Aryl imidazylates and aryl sulfates as electrophiles in metal-free ArSN1 reactions

Article abstract of DOI:10.1021/jo502172c

Some oxygen-bonded substituents were investigated as leaving groups in photoinduced ArS_N1 reactions. Irradiation of aryl imidazylates and of the corresponding imidazolium salts mainly caused homolysis of the ArO-S bond. However, previously unexplored trifluoroethoxy aryl sulfates were found to undergo efficient metal-free arylation. The sulfates were conveniently generated in situ by dissolving the corresponding imidazolium salts in basic 2,2,2-trifluoroethanol.

Full text of DOI:10.1021/jo502172c

Article
pubs.acs.org/joc
Aryl Imidazylates and Aryl Sulfates As Electrophiles in Metal-Free
ArSN1 Reactions
Hisham Qrareya, Stefano Protti, and Maurizio Fagnoni*
Photogreen Lab, Department of Chemistry, University of Pavia, Via Taramelli 12, 27100 Pavia, Italy
S
* Supporting Information
ABSTRACT: Some oxygen-bonded substituents were investigated as leaving groups
in photoinduced ArSN1 reactions. Irradiation of aryl imidazylates and of the
corresponding imidazolium salts mainly caused homolysis of the ArO−S bond.
However, previously unexplored trifluoroethoxy aryl sulfates were found to undergo
efficient metal-free arylation. The sulfates were conveniently generated in situ by
dissolving the corresponding imidazolium salts in basic 2,2,2-trifluoroethanol.
exhibit higher reactivity with respect to the corresponding aryl
INTRODUCTION
■
tosylates,5c pivalates,5e and bromides and chlorides.5h They also
have the advantage of improved stability, cost, and handling
properties over traditional aryl triflates.5c
Phenols are easily converted into a wide range of aryl
electrophiles, including phosphates,1a carbonates,1b carbama-
tes,1c esters and ethers,1d,e sulfamates,1f and sulfonates, that
have been used in various cross-coupling reactions with carbon-
based nucleophiles (mainly organometallic species; see Scheme
1a). Sulfonates constitute the most widely used class of
Little attention has been given, however, to the development
of transition-metal-free approaches for activation of the Ar−O
bond in aryl imidazylates. To the best of our knowledge, the
only case reported deals with the fluoride- induced generation
of o-benzynes from o-(trimethylsilyl)aryl imidazolesulfonates.6
We recently demonstrated that some aryl sulfonates, namely
aryl mesylates,7a triflates,7a and nonaflates,7b can be used as
substrates to achieve metal-free arylations via photoheterolytic
cleavage of the Ar−O bond. In this process, triplet phenyl
cations (I, Scheme 1b) are formed photochemically and
trapped by carbon nucleophiles (NuE).
Scheme 1. Use of Aryl Esters in (a) Transition-Metal-
Catalyzed Reactions and (b) Metal-Free Photochemical
Arylations
We reasoned that (hetero)aromatic sulfonates4 could
function as electrophilic partners for the photogeneration of
cations I. We therefore tested aryl imidazolesulfonates (II,
Scheme 1b), for which no photochemical investigations have
been reported as yet, thereby replacing an S−C bond by an S−
N bond in the leaving group. On the same ground, aryl
sulfurylimidazolium salts (III) were likewise prepared (by N-
methylation of imidazylates II) and tested. Subsequently, the
corresponding trifluoroethyl sulfates (IV, Scheme 1b) were
explored (from the preparative point of view, sulfates IV are
known to be easily prepared from salts III, as previously
reported for the synthesis of TFE-protected 6-sulfated
carbohydrates).8 The virtually unexplored photochemistry of
these aryl sulfates was investigated and gave results radically
different from those reported for sulfonates.9
compounds, and among them, electron-withdrawing perfluor-
oalkyl sulfonates such as triflates2a and nonaflates (ArO-
SO2C4F9)2b have long been used specifically for their excellent
reactivity and the mild reaction conditions required. Other
fluorine-free sulfonates such as mesylates3a and tosylates3b,c
have been likewise used, despite the lower reactivity in cross-
coupling processes due to the high stability of the sulfonate
groups.3a,d,e A recent advance in aromatic chemistry is the use
of the imidazolesulfonate moiety (imidazylate, Imz), previously
used in sugar derivatization via aliphatic nucleophilic
substitution.4 Aryl imidazylates have proven to be efficient
partners in a wide range of cross-coupling procedures5 and
RESULTS
■
The irradiation of aryl imidazylates 1a−d and their
corresponding imidazolium salts 9a−d (Chart 1) was explored
initially. In analogy with other aryl sulfonates,7a,10 compounds
Received: September 22, 2014
© XXXX American Chemical Society
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Chart 1
Table 1. Irradiation of Aryl Sulfonates 1a−d in the Presence
a
of π-Bond Nucleophiles
1a−d absorbed in the UV region at a peak wavelength
depending on the nature of the substituent (from 308 nm for
1b to 255 nm for 1d), with a low emission yield (ΦF up to 0.02;
see Table S1 in Supporting Information).
b
c
nucleophile NuE
byproducts (%)
ArNu (%)
The photoreactivity of these substrates was investigated in
2,2,2-trifluoroethanol (TFE), in the presence of various π-bond
nucleophiles: allyltrimethylsilane (ATMS), 4-pentenoic acid,
benzene, mesitylene, and in selected cases 1-hexyne. A base
(triethylamine, TEA) was added in order to buffer the acid
released during the irradiation. Acetone (10% v/v) was used as
a triplet sensitizer for compounds 1a, 1c, and 1d, also
compensating for the insufficient absorption at 310 nm. The
disappearance quantum yield (Φ−1) was 0.54 for 1b and ca. 0.1
for the other aryl imidazylates (Table 1). Control experiments
were conducted, demonstrating that 1a−d were not consumed
when tubes containing these solutions were covered with
aluminum foil and placed in a multilamp apparatus for 24 h.
