Unidirectional switching between two flavylium reaction networks by the action of alternate stimuli of acid and base

The introduction of an ester group in the flavylium core allowed the reversible conversion between two different flavylium compounds each one exhibiting its own reaction network. An unidirectional switching cycle between 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyrylium and 2-(4-carboxyphenyl)-7-diethylamino-1-benzopyrylium was achieved by means of alternate acid and base stimuli. Addition of base to a methanolic solution of the ester derivative gives rise to the trans-chalcone of the parent carboxylic acid, which upon acidification of the solution forms the respective flavylium cation. This species esterifies under very acidic conditions to restore the original methyl ester derivative. The chemical reaction networks of both compounds were fully characterized from their thermodynamic and kinetic aspects, by a series of pH jumps followed by UV-vis absorption and emission spectroscopy, stopped flow and (1)H NMR. The crystal structure of the trans-chalcone of the ester derivative was unveiled showing a supramolecular structure involving hydrogen bonding.     

Synthesis and characterization of a symmetric bis(7-hydroxyflavylium) containing a methyl viologen bridge

A symmetric bis(flavylium) constituted by two 7-hydroxyflavylium moieties linked by a methylviologen bridge was synthesized. The thermodynamic and kinetics of the network of chemical reactions involving bis(flavylium) and the model compound 7-hydroxy-4'-methylflavylium was completely characterized by means of direct and reverse pH jumps (stopped flow) and flash photolysis. Both compounds follow the usual pH-dependent network of chemical reactions of flavylium derivatives. The equilibrium species of the model compound are the flavylium cation (acidic species) and the trans-chalcone (basic species) with an apparent pK'(a)=2.85. In the case of the bis(flavylium) it was possible to characterize by (1)H NMR spectroscopy three species with different degrees of isomerization: all flavylium, flavylium-trans-chalcone, and all trans-chalcone. Representation of the time-dependent mole fraction distribution of these three forms after a pH jump from equilibrated solutions of all-flavylium cation (lower pH values) to higher pH values, shows that formation of trans-chalcone is not completely stochastic (two independent isomerizations), the isomerization of one flavylium showing a small influence on the isomerization of the other. The radical of the methyl viologen bridge is formed upon reduction of the bis(trans-chalcone) with dithionite. The system is reversible after addition of an oxidant in spite of the occurrence of some decomposition.     

Flavylium Network of Chemical Reactions in Confined Media: Modulation of 3′,4′,7‐Trihydroxyflavilium Reactions by Host–Guest Interactions with Cucurbit[7]uril

In moderately acidic aqueous solutions, flavylium compounds undergo a pH‐, and in some cases, light‐dependent array of reversible chemical reactions. This network can be described as a single acid–base reaction involving a flavylium cation (acidic form) and a mixture of basic forms (quinoidal base, hemiketal and cis and trans chalcones). The apparent pK′_a of the system and the relative mole fractions of the basic forms can be modulated by the interaction with cucurbit[7]uril. The system is studied by using ¹H NMR spectroscopy, UV/Vis spectroscopy, flash photolysis, and steady‐state irradiation. Of all the network species, the flavylium cation possesses the highest affinity for cucurbit[7]uril. The rate of interconversion between flavylium cation and the basic species (where trans‐chalcone is dominant) is approximately nine times lower inside the cucurbit[7]uril.     

Multistate reaction kinetics of 6-hydroxy-4'-(dimethylamino)flavylium driven by pH. A stopped-flow study

The synthetic flavylium salt 6-hydroxy-4'-(dimethylamino)flavylium hexafluorophosphate displays a set of pH-driven chemical reactions in aqueous solutions, involving the formation of hemiketal species and chalcones with cis and trans configurations. Such reactions were studied by steady-state and transient UV-Vis spectroscopy and by stopped-flow techniques. A novel and more generalized kinetic scheme is developed, in order to take account of possible acid/base pairs that occur in the network of chemical reactions as the pH is changed. It is found that the formation of the hemiketal species by hydration of the flavylium is slow, and it is not possible to isolate each process that leads to the formation of the cis-chalcone (hydration and tautomerization reactions). The cis/trans isomerization reaction of cis-chalcone is slow, and the system takes several hours to reach equilibrium after a pH jump at room temperature. In basic conditions, negatively charged trans-chalcones are dominant. Comparison with other flavylium compounds shows that the hydration process is affected mainly by the amino group, while the hydroxyl group influences the tautomerization and isomerization reactions.     

