A conjugate Lewis base-Br?nsted acid catalyst for the sulfenylation of nitrogen containing heterocycles under mild conditions

Article abstract of DOI:10.1039/c6cc09998j

Catalysts that contain a thiourea tethered to a carboxylic acid were found to affect the sulfenylation of indoles and other N-heterocycles on the hour time scale at room temperature. The mild nature of these conditions allowed for the incorporation of diverse functionalities into more complex heterocycles.

Full text of DOI:10.1039/c6cc09998j

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Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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A conjugate Lewis base-Brønsted acid catalyst for the
sulfenylation of nitrogen containing heterocycles under mild
conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Christopher J. Nalbandian,a Eric M. Miller,a Sean T. Toenjes,a and Jeffery L. Gustafsona
DOI: 10.1039/x0xx00000x
www.rsc.org/
A. Notable sulfur-containing compounds in medicinal chemistry and material science
Catalysts that contain a thiourea tethered to a carboxylic acid
were found to affect the sulfenylation of indoles and other N-
heterocycles on the hour time scale at room temperature. The
mild nature of these conditions allowed for the incorporation of
diverse functionalities into more complex heterocycles.
MeHN
O
O
N
S
N
S
N
H
O
S
R
n
O
HN
N
N
PPS
Omeprazole
Axitinib
O
B. Previous work
SR
Aryl sulfides are common functionalities and synthetic
intermediates in drug discovery and material science (Figure
1a). For instance, the kinase inhibitor axitinib is a recent
example of an FDA approved diaryl sulfide, a diaryl sulfide is a
late stage intermediate in the synthesis of omeprazole, and
the industrial polymer polyphenylene sulfide (PPS) has found
numerous industrial applications as high performance thermo-
plastics. Because of these and other examples, the formation
of C-S bonds has received significant attention, with metal
catalyzed cross-couplings comprising the majority of
examples.1,2 The direct formation of C-S bonds from
unfunctionalized starting materials has also received recent
attention with examples of both metal catalyzed C-H3–5
functionalization and aromatic sulfenylation that proceed via
electrophilic aromatic substitution (SEAr).
Ph
O
N
S
Ph
N
S
O
TFA (15 equiv)
CH2Cl2, rt, 4-6h
SR
O
H
N
Me
N
N
N
0.5 mol% MgBr2
90 oC, DMAc
Me
R2
R2
C. This work
N3
10%
OH
O
N
O
HN
N3
OH
S
S
N
N
R
Y
O
O
HN
S
+
NHBoc
NHR
O
S
S
HN Boc
O
N
H
N
H
N
SR
O
N
O
O
O
CF3
Mild sulfenylation of drug-like molecules
Figure 1 a) Representative examples of sulfur-containing compounds in drug discovery
and material science. b) Recent examples of sulfenylations. c) Mild sulfenylations
mediated by catalysts reported within.
Historically, aromatic sulfenylation via SEAr is achieved
using in situ generated sulfenyl halides,6,7 however these
approaches often result in a mixture of sulfenylated and
halogenated products. Kita8 has shown that activation of
quinone O,S-acetals by TMS triflate results in a robust
sulfenylation system, albeit with limited substrate scope with
respect to the substitution on sulfur. Recent examples have
shown that sulfenylation of indoles and pyrroles can be
achieved at elevated temperatures via the in situ formation of
sulfenyl halides by activation of N-sulfenyl imides
(phthalimides or succinimides) with hard halide sources (Figure
1b).9–11 Lewis acids have also been demonstrated to
activate N-sulfenyl imides towards aromatic sulfenylation.12–16
It has also been shown that ortho-thioquinones, generated
from the corresponding N-sulfenyl imides with mild base, are
suitable electrophilic sulfenylation reagents for SEAr.17 Most
recently it18,19 has been shown that super-stoichiometric
amounts of TFA (5-20 equivalents) can mediate sulfenylation
of electron rich arenes by N-thiosuccinimides at room
temperature. While these examples represent useful new
chemistries, they suffer from various issues including a lack of
chemoselectivity and a reliance on elevated temperatures or
harsh conditions that likely preclude them from being applied
in more complex settings such as on natural products or
peptides.
