Temperature dependence of the reduction of phthalic thioanhydrides by NaBH4: Competition between 3-hydroxythiolactone and phthalide formation

Article abstract of DOI:10.1002/1099-0690(200203)2002:6<1033::AID-EJOC1033>3.0.CO;2-S

The reduction of phthalic thioanhydrides with NaBH₄ at between 0 and -20 °C leads to the selective formation of 3-hydroxy-2-thiophthalides, whereas higher temperatures favour the formation of the corresponding phthalide derivatives. A mechanism is proposed that can explain these observations. Kinetic control at low temperatures leads to an improved stability of the intermediate alkoxide, thus allowing isolation of the γ-hydroxythiolactone after subsequent acidification of the reaction mixture. In this way the formation of the thermodynamically favoured phthalide derivative is avoided. Further reduction of the y-hydroxythiolactone with AcOH/57% aqueous HI yields 2-thiophthalides, useful precursors for conducting polymers. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0690(200203)2002:6<1033::AID-EJOC1033>3.0.CO;2-S

FULL PAPER
Temperature Dependence of the Reduction of Phthalic Thioanhydrides by
NaBH4: Competition between 3-Hydroxythiolactone and Phthalide Formation
Iwona Polec,[a] Laurence Lutsen,[a] Dirk Vanderzande,*[a] and Jan Gelan[a]
Keywords: Sulfur heterocycles / Reduction / Lactones
The reduction of phthalic thioanhydrides with NaBH4 at be- ification of the reaction mixture. In this way the formation of
tween 0 and −20 °C leads to the selective formation of 3-
the thermodynamically favoured phthalide derivative is
hydroxy-2-thiophthalides, whereas higher temperatures fa-
avoided. Further reduction of the γ-hydroxythiolactone with
vour the formation of the corresponding phthalide derivat- AcOH/57% aqueous HI yields 2-thiophthalides, useful pre-
ives. A mechanism is proposed that can explain these obser- cursors for conducting polymers.
vations. Kinetic control at low temperatures leads to an im-
proved stability of the intermediate alkoxide, thus allowing
isolation of the γ-hydroxythiolactone after subsequent acid-
(
Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
2002)
Introduction
lic anhydrides by reduction with lithium aluminium hydride
(LAH) in Et2O at Ϫ55 °C. They obtained the expected lac-
tones with satisfactory yields of between 75% and 85%. An-
other example reported by Bailey and Johnson[3] concerns
an efficient procedure using NaBH4 as the reducing agent.
This reaction was carried out in THF (or DMF) at temper-
atures around 0° C, and equimolar quantities of NaBH4
and anhydride were used.
In 1969 Schlessinger and Ponticello[1] reported the reduc-
tion of phthalic thioanhydride (1a) with metal hydrides, giv-
ing different products depending on the hydride type and
reaction conditions (Figure 1).
The introduction of a hydroxyl group at the 3-position of
the lactone ring was first described by Trost et al. in 1980.[7]
This method yields 3-hydroxyphthalide derivatives via
bromine introduction at the lactone ring, followed by its
substitution by a hydroxyl group in a reaction with potas-
sium hydroxide. An alternative route, in which the corres-
ponding phthalic anhydride is reduced with an organopalla-
dium agent, proceeded in very low yields (16%).
Paulussen et al.[8] described the synthesis of 3-benzyl-3-
hydroxy-2-thiophthalide in 1996 from the reaction of 1a
with benzylmagnesium chloride using a literature proced-
ure.[9] The initially obtained disubstituted thiolactone was
dehydrated to the benzylidene-2-thiophthalide, which was
envisaged as a potential chain-stopper in the polymerisation
reaction leading to poly(isothianaphthene) (PITN). Within
a project focused towards PITN derivatives, we have rein-
vestigated the work of Schlessinger and Ponticello[1] and
found a new and efficient synthesis of the 2-thiophthalide
derivatives 3aϪc, passing through the stable 3-hydroxy-2-
thiophthalide intermediates 2aϪc. These compounds were
obtained in the low temperature reduction of phthalic
thioanhydrides 1aϪc with NaBH4 in THF. Furthermore, it
became clear that formation of these hydroxy intermediates
is the result of kinetic control of the process, and that ther-
modynamic control favours the formation of the phthalide
derivatives 4aϪc.
