Chemoselective aromatic C-H insertion of α-diazo-β-ketoesters catalyzed by dirhodium(II) carboxylates

The ability of α-diazo-β-ketoesters bearing a substituent on the benzylic position to undergo aromatic C-H insertion is described. Good to excellent yields of the aromatic C-H insertion products were observed with Rh(2)(tpa)(4) or Rh(2)(esp)(2) catalysts. This is an attractive strategy to prepare tetralins carrying a methyl group on the benzylic position, a structural motif found in several types of natural products.     

Hydrogen bond donating ability of meta and para hydroxy phenoxyl radicals

H-bond complexes between 3- or 4-OH phenoxyl radicals and various H-bond accepting molecules were investigated by experimental and computational methods. The H-bond donating ability (α(2)(H)) of 2,6-di-tert-butyl-4-hydroxyphenoxyl radical (1) was determined as 0.79 ± 0.05 by measuring, using EPR spectroscopy, the variations of the hyperfine splitting constants of 1 as a function of the acceptor concentrations. A computational approach, based on DFT calculations, was employed to estimate the α(2)(H) values for OH-substituted phenoxyl radicals that were not persistent enough to be studied by EPR spectroscopy. The α(2)(H) value calculated for the 2,6-di-methyl analogue of 1 was 0.76, in good agreement with EPR experiments. The α(2)(H) values for 2-methoxy-4-hydroxy (3), 4-hydroxy (4), 4,6-di-methyl-3-hydroxy (5) and 3-hydroxy (6) phenoxyl radicals were computed as 0.77, 0.84, 0.66 and 0.71, respectively, indicating that α(2)(H) values were dependent on the presence of electron donating substituents and on the relative positions of the -OH and -O˙ groups. By correlating the α(2)(H) values for 4 and 6 with their water and gas-phase acidities, an unexpected role of water in promoting proton dissociation from these radicals was evidenced.     

Air-stable Pd(R-allyl)LCl (L= Q-Phos, P(t-Bu)3, etc.) systems for C-C/N couplings: insight into the structure-activity relationship and catalyst activation pathway

A series of Pd(R-allyl)LCl complexes [R = H, 1-Me, 1-Ph, 1-gem-Me(2), 2-Me; L = Q-Phos, P(t-Bu)(3), P(t-Bu)(2)(p-NMe(2)C(6)H(4)), P(t-Bu)(2)Np] have been synthesized and evaluated in the Buchwald-Hartwig aminations in detail, in addition to the preliminary studies on Suzuki coupling and α-arylation reactions. Pd(crotyl)Q-PhosCl (9) was found to be a superior catalyst to the other Q-Phos-based catalysts, and the reported in situ systems, in model coupling reactions involving 4-bromoanisole substrate with either N-methylaniline or 4-tert-butylbenzeneboronic acid. Precatalyst 9 also performed better than the catalysts bearing P(t-Bu)(2)(p-NMe(2)C(6)H(4)) ligand; however, it is comparable to the new crotyl catalysts bearing P(t-Bu)(3) or P(t-Bu)(2)Np ligands. In α-arylation of a biologically important model substrate, 1-tetralone, Pd(allyl)P(t-Bu)(2)(p-NMe(2)C(6)H(4))Cl (15) was found to be the best catalyst. The reason for the relatively higher activity of the crotyl complexes in comparison to the allyl derivatives in C-N bond formation reactions was investigated using X-ray crystallography in conjunction with NMR spectroscopic studies.     

