Preparation, X-ray Structure, and Reactivity of Triisopropylsilyl-Substituted Aryliodonium Salts

Article abstract of DOI:10.1002/ejoc.201500535

(4-Triisopropylsilylphenyl)phenyliodonium tosylate, an aryliodonium salt bearing an extremely bulky, electron-donating triisopropylsilyl (TIPS) substituent in the phenyl ring, was prepared by the reaction of (4-tributyltinphenyl)triisopropylsilane with [h

Full text of DOI:10.1002/ejoc.201500535

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500535
Preparation, X-ray Structure, and Reactivity of Triisopropylsilyl-Substituted
Aryliodonium Salts
Mekhman S. Yusubov,*[a,b] Dmitrii Yu. Svitich,[a] Akira Yoshimura,[c] Brent J. Kastern,[c]
Victor N. Nemykin,[c] and Viktor V. Zhdankin*[c]
Keywords: Hypervalent compounds / Iodine / Silicon / Steric hindrance
(4-Triisopropylsilylphenyl)phenyliodonium tosylate, an aryl-
iodonium salt bearing an extremely bulky, electron-donating
triisopropylsilyl (TIPS) substituent in the phenyl ring, was
prepared by the reaction of (4-tributyltinphenyl)triisoprop-
ylsilane with [hydroxy(tosyloxy)iodo]benzene (Koser’s rea-
gent). The TIPS-substituted aryliodonium tosylate was fur-
gle-crystal X-ray diffraction. Reactions of the TIPS-substi-
tuted arylodonium tosylate with bromide, azide, and thio-
cyanate anions predominantly afforded products of nucleo-
philic substitution in the electron-rich aromatic ring bearing
the TIPS substituent. This unusual result is explained by the
steric effect of the extraordinary bulky para-TIPS substituent
ther converted into (4-triisopropylsilylphenyl)phenyliodon- on the configuration of the reaction intermediate.
ium bromide, the structure of which was established by sin-
used for the generation of radioactive [18F]-radiotracers in
Introduction
positron emission tomography.[2] This application of alkyl-
In recent years, organohypervalent iodine compounds
have attracted significant research activity as versatile and
environmentally benign reagents for organic synthesis.[1]
Aryliodonium salts, Ar2I+X–, belong to an important class
of organohypervalent iodine(III) derivatives with useful re-
activity and broad practical applications in polymer chemis-
try,[1a] biological studies,[1a] and positron emission tomogra-
phy.[2] From a practical viewpoint, particularly important
are the applications of aryliodonium salts as cationic poly-
merization photoinitiators belonging to an important class
of photoacid generators.[3] It has been demonstrated that
alkyl-substituted diphenyliodonium cations bearing a bulky
substituent in the para position, such as the bis(4-tert-but-
ylphenyl)iodonium and 4-cumenyl-4Ј-tolyliodonium salts,
have higher photoacid generation efficiency than the unsub-
stituted diphenyliodonium salts.[4] Alkyl-substituted aryl-
iodonium salts as photoinitiators in general have the advan-
tages of improved solubility, high photocuring ability, and
low toxicity.[5]
and alkoxyaryliodonium salts is possible because of the
higher regioselectivity observed in the reactions of these
substrates with nucleophilic fluoride. In the reaction of un-
symmetrical diaryliodonium salts with nucleophiles, the
identity of the reductively eliminated aryl iodide is typically
dictated by electronic effects and steric factors; in particu-
lar, electron-rich aryl iodides and functionalized electron-
poor aromatic compounds are the main products. The pres-
ence of an electron-donating substituent in the para posi-
tion of an aryliodonium salt is critically important in these
applications.
Considering the importance of aryliodonium salts bear-
ing a bulky, electron-donating substituent in the phenyl
ring, we decided to synthesize triisopropylsilyl (TIPS)-sub-
stituted iodonium salts and to investigate the effect of the
TIPS group on the structure and reactivity of these com-
pounds. It is well known that the electron-rich TIPS group
has extraordinary bulk, which provides steric screening far
beyond the atom to which TIPS is attached, and in ad-
dition, TIPS is a chemically stable group that can be readily
introduced in the aromatic ring by the reaction of a lithiated
aromatic precursor with triisopropylsilyl chloride or tri-
flate.[6] In this paper, we describe the preparation of (4-tri-
isopropylsilylphenyl)phenyliodonium salts and nucleophilic
substitution reactions of the tosylate salt with bromide, az-
ide, and thiocyanate anions.
