(polymeric benzophenone) in photochemical reactions

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The use of insoluble benzoylated polystyrene beads (polymeric benzophenone) in photochemical reactions JOSELUISBOURDELANDE, JOSEPFONT,A N D FRANCISCO SANCHEZ-FERRANDO Departatnent de Quimica Orgrinica, Fac~lltatde CiPncies, Utziversitat Autdnoma cle Barcelona, Bellaterra, Spain

Received March 8, 1982'

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This paper is dedicated to Professor Harry E. Gunning on the occasiotz of his 65th birthday

JOSELUIS BOURDELANDE, JOSEPFONT,and FRANCISCO SANCHEZ-FERRANDO. Can. J. Chem. 61, I007 (1983) Friedel-Crafts benzoylation of 2%-divinylbenzene (DVB) cross-linked polystyrene beads yielded insoluble polymeric analogs of benzophenone which were used as sensitizers in the ( 2 + 2 photocycloadditionof cyclohexene to maleic anhydride and in the E-Z isomerization of (E)-I ,3-pentadiene and methyl (E)-2.2-dimethyl-3,5-hexadienoate.In the first reaction, the use of the polymer-anchored sensitizers resulted in the separation of polymer-attached by-products by simple filtration, although the yield of photoadducts was less than three quarters of that obtained when using benzophenone itself, and the recovered polymers could not be reused. In the photoisomerizations, the compositions of the photostationary states reached when using the polymeric sensitizers (now reusable) were shown to depend on the degree of fu"ctionalizationof the polymers via steric hindrance to the energy transfer process. Intrapolymerically sensitized E-Z photoisomerization was also demonstrated on a polymer containing benzophenone and diene units. Photooxidation of secondary alcohols (isopropanol, cyclohexanol) to ketones and photooxygenation of 2,3-dimethyl-2-butene to 2,3-dimethyl-3-buten-2-yl hydroperoxide (in the presence of oxygen) were other reactions for which polymeric benzophenones could be used. JOSELUISBOURDELANDE, JOSEPFONTet FRANCISCO SANCHEZ-FERRANDO. Can. J. Chem. 61, 1007 (1983). La benzolation selon Friedel et Crafts d'une resine de polystyrkne rCticulCe i 2% de divinylbenzkne (DVB) donne des polymkres insolubles analogues de la benzophknone qu'on utilise comme sensibilisateur dans la photocycloaddition du type 12 + 21 du cyclohexkne sur I'anhydride malkique et dans I'isomCrisation du type E-Zdu pentadikne-I ,3 (E) et du dimCthyl-2,2 hexadikne-3,5 oate de mCthyle (E). Dans la premikre rCaction, I'utilisation d'un sensibilisateur fix6 sur un polymkre conduit i une siparation des produits secondaires fixis sur le polymkre par simple filtration, bien que le rendement en photoadduits reprksente moins de 75% de celui que I'on obtient en utilisant la benzophknone elle-mCme et bien que les polymkres rCcupkrCs ne soient pas rCutilisables. Dans le cas des photoisomCrisations, on montre que les compositions des Ctats ph~tostationnaiies atteints lorsqu'on utilise les sensibilisateurs fixis sur un polymkre (non rkutilisable) dCpendent du taux de fonctionnalisation des polymkres via l'encombrement stCrique qui gkne le processus de transfert d'Cnergie. On a aussi dCmontrC qu'un polymkre contenant une benzophknone et un dikne peut provoquer une photoisomCrisation E-Z sensibiliske a I'intCrieur du polymkre. La photoxydation des alcools secondaires (isopropanol, cyclohexanol) en cCtones et la photoxygtnation du dimkthyl-2,3 butkne-2 en hydroperoxyde de dimCthyl-2,3 butbne-3 yle-2 (en prCsence d'oxygkne) sont d'autres rkactions oh il est possible d'utiliser des benzophCnones fixCes sur un poly mkre. [Traduit par le journal]

