Synthesis of a Cyclic Pentapeptide Mimic of the Active Site His-Tyr Cofactor of Cytochrome <i>c</i> Oxidase

Arylboronic acid based technology provides a mild, regioselective, and nontoxic N-arylation procedure for accessing the unusual N-arylated side chain histidine found in the active site of cytochrome c oxidase (CcO). The N-arylated histidine is elaborated to the complete cytochrome c oxidase cyclic pentapeptide cofactor. Molecular modeling of the cofactor provides insight into the dynamic character of the N-aryl bond

The Pauson-Khand Reaction: A Gas-Phase and Solution-Phase Examination Using Electrospray Ionization Mass Spectrometry

A series of dicobalt hexacarbonyl complexes with charged alkyne ligands were prepared to enable the study of the Pauson–Khand reaction using ESI-MS. The hexacarbonyl complexes can be activated in the gas phase through removal of a CO ligand. The resulting pentacarbonyl ions react readily with alkenes, and no discrimination between alkenes was found for this step, indicating that alkene association is not rate determining in the intermolecular reaction. Solution-phase ESI-MS studies on a system set up for intramolecular reactivity revealed only the hexacarbonyl complex as a detectable intermediate, and the reaction was shown to have a large enthalpy and entropy of activation, consistent with ligand dissociation being rate limiting in the reaction

Using the Rotaxane Mechanical Bond to Enhance Chemical Reactivity

Rates of cycloreversion for squaraine rotaxane mono(endoperoxides) were enhanced by structural modifications that increased cross-component steric destabilization of the inward directed 9,10-anthracene endoperoxide group. The largest rate enhancements were obtained when the surrounding macrocycle contained two 2,6-pyridine dicarboxamide bridging units, which induced a cavity contraction effect. The precursor fluorescent, near-IR, squaraine rotaxanes are effectively photostable because the mono(endoperoxide) products, formed by reaction with photogenerated singlet oxygen, rapidly cyclorevert back to the original squaraine rotaxane

The Pauson-Khand Reaction: A Gas-Phase and Solution-Phase Examination Using Electrospray Ionization Mass Spectrometry

A series of dicobalt hexacarbonyl complexes with charged alkyne ligands were prepared to enable the study of the Pauson–Khand reaction using ESI-MS. The hexacarbonyl complexes can be activated in the gas phase through removal of a CO ligand. The resulting pentacarbonyl ions react readily with alkenes, and no discrimination between alkenes was found for this step, indicating that alkene association is not rate determining in the intermolecular reaction. Solution-phase ESI-MS studies on a system set up for intramolecular reactivity revealed only the hexacarbonyl complex as a detectable intermediate, and the reaction was shown to have a large enthalpy and entropy of activation, consistent with ligand dissociation being rate limiting in the reaction

Synthesis of a Cyclic Pentapeptide Mimic of the Active Site His-Tyr Cofactor of Cytochrome <i>c</i> Oxidase

Arylboronic acid based technology provides a mild, regioselective, and nontoxic N-arylation procedure for accessing the unusual N-arylated side chain histidine found in the active site of cytochrome c oxidase (CcO). The N-arylated histidine is elaborated to the complete cytochrome c oxidase cyclic pentapeptide cofactor. Molecular modeling of the cofactor provides insight into the dynamic character of the N-aryl bond

Unusual Redox Chemistry of Ytterbium Carbazole–Bis(oxazoline) Compounds: Oxidative Coupling of Primary Phosphines by an Ytterbium Carbazole–Bis(oxazoline) Dialkyl

The 1,8-bis­(4′,4′-dimethyloxazolin-2′-yl)-3,6-di-<i>tert</i>-butylcarbazole anion (Czx) forms monomeric, six-coordinate halide complexes of Yb­(II), (Czx)­Yb­(X)­(THF)₂ (X = I (<b>2</b>), Cl (<b>3</b>)), by metathesis of YbX₂ with NaCzx (<b>1</b>) or Na/Hg reduction of (Czx)­Yb­(Cl)₂(THF). The crystal structure of <b>1</b> reveals a polymeric chain structure in which the oxazoline ring bridges to the Na⁺ of an adjacent unit. The iodo complex <b>2</b> serves as a precursor to divalent silylamide, alkyl, and phosphide complexes, (Czx)­Yb­(X)­(THF)_<i>n</i> (<b>4</b>, X = N­(SiMe₃)₂, <i>n</i> = 1; <b>5</b>, X = CH­(SiMe₃)₂, <i>n</i> = 1; <b>7a</b>, X = 2,4,6-Me₃C₆H₂PH, <i>n</i> = 2; <b>7b</b>, X = 2,4,6-Prⁱ₃C₆H₂PH, <i>n</i> = 2). The X-ray structure of <b>4</b> reveals a distorted-trigonal-bipyramidal geometry with the Czx ligand occupying two axial sites and one equatorial site in a pseudo-<i>mer</i> coordination mode. In contrast to the typical metathesis chemistry observed with LiCH­(SiMe₃)₂, an unusual <i>oxidation</i> occurs when <b>2</b> or <b>3</b> is treated with LiCH₂SiMe₃ to generate the previously isolated trivalent alkyl (Czx)­Yb­(CH₂SiMe₃)₂. Trivalent Yb complexes with the Czx ligand also display unusual redox chemistry: rapid <i>reduction</i> to the Yb­(II) phosphides <b>7a</b>,<b>b</b> is observed on treatment of <i>mer,cis</i>-(Czx)­Yb­(Cl)₂(THF) with ArPH^– Na⁺ (<b>6a</b>,<b>b</b>) or, equivalently, on treatment of (Czx)­Yb­(CH₂SiMe₃)₂ with ArPH₂. In both cases, oxidative coupling of the phosphide or phosphine was observed to form <i>meso</i>- and <i>rac</i>-biphosphines, ArPH–PHAr (Ar = 2,4,6-Me₃C₆H₂ (<b>9a</b>), 2,4,6-Prⁱ₃C₆H₂ (<b>9b</b>))