Figure 1. Phthalic thioanhydride reduction (ref.1): i) excess of
LAH, ii) 1 equiv. of LAH, iii) NaBH4/benzene/MeOH
Surprisingly, the literature of the last 30 years does not
contain any other example of the reduction of phthalic
thioanhydrides by metal hydrides, the only similar examples
to be found concern the reduction of the more often utilised
oxygen derivatives.[2Ϫ6] In 1967, Bloomfield and Lee[2] de-
scribed an efficient method to synthesise lactones from cyc-
[a]
Limburg University, Institute for Materials Research (IMO),
Division of Chemistry,
Universitaire Campus, 3590 Diepenbeek, Belgium
Eur. J. Org. Chem. 2002, 1033Ϫ1036
WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ193X/02/0306Ϫ1033 $ 17.50ϩ.50/0
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I. Polec, L. Lutsen, D. Vanderzande, J. Gelan
FULL PAPER
As substrates we used 4,5-dichloro- and 4,5-dime-
thoxyphthalic thioanhydrides (1b؊c) to demonstrate the
generality of this reductive sequence. We found that sodium
borohydride reduction of thioanhydrides containing either
electron donating or electron accepting substituents in the
aromatic ring led to 3-hydroxy derivatives with moderate
efficiency. The separation of the desired compound from
the crude reaction mixture proved to be difficult as both
phthalide and hydroxylated thiophthalide show a similar
polarity — their Rf values are very close to each other in
the TLC.
Results and Discussion
Until now 2-thiophthalide (3a), utilised as one of the pos-
sible starting monomers in the polymerisation leading to
PITN,[10] could be obtained by only a few methods, for ex-
ample from phthalide (4a), upon reaction with PhCH2SNa
and subsequent (CF3CO)2O addition,[11] or from ethyl o-
[12]
toluate upon reaction with NBS, thiourea and NaHCO3
(Figure 2). Recently, Ryu et al.[13] reported the synthesis of
2-thiophthalide (3a) by an intramolecular homolytic substi-
tution of an acyl radical at the sulfur atom in o-iodobenzyl
tert-butyl sulfide, with extrusion of the tert-butyl radical
(Figure 2).
In order to improve the selectivity of this reduction, we
investigated the effect of temperature, and thus we per-
formed the reaction at Ϫ78 °C for 4,5-dichlorophthalic
thioanhydride (1b). The reaction mixture was kept at this
temperature for 1 h after thioanhydride addition, then it
was acidified, also at Ϫ78 °C, followed by warming to am-
bient temperature. Although the rate of reduction was very
slow in these conditions (there was still about 45% of
1
thioanhydride present), it was clearly visible from the H
NMR spectrum that less than 5% of phthalide (4b) had
been formed in the mixture and the reduction led selectively
to 5,6-dichloro-3-hydroxy-2-thiophthalide (2b). When the
reduction was performed at Ϫ78 °C, but the temperature of
the reaction mixture was allowed to increase to ϩ10 °C
before acidification, almost 42% of 4b was present, as estim-
ated from the NMR spectra. The proposed mechanism for
the reduction (Figure 4) may explain or give insight into the
influence of the temperature on the course of the reaction.
Figure 2. The possible pathways leading to 2-thiophthalide.
