Metal-free radical thiolations mediated by very weak bases

Article abstract of DOI:10.1039/c6ob02276f

Aromatic thioethers and analogous heavier chalcogenides were prepared by reaction of arene-diazonium salts with disulfides in the presence of the cheap and weak base NaOAc. The mild and practical reaction conditions (equimolar reagents, DMSO, r.t., 8 h) tolerate various functional groups (e.g. Br, Cl, NO₂, CO₂R, OH, SCF₃, furans). Mechanistic studies indicate the operation of a radical aromatic substitution mechanism via aryl, acetyloxyl, thiyl, and dimsyl radicals.

Full text of DOI:10.1039/c6ob02276f

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Metal-free radical thiolations mediated by very
weak bases†
Cite this: DOI: 10.1039/c6ob02276f
Received 28th October 2016,
Accepted 16th November 2016
Denis Koziakov, Michal Majek and Axel Jacobi von Wangelin*
DOI: 10.1039/c6ob02276f
www.rsc.org/obc
Aromatic thioethers and analogous heavier chalcogenides were
prepared by reaction of arene-diazonium salts with disulfides in
the presence of the cheap and weak base NaOAc. The mild and
practical reaction conditions (equimolar reagents, DMSO, r.t., 8 h)
tolerate various functional groups (e.g. Br, Cl, NO2, CO2R, OH,
SCF3, furans). Mechanistic studies indicate the operation of a
radical aromatic substitution mechanism via aryl, acetyloxyl, thiyl,
and dimsyl radicals.
Introduction
Scheme 2 Thiolation methods of arenediazonium salts.
Sulfur-substituted aromatics constitute a versatile class of
building blocks which find numerous applications in the syn-
thesis of bioactive compounds, materials and fine chemicals under metal-free basic conditions but affords only moderate
(Scheme 1).1 Besides electrophilic aromatic substitutions with yields and bears significant hazard potential due to the inter-
conc. sulfuric acid, various synthetic procedures have been mediacy of explosive azosulfides.5 An elegant alternative is the
reported for the construction of aryl–S bonds from aryl electro- use of disulfides in radical aromatic substitutions which are
philes and sulfur nucleophiles. The most prominent methods initiated by single-electron transfer (SET) and operate under
are transition metal-catalyzed thiolations of aryl halides2 and mild conditions.6 Base-free aromatic thiolations with di-
Sandmeyer-type3 radical substitutions of arenediazonium sulfides were recently realized by organic photoredox-cataly-
salts4 (Scheme 2, top). The latter procedure can be performed sis.7 However, light-mediated processes require the presence of
a suitable photo-sensitizer and irradiation with a powerful
light source, operate at low concentrations over long reaction
times, and generally suffer from the attenuation of light inten-
sity in solution (Lambert–Beer law).
Powerful metal-free electron donors are mostly strong
bases8 and nucleophiles which are incompatible with sensitive
functional groups and protic solvents. However, even weak
bases and nucleophiles can trigger radical aromatic substi-
tutions (SRAr) at electrophilic aryl–X if the elimination of X was
irreversible or a facile bond homolysis can occur.9,10 Following
Scheme 1 Selected compounds containing aryl–S entities.
observations on mechanistically related photo-redox substi-
tutions,7 we directed our attention at SET-mediated reactions
of disulfides with arenediazonium salts.11 Here, we present an
operationally facile protocol which utilizes the cheap, environ-
mentally benign, and weak base sodium acetate (NaOAc) in
Institute of Organic Chemistry, University of Regensburg, Germany.
E-mail: axel.jacobi@ur.de; Fax: +49 (0)941943-4617; Tel: +49 (0)941943-4802
radical aromatic substitutions of arenediazonium salts with
disulfides under mild reaction conditions (Scheme 2, bottom).
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, analytical data of all new compounds. See DOI: 10.1039/c6ob02276f
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Table 2 NaOAc-mediated methylthiolation of arenediazonium saltsa
Method development
Simple inorganic bases are attractive due to their low price,
low toxicity, and facile removal from organic products while
organic bases are mostly more expensive, environmentally pro-
blematic, and less facile to remove from organic phases. We
studied the model reaction of 4-nitrobenzenediazonium tetra-
fluoroborate, containing two π-electrophilic sites, with di-
methyldisulfide (Table 1). Various simple bases were compe-
tent in this thiolation protocol. N,N-Dimethylhydrazine was
most active;12 however, we refrained from further studies due
to the toxicity and hazard potential as carcinogen, explosive,
and groundwater pollutant.13 Strong bases (e.g. KOtBu,
KHMDS, n-BuLi) gave lower selectivities and many by-
products. Polar solvents (acetonitrile, methanol, dimethyl-
sulfoxide (DMSO)) exhibited high reagents solubility and good
yields. Identical reactions in N,N-dimethylformamide (DMF)
underwent competing hydrodediazotation (25%); in benzene
4-nitrobiphenyl was formed (15%). Significantly higher selec-
tivities were observed in the absence of air and moisture.
