Selective deprotection of phenolic polysulfonates

Article abstract of DOI:10.1080/17415990903295694

Nosylates of phenols can be selectively deprotected by thiocresol anions in DMSO. Successful deprotection can be accomplished in molecules containing aryl-appended halides, ethers, aldehydes, alkanesulfonates or arylsulfonates. Nosylate deprotection is accomplished by the CS bond rupture which is believed to proceed by nucleophilic aromatic substitution.

Full text of DOI:10.1080/17415990903295694

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Selective deprotection of phenolic
polysulfonates
Erin E. Chapman a & Richard F. Langler a
a Department of Chemistry, Mount Allison University, Sackville,
NB, Canada, E4L 1G8
Published online: 30 Sep 2009.
To cite this article: Erin E. Chapman & Richard F. Langler (2010): Selective deprotection of
phenolic polysulfonates, Journal of Sulfur Chemistry, 31:1, 19-26
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Journal of Sulfur Chemistry
Vol. 31, No. 1, February 2010, 19–26
Selective deprotection of phenolic polysulfonates
Erin E. Chapman and Richard F. Langler*
Department of Chemistry, Mount Allison University, Sackville, NB, Canada E4L 1G8
(Received 13 July 2009; final version received 27 August 2009)
Nosylates of phenols can be selectively deprotected by thiocresol anions in DMSO. Successful deprotection
can be accomplished in molecules containing aryl-appended halides, ethers, aldehydes, alkanesulfonates
or arylsulfonates. Nosylate deprotection is accomplished by the CS bond rupture which is believed to
proceed by nucleophilic aromatic substitution.
Keywords: sulfonate deprotection; phenols; CS cleavage of p-nitrobenzenesulfonates; nucleophilic
aromatic substitutions; single-electron transfer
1. Introduction
Recently, we have taken on the problem of constructing phenol-ether-derived sulfonates to explore
their biological activities vis à vis malaria, human skin cancer and breast cancer cells (1–5).
A common feature of synthetic planning for molecules with repeating functional groups is the use
of protecting groups. Given that our target molecules include phenolic-derived sulfonate moieties,
it seemed worthwhile to explore the development of a sulfonate protecting group that could be
selectively removed in the presence of other sulfonate functionalities.
Well before our current interest in phenolic sulfonate ethers emerged, a report appeared
describing smooth CS rupture in reactions between p-toluenethiolate anions and p-nitrobenzene-
sulfonates (Scheme 1). The initial report (6) has been, subsequently, placed into context in a
review of ionic reactions of sulfonic acid esters (7, pp 13–24). From the standpoint of mass bal-
ance, Scheme 1 and related reactions, described in (6), must produce phenols (e.g. 4 in Scheme 1),
although no phenols were isolated and properly characterized at that time.
2. Results and discussion
2.1. Deprotections
The p-nitrobenzenesulfonate (nosylate) of 2-naphthol, 6, was prepared and deprotected to provide
2-naphthol 7 in reasonable yield (Scheme 2).
*Corresponding author. Email: rlangler@mta.ca
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2010 Taylor & Francis
DOI: 10.1080/17415990903295694
http://www.informaworld.com

20 E.E. Chapman and R.F. Langler
O2N
SO2OPh + p-CH3(C6H4)SNa
2
1
HMPA
O2N
S
CH3
+
+
PhOH
SO2
3 (67%)
4
CH3
S
+
SO2OPh
5 (16%)
Scheme 1.
ONos
OH
DMSO
+
p-CH3(C6H4)SNa
2
6
7 (70%)
Scheme 2.
Under these same conditions, the corresponding p-cyanobenzenesulfonate furnished
2-naphthol 7 in 55% yield (8), so that further work focussed on nosylate deprotections. Note
that yields for both nosylate preparations and deprotections refer to isolated products.
On the basis of Scheme 2 results, we have concluded that nucleophilic aromatic substitutions, by
mercaptideanions, resultinginCScleavageofphenol-derivednosylatesshowedsomepromiseasa
deprotection strategy. Next, the question of selectivity in the deprotection of mixed polysulfonates
was tackled. The nosylate tosylate 8, upon treatment with the arylthiolate 2/DMSO smoothly
removed the nosylate group and left the tosylate group in place (Scheme 3).
Hence, good selectivity and reasonable yields in nosylate deprotections might be expected.
When Scheme 3 reaction was repeated with sodium phenylmethanethiolate in place of 2, no
tosylate phenol 9 and no benzylic sulfide (corresponds to 3 in Scheme 1) were isolated. Further
work employed 2 as the reagent of choice.
To explore selectivity in the deprotection of polysulfonates, the di-n-butanesulfonate nosy-
late derived from phloroglucinol, 10, was subjected to our standardized deprotection conditions
(Scheme 4).
Earlier attempts (3) to convert symmetrical polyphenolic aromatic rings directly to phenol
polysulfonates (e.g. preparation of 11) proved to be rather frustrating, giving bad mixtures with
low yields. Therefore, we have briefly examined the preparation of a nosylate phenol by attempted
DMSO
TsO
+
p-CH3(C6H4)SNa
ONos
TsO
OH
2
8
9 (61%)
Scheme 3.

