Heck coupling reaction of iodobenzene and styrene using supercritical water in the absence of a catalyst

Heck coupling reaction of iodobenzene and styrene proceeds rapidly and selectively in supercritical water even without any catalyst in the presence of base. Both the choice of base and the reaction conditions had a significant effect on the conversion and the selectivity of the coupling products. The addition of a relatively mild base such as potassium acetate facilitated the cross-coupling reaction, while the hydrolysis of phenyl halide was favored in the presence of a strong base. The conversion and the yields of coupling products increased with increasing temperature, reaching a maximum at 650 K near the critical temperature of water, and then decreased as the temperature was further increased. Water density had a significant influence on the reaction rate, showing nearly 30% augmentation with a slight increase in density from 0.45 to 0.56 g cm(-3), but had less effect on the product selectivity. Two possibilities of the role of water responsible for the noncatalytic Heck coupling reaction in supercritical water, that is, ion and water-catalyzed mechanisms have been considered.     

Controlling a recognition-mediated reaction using a pH switch

The selective recognition-mediated reaction between a nitrone bearing a urea recognition site and a maleimide bearing a proton switchable recognition site can be turned 'on' and 'off' by the addition of base and acid respectively.     

Triplet to ground state intersystem crossing and CIS-trans isomerization in stilbene

The T₁ ? S₀ intersystem crossing process in stilbene is considered adopting a formulation in which the isomerization coordinate can be treated separately. Energy surfaces in agreement with experimental and theoretical results are employed and pertinent spin-orbit coupling is evaluated. The calculated decay rate is in good agreement with the experimental results based on the azulene quenching effect. The qualitative feature that quantum yield for cis formation tends to be higher than the counterpart trans formation is explained. Deuterium effects are discussed and explanations for them are suggested.     

Photoisomerization of stilbene dendrimers: the need for a volume-conserving isomerization mechanisms

Highly branched stilbene dendrimers were synthesized and their photochemical behavior was studied. Even the stilbene dendrimer with molecular weight over 6500 underwent trans-cis isomerization in the excited singlet state within the lifetime of 10 ns. The photoisomerization of C=C double bond of stilbene dendrimers in the excited state may proceed by a volume-conserving novel mechanism such as hula-twist rather than conventional 180 degrees rotation around the C=C double bond based on fluorescence and isomerization experiments.     

Details of the emulsion polymerization of styrene using a reaction calorimeter

An automated reaction calorimeter was used to directly monitor the rate of emulsion polymerization of styrene using different emulsifier (sodium lauryl sulfate) and initiator (potassium persulfate) concentrations. By using this technique in conjunction with off-line measurements of the evolution of the particle size distributions, important details of the process were observed. The classical constant rate period (Interval II) often reported for the batch emulsion polymerization of styrene was not seen in this work. Instead, the experimental results suggest that the end of nucleation and the disappearance of monomer droplets take place at approximately the same conversion (36–40%). From the polymerization rate data, important parameters such as the monomer concentration in the polymer particles and the average number of radicals per particle were calculated. © 1996 John Wiley & Sons, Inc.     

A styrene based water soluble polymer as a reaction medium for photodimerization of aromatic hydrocarbons in water

A styrene based water soluble polymer (polymer-A) has been explored as a host for solubilizing otherwise insoluble aromatic hydrocarbons in water. The increased local concentration of encapsulated aromatic hydrocarbons within the hydrophobic pockets of polymer-A was utilized for performing efficient photodimerization of acenaphthylene (1) and six 9-substituted anthracenes [AnCOOH, AnCHO, AnCH₂OH, AnCH₃, AnBr and AnCN] in water. Photodimerization of these aromatic hydrocarbons were more efficient than in water and yielded dimers even at low concentrations (∼10⁻⁴ M). At the same concentration of anthracenes in organic solvents such as benzene and methanol, no dimers were formed even after 48 h of irradiation. Although the polymer-A was able to increase the local concentration of the reactant aromatics it was unable to orient them towards a single dimer.     

