Regiospecific one-pot synthesis of diaryliodonium tetrafluoroborates from arylboronic acids and aryl iodides

Article abstract of DOI:10.1021/jo8004974

(Chemical Equation Presented) Diaryliodonium salts have recently received considerable attention as mild arylation reagents in organic synthesis. This paper describes a regiospecific, sequential one-pot synthesis of symmetrical and unsymmetrical diaryliodonium tetrafluoroborates, which are the most popular salts in metal-catalyzed arylations. The protocol is fast and high-yielding and has a large substrate scope. Furthermore, the corresponding diaryliodonium triflates can conveniently be obtained via an in situ anion exchange.

Full text of DOI:10.1021/jo8004974

Regiospecific One-Pot Synthesis of Diaryliodonium
Tetrafluoroborates from Arylboronic Acids and Aryl Iodides
Marcin Bielawski, David Aili, and Berit Olofsson*
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm UniVersity,
SE-106 91 Stockholm, Sweden
berit@organ.su.se
ReceiVed March 3, 2008
Diaryliodonium salts have recently received considerable attention as mild arylation reagents in organic
synthesis. This paper describes a regiospecific, sequential one-pot synthesis of symmetrical and
unsymmetrical diaryliodonium tetrafluoroborates, which are the most popular salts in metal-catalyzed
arylations. The protocol is fast and high-yielding and has a large substrate scope. Furthermore, the
corresponding diaryliodonium triflates can conveniently be obtained via an in situ anion exchange.
Introduction
as benzyne generators,13 and also as precursors to 18F-labeled
radioligands.14
Hypervalent iodine reagents are efficient alternatives to toxic
heavy metal-based oxidants and expensive organometallic catalysts
in many organic transformations. Compounds such as iodosylben-
zene, (diacetoxyiodo)benzene, Dess-Martin periodinane and o-
iodoxybenzoic acid (IBX) are frequently employed in natural product
synthesis as mild, selective, and efficient oxidation reagents.1–4
Symmetric diaryliodonium salts are usually preferred in
arylation reactions. However, the aryl moieties in unsymmetrical
salts can often be differentiated electronically or sterically, thus
enabling selective reactions with iodobenzene as the only side
product.6,15 Diaryliodonium salts with halide anions are gener-
ally sparingly soluble in many organic solvents, whereas triflate
and tetrafluoroborate salts have good solubility. Another attrac-
tive property with the latter anions is their weak or nonexistent
nucleophilicity,whichmakesthemeasilyapplicableinsynthesis.6,16
Diaryliodonium salts constitute another class of iodine(III)
reagents, having two carbon ligands bound to iodine. Their use
in organic transformations has recently gained considerable
attention, as their properties resemble those of organometallic
complexes with Hg, Pb, and Pd.5 Diaryliodonium salts are often
applied in metal-catalyzed coupling reactions,5,6 in R-arylations
ofcarbonylcompounds,7–10 asphotoinitiatorsinpolymerizations,11,12
Synthetic routes to diaryliodonium salts usually consist of
two to three reaction steps, commonly followed by an anion
exchange.17–19 Although shorter routes have been reported, they
(1) Wirth, T. Angew. Chem., Int. Ed. 2005, 44, 3656–3665.
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(17) Shah, A.; Pike, V. W.; Widdowson, D. A. J. Chem. Soc., Perkin Trans.
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10.1021/jo8004974 CCC: $40.75 2008 American Chemical Society
Published on Web 05/28/2008

One-Pot Synthesis of Diaryliodonium Tetrafluoroborates
TABLE 1. Optimization of the Synthesis of 3aa
SCHEME 1. One-Pot Synthesis of Diaryliodonium Triflates
entry BF3 ·OEt2 (equiv) step I (min) T (°C) step II (min) yield (%)b
SCHEME 2. Regiospecific Routes to Diaryliodonium Salts
1
2
3
4
5
6
7
8
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.5
2.5
3.0
2.5
<1
15
30
60
30
30
30
30
30
30
30
rt
rt
rt
rt
0
40
rt
rt
rt
rt
rt
60
60
60
60
60
60
30
30
15
15
15
29
59
75
78
47
61
74
80
82
78
83
9
10
11c
a Reaction conditions: 1a (0.27 mmol) and m-CPBA (0.30 mmol)
were dissolved in CH2Cl2 (1 mL), BF3 ·OEt2 was added, and the
reaction was stirred at the indicated temperature for the time given in
step I. 2a (0.30 mmol) was subsequently added at 0 °C, and the mixture
was then stirred at rt for the time given in step II. b Isolated yield. c 1 g
scale, see the Experimental Section for details.
