Photolysis of ((3-(trimethylsilyl)propoxy)phenyl)phenyliodonium salts in the presence of 1-naphthol and 1-methoxynaphthalene

Article abstract of DOI:10.1021/jo000058w

Direct photolysis of ((3-trimethylsilylpropoxy)phenyl)phenyliodonium salts with different counter-anions (Cl^-, SbF₆^-, and B(C₆F₅)₄^-) in methanol leads to products by both heterolytic and homolytic processes. In the presence of 1-naphthol and 1-methoxynaphthalene, products formed by a heterolytic reaction disappear, suggesting an electron-transfer process occurs between excited 1-naphthol/1-methoxynaphthalene and the iodonium salts. In the case of 1-methoxynaphthalene, three phenylated methoxynaphthalene isomers are produced. These are produced as radical coupling products from the phenyl radical and 1-methoxynaphthalene radical cation.

Full text of DOI:10.1021/jo000058w

3484
J . Org. Chem. 2000, 65, 3484-3488
P h otolysis of ((3-(Tr im eth ylsilyl)p r op oxy)p h en yl)p h en yliod on iu m
Sa lts in th e P r esen ce of 1-Na p h th ol a n d 1-Meth oxyn a p h th a len e
Haiyan Gu, Wenqin Zhang, Kesheng Feng, and Douglas C. Neckers*,1
Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43402
Received J anuary 14, 2000
Direct photolysis of ((3-trimethylsilylpropoxy)phenyl)phenyliodonium salts with different counter-
anions (Cl-, SbF6-, and B(C6F5)4-) in methanol leads to products by both heterolytic and homolytic
processes. In the presence of 1-naphthol and 1-methoxynaphthalene, products formed by a
heterolytic reaction disappear, suggesting an electron-transfer process occurs between excited
1-naphthol/1-methoxynaphthalene and the iodonium salts. In the case of 1-methoxynaphthalene,
three phenylated methoxynaphthalene isomers are produced. These are produced as radical coupling
products from the phenyl radical and 1-methoxynaphthalene radical cation.
Sch em e 1
In tr od u ction
In the past two decades, photoinitiated cationic polym-
erization has found a wide range of applications in fields
ranging from UV-curing technologies to the microelec-
tronics industry. Diaryliodonium salts and triarylsulfo-
nium salts, as the most important cationic photoinitia-
tors, have been the subject of significant activity because
of their thermal stability and excellent initiating ef-
ficiency.2
The photochemistry of iodonium salts having nucleo-
philic anions was studied before they were found to be
useful as cationic photoinitiators.3 Since nucleophilic
anions terminate cationic polymerization chains, iodo-
nium salts with such anions cannot be used as initiators.
Photolysis of iodonium salts with nonnucleophilic anions
produces a proton, which is the species that initiates the
polymerization. Early studies suggested a mechanism
involving the photoinduced homolysis of a carbon-iodine
bond, in direct analogy with photolysis of triarylsulfo-
nium salts.4-6
Recent studies have shown that the mechanism of
direct photolysis of iodonium salts is more complex than
originally thought.7,8 It involves heterolytic as well as
homolytic reactions, and the effect of the solvent cage is
considered to be important.9,10 The best current mecha-
nism for the photolysis of diphenyliodonium salts is
shown in Scheme 1.8
Three possible mechanisms have been proposed, namely,
energy transfer, electron transfer, and redox photosen-
sitization. Triplet energy transfer was demonstrated by
using acetone8 and m-trifluoroacetophenone6 as sensitiz-
ers. However, for various reasons not the least of which
is the high triplet energy of most iodonium salts (64 kcal/
mol), it is not practical to use such sensitizers industri-
ally.
In addition to the direct photolysis of diaryliodonium
salts, sensitized photolyses have also been studied.11-18
Electron transfer, in contrast, often occurs.11-15 Sen-
sitized decomposition of iodonium salts in the presence
of anthracene, diethoxyanthracene,7,12 xanthene, ace-
tophenone,13 chlorothioxanthone,14 and thioxanthone de-
rivatives15 have been reported. Steady-state photolysis,
(1) Contribution No. 395 from the Center for Photochemical Sci-
ences.
