Enantioselective H-atom transfer reaction: a strategy to synthesize formaldehyde aldol products

[reaction: see text] Enantioselective radical alkylation of Baylis-Hillman adducts furnished aldol products in good yield and selectivity. The results illustrate that the selectivity in the hydrogen atom transfer is dependent on the size of the ester substituent, with smaller substituents providing better enantioselectivity.     

Photodissociation of pyrrole-ammonia clusters by velocity map imaging: mechanism for the H-atom transfer reaction

The photodissociation dynamics of pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-6) has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. The excited state hydrogen-atom transfer mechanism (ESHT) is evidenced through delayed ionization and presents a threshold around 236.6 nm, in agreement with previous reports. A high resolution determination of the kinetic energy distributions (KEDs) of the products reveals slow (～0.15 eV) and structured distributions for all the ammonia cluster masses studied. The low values of the measured kinetic energy rule out the existence of a long-lived intermediate state, as it has been proposed previously. Instead, a direct N-H bond rupture, in the fashion of the photodissociation of bare pyrrole, is proposed. This assumption is supported by a careful analysis of the structure of the measured KEDs in terms of a discrete vibrational activity of the pyrrolyl co-fragment.     

The new theory of electron transfer: application to the photosynthetic reaction centre

Based on recent developments in the theory of electron transfer, we prove that a non-polar environment is needed to maintain the high efficiency and chemical integrity of the photosynthetic reaction centre. We also determine the Gibbs energy diagram for the primary act of charge separation in photosynthesis, and propose an equivalent circuit that captures the principal features of the entire acceptor side of the electron transport chain in photosystem II.     

Multidimensional reaction coordinate for the excited-state H-atom transfer in perylene quinones: importance of the 7-membered ring in hypocrellins A and B

The excited-state intramolecular H-atom transfer reactions of hypocrellins B and A are compared by using time-resolved absorption and fluorescence upconversion techniques. The hypocrellin B photophysics are well described by a simple model involving one ground-state species and excited-state forward and reverse H-atom transfer with a nonfluorescent excited state. We suggest that excited-state conformational changes are coupled to the H-atom transfer in hypocrellin B just as gauche/anti changes are coupled to the H-atom transfer in hypocrellin A.     

Extrapolating H-atom transfer rate constants via thermochemical kinetics: The effect of tunneling

One of the important applications of chemical kinetics is the attempt to understand complex processes through kinetic modeling. This process frequently requires that rate constants be obtained by extrapolation of data either to higher or lower temperatures than the experimental, or by estimation or correlation with such data. Thermochemical kinetics combined with conventional transition state theory forms a framework from which this may be done. However, rate data for H transfer reactions may have a significant contribution from tunneling. In this work, a one dimensional approach to tunneling, consistent with conventional transition state theory, is taken to show how tunneling affects the extrapolation and correlation of rate constants in thermochemical kinetics. It is concluded that extrapolation and correlation are both quite good even when tunneling comprises 80% of the reaction. However, this is not without limitations, which are discussed. © 1993 John Wiley & Sons, Inc.     

Electron transfer rates and Franck–Condon factors: an application to the early electron transfer steps in photosynthetic reaction centers

Mechanistic aspects of some of the early electron transfer steps occurring in photosynthetic reaction centers are discussed. Starting from the normal modes of the redox cofactors involved in the electron transfer processes, we show how a series of quantities which regulate electron transfer rates, such as (i) the electron transfer active modes, (ii) the intramolecular reorganization energy, and (iii) the mutual couplings between the vibronic states of the donor and the acceptor, can be obtained and used to draw qualitative conclusions on ET rates.     

Cation-modulated electron-transfer channel: H-atom transfer vs proton-coupled electron transfer with a variable electron-transfer channel in acylamide units