Gratifyingly, no excess of reagents was required. The optimized
conditions (equimolar ArN2BF4, NaOAc, Me2S2, 0.2 M in
R′
Yield [%]
R′
Yield [%]
4-OMe
4-Cl
4-NO2
4-OH
4-Ph
2,4,6-Cl3
89
70
85
87
58
56
3-NO2
2-NO2
2-Br
2-SMe
2-CO2Me
1-Naphthyl
77
80
69
73
71
53
a Conditions: arenediazonium tetrafluoroborate (0.6 mmol), dimethyl
disulfide (0.6 mmol), NaOAc (0.6 mmol), DMSO (2 mL), 20 °C, 8 h.
We then applied the optimized conditions to a series of
diversely substituted arenediazonium salts (Table 2). The pro-
tocol proved to be rather general with regard to substituents at
the arene and the disulfide, respectively. Chloro, bromo, ester,
nitro, hydroxy, and naphthyl moities within the diazonium
salts were tolerated. Alkyl disulfides afforded slightly higher
yields than aryl disulfides (Scheme 4). The trifluoromethyl-
sulfanyl moiety, which attracts great interest from pharma-
ceutical programs due to their lipophilic properties, could also
be introduced by reaction with commercial bis(trifluoro-
methyl) disulfide in the same (Scheme 4).14 Sequential radical
5-exo-dig cyclization and thiolation was observed with 2-(pro-
pynyloxy)benzenediazonium tetrafluoroborate to give 3-thio-
ketal benzofurans (Scheme 5).15 The attack of the S-centered
radical on the newly formed double bond is facilitated by the
stabilization of the resulting benzyl radical. On the other
hand, the corresponding 2-allyloxybenzenediazonium salt
DMSO, 18 °C,
8 h) enabled clean conversion to the
arylthioether with minimal formation of anisole as reduction
by-product (<3%). Similar yields were obtained when using
other simple carboxylates (Scheme 3).
Table 1 Selected optimization experimentsa
Equiv. Base
Entry Me2S2 (equiv.)
Yieldb
[%]
Conditions
1
2
3
4
5
6
7
8
5
1
5
5
1.5
1.5
1.5
1.5
1
0.5
1
—
0
66
65
85
85
83
56
29
n-Bu4NI (1.5)
KOtBu (5)
NaOAc (5)
NaOAc (5)
NaOAc (1)
NaOAc (0.5)
NaOAc (0.2)
KOAc (1)
NaOAc (1)
NaOAc (1)
NaOAc (1)
NaOAc (1)
Scheme 4 Preparation of other unsymmetrical aryl thioethers.
16 h
16 h
DMSO, r.t., 8 h
DMSO, r.t., 8 h
H2O/THF/AcMec
9
80
42
10
11
12
13
66/31/73
1
1
MeOH/DMF/MeCNc 77/62/75
DMSO, r.t., 8 h
85d (90)e (83)f
a Optimized conditions: 4-Nitrobenzenediazonium tetrafluoroborate
(0.6 mmol), NaOAc (0.6 mmol) under N2, DMSO (3 mL), r.t., 8 h. b GC
yields vs. internal 1-dodecanenitrile. c Individual reactions in one
solvent. d 80% isolated yield. e After 60 h. f 60 °C, 1 h.
Scheme 3 Similar activity of various sodium carboxylates.
Scheme 5 Base-mediated radical cyclization-thiolation.
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Scheme 6 Procedures with alternative sulfur sources (Ar
=
4-MeOC6H4).
failed to undergo a similarly effective domino reaction but
afforded a different thioether skeleton and many by-products
(Scheme 5, bottom).
Scheme 7 Postulated weak base-mediated SRAr mechanism.
We employed elemental sulfur as alternative thiolation
reagents which indeed led to the formation of symmetrical
diaryl sulfides in moderate to good yields (Scheme 6, top).
While thiols are easy available, they result in the formation of
explosive azosulfide intermediates and under our conditions
gave only moderate to low yields of the desired thioethers. The
direct thiolation with benzenethiol proceeded with moderate
yields in the absence of base. With ethanethiol, mostly hydro-
dediazotation was observed (Scheme 6, bottom).
indirectly by the continuous decrease of the pH value over the
course of the reaction (pH 8 → 4, after aqueous quench)
despite the buffering effect of the weak acid/base pair and the
solvent. Crossover-experiments with dimethyl-disulfide and
diphenyldisulfide resulted in the formation of the mixed
methylphenyl disulfide (MeS-SPh) as byproduct by recombina-
tion of free thiyl radicals.
