[acs symposium series] radiation curing of polymeric materials volume 417 || cationic...

Chapter 7

Cationic Photoinitiators

Rearrangement Reactions from Direct Irradiation of Diarylhalonium Salts

Nigel P. Hacker and John L. Dektar

IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099

The solution photochemistry of diaryliodonium, diarylbromonium and diarylchloronium salts has been studied. Direct irradiation of the diphenylhalonium salts yields 2-, 3- and 4-halobiphenyls, halobenzene, benzene, acetanilide, biphenyl and acid. Similarly, irradiation of di-(4-tolyl)-halonium salts gives the respective halobitolyls, 4-halotoluene, 4-methylacetanilide, bitolyls and acid. The halobiphenyls are formed by in-cage fragmentation-recombination reactions, whereas halobenzene and benzene are cage-escape products. Cleavage of the carbon-halogen by both homolytic and heterolytic pathways is implicated by the identification of benzene or toluene and the respective anilides as escape products.

Onium salt photoinitiators are increasingly being used for radiation curing of polymers in electronic applications. 1 3 Irradiation of onium salts in polymeric media generates acid which on further processing can crosslink acid sensitive monomers (e. g. epoxy functional resins), 4 or cleave acid sensitive groups (e. g. poly(p-t-butoxycarbonyloxystyrene)). Previous mechanistic studies on onium salts proposed both homolytic and heterolytic cleavage pathways to account for product formation. 6 For example, irradiation of triphenylsulfonium salts gives diphenylsulfide, biphenyl and substituted benzenes, and homolytic, 7 and heterolytic,8

cleavage of the carbon-sulfur bond have been proposed from photoproduct analysis. We have recently reported that in addition to diphenylsulfide, rearrangement products result from photolysis of triphenylsulfonium salts. 9 This new rearrangement reaction generates acid and rationalizes the observation, by others, that acid formation exceeds diphenylsulfide formation. 1 0 We report here the direct photolysis of diarylhalonium salts and formation of 2-, 3- and 4-halobiaryls by in-cage fragmentation-recombination reactions, in addition to the escape products, haloarenes.

Experimental

2- and 3-Bromobiphenyl were obtained from Columbia Organics. 4-iodobiphenyl was obtained from Eastman Organic Chemicals. 2-, 3- and 4-Chlorobiphenyl, and

0097-6156/90/0417-0082$06.00/0 ο 1990 American Chemical Society

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In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

7. HACKER & DEKTAR Cationic Photoinitiotors 83

2-iodobiphenyl were obtained from Lancaster Synthesis. 4-Iodotoluene was obtained from Alfa. Acetone and acetonitrile were American Burdick and Jackson UV grade, and were used as received. All other chemicals were obtained from Aldrich Chemical Co. Diaryliodonium salts and the rearrangement products were prepared as previously reported. 1 1 The other halonium salts were prepared by the procedure of Olah et. al. 1 1

Capillary GLC analysis was performed on a Hewlett-Packard 5890 chromatograph equipped with a Hewlett-Packard 7673A autosampler and a Hewlett-Packard 3396A integrator. The column used in all analyses was a J & W Scientific DB-1 (cross-linked methyl silicone) 0.4 μ by 0.18 mm by 20 m.

0.01 M Acetonitrile solutions were irradiated in a Rayonet reactor (λ =' 254 nm), for exploratory studies, or with with a 500 W mercury-xenon lamp focussed through a monochromator (λ = 248 +/- 4 nm), for relative quantum yield studies. Two aliquots were analyzed by GLC before irradiation to account for any partial decomposition (usually found to be less than 0.5%). Three 3.00* mL aliquots were placed in Suprasil cuvettes, sealed with a rubber septum, and purged with argon for 8 min immediately prior to irradiation. After irradiation, the samples were transferred to tubes containing 1.00 mL of hexanes containing a small amount of w-tetradecane as internal standard, and 10.00 mL of 0.5 M NaF^PCX*. The tubes were stoppered and thoroughly mixed. After standing for 4 hr, the hexane layer was removed and analyzed by capillary GLC. The integrator was calibrated against similar concentrations of authentic samples of the photoproducts, which were treated to a similar work-up as the photolysis solutions.