Heterogeneous and homogeneous hydrogenation of styrene and stilbene chromium tricarbonyl complexes

Heterogeneous hydrogenation of the styrene and stilbene chromium tricarbonyl complexes by molecular hydrogen on skeletal nickel and palladium on carbon as catalysts was studied. As compared to styrene and stilbene, their arene chromium tricarbonyl analogs are hydrogenated considerably more slowly, which is related, most likely, to strong adsorption of the π-complexes on the catalyst surface. For the homogeneous hydrogenation of these complexes using a H₂PtCl₆-SnCl₂-LiBr system, styrene and η⁶-styrene chromium tricarbonyl are reduced with a high rate, whereas stilbene and its chromium tricarbonyl complex are hydrogenated very slowly. A possibility of reduction of the unsaturated arene chromium tricarbonyl complexes by sodium borohydride in the presence of cobalt(II) chloride as a catalyst was shown. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 44–47, January, 2007.     

Palladium mediated dehydrogenation in the photochemical cyclization of heterocyclic analogs of stilbene

A method is described for the photocyclization of heterocyclic analogs of stilbene to the corresponding phenanthrene-like systems under a N₂ atmosphere, using a catalytic amount of Pd on carbon (Pd/C) as the “oxidant”.     

Flow injection determination of lactate based on a photochemical reaction using photometric and chemiluminescence detection

A photochemical method for the determination of lactate using a flow-injection system is proposed. The method is based on the decomposition of lactate in the presence of UO2(2+) and Fe3+ upon irradiation with UV or visible light. The Fe2+ produced in the photochemical process was monitored by measuring the absorbance after complexation with ferrozine (lambda max = 562 nm) or the chemiluminescence (CL) intensity in a luminol system without added oxidant. The range of measurements depended on the length of the irradiation time and the detection system used. The detection limits using CL and photometric detection were 2 ng ml-1 and 50 ng ml-1, respectively. The sample throughput was 45 samples h-1. The usefulness of the method was demonstrated by determining lactate levels in blood serum, milk, yoghurt, beer and pharmaceutical preparations.     

Effect of water on the palladium-catalyzed reaction of styrene with iodobenzene

The effect of the amount of water in teh solvent (DMF) on the rate of the reaction of styrene with iodobenzene in the presence of various palladium compounds has been studied. Addition of water to reaction mixture promotes the reaction. The effect of the addition water depends on the nature of the palladium complexes and temperature. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1401–1404, July, 1997.     

Preconcentration and LC analysis of chlorophenols,using a styrene-divinyl-benzene copolymeric sorbent and photochemical reaction detection

Summary A specific HPLC method has been developed for the trace analysis of lower chlorinated aromatic compounds. The method consists of an on-line preconcentration and a post-column reaction step. On-line preconcentration of mono- and dichlorophenols from aqueous samples has been performed using PRP₁, a divinylbenzene-styrene copolymeric sorbent as packing material for both pre-and analytical column. Enrichment factors of over 300 were obtained compared to regular (100 l) loop injections, even for the highly polar monochlorophenols. After reversed-phase separation, post-column photochemical dechlorination is carried out directly in the eluent stream, using a photochemical reactor. Upon dechlorination, fluorescent products are formed, which can be detected selectively. The resulting fluorescence signal shows a linear response to the quantity of solute present over 2 to 3 orders of magnitude (correlation coefficients: 0.990–0.98). For the mono- and dichlorophenols, the detection limit of the photoconversion method is in the lower nanogram range. The method is especially suitable for the analysis of complex matrices such as effluent water samples or biological fluids containing traces of the polar chloroaromatic compounds together with large amounts of other constituents, which interfere when using more general methods of detection like UV absorption. The potential of this technique for automation has been demonstrated by using a microprocessor-controlled column switching unit.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Thermal and photochemical studies with stilbene oxides intramolecular trapping of carbonyl ylide intermediates

The annelated tetrahydrofuran derivatives 14 and 15, resp., are formed in moderate yield by intramolecular trapping reaction of the carbonyl ylide intermediates 9, which are generated either by thermal ring opening of the trans-stilbene oxides 7t/8t or by photolysis of the cis-isomers 7c/8c.     

The “phantom” photochemical singlet state of stilbene and its diphenylpolyene relative

Recently the conception on the cis-trans photoisomerization of stilbene has emerged that this process is governed by a higher excited ¹A_g state which exhibits a minimum at the perpendicular conformation crossing the lowest excited ¹B_u state upon bond rotation. An evaluation of the potential surfaces governing the photoisomerization process by a PPP SCF CI method revealed that there exists indeed such a photochemically active ¹A_g state which in the planar molecule lies about 1 eV above the lowest absorption band and involves the excitations of two electrons from the SCF ground state. Extensions of the calculations to diphenylpolyenes demonstrate that this ¹A_g state is related to the forbidden low-lying doubly excited ¹A_g state observed earlier in these molecules.     

