Catalytic Dienylation: An Emergent Strategy for the Stereoselective Construction of Conjugated Dienes and Polyenes.

Stereoselective construction of conjugated dienes and polyenes has remained an enduring synthetic problem, due to the central roles they play in natural product synthesis, methodology, and medicine. This review focuses on the recent developments in dienylation as an emerging strategy for the direct installation of unsaturated four carbon atom units of conjugated π-systems, outlining the regio- and stereoselectivity, as well as the synthetic scope of reactions with various dienylating reagents and the mechanistic implications of the catalytic cross-coupling processes that are used to enable dienylation.

Comparison of three spot urine formulae and their validation using 24-hour urine sodium for estimation of daily salt intake: a cross-sectional study among Bangladeshi adults.

OBJECTIVE: This study aimed to assess the validity of three commonly used (Tanaka, Kawasaki, INTERSALT) methods based on spot urinary sodium excretion against the 24-hour urinary sodium excretion to estimate the dietary salt intake in Bangladesh. DESIGN: A population-based cross-sectional survey. SETTING: A cross-sectional survey was done in an urban and a rural area of Bangladesh in 2012-2013. PARTICIPANTS: 418 community living residents aged 40-59 years participated in the survey and data of 227 subjects who had complete information were analysed for this validation study. OUTCOME MEASURES: The Bland-Altman method was used to evaluate the agreement between the estimated and measured 24-hour urinary sodium. The estimated average salt intake from Tanaka, Kawasaki and INTERSALT methods were plotted against 24-hour urinary sodium excretion. RESULTS: The mean 24-hour estimated salt intake was 10.0 g/day (95% CI 9.3 to 10.6). The mean estimated urinary salt by Tanaka, Kawasaki and INTERSALT methods were 8.5 g/day (95% CI 8.2 to 8.8), 11.4 g/day (95% CI 10.8 to 12.0) and 8.8 g/day (95% CI 8.6 to 9.0), respectively. Compared with the estimated mean salt intake from 24-hour urine collection, the Bland-Altman plot indicated the mean salt intake was overestimated by the Kawasaki method and underestimated by Tanaka and INTERSALT methods. The linear regression line showed the Kawasaki method was the least biased and had the highest intraclass correlation coefficient (0.57, 95% CI 0.45 to 0.67). CONCLUSION: Tanaka, Kawasaki and INTERSALT methods were not appropriate for the estimation of 24-hour urinary sodium excretion from spot urine samples to assess dietary salt intake in Bangladesh. Among the three methods, the Kawasaki method has the highest agreement with the 24-hour urinary sodium excretion concentration in this population.

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet.

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

Mimicking transition metals in borrowing hydrogen from alcohols.

Borrowing hydrogen from alcohols, storing it on a catalyst and subsequent transfer of the hydrogen from the catalyst to an in situ generated imine is the hallmark of a transition metal mediated catalytic N-alkylation of amines. However, such a borrowing hydrogen mechanism with a transition metal free catalytic system which stores hydrogen molecules in the catalyst backbone is yet to be established. Herein, we demonstrate that a phenalenyl ligand can imitate the role of transition metals in storing and transferring hydrogen molecules leading to borrowing hydrogen mediated alkylation of anilines by alcohols including a wide range of substrate scope. A close inspection of the mechanistic pathway by characterizing several intermediates through various spectroscopic techniques, deuterium labelling experiments, and DFT study concluded that the phenalenyl radical based backbone sequentially adds H+, H˙ and an electron through a dearomatization process which are subsequently used as reducing equivalents to the C-N double bond in a catalytic fashion.

Reduced Phenalenyl in Catalytic Dehalogenative Deuteration and Hydrodehalogenation of Aryl Halides.

Dehalogenative deuteration reactions are generally performed through metal-mediated processes. This report demonstrates a mild protocol for hydrodehalogenation and dehalogenative deuteration of aryl/heteroaryl halides (39 examples) using a reduced odd alternant hydrocarbon phenalenyl under transition metal-free conditions and has been employed successfully for the incorporation of deuterium in various biologically active compounds. The combined approach of experimental and theoretical studies revealed a single electron transfer-based mechanism.

Bicyclic (alkyl)(amino)carbene stabilized zinc(0) complex with singlet biradicaloid ground state.

A storable bicyclic (alkyl)(amino)carbene (BICAAC) stabilized two coordinate zinc(0) complex [(BICAAC)2Zn] (2) was synthesized. DFT calculations reveal that BICAAC plays a decisive role in imparting the stability to 2. This complex activates the C(sp3)-Cl bond of trityl chloride generating the Gomberg's free radical with greater efficiency than metallic Zn powder.

A Bottleable Imidazole-Based Radical as a Single Electron Transfer Reagent.