The results obtained are gathered in Table 1.
d
1a, Φ−1 = 0.081
b
0.5 M ATMS
2a (8), 3a (5)
4a (4)
0.5 M 4-pentenoic acid
1 M benzene
3a (18)
5a (17)
6a (60)
6a (12)
7a (60)
8a (34)
3a (2)
e
1 M benzene
2a (4)
1 M mesitylene
0.5 M 1-hexyne
3a (2)
d
1b, Φ−1 = 0.54
0.5 M ATMS
2b (28)
2b (15)
2b (18)
4b
0.5 M 4-pentenoic acid
1 M benzene
5b
6b
1 M mesitylene
0.5 M 1-hexyne
2b (35), 3b (<1)
7b (19)
8b
2b (15), 3b (<1)
d
1c, Φ−1 = 0.072
Photolysis of 1a in TFE in the presence of a π-bond
nucleophile yielded phenylated compounds ArNu in amounts
that varied considerably with the nucleophile (Chart 2). The
best yields were obtained in the reaction with aromatics to form
biphenyls 6a and 7a (ca. 60% isolated yield). On the other
hand, when 1a is irradiated in the absence of acetone as
sensitizer, a strong decrease in the yield of 6a (down to 12%
yield) was observed, along with the formation of some phenol
2a. Irradiation of 1b, however, gave no phenylated products
other than 7b (20% yield), producing instead only the
deprotected aminophenol 2b along with traces of N,N-
dimethylaniline (3b). Analogous results were observed for
sulfonates 1c and 1d, for which both the yield of arylated
products 4−8c,d and the resulting mass balance were
unsatisfactory in most cases; the corresponding phenols 2
were the main, or exclusive, products observed. The data above
suggest that the product distribution obtained from aryl
imidazylates 1a−d is determined by competition between
ArO−S bond photohomolysis (which leads to phenol 2) and
Ar−OS bond heterolysis (which leads to reduced 3 and to
trapping products 4−8), with the former pathway largely
preferred.
At this point we turned to salts 9a−d, not previously
investigated either in arylation reactions or in photochemistry.
The photophysics of 9a−d are similar to those of demethylated
1a−d (Table S1, Supporting Information). Blank experiments
carried out on 9a, however, demonstrated that this compound
was thermally unstable under the conditions used in Table 1,
since it was converted to sulfate 10a with a rate depending on
the strength of the base used (Scheme 2).
As a matter of fact, in the presence of Cs2CO3 the conversion
took place in a few minutes, whereas with TEA a complete
conversion was achieved in 1 h. We then isolated 10a from 9a
by treatment with basic 2,2,2-trifluoroethanol. The photo-
physics of 10a were similar to those of 1a or 9a or other
methoxyphenyl sulfonates,7 whereas Φ−1 (10a) < Φ−1 (1) (in
neat 2,2,2-trifluoroethanol, the measured Φ−1 for 10a was ca.
0.5 M ATMS
2c (19), 3c (8)
4c (9)
5c
0.5 M 4-pentenoic acid
1 M benzene
2c (10)
2c (2), 3c (2)
2c (5)
6c (39)
7c (16)
1 M mesitylene
0.5 M 1-hexyne
b
2c (6), 3c (11)
8c (1)
d
1d, Φ−1 = 0.087
b
0.5 M ATMS
2d (55)
4d (7)
b
0.5 M 4-pentenoic acid
1 M benzene
5d (4)
b
2d (34)
6d (14)
b
1 M mesitylene
7d (9)
a
Reaction conditions: A nitrogen-saturated solution of sulfonate 1a−d
(0.05 M), Et3N (0.05 M), and the chosen nucleophile in TFE was
irradiated at 310 nm; see Supporting Information. tirr = 24 h (1a, 1c,
1d) or 4 h (1b). Acetone (10% v/v) was added in experiments with
b
aryl sulfonates 1a, 1c, and 1d. Yields determined by gas chromato-
graphic (GC) analysis. Isolated yields. Measured in TFE at 254 nm
(1a, 1c, 1d, 0.015 M) or 310 nm (1b, 0.01 M). Irradiation carried out
in the absence of acetone.
c
d
e
0.03; Table 2). Irradiation of 10a in TFE in the presence of π-
bond nucleophiles led to the phenylated products 4−8a in
yields higher (up to >97%) than those observed from the
irradiation of 1a, and in some cases (see Table 2) a smaller
amount of nucleophile maintained the same yield. We note that
this is the first report of generation of a phenyl cation from an
aryl sulfate. Compounds 9b−d were likewise easily converted
into sulfates 10b−d in basic TFE, as demonstrated by gas
chromatographic−mass spectrometric (GC-MS) analyses (see
Supporting Information).
With these encouraging indications in hand, we decided to
generate sulfates 10a−d in situ by carrying out the arylation
reactions on 9a−d in basic (Cs2CO3) TFE solution (Table 3).
Arylations of 9a gave 4a−7a in similar or slightly lower yields,
compared to experiments starting with pure 10a. Efficient
arylation (in most cases in >70% yield) was likewise found for
sulfates 10b,c (except in synthesis of the allylated compounds
4c,d), despite the non-negligible presence of byproducts 2b and
B
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Chart 2
Table 3. Irradiation of in Situ Generated Aryl Sulfates 10a−d
from Arylsulfonylimidazolium Triflates 9a−d in the Presence
a
of π-Bond Nucleophiles
b
c
nucleophile, NuE
byproducts (%)
ArNu (%)
9a → 10a
2a (8), 3a (4)
2a (8)
0.5 M ATMS
4a (77)
5a (58)
6a (79)
0.5 M 4-pentenoic acid
1 M benzene
2a, 3a (5)
d
d
1 M mesitylene
0.5 M 1-hexyne
3a (0, 1 )
7a (94, 77 )
e
e
3a (7.5, 5 )
8a (63, 73 )
9b → 10b
Scheme 2
0.5 M ATMS,
2b (11), 3b (5)
2b (11), 3b (1)
2b (11), 3b (10)
2b (10), 3b (4)
9c → 10c
4b (72)
5b (79)
6b (78)
7b (70)
0.5 M 4-pentenoic acid
1 M benzene
1 M mesitylene
0.5 M ATMS
2c (13), 3c (1)
4c (7)
0.5 M 4-pentenoic acid
1 M benzene
5c (52)
6c (68)
3c (5)
d
1 M mesitylene
0.5 M 1-hexyne
7c (73, 55 )
e
2c (4), 3c (4)
9d → 10d
2d (31)
8c (30, 54 )
Table 2. Irradiation of 10a in the Presence of π-Bond
a
Nucleophiles
0.5 M ATMS
4d (14)
5d (39)
6d (57)
7d (33)
0.5 M 4-pentenoic acid
1 M benzene
2d (15)
1 M mesitylene
a
Reaction conditions: A nitrogen-saturated solution of 9a−d (0.05
M), Cs2CO3 (0.05 M), and the chosen nucleophile in TFE was
irradiated with 10 × 15 W Hg phosphor-coated lamps (λem = 310 nm,
tirr = 24 h for 9a, 9c, and 9d and 4 h for 9b). Acetone (10% v/v) was
added in the experiments with 9a, 9c, and 9d. Yields determined by
GC analysis. Isolated yields. 0.5 M mesitylene. 1 M 1-hexyne.
b
c
d
e
a
imidazylate 1a was found to release acid upon irradiation, thus
behaving as a photoacid generator (see Supporting Information
for details).