(2) Crivello, J . V. Latent Developments in the Chemistry of Onium
Salts. In Radiation Curing in Polymer Science and Technology;
Fouassier, J . P., Rabek, J . F., Eds.; Elsevier: London, 1993; Vol. II,
Chapter 8, pp 435-471.
(3) Knapczyk, J . W.; Lubinkowski, J . J .; McEwen, W. E. Tetrahedron
Lett. 1972, 35, 3739.
(4) Crivello, J . V. Adv. Polym. Sci. 1984, 62, 1.
(5) Crivello, J . V.; Lee, J . L.; Colon, D. A. Makromol. Chem.
Macromol. Symp. 1988, 13/ 14, 145.
(6) Pappas, S. P.; Pappas, B. C.; Gatechair L. R. J . Polym. Sci.,
Polym. Chem. Ed. 1984, 22, 69.
(11) (a) Crivello, J . V.; Lam, J . H. W. J . Polym. Sci., Polym. Chem.
Ed. 1978, 16, 2441. (b) Crivello, J . V.; Lam, J . H. W. J . Polym. Sci.,
Polym. Chem. Ed. 1979, 17, 1059.
(12) Nelson, E. W.; Carter, T. P.; Scranton A. B. J . Polym. Sci., Part
A: Polym. Chem. 1995, 33, 247.
(7) Devoe, R. J .; Sahyun, M. R. V.; Serpone, N.; Sharma, D. K. Can.
J . Chem. 1987, 65, 2343.
(8) Devoe, R. J .; Sahyun, M. R. V.; Schmidt, E.; Serpone, N.; Sharma,
D. K. Can. J . Chem. 1988, 66, 319.
(13) Pappas, S. P.; Gatechair, L. R.; J ilek, J . H. J . Polym. Sci., Polym.
Chem. Ed. 1984, 22, 77.
(14) Wang, X. Z.; Wang, E. J .; Fouassier, J . P. Acta Chim. Sin. 1992,
58, 492.
(9) Dektar, J . L.; Hacker, N. P. J . Org. Chem. 1990, 55, 639.
(10) Dektar, J . L.; Hacker, N. P. J . Org. Chem. 1991, 56, 1838.
(15) Fouassier, J . P.; Burr, D.; Crivello, J . V. J . Macromol. Sci., Pure
Appl. Chem. 1994, A31, 677.
10.1021/jo000058w CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/10/2000

Photolysis of Iodonium Salts
Sch em e 2
J . Org. Chem., Vol. 65, No. 11, 2000 3485
homolysis. Homolysis leads to an aryl radical and an
aryliodonium radical cation, which in the solvent cage
recombine to form the iodobiaryls 8a -c. The structures
and the possible route to formation of these three isomers
are shown in Scheme 3. After diffusing outside of the
solvent cage, the aryl radical abstracts a H atom from
solvent forming benzene (1) and 2. Therefore, we assign
these two products as products formed by a homolytic
pathway. In the case of heterolysis, a neutral iodoarene
and an aryl cation are formed. The latter is expected to
be trapped by methanol to form anisole (3) and 1-meth-
oxy-4-(3-(trimethylsilyl)propoxy)benzene (4), or by Cl- to
form 9 in the case of chloride salt. We assign these pro-
ducts as those formed by the heterolytic pathway (Scheme
4). How the three isomeric biaryls 7 form is still unclear.
In principle, two possibilities exist. One is electrophilic
substitution of the aryl cation on the arene. The other is
radical substitution of the aryl radical on the arene. Since
neither biphenyl nor 4,4′-(3-(trimethyl)silylpropoxy)bi-
phenyl was detected, out-cage radical coupling is ex-
cluded. Also in methanol, the possibility of electrophilic
substitution is rare. So radical substitution is considered
the route to formation of these products. The para isomer
dominates among these three isomers, and the ratio of
ortho plus meta to para is about 1:10. The structure of
the anion has little effect on the distribution of photo-
products (Table 1). This is consistent with what was
found in the case of diphenyliodonium salts.8
Irradiation of the target iodonium salts in the presence
of 1-naphthol gave just four products. The products
formed by heterolytic reactions and via in-cage recom-
bination disappeared. We attribute this observation to
an electron transfer between the excited naphthol mol-
ecules and the diaryliodonium salts (Scheme 5).