Journal of Sulfur Chemistry 21
ONos
+ 2
CH3(CH2)3SO2O
OSO2(CH2)3CH3
DMSO
10
OH
CH3(CH2)3SO2O
OSO2(CH2)3CH3
11 (35%)
Scheme 4.
monodeprotection of the dinosylate. The p-dinosylate of benzene, 12, was smoothly converted
to the nosylate phenol 13 as shown in Scheme 5.
Note that work with polynosylates is complicated by their very poor solubility in common
organic solvents.
Nosylate deprotection, in the presence of other functionalities, was briefly explored with the
results given in Scheme 6.
The p-bromo substrate 14 was selected for inclusion in this study because p-haloaryl sul-
fonates appear to undergo complicating single-electron transfer (SET) chemistry in some reactions
(7, 9, 10).
Nonetheless, in this case, deprotection appears to have proceeded by means of a nucleophilic
aromatic substitution and gave a reasonable yield.
The vanillin nosylate 14 (Scheme 6) was examined as an exemplary case which provided an
assortment of electrophilic sites to compete for mercaptide anion attack. Although the yield of
the deprotected phenol, 17, was not high, no sulfide aldehyde 18 (Figure 1) was formed in the
reaction shown in Scheme 6.
Competing formation of 18 was the chief concern prior to undertaking the deprotection of 16.
DMSO
NosO
+ 1 p-CH3(C6H4)SNa
ONos
NosO
OH
2
12
13 (58%)
Scheme 5.
DMSO
X
ONos
OH
+ 2
X
Y
Y
14 X = Br; Y = H
15 X = Br; Y = H (58%)
17
16
X = CHO; Y = OCH3
X = CHO; Y = OCH3 (40%)
Scheme 6.

22 E.E. Chapman and R.F. Langler
O
H3C
S
H
H3CO
18
Figure 1. Structure for 18.
2.2. Mechanistic considerations
Earlier, we have explored the mechanism for mercaptide-anion-induced CS bond rupture in aryl
nosylates (6). By means of model nosylates, we established that p-tolyl mercaptide anions would
induce a ratio of CS bond rupture to CO bond rupture of 19:1 in the hypothetical reaction with
nosylate 19 (Figure 2) in HMPA.
ZINDO molecular orbital calculations (6) supported the view that our experimental results
were consistent with nucleophilic aromatic substitution (Scheme 7) rather than SET.
In a related study, Bordwell and Hughes (11) have shown that p-nitrobenzenes, equipped with
good leaving groups, react with 9-substituted fluorenides in DMSO through a Meisenheimer
complex and that neither benzynes nor SET intervenes. In contrast, Sammes et al. (12) have
shown that the displacement of a nitro group from p-dinitrobenzene by substituted phenoxide
O
O
NO2
S
O
NO2
19
Figure 2. Structure for 19.
Na
(C6H4)CH3-p
S
O
N
SO2OAr
Na
O
O
O
SO2OAr
N
S(C6H4)CH3-p
p -O2N(C6H4)S(C6H4)CH3-p
+
3
O
OAr
S
Na
SO2
+ OAr
Na
O
Scheme 7.

Journal of Sulfur Chemistry 23
ions in DMSO is inhibited by radical scavengers that may implicate SET in those reactions.
Furthermore, Shishlov et al. (13–15) have reported fascinating results supporting SET steps
between arylsulfonates and hydroxide ions in aqueous DMSO or lithium metal in dry DMSO.
Typically, a freshly prepared reaction mixture containing mercaptide anions and
p-nitroarylsulfonyl substrates in DMSO or HMPA is black and turns orange/red after a brief
interval. A black solution is consistent with the presence of a species having a loosely held elec-
tron as would be expected for a radical anion. Since our product studies and computational work
(6) implicated nucleophilic aromatic substitution (Scheme 7) as the principal pathway by which
the major products arose, we concluded that SET was, at best, a minor pathway in those reactions.
Reactions described in the current report revealed a new product, p-tolyl p-nitrophenyl
sulfoxide 20 (Figure 3) along with p-tolyl disulfide 21.
In several cases, the sulfoxide was isolated in good purity. Yields ranged from 1 to 10%.
O
p-O2N (C6H4)
S
20
(C6H4)CH3-p
Figure 3. Structure for 20 which is racemic.
_
SET
H3C
S
+
NosOAr
H3C
S
+ NosOAr
DMSO
SET
NosOAr
H3C
S
H3C
S
O
S(CH3)2
O
S(CH3)2
SN2
H3C
S
H3C
S
SN2
O
(CH3)2S
+
H3C
S
O
2
+ (p-CH3(C6H4)S)2
(up to 10%)
H3C
S
21
NosOAr
O
S
p-O2N(C6H4)
(up to 10%)
+
ArO
(C6H4)CH3-p
20
Scheme 8.

