Rate constant measurements for initial addition reactions of radicals at the propagation step of photo‐polymerization as studied by pulsed EPR spectroscopy

Pulsed EPR spectroscopy was employed to determine reaction rate constants at an early stage of addition reactions in radical polymerizations triggered by four initiator radicals, which were generated by photodissociation of four parent molecules. Two monomers (tert‐butylacrylate and tert‐butylmethacrylate) were examined as reactant. Stern–Volmer analysis on the measured decay time of electron spin echo intensity of reacting radicals provides rate constants for addition reactions. We focused on rate constants for the second step reaction between monomer and adduct radical that is produced by the first step addition reaction between initiator radical and monomer. The rate constant measured by pulsed EPR was evaluated by theoretical calculations in the light of (1) enthalpy difference between product radical and reactants and (2) charge transfer interaction between reacting radical and monomer. Copyright © 2016 John Wiley & Sons, Ltd.     

Lewis Acid Coupled Electron Transfer of Metal–Oxygen Intermediates

Redox‐inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal–oxygen intermediates, such as metal–oxo and metal–peroxo complexes. The mechanisms of the oxidative C?H bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)–oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)₃ have been unified as rate‐determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)₃) by iron(III)–oxo complexes. All logarithms of the observed second‐order rate constants of Lewis acid‐promoted oxidative C?H bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)–oxo complexes exhibit remarkably unified correlations with the driving forces of proton‐coupled electron transfer (PCET) and metal ion‐coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)₃ to the metal–oxo moiety has been confirmed for Mn^IV–oxo complexes. The enhancement of the electron‐transfer reactivity of metal–oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox‐inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)–peroxo complexes, resulting in acceleration of the electron‐transfer reduction but deceleration of the electron‐transfer oxidation. Such a control on the reactivity of metal–oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca²⁺ in the oxidation of water to dioxygen by the oxygen‐evolving complex in photosystem II.     

Near-IR excitation transfer and electron transfer in a BF2-chelated dipyrromethane-azadipyrromethane dyad and triad

A molecular dyad and triad, comprised of a known photosensitizer, BF(2)-chelated dipyrromethane (BDP), covalently linked to its structural analog and near-IR emitting sensitizer, BF(2)-chelated tetraarylazadipyrromethane (ADP), have been newly synthesized and the photoinduced energy and electron transfer were examined by femtosecond and nanosecond laser flash photolysis. The structural integrity of the newly synthesized compounds has been established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular-clip-type structure for the triad, in which the BDP and ADP entities are separated by about 14 ? with a dihedral angle between the fluorophores of around 70°. Differential pulse voltammetry studies have revealed the redox states, allowing estimation of the energies of the charge-separated states. Such calculations revealed a charge separation from the singlet excited BDP ((1)BDP*) to ADP (BDP(.+)-ADP(.-)) to be energetically favorable in nonpolar toluene and in polar benzonitrile. In addition, the excitation transfer from the singlet BDP to ADP is also envisioned due to good spectral overlap of the BDP emission and ADP absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from (1)BDP* to ADP (in benzonitrile and toluene) and electron transfer from BDP to (1)ADP* (in benzonitrile, but not in toluene). The kinetic study of energy transfer was measured by monitoring the rise of the ADP emission and revealed fast energy transfer (ca. 10(11) s(-1)) in these molecular systems. The kinetics of electron transfer via (1)ADP*, measured by monitoring the decay of the singlet ADP at λ=820 nm, revealed a relatively fast charge-separation process from BDP to (1)ADP*. These findings suggest the potential of the examined ADP-BDP molecules to be efficient photosynthetic antenna and reaction center models.     

pH-Dependent chemoselective synthesis of alpha-amino acids. Reductive amination of alpha-keto acids with ammonia catalyzed by acid-stable iridium hydride complexes in water

An acid-stable hydride complex [Cp*IrIII(bpy)H]+ {1, Cp* = eta5-C5Me5, bpy = 2,2'-bipyridine} serves as the active catalyst for the highly chemoselective synthesis of alpha-amino acids by reductive amination of alpha-keto acids with aqueous NH3 and HCOO- in water at pH 5-8. pH-dependent catalytic 15N- and 2H-double-labeling has also been accomplished by using 15NH3 and DCOONa, which are ideal amine and hydride ion sources, respectively.     

Mechanistic study on dimerization of acetylene with a Nieuwland catalyst

A mechanistic study on the Nieuwland catalysis for dimerization of acetylene is performed by detecting copper–acetylene and copper–monovinylacetylene π‐complexes and also by examining the kinetics under virtually the same reaction conditions employed in the industrial process. An efficient H/D exchange occurs between acetylene and protons in the Nieuwland catalytic system. Addition of a coordinating ligand to the conventional Nieuwland catalytic system results in improvement of the catalytic activity and selectivity for the acetylene dimerization. The kinetic analysis including the kinetic deuterium isotope effect provides valuable insight into the Nieuwland catalytic mechanism of the dimerization of acetylene. Copyright © 2008 John Wiley & Sons, Ltd.     

Metallocene bis(perfluoroalkanesulfonate)s as air-stable cationic Lewis acids

Zirconocene and titanocene bis(perfluorooctanesulfonate)s were synthesized. In contrast to the corresponding triflates and perchlorates, these compounds are air- and water-stable. They were proved to be ionic on the basis of conductivity measurements and X-ray analysis, allowing these complexes to be stored for months. The strong Lewis acidity of these cationic metallocene species, which was proved by ESR study, enabled catalytic glycosylation.     

Assembly of a Benzo-Fused Bridged Ketone Scaffold from 1,5,10-Enediynes through One-Pot Ruthenium-Catalyzed Cyclization/Iodine-Mediated Oxidative Ring Expansion

In the presence of a cationic Ru catalyst, 1,6-diynes bearing a terminal styryl moiety underwent [2+2+2] cyclization to produce dehydrobiphenylenes fused with a five-membered ring. Although the cycloadducts were unstable toward purification, their one-pot iodine-mediated ring expansion successfully afforded unprecedented bridged ketone products containing a benzo-fused bicyclo[3.2.1] framework.