The mechanism of proton transfer (PT)/electron transfer (ET) in acylamide units was explored theoretically using density functional theory in a representative model (a cyclic coupling mode between formamide and the N-dehydrogenated formamidic radical, FF). In FF, PT/ET normally occurs via a seven-center cyclic proton-coupled electron transfer (PCET) mechanism with a N-->N PT and an O-->O ET. However, when different hydrated metal ions are bound to the two oxygen sites of FF, the PT/ET mechanism may significantly change. In addition to their inhibition of PT/ET rate, the hydrated metal ions can effectively regulate the FF PT/ET cooperative mechanism to produce a single pathway hydrogen atom transfer (HAT) or a flexible proton coupled electron transfer (PCET) mechanism by changing the ET channel. The regulation essentially originates from the change in the O...O bond strength in the transition state, subject to the binding ability of the hydrated metal ions. In general, the high valent metal ions and those with large binding energies can promote HAT, and the low valent metal ions and those with small binding energies favor PCET. Hydration may reduce the Lewis acidity of cations, and thus favor PCET. Good correlations among the binding energies, barrier heights, spin density distributions, O...O contacts, and hydrated metal ion properties have been found, which can be used to interpret the transition in the PT/ET mechanism. These findings regarding the modulation of the PT/ET pathway via hydrated metal ions may provide useful information for a greater understanding of PT/ET cooperative mechanisms, and a possible method for switching conductance in nanoelectronic devices.     

Synthesis of modular thiophene-oxazoline ligands and their application in the asymmetric phenyl transfer reaction to aldehydes

A series of thiophene mono (oxazoline) N,O-ligands with three sites of diversity were synthesized concisely in two steps from the corresponding thiophene carbonitriles. These ligands were applied to the enantioselective phenyl transfer reaction of aldehydes, resulting in the corresponding chiral diaryl methanol products with excellent yields and moderate to good enantioselectivities.     

UHF-AM1 studies on 1,4 and 1,5 intramolecular H-atom transfer reactions in alkenyl radicals

The transition states of intramolecular 1,4 and 1,5 H-atom transfers, from/to primary (p), secondary (s) or tertiary alkyl (t) and primary (p _a), secondary (s _a) or tertiary (t _a) allyl carbon atoms, have been studied at the level of the semiempirical quantum-chemical method AM1 with the UHF approximation. The activation and reaction enthalpies were calculated and compared with experimental data available in the literature and the calculated data obtained for analogous reactions in alkyl radicals. Correlations were found between the activation enthalpies and the dissociation enthalpies of the bonds broken and formed.     

Temperature and pressure dependences of tunneling rate constant: density-functional theory potential-energy surface for H-atom transfer in the fluorene-acridine system

Temperature and pressure dependences of rate constants for solid phase tunneling reactions are analytically considered within the framework of modified theory of radiationless transitions, taking into account the intermolecular and soft intramolecular promotive vibrations of reagents. This treatment allows us to describe theoretically the process of atomic tunneling and the effect of temperature on the potential barrier and reorganization of the reagents. The influence of external pressure appears in our treatment as a static reduction of widths and heights of the potential barrier with hydrostatic compression of the matrix, and also as an increase of frequencies of promotive vibrational modes owing to anharmonicity. The theoretical results are used to interpret experimental data concerning the effect of temperature and pressure on the hydrogen-atom tunneling in the fluorene-acridine reaction system. It has been shown that by taking into account the contributions from reorganization of the reagents, which statically reduce the tunneling barrier and are related to four types of promotive vibrations (translational, librational, and two low-frequency intramolecular modes at 95 and 238 cm(-1)), one can reproduce the experimental data available in the literature. The parameters of the reaction system required for this analysis are calculated from two-dimensional potential-energy surfaces generated at the DFT-B3LYP/6-31G* level.     

Novel biphasic reaction system of ferric chloride dissolved imidazolium hexafluorophosphate and benzotrifluoride: application to electron transfer reaction of cyclopropyl silyl ethers

Ferric chloride (FeCl₃) promoted electron transfer oxidation of bicyclic cyclopropyl silyl ethers was performed in biphasic solution system of 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF₆) and benzotrifluoride (BTF). The resulting chloro-substituted ring-expanded cycloalkanones were treated with an appropriate base to produce substituted cyclic enones. These two-step reactions were successfully devised to proceed in a simpler manner in which the ordinary work-up operations for the former oxidation step, such as water-quench, extraction, and evaporation, were omitted; imidazole was found to be the most suitable base for the latter elimination step.     