24 E.E. Chapman and R.F. Langler
A straightforward rationale for the observation of the sulfoxide 20 in product mixtures would
invoke nucleophilic aromatic substitution arising from nucleophilic attack by p-tolyl sulfenate
anions on a nosylate (e.g. 6 in Scheme 1). The mechanistic proposal in Scheme 8 accounts for the
formation of a modest amount of sulfenate anions. A summary of a good array of circumstantial
evidence for SET steps in reactions of thiolate anions with aryl sulfonates is available (7).
Hence, in our view, the sulfoxide offers intriguing support for SET chemistry as a route to the
minor products, 20 and 21, in these reactions.
3. Conclusions
Phenolic polysulfonates, in which at least one of the sulfonate protecting groups is a nosylate
group, will, in the presence of thiocresol anions, undergo selective removal of the nosylate group(s)
restoringphenolichydroxylgroupsatthosesites. Pastevidencesuggeststhatthereactionsproceed,
principally, by nucleophilic aromatic substitutions (Scheme 7). Current evidence suggests that
SET chemistry may account for the formation of a secondary product, the sulfoxide 20 (Scheme 8).
4. Experimental
4.1. General
Infrared spectra were recorded on a Thermolet Nicolet 2000 spectrophotometer. 1H NMR
(270 MHz) and 13C NMR spectra were obtained on a JEOL JNM-GSX270 Fourier-transform
NMRsystem. Unlessotherwisespecified, allNMRspectrawereobtainedindeuteratedchloroform
using tetramethylsilane as an internal standard. Mass spectra were obtained on a Hewlett-Packard
5988A gas–liquid chromatography mass spectrometer system. Melting point determinations were
done with a Gallenkamp MFB-595 capillary melting point apparatus and are uncorrected.
The staring phenols, 2-naphthol, dihydroquinone, p-bromophenol and vanillin were obtained
from Aldrich Chemicals. The preparation and properties of dibutanesulfonate phenol 11 and
tosylate phenol 9 have been described elsewhere (1, 3).
4.2. Standard nosylate preparation and nosylate properties
The phenolic starting material (ca. 0.5 g) and dry triethylamine (1 equivalent) were added to dry
pyridine (ca. 50 ml). p-Nitrobenzenesulfonyl chloride (1 equivalent) was added slowly over 5 min
and the reaction stirred at ambient temperature for 1 week. Chloroform (200 ml) was added and
the resultant solution washed with 2.5% hydrochloric acid (100 ml aliquots) until the aqueous pH
remained acidic. In the preparation of 6, 8, 10, 14 and 16, damp homogeneous solutions were
obtained. At this point in the preparation of dinosylate 12, the product had precipitated and was
filtered off using impure solid 12 and a damp homogeneous solution.
The damp homogeneous solution was dried (MgSO4), filtered and the solvent evaporated to
afford the target molecule 6, 8, 10, 14 or 16 along with unchanged phenol. At this point in the
preparation of 12, the residue obtained from solvent evaporation was, principally, impure nosylate
phenol 13 (ca. 12% yield).
In each case, except dinosylate 12, the crude product was dissolved in chloroform (200 ml)
and the resultant solution washed with 2.5% sodium hydroxide solution (two 100 ml portions),
dried (MgSO4), filtered and the solvent evaporated. Each target nosylate, except 12, was recrystal-
lized from methanol affording clean product. Solid dinosylate 12 (prepared from dihydroquinone
(0.7 g) and p-nitrobenzenesulfonyl chloride (2.8 g)) was covered with methanol (1750 ml) and
the mixture boiled. The hot mixture was filtered affording clean dinosylate 12 (1.47 g).