Results and Discussion

Exploratory photolysis of 0.01 M acetonitrile solutions of diphenyliodonium triflate gave 2-, 3- and 4-iodobiphenyls, benzene, acetanilide and biphenyl. The iodine-containing photoproducts have strong absorbances compared with the onium salt (Figure 1), and low conversions are necessary to prevent secondary photolysis. The iodonium salt consumption is small and could not be accurately determined by HPLC analysis. However, it is known that onium salts form a complex with cobalt thiocyanate which absorbs at 624 nm. 1 3 Iodonium salt consumption could be accurately determined by mixing the reaction mixtures with an aqueous solution of C0CI2 and NH4CN. and monitoring the disappearance of the cobalt thiocyanate complex (Figure 2). Acid formation was measured by a non-aqueous photometric method using 4-nitrophenoxide indicator. 9 Under these conditions, it was found that the total iodine containing photoproducts were 4.93 χ 10"4 M, the acid formed was 5.5 χ 10"4 M, and that 5.33 χ 10 M of thé iodonium salt was consumed. This excellent agreement for volatile product formation, acid formation, and onium salt consumption, has also been observed upon irradiation of sulfonium salts. 1 4 Similarly, irradiation of diphenylbromonium or diphenylchloronium hexafluorophosphates also yields 2-, 3- and 4-bromobiphenyls or 2-, 3- and 4-chlorobiphenyls, benzene, acetanilide, biphenyl and acid (Figure 3). The relative quantum yields for formation of halogen-containing photoproducts from irradiation of halonium salts in acetonitrile solutions are shown in Table 1. The relative quantum yield (Rel φ) for product formation from iodonium salts is much lower than from chloronium or bromonium salts. Chloronium and bromonium salts are

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84 RADIATION CURING OF POLYMERIC MATERIALS

WAVELENGTH Cn«)

Figure 1: UV Absorption Spectra of Diphenyliodonium Triflate and Photoproducts

CoCl 2 + 2 NH4SCN —> Co(SCN) 2 + 2 N H 4 C I

Co(SCN) 2 + P h 2 I + X ' — » {Ph2I + } 2 Co(SCN) 2 X 2

• 5 Γ

WAVELENGTH (nm)

Figure 2: UV Absorption Spectra Monitoring Ph2I + CF 3 S0 3 " Consumption from Photolysis Mixtures

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7. HACKER & DEKTAR Cationic Photoinitiators 85

known to be less thermally stable than iodonium salts in solution. 1 6 To determine if thermal decomposition was responsible for these differences in Rel φ, solutions of these salts subjected to the same treatment as the irradiated samples but kept in the dark. These solutions showed less than 0.5 % decomposition. Also, substantially more rearrangement products are obtained from irradiation of iodonium salts than chloronium or bromonium salts, and the iodonium salts have an enhanced selectivity for ortho rearrangement products.

Product versus time studies reveal that the rate of biphenyl formation increases faster than the rate of formation of halogen containing products which suggests that biphenyl is a secondary photoproduct. However, while biphenyl is only a trace product from irradiation of the bromonium and chloronium salts, significantly more biphenyl is produced from iodonium salt photolysis. This result suggests that some biphenyl may be formed as a primary photoproduct from iodonium salts. The major reaction pathway for the cage-escape phenyl moieties is reaction with solvent. We have previously shown that triplet sensitization of triphenylsulfonium salts gives 100 % escape products and that the cage-escape phenyl radical reacts with solvent to form benzene. 2 0 The relative yield of benzene (to halobenzene) is 3.5 % for the diphenylchloronium salt, 9 % for the bromonium salt and 20 % for the diphenyliodonium salt. In contrast, cage-escape phenyl cation reacts with the acetonitrile solvent to give acetanilide. Unfortunately, quantification of acetanilide is not precise, leading to errors up to 20%. The relative yield of acetanilide (to halobenzene) is 110 % for the diphenylchloronium salt, 91 % for diphenylbromonium salt, and 78 % for diphenyliodonium salt. However within experimental error, benzene and acetanilide account for all the escape moiety from direct photolysis of diphenylhalonium salts.