Laser flash photolysis studies on radical formation by H-atom abstraction of triplet vitamin K3 and by H-atom transfer via the triplet exciplex between triplet methyldihydroxynaphthalene and benzophenone

Nanosecond laser photolysis techniques were incorporated to obtain (1) the absorption spectra and coefficients of triplet vitamin K₃ (2-methyl-1,4-naphthoquinone, MNQ) and its ketyl radical (2-methylnaphthosemiquinone, 2MNQH^*) in acetonitrile (ACN) as well as to reveal (2) the mechanisms for hydrogen atom abstraction of triplet MNQ (³MNQ^*) from phenol which proceeded in a diffusion process with an efficiency of unity. On the other hand, the hydroxymethylnaphthoxy radical was produced with the benzophenone ketyl radical (BPK) by the hydrogen atom transfer from triplet 2-methyl-1,4-dihydroxynaphthalene (MDHNp) sensitized by triplet benzophenone to benzophenone (BP) via the triplet exciplex. The question to be addressed was, which was produced in the MDHNp-BP system, the 2-methyl or 3-methylnaphthosemiquinone radical? Comparing the absorption spectrum and coefficient of the radical produced via the triplet exciplex with those of the 2MNQH^* obtained by H-atom abstraction of ³MNQ^*, the radical formed with BPK was revealed to be 2MNQH^*. The reasons for the preferable formation of 2MNQH^* are discussed for H-atom abstraction as well as the transfer reactions.     

The correct formulation of the sulfur transfer reagent benzyltriethylammonium tetracosathioheptamolybdate

The recently reported sulfur transfer reagent benzyltriethylammonium tetracosathioheptamolybdate [(PhCH₂)N(C₂H₅)₃]₆[Mo₇S₂₄] [Tetrahedron Lett. 2003, 44, 887] is correctly formulated as benzyltriethylammonium tetrathiomolybdate [(PhCH₂)N(C₂H₅)₃]₂[MoS₄]. The correct formulation explains the unusual sulfur transfer properties of [(PhCH₂)N(C₂H₅)₃]₆[Mo₇S₂₄] observed in the earlier work.     

Multipolymer solution-phase reactions: application to the Mitsunobu reaction

The realization of the first polymer-on-polymer Mitsunobu reaction, in which a polymeric phosphine is used simultaneously with a polymeric azodicarboxylate, is reported. This strategy employs the use of soluble oligomers generated from ring-opening methathesis polymerization. 31P NMR analysis revealed that the two polymers were interacting to generate the Mitsunobu products. Application to several substrates, as well as comparison experiments with other polymeric reagents, is described.     

The application of chiral,non-racemic N-alkylephedrine and N,N-dialkylnorephedrine as ligands for the enantioselective aryl transfer reaction to aldehydes

The catalytic enantioselective arylation of several aldehydes using arylboronic acids as the source of transferable aryl groups is described; the reaction is found to proceed in excellent yields and high enantioselectivities (up to 96% ee) in the presence of a chiral amino alcohol derived from ephedrines and congeners.     

Rate constants and isotope effects for the reaction of H-atom abstraction from RH substrates by PINO radicals

The kinetics of the reactions of hydrogen atom abstraction from the C–H bonds of substrates of different structures by phthalimide-N-oxyl radicals is studied. The rate constants of this reaction are measured and the kinetic isotope effects are determined. It is shown that in addition to the thermodynamic factor, Coulomb forces and donor–acceptor interactions affect the reaction between phthalimide-N-oxyl radicals and substrate molecules, altering the shape of the transition state. This favors the tunneling of hydrogen atoms and leads to a substantial reduction in the activation energy of the process.     

Electron transfer activation of the Diels-Alder reaction: Quantitative relationship to charge transfer excited states

The Diels-Alder cycloaddition of anthracene to tetracyanoethylene (TCNE) is quantitatively compared to alkylmetal insertion under the same reaction conditions. In both systems, the observation of transient charge transfer (CT) absorption bands is related to the presence of 1:1 electron donor-acceptor complexes of anthracene (Ar) and alkylmetal (RM) donors with the TCNE acceptor. The activation free energies ΔG3 for anthracene cycloaddition and alkylmetal insertion are found to be equal to the energies of ion-pair formation, i.e. [Ar⁺TCNE^?] and RM⁺TCNE^?], which are evaluated from the CT transition energies hν_CT. Indeed, the differences in the rates of alkylmetal insertion and anthracene cycloaddition by a factor of more than 10⁹, are shown quantitatively to arise from the differences in ion-pair solvation ΔG^s. The same differences in ΔG^s also apply quantitatively to the free ions, [Ar⁺] and [RM⁺], independently derived from the electrochemical and iron(III) oxidations of alkylmetals and aromatic compounds, respectively, by outer- sphere electron transfer. The charge transfer formulation of the activation process but provides a unifying basis for comparing such diverse processes as Diels-Alder cycloadditions and organometal cleavages, when a common electron-deficient acceptor is employed. The relationship to the concerted mechanisms of the Diels-Alder reaction is discussed.