2′-Hydroxyflavylium: introducing flavanones into the flavylium network of chemical reactions

Chalcones possessing a hydroxyl group in position 2 cyclize to form flavylium salts in acidic media, this reaction being reversible under neutral-basic conditions. On the other hand, chalcones possessing a hydroxyl group in position 2′ cyclize to form flavanones in basic media. By synthesizing 2′-hydroxyflavylium tetrafluoroborate, it was possible to obtain trans-2,2′-dihydroxychalcone that in solution can evolve to 2′-hydroxyflavanone or back to 2′-hydroxyflavylium depending on the pH. The several equilibria established in aqueous solution were fully characterized. The importance of including flavanones into the flavylium network of chemical reactions is briefly exploited.     

Photochromic soft materials: flavylium compounds incorporated into pluronic F-127 hydrogel matrixes

The performance of flavylium-based photochromic systems is increased by their incorporation into Pluronic F-127 matrixes, which switch from polymeric solutions to micelles to gels with changes in temperature depending on copolymer concentration. Two flavylium compounds, 7,4'-dihydroxyflavylium and 7-(N,N-diethylamino)-4-hydroxyflavylium, both exhibiting a small thermal cis-trans isomerization barrier in water were investigated. In the first system the flavylium in the gel photoswitches from the colorless trans-chalcone (Ct) species to the yellow flavylium cation (AH+) with quantum yield Phi=0.04 (25 degrees C) at pH 2.2 or to the orange quinoidal base (A) with quantum yield Phi=0.015 (25 degrees C) at pH 5.2. The photoproducts revert back to their initial form by a thermal process characterized by first-order kinetics; the rate constants exhibit a bell shape variation with pH, with a maximum at pH 4.3 (lifetime 4.2 min). The second system, 7-(N,N-diethylamino)-4-hydroxyflavylium, does not exhibit photochemistry in water but, when incorporated into the Pluronic F-127 gel, switches from yellow to red with a quantum yield of Phi=0.01 at pH 4.9. The respective thermal back reaction takes place with a lifetime of 66.7 min1. The flavylium network of chemical reactions is a good sensor for the detection of not only the critical micelle temperature but also the gelation temperature of Pluronic and like solutions and, in some instances, the exposure to UV and visible radiation.     

Promoting photochromism on flavylium derived 2-hydroxychalcones in aqueous solutions by addition of CTAB micelles

A strategy to obtain photochromism from the network of chemical reactions originated by flavylium compounds in solution is described. This strategy is particularly useful for flavylium salts bearing amino groups which give rise to a variety of beautiful colors but lack photochemistry in water. The trans-chalcone of 7-(N,N-diethylamino)-4'-hydroxyflavylium interacts strongly with CTAB micelles defining a yellow dark state. Upon irradiation, the system switches to a pink-red state emerging from the flavylium cation that is formed inside the micelle and ejected to the bulk aqueous phase. The photochemical product reverts back to the trans-chalcone adduct with the micelle in the dark. The thermodynamics as well as the kinetics of the photochromic system were studied in detail. The best color contrast is obtained at pH = 4.25 with Phi = 0.001 and a recovery lifetime of approximately 3 h. This photochromic system works with no need of changing the pH, which constitutes an important improvement over previously described systems dependent on pH jumps.     