Likewise, para-substituted alkyl- and alkoxyaryliodo-
nium salts are also important reagents that are commonly
[a] Tomsk Polytechnic University,
634050 Tomsk, Russia
E-mail: yusubov@mail.ru
[b] Siberian State Medical University,
634050 Tomsk, Russia
[c] Department of Chemistry and Biochemistry, University of
Minnesota Duluth,
Duluth, MN 55812, USA
E-mail: vzhdanki@d.umn.edu
Results and Discussion
http://www.d.umn.edu/~vzhdanki/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500535.
We investigated several synthetic approaches to the target
(4-triisopropylsilylphenyl)phenyliodonium salts. The first
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M. S. Yusubov, V. V. Zhdankin et al.
SHORT COMMUNICATION
approach was based on the reaction of triisopropylsil-
We found that the reaction of arylstannane 2 with
ylbenzene (PhTIPS) with electrophilic hypervalent iodine [hydroxy(tosyloxy)iodo]arenes (Koser’s reagent) slowly pro-
reagents such as hydroxy(tosyloxy)iodobenzene [PhI(OH)- ceeded at room temperature under an atmosphere of argon
OTs, Koser’s reagent or HTIB], PhIO·Tf2O (Zefirov’s rea- to afford target iodonium tosylate 3 (Scheme 2). Iodonium
gent), PhIO·BF3, PhI(OAc)2/TMSOTf, and other highly salt 3 was isolated in 33% yield as a stable, white solid that
electrophilic activated iodosylarene species.[7] Unfortu- possesses excellent solubility in dichloromethane. The low
nately, all these attempts were unsuccessful and resulted yield is in agreement with typical yields of other aryliodo-
either in the isolation of unchanged PhTIPS or in the for- nium tosylates obtained by this method.[9] The structure of
mation of a black tar. The low reactivity of PhTIPS in these product 3 was confirmed by NMR spectroscopy and high-
reactions is probably related to the extraordinary bulk of resolution ESI mass spectrometry.
the TIPS group, which blocks the phenyl ring from electro-
philic attack.
As the next step, we decided to investigate (4-iodophen-
yl)triisopropylsilane (1) as a key precursor to the target
iodonium salts. Owing to the low reactivity of PhTIPS,
compound 1 could not be prepared by direct oxidative
iodination of the aromatic ring. However, we were able to
synthesize iodide 1 starting from commercially available
1,4-diiodobenzene by iodine–lithium exchange followed by
treatment with triisopropylsilyl triflate (TIPSOTf)
(Scheme 1). Notably, this reaction is extremely sensitive to
nium salts.
moist air and should be performed under an atmosphere of
Scheme 2. Preparation of (4-triisopropylsilylphenyl)phenyliodo-
Iodonium tosylate 3 was further converted into (4-triiso-
propylsilylphenyl)phenyliodonium bromide (4, 34% yield)
by treatment with KBr in boiling acetonitrile for 2 min
(Scheme 2). Heating the mixture for a longer time should be
avoided, because it leads to decomposition of the product.
Iodonium bromide 4 was isolated in the form of a thermally
stable, white, microcrystalline solid. The structure of prod-
uct 4 was established by single-crystal X-ray crystallography
(Figure 1).
argon under absolutely dry conditions. In the presence of
moist air, this reaction gives the respective silyloxy deriva-
tive, 4-IC6H4OTIPS, as a major byproduct. (4-Iodophenyl)-
triisopropylsilane (1) was further converted into (4-tributyl-
tinphenyl)triisopropylsilane (2, Scheme 1), which is the im-
mediate precursor to the target iodonium salt. Stannylation
of iodoarene 1 was performed by a modified Stille coupling
reaction by using hexabutylditin and tetrakis(triphenyl-
phosphine)palladium(0) as the catalyst. This reaction was
performed according to a previously reported procedure
with little modification.[8] Arylstannane 2 was isolated in
the form of a relatively stable clear oil and was identified by
NMR spectroscopy and high-resolution mass spectrometry.
Figure 1. Perspective view of (4-triisopropylsilylphenyl)phenyl-
iodonium bromide (4) with 50% ellipsoid probability. Selected dis-
tances [Å] and angles [°]: I1–C1 2.124(5), I1–C7 2.132(4), I1–Br1
3.238(5), I1–Br1(A) 3.196(5), C1–I1–C7 89.60(17), Br1–I1–Br1(A)
103.27(15), Br1–I1–C1–C6 76.19(12), Br1(A)–I1–C7–C8 87.19(15).
Scheme 1. Synthesis of precursors to (4-triisopropylsilylphenyl)-
phenyliodonium salts.