Reaction conditions: A nitrogen-saturated solution of 9a (0.05 M),
Cs2CO3 (0.05 M), acetone (10% v/v), and the chosen nucleophile in
TFE was irradiated at 310 nm for 24 h. Yields determined by GC
analysis. Isolated yields. Measured at 254 nm (0.01 M 9a). 0.25 M
ATMS. 0.5 M mesitylene.
b
c
d
e
f
DISCUSSION
■
As one might expect, changes in the side group affect the mode
and rate of cleavage in the ground and excited states (Scheme
3). The behavior of imidazylates 1a−d is similar to that
reported for other alkyl aryl sulfonates, where two competing
paths were observed upon irradiation.7,9,10 In this case phenols
2 are observed as the main products resulting from direct
homolysis10 from the singlet states (11a−d) to give phenoxy
radicals (12a−d, Scheme 3, path a). Heterolytic Ar−OS bond
cleavage11 (from triplets 31a−d) to afford triplet phenyl cations
(311+a−d, path b) is observed only as minor path since
intersystem crossing (ISC, from singlets 11a−d to triplets 31a−
d) is slower than the formation of 12a−d. An exception,
however, is the case of 1a, where heterolysis competes to some
extent and arylated compounds were formed in variable
amounts (up to 17% yield for 4 and 5 and 60% yield for
biphenyls 6 and 7; Scheme 3 and Table 1). In the latter case,
the low yield obtained when alkenes were used as nucleophiles
3b in the case of 10b. Apparently, the presence of the sulfate
leaving group allowed for formation of the phenyl cation.
Indeed, a modest arylation was achieved even from sulfate 10d,
in which no electron-donating group is present.11
These findings suggested that imidazylates 1a−d could
likewise be used for in situ formation of sulfates 10a−d by
replacing TEA with cesium carbonate (Cs2CO3). In fact,
compounds 1a−d were slowly (in a few hours) converted
(albeit in part) to sulfates 10a−d in TFE in the presence of
0.05 M Cs2CO3 (higher amounts of base could be detrimental
in this case), in competition with hydrolysis. We then repeated
the same arylation reactions, starting from 10a−d generated in
situ directly from 1a−d. The results, gathered in Table 4, show
that in most cases the arylation yields were intermediate
between those reported in Tables 1 and 3 but consistently
lower than those obtained starting from 9a−d. Finally, aryl
C
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could be explained by the competitive reaction of Me2CO3*
with ATMS (or 4-pentenoic acid) as previously observed in the
photochemistry of related aryl sulfonates7 and caused by the
inefficient energy transfer between Me2CO3* and the aromatic.
Methylation of the imidazole moiety to form imidazolium
salts 9a−d did not substantially modify the photoreactivity of
the resulting aryl sulfonates, and homolysis (path a) remained
the main pathway. The imidazylates (1a−d) and trifluoroe-
thoxysulfates (10a−d) are thermally stable, whereas the
imidazolium salts (9a−d) acted as sulfating agents for the in
situ preparation of trifluoroethyl sulfates 10a−d along path c.
Gratifyingly, in contrast to the aryl imidazylates 1a−d (9a−d),
the photohomolysis of the ArO-S bond in 10a−d played only a
marginal role (path a′), whereas heterolytic cleavage of the Ar−
OS bond took place efficiently (path b′) from the triplet excited
state. The lack of homolysis in sulfates 10a−d can be safely
attributed to an efficient energy-transfer ISC from singlets
110a−d to triplets 310a−d10,16 rather than to a different
stability of the alkoxysulfonyl radical formed, since it has been
previously reported to have similar structural characteristics and
reactivity as the alkanesulfonyl radicals.12
Table 4. Irradiation of in Situ Generated Aryl Sulfates 10a−d
from Aryl Imidazylates 1a−d in the Presence of π-Bond
a
Nucleophiles
b
b
mucleophile NuE
byproducts (%)
1a → 10a
ArNu (%)
0.5 M ATMS
2a (18), 3a (5)
3a (30)
4a (42)
5a (16)
6a (27)
7a (55)
0.5 M 4-pentenoic acid
1 M benzene
2a (2)
1 M mesitylene
1b → 10b
0.5 M ATMS
2b (58), 3b (1)
2b (41)
4b (12)
5b (7)
0.5 M 4-pentenoic acid
1 M benzene
2b (46), 3b (6)
2b (42), 3b (1)
6b (16)
7b (19)
1 M mesitylene
The triplet phenyl cation intermediate generated underwent
reduction to 3a−d (path d), but in the presence of π-bond
nucleophiles, trapping became the main (often exclusive) path
and arylation occurred efficiently (path e, see Table 3). The
formation of phenols 2 by reaction of adventitious water to
311+a−d (path f) can be safely excluded, since solvolysis is
typical of a singlet but not of a triplet phenyl cation as
previously demonstrated.7a Sulfates 10 were likewise formed by
the unprecedented transformation of aryl imidazylates 1 under
basic conditions, although the lack of complete conversion
lowered the arylation yields.
