Showing papers on "Benzophenone published in 2000"

Oxidation of Phenols by Triplet Aromatic Ketones in Aqueous Solution

Abstract: Aromatic ketones efficiently mediate the photo-oxidative degradation of phenols in aerated aqueous solution, a process likely to be relevant in sunlit natural waters. Absolute bimolecular rate constants for the quenching of three model ketone triplets by nine phenols bearing various substituents, from electron-donating alkyl and alkoxy groups to the electron-withdrawing cyano group, were measured by nanosecond laser flash photolysis. Triplet benzophenone (BP) is quenched at nearly diffusion-controlled rates (2.6−5.6 × 109 M-1 s-1). Triplet state quenching of 3‘-methoxyacetophenone (3‘-MAP) and 2-acetonaphthone (2-AN) by the same set of phenols occurs more selectively, with rate constants spanning a range of 1 and more than 2 orders of magnitude, respectively. Quenching rate constants obey a Rehm−Weller relationship to the free energy of electron transfer from the phenol to the ketone triplet. By comparison of the quenching constants with overall photo-oxidation rates obtained by stationary irradiation in ...

Antineoplastic agents. 443. Synthesis of the cancer cell growth inhibitor hydroxyphenstatin and its sodium diphosphate prodrug.

Abstract: A structure-activity relationship (SAR) study of the South African willow tree (Combretum caffrum) antineoplastic constituent combretastatin A-4 (3b) led to the discovery of a potent cancer cell growth inhibitor designated phenstatin (5a) This benzophenone derivative of combretastatin A-4 showed remarkable antineoplastic activity, and the benzophenone derivative of combretastatin A-1 was therefore synthesized The benzophenone, designated hydroxyphenstatin (6a), was synthesized by coupling of a protected bromobenzene and a benzaldehyde to give the benzhydrol with subsequent oxidation to the ketone Hydroxyphenstatin was converted to the sodium phosphate prodrug (6e) by a dibenzyl phosphite phosphorylation and subsequent benzyl cleavage (6a --> 6d --> 6e) While hydroxyphenstatin (6a) was a potent inhibitor of tubulin polymerization with activity comparable to that of combretastatin A-1 (3a), the phosphorylated derivative (6e) was inactive

Ultrasound effects on the photopinacolization of benzophenone.

Abstract: Ultrasonic irradiation is able to modify the course of several photochemical reactions, especially bimolecular, proceeding via triplet states. These effects were illustrated in the study of benzophenone photopinacolization in ethanol. The rates and yields increase when sonication is applied simultaneously to UV irradiation. An explanation is based on a 2-fold effect: (i) light-absorbing transient species undergo sonolytic decomposition, making the photoconversion more efficient, and (ii) sonication induces the triplet state quenching, as shown by Stern-Volmer plots from experiments run in the presence of naphthalene, probably due to the easier collisional deactivation processes favored by the homogeneous distribution of the activated species.

Biomimetic catalysis with immobilised organometallic ruthenium complexes: substrate- and regioselective transfer hydrogenation of ketones.

Abstract: Chloro-(h6-arene) complexes of Ru(II) with N-sulfonyl-1,2-ethylenediamine ligands that have one or two styrene side chains were synthesized and characterized. The chloro ligand was substituted with a diphenylphosphinato ligand and the resulting organometallic complexes are transition state analogs for the Ru-catalyzed transfer hydrogenation of benzophenone. Following the protocol of mol. imprinting, these complexes were copolymd. with ethylene glycol dimethacrylate (EGDMA) in the presence of a porogen. The polymers were ground and sieved, and the phosphinato ligand was substituted with a chloro ligand, thus generating a shape-selective cavity in close proximity to the catalytically active metal center. When tested for their ability to catalyze the redn. of benzophenone, the imprinted polymers showed a significantly higher activity (up to a factor of seven) than control polymers without cavities. Out of a mixt. of seven different arom. and aliph. ketones, benzophenone was preferentially reduced when the imprinted polymer was used. Also, the specificity of the catalyst for diaryl ketones was confirmed in a reaction with a bifunctional substrate, 4-acetyl-benzophenone; the diaryl ketone was reduced faster with the imprinted catalyst than the acetyl group. The opposite regioselectivity was obsd. with the control polymer. Both the activity and the selectivity of the imprinted catalysts are dependent on how the Ru complexes are attached to the polymer backbone. A double connection proved to give superior results.