Electron transfer produces a naphthol radical cation
and diaryliodine radical. The latter decomposes at a
diffusion controlled rate to an iodoarene and the aryl
radical so no heterolytic products or in-cage recombina-
tion products are formed. Although a decrease in the
concentration of 1-naphthol is obvious, no naphthol-
derived photoproducts that could be attributed to cou-
pling of 1-naphthol radical cation and phenyl radical
could be detected. Our explanation is that 1-naphthol
radical cation easily loses a proton forming the naphthoxy
radical. Since naphthoxy radicals often form higher
molecular weight materials, these would not be detected
under our conditions by GC/MS.
laser flash photolysis,6,7 and photo-CIDNP (chemically
induced dynamic nuclear polarization) techniques16 have
been employed to study the details of the mechanism of
the sensitized photolysis of iodonium salts.
Chemical sources of free radicals can also promote
cationic polymerization.16-18 Ledwith and his research
group worked extensively in this area, using benzoin
ether and R-hydroxyacetophenone as radical sources.17
Sundell and co-workers studied many common radicals
and found that molecules containing R-ether radicals or
R-hydroxy radicals can reduce iodonium salts.18 Bi and
Neckers also demonstrated that systems consisting of a
xanthene dye, an aromatic amine, and a diaryliodonium
salt could function as a visible light initiating system for
free radical promoted cationic polymerization.19
In this paper, we report studies of the photolyses of
the title iodonium salts in the presence and absence of
the sensitizers, 1-naphthol and 1-methoxynaphthalene.
It is known that the dihydroxynaphthalene can promote
cationic polymerization as initiated by iodonium salts,20
and we were interested in the interaction between the
iodonium salts and the hydroxynaphthalenes. Product
analysis suggests electron transfer between the excited
1-naphthol/1-methoxynaphthalene molecule and iodo-
nium salt. Diaryliodonium salts containing silane groups
have been shown to increase iodonium salt solubility in
nonpolar solvents.21 Here we used these compounds as
unsymmetrical iodonium salts to get more mechanistic
information about the cleavage pathways.
Resu lts a n d Discu ssion
To provide further evidence for this hypothesis, the
photolysis of these compounds in the presence of 1-meth-
oxynaphthalene was carried out (Table 1). Under these
conditions, products formed by a heterolytic reaction as
well as the iodobiaryls 8 disappear. However, some para
biaryl 7 remains among the photoproducts although the
ortho and meta isomers are not formed. In this case, we
propose the para isomer arises from phenyl radical
substitution on the iodoarene which releases an iodine
atom. In addition to these products, three other new
peaks were observed by GC/MS. These products are
isomers that correspond to an analysis of C17H14O. From
the fragmentaion patterns (219, 191 and 165) we assign
these products to be the isomers 2, 4, 5-phenyl-1-
methoxynaphthalene. A possible mechanism for forma-
tion of these isomers is shown in Scheme 6. The 1-meth-
oxynaphthalene radical cation couples with a phenyl
radical releasing a proton. Since the methyl group is not
a good leaving group in radical displacement processes,
Direct irradiation of the title compound with various
counteranions (Cl-, SbF6-, and B(C6F5)4-) in methanol
gave 13 products in the case of Cl- and 12 products in
the case of the other compounds (Scheme 2). The products
(Table 1) were detected by GC/MS, and the resulting
spectra were compared with those of authentic samples
excluding the iodobiaryl cases.
In Scheme 1 it is shown that, after light absorption,
the diaryliodonium salts undergo both heterolysis and
(16) Goez, M.; Ecert, G.; Muller, U. J . Phys. Chem. A 1999, 103,
5714.
(17) (a) Ledwith, A. Makromol. Chem. Suppl. 1978, 3, 348. (b) Abdul-
Rasoul, F. A. M.; Ledwith, A.; Yagci, Y. Polymer 1978, 19, 1219. (c)
Abdul-Rasoul, F. A. M.; Ledwith, A. Polym. Bull. 1978, 1, 1.