1c → 10c
0.5 M ATMS
2c (16), 3c (4)
2c (8)
4c (27)
5c (7)
0.5 M 4-pentenoic acid
1 M benzene
2c (2), 3c (10)
6c (33)
7c (28)
8c (7)
1 M mesitylene
0.5 M 1-hexyne
2c (6), 3c (5)
1d → 10d
0.5 M ATMS
2d (38)
4d (24)
5d (4)
6d (5)
7d (9)
0.5 M 4-pentenoic acid
1 M benzene
2d (17)
1 M mesitylene
Activation of the Ar−OS bond in aryl sulfates has rarely been
exploited in synthesis. To the best of our knowledge, only diaryl
sulfates have been used as electrophiles in transition metal
arylation procedures and only rarely: in the Kumada synthesis
of biaryls via nickel-catalyzed coupling with Grignard
reactants13a and in the direct C−H ortho arylation of
heterobiaryls catalyzed by a Ru(II) carboxylate complex.13b
The latter reactions, however, always required the use of
transition metal catalysts and, in some cases, of aggressive
nucleophiles.13a On the other hand, cleavage of the ArOSO2O−
R bond in alkyl aryl sulfates (e.g., R = Me)14 is more common
in aliphatic nucleophilic substitution reactions, though the
related activation of the Ar−OSO2OR bond has been not yet
reported. The adoption of a metal-free approach in synthesis
has gained increased attention in recent years and is hoped to
overcome limitations, related to the synthesis and the use of not
readily available and relatively expensive organometallic
reactants, while avoiding the release of stoichiometric amounts
of metal waste as byproducts.15
a
Reaction conditions: A nitrogen-saturated solution of 1a−d (0.05
M), Cs2CO3 (0.05 M), and the chosen nucleophile in TFE was stirred
for 1−2 h and then irradiated with 10 × 15 W Hg phosphor-coated
lamps (λem = 310 nm; tirr = 24 h for 1a, 1c, and 1d and 4 h for 1b).
Acetone (10% v/v) was added in the experiments with 1a, 1c, and 1d.
b
Yields determined by GC analysis.
a
Scheme 3. Efficient and Inefficient Pathways in Irradiation
of Aryl Sulfonates 1a−d and 9a−d and Aryl Sulfates 10a−d
CONCLUSION
■
In the present paper, we report the discovery of a class of new
photoactive substrates, 2,2,2-trifluoroethyl aryl sulfates 10,
capable of photogenerating aggressive phenyl cations that are
smoothly trapped by a wide range of π-bond nucleophiles,
including aromatics, alkynes, and alkenes, to afford valuable
arylation products in a satisfying yield. Interestingly, the
synthesis and isolation of sulfates 10 is not mandatory, since
these electrophiles are quantitatively generated in situ from
sulfonates 9 and (in part) from 1. As a side issue, the
a
Efficient pathways are shown with solid arrows; inefficient ones are
shown with dashed arrows.
D
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Phenyl 1H-Imidazole-1-sulfonate (1d). From phenol (2d, 327 mg,
3.47 mmol), Im2SO2 (1.375 g, 6.94 mmol), and Cs2CO3, (565 mg,
1.87 mmol) in THF (10 mL). Purification by column chromatography
(eluant cyclohexane/ethyl acetate 85:15) afforded 557 mg of 1d
(colorless solid, mp = 36−38 °C, lit.17 mp 32−33 °C, 72% yield).
Spectroscopic data for 1d are in accordance with the literature.5d Anal.
Calcd for C9H8N2O3S: C, 48.21; H, 3.60; N, 12.49. Found: C, 48.1; H,
3.6; N, 12.5.
Synthesis of 1-Aryloxysulfonyl-3-Methyl-1H-imidazol-3-ium
Trifluoromethanesulfonates 9a−d and Sulfate 10a. Compounds
9a−d were synthesized from the corresponding aryl 1H-imidazolesul-
fonates 1a−d by adapting a known procedure.17 A solution of the
chosen imidazylate 1a−d (2 mmol) in dry diethyl ether (8 mL) was
cooled to 0 °C, and then methyl trifluoromethanesulfonate (MeOTf, 2
mmol) was added dropwise. The resulting solution was stirred for 2 h,
and the obtained precipitate was isolated by filtration and washed
repeatedly with cold ether.
1-[(4-Methoxyphenoxy)sulfonyl]-3-methyl-1H-imidazol-3-ium
Trifluoromethanesulfonate (9a). From 1a (508 mg, 2 mmol) and
0.21 mL (2 mmol) of MeOTf in dry ether (8 mL). The obtained
precipitate was filtered and washed with dry cold ether, affording 678
mg of 9a (colorless solid, mp =70−72 °C, 81% yield). Spectroscopic
data for 9a are in accordance with the literature.17 IR (neat, ν/cm−1)
3143, 1594, 1502, 1453, 1258, 1031, 896, 640. Anal. Calcd for
C12H13F3N2O7S2: C, 34.45; H, 3.13; N, 6.70. Found: C, 34.4; H, 3.1;
N, 6.6.
1-{[4-(N,N-Dimethylamino)phenoxy]sulfonyl}-3-methyl-1H-imi-
dazol-3-ium Trifluoromethanesulfonate (9b). From 1b (535 mg, 2
mmol) and 0.21 mL (2 mmol) of MeOTf in dry ether (8 mL), to
afford 768 mg of 9b (colorless solid, decomposes above 85 °C, 89%
yield). 1H NMR (CD3OD, δ) 9.75 (br s, 1H), 8.15 (br s, 1H), 7.85 (br
s, 1H), 7.15−6.75 (AA′BB′, 4H), 4.0 (s, 3H), 3.0 (s, 6H). 13C NMR
(CD3OD, δ) 152.5, 141.9, 141.5 (CH), 127.7 (CH), 125.4 (CH),
123.2 (CH), 114.9 (CH), 41.3 (CH3), 38.2 (CH3). IR (neat, ν/cm−1)
2924, 1596, 1446, 1269, 1160, 1031, 814. Anal. Calcd for
C13H16F3N3O6S2: C, 36.19; H, 3.74; N, 9.74. Found: C, 36.2; H,
3.7; N, 9.7.