Hydroxypyridinechromene and Pyridinechalcone: Two Coupled Photochromic Systems

Substitution of the phenyl group in 2‐hydroxychalcones by a 4‐pyridine unit dramatically changes the network of chemical reactions of this compound: trans‐chalcone‐type ( Ct ), cis‐chalcone‐type ( Cc ), and a hemiketal (hydroxy‐4‐pyridinechromene) ( B ) and their protonated forms are formed, but the presence of a flavylium‐type cation could not be detected even at very acidic pH values. Moreover, whereas in 2‐phenyl‐2‐benzopyrylium compounds B and Cc are generally elusive species whose kinetic processes in aqueous solutions occur on the sub‐second timescale, in the present compound these species equilibrate on a timescale four orders of magnitude lower. Complete characterization of the equilibrium and kinetics of the reaction network could thus be achieved by ¹H NMR spectroscopy and UV/Vis spectrophotometry. The network of chemical reactions exhibits cis–trans photoisomerization, as well as photochromism between the hemiketal and the chalcone‐type species. The irradiation of Ct in MeOH/H₂O (1:1) at 365 nm produces B almost quantitatively through two consecutive photochemical reactions: Ct → Cc photoisomerization followed by Cc → B photo ring closure with a global quantum yield of 0.02. On the other hand, irradiation of B at 254 nm leads to a photostationary state composed by 80 % Ct and 20 % B , with a quantum yield of 0.21.     

Multistate/multifunctional systems. A thermodynamic,kinetic, and photochemical investigation of the 4'-dimethylaminoflavylium compound

The 4'-dimethylaminoflavylium ion in aqueous solution undergoes an intricate network of chemical reactions controlled by pH and light excitation. It is shown that nine different forms are involved, including two species that are not present in previously investigated compounds of the flavylium family. The thermodynamic and kinetic constants of the equilibria and interconversion processes have been obtained by pH jump (included stopped-flow) experiments. The photochromic properties exhibited by the trans/cis chalcone forms have been investigated. The peculiar aspect of 4'-dimethylaminoflavylium, as compared to previously investigated compounds of the same family, is a close to planarity structure, as demonstrated by the X-ray analysis on the parent 4'-aminoflavylium compound (2.3 degrees torsion angle between the benzopyrylium and benzene ring). The results obtained show that the flavylium cation is strongly stabilized by the electron-donor character of the dimethylamino substituent on the benzene ring. The donor-acceptor interaction makes both the protonation of the amino group and the hydration of the flavylium cation difficult, with consequences on the tautomerization and cis/trans isomerization reactions. The multistate/multifunctional properties of 4'-dimethylaminoflavylium have been discussed in the frame of write-lock-read-unlock-erase cycles.     

Identification of 7,4'-dihydroxy-5-methoxyflavylium in "Dragon's blood": to be or not to be an anthocyanin

The compound 7,4'-dihydroxy-5-methoxyflavylium (dracoflavylium) was identified as the major red colorant in samples of the resin "dragon's blood", extracted from the tree Dracaena draco. The complex network of reversible chemical reactions that dracoflavylium undergoes in aqueous solution is fully described; for the first time, all the equilibrium constants that enable a complete characterisation of the system have been obtained (K'(a)=1.6 x 10(-4), K(a1)=1.0 x 10(-4), K(a2)=3.2 x 10(-8), K(Ct1)=1.0 x 10(-7), K(Ct2)=1.3 x 10(-10)). It is concluded that the red colour is due to a stable quinoid base, A, which is the major species at pH 4-7. It is further shown that this compound does not fit the commonly accepted definitions of anthocyanidin nor 3-deoxyanthocyanidin. Similarly to synthetic flavylium salts, the natural compound 7,4'-dihydroxy-5-methoxyflavylium gives rise to several species (multistate system) reversibly interconverted by external stimuli, such as pH.     