Asymmetric intramolecular cyclopropanation of diazo compounds with metallosalen complexes as catalyst: structural tuning of salen ligand

Intramolecular cyclopropanation of alkenyl α-diazoacetates and alkenyl diazomethyl ketones was examined by using optically active (ON⁺)Ru(II)(salen) and Co(II)(salen) complexes as catalysts. For the cyclization of 2-alkenyl α-diazoacetates, Co(II)(salen) complexes 9 and 10 were found to be superior catalysts to the corresponding (ON⁺)Ru(II)(salen) complexes 4 and 5. On the other hand, (ON⁺)Ru(II)(salen) complex 2 was found to be the catalyst of choice for the cyclization of 3-alkenyl diazomethyl ketones, and complex 4 was found to be a good catalyst for the cyclization of (E)-4-alkenyl diazomethyl ketones. The present study demonstrates that metallosalen complexes, especially optically active (ON⁺)Ru(II)(salen) and Co(II)(salen) complexes, can serve as efficient catalysts for the cyclization of alkenyl diazocarbonyl compounds, if a suitable salen ligand is used as the chiral auxiliary.     

Iron-catalyzed oxidative coupling of alkylamides with arenes through oxidation of alkylamides followed by Friedel-Crafts alkylation

FeCl(3) in combination with t-BuOOt-Bu as an oxidant was found to be an efficient catalyst for oxidation of alkylamides to α-(tert-butoxy)alkylamides. FeCl(2) and CuCl showed, respectively, almost the same and slightly lower activities compared with FeCl(3) in the tert-butoxylation of N-phenylpyrrolidone (1a), whereas no tert-butoxylated product was obtained by use of Fe(OTf)(3), RuCl(3), or Zr(OTf)(4). FeCl(3) was found to be effective also as a catalyst for the Friedel-Crafts alkylation with thus obtained α-(tert-butoxy)alkylamides. The Friedel-Crafts alkylation proceeded smoothly also in the presence of a catalytic amount of Fe(OTf)(3), RuCl(3), or Zr(OTf)(4). In contrast, FeCl(2) and CuCl, which showed certain activity toward the tert-butoxylation, failed to promote the Friedel-Crafts alkylation. Among the transition metal complexes thus far examined, only FeCl(3) showed high catalytic activities for both the oxidation and the Friedel-Crafts alkylation. The bifunctionality of FeCl(3) was utilized for the oxidative coupling of alkylamides with arenes through a tandem reaction consisting of oxidation of alkylamides to α-(tert-butoxy)alkylamides and the following Friedel-Crafts alkylation. The FeCl(3)-catalyzed oxidative coupling is applicable to a wide variety of alkylamides and arenes, though a combination of FeCl(3) with Fe(OTf)(3) was found to be effective for the reaction of arenes with low nucleophilicity. A Fe(II)-Fe(III) catalytic cycle is concerned with the tert-butoxylation, whereas a Fe(III) complex as a Lewis acid catalyzes the Friedel-Crafts alkylation.     

Synthesis and NMR characterization of cobalt(III) complexes with triethylenetetramine, 2,2-bipyridine and 1,10-phenanthroline

The compounds α-cis?[Co(trien)(bipy)]Cl₃ and α-cis?[Co(trien)(phen)]Cl₃ were synthesized and characterized by one- and two-dimensional NMR spectroscopy. Compared to α-cis?[Co(trien)(NO₂)₂]Cl, the proton spectra of these two complexes were spread to a wider spectral width. With the aid of two-dimensional experiments, it was possible to assign three multiplets to specific protons, and the remaining multiplet was found to arise from overlap of three separate resonances.     

Diverse structures and adsorption properties of quasi-Werner-type copper(II) complexes with flexible and polar axial bonds

