Keto-enol tautomerism of ß-diketo molecules in the presence of graphitic materials through π-π stacking.

The pseudo aromatic structures of the enol forms of ß-diketo molecules are stabilized on the surface of graphitic materials through π-π interaction. This phenomenon has been studied through a relative binding energy calculation using density functional theory. The intermolecular interaction as well as the relative stability of the keto or enol tautomer is also influenced by the functional groups attached to the graphitic materials. The theoretical results are supported by spectroscopic evidence. Our study with three different graphitic materials, with a comparable extent of π-electrons and acid functionalities, reveals that π-π interaction is the main governing factor for the stability of the enol forms. Then comes the role of intermolecular H-bonding between the adsorbate and adsorbent. This can stabilize both the keto and enol tautomers, according to the arrangements of the functional groups and the geometry of the ß-diketo molecules. Acid groups on the adsorbent can enhance enolization through H-bonding, but an excess of functional groups may decrease the possibility of π-π interaction by disrupting the π-clouds of the graphitic surface and pushing the adsorbate and adsorbent away from each other beyond a π-π stacking distance. In that situation, H-bonding becomes crucial for determining the relative stability. Our results indicate that graphitic materials with acid functionalities across their edges, and ample π-cloud, are the most suitable catalysts for enolization.

Visible Light-Mediated Thiolation of Substituted 1,4-Naphthoquinones Using Eosin Y as a Photoredox Catalyst.

In the presence of eosin Y, a visible light-induced one-step procedure (isolated yield of ≥75%) for thiolation of substituted 1,4-naphthoquinones using various aromatic and aliphatic thiols at room temperature is described herein. The rate-determining step of the reaction is thiyl radical generation, and the radical was characterized by high-resolution mass spectrometry. Cost effectiveness, operational simplicity, a short reaction time, high atom economy, and a very good yield make this photoredox-mediated process a useful alternative to the transition metal (e.g., Cu, Ag, and Pd)-catalyzed coupling reaction of quinones with thiols or disulfides.

Eosin Y Catalyzed Photochemical Synthesis of Arylated Phenothiazones.

In the presence of Eosin Y (EY), the synthesis of substituted phenothiazones was carried out efficiently using various substituted 2-aminothiophenol, diazonium salts, and 1,4-napthaquinones (1,4-NQ) at room temperature (RT) (condition: green LED of 525 nm, 44 W; reaction time: 8 h, isolated yield: 68-90%). A fluorescence quenching experiment and density functional theory (DFT) calculations suggested that the triplet photoexcited state of EY (EY*; τT = 320 ± 10 ns) converts to EY+• via oxidative quenching by ArN2 + (-1.11 V vs SCE for EY* to EY+•) initially. Thiyl and aryl radicals were captured as TEMPO adducts in high-resolution mass spectroscopy (HRMS). The reaction was not inhibited by the addition of a singlet oxygen quencher such as 1,4-diazobicyclo [2.2.2] octane (DABCO), which suggests that singlet oxygen is not participated.

Bio-inspired Cu(II) amido-quinoline complexes as catalysts for aromatic C-H bond hydroxylation.

Cu(II) complexes supported by tetradentate amido-quinoline acyclic ligands (L1 & L2) have been synthesized, characterized, and employed as catalysts for aromatic C-H hydroxylation using H2O2 as an oxidant in the absence of an external base with a high selectivity of around 90% for phenols via the non-radical pathway (TON ≥720). The KIE value, various spectroscopic studies and DFT calculation supported the involvement of Cu(II)-OOH species.

Photochemical C-H Arylation of Napthoquinones Using Eosin Y.

A visible-light-mediated C-H arylation of substituted 1,4-napthoquinones (1,4-SNQ) and 1,2-napthoquinone (1,2-NQ) with diazonium salt using a photocatalyst eosin Y at room temperature in a single step (isolated yield of ≥75%) is described in this report. The rate-determining step of the reaction is aryl radical generation, which was trapped by high-resolution mass spectrometry. Cost-effectiveness, operational simplicity, a short reaction time, high atom economy, and a good yield make this photoredox-mediated process a valuable alternative to the transition-metal (Fe, Cu, Pd, etc.)-catalyzed reaction.

C-H bond chlorination using nickel(II) complexes of tetradentate amido-quinoline ligands.

Ni(II)-tetradentate amido-quinoline complexes effectively catalysed C-H chlorination of a series of hydrocarbons in the presence of NaOCl and acetic acid. The bond dissociation energy of the C(sp3)-H bond of the substrates varies from 99.3 kcal mol-1 (cyclohexane) to 87 kcal mol-1 (ethyl benzene). Exclusively chlorinated products (TON: 220 for cyclohexane) were obtained without any hydroxylated products, thus mimicking the activity of the halogenase enzyme.