(18) Sundell, P. E.; J onsson, S.; Hult, A. J . Polym. Sci., Part A:
Polym. Chem. 1991, 29, 1525.
(19) Bi, Y.; Neckers, D. C. Macromolecules 1994, 27, 3683.
(20) Feng, K. S.; Neckers, D. C. U.S. Patent applied for.
(21) Ger. Offen. DE 4, 4142, 3237, 1993.

3486 J . Org. Chem., Vol. 65, No. 11, 2000
Ta ble 1. Con cen tr a tion sa a n d Distr ibu tion s (%)b of P h otop r od u cts
Gu et al.
X-
EDf
1
2
3
4
9
5
6
7
8c
Cl
Cl
Cl
SbF6
SbF6
none
NAd
MNe
none
NA
MN
none
NA
14a (13)b
15 (22)
15.8 (27)
5.2 (12)
15 (28)
11.3 (30)
6.6 (14)
20 (22)
8.7 (8)
15 (22)
8.9 (15)
4.8 (11)
99 (18)
5.5 (15)
6 (13)
2.8 (3)
3.5 (3)
5.7 (5)
19 (18)
14 (21)
12.6 (22)
8.2 (18)
10 (19)
6.8 (18)
9.4 (20)
11 (12)
15.4 (23)
24 (23)
24 (35)
20.0 (34)
11 (25)
19 (35)
13.2 (35)
13 (29)
50 (54)
21.2 (32)
13 (12)
15 (14)
1 (2)
5.2 (12)
1.2 (2)
2.2 (5)
2.2 (5)
6.4 (14)
3.0 (6)
SbF6
0.6 (2)
5.8 (13)
B(C6F5)4
B(C6F5)4
B(C6F5)4
12 (13)
10.7 (16)
MN
18.7 (28)
0.9 (1)
a
b
Concentrations were obtained from GC data using dodecane as internal standard. Distributions were calculated from the sum of
the concentrations listed. In the case of MN, new products were not included. c Concentration of 8 was estimated using the same coefficients
as for 7. NA is 1-naphthol. The ratio of iodonium salt to NA is 1:5. e MN is 1-methoxynaphthalene. The ratio of iodonium salt to MN is
d
1:5. f Electron donor.
Sch em e 3
Sch em e 4
Gen er a l P r oced u r es for Ir r a d ia tion a n d P r od u ct De-
tection A 4 mL vial containing a dilute (∼0.01 M) solution of
the iodonium salts with or without electron donors (∼0.05 M)
was degassed with dry argon for ca. 10 min. It was then
irradiated for 5 min at ambient temperature in Rayonet PRP-
100 photoreactor equipped with 15 300 nm GEF8T5/BLB
lamps. After photolysis, 5 mL of pentane containing dodecane
as internal standard was added to extract the photoproducts
while 5 mL of water was added. GC/MS was performed to
identify the products, and GC was used to quantify the
concentrations and distributions of the photoproducts.
most of the 1-methoxynaphthalene radical cation is
returned to 1-methoxynaphthalene by reverse electron
transfer. Consequently, the concentration of 1-methoxy-
naphthalene does not change significantly (<5%).
Exp er im en ta l Section
Gen er a l In for m a tion . GC measurements were carried out
on a Hewlett-Packard 5890 instrument (FID detector), with a
30 m J &W DB-1 capillary column. GC/MS spectra were
determined at an ionizing voltage of 70 eV. Elemental analyses
were obtained from Atlantic Microlab Inc. HRMS were ob-
tained from Mass Spectrometry Laboratory of the University
of Illinois at Urbana-Champaign. (3-Chloropropyl)trimethyl-
silane was purchased from Gelest Inc. All other reagents were
purchased from Aldrich Chemical Co. and used as received,
unless otherwise noted.
3-(Tr im eth ylsilyl)p r op yl P h en yl Eth er (I). The title
compound was synthesized from (3-chloropropyl)trimethylsi-
lane and sodium phenolate trihydrate using typical Williamson
ether reaction conditions. After 4 h, water was added to the
reaction mixture and three portions of hexane were used to
extract the product. NaOH solution (0.1 N) and saturated NaCl

Photolysis of Iodonium Salts
J . Org. Chem., Vol. 65, No. 11, 2000 3487
Sch em e 5
Sch em e 6
solution were used to wash the organic layers several times.