Introduction The well developed concept of solid-phase synthesis (I), i.e., the attachment of a reactant to an insoluble matrix (polymeric or inorganic) prior to its reaction (now in heterophase) with soluble reactants, offers the following possibilities when applied to photochemical reactions: (i) anchored substrate, soluble photoreagent (or sensitizer); (ii) anchored photoreagent, soluble substrate; (iii) both substrate and photoreagent anchored. We (2) and others (3) have described the photohalogenation of anchored n-alkanoic acids as an example of case (i). The present paper deals mainly with case (ii), although an example of case (iii) is also presented. Both cases offer great interest, since many of the current approaches to chemical storage or utilization of the energy of sunlight depend on the use of photoredox reagents or sensitizers physically adsorbed or chemically bound to solid insoluble particles of inorganic oxides or organic polymers, in order to produce separation of the individual reacting species involved in each step of the process (4). Actually, some insoluble polymeric (or polymer-bound) photosensitizers have been known for some time. After the pioneering work of Leermakers and James ( 5 ) on the use of polyvinyl phenyl ketone, prepared by bulk poly'Revision received November 26, 1982.

merization of the corresponding monomer, other authors used the solid phase concept and attached known sensitizers to insoluble resins, usually cross-linked polystyrene beads. Several resin-bound photosensitizers have been developed and tested, mainly by Neckers and co-workers, including both soluble and insoluble polystyrylmethyl4-benzoylbenzoate (6), soluble poly(p-(trifluoroviny1)benzophenone) and poly(p(trifluoroviny1)acetophenone) (7), rigid polydivinylbenzophenone (8), and the very useful P-Rose Bengal (9), which is the reagent of choice for the sensitized photochemical generation of singlet oxygen. Benzophenone 1 is well known in organic photochemistry. With its convenient triplet energy of 288 kJ mol-I, it sensitizes many photochemical reactions, such as 12 + 21 photocycloadditions and 2-E photoisomerization of suitable alkenes. In addition, it can also act as a photoreagent, for instance in cycloadditions with alkenes (Paterno-Buchi reaction), or as photooxidant by hydrogen atom abstraction from appropriate donors. One polymeric analog of benzophenone, namely poly-p-styrylmethyl 4-benzoylbenzoate 2, was found by Neckers to sensitize 12 21 photocycloadditions (6). However, in later work by the same group, some doubts were cast on this polymer as a true solid phase sensitizer (10). Another more closely related insoluble polymeric analog of 1, namely polystyryl4-(N,N-dimethy1amino)phenylketone, was found to

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CAN. I . CHEM. VOL. 61, 1983

sensitize the norbornadiene-quadricyclane photoisomerization with the same quantum yield as soluble 4-(N,N-dimethylamino)benzophenone, although at slightly slower rates (1 1). Quite surprisingly, the simplest polymeric analog of benzophenone, i.e., poly-13-styryl phenyl ketone 3, has not been evaluated in photochemical reactions "because of the problem with AlCl, incorporation when polystyrene-divinylbenzene beads are benzoylated under Friedel-Crafts conditions" (6), in spite of the recorded successful uses of 3 in other types of reactions (12- 14). The purpose of the present work is to fill this gap by describing the preparation of AlC1,-free cross-linked poly-pstyryl phenyl ketone 3 (polymeric benzophenone) and its use as sensitizer in the 12 21 photocycloaddition of cyclohexene and maleic anhydride, and in the E-Z photoisomerization of two soluble conjugated dienes and one polymer-bound diene. Also described is the use of 3 in the photooxidation of two secondary alcohols and in the photooxygenation of an alkene to the corresponding hydroperoxide. Preliminary communications have been published on some of these subjects (15, 16).

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Preparation and characterization of polymeric benzophenones Beads of a (98: 2) copolymer of styrene and divinylbenzene (DVB) were benzoylated by reaction with benzoyl chloride in carbon disulfide in the presence of AlC1, (17), a method which is also used for the benzoylation of soluble polystyrene (18). The degree of functionalization to be achieved was limited by the amount of benzoyl chloride and AlC13 used. The resulting polymeric benzophenones were thoroughly washed with a series of solvents and shown to be free of AIC13 by elemental analysis. The degree of functionalization was determined by conversion into the corresponding polymeric oximes 4 and elemental analysis for nitrogen in these derivatives.