Absolute Rate Constants for the Quenching of Reactive Excited States by Melanin and Related 5,6-Dihydroxyindole Metabolites: Implications for Their Antioxidant Activity

Abstract: The triplet-excited state of benzophenone and the singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-P) have been employed as kinetic probes to obtain information on the antioxidant activity of the skin and eye pigment melanin and its biogenetic precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). The excited states were generated by the laser-flash photolysis technique and their reaction kinetics was examined by time-resolved transient absorption or fluorescence spectroscopy, respectively. The reaction between triplet benzophenone and DHI produced with unit efficiency the corresponding 6O-centered semiquinone radical, which was characterized by its characteristic transient absorption. The quenching rate constants for DHI (3.1 – 8.4 × 109 M−1 s−1) and DHICA (3.3–5.5 × 109 M−1 s−1) were near the diffusion-controlled limit, indicating excellent antioxidant properties. Kinetic solvent effects were observed. The reactivity of synthetic melanin,...

The efficient intramolecular sensitisation of terbium(III) and europium(III) by benzophenone-containing ligands

Abstract: The europium and terbium complexes of an octadentate macrocyclic ligand incorporating benzophenone have been prepared; excitation of the ketone, which has a triplet quantum yield of unity, results in very efficient sensitisation of the emissive states of the bound metal ions.

Probing the Surface Polarity of Various Silicas and Other Moderately Strong Solid Acids by Means of Different Genuine Solvatochromic Dyes

Abstract: Reichardt's ET(30) as well as Kamlet−Taft's α (hydrogen-bond-donor acidity) and π* (dipolarity/polarizability) values of various solid acids, e.g., silicas, aluminas, alumosilicates, titanium dioxides, and alumina-functionalized silica particles, are presented. The ET(30) values of the solid acids were directly measured by UV/vis spectroscopy using 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1a) and an eicosafluorine-substituted derivative of 1a (1b) as surface polarity indicators. Kamlet−Taft's α and π* parameters for the various solid acids are analyzed by means of Fe(phen)2(CN)2 [cis-dicyano-bis(1,10-phenanthroline)iron(II)] (2) and Michler's ketone [4,4‘-bis(N,N-dimethylamino)benzophenone] (3) as solvatochromic surface polarity indicators. The chemical interpretation of the α and π* parameters and the nature of the surface sites which they reflect are discussed. The correspondence of the UV/vis spectroscopic results to those of the IR-sensitive probe benzophenone and the fluorescence probe ...

Metastable β-phase of benzophenone: independent structure determinations via X-ray powder ­diffraction and single crystal studies

Abstract: Benzophenone was the first organic molecular material to be identified as polymorphic. It is well known that benzophenone crystallizes in a stable orthorhombic α-form (m.p. 321 K) with space group P212121 and a = 10.28, b = 12.12, c = 7.99 A, [Girdwood (1998). Ph.D. thesis. Strathclyde University, Glasgow, Scotland]. Here we report two separate structure determinations of the metastable β-form (m.p. 297–299 K). Crystalline material of the metastable polymorph was obtained from a melt supercooled to ∼243 K. The structure was determined from X-ray powder diffraction data by employing a novel, computational systematic search procedure to identify trial packing arrangements for subsequent refinement. Unit-cell and space-group information, determined from indexing the powder diffraction data, was used to define the search space. The structure was also determined from single-crystal diffraction data at room temperature and at 223 K. The metastable phase is monoclinic with space group C2/c and a = 16.22, b = 8.15, c = 16.33 A, β = 112.91° (at 223 K). The structures derived from the individual techniques are qualitatively the same. They are compared both with each other and with the stable polymorph and other benzophenone derivatives.