Journal of Sulfur Chemistry 25
The recrystallized nosylate of 2-naphthol, 6, was obtained in 33% yield (mp 106.0–107.5 ◦C).
C16H11NO5S requires C, 58.4; H, 3.4. Found C, 58.0; H, 3.2%. 6 had IR 1531, 1403, 1351,
1189 cm−1. 1H NMR (270 MHz) δ 7.07 (d of d, J = 8.9 Hz, J = 2.5 Hz, 1H), 7.50 (m, 3H), 7.78
(m, 3H), 8.04 (d, J = 8.9 Hz, 2H), 8.34 (d, J = 8.9 Hz, 2H). 13C NMR δ 119.8, 120.5, 124.4,
126.9, 127.3, 127.92, 127.96, 130.0, 130.3, 132.1, 133.4, 141.1, 146.7, 151.0. GCMS: 329 (M+·,
13%), 143 (55%), 115 (100%).
The recrystallized nosylate tosylate 8 was obtained in 17% yield (mp 164.5–166.5 ◦C).
C19H15NO8S2 requires C, 50.8; H, 3.4. Found C, 50.7; H, 3.5%. 8 had IR 1597, 1529, 1406,
1
1367, 1149 cm−1. H NMR (270 MHz) δ 2.47 (s, 3H), 6.97 (s, 4H), 7.34 (d, J = 8.2 Hz, 2H),
7.70 (d, J = 8.2 Hz, 2H), 8.30 (d, J = 8.9 Hz, 2H), 8.41 (d, J = 8.9 Hz, 2H). 13C NMR δ 21.8,
123.4, 124.1, 124.5, 128.5, 129.94, 129.98, 132.0, 140.7, 145.9, 147.3, 148.4, 151.2. GCMS: 449
(M+·, 11%), 419 (46%), 155 (100%), 91 (64%).
The recrystallized nosylate dibutanesulfonate 10 was obtained in 41% yield (mp 106.0–
107.0 ◦C). C20H25NO11S3 requires C, 43.5; H, 4.6. Found C, 43.7; H, 4.6%. 10 had IR 1531,
1377, 1351, 1170 cm−1. 1H NMR (270 MHz) δ 0.98 (t, 6H), 1.50 (sex, 4H), 1.92 (quin, 4H), 3.25
(t, 4H), 6.90 (d, J = 2.0 Hz, 2H), 7.11 (t, J = 2.0 Hz, 1H), 8.01 (d, J = 8.7 Hz, 2H), 8.39 (d,
J = 8.7 Hz, 2H). 13C NMR δ 13.4, 21.4, 25.4, 51.1, 115.7, 116.1, 124.8, 130.0, 139.7, 149.3,
149.6, 151.4. GCMS: 551 (M+·, 0.1%), 366 (25%), 246 (65%), 126 (100%).
The dinosylate 12 was obtained in 48% yield. C18H12N2O10S2 requires C, 45.0; H, 2.5. Found:
C, 45.4; H, 2.5%. 12 had IR 1532, 1404, 1348, 1146 cm−1. 1H NMR (270 MHz, DMSO-d6) δ 7.11
(s, 4H), 8.11 (d, J = 8.9 Hz, 4H), 8.43 (d, J = 8.9 Hz, 4H). 13C NMR (DMSO-d6) δ 124.6, 125.5,
130.6, 139.5, 147.8, 151.7. MS(DIP): 480 (M+·, 16%), 355 (15%), 294 (38%), 186 (100%).
Preparation of dinosylate 12 also furnished the nosylate phenol 13 in 9% yield after recrys-
tallization from 2:1 chloroform:carbon tetrachloride (mp 132.0–134.0 ◦C). C12H9NO6S requires
C, 48.8; H, 3.1. Found: C, 49.2; H, 3.4%. 13 had IR 3479, 1525, 1502, 1404, 1361, 1147 cm−1
.
1H NMR (270 MHz) δ 5.24 (s, 1H), 6.71 (d, J = 9.1 Hz, 2H), 6.82 (d, J = 9.1 Hz, 2H), 8.00 (d,
J = 8.9 Hz, 2H), 8.35 (d, J = 8.9 Hz, 2H). 13C NMR δ 116.4, 123.3, 124.3, 130.0, 140.9, 142.7,
151.0, 154.9. GCMS: 123 (4%), 109 (M+· – C6H4NO4S, 100%), 81 (17%).
The recrystallized bromonosylate 14 was obtained in 39% yield (mp 125.5–126.2 ◦C).
C12H8BrNO5S requires C, 40.2; H, 2.3. Found: C, 40.0; H, 2.2%. 14 had IR 1530, 1392, 1348,
1172 cm−1. 1H NMR (270 MHz) δ 6.89 (d, J = 8.9 Hz, 2H), 7.44 (d, J = 8.9 Hz, 2H), 8.02 (d. J =
9.0 Hz, 2H), 8.38 (d. J = 9.0 Hz, 2H). 13C NMR δ 121.4, 123.9, 124.5, 129.9, 133.2, 140.7, 148.1,
151.2. GCMS: 359 (M+ · +2, 14%), 357 (M+·, 13.4%), 186 (32%), 173 (100%), 171 (98.8%).
The recrystallized aldehyde nosylate 16 was obtained in 34% yield (mp 151.0–153.0 ◦C).
C14H11NO7S requires C, 49.9; H, 3.3. Found: C, 50.1; H, 3.5%. 16 had IR 1705, 1533, 1420, 1348,
1142 cm−1. 1H NMR (270 MHz) δ 3.62 (s, 3H), 7.38 (d, J = 1.6 Hz, 1H), 7.41 (d, J = 8.0 Hz,
1H), 7.47 (d of d, J = 8.0 Hz, J = 1.6 Hz, 1H), 9.93 (s, 1H). 13C NMR δ 55.9, 111.3, 124.1,
124.4, 124.7, 129.9, 136.3, 141.8, 142.3, 151.0, 152.2, 190.5. GCMS: 151 (M+· – C6H4NO4S,
100%), 122 (9%), 95 (36%).
4.3. Nosylate deprotections
Nosylate deprotections were carried out in the manner described below for the exemplary case of
6. Sodium metal (19 mg, 0.83 mmol) was dissolved in methanol (10 ml) and thiocresol (107 mg,
0.86 mmol) added. The solution was stirred for 5 min and the solvent evaporated. DMSO (15 ml)
was added and the mixture stirred to dissolve the salt. The nosylate of 2-naphthol, 6, (251 mg,
0.76 mmol) was added in small portions over 5 min. The reaction mixture was stirred at ambi-
ent temperature for 2 h. Water (400 ml) and 10% hydrochloric acid (25 ml) were added and the
resultant mixture extracted with diethyl ether (three 100 ml aliquots). The combined ether layers