A pale yellow liquid was obtained in 94% yield after evapora-
tion of the hexane. The compound was characterized by 1H
NMR and HR-MS. 1H NMR (CDCl3, 200 Hz): δ 7.24-7.32 (m,
2H), 6.87-6.93 (m, 3H), 3.91 (t, 2H, J ) 7.0 Hz), 0.57-0.66
(m, 2H), -0.02 to 0.06 (m, 9H). HR-MS: calcd for m/z 208.1284,
found m/z 208.1288.
(3-(Tr im et h ylsilyl)p r op oxyp h en yl)p h en yliod on iu m
Ch lor id e (II). The chloride salt was synthesized from I and
iodobenzene diacetate by following a literature procedure.3 The
crude product was purified by recrystallization from acetoni-
trile to obtain white crystals, mp 169-170 °C. 1H NMR (CDCl3,
200 MHz): δ 7.85-7.98 (m, 4H), 7.21-7.41 (m, 3H), 6.74 (d,
2H, J ) 8.8 Hz), 3.84 (t, 3H, J ) 6.6 Hz), 1.64-1.79 (m, 2H),
0.49-0.57 (m, 2H), -0.07 to 0.06 (m, 9H). 13C NMR (CDCl3,
50 MHz): δ 161.1 (C), 136.7 (CH), 134.2 (CH), 130.9 (CH),
130.4 (CH), 120.6 (C), 117.3 (CH), 108.8 (C), 70.8 (CH2), 23.5
(CH2), 12.4 (CH2), -1.8 (CH3). Anal. Calcd: C, 48.40; H, 5.42.
Found: C, 48.19; H, 5.40.
Fouassier.22 The19F NMR spectrum was consistent with that
found in the literature.
(3-(Tr im eth ylsilyl)pr opoxyph en yl)ph en yliodon iu m Tet-
r a k is(p en ta flu or op h en yl)bor a te. The title compound was
obtained from metathesis of the chloride salt II and KB(C6F5)4
in CH2Cl2. The crude product was purified by filtering through
a thin layer of neutral alumina to produce a solid, mp 48-51
1
°C. H NMR (CDCl3, 200 MHz): δ 7.67-7.83 (m, 5H), 7.47-
7.56 (m, 2H), 6.98 (d, J ) 9.6 Hz), 3.96 (t, 2H, J ) 6.6 Hz),
1.65-1.83 (m, 2H), 0.53-0.62 (m, 2H), -0.05 to 0.06 (m, 9H).
13C NMR (CDCl3, 50 MHz): δ 164.1 (C), 150.5 (d, C, J ) 23.9
Hz), 140.5 (d, C, J ) 23.5 Hz), 138.6 (C), 137.5 (CH), 133.9
(CH), 133.8 (CH), 133.5 (CH), 119.8 (CH), 112.1 (C), 97.3 (C),
71.7 (CH2), 23.4 (CH2), 12.3 (CH2), -2.0 (CH3). 19F NMR
(CDCl3): δ -132.8 (sm, 2F), -162.8(m, 1F), -166.7 (sm, 2F).
Anal. Calcd: C, 46.26; H, 2.22. Found: C, 46.24; H, 2.24.
3-(Tr im eth ylsilyl)p r op yl 4-Ch lor op h en yl Eth er . The
title compound was synthesized from 4-chlorophenol and (3-
chloropropyl)trimethylsilane using typical Williamson ether
reaction conditions. The product was isolated and purified as
above (93%), and then characterized by 1H NMR and 13C NMR.
1H NMR (CDCl3, 200 MHz): δ 7.15-7.21 (m, 2H), 6.74-6.74
(m, 2H), 3.84 (t, 2H, J ) 7.0 Hz), 1.67-1.82 (m, 2H), 0.52-
0.61 (m, 2H), -0.05 to 0.02 (m, 9H). 13C NMR (CDCl3, 50
MHz): δ 157.7 (CH), 129.2 (C), 125.2 (CH), 115.7 (C), 70.9
(CH2), 23.8 (CH2), 12.5 (CH2), -1.8 (CH3). HR-MS: calcd for
m/e 242.0895, found m/e 242.0893.