Alkali and alkaline-earth metal ketyl complexes: isolation, structural diversity, and hydrogenation/protonation reactions

Abstract: The use of hexamethylphosphoric triamide (HMPA) as a stabilizing ligand allowed successful isolation of a series of structurally characterizable alkali metal and calcium ketyl complexes. Reaction of lithium and sodium with one equivalent of fluorenone and reaction of sodium with one equivalent of benzophenone in THF, followed by addition of two equivalents of HMPA, yielded the corresponding ketyl complexes 1, 2, and 11, respectively, as microketyl-bridged dimers. If one equivalent of HMPA was used in the reaction of sodium with fluorenone, a further aggregated complex, the mu3-ketyl-bridged tetramer 3, was isolated, whereas analogous reaction of benzophenone with sodium afforded the trimeric ketyl complex 13, rather than a simple benzophenone analogue of 3. In the reaction of potassium with fluorenone, the use of two equivalents of HMPA gave the tetramer 4, rather than a dimeric complex analogous to 1 or 2. Compared to the tetrameric sodium complex 3, there is an extra HMPA ligand that bridges two of the four K atoms in 4. When 0.5 equiv of HMPA was used in the above reaction, complex 5, a THF-bridged analogue of 4, was isolated. In the absence of HMPA, the reaction of sodium with an excess of fluorenone yielded the tetrameric ketyl complex 6, in which two of the four Na atoms are each terminally coordinated by a fluorenone ligand, and the other two Na atoms are coordinated by a THF ligand. Two bridging THF ligands are also observed in 6. Reaction of 1,2-bis(biphenyl-2,2'-diyl)ethane-1,2-diol (7) with two equivalents of LiN(SiMe3)2 or NaN(SiMe3)2 in the presence of four equivalents of HMPA easily afforded 1 or 2, respectively, via C-C bond cleavage of a 1,2-diolate intermediate. The reaction of calcium with two equivalents of fluorenone or benzophenone in the presence of HMPA gave the corresponding complexes that bear two independent ketyl ligands per metal ion. In the presence of 3 or four equivalents of HMPA, the fluorenone ketyl complex was isolated in a six-coordinate octahedral form (10), while the benzophenone ketyl complex was obtained as a five-coordinate trigonal bipyramid (13). The radical carbon atoms in both benzophenone ketyl and fluorenone ketyl complexes are still in an sp2-hybrid state. However, in contrast with the planar configuration of the whole fluorenone ketyl unit, the radical carbon atom in a benzophenone ketyl species is not coplanar with any of the phenyl groups; this explains why benzophenone ketyl is more reactive than fluorenone ketyl. Hydrolysis of 2 or 11 with 2N HCI yielded the corresponding pinacol-coupling product, while treatment of 2 or 11 with 2-propanol, followed by hydrolysis, gave the pairs fluorenone and fluorenol or benzophenone and benzhydrol, respectively. A possible mechanism for these reactions is proposed.

Synthesis of the Benzophenone Fragment of Balanol via an Intramolecular Cyclization Event

Abstract: Studies are reported on the use of either a 7-exo radical cyclization or an intramolecular Heck reaction as the key step for the construction of the benzophenone fragment of the PKC inhibitor, balanol. Whereas, the former approach was unsuccessful, the Heck reaction proved to be viable for the coupling of two fully functionalized aryl subunits affording regioselectively a biaryl seven-membered lactone with an exocyclic alkene as the major component, in contrast to the competing eight-membered ring lactone. Hydrolysis of the lactone followed by oxidative cleavage of the alkene with ruthenium tetraoxide completed this short synthesis of the benzophenone unit.