26 E.E. Chapman and R.F. Langler
were evaporated and the residue covered with 2.5% hydrochloric acid (100 ml). The resultant
mixture was extracted with diethyl ether (three 100 ml portions). The combined organic layers
were dried (MgSO4), filtered and the solvent evaporated to provide crude product (0.288 g).
Chloroform (100 ml) was added and the resultant solution extracted with 2.5% sodium hydrox-
ide (two 50 ml portions). The organic layer was dried (MgSO4), filtered and the solvent evaporated
to provide impure sulfide 3 (154 mg). The aqueous layers were combined, acidified with concen-
trated hydrochloric acid (10 ml) and backextracted with chloroform (three 100 ml aliquots). The
combined organic layers were dried (MgSO4), filtered and the solvent evaporated furnishing
1
2-naphthol (77 mg). The 2-naphthol, so obtained, was identical to the authentic material by H
NMR, 13C NMR and GCMS.
Impure sulfide 3 was chromatographed on silica gel (15 g) employing 6:1 petroleum
ether:chloroform (thirty 15 ml fractions), followed by chloroform (thirty 15 ml fractions).
Fraction 2 furnished p-tolyl disulfide 21 (15 mg). Fractions 4–8 were combined and concen-
trated affording clean sulfide 3 (117 mg) which was identical to the previously described material
(6) by 1H NMR, 13C NMR and GCMS. Fractions 31 and 32 were combined and concentrated to
give sulfoxide 20 (20 mg). Although some chemistry of 20 was outlined earlier (16), no spectra
1
were described. 20 has H NMR (270 MHz) δ 2.35 (s, 3H), 7.26 (d, J = 8.2 Hz, 2H), 7.64 (d,
J = 8.2 Hz, 2H), 7.80 (d, J = 8.9 Hz, 2H), 8.28 (d, J = 8.9 Hz, 2H). 13C NMR δ 21.5, 124.4.
125.2, 125.3, 130.5, 141.3, 142.9, 149.2, 153.3. GCMS: 261 (M+·, 30%), 245 (93%), 213 (93%),
107 (100%).
The procedure for the deprotection of the dinosylate 12 differed from the representative pro-
cedure, outlined above, in three ways. Dissolution of 12 in DMSO required warming to 90 ◦C
and the reaction time was extended to 3 h. Initial extractive work-up with diethyl ether led to
precipitation of unchanged starting material (41%) which was filtered off before the second cycle
of ether extractions was initiated.
Yields for the deprotections are given in Schemes 2–6.
Acknowledgement
The authors gratefully acknowledge technical assistance from D. Durant, R. Smith and B. McNally.
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