We have recently demonstrated that Lewis bases such as
triphenylphosphine sulfide are effective catalysts for the
halogenation of aromatics with N-halosuccinimides.20,21 With
this work in mind we set out to determine if Lewis base
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Journal Name
O
View Article Online
S
O
DOI: 10.1039/C6CC09998J
N
SR
10% catalyst 6
N
SPh
+
N
Y
N
Y
O
DCM (0.2 M), rt, time
R
R
2a
S
O
9-15
N
SPh
10% catalyst
SPh
N
solvent, rt, time
1
3
SPh
catalysts:
N
H
N
H
N
H
Se
P
9
11
3h, 91%
10
1h, 93%
1h, 94%
O
O
4
H
N
SPh
SPh
NHBoc
SPh
O
O
S
S
F3C
F3C
Cl
SPh
N
Cl
HN
HN
NH
NH
N
H
N
NH
NH
N
Cl
Cl
HO2C
HO2C
14
15
24h, 69%
12
13
4h, 41%
5
6: 88%a, 6h
4: 87%b, 6h
6 (Racemic)
F3C
Cl
F3C
Cl
Cl
6: 82%c, 3h
4: 63%d, 3h
Cl
Cl
O
S
F3C
Scheme 1. Reactions were performed at room temperature by the addition of 1
equivalent of substrate, 0.1 equivalent of catalyst, 1.1 equivalent of reagent at a
concentration of 0.2M. Isolated yields represent an average of two trials. a1 equivalent
of TFA was added. b0.1 equivalent of 4 with 1 equivalent of TFA was used. c0.5
equivalents of TFA was added. d0.1 equivalent of 4 with 0.5 equivalent of TFA was used.
CO2H
F3C
Cl
HN
Cl
NH
H
N
NH
S
Cl
HO2C
Cl
CF3
O
N
H
F3C
Cl
N
H
Cl
Cl
8
7
Entry
1a,b
Catalyst Additive
Solvent
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CD2Cl2
CDCl3
CD2Cl2
CDCl3
CDCl3
H2O
Time
Yield
0
0
previously developed by Seidel25 and possesses a thiourea (a
None
None
None
1 eq. TFA
None
2 Hours
2 Hours
2 Hours
5 Min.
2 Hours
2 Hours
30 Min.
5 Min.
2 a,b
3 a,b
4 a,b
5 a,b
6 a,b
7 a,b
8 a,b
9 a,b
10 a,b
11 a,b
12 c,d
known Lewis base)26,27 tethered to
a tetra-chlorinated
4
4
5
6
6
6
6
7
8
6
0
carboxylic acid was able to catalyse sulfenylation in the
absence of added TFA (Table 1, entry 5), albeit on a prolonged
time scale compared to that of entry 4.
We next evaluated the effect of changing the orientation of
the thiourea and Brønsted acid relative to each other, finding
1 eq. TFA
99
34
81
80
99
99
70
3
None
None
None
1 eq. TFA
1 eq. TFA
None
racemic catalyst 6, which is based on
a
cis-1,2-
5 Min.
cyclohexyldiamine scaffold, to be a more effective catalyst
than 5 (Table 1, Entry 6). A noticeable jump in reaction rate
was observed when running this reaction in dichloromethane
(Table 1, Entry 7), allowing this chemistry to be complete in
under an hour. While our goal was to avoid the use of strong
2 Hours
2 Hours
1 Hour
None
None
69
Table 1 Reactions were performed at room temperature by the addition of .03
mmol of 1, 0.003 mmol of catalyst, and 600 μL of solvent into an NMR tube,
followed by the addition of 0.033 mmol of 2a. aSolvents were run through a short acid, it is worth noting that the combination of one equivalent
column of basic alumina prior to reaction. bPercent conversions by 1H NMR
TFA and 10% of 6 in (Table 1, entries 8, 9) resulted in full
conversion to product in minutes. 6 Also mediated this
chemistry in water/methanol mixtures (Table 1, Entry 12),
suggesting this chemistry may be applicable to challenges in
represent an average of three trials using tetramethylsilane as internal standard.