1-[(4-tert-Butylphenoxy)sulfonyl]-3-methyl-1H-imidazol-3-ium
Trifluoromethanesulfonate (9c). From 1c (561 mg, 2 mmol) and
0.21 mL (2 mmol) of MeOTf in dry ether (8 mL). The obtained
precipitate was filtered and washed with dry cold ether to give 818 mg
of 9c (colorless solid, mp =149−151 °C, 90% yield). 1H NMR
(CD3OD, δ) 9.80 (s, 1H), 8.10 (s, 1H), 7.90 (s, 1H), 7.60−7.15
(AA′BB′, 4H), 4.00 (s, 3H), 1.35 (s, 9H). 13C NMR (CD3OD, δ)
154.6, 148.9, 141.3 (CH), 129.4 (CH), 127.7 (CH), 122.9 (CH),
122.0 (CH), 38.1 (CH3), 36.0, 31.8 (CH3). IR (neat, ν/cm−1) 3180,
2923, 1448, 1262, 837. Anal. Calcd for C15H19F3N2O6S2: C, 40.54; H,
4.31; N, 6.30. Found: C, 40.5; H, 4.3; N, 6.3.
3-Methyl-1-(Phenoxysulfonyl)-1H-imidazol-3-ium Trifluorome-
thanesulfonate (9d). From 1d (449 mg, 2 mmol) and MeOTf
(0.21 mL, 2 mmol) in dry ether (8 mL). The obtained precipitate was
filtered and washed with dry cold ether to afford 583 mg of 9d
(colorless solid, mp = 80−82 °C, lit.17 mp 79−80 °C, 86% yield.
Spectroscopic data for 9d are in accordance with the literature.17 IR
(neat, ν/cm−1) 2924, 1654, 1269, 1161, 1032, 874, 692. Anal. Calcd
for C11H11F3N2O6S2: C, 34.02; H, 2.86; N, 7.21. Found: C, 33.9; H,
2.9; N, 7.2.
4-Methoxyphenyl 2,2,2-Trifluoroethyl Sulfate (10a). Compound
9a (510 mg, 1.22 mmol) was treated with 467 mg (1.22 mmol) of
Cs2CO3 in TFE (24.5 mL). The resulting solution was stirred for 1 h,
and then the solvent was evaporated. Purification of the residue by
column chromatography (eluant neat hexane) afforded 316 mg of 10a
(90% yield, colorless oil). 1H NMR (CD3COCD3) δ 7.35−6.90
(AA′BB′, 4H), 4.75−4.65 (q, 2H, J = 8 Hz), 3.80 (s, 3H). 13C NMR
(CD3COCD3) δ 158.7, 143.3, 122.0 (CH), 114.9 (CH), 67.7 (CH2),
55.6 (CH3). IR (neat, ν/cm−1) 2954, 1504, 1417, 1170, 892, 838. GC-
MS (m/z) 286 (15), 123 (100), 95 (20), 83 (5). Anal. Calcd for
C9H9F3O5S: C, 37.77; H, 3.17. Found: C, 37.8; H, 3.2. Quantitative
formation of sulfates 10b−d from 9b−d in TFE in the presence of
Cs2CO3 has been confirmed by GC-MS analyses of the resulting
photohomolysis of aryl imidazylates 1 could potentially be
exploited for the photoinduced release of strong acid, as
demonstrated during the photolysis of 1a (see Supporting
Information).
EXPERIMENTAL SECTION
■
General Information. 1H and 13C NMR spectra were recorded on
1
a 300 MHz spectrometer. Attributions were made on the basis of H
and 13C NMR, as well as distortionless enhancement by polarization
transfer (DEPT)-135 experiments; chemical shifts are reported in ppm
downfield from tetramethylsilane (TMS). Photochemical reactions
were performed by using nitrogen-purged solutions in quartz tubes.
Irradiations were performed in a multilamp reactor fitted with 10 15-W
phosphor-coated lamps (emission maximum 310 nm). Quantum
yields (Φ−1) of compounds 1a−d and 10a have been measured on a
0.015 M (1a, 1c, 1d) or 0.01 M (1b, 10a) solution in TFE (irradiation
1 × 15 W Hg lamp). Workup of the photolytes involved concentration
in vacuo and chromatographic separation on silica gel. Solvents of
HPLC purity were employed in the photochemical reactions. All the
employed π bond nucleophiles [allyltrimethylsilane (ATMS), 4-
pentenoic acid, 1-hexyne, benzene, and mesitylene] were commercially
available and used as received.
Synthesis of Aryl Imidazolesulfonates 1a−d. Compounds 1a−
d were prepared from the corresponding phenols as previously
reported.5b A 50 mL round-bottom flask was charged with the
corresponding phenol 2a−d (3.47 mmol), 1,1′-sulfonyldiimidazole5b
(Im2SO2, 6.94 mmol), and cesium carbonate (Cs2CO3, 1.87 mmol) in
tetrahydrofuran (THF, 10 mL). The reaction was stirred at room
temperature for 16 h, and then the solvent was evaporated. Ethyl
acetate (20 mL) was added to the resulting residue, and the obtained
solution was cooled to 0 °C and treated with saturated aqueous
NH4Cl. The layers were separated and the aqueous layer was washed
further with ethyl acetate (2 × 10 mL). The combined organic extracts
were washed with brine (10 mL) and water (10 mL) and dried over
MgSO4, and the solvent was removed under reduced pressure. The
crude residue was purified by column chromatography (eluant
cyclohexane/ethyl acetate).
4-Methoxyphenyl 1H-Imidazole-1-sulfonate (1a). From 4-me-
thoxyphenol (2a, 430 mg, 3.47 mmol), Im2SO2 (1.375 g, 6.94 mmol),
and Cs2CO3 (0.565 g, 1.87 mmol) in THF (10 mL). Purification via
column chromatography (eluant cyclohexane/ethyl acetate 9:1)
afforded 775 mg of 1a (viscous oil, 88% yield). Spectroscopic data
for 1a are in accordance with the literature.5d Anal. Calcd for
C10H10N2O4S: C, 47.24; H, 3.96; N, 11.02. Found: C,47.2; H, 4.0; N,
11.0.
4-(N,N-Dimethylamino)phenyl 1H-Imidazole-1-sulfonate (1b).
