Javier I. Bardagi

Reduction of aryl halides by consecutive visible light-induced electron transfer processes

Doubling up on optically driven catalysis During photosynthesis, plants absorb light from the Sun four consecutive times before they accumulate enough energy to make oxygen from water. In contrast, when chemists harness light energy to promote reactivity, they tend to rely on single discrete absorption events. Ghosh et al. now show that a particular dye molecule can channel the combined energy from two absorbed photons to the reduction and subsequent coupling reactions of aryl halide molecules. The method expands the reach of photocatalysis to a broader range of compounds, such as chlorides, which are too stable to breach with a single photon. Science, this issue p. 725 A dye molecule channels the energy of two visible photons into aryl substitution reactions. Biological photosynthesis uses the energy of several visible light photons for the challenging oxidation of water, whereas chemical photocatalysis typically involves only single-photon excitation. Perylene bisimide is reduced by visible light photoinduced electron transfer (PET) to its stable and colored radical anion. We report here that subsequent excitation of the radical anion accumulates sufficient energy for the reduction of stable aryl chlorides giving aryl radicals, which were trapped by hydrogen atom donors or used in carbon-carbon bond formation. This consecutive PET (conPET) overcomes the current energetic limitation of visible light photoredox catalysis and allows the photocatalytic conversion of less reactive chemical bonds in organic synthesis.

Synthesis of 6-Substituted 2-Pyrrolyl and Indolyl Benzoxazoles by Intramolecular O-Arylation in Photostimulated Reactions

The synthesis of a series of 6-substituted 2-pyrrolyl and 2-indolyl benzoxazoles by photostimulated C-O cyclization of anions from 2-pyrrole carboxamides, 2-indole carboxamides, or 3-indole carboxamides has been found to proceed in good to excellent yields (41-100%) in DMSO and liquid ammonia. The pyrrole and indole carboxamides are obtained in good to very good isolated yields by an amidation reaction of different 2-haloanilines with 2-carboxylic acid of pyrrole and 2- or 3-carboxylic acid of indole. To explain the regiochemical outcome of these reactions (C-O arylation vs C-N or C-C arylation), a theoretical analysis was performed using DFT methods and the B3LYP functional.

Short Access to 6-Substituted Pyrimidine Derivatives by the SRN1 Mechanism. Synthesis of 6-Substituted Uracils through a One-Pot Procedure

The synthesis of 6-substituted 2,4-dimethoxypyrimidines with different nucleophiles was accomplished with good to excellent yields (50-95%) through S(RN)1 reactions, starting from commercially available 6-chloro-2,4-dimethoxypyrimidine (1). Hydrolysis of these derivatives gave access to 6-substituted uracils with good yields and short times by the use of microwave irradiation. The preparation of uracils from 1 without the isolation of 2,4-dimethoxypyrimidine derivatives affords a rapid access to these compounds in good yields and excellent purity by avoiding an unnecessary step of purification.

Advances in the Synthesis of 5- and 6-Substituted Uracil Derivatives

Fil: Bardagi, Javier Ivan. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Cordoba. Instituto de Investigaciones en Fisico-quimica de Cordoba. Universidad Nacional de Cordoba. Facultad de Ciencias Quimicas. Instituto de Investigaciones en Fisico-quimica de Cordoba; Argentina

Initiation in Photoredox C–H Functionalization Reactions. Is Dimsyl Anion a Key Ingredient?

Previous studies have reported the arylation of unactivated arenes with ArX, base (KOtBu or NaOtBu), and an organic additive at high temperatures. Recently, we showed that this reaction proceeds in the absence of additives at rt but employs UV-vis light. However, details of mechanisms that can use a photoinduced base-promoted homolytic aromatic substitution reaction (photo-BHAS) have remained elusive until now. This work examines different mechanistic routes of the essential electron-transfer step (ET) of this reaction in order to identify a possible path for the formation of 1-adamantyl radicals from 1-haloadamantanes (initiation step). On the basis of photochemical and photophysical experiments and computational studies, we propose an unprecedented initiation step that could also be applied to other ET reactions performed in DMSO. For the first time, it is reported that dimsyl anion, formed from a strong base and DMSO (solvent), is responsible for inducing the initiation by a photo-BHAS process on alkyl halides.

Reply to “Photoredox Catalysis: The Need to Elucidate the Photochemical Mechanism”

We have reported in our recent Communication the advantages of combining the best features of visible light absorbing tris(2,2’-bipyridine)ruthenium(II), Ru(bpy)3 2+, and visible light transparent, however, strongly reducing, polycyclic aromatic hydrocarbons via a simple triplet–triplet energy transfer process in synthetic photoredox catalysis. Balzani, Ceroni et al. question in their Correspondence the formation of a pyrene radical anion as proposed reactive intermediate, as the electron transfer from the amine to a pyrene triplet state is endothermic based on the estimated values derived from literature redox potentials.

Anions involved in the initiation of the thermally induced SRN1 reaction for α-arylation of ketones

The SRN1 reaction between acetophenone enolate and PhI is thermally induced and accelerated by microwave irradiation to give the corresponding substitution product 1,2-diphenylethanone in a 50% yield in DMSO at 70 °C. Regarding the mechanism of initiation, in this reaction, acetophenone enolate, tert-butoxyde anion and dimsyl anion (the ionic form of the solvent) could promote the initial electron transfer to start the radical reaction. Comparative studies on the PhI dehalogenation promoted by the different anions were conducted in DMSO under microwave irradiation and by quantum calculations. The dimsyl anion shows a higher iodide generation even at lower concentrations than acetophenone enolate and tBuO−. Likewise, DFT calculation by B3PW91, M062X and PBE0 shows the dymsyl anion to be the best electron donor. While the three anions can initiate the radical reaction, the reactivity order found locates the dimsyl anion in first place, followed by the enolate of acetophenone and then the alkoxide. The results reported herein allow a greater understanding of the initiation process with tert-butoxide solutions in DMSO.