Phase-dependent photochromism of a lactone-stabilized chromene from a flavylium reaction network

New trans‐2‐hydroxychalcones bearing a carboxylate group at position 2′ ( Ct ^?) were synthesized (compounds 2 and 3 ). These compounds lead to a network of chemical reactions depending on pH value, light, and solvent. In water, when the pH value is lowered, the ionized trans‐chalcone is protonated and the flavylium cation A H⁺ is formed at very acidic pH values through hemiketal B and cis‐chalcone Cc , with global acidity constants of pK′_a ≤?1 and ≈0.1, respectively, for 2 and 3 . The electron‐acceptor character of the carboxylic substituent not only increases the observed acidity of the flavylium cation, but also decreases the rate of the ring‐opening/‐closing from a subsecond timescale to hours relative to model compound 1 (without carboxylate). The photochemistry of the network was studied in detail by means of continuous irradiation, monitored by UV/Vis absorption and ¹H and ¹³C NMR spectroscopic analysis. Although compound 3 is only slightly photoactive, compound 2 ( Ct^? ) reacts in aqueous solutions (λ_irr=313 nm) to form B^? and Cc^? , with a global quantum yield of 0.15, and fully reverts back to Ct^? with a rate constant of k=6.7×10^?5 s^?1. The flavylium cation is no longer formed in methanol, and irradiation of Ct^? leads to the formation of B ^? and the new lactone‐trapped chromene species La . The formation of La takes place through a sequence of three photochemical steps: photoisomerization of Ct ^?, photo‐ring‐closing reaction of Cc ^?, and photolactonization of B ^?. Only the cis/trans isomerization and ring‐closing reactions are thermally reversible on a timescale of seconds and hours, respectively. A photochromic system was achieved in rigid matrices of methanol (at 77 K) and 1‐dodecanol (5 °C) by irradiating lactone La to give a red ortho‐quinone allide through a photo‐ring‐opening reaction; the color disappears with a rate constant of k=1.25×10^?2 s^?1 in 1‐dodecanol at 5 °C.     

Electrophoretic investigation of the boron cluster anion [7-(2'-pyridyl)-nido-7,8-dicarbaundecaborate]- and its protonated zwitterionic product

ACN is a better solvent than methanol for both [NMe(4)] [7-(2'-pyridyl)-nido-7,8-C(2)B(9)H(11)] and its protonated anion. The investigated laboratory preparations of the salt and of its protonated anion are electrophoretically pure solids stable for 2 months at 4 degrees C. At a longer storage, the solid salt is more stable than the solid protonated anion. In the 40:60 v/v water-methanol solvent, decomposition products of the salt anion are detectable after one-week storage of the salt solution at 4 degrees C. The protonated anion does not decompose for almost 1 year in water-organic solutions at 4 degrees C. The exchange of the proton between the protonated anion and the solution is reversible and fast at room temperature. The pH dependence of the mobility of the [7-(2(-pyridyl)-nido-7,8-C(2)B(9)H(11)](-) anion reveals that the basicity of the nitrogen atom in the pyridine ring is not significantly affected by the bonding of the pyridyl group to the nido-7,8-C(2)B(9)H(11) cluster in position 7 and that the proton from the solution is accepted by the nitrogen atom in the 2-pyridyl ring. The UV-spectra of the salt and of its protonated anion indicate that the accepted proton is probably slightly shifted to the open face of the nido-7,8-C(2)B(9)H(11) cluster. The [1](-) is chiral.     

Kinetics of the exchange reactions between Gd(DTPA)2-, Gd(BOPTA)2-, and Gd(DTPA-BMA) complexes, used as MRI contrast agents, and the triethylenetetraamine-hexaacetate ligand