Visible-Light-Mediated Synthesis of Substituted Phenazine and Phenoxazinone Using Eosin Y as a Photoredox Catalyst.

This paper describes an efficient, sustainable, one-step procedure for synthesizing substituted phenazines and phenoxazinones from commercially available ortho-substituted aromatic amines with very good yield (≥80%) in water. The procedure uses eosin Y (EY) as a photoredox catalyst at room temperature (RT). The highly reactive o-quinone-diimine or o-quinone-imine intermediate was characterized by the HR-MS technique.

Phosphorous-Doped Graphitic Material as a Solid Acid Catalyst for Microwave-Assisted Synthesis of ß-Ketoenamines and Baeyer-Villiger Oxidation.

Synthesis of phosphorous-doped graphitic materials (P-Gc) using phytic acid as a precursor was done in a microwave oven in a cost- and time-effective green way. The material was used as a solid acid catalyst for microwave (MW)-assisted synthesis of ß-ketoenamines and Baeyer-Villiger (BV) oxidation. In the case of BV oxidation, hydrogen peroxide (H2O2) was used as a green oxidant. For ß-ketoenamines, in most cases, 100% conversion with an ∼95% yield was achieved in ethyl acetate medium. In solvent-free conditions, the yield of ß-ketoenamines was ∼75%. A kinetic study suggested that the resonance stabilization of the positive reaction center happens in the transition state for ß-ketoenamine synthesis. In BV oxidation, cyclic ketones were converted to their corresponding cyclic esters in good to high yields (∼80% yield) in a shorter reaction time (6-20 min). As per our knowledge, this is the first report of BV oxidation catalyzed by a heteroatom-doped graphitic material. For BV oxidation, the phosphoric acid functional groups present in P-Gc might increase the electrophilicity of the carbonyl group of the ketones to compensate for the weakness of H2O2 as a nucleophile and a spiro-bisperoxide intermediate has been identified in high-resolution mass spectrometry.

Hypovalent titanium and Ti(II)-Ti(III) interconversions.

Treatment of pink titanium(iii) triflate (0.045 M) with HF in triflic acid (CH(3)SO(3)H) converts Ti(iii) rapidly to a 1 : 1 mixture of Ti(IV) and green Ti(II): (2 Ti(III) + 4 HF --> TiF(4) + Ti(II) + 4 H(+)). This disproportionation is half complete when [HF] added is 0.027 M. Substituted 1,4-benzoquinones are reduced rapidly by Ti(iii) in the absence of fluoride, yielding straightforward logarithmic curves, but reactions of the same quinones with Ti(ii) in fluoride media exhibit more complex profiles, the major portions of which are zero order in oxidant. These reactions are strongly catalyzed by added Ti(iv). Analyses of complex curves are consistent with a reaction sequence initiated by Ti(ii)-Ti(iv) disproportionation, forming Ti(iii), which reacts with the quinone, yielding the quinhydrone, QH . The latter is rapidly reduced by Ti(ii). Values of rate constants obtained from these analyses are in agreement with those for reductions of quinones by Ti(iii), in the absence of fluoride.

Reactions of aquatitanium(II) with hypervalent chromium species.

Titanium(ii) solutions, prepared by dissolving titanium metal in triflic acid and HF, react readily with derivatives of Cr(vi), Cr(v) and Cr(iv). Reductions of Cr(vi) and Cr(iv), carried out with [Co(NH(3))(5)Br](2+) as a scavenger for Cr(ii), yield no detectable Co(2+), indicating that 2e(-) steps, bypassing Cr(v) and Cr(iii), are not operative. Catalysis by Ti(iv) is observed only for reduction of Cr(vi). Rates for reduction by Ti(iii) are remarkably similar to those for the corresponding reductions by Ti(ii). The addition of Ti(iii) to triflic acid and HF results in rapid and quantitative disproportionation to Ti(ii) and Ti(iv).

Reductions of oxo species by aquatitanium(II) as catalyzed by titanium(IV).

Titanium(II) solutions, prepared by dissolving titanium wire in triflic acid + HF, contain equimolar quantities of Ti(IV). These preparations react readily with oxidizing species such as VO(2)(+), substituted benzoquinones, and the biguanide complex of Mn(IV). Reactions with excess Ti(II) yield Ti(III). Reductions of 1,4-benzoquinones and the Mn(IV) complex are catalyzed by added Ti(IV) but stoichiometry is unaffected. Rate laws for the Ti(IV)-catalyzed reactions are consistent with formation of a Ti(II)-Ti(IV) complex (K(formation) 4 x 10(2) M(-1)), which, in some cases, also partakes in contributing deprotonated and halogen-catalyzed paths.