The quasi-Werner-type copper(II) complex, [Cu(PF(6))(2)(4-mepy)(4)] (1), in which 4-mepy is the 4-methylpyridine ligand, has flexible and polar axial bonds of Cu-PF(6). Flexibility of the Cu-PF(6) bonds induces diverse and unprecedented guest-inclusion structures, such as {[Cu(PF(6))(2)(4-mepy)(4)][Cu(PF(6))(4-mepy)(4)(acetone)]·PF(6)·4acetone} (γ-1?2.5acetone), {[Cu(PF(6))(2)(4-mepy)(4)][Cu(PF(6))(4-mepy)(4)(2-butanone)]·PF(6)·3.5(2-butanone)} (γ-1?2.25(2-butanone)), {[Cu(PF(6))(2)(4-mepy)(4)][Cu(PF(6))(4-mepy)(4)(H(2)O)]·PF(6)·4benzene} (γ-1?0.5H(2)O·2benzene), and {[Cu(PF(6))(2)(4-mepy)(4)]·2benzene} (γ-1?2benzene). Exposure of the dense form, α-1, to benzene vapor affords the benzene-inclusion complex {[Cu(PF(6))(2)(4-mepy)(4)]·2benzene} (γ-1?2benzene), all benzene guests of which are easily removed by vacuum drying, reforming guest-free, dense α-1' with smaller sized crystals than α-1. In contrast to α-1, which shows almost no CO(2) adsorption, α-1' adsorbs CO(2) gas with structural transformations, this being the first example that exhibits adsorption of gas in a dense Werner-type complex and a drastic change in adsorption properties depending on the size of the crystals.     

Polytypism and oxo-tungstate polyhedra polymerization in novel complex uranyl tungstates

Three new uranyl tungstates, α-, β-Cs(2)[(UO(2))(2)(W(2)O(9))], and Rb(6)[(UO(2))(7)(WO(5))(2)(W(3)O(13))O(2)], have been obtained by high temperature solid state reactions. All three compounds display novel structure topologies: α- and β-Cs(2)[(UO(2))(2)(W(2)O(9))] are based upon layers with a new topology that can be related to the uranophane topology; Rb(6)[(UO(2))(7)(WO(5))(2)(W(3)O(13))O(2)] is a rare example of a non-molecular inorganic phase with layers containing oxo-tungstate trimers. The structural relationship between α- and β-Cs(2)[(UO(2))(2)(W(2)O(9))] can be assigned to polytypism.     

Enhanced photochemical activity of α-Fe2O3 films supported on SrTiO3 substrates under visible light illumination

The visible light photochemical reactivity of a 50 nm thick α-Fe(2)O(3)(0001) (hematite) film on a SrTiO(3)(111) substrate is compared to the reactivities of bulk hematite and the same film supported by α-Al(2)O(3)(0001). The hematite film supported by SrTiO(3)(111) is far more reactive then the other two cases.     

Diasteroselective preparation of cyclopropanols using methylene bis(iodozinc)

A diastereoselective synthesis of trans-2-substituted cyclopropanols is outlined. Bimetallic CH(2)(ZnI)(2) was found to react with α-chloroaldehydes to give cyclopropanols in yields of 64-89% and dr's ≥ 10:1. The high trans-selectivity resulted from equilibration of the cyclopropoxide intermediates.     

Characterization and dynamics of substituted ruthenacyclobutanes relevant to the olefin cross-metathesis reaction

The reaction of the phosphonium alkylidene [(H(2)IMes)RuCl(2)=CHP(Cy)(3))](+) BF(4)(-) with propene, 1-butene, and 1-hexene at -45 °C affords various substituted, metathesis-active ruthenacycles. These metallacycles were found to equilibrate over extended reaction times in response to decreases in ethylene concentrations, which favored increased populations of α-monosubstituted and α,α'-disubstituted (both cis and trans) ruthenacycles. On an NMR time scale, rapid chemical exchange was found to preferentially occur between the β-hydrogens of the cis and trans stereoisomers prior to olefin exchange. Exchange on an NMR time scale was also observed between the α- and β-methylene groups of the monosubstituted ruthenacycle (H(2)IMes)Cl(2)Ru(CHRCH(2)CH(2)) (R = CH(3), CH(2)CH(3), (CH(2))(3)CH(3)). EXSY NMR experiments at -87 °C were used to determine the activation energies for both of these exchange processes. In addition, new methods have been developed for the direct preparation of metathesis-active ruthenacyclobutanes via the protonolysis of dichloro(1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)(benzylidene) bis(pyridine)ruthenium(II) and its 3-bromopyridine analogue. Using either trifluoroacetic acid or silica-bound toluenesulfonic acid as the proton source, the ethylene-derived ruthenacyclobutane (H(2)IMes)Cl(2)Ru(CH(2)CH(2)CH(2)) was observed in up to 98% yield via NMR at -40 °C. On the basis of these studies, mechanisms accounting for the positional and stereochemical exchange within ruthenacyclobutanes are proposed, as well as the implications of these dynamics toward olefin metathesis catalyst and reaction design are described.     

