Synthesis and Photophysical Properties of 3-Substituted-1H-Indazoles: A Pd-Catalyzed Double C-N Bond Formation Strategy via 1,6-Conjugate Addition.

A Pd-catalyzed cascade process for the direct synthesis of 3-substituted-1H-indazole employing p-quinone methide (p-QM) and arylhydrazine through Pd-catalyzed double C-N bond formation via 1,6-conjugate addition is reported. This reaction strategy affords efficient and practical access to synthetically important diverse 3-substituted-1H-indazoles in good yields. The photophysical properties of the synthesized 3-substituted-1H-indazoles are investigated, and some of them showed very good fluorescence properties with quantum yields up to 85%. Also, the synthesized 3-substituted-1H-indazole exhibits an acid-sensitive fluorescence turn-off activity.

Pd-Catalyzed Alkyne and Aryne Annulations: Synthesis and Photophysical Properties of π-Extended Coumarins.

A Pd-catalyzed alkyne and aryne annulation strategy via C-H activation has been implemented for the synthesis of π-extended coumarins. This synthetic strategy provides a wide range of π-extended coumarins in moderate to good yields with good functional group compatibility. Photophysical properties of the synthesized π-extended coumarins have been evaluated, and some of them show interesting fluorescent properties. Three of the synthesized coumarins have been unambiguously established by a single-crystal XRD study.

Indane-Fused Spiropentadiene Chromanones: A Pd-Catalyzed Spiroannulation Followed by Cyclization via C-H Activation Strategy.

Pd-catalyzed spiroannulation of 4-bromocoumarin with alkynes has been illustrated. The reaction highlights an interesting process for cascade formation of two five-membered rings through spiroannulation followed by cyclization via C-H activation. This method offers an attractive platform for the synthesis of a broad range of indane-fused spiropentadiene chromanones in good yields.

Correction: tert-Butylhydroperoxide (TBHP) mediated oxidative cross-dehydrogenative coupling of quinoxalin-2(1H)-ones with 4-hydroxycoumarins, 4-hydroxy-6-methyl-2-pyrone and 2-hydroxy-1,4-naphthoquinone under metal-free conditions.

Correction for 'tert-Butylhydroperoxide (TBHP) mediated oxidative cross-dehydrogenative coupling of quinoxalin-2(1H)-ones with 4-hydroxycoumarins, 4-hydroxy-6-methyl-2-pyrone and 2-hydroxy-1,4-naphthoquinone under metal-free conditions' by Suraj Sharma et al., Org. Biomol. Chem., 2020, 18, 6537-6548, DOI: .

tert-Butylhydroperoxide (TBHP) mediated oxidative cross-dehydrogenative coupling of quinoxalin-2(1H)-ones with 4-hydroxycoumarins, 4-hydroxy-6-methyl-2-pyrone and 2-hydroxy-1,4-naphthoquinone under metal-free conditions.

We report an efficient and atom-economical method of C-3 functionalization of quinoxalin-2(1H)-ones with 4-hydroxycoumarins, 4-hydroxy-6-methyl-2-pyrone, and 2-hydroxy-1,4-naphthoquinone via the free radical cross-coupling pathway under metal-free conditions. tert-Butylhydroperoxide (TBHP) smoothly promotes the reaction furnishing the cross-dehydrogenative coupling (CDC) products in very good to excellent yields. The protocol neither uses any toxic reagents nor metal catalysts to carry out the reaction, and all the products have been obtained without column chromatography purification. Different radical trapping experiments with 2,2,6,6-tetramethylpiperidine-1-oxyl, butylated hydroxytoluene, and diphenyl ethylene confirm the involvement of radicals.

Three-Component Coupling Reactions of Aryne, DMSO, and Activated Alkyne: Stereoselective Synthesis of 2-[( o-Methylthio)aryloxy]-Substituted Dialkyl Maleates.

A transition-metal-free coupling reaction of aryne, DMSO, and activated alkyne for the synthesis of 2-[( o-methylthio)aryloxy]-substituted dialkyl maleates is reported. This cascade process is associated with several bond cleavage as well as bond formation reactions in one pot. One of our synthesized maleates has been unambiguously established by single-crystal XRD studies. This methodology allows preparation of trisubstituted vinyl ethers with excellent stereospecificity.

