Intramolecular and intermolecular kinetic isotope effects (KIE) in the nitrosoarene ene reaction: experimental evidence for reversible intermediate formation

The intramolecular and intermolecular kinetic isotope effects (KIE) have been determined for the nitrosoarene ene reaction with deuterium-stereolabeled 2,3-dimethyl-2-butenes (TME). trans-TME-d(6) (k(H)/k(D) = 3.0) and gem-TME-d(6) (k(H)/k(D) = 4.0) show large intramolecular primary isotope effects. In contrast, the intramolecular competition in cis-TME-d(6) (k(H)/k(D) = 1.5) and the intermolecular competition for the TME-d(0)/TME-d(12) pair (k(H)/k(D) = 1.98) show considerably smaller, but mechanistically significant kinetic isotope effects. The latter fact is rationalized in terms of reversible formation of a three-membered-ring intermediate, namely the aziridine N-oxide, or a similar unsymmetrical, polarized diradical in the first step of the reaction. Such reversibility has also been implied earlier for triazolinedione (TAD) and singlet oxygen ((1)O(2)) with deuterium-stereolabeled 2-butenes, but of the three enophiles, ArNO is the most sensitive toward reversibility, which is due to its moderate reactivity and its high steric demand.     

Direct electrochemistry of cytochrome c on nanotextured diamond surface

Nanotextured diamond surfaces with geometrical properties close to protein dimensions were used for the realization of direct electron transfer of cytochrome c (cyt c) without any covalent bonding. The peroxidase activity of native and denatured cyt c was also investigated. Cyclic voltammograms of native cyt c show quasi-reversible electron transfer reactions, while no heme redox activity is detected for denatured cyt c. Unfolding (denaturation) of cyt c can be achieved in the presence of hydrogen peroxide. Partially or fully denatured cyt c showed higher peroxidase activity than native cyt c. This is because denatured cyt c loses its tertiary structure and hydrogen peroxide is easier to access the heme redox center. The apparent Michaelis–Menten constant K_m for native and denatured cyt c has been determined to be 0.23 mM and 0.08 mM.     

On the electronic character of localized singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals: substituent effects on the lifetime

Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe > OMe, CN > CN, CN > OMe, H > Cl, Cl approximately CN, H approximately Me, Me > H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures.     

Total Synthesis of α-Tocopherol through Enantioselective Iridium-Catalyzed Fragmentation of a Spiro-Cyclobutanol Intermediate

A conceptionally new strategy for the asymmetric (2R-selective) synthesis of α-tocopherol (vitamin E) was developed. In the stereocontrolled key step, a prochiral spiro[chromane-2,3′-cyclobutanol] unit is effectively desymmetrized under C−C bond activation in an unprecedented iridium-catalyzed transformation using (S)-DTBM-SegPhos as a chiral ligand (e.r. 97:3). To complete the synthesis, the side chain was attached through Ru-catalyzed cross-metathesis employing an alkene derived from (R,R)-hexahydrofarnesol. To suppress epimerization during the final hydrogenation, PtO₂ had to be used as a catalyst instead of Pd/C. In an alternative approach (employing a propargyl-substituted spiro-cyclobutanol), the side chain was constructed prior to the Ir-catalyzed ring fragmentation (>99:1 d.r.) through enyne cross-metathesis (using an alkene derived from (R)-dihydrocitronellal) followed by Cr-catalyzed 1,4-hydrogenation and (diastereoselective) Pfaltz hydrogenation of the resulting triple-substituted olefin. The work demonstrates the potential of iridium catalysis for enantioselective C−C bond activation.     

Extension of longwavelength IR photovoltaic detector operation to near room-temperatures

Practical realization of near room temperature (230–300 K) longwavelength (5–12 μm) photovoltaic detectors is reported. The devices are epitaxial n⁺ip photodiodes operated at ambient temperature or with a simple, two-stage thermoelectric cooling. The performance of the photodiodes has been improved by the use optimized composition and doping profile structures. The tunnel currents were minimized by interfacing the n⁺ and p-type layers with a thin (0.5 μm) lightly doped i-region. The quantum efficiency has been increased by the use of backside reflector. Further improvement of performance was achieved by the use of monolithic optical immersion. Large area devices with useful performance were obtained by the use of small close-spaced elements connected in series. The near room temperature photovoltaic detectors are of particular significance for very low and very high frequency applications.