Reduction of 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1H-imidazol-3-ium chloride (1) resulted in the formation of the first structurally characterized imidazole-based radical 2. 2 was established as a single electron transfer reagent by treating it with an acceptor molecule tetracyanoethylene. Moreover, radical 2 was utilized as an organic electron donor in a number of organic transformations such as in activation of an aryl-halide bond, alkene hydrosilylation, and in catalytic reduction of CO2 to methoxyborane, all under ambient temperature and pressure.

Designing a Cr-catalyst bearing redox non-innocent phenalenyl-based ligand towards hydrosilylative CO2 functionalization.

Herein, we report the synthesis of a Cr(iii)-complex bearing a redox non-innocent phenalenyl-based ligand and its use as a catalyst for SET mediated hydrosilylative reduction of carbon dioxide towards formylation of primary amides under mild conditions. A preliminary mechanistic picture for this transformation has been proposed by isolation and characterization of several reactive intermediates.

Switching between mono and doubly reduced odd alternant hydrocarbon: designing a redox catalyst.

Since the early Hückel molecular orbital (HMO) calculations in 1950, it has been well known that the odd alternant hydrocarbon (OAH), the phenalenyl (PLY) system, can exist in three redox states: closed shell cation (12π e-), mono-reduced open shell neutral radical (13π e-) and doubly reduced closed shell anion (14π e-). Switching from one redox state of PLY to another leads to a slight structural change owing to its low energy of disproportionation making the electron addition or removal process facile. To date, mono-reduced PLY based radicals have been extensively studied. However, the reactivity and application of doubly reduced PLY species have not been explored so far. In this work, we report the synthesis of the doubly reduced PLY species (14π e-) and its application towards the development of redox catalysis via switching with the mono-reduced form (13π e-) for aryl halide activation and functionalization under transition metal free conditions without any external stimuli such as heat, light or cathodic current supply.

NHC-catalyzed silylative dehydration of primary amides to nitriles at room temperature.

Herein we report an abnormal N-heterocyclic carbene catalyzed dehydration of primary amides in the presence of a silane. This process bypasses the energy demanding 1,2-siloxane elimination step usually required for metal/silane catalyzed reactions. A detailed mechanistic cycle of this process has been proposed based on experimental evidence along with computational study.

A K-arylacetylide complex for catalytic terminal alkyne functionalization using KOtBu as a precatalyst.

Herein we report a transition metal free catalytic terminal alkyne functionalization across the C-X triple bond (X = CH and N) with E-selective homo (alkyne-alkyne) and head-to-tail selective hetero (alkyne-nitrile) dimerization. A series of stoichiometric reactions enabled us to crystallize a reactive organometallic intermediate K-arylacetylide complex which was characterized by X-ray crystallography, indicating that an ionic mechanism is operative.

Primary amides to amines or nitriles: a dual role by a single catalyst.

We report a manganese-catalyzed hydrosilylative reduction of various primary amides to amines (25 examples). On simple modification of the reaction conditions such as in the presence of a catalytic amount of secondary amide, the same catalyst can transform the primary amides into intermediate nitrile compounds (16 examples) in excellent yields. This is the first example where such a controlled catalytic transformation of primary amides to amines or nitriles with a single catalyst has been demonstrated.

Transition-Metal-Free Catalytic Carboalkoxylation of Styrenes at Room Temperature.

Herein, we describe the first transition-metal-free catalytic carboalkoxylation of styrenes with aryl diazonium salts by Meerwein addition in the presence of a phenalenyl ligand at room temperature without requiring any light stimulation. This three-component reaction allows facile difunctionalization of styrene derivatives with various alcohols (such as 1, 2, and 3°) as the source of alkoxy group during this transformation. The key intermediates and the transition states involved in this reaction path were unraveled by a series of control experiments coupled with density functional theory calculations. The full mechanistic investigation provides an understanding of the selectivity toward carboalkoxylation (Meerwein arylation addition elimination) in the presence of various alcohols over the simple arylation to multiple bond (Meerwein arylation-elimination) reaction.

Transition metal-free catalytic reduction of primary amides using an abnormal NHC based potassium complex: integrating nucleophilicity with Lewis acidic activation.

An abnormal N-heterocyclic carbene (aNHC) based potassium complex was used as a transition metal-free catalyst for reduction of primary amides to corresponding primary amines under ambient conditions. Only 2 mol% loading of the catalyst exhibits a broad substrate scope including aromatic, aliphatic and heterocyclic primary amides with excellent functional group tolerance. This method was applicable for reduction of chiral amides and utilized for the synthesis of pharmaceutically valuable precursors on a gram scale. During mechanistic investigation, several intermediates were isolated and characterized through spectroscopic techniques and one of the catalytic intermediates was characterized through single-crystal XRD. A well-defined catalyst and isolable intermediate along with several stoichiometric experiments, in situ NMR experiments and the DFT study helped us to sketch the mechanistic pathway for this reduction process unravelling the dual role of the catalyst involving nucleophilic activation by aNHC along with Lewis acidic activation by K ions.