(ref.[10]): i) PhCH2SH/NaH; DMF, reflux, N2, ii) H3Oϩ, iii)
(CF3CO)2O. (ref.[11]): iv) NBS, hν, CCl4, v) thiourea, acetone, vi)
NaHCO3, heat. (ref.[12]): vii) CO, 80 atm., Bu3SnH, AIBN, benzene,
100 °C, 5h
In the process of reinvestigating the findings of Schless-
inger and Ponticello,[1] we discovered that the reduction of
phthalic thioanhydride (1a) by NaBH4 in THF at 0 °C led
to the formation of 3-hydroxy-2-thiophthalide (2a) with an
excellent yield of about 95%. Zn/AcOH reduction of
phthalic thioanhydride (1a) also gave this hydroxylated thi-
olactone, although with a somewhat lower but still satisfact-
ory yield (approx. 70%). However, small amounts of phthal-
ide (4a) formed during the reduction process (GC-MS ana-
lysis). The synthesised intermediate underwent further re-
duction with AcOH/57% aq. HI solution.[14] The expected
2-thiophthalide (3a) was obtained with a good yield of
about 75%. Following the same procedure we also obtained
5,6-disubstituted derivatives of 2-thiophthalide (3bϪc), as
well as their 3-hydroxylated intermediates 2bϪc (Figure 3).
Figure 4. Mechanism of 3-hydroxy-2-thiophthalide and phthalide
formation
The first step is the formation of an alkoxide. The relative
stability of this intermediate at low temperature guarantees
that the hydroxyl compound will be obtained after acid-
ification of the reaction medium. In the case of electron
donating groups (such as methoxy groups) we obtained a
high selectivity of this reduction at Ϫ20 °C (only about 5%
of the phthalide 4c was formed). In the case of electron
acceptors (such as chloro substituents) twice as much of
Figure 3. Synthesis of 2-thiophthalide from phthalic thioanhydride:
i) NaBH4/THF, 0 °C, ii) AcOH/HI 57% aq., heat
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Eur. J. Org. Chem. 2002, 1033Ϫ1036

Temperature Dependence of the NaBH4 Reduction of Phthalic Thioanhydrides
FULL PAPER
mixture was kept at 0 °C for 1 h, then the ice-bath was removed
and the reaction mixture was kept at room temp. for 0.5 h. It was
then cooled to 0 °C and 6  HCl was added dropwise at this tem-
perature to make the reaction mixture acidic. The THF was then
evaporated under reduced pressure, and the product was extracted
with Et2O or EtOAc (3 ϫ 50 mL). The combined organic layers
were dried over MgSO4, the mixture filtered and the solvent evap-
orated. The remaining crude substance was purified by recrystallis-
ation from hexane/EtOAc (9:1, v/v) to give the product as slightly
beige crystals (15.9 g; 96%).
the by-product was formed at the same temperature. These
results point to a kinetic control in the formation of the 3-
hydroxy-2-thiophthalides 2aϪc. At temperatures higher
than 0 °C thermodynamic control takes over and conver-
sion into an intermediate aldehyde and further reduction
could explain the formation of the phthalide derivatives.
These results are also in agreement with earlier work[8] in
which a similar behaviour to form phthalide derivatives ra-
ther than thiophthalide derivatives was observed at higher
temperatures.
Method B: Zn granules (30 mesh diameter; 1.38 g, 20 mmol) and
phthalic thioanhydride (1a; 1.64 g, 10 mmol) were refluxed in
20 mL of AcOH for 24 h. The mixture was then cooled to room
temp., 40 mL of water was added, and extraction of the reaction
mixture was carried out with 3 ϫ 30 mL of EtOAc. The organic
layers were washed with 5% aq. NaHCO3 to pH ഠ 6, separated,
dried over MgSO4, and the solvent was evaporated. 1.1 g of prod-
uct was obtained (69%) as a slightly yellow solid. M.p. from
method B: 108Ϫ112 °C, from method A: 108.4Ϫ111.4 °C. Rf (hex-
ane/EtOAc, 8:2, v/v) ϭ 0.15 (ind. UV254). GC-MS: m/z (%) ϭ 166
(100) [Mϩ], 148 [Mϩ Ϫ H2O], 138 [Mϩ Ϫ CO], 133 [Mϩ Ϫ SH], 105
[Mϩ Ϫ CH(OH)S], 77 [Mϩ Ϫ CH(OH)SCO]. 1H NMR (CDCl3,
300 MHz): δ ϭ 2.96 (s, 1 H, OH), 6.69 (s, 1 H, CHOH), 7.50Ϫ7.75
(m, 4 H, arom.). HR-MS for C8H6O2S: calcd. 166.00885; found
166.00914.