(3-(Trimethylsilyl)propoxyphenyl)phenyliodonium Hexa-
flu or oa n tim on ia te. The title compound was synthesized from
the chloride salt II and NaSbF6 by following a modified
procedure as described by Crivello.4 Instead of filteration of
the precipitate after the addition of water, methylene chloride
was used to extract the product. After rotatory evaporation, a
1
waxy solid was obtained, mp 93-95 °C. H NMR (CDCl3, 200
MHz): δ 7.95-7.99 (m, 4H), 7.41-7.64 (m, 3H), 6.93 (d, J )
9.2 Hz), 3.91 (t, 2H, J ) 6.6 Hz), 1.67-1.83 (m, 2H), 0.52-
0.61 (m, 2H), -0.06 to 0.06 (m, 9H). 13C NMR (CDCl3, 50
MHz): δ 163.3 (C), 137.9 (CH), 134.7 (CH), 133.0 (CH), 132.7
(CH), 119.1 (CH), 112.7 (C), 98.8 (C), 71.4 (CH2), 23.3 (CH2),
12.4 (CH2), -1.8 (CH3). Anal. Calcd: C, 33.41; H, 3.74.
Found: C, 33.60; H, 3.73.
3-(Tr im eth ylsilyl)p r op yl 4-Iod op h en yl) Eth er . The title
compound was synthesized from 4-iodophenol and (3-chloro-
propyl)trimethylsilane using typical Williamson ether reaction
conditions and purified as mentioned above. The yield was
91%. The product was characterized by 1H NMR, 13C NMR,
P ota ssiu m Tetr a k is(p en ta flu or op h en yl)bor a te. This
compound was obtained following a procedure described by
(22) Castellanos, F.; Fouassier, J . P.; Priou, C.; Cavezzan, J . J . Appl.
Polym. Sci. 1996, 60, 705.

3488 J . Org. Chem., Vol. 65, No. 11, 2000
Gu et al.
and HR-MS. 1H NMR (CDCl3, 200 MHz): δ 7.49-7.54 (m, 2H),
6.61-6.67 (m, 2H), 3.85 (t, 2H, J ) 6.6 Hz), 1.67-1.83 (m, 2H),
0.52-0.61 (m, 2H), -0.02-0.06 (m, 9H). 13C NMR (CDCl3, 50
MHz): δ 158.9 (CH), 138.1 (C), 116.8 (C), 82.4 (CH), 70.7 (CH2),
23.7 (CH2), 12.5 (CH2), -1.8 (CH3). HR-MS: calcd for m/z
334.0250, found m/z 334.0243.
3-(Tr im eth ylsilyl)p r op yl 4-P h en ylp h en yl Eth er . The
title compound was synthesized from 4-phenylphenol and (3-
chloropropyl)trimethylsilane using typical Williamson ether
reaction conditions. A white waxy solid, mp 44-45 °C, was
obtained (92%). The product was characterized by 1H NMR
and 13C NMR. 1H NMR (CDCl3, 200 MHz): δ 7.22-7.54 (m,
7H), 6.89-6.97 (m, 2H), 3.92 (t, 2H, J ) 6.6 Hz), 1.70-1.86
(m, 2H), 0.54-0.63 (m, 2H), -0.05 to 0.06 (m, 9H). 13C NMR
(CDCl3, 50 MHz): δ 158.6 (C), 140.8 (C), 133.5 (C), 128.7 (CH),
128.1 (CH), 126.7 (CH), 126.6 (CH), 70.7 (CH2), 23.9 (CH2),
12.6 (CH2), -1.7 (CH3). HR-MS: Calcd for m/z 284.159 64,
found m/z 284.159 65.
Ack n ow led gm en t. The authors gratefully acknowl-
edge financial support from the United Soybean Board
(Project No. 9412). We also thank Dr. J ohn Malpert for
helpful discussions. One of us (H.G.) is a Fellow of the
McMaster Endowment.
J O000058W