c30% MeOH in DI water was used 0.2M to 1. dObtained as an isolated yield as an
average of two trials on a 0.043mmol scale to 1.
more complex biological settings including the sulfenylation of
peptides, proteins, and polar natural products.
catalysis could also mediate aromatic sulfenylation. We initially
studied the sulfenylation of indole with N-
1
The increased activity of 6 led us to evaluate other
catalysts with different relative orientations and distances
between the carboxylic acid and thiourea (Table 1, Entries 10,
11). Catalyst 7, where the thiourea and carboxylic acid are
linked via a cis-1,4-cyclohexyldiamine scaffold, proved only
slightly less efficient than 6. On the other hand, catalyst 8,
where the catalyst moieties are tethered by a more flexible
ethylene diamine linker, did not mediate significant
sulfenylation. This data suggests that the relative orientation
of the thiourea and carboxylic acid is a key factor for catalysis.
We next sought to determine the substrate scope of this
chemistry (Scheme 1). For indole, we observed clean
sulfenylation at the more electron rich C-3 position, recovering
94% of 9. This is complementary to Cossy’s sulfenylation
conditions19 which result in C-2 sulfenylation, due to an acid
mediated rearrangement of C-3 sulfenylated product.
thiophenylsuccinimide (2a), observing no reaction in the
presence or absence of one equivalent TFA (Table 1, entries
1,2). While Lewis bases proved ineffective in the absence of
TFA (i.e. Table 1, entry 3), we found that the combination of
10% triphenylphosphine selenide 4 and 1 equivalent of TFA
(Table 1, entry 4) resulted in rapid conversion to 3 on the
minute time scale. This synergy between Brønsted acid and
Lewis base has previously been observed by Denmark22–24 in
the context of olefin sulfenylation.
As our overarching goal was to make aromatic
sulfenylation applicable to more complex molecules,
particularly those with with acid sensitive functionalities, we
were concerned by the use of stoichiometric strong acid in
entry 4. Because of this, we set out to determine if we could
exploit the observed acid-base synergy in the context of a
single catalyst. We quickly found that catalyst 5 which was
2 | J. Name., 2012, 00, 1-3
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COMMUNICATION
O
O
View Article Online
S
SR
10% catalyst 6
SR
10% catalyst 6
N
DOI: 10.1039/C6CC09998J
+
N
SR
+
N
Y
DCM (0.2 M), rt, time
N
R
R
Y
N
H
N
Y
Y
N3
DCM (0.2 M), rt, time
O
H
O
2a-2e
16-21
22-27
2b
H
N
O
Boc
HN
O
O
S
S
N
N
S
O
H
N3
NHAc
O
N
N
N
S
N3
O
O
O
N3
O
O
S
O
N
O
O
2b
N
H
O
S
2a
2c
S
S
S
N
O
O
N
CF3
N
H
S
N
N3
22
24h, 49%a,b
2d
O
24
2d, 62%a
23
3h, 77%
O
2e
O
CF3
O
O
S
OH
OH
NH
N3
Boc
HN
O
O
NHBoc
H
N
S
O
S
N
H
N
H
N
H
S
HN Boc
N
N
O
O
H
H
O
N3
53%a , 4h
25
18
N3
HN
16
5h, 84%
O
17
S
18h, 58%a
N3
4h, 77%
O
S
CF3
NHAc
S
S
NH
Boc
H
N
O
HN
N3
N
O
S
O
NBOM
O
N
N
SPh
HN
O
H
N
H
H
N
H
S
N3
NH
20
O
HO
Cbz
19
8h, 87%
6: 73%a, 24h
4: 25%b, 24h
21
24h, 95%
6: 96%a, 4h
4: 76%c, 4h
6: 85%a, 19h
15 eq. TFA: 0%d, 19h
27
Scheme 2 Reactions were performed at room temperature by the addition of 1
equivalent of substrate, 0.1 equivalent of catalyst, 1.1 equivalent of reagent at a
concentration of 0.2M. Isolated yields represent an average of two trials. a1 equivalent
of TFA was added. b0.1 equivalent of 4 with 1 equivalent of TFA was used.
