Hydroxybenzophenones

The substituted 2-hydroxybenzophenone, 2-(2-hydroxy-4-di-n-butylamino)-benzoylbenzoic acid underwent reaction with 4-methoxy-2-methyldiphenyl-amine in concentrated sulphuric acid at 10-1 during 20 hours followed by work-up in alkaline solution and a final treatment by refluxing in hot toluene for 2 hours to give 2-anilino-3-methyl-6-dibutylaminofluoran in 88% yield (ref.81). 

2-Hydroxybenzophenone, introduced into a mixture of its 0-cyanide derivative in ether containing finely powdered potassium hydroxide with vigorous stirring and reaction at ambient temperature during 1 hour led to 2-(2-benzoylphenoxy - 

A vigorously stirred mixture of 9-hydroxy-1-oxophenalene and 40% aqueous methylamine heated at 125 C under pressure (150psi) for 1 hour in a Monel 

A double Michael addition has been observed in the intermolecular reaction of the a, p-unsaturatedacylanthraquinone, 1 -hydroxy-2-(1 -oxopent-2-enyl)anthra-quinone illustrated with tert-butyl cyanoacetate followed by an intramolecular cyclisation of the product. The whole sequence was conducted at ambient temperature during 6 hours in dimethylformamide to give a substituted benzohydroxyanthraquinone in 75% yield (ref.84). 

In the same series, 1-hydroxy-2-(3-phenylprop-2-ynol)anthraquinone when stirred in piperidine for 1/2 hour at 20°C and then added as a concentrate from chloroform extraction to silica gel followed by work-up afforded 2-phenyl-4H-anthra[1,2b]pyran-4,7,12-trione in 82% yield (ref.85). 

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Hydroxybenzophenones represent the largest and most versatile class of ulbaviolet stabilizers that are used to protect materials from the degradative effects of ulbaviolet radiation. They function by absorbing ultraviolet radiation and by quenching elecbonically excited states. 

Additives used include plasticisers such as diphenyl diethyl ether, ultraviolet light absorbers such as 5-chloro-2-hydroxybenzophenone (1-2% on the polymer) and stabilisers such as phenoxy propylene oxide. 

UV absorbers have been found to be quite effective for stabilization of polymers and are very much in demand. They function by the absorption and harmless dissipation of the sunlight or UV-rich artificial radiation, which would have otherwise initiated degradation of a polymer material. Meyer and Geurhart reported, for the first time in 1945 [10], the use of UV absorber in a polymer. They found that the outdoor life of cellulose acetate film was greatly prolonged by adding phenyl salicylate (salol) [10]. After that, resorcinol monobenzoate, a much more effective absorber, was introduced in 1951 [11] for stabilization of PP, but salol continued to be the only important commercial stabilizer for several years. The 2,4-dihydroxybenzophenone was marketed in 1953, followed shortly by 2-hydroxy-4-methoxybenzophenone and other derivatives. Of the more commonly known UV absorbers, the 2-hydroxybenzophenones, 2-hy-droxy-phenyl-triazines, derivatives of phenol salicylates, its metal chelates, and hindered amine light stabilizers (HALS) are widely used in the polymer industry. 

Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

The five-coordinate complexes Ir(CO)(PPh3)2L, where HL = /3-diketone, A-benzoyl-A-phenyl-hydroxylamine, salicylaldehyde, 8-hydroxyquinoline, 2-hydroxybenzophenone, 2-hydroxy-8-methoxybenzophenone, were prepared from [Ir(CO)(PPh3)2Cl].632 The resulting compounds all underwent oxidative addition reactions with Br2. Reaction of [(cod)2IrCl]2 with N-substituted 3-hydroxy-2-methyl-4-pyridine gives the bichelated complex (389). 33... 

The apparatus used for picosecond flash spectroscopy on these systems has been described before(8 10). Figure 3a and b show typical transient absorption data obtained on 2-hydroxybenzophenone and the copolymer. Summary of these spectral data are given in Table 3. The transient observed at the shortest delay time (7ps) is the first excited singlet in all systems. The spectral data (at delay times > 50ps) permit placement of upper limits on triplet yields in CH2CI2 for both 2-hydroxy benzophenone itself and the copolymerized chromophore. 