From 476 mg (3.47 mmol) of 4-(N,N-dimethylamino)phenol
(2b),16 1.375 g (6.94 mmol) of Im2SO2, and 565 mg (1.87 mmol)
of Cs2CO3 in THF (10 mL). Purification via column chromatography
(eluant cyclohexane/ethyl acetate 9:1) afforded 650 mg of 1b
(colorless solid, mp = 62−64 °C, 70% yield). 1H NMR (CD3COCD3,
δ) 7.70 (s, 1H), 7.25−7.30 (s, 1H), 7.20 (s, 1H), 6.80−6.60 (4H,
AA′BB′), 2.95 (s, 6H). 13C NMR (CD3COCD3, δ) 149.5, 139.8, 137.5
(CH), 131.0 (CH), 121.7 (CH), 118.2 (CH), 112.9 (CH), 40.7
(CH3). IR (neat, ν/cm−1) 3320, 2924, 1416, 1048, 947. Anal. Calcd
for C11H13N3O3S: C, 49.43; H, 4.90; N, 15.72. Found: C, 49.5; H, 4.8;
N, 15.7.
4-tert-Butylphenyl 1H-Imidazole-1-sulfonate (1c). From 4-tert-
butylphenol (2c, 521 mg, 3.47 mmol), Im2SO2 (1.375 g, 6.94 mmol),
and Cs2CO3 (565 mg, 1.87 mmol) in THF (10 mL). Purification by
column chromatography (eluant cyclohexane/ethyl acetate 9:1)
afforded 700 mg of 1c (colorless solid, mp = 45−47 °C, 72% yield).
1H NMR (CD3COCD3, δ) 7.90 (s, 1H), 7.40−7.35 (2H, part of the
AA′BB′ system), 7.35−7.30 (s, 1H), 7.20 (s, 1H), 7.09−6.95 (2H, part
of the AA′BB′ system), 1.30 (s, 9H). 13C NMR (CD3COCD3, δ)
153.1, 148.3, 137.5 (CH), 131.2 (CH), 127.1 (CH), 120.5 (CH),
118.2 (CH), 34.6, 31.1 (CH3). IR (neat, ν/cm−1) 3131, 2966, 1504,
1428, 886. Anal. Calcd for C13H16N2O3S: C, 55.70; H, 5.75; N, 9.99.
Found: C, 55.6; H, 5.8; N, 9.9.
E
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The Journal of Organic Chemistry
Article
solutions. 10b: GC-MS (m/z) 299 (18), 136 (100), 108 (10), 65 (5).
10c: GC-MS (m/z) 312 (10), 297 (100), 267 (10), 91 (5). 10d: GC-
MS (m/z) 256 (50), 93 (100), 77 (10), 65 (90).
5-(N,N-Dimethyl-4-aminobenzyl)dihydrofuran-2-one (5b). From
488 mg of 9b, (1.5 mmol, 0.05 M), 488 mg of Cs2CO3 (1.5 mmol,
0.05 M), and 1.3 mL of 4-pentenoic acid (15 mmol, 0.5 M) in TFE
(30 mL), irradiated for 4 h. Purification by column chromatography
(eluant cyclohexane/ethyl acetate 95:5) afforded 260 mg of 5b
(colorless solid, mp = 51−53 °C, lit.24b mp 54−57 °C, 79% yield).
Spectroscopic data for 5b are in accordance with the literature.19 Anal.
Calcd for C13H17NO2: C, 71.21; H, 7.81; N, 6.39. Found: C, 71.3; H,
7.8; N, 6.4.
4-(N,N-Dimethylamino)biphenyl (6b). From 647 mg (1.5 mmol,
0.05 M) of 9b, 488 mg (1.5 mmol, 0.05 M) of Cs2CO3, and 2.67 mL
(30 mmol, 1.0 M) of benzene in TFE (30 mL), irradiated for 4 h.
Purification by column chromatography (eluant neat cyclohexane)
afforded 231 mg of 6b (colorless solid, mp = 119−121 °C, lit.25 mp
120−123 °C, 78% yield). Spectroscopic data for 6b are in accordance
with the literature.25 Anal. Calcd for C14H15N: C, 85.24; H, 7.66; N,
7.10. Found: C, 85.3; H, 7.6; N, 7.1.
4-(N,N-Dimethylamino)-2′,4′,6′-trimethylbiphenyl (7b). From
647 mg (1.5 mmol, 0.05 M) of 9b, 488 mg (1.5 mmol, 0.05 M) of
Cs2CO3, and 4.17 mL (30 mmol, 1.0 M) of mesitylene in TFE (30
mL), irradiated for 4 h. Purification by column chromatography
(eluant neat cyclohexane) afforded 252 mg of 7b (colorless solid, mp
= 103−106 °C, lit.24 mp 105−108 °C, 70% yield). Spectroscopic data
for 7b are in accordance with the literature.24 13C NMR (CDCl3, δ)
136.6, 136, 130 (CH), 127.9 (CH), 112.5 (CH), 40.8 (CH3), 20.9
(CH3), 20.8 (CH3). Anal. Calcd for C17H21N: C, 85.30; H, 8.84; N,
5.85. Found: C, 85.3; H, 8.8; N, 5.9.
Synthesis of 4-tert-Butyl Compounds 4c−8c. 1-tert-Butyl-4-
allylbenzene (4c). From 667 mg of 9c (1.5 mmol, 0.05 M), 488 mg of
Cs2CO3 (1.5 mmol 0.05 M), 3 mL of acetone (10% v/v), and 2.4 mL
of ATMS (15 mmol, 0.5 M) in TFE (30 mL), irradiated for 24 h.
Purification by column chromatography (eluant neat hexane) afforded
18 mg of 4c (oil, 7% yield). Spectroscopic data for 4c are in
accordance with the literature.26 Anal. Calcd for C13H18: C, 89.59; H,
10.41. Found: C, 89.5; H, 10.4.
Preparative Irradiations. Irradiation of Aryl Imidazylates 1a−d.
A 0.05 M solution of sulfonates 1a−d, the chosen π-bond nucleophile
(0.5−1.0 M), 0.05 M Et3N, and, when required, acetone (10% v/v) in
TFE was nitrogen-purged in a quartz tube and then irradiated at 310
nm. The reaction course was followed by means of GC and HPLC
analyses. GC yields of compounds 2a−d, 3a−d, 5d, and 7d were
determined by comparison with either commercial standards or
synthesized compounds.
Irradiation of Sulfonates 9a−d. A 0.05 M solution of sulfonate
9a−d, the chosen π-bond nucleophile (0.5−1.0 M), 0.03 M Cs2CO3,
and, when required, acetone (10% v/v) in TFE was nitrogen-purged in
a quartz tube and then irradiated at 310 nm. The reaction course was
followed by means of GC and HPLC analyses. GC yields of
compounds 2a−d, 3a−d, 5d, and 7d were determined by comparison
with either commercial standards or synthesized compounds.