Ground- and triplet excited-state properties correlation: a computational CASSCF/CASPT2 approach based on the photodissociation of allylsilanes

Excited-state properties, although extremely useful, are hardly accessible. One indirect way would be to derive them from relationships to ground-state properties which are usually more readily available. Herewith, we present quantitative correlations between triplet excited-state (T?) properties (bond dissociation energy, D?(T?), homolytic activation energy, E(a)(T?), and rate constant, k(r)) and the ground-state bond dissociation energy (D?), taking as an example the photodissociation of the C-Si bond of simple substituted allylsilanes CH?=CHC(R1R2)-SiH? (R1 and R2 = H, Me, and Et). By applying the complete-active-space self-consistent field CASSCF(6,6) and CASPT2(6,6) quantum chemical methodologies, we have found that the consecutive introduction of Me/Et groups has little effect on the geometry and energy of the T? state; however, it reduces the magnitudes of D?, D?(T?) and E(a)(T?). Moreover, these energetic parameters have been plotted giving good linear correlations: D?(T?) = α? + β? · D?, E(a)(T?) = α? + β? · D?(T?), and E(a)(T?) = α? + β? · D? (α and β being constants), while k(r) correlates very well to E(a)(T?). The key factor behind these useful correlations is the validity of the Evans-Polanyi-Semenov relation (second equation) and its extended form (third equation) applied for excited systems. Additionally, the unexpectedly high values obtained for E(a)(T?) demonstrate a new application of the principle of nonperfect synchronization (PNS) in excited-state chemistry issues.     

Reactivity of a Sterically Unencumbered α-Borylated Phosphorus Ylide towards Small Molecules

The influence of substituents on α-borylated phosphorus ylides (α-BCPs) has been investigated in a combined experimental and quantum chemical approach. The synthesis and characterization of Me₃PC(H)B(iBu)₂ ( 1 ), consisting of small Me substituents on phosphorous and iBu residues on boron, is reported. Compound 1 is accessible through a novel synthetic approach, which has been further elucidated through DFT studies. The reactivity of 1 towards various small molecules was probed and compared with that of a previously published derivative, Ph₃PC(Me)BEt₂ ( 2 ). Both α-BCPs react with NH₃ to undergo heterolytic N−H bond cleavage. Different di- and trimeric ring structures were observed in the reaction products of 1 with CO and CO₂. With PhNCO and PHNCS, the expected insertion products [Me₃PC(H)(PhNCO)B(iBu)₂] and [Me₃PC(H)(PhNCS)B(iBu)₂], respectively, were isolated.     

Polymer-supported haloarene chromium dicarbonyl isonitrile complexes: a study of their synthesis and reactivity

Different arene Cr(CO)(3) complexes were supported on a polystyrene isonitrile resin by photochemical-promoted replacement of a chromium carbonyl ligand by the NC group. The supported complexes proved to be stable and were successfully used for further transformations. In particular, the reactivity of dichlorobenzene complexes to different nucleophiles was investigated and found to be comparable with that of the parent Cr(CO)(3) complexes.     