Acid-base testing of catalysts using 1-butene isomerization as test reaction

Catalysts tested were classified into two groups according to the initial ratio ofcis- andtrans-2-butene in the products. CaY, Fe-PILC and Al-PILC proved to be acidic while LiY, NaN₃/NaY and calcined hydrotalcite (HT) basic materials.     

Benzil-tethered precipitons for controlling solubility: a round-trip energy-transfer mechanism in the isomerization of extended stilbene analogues

We are investigating photoresponsive molecules called "precipitons" that undergo a solubility change co-incident with isomerization. Isomerization can be induced by light or by catalytic reagents. Previous work demonstrated that covalent attachment of a metal complex, Ru(II)(bpy)3, greatly accelerates photoisomerization and influences the photostationary state. In this paper, we describe precipitons (1,2-biphenylethenes; analogous to stilbenes) that are activated by a covalently attached organic sensitizer (benzil). We find that isomerization of these stilbene analogues is little effected by the presence of benzil in solution but that the intramolecular benzil effect is to increase the rate of isomerization and to significantly change the photostationary state. What is most interesting about these observations is that the precipiton is the primary chromophore in this bichromophoric system (precipiton absorbance is many times greater than benzil absorbance in the 300-400 nm range), yet the neighboring benzil has a significant effect on the rate and the photostationary state. The effect of unattached benzil on the rate was small, about a 24% increase in rate as compared with 4-6-fold changes for an attached benzil. We speculate that the isomerization process occurs by a "round-trip" energy-transfer mechanism. Initial excitation of the precipiton chromophore initiates a sequence that includes (1) formation of the precipiton singlet state, (2) singlet excitation transfer from the precipiton unit to the benzil, (3) benzil-centered intersystem crossing to the localized benzil triplet state, (4) triplet energy transfer from the benzil moiety back to the precipiton, and (5) isomerization.     

Controlling coupling reaction of EDC and NHS for preparation of collagen gels using ethanol/water co-solvents

To control the crosslinking rate of the collagen gel, ethanol/water co-solvent was adopted for the reaction solvent for the collagen microfibril crosslinking. Collagen gel was prepared by using EDC and NHS as coupling agents. Ethanol did not denaturate the helical structure of the collagen and prevented the hydrolysis of EDC, but showed the protonation of carboxylate anions. In order to control the intra- and interhelical crosslink of the collagen triple helix, variations of the mole ratio of carboxyl group/EDC/NHS, and of the ethanol mole concentration were investigated. Increase in the EDC ratio against the carboxyl group increased the crosslinking rate. Furthermore, an increase in the ethanol mole concentration resulted in an increase of the crosslinking rate until ethanol mole concentration was 0.12, but showed gradual decrease as the ethanol mole concentration was further increased. This is because the adsorption of solvent by the collagen gel, protonation of carboxylate anion, and hydrolysis of EDC is at its most optimum condition for the coupling reaction when the ethanol mole concentration is 0.12. The re-crosslinking of the collagen gel showed an increase in the crosslinking rate, but did not show further increase when the coupling reaction was executed for the third time. This implied that the highest possible crosslinking rate for the intra- and interhelical is approximately 60% when EDC/NHS is used.     

Heterogeneous reductive isomerization reaction using catalytic Pd/C and H2

[reaction: see text] A highly selective catalytic reductive isomerization reaction is described. The extremely mild and neutral reaction conditions (10% Pd/C, H2, and MeOH at 0 degrees C) tolerate a wide range of functional groups and generally result in excellent yields. Mechanistic studies suggest that this reaction does not proceed via a stepwise reduction/elimination sequence or a pi-allylpalladium intermediate.     

Potential energy curves and photochemical cis-trans isomerism in styrene

Cis-trans photoisomerism of styrene is investigated from a theoretical point of view. Curves of the potential energy as a function of the rotation angle (ω) around the ethylenic bond were obtained for the ground state and a few excited states by the CIPSI PCILO method. The potential surface of the lowest excited singlet is found to have both an absolute minimum at ω = π/2 and a small relative minimum at ω = 0. These findings are consistent with the singlet mechanism for the direct cis-trans photoconversion. The origin of the two energy minima in the S₁ potential surface is explained in terms of weights of the local excitations in the zero-order wavefunction.