The three polymeric benzophenones prepared in this way were shown to contain 0.95, 2.2, and 3.2 mequiv. g-' of carbony1 group. This can also be expressed as 11%, 29%, and 50% functionalization of the phenyl groups of the original polystyrene. For convenience, we shall refer to these three polymeric benzophenones as P-1 1, P-29, and P-50, respectively. 'The photostability of all three polymers was checked by irradiation (A > 290 nm) for 15 h in benzene or acetonitrile under nitrogen. This very prolonged irradiation released in soluble form less than 0.1% of a nonvolatile material which showed no peak of benzophenone by glc. The recovered irradiated polymers showed ir spectra almost identical to those of the starting materials, except for a very small broad absorption in the associated hydroxyl region. Furthermore, very little loss of activity was found when using P-29 pre-irradiated for 14 h in Z / E photoisomerizations (Fig. 2, curves 12 and 13). These

data imply the occurrence of some (although very little) photodegradation of the polymers by backbone hydrogen atom abstraction, followed by cross-linking. Thus, P-1 1, P-29, and P-50 were shown to be rather photostable in the absence of substrates. These results should be compared with those obtained by Neckers (10) when evaluating his ester-type polymeric benzophenone 2 (6), which clearly showed the occurrence of up to 50% photodecarboxylation with release of up to 20% free benzophenone during irradiation for 6 h.

'2 @--cH2-

RH

+ C 0 2 + .C6H4-COPh

Ph2C0 (ref. 6 ) 1

Polymeric benzophenones as sensitizers (a) Setlsitized 12 + 2 1 photocycloadclitiot~of cyclolzexene and maleic anhydricle The photocycloaddition between cyclohexene 5 and maleic anhydride 6 gives mixtures of diastereomeric bicyclol4.2.01octane-7,8-dicarboxylic anhydrides 7 and can be performed by unsensitized direct irradiation of the charge-transfer complex between 5 and 6 (19) or under sensitization by benzophenone 1 (20). We now describe the use of polymeric benzophenones as sensitizers for this reaction, the results being compared with those obtained when using 1 in the same conditions. In our hands, some by-products to be expected in the reaction sensitized by 1 were bi-2-cyclohexen-1-yl 8, cyclohex-2-enyldiphenylmethanol 9, and 7,7-diphenyl-6-heptenal 10, as well as benzopinacol 11 and benzhydrol 12. Of course, by-products of types 9-12, if formed, could not be released to the solution when using the polymeric sensitizers P-11, P-29, or P-50. Thus, after filtration, the crude reaction mixture contained only adducts 7 and by-product 8, besides starting materials.

Formation of 7 as a function of irradiation time was determined by glc. The results are given in Fig. 1. As seen in this figure, the polymeric sensitizers were less efficient than 1, and since the "concentrated" P-50 gave lower yields of 7 than the "diluted" P-11, steric hindrance is likely to play a major role. We conclude that energy transfer to the bulky charge-transfer complex between 5 and 6 is more sterically hindered when using the more highly functionalized polymeric sensitizer. Other factors being the same, yields of 7 increased when performing these reactions in the presence of acetonitrile as a cosolvent, probably due to the low solubility of maleic anhy-

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BOURDELANDE E T AL

FIG. I . Photocycloaddition of cyclohexene 5 and maleic anhydride 6. Solid lines: yields of adducts 7 upon irradiation of 6 (2 g, 20.4 mmol) and 5 (10 mL) in acetonitrile (20 mL) in the presence of: curve 1, Ph2C0 (273 mg, 1.5 mmol); curve 2 , PhzCO (273 mg, 1.5 mmol) and unfunctionalized polystyrene (2.38 g); curve 3, P-11 (2.7 g, 2.56 mequiv.); curve 4, P-50 (470 mg, 1.5 mequiv.); curve 5, unfunctionalized polystyrene (2.38 g); curve 6, no carbonyl compound; curve 7, P-11 recovered from experiment 3 (curve 3). Dashed lines: yields of adducts 7 upon irradiation of 6 (2 g, 20.4 mmol) in 5 (30 mL) in the presence of: curve 8, PhzCO (273 mg, 1.5 mmol); curve 9, P-11 (2.7 g, 2.56 mequiv.); curve 10, P-50 (470 mg, 1.5 mequiv.); curve 11, no carbonyl compound.