Aryne insertion into the P[double bond, length as m-dash]O bond: one-pot synthesis of quaternary phosphonium triflates.

A novel transition-metal free synthetic strategy has been developed for the direct synthesis of quaternary phosphonium triflates via insertion of aryne into phosphine oxide. This methodology provides good yields of quaternary phosphonium salts and one of the synthesized phosphonium salts has been unambiguously established by single crystal XRD study. Preliminary mechanistic studies suggest that the reaction proceeds via a sequential [2 + 2] cycloaddition followed by the o-arylation and protonation pathway.

Visible light triggered photo-decomposition of vinyl azides to (E)-stilbene derivatives via 1,2-acyl migration.

An efficient photocatalyst-free visible light assisted synthetic route to various (E)-stilbene derivatives was developed. The reaction proceeds through a denitrogenative photo-decomposition of vinyl azides into 2H-azirines followed by neighboring amino group assisted ring opening, 1,2-acyl migration and enolization. The photochemical reaction offers light harvesting without any photocatalyst to access a wide variety of substituted (E)-stilbenes in moderate to high yields.

2,3-Diaroyl benzofurans from arynes: sequential synthesis of 2-aroyl benzofurans followed by benzoylation.

A cascade synthetic strategy for the direct synthesis of 2-aroyl benzofurans from aryne precursors has been developed. This reaction proceeds via C-O and C-C bond cleavage as well as C-O and C-C bond formation in a single reaction vessel. The methodology provides good yields of 2-aroyl benzofurans and tolerates a variety of functional groups. The synthesized 2-aroyl benzofurans were further benzoylated at 3-positions and one of the synthesized 2,3-diaroyl benzofuran structures was confirmed unambiguously by X-ray crystallography.

Proton reduction by a nickel complex with an internal quinoline moiety for proton relay.

The complex Ni(DQPD) (where DQPD = deprotonated N(2),N(6)-di(quinolin-8-yl)pyridine-2,6-dicarboxamide (DQPDH2)) behaves as a visible light driven active catalyst to reduce protons from water when employed with the photosensitizer fluorescein (Fl) and triethylamine (TEA) as the sacrificial electron donor. The photocatalytic system shows very high activity, attaining 2160 turnovers and an initial turnover rate of 0.032 s(-1) with respect to the catalyst. The proposed electrocatalytic mechanism is of the CECE type (C is a chemical step protonation and E is the electrochemical step reduction), where the Ni(DQPD) catalyst undergoes rapid protonation at the non-coordinating nitrogen atom of the quinoline before undergoing reduction. The location of the pendant base is a key factor such that the N-H resulting from the protonation of the non-coordinating nitrogen atom of the quinoline is properly located to interact with the Ni-H hydride leading to heterocoupling between protons and hydrides. Theoretical calculations for the catalytic system were carried out using the density functional level of theory (DFT) and are consistent with a mechanism for catalysis in a polypyridine nickel system. This is the first report of a polypyridine based nickel catalyst where the pendant base is responsible for the internal proton relay towards the metal center through the heterocoupling between protons and hydrides to generate hydrogen.

Stepwise formation of a pentanuclear Ni4Cu heterometallic complex exhibiting a vertex-sharing defective double-cubane core and diphenoxo- and phenoxo/azide bridging groups: a magnetostructural and DFT theoretical study.

Sequential reaction of a N5O3 octadentate tripodal ligand with Ni(2+) and subsequently with Cu(2+) and azide ligand afforded the first example of a heterobridged (phenoxo/µ(1,1)-azido) pentanuclear heterometallic (Ni4Cu) compound, which exhibits a centrosymmetric vertex-sharing defective double-cubane structure. The study of the magnetic properties reveals that the compound shows ferromagnetic interaction interactions, leading to an S = 9/2 spin ground state. Density functional theory calculations on the X-ray structure and model compounds predict ferromagnetic interactions through the magnetic exchange pathways involving each couple of metal ions.

Ambidexterity and Left-Handedness Induced by Geminally Disubstituted γ Amino Acid Residues in Chiral 310 Helices.