From the results above it is possible to conclude that un-
der the conditions used by us for the phthalic thioanhydride
reduction, the reaction is chemoselective in favour of the 3-
hydroxylated thiolactone (2). In the case of the experiments
described by Schlessinger and Ponticello[1] it is likely that
the thioanhydride ring is opened and sulfur is liberated as
H2S after hydrolysis of the basic salt of the thioacid, fol-
lowed by the phthalide ring closure. Comparing both sets
of results we can conclude that the nature of the final prod-
uct is strongly dependent on the temperature and the solv-
ent used for the reduction.
Considering the simplicity of the process and the readily
available reducing agents, the method of 2-thiophthalide (3)
preparation described above could be useful for the syn-
thesis of PITN and its derivatives. The 3-hydroxylated inter-
mediates we obtained are new and stable chemical struc-
tures, and may be useful in the synthesis of conducting
polymers.
Synthesis of 2-Thiophthalide (3a): 3-Hydroxy-2-thiophthalide (2a;
15.9 g, 96.1 mmol) was added to a solution of 50 mL of acetic acid
and 35 g of 57% hydroiodic acid (aqueous solution),. The mixture
was kept at 125 °C for 1 h. It was then cooled to room temp., and
poured into an aq. solution of 1  NaOH (350 mL) containing 35 g
of NaHSO3. Next, the mixture was extracted with EtOAc (3 ϫ
100 mL). The organic layers were dried over MgSO4 and decol-
ourized by treating with activated charcoal, then the solution was
filtered and the solvent was evaporated. The obtained crude sub-
stance was recrystallised from hexane/EtOAc (9:1, v/v) to give the
product as yellow crystals (10.7 g, 74%). M.p. 53.5Ϫ54.5 °C (58Ϫ60
°C: ref.[8,11] ). Rf (hexane/EtOAc, 8:2, v/v) ϭ 0.39 (ind. UV254). GC-
MS: m/z (%) ϭ 150 (100%) [Mϩ], 121 (100) [Mϩ Ϫ CHO], 105 [Mϩ
Experimental Section
General Methods: All solvents used in the synthesis were distilled
before use. THF was refluxed under nitrogen with sodium metal
and benzophenone until a blue colour persisted, and was then dis-
tilled. Phthalic thioanhydride (1a), 4,5-dichlorophthalic thioanhyd-
ride (1b) and 4,5-dimethoxyphthalic thioanhydride (1c) were ob-
tained according to existing literature procedures,[15,16] and were
analysed by GC-MS spectrometry, which confirmed their molecu-
lar mass as 164 [Mϩ], 232 [Mϩ] and 224 [Mϩ], respectively. Sodium
borohydride 98ϩ% powder and Zn 30 mesh granules were pur-
chased from Aldrich. Hydroiodic acid (57% H2O solution), and the
other substances used for the syntheses were analytically pure and
were purchased from Acros Chimica. 1H NMR spectra were ob-
tained with a Varian Unity 300 spectrometer. For all synthesized
substances spectra were recorded in deuterated chloroform; the
chemical shift at δ ϭ 7.24 (relative to TMS) for the residual pro-
tonated solvent was used as reference.
