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1.
Chem Commun (Camb) ; 55(48): 6838-6841, 2019 Jun 11.
Artigo em Inglês | MEDLINE | ID: mdl-31093623

RESUMO

Here we report the application of dual nickel/photoredox catalysis to the allylation of aliphatic, aromatic and heteroaromatic aldehydes by using commercially available reagents. The process utilizes the combination of a Ni(ii) complex, [Ru(bpy)3]2+ as a photoredox catalyst, and allylacetate under blue LED irradiation, and allows the synthesis of a large variety of homoallylic alcohols.

2.
Chem Commun (Camb) ; 54(72): 10044-10047, 2018 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-30039815

RESUMO

Here we report the application of readily prepared and available coumarin dyes for photoredox catalysis, which are able to mimic powerful reductant [Ir(iii)] complexes. Coumarin derivatives 9 and 10 were employed as photoreductants in pinacol coupling and in other reactions, in the presence of Et3N as a sacrificial reducing agent. As the electronic, photophysical, and steric properties of coumarins could be varied, a wide applicability to several classes of photoredox reactions is predicted.

3.
Angew Chem Int Ed Engl ; 57(19): 5454-5458, 2018 05 04.
Artigo em Inglês | MEDLINE | ID: mdl-29543370

RESUMO

The herein reported visible-light-activated catalytic asymmetric [3+2] photocycloadditions between cyclopropanes and alkenes or alkynes provide access to chiral cyclopentanes and cyclopentenes, respectively, in 63-99 % yields and with excellent enantioselectivities of up to >99 % ee. The reactions are catalyzed by a single bis-cyclometalated chiral-at-metal rhodium complex (2-8 mol %) which after coordination to the cyclopropane generates the visible-light-absorbing complex, lowers the reduction potential of the cyclopropane, and provides the asymmetric induction and overall stereocontrol. Enabled by a mild single-electron-transfer reduction of directly photoexcited catalyst/substrate complexes, the presented transformations expand the scope of catalytic asymmetric photocycloadditions to simple mono-acceptor-substituted cyclopropanes affording previously inaccessible chiral cyclopentane and cyclopentene derivatives.

4.
Phys Chem Chem Phys ; 20(12): 8071-8076, 2018 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-29516066

RESUMO

A mechanism based on the sequential absorption of two photons by the components of a redox couple has been recently proposed for catalysis of the energetically demanding reduction of aryl halides. Here, we analyze the suggested photochemical mechanism of this reaction, which employs perylenediimide (PDI) as a photocatalyst, on the basis of spectroscopic, electrochemical and electron paramagnetic resonance data. Our results indicate that the photoexcited PDI radical anion (*PDI˙-) cannot play the role of a photosensitizer in the aforementioned process. Instead, the reduction of 4'-bromoacetophenone likely involves *PDI˙- decomposition products. The extremely short lifetime of the photoexcited transient species, as *PDI˙-, is a major general limitation for photocatalytic schemes based on sequential two-photon excitation. In order to better understand the potential of such schemes, we discuss them in the context of the Z-scheme in natural photosynthesis.

5.
Angew Chem Int Ed Engl ; 56(42): 12820-12821, 2017 10 09.
Artigo em Inglês | MEDLINE | ID: mdl-28857385

RESUMO

The photocatalytic mechanism reported in a recent Communication to produce the radical anion of pyrenes postulates a highly endergonic electron transfer process. An analysis of the thermodynamics is reported together with the proposal of an alternative thermodynamically feasible mechanism.

6.
Chemistry ; 23(26): 6380-6390, 2017 May 05.
Artigo em Inglês | MEDLINE | ID: mdl-28263437

RESUMO

A shape-persistent molecule, featuring four bipyridinium units, has been synthesized that upon reduction undergoes intermolecular pimerization because of the rigid architecture of the molecule. The pimerization process has been investigated by a variety of techniques, such as absorption measurements, EPR spectroscopy, as well as gamma and pulse radiolysis, and compared with the behavior of a model compound. Computational studies have also been performed to support the experimental data. The most interesting feature of the tetramer is that pimerization occurs only above a threshold concentration of monoreduced species, on the contrary to the model compound. Furthermore, there is an increase of the apparent pimerization constant by increasing the concentration of reduced bipyridinium units. These results have been interpreted by the fact that pimerization is favored in the tetrahedrally shaped molecule because of a cooperative mechanism. Each multiply reduced molecule can indeed undergo multiple intermolecular interactions that enhance the stabilization of the system, also leading to hierarchical supramolecular growth. The resulting supramolecular system formed by such intermolecular pimerization should exhibit a diamond-like structure, as suggested by a simplified modeling approach. The intermolecular nature of the pimerization process occurring in the tetramer has been demonstrated by measuring the corresponding bimolecular rate constant by pulsed radiolysis experiments.

7.
Chemistry ; 21(50): 18052-6, 2015 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-26509744

RESUMO

Photocatalysis enables the cascade reactions of indoles and CBr4 in MeOH through a C(sp(2) )H functionalization/methanolysis sequence. The title reaction provides an efficient access to indole 2- and 3-carboxylates in a single operation (no preinstallation of protecting as well as directing groups was required) with good yields under mild reaction conditions.

8.
Chemistry ; 20(23): 7054-60, 2014 Jun 02.
Artigo em Inglês | MEDLINE | ID: mdl-24825676

RESUMO

The tetrahedral, shape-persistent molecule 1(4+), containing four pyridylpyridinium units connected through a central carbon atom, exhibits unexpected photophysical properties including a substantially redshifted absorption (2350 cm(-1)) and a very strong fluorescence (Φem = 40 %), compared with the monomer 2(+) (Φem = 0.4 %). Density functional theory calculations on the structure and spectroscopic properties of 1(4+) and 2(+) show that exciton interactions, homoconjugation, and orbital nature account for the observed differences in their photophysical properties. The protonated tetramer binds four cucurbit[7]uril molecules and the host/guest interactions can be controlled by chemical (acid/base) as well as redox stimuli.


Assuntos
Hidrocarbonetos Aromáticos com Pontes/química , Imidazóis/química , Compostos de Piridínio/química , Técnicas Eletroquímicas , Espectroscopia de Ressonância Magnética , Oxirredução , Compostos de Piridínio/síntese química , Teoria Quântica , Água/química
9.
Chemphyschem ; 13(1): 221-5, 2012 Jan 16.
Artigo em Inglês | MEDLINE | ID: mdl-22109993

RESUMO

The rotational spectrum of the tetrahydrofuran-krypton van der Waals complex has been investigated by pulsed-jet Fourier transform microwave spectroscopy. The spectra of the (84)Kr and (86)Kr isotopologues have been assigned and the krypton atom is located nearly over the oxygen atom, almost perpendicular to the COC plane. Each rotational transition is split into two component lines due to, according to the observed Coriolis coupling term between the tunneling states, the residual pseudorotational effects of the ring in the complex. The splitting between the two vibrational sublevels is 87.462(2) and 87.062(2) MHz for the (84)Kr and (86)Kr isotopologues, respectively. These splittings have been used to determine the barrier to inversion, B(2) = 67 cm(-1). The dissociation energy has been estimated to be 3.7 kJ mol(-1) from centrifugal distortion effects.

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