Distinctive Aspects in Aquation, Proton-Coupled Redox, and Photoisomerization Reactions between Geometric Isomers of Mononuclear Ruthenium Complexes with a Large-π-Conjugated Tetradentate Ligand.

Geometric isomers of mononuclear ruthenium(II) complexes, distal-/proximal-[Ru(tpy)(dpda)Cl]+ (d-/p-RuCl, tpy = 2,2':6',2″-terpyridine, dpda = 2,7-bis(2-pyridyl)-1,8-diazaanthracene), were newly synthesized to comprehensively investigate the geometric and electronic structures and distinctive aspects in various reactions between isomers. The ultraviolet (UV)-visible absorption spectra of d-/p-RuCl isomers show intense bands for metal-to-ligand charge transfer (MLCT) at close wavelengths of 576 and 573 nm, respectively. However, time-dependent density functional theory (TD-DFT) calculations suggest that the MLCT transition of d-RuCl involves mainly single transitions to the π* orbital of the dpda ligand in contrast to mixing of the π* orbitals of the dpda and tpy ligands for p-RuCl. The aquation reaction (1.5 × 10-3 s-1) of p-RuCl to yield proximal-[Ru(tpy)(dpda)(OH2)]2+ (p-RuH2O) is faster than that (5.3 × 10-6 s-1) of d-RuCl in D2O/CD3OD (4:1 v/v) by three orders of magnitude, which resulted from the longer Ru-Cl bond by 0.017 Å and the distorted angle (100.2(3)°) of Cl-Ru-N (a nitrogen of dpda, being on a tpy plane) due to the steric repulsion between Cl and dpda for p-RuCl. Electrochemical measurements showed that d-RuH2O undergoes a 2-step oxidation reaction of 1H+-coupled 1e- processes of RuII-OH2/RuIII-OH and RuIII-OH/RuIV═O at pH 1-9, whereas p-RuH2O undergoes a 1-step oxidation reaction of a 2H+-coupled 2e- process of RuII-OH2/RuIV═O in the pH range of pH 1-10. The irreversible photoisomerization from d-RuH2O to p-RuH2O was observed in aqueous solution with an internal quantum yield (Φ) of 5.4 × 10-3% at 520 nm, which is lower compared with Φ = 1.1-2.1% of mononuclear Ru(II) aquo complexes with similar bidentate ligands instead of dpda by three orders of magnitude. This is possibly ascribed to the faster nonradiative decay rate from the excited 3MLCT state to the ground state for d-RuH2O due to the lower π* level of dpda ligands according to the energy-gap law: the rate decreases exponentially with the increasing energy gap.

Complex formation of silver(I) ions with a glucosinolate derivative: structural and mechanistic insights into myrosinase-mimicking C-S bond cleavage.

The crystal structure of an unprecedented silver complex of O-acetylsinigrin has shown chelating bonds of the sulfated thiohydroximate and an η2-bond of the ethylene moiety with Ag(i). Mechanistic studies on the formation and decomposition of the complex by the 1H NMR measurements and theoretical calculations with the DFT method indicated relevance to the glucosinolate degradation in biological systems.

Enantioselective Reaction of 2H-Azirines with Oxazol-5-(4H)-ones Catalyzed by Cinchona Alkaloid Sulfonamide Catalysts.

The enantioselective reaction of 2H-azirines with oxazol-5-(4H)-ones (oxazolones) using a cinchona alkaloid sulfonamide catalyst has been developed. The reaction proceeded at the C-2 position of oxazolones to afford products with consecutive tetrasubstituted stereogenic centers in high yield with high diastereo- and enantioselectivity. The obtained aziridines were converted into various chiral compounds without loss of enantiopurity.

Enantioselective Vinylogous Mannich Reaction of Acyclic Vinylketene Silyl Acetals with Ketimines Using Chiral Bis(imidazoline)-Cu(II) Catalysts.

The enantioselective vinylogous Mannich reaction of ketimines derived from isatins with acyclic vinylketene silyl acetals has been developed using a chiral bis(imidazoline)-Cu(II) catalyst. A series of chiral 3-aminooxindole derivatives bearing tetra-substituted stereogenic centers with an α,ß-unsaturated ester moiety were obtained in excellent yields (up to 99%) with high enantioselectivities (up to 98% ee). Enantioselective bisvinylogous Mannich reaction to ketimines derived from isatins also afforded product with high enantioselectivity. Based on the experimental investigations, a possible transition state has been proposed to explain the origin of the asymmetric induction.