Incorporation of cationic electron donor of Ni-pyridyltetrathiafulvalene with anionic electron acceptor of polyoxometalate

A new salt-[Ni(II)(DMSO)(5)(TTFPy)](2)[α-SiW(12)O(40)] (1)-based on polyoxometalates was prepared by coordinating a cationic electron donor of pyridyltetrathiafulvalene (TTFPy) with Ni(II). Although the TTFPy molecule did not form a salt with the anionic α-[SiW(VI)(12)O(40)](4-) because of the weak charge-transfer (CT) interaction, the coordination of Ni with the pyridyl moiety permitted salt formation driven by electrostatic interaction, giving a single crystal of 1. Crystallographic analysis, UV-vis and IR spectroscopy and electrochemical characterization revealed that the fully oxidized α-[SiW(VI)(12)O(40)](4-) was crystallized with the neutral TTFPy moiety from the acetonitrile solution because of the low electron-withdrawing ability of α-[SiW(VI)(12)O(40)](4-), forming a brown-orange crystal. The crystal colour quickly turned to black by immersing in methanol, due to CT from TTF moiety to α-[SiW(VI)(12)O(40)](4-), which was caused by the solvent effect. Increase in the solvent acceptor number from 18.9 for acetonitrile to 41.3 for methanol resulted in the enhancement of the electron withdrawing ability of α-[SiW(VI)(12)O(40)](4-) by 0.317 V in methanol.     

Isolations and characterization of highly water-soluble dimeric lanthanide citrate and malate with ethylenediaminetetraacetate

Highly water-soluble lanthanum and cerium citrates or malates with ethylenediaminetetraacetate (NH(4))(8)[Ln(2)(Hcit)(2)(EDTA)(2)]·9H(2)O [Ln = La, 1; Ce, 2], K(8)[La(2)(Hcit)(2)(EDTA)(2)]·16H(2)O (3) and K(6)[Ln(2)(Hmal)(2)(EDTA)(2)]·14H(2)O [Ln = La, 4; Ce, 5] (H(4)cit = citric acid, H(3)mal = malic acid, and H(4)EDTA = ethylenediaminetetracetic acid) were prepared from the reactions of lanthanide ethylenediaminetetraacetate trihydrates with citric or malic acid at pH 5.0-6.5. These compounds were characterized by elemental analyses, IR, TG-DTG, solution (13)C{(1)H} NMR, solid state (13)C NMR spectra and X-ray structural analyses. The main structural feature of the compounds consists of a dinuclear unit deca-coordinated by EDTA and citrate or malate. The α-hydroxy and α-carboxy groups of citrate and malate chelate in five-membered ring with one lanthanide ion, while one of the β-carboxy group coordinates with the other lanthanide ion, forming a dimeric structure. The other pendent β-carboxy groups in 1-3 form very strong intramolecular hydrogen bond with α-hydroxy groups [O1O7 2.594(4), 2.587(8) and 2.57(1) ? for 1-3 respectively]. (13)C NMR spectra of the lanthanum compounds show obvious downfield shifts based on solid and solution NMR measurements, indicating the coordinations of mixed-ligand in lanthanum complexes, while highfield shifts are observed in cerium complexes.     

Mechanistic insight into the N=N bond-cleavage of azo-compounds that was induced by an Al-Al-bonded compound [L(2-)Al(II)-Al(II)L(2-)

An α-diimine-stabilized Al-Al-bonded compound [L(2-)Al(II)-Al(II)L(2-)] (L = [{(2,6-iPr(2)C(6)H(3))NC(Me)}(2)]; 1) consists of dianionic α-diimine ligands and sub-valent Al(2+) ions and thus could potentially behave as a multielectron reductant. The reactions of compound 1 with azo-compounds afforded phenylimido-bridged products [L(-)Al(III)(μ(2)-NPh)(μ(2)-NAr)Al(III)L(-)] (2-4). During the reaction, the dianionic ligands and Al(2+) ions were oxidized into monoanions and Al(3+), respectively, whilst the [NAr](2-) imides were produced by the four-electron reductive cleavage of the N=N double bond. Upon further reduction by Na, the monoanionic ligands in compound 2 were reduced to the dianion to give [(L(2-))(2)Al(III)(2)(μ(2)-NPh)(2)Na(2)(thf)(4)] (5). Interestingly, when asymmetric azo-compounds were used, the asymmetric adducts were isolated as the only products (compounds 3 and 4). DFT calculations indicated that the reaction was quite feasible in the singlet electronic state, but the final product with the triplet-state monoanionic ligands could result from an exothermic singlet-to-triplet conversion during the reaction process.     