1
Ϫ CH2S], 78 [Mϩ Ϫ CH2SCO]. H NMR (CDCl3, 300 MHz): δ ϭ
4.45 (s, 2 H, CH2), 7.40Ϫ7.70 (m, 4 H, arom.). HR-MS for
C8H6OS: calcd. 150.01394; found 150.01665.
Synthesis of 5,6-Dichloro-3-hydroxy-2-thiophthalide (2b): 4,5-
Dichlorophthalic thioanhydride (1b; 7.4 g, 31.9 mmol) [previously
recrystallized from hexane/EtOAc (m.p. 95.5Ϫ96.4 °C)] dissolved
in 100 mL of THF was added to a suspension of NaBH4 (1.2 g,
31.9 mmol) in THF (50 mL) at Ϫ20 °C (NaCl-ice bath). After the
substrate addition, the reaction mixture was kept at Ϫ20 °C for
2 h, and then left in the cooling bath until its temperature reached
0 °C (ഠ1 h).The solution was then acidified with dilute HCl, the
solvent was evaporated and the residue was extracted with diethyl
ether or ethyl acetate (3 ϫ 150 mL). The organic layer was dried
over MgSO4, filtered, the ether was distilled off, and the obtained
solid was washed with 50 mL of hexane. The hexane was then fil-
tered off and the pale pink solid was dried in the air to 6.25 g of
product (84%). M.p. 129.0Ϫ131.0 °C. Rf (hexane/EtOAc, 7:3,
v/v) ϭ 0.48 (ind. UV254). GC-MS: m/z (%) ϭ 234 (80) [Mϩ], 201
GC-MS analyses were performed on TSQ-70 and Voyager mass
spectrometers (Thermoquest); capillary column: Chrompack
CPsil5CB or CPsil8CB. Melting points (uncorrected) were meas-
ured on a digital melting point apparatus, Electrothermal IA 9000
series. TLC analyses were made on Merck aluminium sheets, 20 ϫ
(90) [Mϩ Ϫ SH], 188 (100) [Mϩ Ϫ CH2S], 173 (100) [Mϩ
Ϫ
20 cm, covered with silica gel 60 F254
.
CH2OSH]. 1H NMR (CDCl3, 300 MHz): δ ϭ 6.65 (s, 1 H, CHOH),
7.82 (s, 1 H, arom.), 7.83 (s, 1 H, arom.). HR-MS for C8H4O2SCl2:
calcd. 233.93091; found 233.93128.
Synthesis of 3-Hydroxy-2-thiophthalide (2a). Method A: NaBH4
(2.4 g, 63 mmol) was refluxed for 0.5 h in 100 mL of dry THF un-
der N2 atm. The suspension was then cooled to 0 °C and at this
temp. phthalic thioanhydride (1a; 16.4 g, 100 mmol) in 100 mL of Synthesis of 3-Hydroxy-5,6-dimethoxy-2-thiophthalide (2c): In an
dry THF was added dropwise. After the addition was complete, the
analagous procedure to that described above 4,5-dimethoxy-
Eur. J. Org. Chem. 2002, 1033Ϫ1036
1035

I. Polec, L. Lutsen, D. Vanderzande, J. Gelan
FULL PAPER
phthalic thioanhydride (1c; 1.6 g, 7.1 mmol) [previously recrystallised
from hexane/EtOAc (m.p. 197.3Ϫ198.5 °C; ref.[15] m.p. 181.3 °C)]
dissolved in 30 mL of THF was reduced with NaBH4 (0.27 g,
7.1 mmol) to give the product (1.18 g, 73%) as a pink solid. M.p.
123.5Ϫ126.0 °C. Rf (hexane/EtOAc, 6:4, v/v) ϭ 0.16 (ind. UV254).
