A theoretical study of the [4 + 4] dimerization of thioformylketene

[reaction and structure: see text] A theoretical study (B3LYP and G3MP2B3) of the dimerization of thioformylketene (1) was performed. Four pathways-two [4 + 2] pathways with thioformylketene (1), one [4 + 4] pathway with 1, and one [4 + 2] pathway involving 1 and thietone (11)-were considered. Interestingly, the [4 + 4] pathway with 1 had the lowest barrier (3.8 kcal/mol). The geometry of the transition state TS14 is unusual, with the forming bonds in the plane of the ketene. This suggests that the reaction is pseudopericyclic.     

Photophysical Properties of Coumarin‐30 Dye in Aprotic and Protic Solvents of Varying Polarities¶

Experimental results on various photophysical properties of coumarin‐30 (C30) dye, namely, Stokes' shift (Δv), fluorescence quantum yield (τ_f), fluorescence lifetime (τ_f), radiative rate constant (k_f) and nonradiative rate constant (k_nr), as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Δf= [(ε ‐ 1)/(2ε+ 1) ‐ (n² ‐ 1)/ (2n²+ l)], they show unusual deviations in nonpolar solvents at one end and in high‐polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: ( 1 ) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7‐NEt₂ substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone π cloud; ( 2 ) in medium to higher polarity solvents, the dye exists in a polar intra‐molecular charge transfer structure, where the 7‐NEt₂ group and the 1,2‐benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone π cloud; ( 3 ) in protic solvents, the dye‐solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and ( 4 ) in high‐polarity protic solvents, the excited C30 undergoes a new activation‐controlled nonradiative deexcitation process because of the involvement of a twisted intra‐molecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity‐dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.     

Degradation of commercially important polyhydroxyalkanoates in tropical mangrove ecosystem

This paper presents the degradation trends of selected polyhydroxyalkanoate (PHA) films in a tropical mangrove environment. The biodegradability of homopolymer poly(3-hydroxybutyrate) [P(3HB)] and its co-polymers, poly(3-hydroxybutyrate-co-5 mol% 3-hydroxyvalerate) [P(3HB-co-5 mol% 3HV)] and poly(3-hydroxybutyrate-co-5 mol% 3-hydroxyhexanoate) [P(3HB-co-5 mol% 3HHx)], was investigated along with P(3HB) films containing 38 wt% titanium dioxide (TiO₂) [P(3HB)-38 wt% TiO₂]. The degradation of these formulations was monitored for 8 weeks at three different zones in an intermediate mangrove compartment along Sungai Pinang, adjacent to a famous fishing village on south of Penang Island. The degradation rate was observed both on the surface and in the sediment and was expressed in percentage of weight loss. The microbial enumeration done using sediment from the different zones indicated similar colony-forming unit (CFU) counts even though differences were noticed in the degradation profile of the various films in the respective zones. The results obtained revealed that co-polymers disintegrated at similar or higher rate than the homopolymer, P(3HB). However, the incorporation of TiO₂ into PHB films caused the degradation rate of P(3HB)-38 wt% TiO₂ composite film to be far slower than all the other PHA films. The overall rate of degradation of all PHA films placed on the sediment surface was slower than those buried in the sediment. Microscopic analyses showed that the surface morphology of P(3HB-co-5 mol% 3HHx) was more porous compared to P(3HB) and P(3HB-co-5 mol% 3HV) films, which may be an important factor for its rapid degradation.     

Stopped-flow kinetics of tetrazine cycloadditions; experimental and computational studies toward sequential transition states

The Diels-Alder cycloadditions of tetrazines (1) with alkynes (2) are expected to give bicyclic adducts (3). Kinetic measurements of the cycloadditions of 1a and 1b with 2a give DeltaG(++) = 19.2 +/- 1.0 and 11.5 +/- 1.2 kcal/mol, respectively. Stopped-flow UV studies on the reaction of 1b with 2a show an isosbestic point at 428 nm; this places an upper limit of 11.6 +/- 2.6 kcal/mol on DeltaG(++) for loss of N(2) from the putative bicyclic intermediate 3b. Calculations (B3LYP/6-31G(d,p) + ZPVE) of transition structures for the reaction of tetrazinediacid 1d with propynylamine 2c are consistent with the experimental results for the reaction of 1b with 2a. This and several related model systems reveal two interesting features of the calculated energy surfaces. First, there may be no barrier for the loss of nitrogen from structures 3 and thus there may be two sequential transition states. This also extends Berson's correlation of activation energy with reaction energy in pericyclic reactions to significantly lower barriers. Second, for the cycloaddition of 4e and 2c, there is neither an intermediate nor a transition state between TS3e and the final product 6e. It appears that the energy surface "turns a corner" in the vicinity of a structure resembling 5e. This is not a mathematically well-defined point but has chemical consequences in that the overall exothermicity of the reaction from 4e to 6e is not felt in TS3e.     