Other mechanistic scenarios could be excluded based on
the following instructive experiments and considerations: (i)
The operation of the acetate anion as single-electron transfer
reagent19 is discouraged by the lack of any detectable CO2 for-
mation by head-space MS analyses and gas phase absorptions
into aqueous Ba(OH)2. (ii) A direct disulfide-mediated electron
Mechanistic studies
Reaction progress analysis documented an immediate onset of
reactivity within less than 10 s, more than 50% conversion
within 1 h, and a significantly slower turnover after 1–2 h. This
behaviour could be a consequence of two competing reaction
mechanisms: a rapid base-induced thiolation and a slow
radical chain propagation.16 The operation of the latter is also
supported by experiments with catalytic amounts of NaOAc
(20 mol% and 50 mol%) which afforded 29% and 56% yield,
respectively, after 16 h. Further mechanistic insight has
already been gained by the initial optimization experiments
(see Table 1). The stoichiometry of the reaction (equimolar
ArN2BF4, R2S2, and NaOAc) indicates a loss channel of one
half of the disulfide molecule which is not electron transfer in
nature (RS• → RS+ + e−)16 as such a scenario could operate with
catalytic amounts of base.
transfer was already disproven in entry
1 of Table 1.
Consistently, a solution of arenediazonium salt and NaOAc in
DMSO underwent significant hydrodediazotation after 30 min
in the absence of disulfides; deuterium incorporation from
DMSO-d6 was also observed (Scheme 8, top). (iii) Base-
We postulate a mechanism that involves homolysis of the
initially formed diazoacetate (Scheme 7). The resultant aryl
and acetyloxyl radicals engage in orthogonal onward reactions.
The former reacts with the disulfide upon release of a thiyl
radical which is trapped by a dimsyl radical derived from H
atom transfer (HAT) with acetyloxyl. The presence of the aryl
radical intermediate Ar• was confirmed by radical trapping
with TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)-oxyl in a stan-
dard reaction. The aryl radical intermediate attacks the di-
sulfide in an SH2 fashion by homolysis of the S–S bond in a
single step.17 The resulting DMSO-SR adducts (R = Me, Ph)
were detected by ESI-MS (Scheme 7, bottom left).7a,18 The
generation of acetic acid as major byproduct was monitored Scheme 8 Experimental and calculated H atom transfer (HAT).
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washed with cold diethylether (2 × 10 mL) and dried on air to
give analytically pure arenediazonium salts.
General procedure for base-induced thiolation, selenylation
and telluration
Scheme 9 Selenations and tellurations under similar conditions.
A vial (5 mL) was charged with a magnetic stir bar, the arenedia-
zonium salt (0.5 mmol), the disulfide (0.5 mmol) and sodium
acetate (0.5 mmol) and capped with a rubber septum. The vial
was purged with N2 (5 min). Dry DMSO (2.5 mL) was added.
After 8 h of stirring, water (5 mL) was added to give an emulsion,
which was extracted with diethylether (3 × 5 mL). The organic
phases were washed with brine (5 mL) and dried (MgSO4). The
solvent was evaporated in vacuo; the residue was purified by SiO2
flash column chromatography (pentane/ethyl acetate).
mediated aryl radical formation from arene-diazonium salts
could in principle proceed by SET from the diazotate anion.20
On thermodynamic grounds, the diazotate anion/radical
appears to be a reasonable 1e-redox couple under weakly basic
conditions.21 However, this mechanism requires the operation
of a highly selective H atom transfer (HAT) from a suitable
H-donor (DMSO) to the azotate radical19 but not to the aryl
radical intermediate. A DFT-based analysis excludes such
mechanism because the HAT to the aryl radical is thermo-
dynamically and kinetically highly favoured (Scheme 8,
bottom).20,22 (iv) The operation of a dimsyl anion-mediated
Calculations
Geometries and energies were calculated using Gaussian
G09W.21a DFT calculations were performed using M06-2X
functional,21b and 6-31+G(d,p) basis set. Geometry optimi-
zations and single point calculations were done using the
polarized continuum model to account for solvation effects.
S
RN1 mechanism can be excluded by the large pKa difference
of acetic acid (∼12) and DMSO (∼35) in DMSO.23
Under identical conditions, the general protocol was
applied to the synthesis of arylselenoethers and aryltelluro-
ethers (Scheme 9).24 Employment of the diphenyl dichalco-
genides afforded very good yields of the methyl 2-phenyl-
carboxylate derivatives and moderate yields of nitrobenzene
derivatives. Mechanistic studies on related systems were per-
formed by Pandey and coworkers which reported the facile
SET oxidation of diselenides in the presence of suitable accep-
tors and subsequent mesolysis to a phenylselenyl radical and
phenylselenyl cation.6b,25
Acknowledgements
D. K. thanks the Bayhost program of the State of Bavaria
(2014–2015) and the Fonds der Chemischen Industrie (FCI,
2015–2017) for doctoral fellowships. M. M. is a fellow of the
Graduate School on “Chemical Photocatalysis” (GRK 1626) of
the Deutsche Forschungsgemeinschaft (DFG).
Notes and references
Conclusion
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We have developed a new metal-free thiolation method for syn-
thesis of arylsulfides which employs the cheap, weak, and
environmentally benign base sodium acetate. Experimental
and theoretical studies indicated the operation of an initial
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Applications to the synthesis of various functionalized
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