A new mechanism of benzophenone photoreduction in photoinitiated crosslinking of polyethylene and its model compounds

Abstract: The photolytic products and a new photoreduction mechanism of benzophenone (BP) as a photoinitiator in the photocrosslinking of polyethylene (PE) and its model compounds (MD) have been studied by means of fluorescence, ESR, 13C and 1H NMR spectroscopy. The fluorescence spectra from the PE and MD systems demonstrate that the main photoreduction product of BP (PPB) is benzpinacol formed by the recombination of two diphenylhydroxymethyl (K•) radical intermediates. The ESR spectrum obtained from the UV irradiation of the MD/BP system gives positive evidence of K• radicals. Two new PPB products: an isomer of benzpinacol with quinoid structure, 1-phenyl-hydroxymethylene-4-diphenyl-hydroxymethyl-2,5-cyclohexa-diene and three kinds of α-alkylbenzhydrols have been detected and identified for the first time by 13C and 1H NMR spectroscopy from the MD systems. The latter could be formed by the reactions of K• radicals with alkyl radicals produced by hydrogen abstraction of the excited triplet state 3(BP)* from polyethylene or its model compounds. These results provide new experimental evidence for elucidating the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 999–1005, 2000

Synthesis of novel optically active poly(ester-imide)s with benzophenone linkages by microwave-assisted polycondensation

Abstract: A new simple and rapid polycondensation reaction of 4,4'-carbonyl-bis(phthaloyl-L-alanine)diacid chloride [N,N'-(4,4'-carbonyldiphthaloyl)]bisalanine diacid chloride with several diphenols, such as bisphenol-A, phenolphthalein, 1,8-dihydroxyanthraquinone, 4,4'-dihydroxybiphenyl, 1,5-dihydroxynaphthalene and hydroquinone, in the presence of a small amount of a polar organic medium such as o-cresol was performed using a domestic microwave oven. The polycondensation reaction proceeded rapidly and was almost complete within 12 min to give a series of poly(ester-imide)s with inherent viscosities of about 0.35-0.58 dl g -1 . The resulting poly(ester-imide)s were obtained in high yield and are optically active and thermally stable. All the above compounds have been fully characterized by IR spectroscopy, elemental analysis, inherent viscosity (η inh ), solubility test and specific rotation. Thermal properties of the poly(ester-imide)s have been investigated using thermal gravimetric analysis (TGA).

Triplet Energy Transfer and Triplet Exciplex Formation of Benzophenone

Abstract: Photochemical and photophysical aspects of (1) triplet energy transfer (TET) from triplet benzophenone (BP) to naphthalene derivatives (NpD) and (2) formation and decay processes of triplet exciplexes between 3NpD* and BP have been studied by 355-nm laser photolysis techniques in the liquid phase. As an initial event, TET occurs from 3BP* to NpD competing with H-atom transfer (HT), electron transfer (ET), and induced quenching (IQ) on the nanosecond time scale. The TET rate constant increases with an increase of solvent polarity while that of HT decreases, indicating that the 3(n, π*) state of BP is slightly mixed with 1(π, π*) in polar media. After formation of 3NpD* by TET from 3BP*, chemical reactions via triplet exciplexes between 3NpD* and BP having loose sandwich-like structures with weak charge transfer character take place in the microsecond time scale, depending on the substituent groups of NpD. (1) HT from triplet 2-naphthylammoniun ion and triplet naphthol to BP occurs efficiently to yield the ...