The 4,4'-ditoIylhalonium salts were studied to give a better understanding of the escape aryl moieties. Photolysis of solutions of 4,4'-ditolyliodonium, 4,4'-ditolylbromonium, or 4,4'-ditolylchloronium hexafluorophosphate gives 2- and 3- iodobitolyl, 2- and 3-bromobitolyl, or 2- and 3-chlorobitolyl, toluene, 4- methylacetanilide, isomeric bitolyls and acid (Figure 4). Two of the isomeric bitolyls, 3,4'-bitolyl and 2,4'-bitolyl, are secondary photoproducts. While the 3,4'-bitolyl is a trace product from halonium salt photolysis, the 2,4'-bitolyl is not detected at low conversions. However prolonged photolysis of the halonium salt does produce detectable amounts of the 2,4'-bitolyl, as does irradiation of 3-halobitolyl. 4,4'-Bitolyl is a primary photoproduct and can be formed from dimerization of 4-tolyl radical, or by ipso attack of 4-tolyl radical with halotoluene radical cation. As with the diphenylhalonium series, significantly more bitolyls are obtained from photolysis of the iodonium salts than the bromonium or chloronium salts. The cage-escape tolyl moieties, 4-tolyl radical and 4-tolyl cation, react with solvent to yield toluene and 4-methylacetanilide respectively. The relative yield of toluene (to 4-halotoluene) is 6.4 % for the 4,4'-ditolylchloronium salt, 11 % for the 4,4'-ditolylbromonium salt and 38 % for the 4,4'ditolyliodonium salt. Similarly, the relative yield of 4-methylacetanilide (to 4-halotoluene) decreases from 4,4'-ditolylchloronium salt to 4,4'-ditolylbromonium salt and to the 4,4'ditolyliodonium salt.

Diphenylbromonium, diphenylchloronium and 4-chlorodiphenylchloronium hexafluorophosphatcs have previously been evaluated as photoinitiators, but the

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Figure 3: Products from Photolysis of Diphenylhalonium Salts

Figure 4: Products from Photolysis of 4,4'Ditolylhalonium Salts

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Table 1. Product Quantum Yields upon Direct Irradiation of 0.01 M Sait Solutions in Acetonitrile, λ = 248 nm (Ph = Phenyl, 4-Tol = 4-Tolyl)

Compound Rel φ ArX 2-XBP 3-XBP 4-XBP

Ph2Cl + PF 6- 0.96 84 8 5 3

Ph 2Br + PF6" 1.00 81 9 6 4

Ph2I + CF 3 S0 3 - 0.66 74 19 3 4

4-Tol2Cl + PF6* 0.89 85 5 10 -4-Tol2Br+ PF6" 0.90 82 11 6 -4-Tol2I+ PF6" 0.55 71 23 6 -

rearrangement reaction was not observed. 1 6 Earlier studies on diaryliodonium salt photolyses have reported recombination products. Irradiation of 4,4'-di-te>7-butyldiphcnyliodonium tetrafluoroborate and and diphenyliodonium hexafluoroarsenate 1 8 gave only traces of the respective iodobiaryls. However, a more recent study on diphenyliodonium hexafluorophosohate, reported an 18 % yield of 4-iodobiphenyl and 2-iodobiphenyl in a 80:20 ratio. 1 9 Our results here indicate that direct photolysis of diaryliodonium salts gives iodobiaryls in 20 - 30 % yield. In the case of the diphenyliodonium salt we observe all three iodobiphenyl isomers in a ratio of 75:25 for the 2-isomer to the 3- and 4-isomers. This is similar to the results from our studies on triphenylsulfonium, triphenylselenium and triphenyltelluronium salts, where the 2-substituted biphenyl is always the major isomer. 9 ' 1 4 We also see the rearrangement products, the isomeric halobiaryls, in 15-20 % yield from irradiation of the diarylbromonium and diarylchloronium salts. Our results here indicate that direct photolysis of diarylhalonium salts is similar to the photochemstry of triarylsulfonium salts. Acid is produced from halobiaryl formation by rearrangement of the halonium salt to give the cyclohexadienyl cation. Aromatization of the cyclohexadienyl cation gives halobiaryl and acid. The cyclohexadienyl cation can be formed by in-cage recombination from either of the aryl cation - haloarene or aryl radical - haloarene radical cation pairs (Figure 5). These pairs of intermediates can also form acid by the previously reported cage-escape reactions. 1 ~3, 6 ' 1 7 ' 1 8

The escape arene fragments represent evidence for both homolytic and heterolytic cleavage of the carbon-halogen bond. The homolytic pathway gives phenyl radical and halobenzene radical cation from irradiation of the halonium salt. Phenyl radical reacts with solvent to give benzene, whereas dimerization to biphenyl is a minor process. Similarly, toluene is formed from reaction of 4-tolyl radical, the escape arene fragment from homolysis of the 4,4'-ditolylhalonium salts, with solvent. The heterolytic pathway gives aryl cation and haloarene from irradiation of the halonium salt. The aryl cation reacts with solvent to give the respective anilide (acetanilide or 4-methylacetanilide). An anilide were previously reported as a trace photoproduct from irradiation of 4,4'-di-/er/-butyldiphenyliodonium tetrafluoroborate and a minor photosolvolysis pathway was proposed for it's formation. 1 7 Our results indicate that anilides are major primary photoproducts from direct irradiation of diarylhalonium salts and that solvolysis occurs via the phenyl cation. The homolytic cleavage to give aryl radical accounts for a maximum of 40 % of the observed escape products from photolysis of any of the six diarylhalonium salts studied.