require the use of toxic chromium reagents20 or inorganic
iodine(III) reagents that need to be prepared in advance.21–23
During the past few years, some one-pot procedures starting
from arenes and iodoarenes or iodine have been developed,
yielding both unsymmetrical and symmetrical salts.24–28 The
one-pot reaction developed in our laboratory employs m-CPBA
and triflic acid, delivering diaryliodonium triflates in high yields
and short reaction times (Scheme 1).24,25
A common feature of almost all synthetic routes is the
electrophilic aromatic substitution of an arene onto an iodine(III)
intermediate. High para selectivity is usually obtained, which
limits the number of salts that can be obtained by these methods.
Symmetric salts with ortho and meta substitutents are only
accessible by a limited number of routes. These employ
preformed iodine(III) reagents and lithiated arenes,29,30 arylbo-
ronic acids,31,32 stannanes,19,33 or silanes34 (Scheme 2).
envisioned a regiospecific one-pot reaction starting from io-
doarenes and a suitably activated arene source. Arylboronic acids
were selected due to their high reactivity and low toxicity
compared to silanes and stannanes, respectively. The protocol
would employ m-CPBA, which has recently been used in several
iodine oxidations,35–39 and a suitable acid, the anion of which
would end up in the salt. The use of boron trifluoride etherate
was deemed interesting, as it could give rise to diaryliodonium
tetrafluoroborates without an extra anion-exchange step.40–42
Tetrafluoroborate salts have been employed in several recent
papers on Pd-catalyzed arylation reactions,6,43,44 but there is no
general way to synthesize them.31,45–48
When the model substrates iodobenzene (1a) and phenylbo-
ronic acid (2a) were reacted in the presence of m-CPBA49 and
boron trifluoride at room temperature, diphenyl salt 3a was
indeed formed, albeit in low yield (Table 1, entry 1). An
unwanted reaction between m-CPBA and 2a was observed,
Results and Discussion
To widen the scope of easily accessible diaryliodonium salts
and circumvent the need for preformed iodine(III) reagents, we
(35) Dohi, T.; Maruyama, A.; Yoshimura, M.; Morimoto, K.; Tohma, H.;
Kita, Y. Angew. Chem., Int. Ed. 2005, 44, 6193–6196.
(19) Zhdankin, V. V.; Scheuller, M. C.; Stang, P. J. Tetrahedron Lett. 1993,
34, 6853–6856.
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Chem. Soc. 2005, 127, 12244–12245.
(20) Kazmierczak, P.; Skulski, L. Synthesis 1995, 1027–1032.
(21) Zefirov, N. S.; Kasumov, T. M.; Koz’min, A. S.; Sorokin, V. D.; Stang,
P. J.; Zhadankin, V. V. Synthesis 1993, 1209–1210.
(22) Stang, P. J.; Zhdankin, V. V.; Tykwinski, R.; Zefirov, N. S. Tetrahedron
Lett. 1992, 33, 1419–1422.
(37) Yamamoto, Y.; Togo, H. Synlett 2006, 798–800.
(38) Togo, H.; Yamamoto, Y. Synlett 2005, 2486–2488.
(39) Tohma, H.; Maruyama, A.; Dohi, T.; Shiro, M.; Morita, T.; Kita, Y.
Angew. Chem., Int. Ed. Engl. 2004, 43, 3595–3598.
(40) Ochiai, M.; Kitagawa, Y.; Takayama, N.; Takaoka, Y.; Shiro, M. J. Am.
Chem. Soc. 1999, 121, 9233–9234.
(23) Stang, P. J.; Zhdankin, V. V.; Tykwinski, R.; Zefirov, N. S. Tetrahedron
Lett. 1991, 32, 7497–7498.
(41) Gronheid, R.; Lodder, G.; Ochiai, M.; Sueda, T.; Okuyama, T. J. Am.
Chem. Soc. 2001, 123, 8760–8765.
(24) Bielawski, M.; Olofsson, B. Chem. Commun. 2007, 2521–2523.