Spectroscopic and theoretical studies of the excited states of fenofibric acid and ketoprofen in relation with their photosensitizing properties

Abstract: A study of the different excited states of fenofibric acid (FB) and ketoprofen (KP), two aromatic acids derived from benzophenone, has been achieved by absorbance and low temperature emission spectroscopy to take into account their photochemical reactivity and their in vitro and in vivo photosensitizing properties. These experiments have shown that FB mainly exhibits a singlet-singlet transition of ππ* character, the nπ* transition being undetectable, in contrast with KP. In phosphate buffer as well as in ethanol or in isopentane, the lowest triplet excited state of fenofibric acid appears to be, as for KP, an nπ* triplet state with characteristics similar to those of benzophenone. These assignments were fairly well supported by theoretical calculations performed at the TD-DFT level. The transient formation of a ketyl radical detected in flash photolysis experiments performed in ethanol confirms that the photochemical reactivity of FB towards hydrogen abstraction in ethanol is typically that of an nπ* triplet state. The good agreement between the experimental and theoretical results is discussed with respect to those previously obtained by other theoretical methods.

Role of phenolic derivatives in photopolymerization of an acrylate coating

Abstract: The photopolymerization of acrylate resins on wood surfaces suffers from retardation and inhibition effects due to the phenolic derivatives present at the interface. This article details the study of the effect of a set of phenolic compounds on the initiation step. The global effect was recorded by differential scanning calorimetry and photocalorimetry. A comparison between a direct photocleavable initiator such as 2,2-dimethyloxy-2-phenylacetophenone (DMPA) and a two-component system like benzophenone/N-methyldiethanolamine (BP/MDEA) suggests that the retardation effect observed in the latter case is due to the interaction between phenols and the triplet state of BP. Subsequently, nanosecond transient absorption (NTA) spectroscopy was used to measure in acetonitrile the quenching rate constants k Q . A hydrogen abstraction occurred, and the ketyl radical quantum yield was also determined by NTA experiments. In comparison with the photoreduction mechanisms proposed in the literature, the high k Q values obtained were tentatively correlated to the half-wave oxidation potentials of phenols in order to discuss the involvement of an electron transfer within the reaction. Some EPR experiments were done to confirm in situ the photoreduction process at the wood surface and the creation of phenoxyl radicals. The interaction of phenols with some initiating radicals was also studied.

Study of the photophysics and energy transfer of 9,10-diphenylanthracene in solution

Abstract: Photophysics of 9,10-diphenylanthracene (DPA) in benzene solution was studied with time-resolved thermal lensing (TRTL) technique. The TRTL signal was composed of the fast and slow-rising components. The heat conversion efficiency, which is defined as the ratio of energy released as heat against energy absorbed, was determined to be 0.31±0.01, revealing that the fluorescence quantum yield is fairly large (0.88±0.03), while the quantum yield for triplet formation is quite small (0.04±0.02) by analyzing the slow component of the TRTL signal. The triplet–triplet (T–T) absorption of DPA in benzene appeared for the wavelength region of 420–470 nm, obtained by energy transfer with triplet benzophenone used as a photosensitizer. The quenching rate constant of DPA by triplet benzophenone was determined to be (3.6±0.3)×10 9 M −1 s −1 with Stern–Volmer plots. The intrinsic energy transfer rate was also estimated by analyzing the time profile of the TRTL signal.

Nature of the lowest triplet states of 4′-substituted N-phenylphenothiazine derivatives

Abstract: The nature of the lowest triplet state of the donor–acceptor N-phenylphenothiazine derivatives has been studied by means of phosphorescence and EPR spectroscopy in various low temperature glasses. The triplet excitation of N-phenylphenothiazine and N-(p-anisyl)-phenothiazine is localised within the phenothiazine subunit. On the contrary, the 77 K phosphorescence of the molecules containing an electron acceptor group (i.e. –CN, –COCH3 or –COC6H5 at the 4′ position) is similar to that for benzonitrile, acetophenone or benzophenone, respectively, although the energy levels of their T1 states are higher than that of phenothiazine. With increasing temperature, however, their phosphorescence becomes similar to that characteristic for phenothiazine. This finding has been explained in terms of the excited-state intramolecular energy transfer accompanied by the planarisation of the phenothiazine kernel. The results of crystallographic investigations support this hypothesis.