Synthesis of 4-Methoxy Compounds 4a−8a. 1-(2-Propenyl)-
4-methoxybenzene (4a). From 628 mg of 9a (1.5 mmol, 0.05 M),
488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL of acetone (10% v/v),
and 2.4 mL of ATMS (15 mmol, 0.5 M) in TFE (30 mL), irradiated
for 24 h. Purification by column chromatography (neat cyclohexane)
afforded 171 mg of 4a (oil, 77% yield). Spectroscopic data for 4a are in
accordance with the literature.18 Anal. Calcd for C10H12O: C, 81.04; H,
8.16. Found: C, 81.0; H, 8.2.
5-(4-Methoxybenzyl)dihydrofuran-2(3H)-one (5a). From 628 mg
of 9a (1.5 mmol, 0.05 M), 488 mg of Cs2CO3 (0.05 M, 1.5 mmol), 3
mL of acetone (10% v/v), and 1.3 mL of 4-pentenoic acid (15 mmol,
0.5 M) in TFE (30 mL), irradiated for 24 h. Purification by column
chromatography (eluant cyclohexane/ethyl acetate 8:2) afforded 180
mg of 5a (oil, 58% yield). Spectroscopic data for 5a are in accordance
with the literature.19 Anal. Calcd for C12H14O3: C, 69.88; H, 6.84.
Found: C, 69.9; H, 6.9.
4-Methoxybiphenyl (6a). From 628 mg of 9a (1.5 mmol, 0.05 M),
488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL of acetone (10% v/v),
and 2.67 mL of benzene (30 mmol, 1 M) in TFE (30 mL), irradiated
for 24 h. Purification by column chromatography (eluant cyclohexane/
ethyl acetate 97:3) afforded 219 mg of 6a (colorless solid, mp = 87−89
°C, lit.20 mp 86−88 °C, 79% yield). Spectroscopic data for 6a are in
accordance with the literature.21 Anal. Calcd for C13H12O: C, 84.75; H,
6.57. Found: C, 84.7; H, 6.6.
5-(4-tert-Butylbenzyl)dihydrofuran-2(3H)-one (5c). From 667 mg
of 9c (1.5 mmol, 0.05 M), 488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3
mL of acetone (10% v/v), and 1.3 mL of 4-pentenoic acid (15 mmol,
0.5M) in TFE (30 mL), irradiated for 24 h. Purification by column
chromatography (eluant cyclohexane/ethyl acetate 8:2) afforded 181
mg of 5c (oil, 52% yield). Spectroscopic data for 5c are in accordance
with the literature.26 Anal. Calcd for C15H20O2: C, 77.55; H, 8.68.
Found: C, 77.5; H, 8.7.
4′-Methoxy-2,4,6-trimethyl-1,1′-biphenyl (7a). From 628 mg of
9a (1.5 mmol, 0.05 M), 488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL
of acetone (10% v/v), and 4.17 mL of mesitylene (30 mmol, 1.0 M) in
TFE (30 mL), irradiated for 24 h. Purification by column
chromatography (eluant cyclohexane/ethyl acetate 97:3) afforded
318 mg of 7a (colorless solid, mp = 68−70 °C, lit.22 mp 69−71 °C,
94% yield). Spectroscopic data for 7a are in accordance with the
literature.22 Anal. Calcd for C16H18O: C, 84.91; H, 8.02. Found: C,
84.9; H, 8.0. Compound 7a was obtained in 77% yield when 0.5 M
mesitylene was used.
1-(Hex-1-ynyl)-4-methoxybenzene (8a). From 628 mg of 9a (1.5
mmol, 0.05 M), 488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL of
acetone (10% v/v), and 1.7 mL of 1-hexyne (15 mmol, 0.5 M) in TFE
(30 mL). The solution was nitrogen-purged in quartz tubes and then
irradiated for 24 h. Purification by column chromatography (eluant
neat hexane) afforded 178 mg of 8a (oil, 63% yield). Spectroscopic
data for 8a are in accordance with the literature.23 Anal. Calcd for
C13H16O: C, 82.94; H, 8.57. Found: C, 83.0; H, 8.6. Compound 8a
was obtained in 73% yield when 1 M 1-hexyne was used.
Synthesis of 4-(N,N-Dimethylamino) Compounds 4b−7b. 4-
(Propen-2-yl)-N,N-dimethylanisole (4b). From 647 mg (1.5 mmol,
0.05 M) of 9b, 488 mg (1.5 mmol, 0.05 M) of Cs2CO3, and 2.4 mL
(15 mmol, 0.5 M) of ATMS in TFE (30 mL), irradiated for 4 h.
Purification by column chromatography (eluant cyclohexane/ethyl
acetate 98:2) afforded 174 mg of 4b (oil, 72% yield). Spectroscopic
data for 4b are in accordance with the literature.24a Anal. Calcd for
C11H15N: C, 81.94; H, 9.38; N, 8.69. Found: C, 81.9; H, 9.5; N, 8.7.
4-tert-Butylbiphenyl (6c). From 667 mg of 9c (1.5 mmol, 0.05 M),
488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL of acetone (10% v/v),
and 2.67 mL of benzene (30 mmol, 1 M) in TFE (30 mL). The
solution was nitrogen-purged in quartz tubes and then irradiated for 24
h. Purification by column chromatography (eluant neat hexane)
afforded 216 mg of 6c (colorless solid, mp = 45−47 °C, lit.26 mp 49−
51 °C, 68% yield). Spectroscopic data for 6c are in accordance with
the literature.26 Anal. Calcd for C16H18: C, 91.37; H, 8.63. Found: C,
91.4; H, 8.6.
4′-tert-Butyl-2,4,6-trimethylbiphenyl (7c). From 667 mg of 9c (1.5
mmol, 0.05 M), 488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL of
acetone (10% v/v), and 4.17 mL of mesitylene (30 mmol, 1 M) in
TFE (30 mL), irradiated for 24 h. Purification by column
chromatography (eluant neat hexane) afforded 276 mg of 7c (colorless
solid, mp = 108−110 °C, lit.26 mp 108−110 °C, 73% yield).
