observed in the O 1s XAS spectra of SrCe 1-x Yb x O 3 . The XAS structures and their temperature dependence reflect the activation energy estimated from electrical conductivity. These facts indicate that the conductivity is achieved by deep acceptor-induced level lying in the middle of the band gap.

Regioselective and Stereospecific Dehydrogenative Annulation Utilizing Silylium Ion-Activated Alkenes.

Treatment of dialkylbenzylsilanes (1) with trityl tetrakis(pentafluorophenyl)borate (TPFPB) afforded the corresponding silylium ions in equilibrium with their intra- or intermolecular π-complexes, which underwent dehydrogenative annulation with various alkenes to form 1,2,3,4-tetrahydro-2-silanaphthalenes (4) in up to 82% isolated yield. Sterically bulkier substituents on the silicon atom tended to increase the yield of cyclic products 4. The annulation products retained the stereochemistry in cases of the reactions using internal alkenes. The use of diisopropyl(1-naphthyl)silane (2) instead of 1 also resulted in annulation to obtain the 2,3-dihydro-1-sila-1H-phenalene derivatives 6. Electrophilic aromatic substitution at the 8-position was predominant, despite the two potentially reactive positions on the naphthyl group. The steric hindrance of the naphthyl group prevented addition of the cis-alkene to the silylium ion, which would considerably decrease yields of the desired products from 2 compared to those from 1.

Novel Porous 3D Network Structure Formed by Self-Assembly of Tetrakis(phthalimide) Palladium(II) Complex

A disodium tetrakis(phthalimide) palladium(II) complex, [Na2Pd(phthal)4] (1), was synthesized by reaction of four phthalimide molecules with Pd(OAc)2 in the presence of Et3N. The above and below axial sites of the Pd coordination plane formed the spaces suitable for capturing Na+ ions, which were coordinated with the four imidato oxygen atoms. The complex 1 was linked through the Na+ ions bridged by the water molecules, building an infinite chain structure along the crystallographic a axis. The four phthalimidato moieties contacted in the four orthogonal directions together through their π-π stacking interaction. Such a self-assembly of molecule 1 constructed nanopores with a pore size of ca. 3 × 4 Å in the crystals.