Hydroxyaromatic compounds

The first step of the procedure illustrates a general way of preparing aryl iert-butyl ethers.416 The second step is the best way to prepare 2-hydroxythiophene, inasmuch as the yield is good and ter/-butyl perbenzoate is a readily available perester that is relatively stable. The same procedure has been used to convert several other haloaromatic compounds to hydroxyaromatic compounds in good yield4 and is probably quite general. 

The emission spectra of 10-CPT in water-methanol mixtures exhibits dual fluorescence (Fig. 1 left). The appearance of the low energy emission band at 570 nm for 10-CPT in a neat methanol solution indicates an efficient PTTS process. The large fluorescence quantum yield and similarity of the emission at neutral and basic solutions is evidence of the excited anion (RO ) formation, in contrast to 6HQ, for which double PTTS leads to the tautomer [2], With the increase of water content in the mixtures, we observed a substantial decrease in the fluorescence intensity of the nondissociated form of 10-CPT at 430 nm and a concomitant increase of RO " intensity at 570 nm. This is a well-known effect in hydroxyaromatic compounds [4] and is attributed to the increase of the protolytic photodissociation rate with increasing water concentration. 

H -tetramethylbenzidine in anionic-cationic mixed micelles has been studied in detail by ESR . The photochemistry of the semi-oxidised forms of eosin Y and rose bengal have been investigated in colloidal solutions. Relevant to the fluorescence of proteins is a study of fluorescence quenching of indolic compounds by amino-acids in SOS, CTAB, and CTAC micelles O Rate constants for proton transfer of several hydroxyaromatic compounds have been measured in a variety of surfactant solutions. Photoprotolytic dissociation does not require exit of the reactant molecules from the micelles. Micellar solutions can be used to improve the fluorescence determination of 2-naphthol by inhibiting proton transfer or proton inducing reactions z2. jpe decay of the radical pair composed of diphenylphosphonyl and 2,4,6-trimethyl benzoyl radicals in SDS is affected by magnetic... 

Finally, the importance of GC/MS techniques for the analysis of hydroxyaromatic compounds generated during microsomal hydroxylation of benzene derivatives is mentioned here. Using various partially ring-deuteriated substituted benzenes, including biphenyl, evidence for direct aromatic hydroxylation was gained from the careful mass spectrometric tracing of the fate of the label in the various silyl-derivatized hydroxylation products . 

The carboxylation of hydroxyaromatic compounds in the form of alkaline salts (the Kolbe Schmitt reaction) is well known and applied in the synthesis of acids such as salicylic acid, para-hydroxybenzoic acid and orf/to-cresotinic acid. 

In this reaction, the course of condensation and the type of products formed depend on several factors, such as the nature of hydroxyaromatic compounds, amines, aldehydes, and the actual ratio of the components in the reaction system. For examples, besides the initial ammonia, this reaction has been extended successfully to both primary amines and secondary amines and among the secondary amines, the reaction particularly works for dimethylamine, diethylamine, and piperidine. In addition, when one of the secondary amines exists in large excess or when dimethylamine or ammonia is removed from the reaction system via vaporization, good yields of pure products are often achievable. Besides the commonly used -naphthol, many other hydroxyaromatic compounds are suitable for this reaction as well, such as phenols, and quinolinols. " It is found that among kojic acid, 8-quinolinol, and 8-hydroxquinaldine, kojic acid is the most reactive compound for the Betti reaction, which reacts with benzylideneaniline in less than 1 day... 

Five hydroxyaromatic compounds (1- and 2-hydroxy-, and 2,7-dihydroxynaph-thalene, 5-hydroxy-1,4-naphthoquinone, 4,4 -dihydroxybiphenyl) were extracted hom water and baseline resolved on a phenyl colunm (2 = 254 nm). Elution was Complete in 15 min using a 22/78 acetonitrile/water (0.1 M acetate at pH 4.7) mobile phase [901]. The stated linear range was 0.002-10 mg/L with detection limits of 0.7-3.2 pg/mL (analyte dependent). 

Photoprotolytic reactions in micellar solutions were systematically studied by the authors of the present review with coworkers [15, 16, 66, 120-122]. Effective dissociation rate constants ki for a set of hydroxyaromatic compounds in anionic, cationic and uncharged micelles were determined from the dependence of fluorescence quantum yields on the concentration of micelles [Eq. (22)] or on the concentration of acid [Eq. (49)]. The latter method was also used for determining the effective equilibrium constants and effective rate constants of the reverse protonation reaction [Eq. (49)]. The data obtained are presented in Table 3. 

A noteworthy compensation effect is observed in cationic micelles a smaller increase of kjk-i is found for hydroxyaromatic compounds with smaller pK values and vice versa. 