Enantioselective aza-Friedel-Crafts reaction of cyclic ketimines with indoles using chiral imidazoline-phosphoric acid catalysts.

An enantioselective aza-Friedel Crafts reaction of cyclic 4-aryl-3-oxo-1,2,5-thiadiazol-1,1-oxides as cyclic ketimines with indoles was developed. High enantioselectivities were observed for the reaction of various cyclic ketimines with indoles using chiral imidazoline-phosphoric acid catalysts. The obtained products can be converted to chiral α-amino amide and hydantoin.

Catalytic Enantioselective Reaction of 2H-Azirines with Thiols Using Cinchona Alkaloid Sulfonamide Catalysts.

The first catalytic enantioselective reaction of 2H-azirines with thiols has been developed. The obtained aziridines can be converted to optically active oxazolines, aziridylamides, or α-sulfonyl esters. Transformation of these optically active aziridines showed that 2H-azirines act as ß,ß-dicarbocationic amine synthons.

Catalytic Enantioselective Reaction of Allenylnitriles with Imines Using Chiral Bis(imidazoline)s Palladium(II) Pincer Complexes.

The first highly enantioselective reaction of allenylnitriles with imines has been developed. Excellent yields and enantioselectivities were observed for the reaction with various imines using chiral Phebim-PdII complexes. This process offers a simple and efficient synthetic route for various functionalized α-vinylidene-ß-aminonitriles and their derivatives.

Enantioselective Oxidative Ring-Opening Reaction of Aziridines with α-Nitroesters Using Cinchona Alkaloid Amide/Nickel(II) Catalysts.

The enantioselective oxidative ring-opening reaction of aziridines with α-nitroacetates has been developed. Good yields and enantioselectivity were observed for the reaction of various aziridines using a novel cinchona alkaloid amide/NiBr2 catalyst. Both enantiomers of products could be obtained by using pseudoenantiomeric chiral catalysts. This process offers an efficient route for the synthesis of α-aminoketones.

Catalytic Enantioselective Reaction of α-Phenylthioacetonitriles with Imines Using Chiral Bis(imidazoline)-Palladium Catalysts.

The catalytic enantioselective reaction of α-phenylthioacetonitriles with imines has been developed. The reaction of various imines proceeds in good yields and diastereo- and enantioselectivities in the presence of chiral bis(imidazoline)-palladium catalysts. The obtained products can be converted into ß-aminonitrile or ß-aminoamide compounds without loss of enantiopurity.

Catalytic Enantioselective Reaction of α-Aminoacetonitriles Using Chiral Bis(imidazoline) Palladium Catalysts.

The catalytic enantioselective reaction of diphenylmethylidene-protected α-aminoacetonitriles with imines has been developed. Good yields and diastereo- and enantioselectivities were observed for the reaction of various imines using chiral bis(imidazoline)/Pd catalysts. The reaction of α-aminonitriles with di-tert-butyl azodicarboxylate afforded chiral α,α-diaminonitriles in high yields with high enantioselectivities.

A novel square-planar Ni(II) complex with an amino-carboxamido-dithiolato-type ligand as an active-site model of NiSOD.