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Table 2. Product Distribution from Direct Irradiation of 0.01 M Sulfonium Salt Solutions, λ = 248 nm (Ph = Phenyl, 4-Tol = 4-Tolyl)

Compound Solvent % Ar2S % Rearr.

% ArH % ArR

Ph3S^ CF3SO3" CH 3 CN 40 60 -- 70 Ph3S" CF3S03" CH 3 OH 38 62 - 77 4-Tol3S+ CF 3S0 3- CH 3 CN 32 67 52 72 4-Tol3S + CF3S03" CH 3 OH 32 68 40 45

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide.20

However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. 2 1 Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts.

There are some major differences between the photochemistry of diarylhalonium salts and triarylsulfonium salts (Table 2). Considerably more rearrangement products are detected from irradiation of sulfonium salts. The rearrangement products account for 60 - 70 % of the reaction of sulfonium salts whereas rearrangement is only 10-25 % of the halonium salt reaction. The intermediates produced from heterolysis of the onium salt are halobenzene or diphenylsulfide and phenyl cation. The reaction between phenyl cation and the respective arene to give the rearrangement product can be considered an electrophilic substitution. It is well known that diphenylsulfide is much more reactive towards electrophiles than halobenzene, and so should be more likely give recombination products. The escape phenyl moiety gives products mainly derived from phenyl cation from direct irradiation of the halonium salts salts, whereas phenyl cation accounts for only 50 - 70 % of the escape phenyl moiety from sulfonium salts. This suggests that the phenyl radical pair of intermediates is more stable than the phenyl cation pair for sulfonium salts than for halonium salts. The phenyl cation - halobenzene pair can convert to the phenyl radical pair by an electron transfer (Figure 6). The observation that products derived from both pairs of intermediates are detected, suggests that there may be interconversion between the phenyl cation pair and phenyl radical pair. From the oxidation potentials of the intermediates, phenyl radical, diphenylsulfide, the relative stability of the phenyl cation - diphenylsulfide pair to the phenyl radical -diphenylsulfinyl radical cation pair is estimated to be ΔΕ = -16 kcal / mole with

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HACKER & DEKTAR Cationic Photoinitiators

Figure 5: Acid Generation from Formation of Rearrangement Products

P ^ Z " - ^ - » [Pr^X + Z"]* > PhX- Ph+ Ζ

PhX Ph+ Z" PhX*' Ph" Ζ

PhX Ph+ Z~ - > Ph-PhX + PhX + PhR + HZ

PhX+" Ph* Z" ——> Ph-PhX + PhX + PhH + HZ

Figure 6: Mechanism for Photodecomposition of Diarylhalonium Salts

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the phenyl radical - diphenylsulfinyl radical cation pair more stable. From similar estimates for the halogen series, the phenyl radical - halobenzene radical pair is only 0 - 6 kcal / mole more stable than the phenyl cation - halobenzene pair. Thus it is more likely that the phenyl cation pair of intermediates will participate in the photochemistry of diarylhalonium salts than triarylsulfonium salts.

Conclusions

Direct irradiation of diarylhalonium salts results in formation of 2-, 3- and 4-halobiaryls by an in-cage fragmentation-recombination reaction, in addition to the escape product, haloarene. The cage and escape products are formed from both homolytic and heterolytic fragmentation of the carbon-halogen bond, with the heterolytic process being more dominant. Iodonium salts give more in-cage products and more homolytic cleavage products than bromonium or chloronium salts. The Rel φ is lower for iodonium salt photolysis than bromonium or chloronium photodecomposition.

References

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18. S. P. Pappas, B. C. Pappas, L. R. Gatechair, J. H. Jilek and W. Schnabel, J. Polym. Sci., Polym. Chem. Ed., 1984, 22, 69.

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[acs symposium series] radiation curing of polymeric materials volume 417 || cationic...

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