Figure 3. Transient absorption spectroscopy on 2-hydroxybenzophenone and its derivative copolymer (A) 2-hydroxybenzophenone in EtOH solution and 2-hy-droxybenzophenone in CHtClt solution (B) copolymer in CHuCU. solution...

It is primarily the prospect of reduced mobility and volatility, with the expected improvement in long-term performance, which motivates the preparation of polymeric ultraviolet stabilizers. Work in this area was reviewed thoroughly by Bailey and Vogl in 1976 (1), and more recently by the author (2). The present paper describes recent synthetic work involving four classes of effective ultraviolet stabilizers salicylate esters (I), 2-hydroxybenzophenones (II), a-cyano-3-phenyl-cinnamates (III) and hydroxyphenylbenzotriazoles (IV). In each... 

Radical polymerizations of vinyl-substituted ultraviolet stabilizers were accomplished with azobisisobutyronitrile (AIBN) as initiator, with careful exclusion of oxygen. Copolymerization was also readily achieved. The following sections describe in detail the preparation of polymeric ultraviolet stabilizers from salicylate esters, 2-hydroxybenzophenones, a-cyano-p-phenyl-cinnamates and hydroxyphenylbenzotriazoles. 

Each of the derivatives may be regarded as a substituted styrene, and classical styrene syntheses have been employed. Radical polymerization of the phenolic monomers (salicylate esters, 2-hydroxybenzophenones and hydroxyphenylbenzotriazoles) proceeds normally with AIBN as initiator, at least when oxygen is carefully excluded. It is expected that polymeric ultraviolet stabilizers, perhaps in combination with conventional stabilizer will make an important contribution to photostabilization technology. 

The capability of 2-hydroxybenzophenone derivatives to dissipate light energy has been ascribed to rapid deactivation of the excited singlet state by intramolecular interaction between the carbonyl and hydroxyl groups, possibly involving reversible H-transfer. These proposals are outlined in Scheme I, where P and PP represent the polymer and photoproduct, respectively. 

Many UV absorbers are based upon either 2-hydroxybenzophenone or 2-hydroxytriazole derivatives, (the corresponding 3-hydroxy-compounds are not effective). These work via the following mechanisms ... 

We can illustrate the application of PAC to a simple photochemical reaction. Acetone is readily excited to its singlet excited state which rapidly undergoes efficient intersystem crossing to its triplet state. The triplet state decays in solution primarily by radiationless decay. The PAC experimental waveforms obtained from the photoexcitation of acetone in air and argon-saturated cyclohexane are shown in Fig. 1. In addition, the waveform obtained from the calibration compound 2-hydroxybenzophenone is also shown. 

Figure 1. Experimental waveforms from excitation (266 nm) of 2-hydroxybenzophenone (T-Wave, and acetone in air-saturated (E-Wave(l)) and argon-saturated (E-Wave(2)) cyclohexane. (With permission from Joshua L. Goodman.)...

Benorylate (315) [4 -(acetamido)phenyl-2-acetoxybenzoate] is another example. It is the ester between two well-known antiinflamatory drugs, aspirin and paracetamol, and is employed in rheumatoid arthritis therapy. In view of the chemical structure with three photolabile groups (two esters and one amide), its possible phototoxicity has been investigated. From the preparative irradiations, it has been concluded that the PFR takes place with breaking of the central C—O bond to yield 5-acetamido-2 -acetoxy-2-hydroxybenzophenone (316). This product undergoes transacetylation to 5 -acetamido-2 -acetoxy-2-hydroxy-benzophenone (318) (Scheme 80) [300]. 

Very early in the study of photosensitization it was discovered that salicylaldehyde, 2,4-dihydroxybenzophenone, and 2-hydroxy-4-methoxy-benzophenone do not sensitize piperylene isomerization, indicating that enolization is much faster than quenching by the diene.37 Later it was shown that 2-hydroxybenzophenone would sensitize the dimerization of isoprene in concentrated solution, but that the reaction was much less efficient than when benzophenone was used as a sensitizer (Table I).36... 