To understand the role of the unique equatorial coordination environment at the active center in nickel superoxide dismutase (NiSOD), we prepared a novel Ni(II) complex with an amino-carboxamido-dithiolato-type square-planar ligand (1, [Ni(2+)(L1)](-)) as a model of the NiSOD active site. Complex 1 has a low-spin square-planar structure in all solvents. Interestingly, the absorption wavelength and ν(C═O) stretching vibrations of 1 are affected by solvents. This provides an indication that the carbonyl oxygens participate in hydrogen-bonding interactions with solvents. These interactions are reflected in the redox potentials; the peak potential of an anodic wave (Epa) values of Ni(II)/Ni(III) waves for 1 are shifted to a positive region for solvents with higher acceptor numbers. This indicates that the disproportionation of superoxide anion by NiSOD may be regulated by hydrogen-bonding interactions between the carboxamido carbonyl and electrophilic molecules through fine-tuning of the redox potential for optimal SOD activity. Interestingly, the Epa value of the Ni(III)/Ni(II) couple in 1 in water (+0.303 V vs normal hydrogen electrode (NHE)) is similar to that of NiSOD (+0.290 V vs NHE). We also investigated the superoxide-reducing and -oxidizing reactions of 1. First, 1 reacts with superoxide to yield the superoxide-bound Ni(II) species (UV-vis: 425, 525, and ∼650 nm; electron paramagnetic resonance (EPR) (4 K): g// = 2.21, g⊥ = 2.01; resonance Raman: ν((16)O-(16)O)/ν((18)O-(18)O) = 1020/986 cm(-1)), which is then oxidized to Ni(III) state only in the presence of both a proton and 1-methylimidazole, as evidenced by EPR spectra. Second, EPR spectra indicate that the oxidized complex of 1 with 1-methylimidazole at the axial site can be reduced by reaction with superoxide. The Ni(III) complex with 1-methylimidazole at the axial site does not participate in any direct interaction with azide anion (pKa 4.65) added as mimic of superoxide (pKa 4.88). According to these data, we propose the superoxide disproportionation mechanism in superoxide-reducing and -oxidizing steps of NiSOD in both Ni(II) and Ni(III) states.

Coupling of an N-heterocyclic carbene on iron with alkynes to form η5-cyclopentadienyl-diimine ligands.

A cyclometalated N-heterocyclic carbene ligand in a half-sandwich iron complex was found to couple with alkynes, leading to a unique type of ring opening of the carbene ligand and the formation of ferrocenyl-diimine complexes. An intermediary iron complex obtained from the reaction with phenylacetylene reveals that the ring opening follows the formation of a fused heterocycle consisting of an imidazole ring and two alkynes.

Reduction of carbon monoxide by a tetrakis(aryloxide)diniobium complex having four bridging hydrides.

The use of a sterically demanding aryloxide ligand allows us to isolate a tetrakis(aryloxide)diniobium complex bridged by four hydride ligands. This hydride aryloxide undergoes reduction of CO via hydrogenation and deoxygenation, resulting in formation of a methylidene species. Further carbonylation gives a vinylidene complex.

Multielectron reduction of diazoalkane and azides via reversible cyclometalation in ditantalum complexes.

Reactions of the cyclometalated hydride complexes with diazo and azide substrates are reported. While diazoalkane undergoes four-electron N-N bond cleavage, azides undergo two-electron N(2) extrusion.

Naphthalene and anthracene complexes sandwiched by two {(Cp*)Fe(I)} fragments: strong electronic coupling between the Fe(I) centers.

The reactions of the half-sandwich iron(II) complex [FeCl(Cp*)(tmeda)] (1; Cp*=η(5)-C(5)Me(5), TMEDA=N,N,N',N'-tetramethylethylenediamine) with potassium naphthalenide or potassium anthracenide gave the diamagnetic complexes [(Cp*)Fe(µ-polyarene)Fe(Cp*)] (polyarene=naphthalene (2), anthracene (3a)), which have two {(Cp*)Fe} units bound to opposite faces of the polyarene. One of two {(Cp*)Fe} units in 3a is located over the central ring of anthracene while the other is positioned over an outer ring. The {(Cp*)Fe} unit bound to the central ring of 3a migrates to the outer ring upon heating in the solid state to give the isomer 3b. The electrochemical potential separations between successive one-electron redox events for complexes 2 and 3b are large. The mixed valence complexes [2](+) and [3b](+) were synthesized by chemical oxidation. The mixed-valence complex [3b](+) is charge delocalized on the Mössbauer timescale at 78 K, and its absorption spectrum shows an intervalence charge-transfer band. Complex [2](+) exhibits two absorption bands in the near-IR region and a slightly broadened doublet in the Mössbauer spectrum. DFT calculations were carried out to examine the electronic structures of these dinuclear iron(I) complexes to elucidate the factors responsible for their diamagnetism and to determine the degree of charge delocalization in the mixed-valence complexes.

Palladium-catalyzed coupling reactions of tetrafluoroethylene with arylzinc compounds.