Phytochemical constituents from dietary plant Citrus hystrix

Chemical investigation of the fruit peel of dietary plant Citrus hystrix offered two new flavones 5,6,4′-trihydroxypyranoflavone I and 5,4′-dimethyl-6-prenylpyranoflavone XIII besides 11 known compounds. The structures of all compounds were elucidated with the aid of suitable analytical methods like 1D, 2D-NMR, mass and single crystal X-ray analysis. An X-ray crystal study of compound II was done for the first time and the compounds I–VI, XI and XII are hitherto not reported from this plant. Biological studies revealed that compound I found to have a good antidiabetic and antiacetylcholinesterase activities meanwhile compounds II, III and V showed a significant free radical scavenging ability as well as antioxidant capacity. In addition, compounds I, IV, V and VI showed cytotoxicity against U87, A549 and MCF-7 cells. Overall, the new compound I showed valuable bioactive properties. Due to insufficient quantity of compound XIII, biological studies were not done.     

Mechanistic Study of the Titanocene(III)‐Catalyzed Radical Arylation of Epoxides

An atom‐economical and catalytic arylation of epoxide‐derived radicals is described. The key step of the catalytic system is a sequential electron and proton transfer for the rearomatization of the radical σ‐complex and catalyst regeneration. Kinetic, computational, spectroscopic, and cyclovoltammetric investigations highlight the key issues of the reaction mechanism and catalyst stabilization by collidine hydrochloride. Studies employing radicophiles rule out the participation of cations as reactive intermediates.     

Theoretical assessment of antioxidant property of polyproponoid and its derivatives

Structural Chemistry - Derivatives of parent molecules possess similar structural activity which makes them to be the topic of equal interest. In the present work, a naturally occurring acid...     

Mechanistic study of samarium diiodide-HMPA initiated 5-exo-trig Ketyl-Olefin coupling: the role of HMPA in post-electron transfer steps

The mechanistic importance of HMPA and proton donors (methanol, 2-methyl-2-propanol, and 2,2,2-trifluoroethanol) on SmI2-initiated 5-exo-trig ketyl-olefin cyclizations has been examined using stopped-flow spectrophotometric studies. In the presence of HMPA, the rate order of proton donors was zero and product studies showed that they had no impact on the diastereoselectivity of the reaction. Conversely, reactions were first-order in HMPA, and the additive displayed saturation kinetics at high concentrations. These results were consistent with HMPA being involved in a rate-limiting step before cyclization, where coordination of the intermediate ketyl to the sterically congested Sm(III)HMPA both stabilizes the intermediate and inhibits cyclization. Liberation of the contact ion pair through displacement by an equivalent of HMPA provides a solvent-separated ion pair releasing the steric constraint to ketyl-olefin cyclization. The mechanism derived from rate studies shows that HMPA is important not only in increasing the reduction potential of Sm(II) but also in enhancing the inherent reactivity of the radical anion intermediate formed after electron transfer through conversion of a sterically congested contact ion pair to a solvent-separated ion pair. The mechanistic complexity of the SmI2-HMPA-initiated ketyl-olefin cyclization is driven by the high affinity of HMPA for Sm(III), and these results suggest that simple empirical models describing the role of HMPA in more complex systems are likely to be fraught with a high degree of uncertainty.     

A convenient pathway to Sm(II)-mediated chemistry in acetonitrile

In this communication we show that the instability of samarium diiodide (SmI(2)) in acetonitrile is a consequence of ionization of the reductant in this solvent. Samarium triflate (Sm(OTf)(2)) is exceptionally stable in acetonitrile for periods over six months and can be used with appropriate additives to initiate a ketyl-olefin coupling reaction in high yield.