Phenalenyl Based Aluminum Compound for Catalytic C-H Arylation of Arene and Heteroarenes at Room Temperature.

Main group metal based catalysis has been considered to be a cost-effective alternative way to the transition metal based catalysis, due to the high abundance of main group metals in the Earth's crust. Among the main group metals, aluminum is the most abundant (7-8%) in the Earth's crust, making the development of aluminum based catalysts very attractive. So far, aluminum based compounds have been popularly used as Lewis acids in a variety of organic reactions, but chemical transformation demanding a redox based process has never utilized an Al(III) complex as a catalyst. Herein, we tuned the redox noninnocence behavior of a phenalenyl ligand by coupling with Al(III) ion, which subsequently can store the electron upon reduction with K to carry out direct C-H arylation of heteroarenes/mesitylene at ambient temperature. A mechanistic investigation revealed that a three-electron reduced phenalenyl based triradical aluminum(III) complex plays the key role in such catalysis. The electronic structure of the catalytically active triradical species has been probed using EPR spectroscopy, magnetic susceptibility measurements, and electronic structure calculations using a DFT method.

An Iron-Based Long-Lived Catalyst for Direct C-H Arylation of Arenes and Heteroarenes.

Direct C-H arylation of arenes and heteroarenes to biaryls at ambient temperature has been accomplished using a phenalenyl-supported iron(III) catalyst. The present catalyst requires a chemical reductant such as potassium and functions without any light stimulation. C-H arylation of various heteroarenes including pyridine as well as unactivated arene such as benzene delivered good to excellent yield (28 examples, up to 92 %) at room temperature. A combined effort based on experiments and theoretical calculations established that a phenalenyl-based radical species (generated by chemical reduction of the iron(III) coordinated phenalenyl complex) plays key role during the catalysis. Furthermore, this catalyst displayed remarkable stability during the catalysis, as evident from the fact that it was still usable over ten consecutive catalytic runs without losing its catalytic efficiency.

Integrating Organic Lewis Acid and Redox Catalysis: The Phenalenyl Cation in Dual Role.

In recent years, merging different types of catalysis in a single pot has drawn considerable attention and these catalytic processes have mainly relied upon metals. However, development of a completely metal free approach integrating organic redox and organic Lewis acidic property into a single system has been missing in the current literature. This study establishes that a redox active phenalenyl cation can activate one of the substrates by single electron transfer process while the same can activate the other substrate by a donor-acceptor type interaction using its Lewis acidity. This approach has successfully achieved light and metal-free catalytic C-H functionalization of unactivated arenes at ambient temperature (39 entries, including core moiety of a top-selling molecule boscalid), an economically attractive alternative to the rare metal-based multicatalysts process. A tandem approach involving trapping of reaction intermediates, spectroscopy along with density functional theory calculations unravels the dual role of phenalenyl cation.

Tuning the redox non-innocence of a phenalenyl ligand toward efficient nickel-assisted catalytic hydrosilylation.

In this report, a ligand-redox assisted catalytic hydrosilylation has been investigated. A phenalenyl ligand coordinated nickel complex has been utilized as an electron reservoir to develop a base metal-assisted catalyst, which very efficiently hydrosilylates a wide variety of olefin substrates under ambient conditions. A mechanistic investigation revealed that a two-electron reduced phenalenyl based biradical nickel complex plays the key role in such catalysis. The electronic structure of the catalytically active biradical species has been interrogated using EPR spectroscopy, magnetic susceptibility measurements, and electronic structure calculations using a DFT method. Inhibition of the reaction by a radical quencher, as well as the mass spectrometric detection of two intermediates along the catalytic loop, suggest that a single electron transfer from the ligand backbone initiates the catalysis. The strategy of utilising the redox reservoir property of the ligand ensures that the nickel is not promoted to an unfavorable oxidation state, and the fine tuning between the ligand and metal redox orbitals elicits smooth catalysis.

A new face of phenalenyl-based radicals in the transition metal-free C-H arylation of heteroarenes at room temperature: trapping the radical initiator via C-C σ-bond formation.

The radical-mediated transition metal-free approach for the direct C-H bond functionalization of arenes is considered as a cost effective alternative to transition metal-based catalysis. An organic ligand-based radical plays a key role by generating an aryl radical which undergoes a subsequent functionalization process. The design principle of the present study takes advantage of a relatively stable odd alternant hydrocarbon-based phenalenyl (PLY) radical. In this study, the first transition metal-free catalyzed direct C-H arylation of a variety of heteroarenes such as azoles, furan, thiophene and pyridine at room temperature has been reported using a phenalenyl-based radical without employing any photoactivation step. This protocol has been successfully applied to the gram scale synthesis of core moieties of bioactive molecules. The phenalenyl-based radical initiator has been characterized crystallographically by trapping it via the formation of a C-C σ-bond between the phenalenyl radical and solvent-based radical species.