(25) Bielawski, M.; Zhu, M.; Olofsson, B. AdV. Synth. Catal. 2007, 349,
2610–2618.
(42) For a discussion on the formation of BF4 anions from BF3, see: Blid,
J.; Brandt, P.; Somfai, P. J. Org. Chem. 2004, 69, 3043–3049.
(43) Deprez, N. R.; Kalyani, D.; Krause, A.; Sanford, M. S. J. Am. Chem.
Soc. 2006, 128, 4972–4973.
(26) Zhu, M.; Jalalian, N.; Olofsson, B. Synlett 2008, 592–596.
(27) Hossain, M. D.; Ikegami, Y.; Kitamura, T. J. Org. Chem. 2006, 71,
9903–9905.
(44) Kalyani, D.; Deprez, N. R.; Desai, L. V.; Sanford, M. S. J. Am. Chem.
Soc. 2005, 127, 7330–7331.
(28) Hossain, M. D.; Kitamura, T. Tetrahedron 2006, 62, 6955–6960.
(29) Pirguliyev, N. S.; Brel, V. K.; Akhmedov, N. G.; Zefirov, N. S. Synthesis
2000, 81–83.
(45) Ochiai, M.; Toyonari, M.; Sueda, T.; Kitagawa, Y. Tetrahedron Lett.
1996, 37, 8421–8424.
(30) Kitamura, T.; Kotani, M.; Fujiwara, Y. Tetrahedron Lett. 1996, 37, 3721–
3722.
(46) Chen, D.-W.; Ochiai, M. J. Org. Chem. 1999, 64, 6804–6814.
(47) Kasumov, T. M.; Brel, V. K.; Koz’min, A. S.; Zefirov, N. S. Synthesis
1995, 775–776.
(31) Ochiai, M.; Toyonari, M.; Nagaoka, T.; Chen, D.-W.; Kida, M.
Tetrahedron Lett. 1997, 38, 6709–6712.
(48) Yoshida, M.; Osafune, K.; Hara, S. Synthesis 2007, 1542–1546.
(49) In our first report on m-CPBA oxidation of aryl iodides, the m-CPBA
was used without drying (ref 24). However, new, commercially available cans
of m-CPBA were found to contain large and variable amounts of H2O, and drying
under vacuum at room temperature for 1 h was necessary to obtain reproducible
results. The percentage of active oxidizing agent was determined by iodometric
titration; see the Supporting Information for further details.
(32) Carroll, M. A.; Pike, V. W.; Widdowson, D. A. Tetrahedron Lett. 2000,
41, 5393–5396.
(33) Pike, V. W.; Butt, F.; Shah, A.; Widdowson, D. A. J. Chem. Soc., Perkin
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1543–1544.
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Bielawski et al.
which could explain the low yield. Fortunately, a short preoxi-
dation time before addition of 2a was sufficient to dramatically
increase the yield of 3a (entries 2-4). Temperature variation
did not improve the results, either during the preoxidation
(entries 5 and 6) or in the second step. The use of 2.5 equiv of
BF3 ·OEt2 resulted in a faster reaction, and 3a was isolated in
high yield after only 45 min reaction time (entries 8 and 9).
Salt 3a was easily isolated by fast elution of the crude reaction
mixture through a silica plug followed by precipitation from
diethyl ether.50 Furthermore, the protocol was easily scaled up
to 1 g without loss in yield or purification efficiency (entry 11).
To investigate the scope of this reaction, the optimized
conditions were subsequently applied to other substrates.
Iodobenzene was reacted with electron-deficient and electron-
rich arylboronic acids 2 to give unsymmetrical salts 3b-p in
high yields (Table 2). The halide-substituted arylboronic acids
2b-f participated excellently in the reaction, yielding ortho-,
meta- and para-substituted salts 3b-3f (entries 2-6). Likewise,
o- and m-methyl-substituted boronic acids 2g,h delivered salts
3g,h (entries 7 and 8). Sterically hindered substrates like 2,6-
dimethylboronic acid 2i could also be employed (entry 9).