The development of metal complexes, particularly those based on nickel, resulted in compounds which exhibited relatively low extinction coefficients in the near ultraviolet region and yet in many instances were found to be superior in performance to the currently available ortho-hydroxyaromatic compounds [3,5]. This finding resulted in a search for some other feasible mechanistic function. As carbonyl photolysis was considered to be an important initiation process which gives rise to both free radicals and backbone cleavage, then deactivation or quenching of the excited-state precursor should prove an effective means of photoprotection. Effective structures in this respect were tris(dibenzoylmethanato) chelates of Fe and Cr (Structure 5), nickel oxime chelates (Structure 6), and the nickel complex of Structure 7, nick-el(ii) 2,2 -thiobis(4-f-octylphenolato)- -butylamine. 

It has been foimd that a derivative of an alkylated hydroxyaro-matic compound derived from a conventional PIB and a high vinylidene PIB is equally effective and may be more effective as a fuel additive than a derivative of an alkylated hydroxyaromatic compound derived from a high vinylidene PIB. The preparation of a PIB alkylated phenol and the Mannich reaction runs as follows (27) ... 

ABSTRACT. Kinetics of proton transfer photoreactions in simple model systems is analyzed from the point of view of reaction kinetics in microphases. Protolytic photodissociation of some hydroxyaromatic compounds ArOH ( 1- and 2-na-phthol, chlorosubstituted naphthols ) was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of at least two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergo photodissociation, but another do not dissociate. In micelles only minor fraction ( few per cent ) of ArOH molecules do not take part in excited-state proton transfer reaction. These phenomena reflect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles. A correlation between proton transfer rate constants and equilibrium constants in microphases similar to that in homogeneous solutions is observed. Microphase approach give a possibility to discuss reactions in dynamical organized molecular systems in terms of classical chemical kinetics. 

To calculate rate constants of protolytic dissociation of hydroxyaromatic compounds in the singlet excited state in the micellar phase we studied the dependence of fluorescence quantum yields and fluorescence kinetics of undissociated (f>) and dissociated (p ) forms of hydroxyaromatic compounds in the micellar solutions on the concentration of hydroxo-nium ions in the solution. Similarly to aqueous solutions proton transfer photoreactions in micellar solutions can be described by simple kinetic scheme ... 

TABLE 2. Rate constants of protolytic photoreactions of the the hydroxyaromatic compounds in microgeterogeneous systems... 

For protolytic photodissociation of hydroxyaromatic compounds we may expect that in anionic micelles the excited anions of hydroxyaromatic compounds having the same sign of charge will leave the micelles, and in the cationic micelles - hydrogen ions will leave micelles. Exit rate constants of excited anions of hydroxyaromatic compound from the micelles were determined using the non-solubilized fluorescence quenchers. Simultaneously, we have proved that it is in the micellar phase that protolytic dissociation does proceed and not as a result of the preliminary exit of excited molecules of hydroxyaromatic compounds from the micellar phase to an aqueous one. 

TABLE 5. Fluorescence decay parameters of hydroxyaromatic compounds in microheterogeneous systems. 

Now we shall analyze the regularities in rate constants of protolytic photodissociation of hydroxyaromatic compounds in the micellar phase and in the membranes of vesicles ( Tables 1 and 5 ). For one and the same compounds in cationic micelles the acidity constants are usually greater than in the aqueous solution ( pA decreases ), and dissociation rate... 

Enhanced Acidity of Hydroxyaromatic Compounds in the Excited Singlet State. .. 39-2... 

The excited state properties of hydroxyaromatic compounds (phenols, naphthols, etc) are of interest to a wide audience in chemistry, including those interested in the environmental decomposition of phenols, chemical physicists interested in the very fast dynamics of excited-state proton transfer (ESPT) and excited-state intramolecular proton transfer (ESIPT), physical chemists interested in photoionization and the photochemical pathways for phenoxyl radical formation, and organic photochemists interested in the mechanisms of phenol and hydroxyarene photochemistry. Due to space limitations, this review is restricted to molecular photochemistry of hydroxyaromatic compounds reported during the last three decades that are of primary interest to organic photochemists. It also includes a brief section on the phenomenon of enhanced acidity of phenols and other hydroxyaromatics because this is central to hydroxyarene photochemistry and forms the basis of much of the mechanistic photochemistry to be discussed later on. Several reviews that offer related coverage to this work have also appeared recently. This review does not cover aspects of electron photoejection from phenols or phenolate ions (and related compounds such as tyrosine) or phenol OH homolysis induced photochemically, as shown in Eq. (39.1), as these are adequately covered elsewhere ... 

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