Use of Competition Kinetics with Fast Reactions of Grignard Reagents

Abstract: Competition kinetics are useful for estimation of the reactivities of Grignard reagents if the reaction rates do not differ widely and if exact rates are not needed. If the rate of mixing is slower than the rate of reaction, the ratios between the rates of fast and slow reagents are found to be too small. This is concluded from experiments in which results obtained by competition kinetics are compared with results obtained directly by flow stream procedures. A clearer picture of the reactivity ratios is obtained when the highly reactive reagent is highly diluted with its competitor. A fast reagent may account for almost all the product even when present as only 1 part in 100 parts of the competing agent. In this way allylmagnesium bromide is estimated to react with acetone, benzophenone, benzaldehyde, and diethylacetaldehyde ca. 1.5 x 10(5) times faster than does butylmagnesium bromide. The rates found for the four substrates do not differ significantly, and it seems possible that there is a ceiling over the rate of reaction of this reagent, for example, caused by diffusion control. This may explain that competition kinetics using allylmagnesium bromide have failed to show kinetic isotope effects or effects of polar substituents with isotopically or otherwise substituted benzophenones. A recently reported alpha-deuterium secondary kinetic isotope effect for the reaction of benzaldehyde with allylmagnesium bromide was observed at -78 degrees C, but was absent at room temperature. It is suggested that the reaction of benzophenone and benzaldehyde with allylmagnesium bromide has a radical-concerted mechanism since no radical-type products are produced and since no color from an intermediate ketyl is observed even at -78 degrees C.

Diastereoselective photosensitised radical addition to fumaric acid derivatives bearing oxazolidine chiral auxiliaries

Abstract: Various fumaric acid diamides were alkylated by reaction with photogenerated radicals. The radicals were produced either by benzophenone triplet hydrogen abstraction (from 1,3-dioxolane, 2-methyl-1,3-dioxolane and adamantane) or via photoinduced electron transfer sensitised by DCN/biphenyl from stannanes (t-BuSnPh3 and Bu4Sn). When chiral substrates were employed the reaction occurred with a good degree of stereoselectivity even when acyclic alkyl radicals were involved.

Photochemical alkylation of ketene dithioacetal S,S-dioxides. An example of captodative olefin functionalization

Abstract: Radical alkylation of some ketene dithioacetal S,S-dioxides failed through the tin hydride promoted chain process but was successfully performed through stoichiometric photochemical initiation, either by electron transfer or hydrogen abstraction. In the first case, alkyl radicals were produced from tetralkylstannanes (t-Bu-, i-Pr-, n-Bu-SnR(3)) via radical cation fragmentation, while in the second case these were produced from alkanes (cyclohexane, adamantane) by benzophenone triplet. When bulky radicals (t-Bu, adamantyl) were involved, the addition occurred with complete diastereoselectivity.

Iron-Mediated Synthetic Routes to Unsymmetrically Substituted, Sterically Congested Benzophenones

Abstract: A new synthetic route to unsymmetrically substituted benzophenones, relying upon iron-mediated reactions in all of the key steps, is described. The key buiding block, η6-2-chloro-carbomethoxybenzene-η5-cyclopentadienyl iron hexafluorophosphate (4), is reacted with a variety of substituted phenols, providing diaryl ether complexes (6). After hydrolysis of the ester functionalities, these complexes are subjected to Friedel−Crafts conditions. The efficiency of this intramolecular acylation reaction is very much dependent upon the substituents on the phenols. If these are appropriately chosen, xanthone complexes are isolated in fair to good yields. Regioselective ring-opening, using oxygen-nucleophiles, delivers substituted benzophenone complexes. After regioselective nucleophilic addition of cyanide ion, performed in the presence of DDQ, highly substituted benzophenones are isolated. To demonstrate the applicability of the new route, a formal synthesis of the benzophenone moiety of the protein kinase C inhib...