Unusual Redox Chemistry of Ytterbium Carbazole–Bis(oxazoline) Compounds: Oxidative Coupling of Primary Phosphines by an Ytterbium Carbazole–Bis(oxazoline) Dialkyl

The 1,8-bis­(4′,4′-dimethyloxazolin-2′-yl)-3,6-di-<i>tert</i>-butylcarbazole anion (Czx) forms monomeric, six-coordinate halide complexes of Yb­(II), (Czx)­Yb­(X)­(THF)₂ (X = I (<b>2</b>), Cl (<b>3</b>)), by metathesis of YbX₂ with NaCzx (<b>1</b>) or Na/Hg reduction of (Czx)­Yb­(Cl)₂(THF). The crystal structure of <b>1</b> reveals a polymeric chain structure in which the oxazoline ring bridges to the Na⁺ of an adjacent unit. The iodo complex <b>2</b> serves as a precursor to divalent silylamide, alkyl, and phosphide complexes, (Czx)­Yb­(X)­(THF)_<i>n</i> (<b>4</b>, X = N­(SiMe₃)₂, <i>n</i> = 1; <b>5</b>, X = CH­(SiMe₃)₂, <i>n</i> = 1; <b>7a</b>, X = 2,4,6-Me₃C₆H₂PH, <i>n</i> = 2; <b>7b</b>, X = 2,4,6-Prⁱ₃C₆H₂PH, <i>n</i> = 2). The X-ray structure of <b>4</b> reveals a distorted-trigonal-bipyramidal geometry with the Czx ligand occupying two axial sites and one equatorial site in a pseudo-<i>mer</i> coordination mode. In contrast to the typical metathesis chemistry observed with LiCH­(SiMe₃)₂, an unusual <i>oxidation</i> occurs when <b>2</b> or <b>3</b> is treated with LiCH₂SiMe₃ to generate the previously isolated trivalent alkyl (Czx)­Yb­(CH₂SiMe₃)₂. Trivalent Yb complexes with the Czx ligand also display unusual redox chemistry: rapid <i>reduction</i> to the Yb­(II) phosphides <b>7a</b>,<b>b</b> is observed on treatment of <i>mer,cis</i>-(Czx)­Yb­(Cl)₂(THF) with ArPH^– Na⁺ (<b>6a</b>,<b>b</b>) or, equivalently, on treatment of (Czx)­Yb­(CH₂SiMe₃)₂ with ArPH₂. In both cases, oxidative coupling of the phosphide or phosphine was observed to form <i>meso</i>- and <i>rac</i>-biphosphines, ArPH–PHAr (Ar = 2,4,6-Me₃C₆H₂ (<b>9a</b>), 2,4,6-Prⁱ₃C₆H₂ (<b>9b</b>))

Using the Rotaxane Mechanical Bond to Enhance Chemical Reactivity

Rates of cycloreversion for squaraine rotaxane mono(endoperoxides) were enhanced by structural modifications that increased cross-component steric destabilization of the inward directed 9,10-anthracene endoperoxide group. The largest rate enhancements were obtained when the surrounding macrocycle contained two 2,6-pyridine dicarboxamide bridging units, which induced a cavity contraction effect. The precursor fluorescent, near-IR, squaraine rotaxanes are effectively photostable because the mono(endoperoxide) products, formed by reaction with photogenerated singlet oxygen, rapidly cyclorevert back to the original squaraine rotaxane

Synthesis of a Cyclic Pentapeptide Mimic of the Active Site His-Tyr Cofactor of Cytochrome <i>c</i> Oxidase

Arylboronic acid based technology provides a mild, regioselective, and nontoxic N-arylation procedure for accessing the unusual N-arylated side chain histidine found in the active site of cytochrome c oxidase (CcO). The N-arylated histidine is elaborated to the complete cytochrome c oxidase cyclic pentapeptide cofactor. Molecular modeling of the cofactor provides insight into the dynamic character of the N-aryl bond

Synthesis and characterization of low-melting ferrocenyl salts: a study of thermal and photochemical redox reactions

The syntheses of bis(trifluoromethane) sulfonimide N-(ferrocenylmethyl) N,N,N-trimethyl ammonium salt 1 and bis(trifluoromethane) sulfonimide N′,N-bis(ferrocenylmethyl) inmidazolium salt 2 are reported. The compounds show low melting points, 76° and 125 °C, respectively, and were characterized by spectroscopic techniques 1H-NMR, mass spectrometry, FT-IR spectroscopy, cyclic voltammetry, and XRD crystallographic analysis. Cyclic voltammetry results demonstrate that both salts have a quasi-reversible electrochemical behavior, similar to the reversible one shown by ferrocene under the same conditions. In the case of 1, a coupled chemical reaction was detected at slow scan rates after the oxidation. Transient species in the redox processes were investigated using flash photolysis and pulse radiolysis. In both cases, transient spectra of the products were similar to transient spectra observed for ferrocene. The decay kinetics of the oxidized and reduced transients were examined and mechanistic information was obtained. The experimental results show the potential uses of the two complexes as catalytic materials for electrochemical applications.</p