Search

A new reaction mode for triarylboranes under photochemical conditions was discovered. Photoirradiation of dimesitylboryl-substituted (hetero)arenes produced spirocyclic boraindanes, where one of the C−H bonds in the ortho-methyl groups of the mesityl substituents was formally added in a syn fashion to a C−C double bond of the (hetero)aryl group. Quantum chemical calculations and laser flash photolysis measurements indicated that the reaction proceeds through a [1,6]-sigmatropic rearrangement. This behavior is reminiscent of the photochemical reaction mode of arylalkenylketones, thus demonstrating the isosteric relation between tricoordinate organoboron compounds and the corresponding pseudo-carbocationic species in terms of pericyclic reactions. Despite the disrupted π-conjugation, the resulting spirocyclic boraindanes exhibited a characteristic absorption band at relatively long wavelengths ( λ=370-400 nm). [ABSTRACT FROM AUTHOR]

Adiabatic photochemical reactions of cyclopropanes possessing a benzophenone moiety ( trans- and cis- 5), which generate a luminescent excited biradical 1 6••*, were studied. Various photochemical and spectroscopic techniques were used for this purpose. Excitation (350 nm, n, π* band) of 5 at 77 K affords the lowest excited state 1 5* that undergoes intersystem crossing to form 3 5*, which deactivates through phosphorescence. In contrast, 295-nm excitation ( π, π* band) of 5 affords the higher electronically excited state 1 5**, which undergoes adiabatic C-C bond cleavage reaction to form the excited biradical 1 6••* that luminesces at 630 nm. The results suggest that 5 follows 2 photoreaction modes depending on the excitation wavelength, a finding that disobeys Kasha's rule. [ABSTRACT FROM AUTHOR]

Two analogs of blestriarene C (4,4'-dimethoxy-1,1'-biphenanthrene-2,2',7,7'-tetraol) bearing no 7,7'-dihydroxy ( 3) and 4,4'-dimethoxy groups 4 were prepared. Unlike blestriarene C ( 1), compounds 3 and 4, as well as 1,1'-biphenanthrene-2,2'-diol ( 5), do not racemize under fluorescent lamp illumination. Cyclic voltammetry analysis reveals that compound 1 has a lower half-wave potential ( E1/2) than compounds 3, 4, 5, suggesting that a redox cycle is involved in the . Compound 1 racemizes by absorbing UV light corresponding to the 1 Lb band. During the reaction, no side products are observed. The is significantly inhibited under nitrogen. Based on these observations, we propose a feasible mechanism for the easy of compound 1, which is mediated by a cation radical generated in situ by a reversible photo-induced oxygen oxidation. Chirality 27:479-486, 2015. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Product analyses and nanosecond time-resolved spectroscopy on laser flash photolysis were studied for the photoinduced electron-transfer reaction of 3,4-di(α-styryl)furan (6a). A combination of these results, kinetic, density functional theoretical (DFT), and time-dependent DFT analyses enabled assignment of the absorption to the tetramethyleneethane (TME)-type radical cation (7a·+, λmax = 392 nm) and the corresponding singlet biradical (17a**, λmax = 661 nm). These two intermediates were mechanistically linked to each other with a facile back electron-transfer reaction. The present studies provide a new method for the generation of aryl-substituted TME-type intermediates. [ABSTRACT FROM AUTHOR]

Abstract: Nanosecond time-resolved UV/vis absorption spectroscopy on laser flash photolysis and calculations based on (time-dependent) density functional theory for radical cations of 7-benzhydrylidenebenzonorbornene ( 3 ) and 7-benzhydrylidene-2,3-dimethylenenorbornane ( 4 ) indicated their contrasting molecular geometry and electronic structures, suggesting the nonclassical and classical nature of 3 •+ and 4 •+ , respectively, which originated from the different patterns of electronic coupling between the C-7–C-8 and C-2–C-3 subunits. [Copyright &y& Elsevier]

Abstract: A photoinduced electron-transfer reaction of 2,2-dianisyl-4-isopropylidene-3,3-dimethylcyclobutanone ( 1 ) in benzene gave two unexpected CA-adducts, 5,5-dianisyl-4,4-dimethyl-3-[1-(4-hydroxy-2,3,5,6-tetrachlorophenyl)methylethylidene]dihydrofuran-2-one ( 7 ) and 2,2-dianisyl-4-(4-hydroxy-2,3,5,6-tetrachlorophenoxy)-4-isopropenyl-3,3-dimethylcyclobutanone ( 8 ), while the reaction in acetonitrile did not, suggesting significant solvent effects on the product distribution. [Copyright &y& Elsevier]

Abstract: Mechanistic studies and product analyses were performed on the single electron transfer (SET) promoted photoamination reaction between 2,5-dimethylhexa-2,4-diene (1) and 9-cyanophenanthrene in order to gain a comprehensive understanding of the effects of additive metal salts. The product selectivity of this process was found to be dramatically increased by the addition of Mg2+ salts. An investigation involving the analyses of fluorescence and time-resolved absorption spectroscopic properties reveals that Mg2+ enhances the rate of the initial SET step and stabilizes the resulting radical ion pair in this photoreaction. [Copyright &y& Elsevier]

Photochemical reactions of 2,5-bis[2,2-di(thien-2-yl)ethenyl]thiophene ( 1a ) and its tetramethyl derivative 1b , under direct photoexcitation, and photoinduced electron-transfer conditions, were studied. Our initial prediction was that 1b would undergo a photoreaction as part of a reversible photochromic system, while a reaction of its non-methyl substituted analog 1a would undergo a typical double sequence of 6π-electrocyclization (6π-EC)/aromatization (AR) (double dehydrogenation, 2H) to give 7,11-di(thien-2-yl)trithia[5]helicene ( 2a ). In fact, we observed that photoirradiation of 1b leads to formation of the 7,11-bis(3-methylthien-2-yl)trithia[5]helicene ( 2b ). In this process, 2b is formed via a double sequence of 6π-EC/AR (dehydrogenation/demethylation, H/ Me). Moreover, the yield of formation of 2b is much higher than that of 2a , which reacts through a double sequence of 6π-EC/AR ( 2H). Thus, this photochemically-induced paradoxical cascade reaction based on the newly uncovered reactivity serves as an efficient method to construct the trithia[5]helicene framework. [ABSTRACT FROM AUTHOR]

This review focuses on recent advances that have been made in conducting synthetically useful organic photochemical reactions by using microflow reactors. Attention is given to the utilization of this technique in the “scale-up” of a variety of photochemical processes including intermolecular photocycloadditions, intramolecular photocycloadditions and photocyclizations, photoadditions, photoreductions, photoisomerizations, photosubstitutions, photooxidations, photorearrangements, and heterogeneous photocatalytic reactions. In many examples, the use of the microflow method is compared to those carried out in batch systems. Finally, advantages and disadvantages of microflow photoreactions along with the possible employment of this approach to scale-up industrial photochemical processes are discussed. [ABSTRACT FROM AUTHOR]

Solvent, temperature, and excitationwavelength significantly affectedthe photochemical outcomes of a naphthalene–dicyanoethene systemtethered by different number (n) of methylene groups(1–3). The effect of irradiationwavelength was almost negligible for 2abut pronouncedfor 3a. The temperature dependence and theoretical calculationsindicated the diversity of exciplex conformations, an ensemble ofwhich can be effectively altered by changing excitation wavelengthto eventually switch the regioselectivity of photoreactions. [ABSTRACT FROM AUTHOR]