Crossed Intermolecular [2+2] Cycloaddition of Styrenes by Visible Light Photocatalysis

Visible-light-promoted syntheses of β-keto sulfones from alkynes and sulfonylhydrazides

Functionalized β-keto sulfones were prepared under the synergistic interactions of visible light irradiation, Ru(bpy)₃Cl₂, oxygen, KI, and NaOAc basic additive.

Characterizing Chain Processes in Visible Light Photoredox Catalysis

The recognition that Ru(bpy)32+ andsimilar visible light absorbing transition metal complexes can be photocatalysts for a variety of synthetically useful organic reactions has resulted in a recent resurgence of interest in photoredox catalysis. However, many of the critical mechanistic aspects of this class of reactions remain poorly understood. In particular, the degree to which visible light photoredox reactions involve radical chain processes has been a point of some disagreement that has not been subjected to systematic analysis. We have now performed quantum yield measurements to demonstrate that threerepresentative, mechanistically distinct photoredox processes involve product-forming chain reactions. Moreover, we show that the combination of quantum yield and luminescence quenching experiments provides a rapid method to estimate the length of these chains. Together, these measurements constitute a robust, operationally facile strategy for characterizing chain processes in a wide range of visible light photoredox reactions.

Tailoring flavins for visible light photocatalysis: organocatalytic [2+2] cycloadditions mediated by a flavin derivative and visible light

A new application of flavin derivatives in visible light photocatalysis was found. 1-Butyl-7,8-dimethoxy-3-methylalloxazine, when irradiated by visible light, was shown to allow an efficient cyclobutane ring formation via an intramolecular [2+2] cycloaddition of both styrene dienes, considered as electron-rich substrates, and electron-poor bis(arylenones), presumably proceeding via an energy transfer mechanism.

Syntheses of Sceptrins and Nakamuric Acid and Insights into the Biosyntheses of Pyrrole-Imidazole Dimers

Sceptrins and nakamuric acid are structurally unique antibiotics isolated from marine sponges. Recent studies suggest that the biosynthesis of these dimeric pyrrole-imidazole alkaloids involves a single-electron transfer (SET)-promoted [2+2] cycloaddition to form their cyclobutane core skeletons. We describe herein the biomimetic syntheses of racemic sceptrin and nakamuric acid. We also report the asymmetric syntheses of sceptrin, bromosceptrin, and dibromosceptrin in their natural enantiomeric form. We further provide mechanistic insights into the pathway selectivity of the SET-promoted [2+2] and [4+2] cycloadditions that lead to the divergent formation of the sceptrin and ageliferin core skeletons. Both the [2+2] and [4+2] cycloadditions are stepwise reactions, with the [2+2] pathway kinetically and thermodynamically favored over the [4+2] pathway. For the [2+2] cycloaddition, the dimerization of pyrrole-imidazole monomers is rate-limiting, whereas for the [4+2] cycloaddition, the cyclization is the slowest step.

Cobalt-Catalyzed [2π + 2π] Cycloadditions of Alkenes: Scope, Mechanism, and Elucidation of Electronic Structure of Catalytic Intermediates

Aryl-substituted bis(imino)pyridine cobalt dinitrogen compounds, (^RPDI)CoN₂, are effective precatalysts for the intramolecular [2π + 2π] cycloaddition of α,ω-dienes to yield the corresponding bicyclo[3.2.0]heptane derivatives. The reactions proceed under mild thermal conditions with unactivated alkenes, tolerating both amine and ether functional groups. The overall second order rate law for the reaction, first order with respect to both the cobalt precatalyst and the substrate, in combination with electron paramagnetic resonance (EPR) spectroscopic studies established the catalyst resting state as dependent on the identity of the precatalyst and diene substrate. Planar S =¹/₂ κ³-bis(imino)pyridine cobalt alkene and tetrahedral κ²-bis(imino)pyridine cobalt diene complexes were observed by EPR spectroscopy and in the latter case structurally characterized. The hemilabile chelate facilitates conversion of a principally ligand-based singly occupied molecular orbital (SOMO) in the cobalt dinitrogen and alkene compounds to a metal-based SOMO in the diene intermediates, promoting C–C bond-forming oxidative cyclization. Structure–activity relationships on bis(imino)pyridine substitution were also established with 2,4,6-tricyclopentyl-substituted aryl groups, resulting in optimized catalytic [2π + 2π] cycloaddition. The cyclopentyl groups provide a sufficiently open metal coordination sphere that encourages substrate coordination while remaining large enough to promote a challenging, turnover-limiting C(sp³)–C(sp³) reductive elimination.