GC-MS: m/z (%) ϭ 226 [Mϩ, 45%), 193 [Mϩ Ϫ SH, 100%), 165
[Mϩ Ϫ SC(OH)]. 1H NMR: (CDCl3, 300 MHz, ppm relative to
TMS): 3.89 (s, 3 H, OCH3), 3.97 (s, 3 H, OCH3), 6.56 (d, 1 H,
CHOH), 7.10 (s, 1 H, arom.), 7.12 (s, 1 H, arom.). HR-MS Calcd.
for C10H10O4S: 226.02998; found 226.03006.
Acknowledgments
We would like to thank the research council of Limburg University
for a postdoctoral grant for I. P. and the FWO for financial support
of the project. We also thank Rene De Boer from the Department
of Organic Chemistry at the Catholic University of Leuven for HR-
MS analyses.
´
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Synthesis of 5,6-Dichloro-2-thiophthalide (3b): 5,6-Dichloro-3-hy-
droxy-2-thiophthalide (2b; 4.7 g, 20 mmol) was added to a solution
of 10 mL of acetic acid and 7.0 g of 57% aq. soln. hydroiodic acid.
The mixture was stirred at 110 °C for 3 h. It was then cooled to
room temp. and poured into 70 mL of 1  NaOH, containing 7.0 g
(67.3 mmol) of NaHSO3. The work up was the same as for 3a. The
crude substance was purified by column chromatography on silica
gel (eluent: hexane/EtOAc, in gradient, start 95:5, end 65:35,
v/v) to give 2.1 g (49%) of product as slightly yellow crystals. M.p.
161.0Ϫ163.2 °C. Rf (hexane/EtOAc, 7:3, v/v) ϭ 0.72 (ind. UV254).
GC-MS: m/z (%) ϭ 218 (80) [Mϩ], 189 (100) [Mϩ Ϫ CHO], 146
[Mϩ Ϫ CSCO]. 1H NMR (CDCl3, 300 MHz): δ ϭ 4.41 (s, 2 H,
CH2, thiolactone cyclic ring), 7.64 (s, 1 H, arom.), 7.86 (s, 1 H,
arom.). HR-MS for C8H4OSCl2: calcd. 217.93599; found
217.93603.
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Ϫ derivativen in de synthese van poly(isothianafteen)’’, 1996,
Limburgs Universitair Centrum, Instituut voor materiaal Ond-
erzoek, Dept. Chem.
Synthesis of 5,6-Dimethoxy-2-thiophthalide (3c): 5,6-Dimethoxy-3-
hydroxy-2-thiophthalide (2c; 0.91 g, 4 mmol) was added to a solu-
tion of 2 mL of acetic acid and 1.4 g of 57% aq. hydroiodic acid.
The mixture was stirred at 120 °C for 5 h. It was then cooled to
room temp. and poured into 20 mL of 1  NaOH containing 1.4 g
(13.4 mmol) of NaHSO3. The workup was the same as for 3a. After
separation of the product by column chromatography on silica gel
(eluent: hexane/EtOAc, in gradient, start 95:5, end 65:35, v/v), 0.4 g
of pure product was collected as slightly yellow crystals (48%). M.p.
150.0Ϫ152.5 °C. Rf (hexane/EtOAc,7:3, v/v) ϭ 0.56 (ind. UV254).
GC-MS: m/z (%) ϭ 210 (100) [Mϩ], 182 (65) [Mϩ Ϫ CO], 167 [Mϩ
Ϫ CHS], 139 [Mϩ Ϫ CSCO]. 1H NMR (CDCl3, 300 MHz): δ ϭ
3.91 (s, 3 H, OCH3], 3.95 (s, 3 H, OCH3), 4.35 (s, 2 H, CH2, thio-
lactone ring), 6.93 (s, 1 H, arom.), 7.20 (s, 1 H, arom.). HR-MS for
C10H10O3S: calcd. 210.03507; found 210.03557.
[11]
[12]
[13]
[14]
[15]
[16]
Received September 20, 2001
[O01467]
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Eur. J. Org. Chem. 2002, 1033Ϫ1036