The synthesis of electron-deficient iodonium salts generally
requires heating and prolonged reaction times. It was therefore
surprising that salts 3j-m, obtained from electron-deficient
boronic acids with varying substitution patterns, could be
isolated in good yields in only 45 min (entries 10-13). Electron-
rich substrates, such as p-methoxy- and 1-naphthylboronic acids
2o,p, delivered salts 3o,p in high yields when the reactions were
performed at low temperature (entries 15 and 16). Unfortunately,
m-methoxyphenylboronic acid did not give the expected meta-
substituted iodonium salt. A para-substituted salt was obtained
instead, presumably via the electrophilic aromatic substitution
pathway, which reflects a limitation to the protocol.
The synthesis of symmetric diaryliodonium tetrafluoroborates
was subsequently investigated. Substituted symmetric salts are
generally difficult to obtain, as the system either becomes too
unreactive (electron-poor substrates) or too reactive (electron-rich
substrates). Reported procedures are generally limited in scope and
give moderate yields.14–16 Gratefully, our protocol delivered both
electron-poor and electron-rich salts, as depicted in Table 3.
The halogenated iodoarenes 1b and 1c were smoothly oxidized
and coupled with 2b and 2e, respectively, yielding symmetric salts
3q,r (entries 2 and 3). Likewise, 2-iodotoluene (1d) and o-methyl-
substituted boronic acid 2g gave salt 3s (entry 4). Again, the
deactivated substrates showed high reactivity, giving salts 3t-v
within 1.5 h at rt (entries 5-7). The highly activated iodoarenes
1h and 1i also participated in the reactions with the corresponding
arylboronic acids (2o,p), but even at -78 °C side reactions took
place and moderate yields were obtained (entries 8 and 9). An
alternative route to highly activated symmetrical diaryliodonium
tetrafluoroborates is the conversion from the corresponding tosylates
with an in situ anion exchange.26
acid route could be used also to obtain diaryliodonium triflates
with previously unobtainable substituents. Although the reaction
indeed worked with triflic acid, higher yields were obtained
when boron trifluoride was used followed by an in situ anion
exchange with triflic acid. This is exemplified by the synthesis
of bis(2-fluorophenyl)iodonium triflate (4), which was isolated
in 75% yield by addition of triflic acid to the crude mix-
ture after complete conversion to diaryliodonium salt 3q
(Scheme 3).
Conclusions
To summarize, we have demonstrated an efficient and fast
one-pot synthesis of symmetric and unsymmetric diaryliodonium
tetrafluoroborates from iodoarenes and arylboronic acids. Both
electron-deficient and electron-rich salts can be synthesized in
a regiospecific manner, and the substitution pattern can easily
be varied. An in situ anion exchange with triflic acid gives access
also to the corresponding diaryliodonium triflates.
Experimental Section
General Procedure for the Synthesis of Salts 3a-v. m-
Chloroperbenzoic acid (81% active oxidant, 61 mg, 0.29 mmol)
was dissolved in CH2Cl2 (1 mL). To the solution was added aryl
iodide 1a (0.26 mmol) followed by BF3 ·OEt2 (81 µL, 0.65 mmol)
at room temperature. The resulting yellow solution was stirred at
rt for 30 min and then cooled to 0 °C, and arylboronic acid 2b (40
mg, 0.29 mmol) was added. After 15 min of stirring at rt, the crude
reaction mixture was applied on a silica plug (0.8 g) and eluted
with CH2Cl2 (10 mL) to remove unreacted ArI and m-CBA,
followed by CH2Cl2/MeOH (30 mL, 20:1), to elute the product,
leaving any boric acid derivatives on the column. The latter solution
was concentrated, and diethyl ether (1 mL) was added to the residue
to induce a precipitation of salt 3b, with any iodine(III) intermedi-
ates and BF3 derivatives remaining in solution. (If precipitation is
hard to obtain, a small amount of CH2Cl2 can be added.) The
solution was allowed to stir for 15 min, and then the ether phase
was decanted,51 and the solid was washed twice more with diethyl
ether (2 × 1 mL) and then dried in vacuo to give pure diaryliodo-
nium tetrafluoroborate salt 3b in 88% yield as a white solid: mp
141-143 °C; 1H NMR (400 MHz, DMSO-d6) δ 8.42 (1 H, ddd, J
) 7.6, 6.0, 1.6 Hz), 8.23 (2 H, d, J ) 7.2 Hz), 7.77-7.64 (2 H,
m), 7.62-7.52 (3 H, m), 7.38 (1 H, td, J ) 8.0, 1.2 Hz); 13C NMR
(100 MHz, DMSO-d6) δ 159.2 (d, J ) 248 Hz), 137.1, 135.6 (d,
J ) 8 Hz), 135.1, 132.3, 132.0, 127.7 (d, J ) 3 Hz), 117.0, 116.8,
103.8 (d, J ) 23.3 Hz); 19F NMR (376 MHz, DMSO-d6) δ -97.45
(1 F), [-147.77, -147.82] (4 F); HRMS (ESI) calcd for C12H9FI
([M - BF4-]+) 298.9727, found 298.9718.
