Walter D. Guerra

Bio: Walter D. Guerra is an academic researcher from National University of Cordoba. The author has contributed to research in topics: Intramolecular force & Electron transfer. The author has an hindex of 4, co-authored 8 publications receiving 76 citations. Previous affiliations of Walter D. Guerra include National University of Distance Education.

“Transition-metal-free” synthesis of carbazoles by photostimulated reactions of 2′-halo[1,1′-biphenyl]-2-amines.

Abstract: An efficient and simple protocol for the preparation of a series of 9H-carbazoles by photostimulated SRN1 substitution reactions is presented. Substituted 9H-carbazoles were synthesized in low to excellent yields (up to 96%) through an intramolecular C–N bond formation of 2′-halo[1,1′-biphenyl]-2-amines by the photoinitiated SRN1 mechanism under mild and “transition-metal-free” conditions. The biphenylamines used as substrates were obtained with isolated yields ranging from 21% to 84% by two approaches: (A) the cross-coupling Suzuki–Miyaura reaction and (B) the radical arylation of anilines. Some key aspects of the proposed mechanism were evaluated at the B3LYP/6-311+G* level.

Synthesis of Dibenzosultams by "Transition-Metal-Free" Photoinduced Intramolecular Arylation of N-Aryl-2-halobenzenesulfonamides.

Abstract: A new and general synthetic route to prepare dibenzosultams is here reported. This approach involves the synthesis of N-aryl-2-halobenzenesulfonamides (3), followed by intramolecular C–C photoinduced arylation under soft conditions without the use of “Transition Metal”. The photostimulated reactions exhibit very good tolerance to different substituent groups with good to excellent isolated yields (42–98%) of products. Moreover, it is shown that LED (λ = 395 nm) is an efficient light energy source to initiate efficiently the reactions. Theoretical inspection of the mechanism was made to probe the involvement of the radical-anion SRN1 process.

Transition-Metal-Free and Visible-Light-Mediated Desulfonylation and Dehalogenation Reactions: Hantzsch Ester Anion as Electron and Hydrogen Atom Donor

Abstract: Novel approaches for N- and O-desulfonylation under room temperature (rt) and transition-metal-free conditions have been developed. The first methodology involves the transformation of a variety of N-sulfonyl heterocycles and phenyl benzenesulfonates to the corresponding desulfonylated products in good to excellent yields using only KOtBu in dimethyl sulfoxide (DMSO) at rt. Alternately, a visible light method has been used for deprotection of N-methyl-N-arylsulfonamides with Hantzsch ester (HE) anion serving as the visible-light-absorbing reagent and electron and hydrogen atom donor to promote the desulfonylation reaction. The HE anion can be easily prepared in situ by reaction of the corresponding HE with KOtBu in DMSO at rt. Both protocols were further explored in terms of synthetic scope as well as mechanistic aspects to rationalize key features of desulfonylation processes. Furthermore, the HE anion induces reductive dehalogenation reaction of aryl halides under visible light irradiation.

Managing the Redox Potential of PCET in Grotthuss-Type Proton Wires.

Abstract: Expanding proton-coupled electron transfer to multiproton translocations (MPCET) provides a bioinspired mechanism to transport protons away from the redox site. This expansion has been accomplished by separating the initial phenolic proton donor from the pyridine-based terminal proton acceptor by a Grotthuss-type proton wire made up of concatenated benzimidazoles that form a hydrogen-bonded network. However, it was found that the midpoint potential of the phenol oxidation that launched the Grotthuss-type proton translocations is a function of the number of benzimidazoles in the hydrogen-bonded network; it becomes less positive (i.e., a weaker oxidant) as the number of bridging benzimidazoles increases. Herein, we report a strategy to maintain the high redox potential necessary for oxidative processes relevant to artificial photosynthesis, e.g., water oxidation and long-range MPCET processes for managing protons. The integrated structural and functional roles of the benzimidazole-based bridge provide sites for substitution of the benzimidazoles with electron-withdrawing groups (e.g., trifluoromethyl groups). Such substitution increases the midpoint potential of the phenoxyl radical/phenol couple so that proton translocations over ∼11 Å become thermodynamically comparable to that of an unsubstituted system where one proton is transferred over ∼2.5 Å. The extended, substituted system maintains the hydrogen-bonded network; infrared spectroelectrochemistry confirms reversible proton translocations from the phenol to the pyridyl terminal proton acceptor upon oxidation and reduction. Theory supports the change in driving force with added electron-withdrawing groups and provides insight into the role of electron density and electrostatic potential in MPCET processes associated with these Grotthuss-type proton translocations.

Dialkylbiaryl phosphines in Pd-catalyzed amination: a user's guide

Abstract: Dialkylbiaryl phosphines are a valuable class of ligand for Pd-catalyzed amination reactions and have been applied in a range of contexts. This perspective attempts to aid the reader in the selection of the best choice of reaction conditions and ligand of this class for the most commonly encountered and practically important substrate combinations.

Concerted Nucleophilic Aromatic Substitution Reactions.

Abstract: Recent developments in experimental and computational chemistry have identified a rapidly growing class of nucleophilic aromatic substitutions that proceed by concerted (cSN Ar) rather than classical, two-step, SN Ar mechanisms. Whereas traditional SN Ar reactions require substantial activation of the aromatic ring by electron-withdrawing substituents, such activating groups are not mandatory in the concerted pathways.

Synthesis of Carbazoles by a Merged Visible Light Photoredox and Palladium-Catalyzed Process

Abstract: Carbazoles have attracted great interest in recent years for a variety of applications in organic and medicinal chemistry as well as in materials science. In this work, an efficient method for the synthesis of carbazoles through the intramolecular C–H bond amination of N-substituted 2-amidobiaryls has been developed. Under visible light and an aerobic atmosphere, the transformation requires only catalytic amounts of Pd(OAc)2 and [Ir(dFppy)2phen]PF6 (dFppy = 2-(2,4-difluorophenyl)pyridine; phen = 1,10-phenanthroline), the latter of which is utilized in synthetic chemistry for the first time. Spectroscopic and electrochemical studies revealed that the reaction is initiated by photoinduced electron transfer from a palladacyclic intermediate, formed from the 2-amidobiaryl and PdII species, to the photoexcited Ir catalyst. This step triggers reductive elimination in a PdIII-containing palladacycle to produce the carbazole and a PdI species. The one-electron-reduced photocatalyst is reoxidized by O2 to generate...

Construction of Benzo[c]carbazoles and Their Antitumor Derivatives through the Diels-Alder Reaction of 2-Alkenylindoles and Arynes.

Abstract: The direct assembly of benzo[c]carbazole derivatives via the Diels–Alder reaction of arynes and easily accessible 2-alkenylidoles was reported. By employing different aryne precursor loads, 6,7-dihydrobenzo[c]carbazoles or aryl-substituted 7,11b-dihydrobenzo[c]carbazoles could be controllably generated in good to excellent yields under a nitrogen atmosphere. On the other hand, when the reaction was conducted under oxygen, oxidated/aromatized product benzo[c]carbazoles could be generated directly with high selectivity and efficiency in a one-step manner. Interestingly, the benzo[c]carbazole-5-carboxamide amidation derivatives of the above products showed good antitumor activities. The inhibitory effect of these molecules against cancer cells was also described.