An approach for the synthesis of nakamuric acid

The biosynthesis of dimeric pyrrole-imidazole alkaloids is likely mediated by enzyme-catalyzed reversible single-electron transfer (SET) cycloaddition. We now show that Ir(ppy)₃ can promote SET-mediated formal [2+2] and [4+2] cycloaddition reactions of pyrrole-imidazole alkaloids-related substrates under photolytic conditions. This biomimetic approach is useful for the construction of the core skeleton of nakamuric acid and sceptrin.

Syntheses of Cyclic Guanidine-Containing Natural Products

Naturally occurring guanidine derivatives frequently display medicinally useful properties. Among them, the higher order pyrrole-imidazole alkaloids, the dragmacidins, the crambescidins/batzelladines, and the saxitoxins/tetradotoxins have stimulated the development of many new synthetic methods over the past decades. We provide here an overview of the syntheses of these cyclic guanidine-containing natural products.

A bio-inspired total synthesis of tetrahydrofuran lignans

Lignan natural products comprise a broad spectrum of biologically active secondary metabolites. Their structural diversity belies a common biosynthesis, which involves regio- and chemoselective oxidative coupling of propenyl phenols. Attempts to replicate this oxidative coupling have revealed significant challenges for controlling selectivity, and these challenges have thus far prevented the development of a unified biomimetic route to compounds of the lignan family. A practical solution is presented that hinges on oxidative ring opening of a diarylcyclobutane to intercept a putative biosynthetic intermediate. The effectiveness of this approach is demonstrated by the first total synthesis of tanegool in 4 steps starting from ferulic acid, as well as a concise synthesis of the prototypical furanolignan pinoresinol.

Cross dehydrogenative coupling (CDC) of aldehydes with N-hydroxyimides by visible light photoredox catalysis

A visible light mediated cross-dehydrogenative-coupling (CDC) reaction has been developed for the synthesis of N-hydroxyester derivatives from aldehydes and N-hydroxyimides.

[2+2] cycloaddition of 1,3-dienes by visible light photocatalysis

[2+2] photocycloadditions of 1,3-dienes represent a powerful yet synthetically underutilized class of reactions. We report that visible light absorbing transition metal complexes enable the [2+2] cycloaddition of a diverse range of 1,3-dienes. The ability to use long-wavelength visible light is attractive because these reaction conditions tolerate the presence of sensitive functional groups that might be readily decomposed by the high-energy UVC radiation required for direct photoexcitation of 1,3-dienes. The resulting vinylcyclobutane products are poised for a variety of further diversification reactions, and this method is consequently expected to be powerfully enabling in the synthesis of complex organic targets.

[3+2] Photooxygenation of aryl cylopropanes via visible light photocatalysis

We report that Ru(bpz)₃²⁺ is an excellent sensitizer for the photooxygenation of aryl cyclopropanes upon irradiation with visible light. The effectiveness of this photocatalyst enables the synthesis of a range of five-membered endoperoxides in excellent yield with quite low (0.5 mol%) catalyst loadings even when standard household light sources are utilized.

Solar synthesis: prospects in visible light photocatalysis

Chemists have long aspired to synthesize molecules the way that plants do-using sunlight to facilitate the construction of complex molecular architectures. Nevertheless, the use of visible light in photochemical synthesis is fundamentally challenging because organic molecules tend not to interact with the wavelengths of visible light that are most strongly emitted in the solar spectrum. Recent research has begun to leverage the ability of visible light-absorbing transition metal complexes to catalyze a broad range of synthetically valuable reactions. In this review, we highlight how an understanding of the mechanisms of photocatalytic activation available to these transition metal complexes, and of the general reactivity patterns of the intermediates accessible via visible light photocatalysis, has accelerated the development of this diverse suite of reactions.