The kinetics of ligand exchange reactions occurring between the Gd(DTPA), Gd(BOPTA), and Gd(DTPA-BMA) complexes, used as contrast agents in MRI, and the ligand TTHA, have been studied in the pH range 6.5-11.0 by measuring the water proton relaxation rates at 25 °C in 0.15 M NaCl. The rates of the reactions are directly proportional to the concentration of TTHA, indicating that the reactions take place with the direct attack of the H(i)TTHA((6-i)-) (i = 0, 1, 2 and 3) species on the Gd(3+) complexes, through the formation of ternary intermediates. The rates of the exchange reactions of the neutral Gd(DTPA-BMA) increase when the pH is increased from 6.5 to 9, because the less protonated H(i)TTHA((6-i)-) species can more efficiently attack the Gd(3+) complex. The rates of the exchange reactions of [Gd(DTPA)](2-) and [Gd(BOPTA)](2-) also increase from pH 8.5 to 11, but from 6.5 to 8.5 an unexpected decrease was observed in the reaction rates. The decrease has been interpreted by assuming the validity of general acid catalysis. The protons from the H(i)TTHA((6-i)-) species (i = 2 and 3) can be transferred to the coordinated DTPA or BOPTA in the ternary intermediates when the dissociation of the Gd(3+) complexes occurs faster. The kinetic inertness of Gd(DTPA), Gd(BOPTA), and Gd(DTPA-BMA) differs very considerably; the rates of the ligand exchange reactions of Gd(DTPA-BMA), thus the rates of its dissociation, are 2 to 3 orders of magnitude higher than those of Gd(DTPA) and Gd(BOPTA). The rates of the ligand exchange reactions increase with increasing concentration of the endogenous citrate, phosphate, or carbonate ions at a pH of 7.4, but the effect of citrate and phosphate is negligible at their physiological concentrations. The increase in the reaction rates at the physiological concentration of the carbonate ion is significant (20-60%), and the effect is the largest for the Gd(DTPA-BMA) complex.     

Isomerization between 2-(2,4-Dihydroxystyryl)-1-benzopyrylium and 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium

2-Phenyl-1-benzopyrylium (flavylium) and 2-styryl-1-benzopyrylium (styrylflavylium) cations establish in aqueous solution a series of equilibria defining chemical reaction networks responsive to several stimuli (pH, light, redox potential). Control over the mole fraction distribution of species by applying the appropiate stimuli defines a horizontal approach to supramolecular chemistry, in agreement with the customary bottom-up approach toward complex systems. In this work, we designed an asymmetric styrylchalcone able to cyclize in two different ways, producing two isomeric styrylflavylium cations whose chemical reaction networks are thus interconnected. The chemical reaction networks of 2-(2,4-dihydroxystyryl)-1-benzopyrylium (AH(+)) and 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium (AH(+)(iso)) comprise the usual species observed in flavylium-derived networks, in this case, the styryl derivatives of quinoidal bases, hemiketals, and chalcones. The thermodynamics and kinetics of the crossed networks were characterized by the use of UV-vis absorption and NMR spectroscopy as well as time-resolved pH jumps followed by stopped-flow. The two styrylflavylium cations are connected (isomerize) through two alternative intermediates, the asymmetric trans-styrylchalcone (Ct) and a spiropyran-type intermediate (SP). At pH = 1, AH(+) slowly evolves (k(obs) ≈ 10(-5) s(-1)) to a mixture containing 62% AH(+)(iso) through the Ct intermediate, while at pH = 5, the SP intermediate is involved. The observed rate constants for the conversion of the styrylflavylim cations into equilibrium mixtures containing essentially Ct follow a pH-dependent bell-shaped curve in both networks. While at pH = 1 in the dark, AH(+) evolves to an equilibrium mixture containing predominantly AH(+)(iso), irradiation at λ > 435 nm induces the opposite conversion.     

Kinetics of the reaction of aqueous iron(vi) (FeVIO42-) with ethylenediaminetetraacetic acid

The reaction of aqueous iron(vi) (FeVIO42-, Fe(vi)) with ethylenediaminetetraacetic acid (EDTA) was studied kinetically as a function of pH (1.98-12.40) and temperature (15-45 degrees C) using a stopped flow kinetic technique. The rate law for the reaction of Fe(vi) with EDTA was found to be first-order with respect to each reactant over the entire studied pH range. The observed rate constants, k, decrease with an increase in pH, varying from 4.19 x 10(4) to 8.60 x 10(-2) M(-1) s(-1) over the pH range. The speciation of Fevi (H3FeO4+, H2FeO4, HFeO4-, and FeO42-) and EDTA (H4Y, H3Y-, H2Y2-, HY3-, and Y4-, Y = EDTA) species was used to explain the pH dependence of the k values. From the temperature effect on k at pH 5.4, 7.1, and 9.2, activation parameters, DeltaS(double dagger) and DeltaH(double dagger), were obtained for the reactions of Fe(VI) with EDTA. The values of DeltaS(double dagger) for the reactions were found to be negative, implying a highly ordered transition state in the reaction. The DeltaH(double dagger) for the reaction at pH 7.1 and 9.2 showed similar values within experimental error. Using the observed enthalpy parameters and the enthalpy of deprotonation of HFeO4- and EDTA species (HEDTA3- and H2EDTA2-), the enthalpy of deprotonation of H2FeO4 (DeltaH0H2FeO4) was determined as 5.7 +/- 3.0 kJ mol(-1). The reactivity of Fe(VI) with aminopolycarboxylates (APCs) was also studied in alkaline medium. The order of reactivity was determined as primary > secondary > tertiary, which suggests that FeVIO42- attacks at the nitrogen atom sites of APCs.     