Large-Scale Synthesis of Diphenyliodonium Tetrafluorobo-
rate (3a). m-Chloroperbenzoic acid (81% active oxidant, 640 mg,
3.0 mmol) was dissolved in CH2Cl2 (10 mL). To the solution was
added iodobenzene (1a, 310 µL, 2.7 mmol) followed by slow
addition of BF3 ·OEt2 (850 µL, 6.8 mmol) at room temperature.
The resulting yellow solution was stirred at rt for 30 min and then
cooled to 0 °C, and phenylboronic acid (2a, 370 mg, 3.0 mmol)
was added. After 15 min of stirring at rt, the crude reaction mixture
was applied on a silica plug (6.0 g) and eluted with CH2Cl2 (60
mL) followed by CH2Cl2/MeOH (120 mL, 20:1). The latter solution
was concentrated, and diethyl ether (10 mL) was added to the
residue to induce a precipitation. The solution was allowed to stir
for 15 min, and then the ether phase was decanted. The solid was
washed twice more with diethyl ether (2 × 10 mL) and then dried
Although the yields in entries 5-9 are moderate, the synthesis
should be appealing due to its simplicity, short reaction time,
easy purification, and large substrate scope.
Diaryliodonium triflates are frequently applied in synthesis.
As shown in Scheme 1, we have previously developed an
efficient one-pot synthesis of triflate salts. That methodology is
for mechanistical reasons limited to certain substitution patterns,
and it was therefore of interest to investigate whether the boronic
(51) Decantation of the filtrate proved to be better than filtration on a glass
filter funnel, as the solid was initially rather sticky. After the washing procedure
described above, it formed a solid that was easily collected.
(50) This purification was found both fast and highly efficient, and 3a was
obtained with high purity without need for recrystallization.
4604 J. Org. Chem. Vol. 73, No. 12, 2008

One-Pot Synthesis of Diaryliodonium Tetrafluoroborates
TABLE 2. Synthesis of Salts 3 from PhI (1a) and Arylboronic Acids 2a
a Reaction conditions: 1a (1.0 equiv) and m-CPBA (1.1 equiv) were dissolved in CH2Cl2 (1 mL), BF3 ·OEt2 (2.5 equiv) was added, and the reaction
was stirred at rt for 30 min. 2 (1.1 equiv) was subsequently added at 0 °C, and the mixture was then stirred at rt for 15 min. b Isolated yield. c 2 was
added at -78 °C.
in vacuo to give salt 3a in 83% yield (825 mg). Analytical data
were in agreement with previous reports; see the Supporting
Information.
Modified Synthesis of Electron-Rich Salts 3w and 3x. m-
Chloroperbenzoic acid (81% active oxidant, 68 mg, 0.30 mmol)
was added to a sealed tube and dissolved in CH2Cl2 (1 mL). To
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Bielawski et al.
TABLE 3. Synthesis of Symmetric Salts 3 from Aryl Iodides 1 and Arylboronic Acids 2a
a Reaction conditions: 1 (1.0 equiv.) and mCPBA (1.1 equiv.) were dissolved in CH2Cl2 (1 mL), BF3 ·OEt2 (2.5 equiv.) was added and the reaction
was stirred at rt for 30 min. 2 (1.1 equiv.) was subsequently added at 0 °C, the mixture was then stirred at rt for 15 min. b Isolated yield. c 60 min
preoxidation time. d In acetonitrile. e Special conditions were required, see the Experimental Section.