Regioselective synthesis of trifluoromethyl group containing allylic amines by palladium-catalyzed allylic amination and sequential isomerization

The palladium-catalyzed regioselective allylic amination of the α-trifluoromethyl group-substituted allyl acetate has been accomplished using Pd(OAc)₂/DPPE and [Pd(π-allyl)(cod)]BF₄/DPPF as catalysts. The selective formation of the γ-product was attained in the presence of Pd(OAc)₂/DPPE, while the α-product was obtained using [Pd(π-allyl)(cod)]BF₄/DPPF. We also succeeded in the regioselective synthesis of the enantiomerically enriched aminated product from chiral allyl acetate using Pd(OAc)₂/DPPE and [Pd(π-allyl)(cod)]BF₄/(S)-BINAP. Furthermore, we found that kinetic resolution had occurred during the isomerization step from the γ-type product to the α-type product by the [Pd(π-allyl)(cod)]BF₄/(S)-BINAP catalyst.     

Photoreduction of 99Tc pertechnetate by nanometer-sized metal oxides: new strategies for formation and sequestration of low-valent technetium

Technetium-99 ((99)Tc) (β(-)(max): 293.7 keV; t(1/2): 2.1 × 10(5) years) is a byproduct of uranium-235 fission and comprises a large component of radioactive waste. Under aerobic conditions and in a neutral-basic environment, the pertechnetate anion ((99)TcO(4)(-)) is stable. (99)TcO(4)(-) is very soluble, migrates easily through the environment and does not sorb well onto mineral surfaces, soils, or sediments. This study moves forward a new strategy for the reduction of (99)TcO(4)(-) and the chemical incorporation of the reduced (99)Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), α(2)-[P(2)W(17)O(61)](10-), that can be photoactivated in the presence of 2-propanol to transfer electrons to (99)TcO(4)(-) and incorporate the reduced (99)Tc covalently into the α(2)-framework to form the (99)Tc(V)O species, (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-). This occurs via the formation of an intermediate species that slowly converts to (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-). Extended X-ray absorption fine structure and X-ray absorption near-edge spectroscopy analysis suggests that the intermediate consists of a (99)Tc(IV) α(2)- species where the (99)Tc is likely bound to two of the four W-O oxygen atoms in the α(2)-[P(2)W(17)O(61)](10-) defect. This intermediate then oxidizes and converts to the (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-) product. The reduction and incorporation of (99)TcO(4)(-) was accomplished in a "one pot" reaction using both sunlight and UV irradiation and monitored as a function of time using multinuclear nuclear magnetic resonance and radio thin-layer chromatography. The process was further probed by the "step-wise" generation of reduced α(2)-P(2)W(17)O(61)(12-) through bulk electrolysis followed by the addition of (99)TcO(4)(-). The reduction and incorporation of ReO(4)(-), as a nonradioactive surrogate for (99)Tc, does not proceed through the intermediate species, and Re(V)O is incorporated quickly into the α(2)-[P(2)W(17)O(61)](10-) defect. These observations are consistent with the periodic trends of (99)Tc and Re. Specifically, (99)Tc is more easily reduced compared to Re. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and the mechanisms of the reduction and incorporation of (99)Tc into a material.     

Block copolymer assisted synthesis of porous α-Ni(OH)(2) microflowers with high surface areas as electrochemical pseudocapacitor materials

Porous α-Ni(OH)(2) microflowers are successfully synthesized via a one-step aqueous-phase reaction assisted by block copolymers under mild conditions. The electrochemical measurement demonstrates that the α-Ni(OH)(2) microflowers calcined at 200 °C are capable to deliver a specific capacity of 1551 F g(-1) in 6 M KOH solution, suggesting their high potential as a novel electrochemical pseudocapacitor.