The hypervalent iodine ion in 4 was found in pseudo-
The most common modern synthetic approach to substi- square-planar coordination established by two covalent I–
tuted aryl(phenyl)iodonium salts utilizes a very mild reac- C bonds and two close contacts with bromide anions. Both
tion of arylstannanes with [hydroxy(tosyloxy)iodo]arenes.[9] I–C bonds are within the typical range for IIII compounds,
In principle, the target (4-triisopropylsilylphenyl)phenyl- whereas the C–I–C bond angle is close to an expected IIII
iodonium tosylate can be obtained by using two possible T-shaped geometry (Figure 1). The I–Br contacts have dis-
combinations of reactants: PhSnBu3 + ArI(OH)OTs or tances smaller than the sum of the van der Waals radii of
ArSnBu3 + PhI(OH)OTs. Our attempts to oxidize aryl iod- iodine and bromine. Each bromide anion forms two close
ide 1 to the respective hypervalent iodine reagents 4-TIP- contacts with two cations of 4. Such Br–I–(Br–I)n contacts
SC6H4I(OAc)2 and 4-TIPSC6H4I(OH)OTs were unsuccess- are oriented along the crystallographic a axis and are re-
ful, and therefore, we concentrated our efforts on the sec- sponsible for the observed polymeric structure of 4. The
ond approach.
Br–I–Br angle is slightly larger than the expected 90° angle
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Triisopropylsilyl Substituted Aryliodonium Salts
for pseudo-square-planar geometry around an iodine cen- in ligand coupling with the nucleophile, and this results in
ter. Both aromatic rings in structure of 4 are almost perpen- TIPS-substituted products 5–7.[12]
dicular to the 2C–I–2Br plane. The silicon atom in 4 has
expected tetrahedral configuration and typical Si–C bond
lengths.
Considering the practical importance of nucleophilic
Conclusions
In summary, we prepared (4-triisopropylsilylphenyl)-
phenyliodonium tosylate, an aryliodonium salt bearing an
extremely bulky, electron-donating triisopropylsilyl (TIPS)
substituent in the phenyl ring. The TIPS-substituted aryl-
odonium tosylate was further converted into (4-triisoprop-
ylsilylphenyl)phenyliodonium bromide, the structure of
which was established by single-crystal X-ray diffraction.
Reactions of the TIPS-substituted arylodonium tosylate
with bromide, azide, and thiocyanate anions predominantly
afforded products of nucleophilic substitution in the elec-
tron-rich aromatic ring bearing the TIPS substituent. This
unusual result can be explained by steric effects of the ex-
traordinary bulky para-TIPS substituent on the configura-
tion of the reaction intermediate.
substitution reactions of aryliodonium salts in synthetic or-
ganic chemistry, we investigated reactions of tosylate salt 3
with several common nucleophiles. These reactions should
proceed in agreement with the general regioselectivity
pattern of reactions of nonsymmetrical iodonium salts with
nucleophiles, in which substitution occurs predominantly in
the more electron-deficient aromatic ring.[10]
The results of the reactions of tosylate salt 3 with brom-
ide, azide, and thiocyanate anions are summarized in
Scheme 3. The reactions were performed by heating iodo-
nium salt 3 with the nucleophiles in acetonitrile or aqueous
acetonitrile under reflux conditions. The reactions were
monitored by NMR spectroscopy, and upon completion of
the reactions, the organic products were separated from the
inorganic salts and dried in vacuo for 10 h, which resulted
in complete removal of the volatile monosubstituted benz-
enes from the mixtures to leave only nonvolatile TIPS-sub-
stituted products. The ratio of TIPS-substituted reaction
Experimental Section
(4-Triisopropylsilylphenyl)phenyliodonium Tosylate (3): (4-Tributyl-
tinphenyl)triisopropylsilane (2; 105 mg, 0.2 mmol), PhI(OTs)OH
(78.4 mg, 0.2 mmol), and CH2Cl2 (3 mL) were placed in a flask,
and the flask was purged with argon and then stirred for 48 h. The
mixture was then evaporated to remove the solvent, which left a
white solid. Diethyl ether (5 mL) was then added to the flask, and
the resulting white suspension was stirred for 8 h. The precipitate
was filtered to afford tosylate 3 as a white solid, yield 40 mg (33%);
1
products was determined by H NMR spectroscopy.