Organofluorine compounds are widely used in all aspects of the chemical industry. Although tetrafluoroethylene (TFE) is an example of an economical bulk organofluorine feedstock, the use of TFE is mostly limited to the production of poly(tetrafluoroethylene) and copolymers with other alkenes. Furthermore, no catalytic transformation of TFE that involves carbon-fluorine bond activation has been reported to date. We herein report the first example of a palladium-catalyzed coupling reaction of TFE with arylzinc reagents in the presence of lithium iodide, giving α,ß,ß-trifluorostyrene derivatives in excellent yields.

C-H bond activation/borylation of furans and thiophenes catalyzed by a half-sandwich iron N-heterocyclic carbene complex.

A coordinatively unsaturated iron-methyl complex having an N-heterocyclic carbene ligand, [Cp*Fe(L(Me))Me] (1; Cp*=eta(5)-C(5)Me(5), L(Me)=1,3,4,5-tetramethyl-imidazol-2-ylidene), is synthesized from the reaction of [Cp*Fe(TMEDA)Cl] (TMEDA=N,N,N',N'-tetramethylethylenediamine) with methyllithium and L(Me). Complex 1 is found to activate the C-H bonds of furan, thiophene, and benzene, giving rise to aryl complexes, [Cp*Fe(L(Me))(aryl)] (aryl=2-furyl (2), 2-thienyl (3), phenyl (4)). The C-H bond cleavage reactions are applied to the dehydrogenative coupling of furans or thiophenes with pinacolborane (HBpin) in the presence of tert-butylethylene and a catalytic amount of 1 (10 mol% to HBpin). The borylation of the furan/thiophene or 2-substituted furans/thiophenes occurs exclusively at the 2- or 5-positions, respectively, whereas that of 3-substituted furans/thiophenes takes place mainly at the 5-position and gives a mixture of regioisomers. Treatment of 2 with 2 equiv of HBpin results in the quantitative formation of 2-boryl-furan and the borohydride complex [Cp*Fe(L(Me))(H(2)Bpin)] (5). Heating a solution of 5 in the presence of tert-butylethylene led to the formation of an alkyl complex [Cp*Fe(L(Me))CH(2)CH(2)tBu] (6), which was found to cleave the C-H bond of furan to produce 2. On the basis of these results, a possible catalytic cycle is proposed.

C-H bond activation of heteroarenes mediated by a half-sandwich iron complex of N-heterocyclic carbene.

Half-sandwich iron complexes of N-heterocyclic carbenes, Cp*Fe(L(R))Cl (2a; L(Mes) = 1,3-dimesityl-imidazol-2-ylidene, 2b; L(iPr) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Cp* = eta(5)-C5Me5), have been synthesized by the reaction of Cp*Fe{N(SiMe3)2} (1) with the corresponding imidazolium salts. Treatment of 2a with either methyllithium or phenyllithium replaces the chloride with either a methyl or a phenyl group, generating Cp*Fe(L(Mes))R (3a; R = Me, 3b; R = Ph). These complexes, in turn, undergo cyclometalation at elevated temperatures, and Cp*Fe{kappa2-(C,C)-L'(Mes)} (4; L'(Mes) = CH2C6H2-3,5-Me2-2-(3-mesityl-imidazol-2-ylidene-1-yl)) was isolated. On the other hand, methylation of 2b at room temperature leads directly to the formation of a cyclometalated complex, Cp*Fe{kappa2-(C,C)-L'(iPr)} (6; L'(iPr) = CH2CH(CH3)(3-isopropyl-4,5-dimethylimidazol-2-ylidene-1-yl)). The Fe(II) center of 6 traps atmospheric dinitrogen reversibly to produce a dinuclear end-on N2 complex [Cp*Fe{kappa2-(C,C)-L'(iPr)}]2(mu-eta(1):eta(1)-N2) (7). Complex 6 also promotes C-H bond activation of thiophene, furan, benzothiophene, and benzofuran at room temperature. In these reactions, C-H bond cleavage occurred exclusively at the 2-position of the rings, generating Cp*Fe(L(iPr))(2-C4H3E) (8; E = S, 9; E = O) and Cp*Fe(L(iPr))(2-C8H5E) (10; E = S, 11; E = O), while C-H cleavage took place mainly at the 4-position in the case of pyridine. Coupling reactions between heteroarenes and catecholborane (HBcat) can be carried out by treatment of 6 with heteroarenes followed by the addition of excess HBcat, giving rise to 2-boryl-heteroarenes and the borohydride complex Cp*Fe(L(iPr))(H2Bcat) (14).