dride in cyclohexene. Formation of the oxidative dimer 8 shows the photooxidizing ability of 1 and its polymeric analogs (see below). The polymeric benzophenones recovered after their use in these sensitized cycloadditions did not show the original ketone carbonyl band in their ir spectra, bands at 1700- 1780 (broad) and 1860 cm-I as well as some associated hydroxyl bands at 3100-3600 cm-' being observed instead. These recovered polymers could not be reused, and their 1780- 1860 cm-' bands, so similar to those of succinic anhydride, were taken as a clear indication of the anchoring of part of the maleic anhydride to the phenyl rings of the polymer units by 12 21 photocycloaddition followed by conventional Diels-Alder cycloaddition, a process known to happen for benzene (21) and soluble polystyrene (22). Attempts to determine the amount of polymer-bound maleic anhydride (by formation of the corresponding p-bromophenacyl ester, followed by Br analysis) failed, since most anhydride survived the treatment, as shown by ir.

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(b) Sensitized E-Z photoisomerization of conjugated dienes The E-Z photoisomerization of (E)- 1,3-pentadiene, (E)-13, sensitized by 1 , is a well-known process (23) characterized by the triplet energies of substrate (E)-13 (247 kJ mol-I), product (2)-13 (238 kJ mol-I), and sensitizer 1 (288 kJ mol-'), and by a photostationary Z/E ratio of 0.82 + 0.04 when using sensitizers having triplet energies well above those of the dienes, as is the case with 1. We have performed this photoisomerization using polymeric benzophenones instead of 1. The Z/E ratio was determined by glc as a function of irradiation time. The photoisomerization of methyl (E)-2,2-dimethyl-3,5-hexadienoate,(E)-14, prepared by known procedures (24), was also examined. The results of both series are given in Fig. 2. As shown in this figure, photostationary states were reached slightly more slowly when using

polymeric benzophenones as sensitizers instead of 1. The polymeric sensitizers could be reused without producing changes in the composition of the photostationary states, although these states were approached at somewhat smaller rates.' One important feature of the polymeric sensitizers was the decrease in the photostationary Z/E ratio which went along with the increase in the degree of functionalization of the polymer, clearly shown in the case of 14. This effect appears to be of steric nature. Indeed, steric hindrance to energy transfer from triplet sensitizer to ground state alkene results in an increased photostationary proportion of the alkene isomer having the higher triplet energy (25). In our case such an effect should favour (E)-13 over (2)-13, according to the known triplet energies of these two isomers, and we assume that analogously (E)-14 would also be favoured over (2)-14 . In the case of 13 this steric hindrance to energy transfer was operative only when using the more concentrated polymeric sensitizer, P-50. On the somewhat bulkier diene 14, even the lightly functionalized polymer P-11 showed this effect. We therefore conclude that increasing the degree of functionalization of the polymeric benzophenones affects the photostationary Z/E ratios via steric hindrance of the energy transfer step. The same effect on the photostationary Z/E ratios can be obtained by means of the use of sensitizers with lower triplet energy, particularly if this energy approaches that of the phantom triplet of the alkene 'A referee points out that light transmittance/reflectance of the polymer beads changes on swelling in aromatic solvents, and that any change in this transmittance/reflectance ratio would affect the absorption of light and therefore the rate of the photochemical reaction. Nevertheless, the only type of reactions performed in benzene were these photoisomerizations and, although some changes could be observed in the rates of these reactions, the major finding was the change in the cornposition of the photostationary states, an effect which cannot be due to changes in transmittance/reflectance ratio.

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BOURDELANDE ET AL.

101 1

TABLEI . Intrapolymerically

sensitized photoisomerization polymer-bound d i e n e s 15"

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Polymer

the presence of triethylamine, giving the desired difunctional polymers 15, which were shown to be devoid of chlorine by elemental analysis. We assumed that the degree of ester functionalization of polymers 15 was the same as the chloromethyl contents of their immediate precursors. In this way, difunctional polymers 15a and 15b were prepared. Both polymers released only the E diene ester (E)-14 when transesterified with refluxing methanolic hydrochloric acid in dioxane (2). This shows the configurational stability of the double bond towards the transesterification conditions. On the other hand, the transesterification was far from complete, as shown by the low (