Applications of C-H functionalization logic to cyclobutane synthesis

The application of C-H functionalization logic to target-oriented synthesis provides an exciting new venue for the development of new and useful strategies in organic chemistry. In this article, C-H functionalization reactions are explored as an alternative approach to access pseudodimeric cyclobutane natural products, such as the dictazole and the piperarborenine families. The use of these strategies in a variety of complex settings highlights the subtle geometric, steric, and electronic effects at play in the auxiliary guided C-H functionalization of cyclobutanes.

Redox mediators in visible light photocatalysis: photocatalytic radical thiol-ene additions

Synthetically useful radical thiol-ene reactions can be initiated by visible light irradiation in the presence of transition metal polypyridyl photocatalysts. The success of this method relies upon the use of p-toluidine as an essential additive. Using these conditions, high-yielding thiol-ene reactions of cysteine-containing biomolecules can be accomplished using biocompatibile wavelengths of visible light, under aqueous conditions, and with the thiol component as the limiting reagent. We present evidence that p-toluidine serves as a redox mediator that is capable of catalyzing the otherwise inefficient photooxidation of thiols to the key thiyl radical intermediate. Thus, we show that co-catalytic oxidants can be important in the design of synthetic reactions involving visible light photoredox catalysis.

Dicyanopyrazine-derived push–pull chromophores for highly efficient photoredox catalysis

Here, we report dicyanopyrazine (DPZ)-derived push–pull chromophores, easily prepared and tunable organic compounds, as new kinds of photoredox catalysts.

Visible-light radical reaction designed by Ru- and Ir-based photoredox catalysis

New and easy protocols for radical reactions have been developed.

Reductive Cyclizations of Nitroarenes to Hydroxamic Acids by Visible Light Photoredox Catalysis

We have developed a photocatalytic reduction of nitroarenes as an efficient, chemoselective route to biologically important N-phenyl hydroxamic acid scaffolds. Optimal conditions call for 2.5 mol% of a ruthenium photocatalyst, visible light irradiation, and a dihydropyridine terminal reductant. Because of the mild nature of the visible light activation, functional groups that might be sensitive to other non-photochemical reduction methods are easily tolerated.

Visible Light Photocatalysis: The Development of Photocatalytic Radical Ion Cycloadditions

Photochemistry has the potential to significantly impact multiple aspects of chemical synthesis, in part because photoinduced reactions can be used to construct molecular architectures that would otherwise be difficult to produce. Nevertheless, organic chemists have been slow to embrace photochemical synthesis because of technical complications associated with the use of ultraviolet light. Our laboratory has been part of an effort to design synthetically useful reactions that utilize visible light. This strategy enables the synthesis of a diverse range of organic structures by generation of a variety of reactive intermediates under exceptionally mild conditions. This Perspective article describes the reasoning that led to the conception of our first experiments in this area, the features of our reaction design that have been most powerful in the discovery of new processes, and a few of the possible future areas in which visible light photocatalysis might have a large impact.

Visible light-induced 3-sulfenylation of N-methylindoles with arylsulfonyl chlorides

The synthesis of 1-methyl-3-(arylthio)-1H-indoles has been achieved by the photoredox reaction of N-methylindoles with readily available arylsulfonyl chlorides in moderate yields.

Synthesis of cyclobutane lignans via an organic single electron oxidant-electron relay system

A direct method to synthesize lignan cyclobutanes and analogs via photoinduced electron transfer is presented. A variety of oxygenated alkenes are employed to furnish terminal or substituted cyclobutane adducts with complete regiocontrol, yielding cycloadducts with trans stereochemistry. Key to minimizing competing cycloreversion is the inclusion of an aromatic electron relay (ER). This method has been adapted to the synthesis of the natural products magnosalin and pellucidin A.