SCHEME 3. One-Pot Synthesis with in Situ Anion Exchange
the solution was added aryl iodide 1 (0.27 mmol), and the reaction
mixture was placed in an 80 °C preheated oil bath. After 10 min,
the vial was cooled to -78 °C, and a 0 °C mixture of BF3 ·OEt2
(85 µL, 0.68 mmol) and arylboronic acid 2 (37 mg, 0.30 mmol),
dissolved in CH2Cl2 (1 mL), was transferred to the cooled reaction
mixture through a cannula. The resulting dark solution was stirred
for 30 min at -78 °C and brought up to room temperature, and the
product was isolated as described in the general procedure. Bis(4-
methoxyphenyl)iodonium tetrafluoroborate (3w): isolated in 46%
yield as a gray solid; mp 170 °C dec; 1H NMR (400 MHz, DMSO-
d6) δ 8.12 (4 H, d, J ) 7.2 Hz), 7.06 (4 H, d, J ) 7.2 Hz), 3.79 (6
H, s); 13C NMR (100 MHz, DMSO-d6) δ 161.8, 136.8, 117.3, 105.9,
55.7; 19F NMR (376 MHz, DMSO-d6) δ [-147.78, -147.84] (4
F); HRMS (ESI) calcd for C14H14IO2 ([M - BF4-]+) 341.0033,
found 341.0029. Bis(1-naphthyl)iodonium tetrafluoroborate (3x):
DMSO-d6) δ 137.3, 134.2, 133.4, 130.9, 129.7, 129.4, 128.8, 128.0,
127.5, 119.2; 19F NMR (376 MHz, DMSO-d6) δ [-147.78,
-147.84] (4 F); HRMS (ESI) calcd for C20H14I ([M - BF4-]+)
381.0135, found 381.0136.
Synthesis of Triflate Salt 4 via Tetrafluoroborate Salt 3q.
m-Chloroperbenzoic acid (81% active oxidant, 58 mg, 0.27 mmol)
was dissolved in CH2Cl2 (1 mL). To the solution was added
2-fluoroiodobenzene (1b, 0.25 mmol) followed by BF3 ·OEt2 (78
µL, 0.62 mmol) at room temperature. The resulting yellow solution
was stirred for 30 min and cooled to 0 °C, followed by addition of
2-fluorophenylboronic acid (2b, 38 mg, 0.27 mmol). After 15 min
of stirring at rt, triflic acid (24 µL, 0.27 mmol) was added at room
temperature, and the mixture was stirred for an additional 15 min
then isolated as described in the general procedure to give 4 in
75% yield as a white solid: mp 181-183 °C; 1H NMR (400 MHz,
DMSO-d6) δ 8.41 (2 H, ddd, J ) 8.0, 6.4, 1.6 Hz), 7.74 (2 H, m),
1
isolated in 37% yield as a gray solid; mp 155 °C dec; H NMR
(400 MHz, DMSO-d6) δ 8.88 (2 H, dd, J ) 7.6, 0.8 Hz), 8.49 (2
H, d, J ) 8.8 Hz), 8.21 (2 H, d, J ) 8.0 Hz), 8.01 (2 H, d, J ) 8.0
Hz), 7.81 (2 H, app td, J ) 6.8, 1.2 Hz), 7.68 (2 H, app td, J )
8.0, 0.8 Hz), 7.57 (2 H, app td, J ) 7.8 Hz); 13C NMR (100 MHz,
7.59 (2 H, td, J ) 8.4, 1.2 Hz), 7.38 (2 H, td, J ) 8.0, 1.3 Hz); 13
C
NMR (100 MHz, DMSO-d6) δ 159.1 (d, J ) 248.4 Hz), 137.1,
135.8 (d, J ) 7.7 Hz), 127.8, 120.8 (q, 320.5 Hz, F3SO3-), 116.8
(d, J ) 22.1 Hz), 104.1 (d, J ) 24.0 Hz); 19F NMR (376 MHz,
4606 J. Org. Chem. Vol. 73, No. 12, 2008

One-Pot Synthesis of Diaryliodonium Tetrafluoroborates
DMSO-d6) δ -77.3 (3 F, s, CF3SO3-), -97.43 (2 F, m); HRMS
(ESI) calcd for C12H8F2I ([M - OTf-]+) 316.9633, found 316.9624.
Supporting Information Available: General experimental
conditions, analytical data, and NMR spectra for salts 3 and 4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was financially supported by
the Swedish Research Council and the Department of Organic
Chemistry at Stockholm University.
JO8004974
J. Org. Chem. Vol. 73, No. 12, 2008 4607