Chirality is an omnipresent feature in nature's architecture starting from simple molecules like amino acids to complex higher-order structures viz. proteins, DNA, and RNA. The L configuration of proteinogenic amino acids gives rise to right-handed helices. Ambidexterity is as rare in organisms as in molecules. There are only a few reports of ambidexterity in single-peptide molecules composed of either mixed L and D or achiral residues. Here, we report, for the first time, the ambidextrous and left-handed helical conformations in the chiral nonapeptides P1-P3 (Boc-LUVUγx,xULUV-OMe where U = Aib, x,x = 2,2/3,3/4,4), containing chiral L α amino acid residues, in addition to the usually observed right-handed helical conformation. The centrally located achiral γ residue, capable of adopting both left and right-handed helical conformations, induces its handedness on the neighboring chiral and achiral residues, leading to the observation of both left and right-handed helices in P2 and P3. The presence of a single water molecule proximal to the γ residue induces the reversal of helix handedness by forming distinct and stable water-mediated hydrogen bonds. This gives rise to ambidextrous helices as major conformers in P1 and P2. The absence of the observation of ambidexterity in P3 might be due to the inability of γ4,4 in the recruitment of a water molecule. Experiments (NMR, X-ray, and CD) and density functional theory (DFT) calculations suggest that the position of geminal disubstitution is crucial for determining the population of the amenable helical conformations (ambidextrous, left and right-handed) in these chiral peptides.

{4-[(3-Formyl-4-hy-droxy-phen-yl)diazen-yl]benzoato}triphenyl-tin.

In the title compound, [Sn(C(6)H(5))(3)(C(14)H(9)N(2)O(4))], the Sn atom has a distorted tetra-hedral geometry with one of the carboxyl-ate O atoms and the C atoms from three phenyl groups. The other carboxyl-ate O atom of the benzoate ligand inter-acts weakly with the Sn atom, with an Sn⋯O distance of 2.790 (2) Å, which causes a distortion of the tetra-hedral coordination geometry.

6,6'-Diamino-1,1',3,3'-tetra-methyl-5,5'-(4-chloro-benzyl-idene)bis-[pyrimidine-2,4(1H,3H)-dione].

The title compound, C(19)H(21)ClN(6)O(4), is a 1:2 adduct of p-chloro-benzaldehyde and uracil. It crystallizes with two mol-ecules in the asymmetric unit. The two uracil units in the same mol-ecule are connected by a pair of strong N-H⋯O hydrogen bonds. The packing is stabilized by N-H⋯O, C-H⋯O and C-H⋯N inter-actions.

Coumarin to Isocoumarin: One-Pot Synthesis of 3-Substituted Isocoumarins from 4-Hydroxycoumarins and Benzyne Precursors.

A novel transition-metal-free direct synthesis of 3-substituted isocoumarin from 4-hydroxycoumarin and a benzyne precursor is developed. This synthetic strategy proceeds via C-O and C-C bond cleavage as well as C-O and C-C bond formations in a single reaction vessel by simple treatment with CsF in the absence of catalyst. This methodology affords moderate to good yields of 3-substituted isocoumarins and is tolerant of a variety of functional groups including halide.

µ-Pyridine-bridged copper complex with robust proton-reducing ability.

The binuclear copper complex [Cu(DQPD)]2 (where DQPD = deprotonated N2,N6-di(quinolin-8-yl)pyridine-2,6-dicarboxamide (DQPDH2)) was synthesised and characterised by various spectroscopic as well as electrochemical techniques. The binuclear copper complex was converted into a mononuclear one by the addition of 2 equivalents of pTsOH into [Cu(DQPD)]2. The interconversion between the dimer and monomer complex was studied through UV-Vis spectroscopy and cyclic voltammetry. The mononuclear copper complex showed high catalytic activity towards electrochemical proton reduction using acetic acid as the external proton source in 95 : 5 (v/v) DMF/H2O. It showed an ic/ip (where ic is the catalytic current in the presence of acetic acid and ip is the reduction peak current in absence of acid) value of 24 and a turnover rate (TOF) of 111.70 s-1 at a scan rate of 100 mV s-1 at 25 °C. The [Cu(DQPD)]2 complex evolved hydrogen under the irradiation of visible light in the presence of fluorescein (Fl) as a photosensitizer and triethylamine (TEA) as the sacrificial electron donor with an initial TOF of 0.03 s-1 with respect to the catalyst.