Reactions of a {Mo16}-type polyoxometalate cluster with electrophiles: a synthetic, theoretical and magnetic investigation

A medium-nuclearity mixed-valence polyoxomolybdate [H2Mo16O52]10-={Mo16}(1a) was synthesized using an approach that employed protonated hexamethylenetetramine (HMTAH+) as counter ion and yielded (HMTAH)10 1a.34 H2O (1). The {Mo16} cluster anion exhibits significant nucleophilicity and traps electrophiles such as divalent transition metal ions, resulting in a family of isostructural compounds based on {Mo16M2}-type anions [M(H2O)8H2Mo16O52]6- (M=FeII (2), MnII (3), CoII (4)). The highly reactive nature of the {Mo16} system is also revealed by rearrangement and decomposition reactions of to either slowly form a sodium-bridged heptamolybdate-based chain compound (5) when left in the reaction solution or, in the presence of very high concentrations of electrophiles, to heptamolybdate-based cluster compounds [M2(H2O)9Mo7O24]2- of the {M2Mo7}-type (M=FeII (6), MnII (7)). Compounds were characterised by single crystal X-ray diffraction, elemental analysis, IR spectroscopy, magnetic susceptibility measurements, and density functional theory calculations.     

Functionalization of 7-Hydroxy-pyranoflavylium: Synthesis of New Dyes with Extended Chromatic Stability

This work reports the functionalization of pyranoflavyliums pigment using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride coupling chemistry. Four cinnamic acids were used to establish an ester bond with the hydroxyl group of the pyranoflavylium, namely 4-dimethylamino-, 4-amino-, 4-bromo-, and trans-cinnamic acids. The experimental condition, namely the molar ratios, solvent, and reaction time, were adjusted to obtain higher reaction yields in a reduced period. Excellent reaction yields of 68%, 85%, 94%, and 99% were achieved for 4-amino, trans-, 4-bromo, and 4-dimethylamino pyranoflavylium cinnamates, respectively. The structure of the functionalized pigments was fully clarified using one-dimensional (1H) and two-dimensional (COSY, HSQC, and HMBC) NMR experiments and HRSM analysis. Regardless of the type of functionalization, the UV-Visible spectrum showed a bathochromic shift (red region) on the maximum absorption wavelength and the absence of acid-base reactions throughout a broad pH range in comparison to the pyranoflavylium precursor. This work offers a valuable environmentally friendly, quick, and straightforward alternative to flavylium compounds’ challenging and labor-intensive functionalization, resulting in novel dyes with higher stability and dissimilar chromatic features.     

How and when does an unusual and efficient photoredox reaction of 2-(1-hydroxyethyl) 9,10-anthraquinone occur? A combined time-resolved spectroscopic and DFT study