The enhanced reaction in ethanol is attributed to weakening of the tight, intramolecular hydrogen bond which is believed to facilitate photoenolization of 2-hydroxybenzophenone. t>-Hydroxybutyrophenone undergoes photoenolization in preference to Type II cleavage.25 The enol of o-benzylbenzo-phenone (2) forms a Diels-Alder adduct with dimethyl acetylenedicarboxylate, indicating that reaction with a reactive substrate is possible.30... 

Stabilizers belonging to this class absorb high energy radiation, and by a process of internal conversion provide a pathway for its harmless dissipation throughout the polymer system. Two of the commonly used types of UV absorber are 2-hydroxybenzophenones and 2-hydroxyphenylbenzotriazoles (B-75MI11500, B-79MIU505). 

The furanoid ring in benzo[6 ]furan is susceptible to attack by oxidants. Permanganate and chromic acid give derivatives of 2-hydroxybenzoic acid with compounds unsubstituted at the 3-position, but compounds with a 3-methyl or a 3-aryl substituent give derivatives of 2-hydroxyacetophenone or 2-hydroxybenzophenone. Ozonolysis of benzo[6]furan affords 2-hydroxybenzoic acid, 2-hydroxybenzaldehyde and some catechol produced via its diformate. Before the advent of NMR spectroscopy these methods were used in structural elucidation of benzofuranoid natural products, as in the case of O-methyleuparin (Scheme 26). 

Figure 6.15 Chemical structures of some polymer photostabilizers (a) 2-hydroxybenzophenone, (b) hydroxybenzotriazole, (c) oxanilide...

Figure 6.22 Intramolecular proton transfer in the charge-transfer state of 2-hydroxybenzophenone. The zwitterionic form reverts to the ground state in a time of ps...

Because of its high crystallinity, the hnmopolymer (PVDV) is insoluble in most solvents at room temperature. However, since the regularity of repeating units in the chain is decreased by copolymerization, Saran is soluble in cyclic ethers and aromatic ketones. This copolymer (100 g) is plaslicized by die addition of a-melhyl-benzyl ether (5 g), stabilized against ultraviolet light degradation by 5-chloro-2-hydroxybenzophenone (2.0 g) and heat stabilized by phenoxypropylene oxide (2.0 g). 

Ultraviolet absorbers continue to be the most widely used stabilizers. Such products must have long-term stability to ultraviolet light, be relatively nontoxic, heat stable, have little color, must not sensitize the substrate, and must be priced at levels which the plastics processor can tolerate. The principal classes of chemicals meeting these requirements at present are the 2-hydroxybenzophenones, and 2-(2/-hydroxyphenyl)benzotriazoles, substituted acrylates, and aryl esters. Typical compounds representative of these classes are 2-hydroxy-4-octoxybenzophenone, 2-(2 -hy droxy-5 -me thylphenyl) be nzotn azole, ethyl-2-cyano-3,3-diphenyl acrylate, diinethyl / -methoxybenzylidene maJonate, and / -ter -octylphenyl salicylate. 

Several pharmacological studies were earned out to substantiate the proposed prodrug concept for (/< )-a-methylhistamine (12). The parent compound of this series of azomethines is the 2-hydroxybenzophenone derivative of (ft)-a-methylhistamine which is designated BP 2.94 (36). To evaluate the desired protection against HMT metabolization, BP 2.94 (36) was tested as a possible substrate in the presence of S-adenosylmethionine in comparison to histamine and ( )-a-methylhistamine (12). As a result no methylation of the prodrug 36 was observed, whereas both primary amines displayed similar Km- and Fmax-values [6],... 

Knappe, E., Peteri, D., and Rohdewald, I., Impregnation of chromatographic thin layers with polyesters for the separation and identification of substituted 2-hydroxybenzophenones and other ultraviolet absorbers, Z Anal. Chem., 197, 364, 1963 Chem. Abs., 60, 762g, 1964. 

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