1
m.p. 169–170 °C. H NMR (500 MHz, CDCl3): δ = 7.98–7.96 (m,
2 H), 7.88–7.86 (m, 2 H), 7.67–7.66 (m, 2 H), 7.61 (t, J = 8.5 Hz,
1 H), 7.52–7.51 (m, 2 H), 7.45 (t, J = 7.5 Hz, 2 H), 7.10–7.09 (m,
2 H), 2.32 (s, 3 H), 1.37 (sept, J = 7.0 Hz, 3 H), 1.05–1.03 (m, 18
H) ppm. 13C NMR (125 MHz, DMSO): δ = 140.3, 138.4, 138.0,
135.9, 134.4, 132.7, 132.3, 128.5, 126.0, 117.8, 116.4, 21.3, 18.7,
10.4 ppm. HRMS (ESI): calcd. for C21H30ISi [M – OTs]+ 437.1161;
found 437.1159.
(4-Triisopropylsilylphenyl)phenyliodonium Bromide (4): A mixture
of (4-triisopropylsilylphenyl)phenyliodonium tosylate (3; 14 mg,
0.023 mmol), CH3CN (1.2 mL), H2O (0.1 mL), and KBr (13 mg,
0.109 mmol) was heated at reflux (85 °C) for 2 min until a clear
solution was formed. The resulting solution was cooled slowly to
room temperature, which resulted in precipitation of colorless crys-
tals. The mixture was then placed in a refrigerator overnight. The
precipitate was filtered, washed with Et2O, and dried to afford
bromide 4 (4 mg, 34%); m.p. 152 °C. 1H NMR (500 MHz, CDCl3):
δ = 7.98 (d, J = 8.5 Hz, 2 H), 7.90 (d, J = 8.5 Hz, 2 H), 7.55 (t, J
= 7.5 Hz, 1 H), 7.48 (d, J = 8.5 Hz, 2 H), 7.40 (t, J = 7.5 Hz, 2 H),
1.36 (sept, J = 7.0 Hz, 3 H), 1.03 (d, J = 7.5 Hz, 18 H) ppm. 13C
NMR (125 MHz, CDCl3): δ = 140.3, 138.1, 134.8, 132.9, 131.7,
131.4, 120.8, 120.1, 18.5, 10.7 ppm. HRMS (ESI): calcd. for
C21H30ISi [M – Br]+ 437.1161; found 437.1163.
Scheme 3. Reactions of (4-triisopropylsilylphenyl)phenyliodonium
tosylate with nucleophiles.
To our surprise, reactions with each nucleophile pre-
dominantly afforded products 5–7 resulting from nucleo-
philic substitution in the electron-rich aromatic ring bearing
the TIPS substituent (Scheme 3). This unexpected result
can probably be explained by steric effects of the para-TIPS
substituent, by analogy with the so-called “ortho ef-
fect”.[11,12] It was previously observed that the ortho-methyl-
substituted aromatic ring in a nonsymmetrical diaryliodo-
nium salt shows enhanced reactivity toward nucleophiles,
which is explained by the effect of steric factors on the con-
figuration of the reaction intermediate. In a similar fashion,
the presence of an extremely bulky TIPS group forces the
Ar group to stay predominantly in the equatorial position
X-ray crystallography: Single crystals of (4-triisopropylsilylphenyl)-
phenyliodonium bromide (4) suitable for X-ray crystallographic
analysis were obtained by slow crystallization from a dichlorometh-
ane/hexane solution. X-ray diffraction data of (4-triisopropylsil-
ylphenyl)phenyliodonium bromide were collected with a Rigaku
RAPID II Image Plate system by using graphite-monochromated
of the trigonal bipyramidal intermediate and to participate Mo-Kα radiation (λ = 0.71073 Å) at 123 K. The structure was
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M. S. Yusubov, V. V. Zhdankin et al.
SHORT COMMUNICATION
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full-matrix least-squares refinement on F2 by using the Crystals
for Windows program. Crystal data for (4-triisopropylsilylphenyl)
[2]
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phenyliodonium bromide: C21H30BrISi,
M = 517.36, a =
J. V. Crivello, K. Dietliker, Photoinitiators for Free Radical Cat-
ionic & Anionic Photopolymerisation, Wiley, Chichester, UK,
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9.07770(10) Å, b = 14.3030(2) Å, c = 35.499(3) Å, α = 90.00°, β =
90.00°, γ = 90.00°, V = 4609.1(3), Rf = 4.54.
CCDC-1061829 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
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Acknowledgments
This work was supported by a research grant from the National
Science Foundation (NSF), U.S.A. (CHE-1262479) and by a NSF
instrumentation grant (MRI-0922366). M. S. Y. and D. Y. S. are
also thankful to the Ministry of Education and Science of Russian
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Received: April 27, 2015
Published Online: July 2, 2015
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Eur. J. Org. Chem. 2015, 4831–4834