The photophysics and photochemical reactions of 2-(1-hydroxyethyl) 9,10-anthroquinone (2-HEAQ) were studied using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy techniques and density functional theory (DFT) calculations. In acetonitrile, 2-HEAQ underwent efficient intersystem crossing to the triplet excited state ((2-HEAQ)(3)). A typical photoreduction reaction for aromatic ketones took place via production of a ketyl radical intermediate for 2-HEAQ in isopropanol. In water-containing solutions with pH values between 2 and 10, an unusual photoredox reaction reported by Wan and co-workers was detected and characterized. Observation of the protonated species in neutral and acidic aqueous solutions by fs-TA spectra indicated the carbonyl oxygen of (2-HEAQ)(3) was protonated initially and acted as a precursor of the photoredox reaction. The preference of the photoredox reaction to occur under moderate acidic conditions compared to neutral condition observed using ns-TR(3) spectroscopy was consistent with results from DFT calculations, which suggested protonation of the carbonyl group was the rate-determining step. Under stronger acidic conditions (pH 0), although the protonated (2-HEAQ)(3) was formed, the predominant reaction was the photohydration reaction instead of the photoredox reaction. In stronger basic solutions (pH 12), (2-HEAQ)(3) decayed with no obvious photochemical reactions detected by time-resolved spectroscopic experiments. Reaction mechanisms and key reactive intermediates for the unusual photoredox reaction were elucidated from time-resolved spectroscopy and DFT results. A brief discussion is given of when photoredox reactions may likely take place in the photochemistry of aromatic carbonyl-containing compounds and possible implications for using BP and AQ scaffolds for phototrigger compounds.     

Investigation of the pronounced medium effects observed in the voltammetry of the highly charged lacunary anions [alpha-SiW11O39]8- and [alpha-PW11O39]7-

A detailed study is reported of the influence of protons, metal cations, and media on the redox chemistry of lacunary anions [alpha-SiW11O39]8- and [alpha-PW11O39]7- of high formal negative charge. Each anion displayed a single chemically reversible one-electron reduction process in carefully dried aprotic CH3CN solution. This process was detected at very negative potentials just prior to the solvent limit. Addition of 0.3 equiv of acid gave rise to a new reduction process at considerably less negative potentials, which is attributed to formation of the protonated species [SiW11O38(OH)]7- and [PW11O38(OH)]6-. Voltammograms derived from simulations based on a double-square scheme are in excellent agreement with experiment. Previous data reported the presence of several processes in CH3CN and appear to have been influenced by the presence of protons and/or adventitious water. Not surprisingly, protonation reactions coupled to charge transfer contribute significantly to the voltammetry of these lacunary anions in buffered aqueous media over the pH range 2-6. A multi-square-scheme mechanism allowed the essential thermodynamic and kinetic features of this system to be captured and an assessment of the relative significance of possible individual pathways. The high formal anionic charges of [SiW11O39]8- and [PW11O39]7- appear to provide highly basic reduced forms that are able to abstract protons from water to produce protonated species which are reduced at potentials more than a volt less negative than those for the processes [SiW11O39]8-/9- and [PW11O39]7-/8- found in dry aprotic media.     

General base and general acid catalyzed intramolecular aminolysis of esters. Cyclization of esters of 2-aminomethylbenzoic acid to phthalimidine

Plots of log k(0) vs pH for the cyclization of trifluoroethyl and phenyl 2-aminomethylbenzoate to phthalimidine at 30 degrees C in H(2)O are linear with slopes of 1.0 at pH >3. The values of the second-order rate constants k(OH) for apparent OH(-) catalysis in the cyclization reactions are 1.7 x 10(5) and 5.7 x 10(7) M(-)(1) s(-)(1), respectively. These rate constants are 10(5)- and 10(7)-fold greater than for alkaline hydrolysis of trifluoroethyl and phenyl benzoate. The k(OH) for cyclization of the methyl ester is 7.2 x 10(3) M(-)(1) s(-)(1). Bimolecular general base catalysis occurs in the intramolecular nucleophilic reactions of the neutral species. The value of the Bronsted coefficient beta for the trifluoroethyl ester is 0.7. The rate-limiting step in the general base catalyzed reaction involves proton transfer in concert with leaving group departure. The mechanism involving rate-determining proton transfer exemplified by the methyl ester in this series (beta = 1.0) can then be considered a limiting case of the concerted mechanism. General acid catalysis of the neutral species reaction or a kinetic equivalent also occurs when the leaving group is good (pK(a) 相似文献