Spectroscopic data for 7c are in accordance with the literature.21 Anal.
Calcd for C19H24: C, 90.42; H, 9.58. Found: C, 90.4; H, 9.7.
1-tert-Butyl-4-(hex-1-ynyl)benzene (8c). From 667 mg of 9c (1.5
mmol, 0.05 M), 488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL of
acetone (10% v/v), and 1.7 mL of 1-hexyne (15 mmol, 0.5 M) in TFE
(30 mL), irradiated for 24 h. Purification by column chromatography
(eluant neat hexane) afforded 96 mg of 8c (colorless oil, 30% yield).
1H NMR (CDCl3, δ) 7.45−7.25 (AA′BB′, 4H), 2.45−235 (t, 2H, J = 7
Hz), 1.7−1.5 (m, 4H), 1.35 (s, 9H), 1.0−0.9 (t, 3H, J = 7 Hz); 13C
NMR (CDCl3, δ) 150.5, 131.1 (CH), 125.0 (CH), 121.0, 89.5, 80.4,
34.5, 31.1 (CH3), 30.8 (CH2), 21.9 (CH2), 19.0 (CH2), 13.5 (CH3).
IR (neat, ν/cm−1) 2961, 2250, 1466, 1364, 1269, 834. Anal. Calcd for
F
dx.doi.org/10.1021/jo502172c | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
C16H22: C, 89.65; H, 10.35. Found: C, 89.7; H, 10.4. Compound 8c
was obtained in 54% yield when 1 M 1-hexyne was used.
(5) (a) Shirbin, S. J.; Boughton, B. A.; Zammit, S. C.; Zanatta, S. D.;
Marcuccio, S. M.; Hutton, C. A.; Williams, S. J. Tetrahedron Lett. 2010,
51, 2971−2974. (b) Civicos, J. F.; Alonso, D. A.; Najera, C. Adv. Synth.
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Synthesis of 4-Unsubstituted Compounds 5d and 7d. 5-
Benzyldihydrofuran-2(3H)-one (5d). From 582 mg of 9d (1.5 mmol,
0.05 M), 488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL of acetone
(10% v/v), and 1.3 mL of 4-pentenoic acid (15 mmol, 0.5 M, 15
mmol) in TFE (30 mL), irradiated for 24 h. Purification by column
chromatography (eluant cyclohexane/ethyl acetate 8:2) afforded 104
mg of 5d (oil, 39% yield). Spectroscopic data for 5d are in accordance
with the literature.27 Anal. Calcd for C11H12O2: C, 74.98; H, 6.86.
Found: C, 75.0; H, 6.9.
2,4,6-Trimethylbiphenyl (7d). From 582 mg of 9d (1.5 mmol, 0.05
M), 488 mg of Cs2CO3 (1.5 mmol, 0.05 M), 3 mL of acetone (10% v/
v), and 4.17 mL of mesitylene (30 mmol, 1 M) in TFE (30 mL),
irradiated for 24 h. Purification by column chromatography (eluant
neat cyclohexane) afforded 97 mg of 7d (oil, 33% yield). Spectroscopic
data for 7d are in accordance with the literature.28 Anal. Calcd for
C15H16: C, 91.78; H, 8.22. Found C, 91.8; H, 8.2.
(d) Cívicos, J. F.; Alonso, D. A.; Naj
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(12) Chatgilialoglu, C.; Griller, D.; Rossini, S. J. Org. Chem. 1989, 54,
2734−2737.
ASSOCIATED CONTENT
■
S
* Supporting Information
One table listing photophysical properties of 1a−d, 9a−d, and
10a; additional text and one figure showing efficiency of acid
1
photorelease from 1a; H and 13C NMR spectra of 1a−d, 5d,
7d, 8c, 9a−d, and 10a. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Fax +39 0382 987323; tel +39 0382 987198; e-mail fagnoni@
unipv.it.
(13) (a) Guan, B.-T.; Lu, X.-Y.; Zheng, Y.; Yu, D.-G.; Wu, T.; Li, K.-
L.; Li, B.-J.; Shi, Z.-J. Org. Lett. 2010, 12, 396−399. (b) Ackermann, L.;
Pospech, J.; Potukuchi, H. K. Org. Lett. 2012, 14, 2146−2149.
(14) See, for example, Buncel, E.; Chuaqui, C. J. Org. Chem. 1980, 45,
2825−2830.
Notes
The authors declare no competing financial interest.
(15) See, for instance, Liu, W.; Cao, H.; Zhang, H.; Zhang, H.;
Chung, K. O.; He, C.; Wang, H.; Kwong, F. Y.; Lei, A. J. Am. Chem.
Soc. 2010, 132, 16737−16740.
ACKNOWLEDGMENTS
■
This work was supported by the Fondazione Cariplo (Grant
2012-0186). S.P. and H.Q. acknowledge MIUR, Rome (FIRB-
project RBFR08J78Q) and the Italian Ministry of Foreign
Affairs (E-PLUS project, Enhancement of the Palestinian
University System), respectively, for financial support. We
thank Professor A. Albini (University of Pavia) and Professor P.
E. Hoggard (Santa Clara University) for fruitful discussions. We
are grateful to Dr. D. Ravelli, Dr. D. Gozzini, and A. Gandini
(University of Pavia) for assistance.
(16) De Carolis, M.; Protti, S.; Fagnoni, M.; Albini, A. Angew. Chem.,
Int. Ed. 2005, 44, 1232−1236.
(17) Desoky, A. Y.; Hendel, J.; Ingram, L.; Taylor, S. D. Tetrahedron
2011, 67, 1281−1287.
(18) Scrivanti, A.; Beghetto, V.; Bertoldini, M.; Matteoli, U. Eur. J.
Org. Chem. 2012, 264−268.
(19) Protti, S.; Fagnoni, M.; Albini, A. J. Am. Chem. Soc. 2006, 128,
10670−10671.
(20) Tanimoro, K.; Ueno, M.; Takeda, K.; Kirihata, M.; Tanimori, S.
J. Org. Chem. 2012, 77, 7844−7849.
(21) Sreedhar, B.; Yada, D.; Reddy, P. S. Adv. Synth. Catal. 2011, 353,
2823−2836.
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