15 eq. TFA: 0%d, 4h
26
Scheme 3 Sulfenylation of biologically relevant molecules using 2b. Reactions were
performed at room temperature by the addition of 1 equivalent of substrate, 0.1
equivalent of catalyst, and 1.1 equivalent of reagent at a concentration of 0.2M in
substrate. Isolated yields represent an average of two trials. a1 equivalent of TFA was
added. bA 9:1 mixture of DCM/MeOH was used as the solvent. c0.1 equivalent of 4 with
1 equivalent of TFA was used. dNo catalyst, 15 equivalents of TFA at a concentration of
0.5M in substrate with 1 equivalent of reagent.
Several C-3 and C-2 substituted indoles also proceeded
smoothly to give 10, 11, and 12 in good yields. Pyrrole was also
amenable to this chemistry to give 13, however at lower yields
due to the formation of multiple constitutional isomers. tert-
Butoxy carbonyl (boc) protected tryptophan also sulfenylated
rapidly to yield 82% of 14 with no observed deprotection..
Finally, azaindoles, which are markedly less electronically
activated than indoles, sulfenylated to give 15 in good yield,
notably very little azaindole sulfenylation was observed under
TFA conditions, perhaps due to protonation of the 7-nitrogen.
It should be noted that conversion to 12 and 14 required
some added TFA in addition to 10% 6 to ensure the reaction
proceeded on a timely manner. For these cases the conditions
in table 1, entry 4 (4, 1 equivalent TFA) proved comparable (for
12) or noticeably less efficient (for 14).
employed to modulate the lipophilicity of drug candidates.29
Bis-N-thiosuccinimides 2d was also synthesized, allowing for
the synthesis of new heterocycles such as 20 in which the C-3
and C-2 are bridged. As before, in some cases the addition of 1
equivalent of TFA was necessary for the reaction to progress in
a timely manner. This is observed especially when using N-
thioalkyl imides, which proved to be less electrophilic
reagents. Notably in some of these instances (i.e 20) the
conditions in Table 1, entry 4 proved to be less efficient.
We next evaluated known biologically active small
molecules and FDA approved drugs. In addition to leading to
new analogs, the addition of an azide using 2b would
represent an efficient way to insert a linker to obtain chimeric
molecules such as PROTACs30 or affinity labelled analogs to
determine the molecular targets of a bioactive.31 We first
looked at Naratriptan32 an FDA approved treatment for
migraines, finding conversion to 22 in moderate isolated yield
with 2b in a mixture 9:1 DCM/MeOH. Melatonin (23) was
quickly converted in good yield, to the expected sulfenylated
product with reagent 2b. We were also able to sulfenylate
biologically active pyrrole 2433 in good yield, albeit a longer
reaction time was needed. Finally, we evaluated peptides as
substrates, observing clean conversion of Boc-Trp-Gly-Gly-Trp-
OMe to doubly sulfenylated peptide 25 with 2b. We were also
able to cleanly sulfenylate Z-Tyr-Trp-OMe and Boc-His(N-Bom)-
Trp-Ome at tryptophan using 6 to give functionalized peptide
26 and 27 in high yields. This highlights the mildness and
specificity of our sulfenylation conditions as these peptides
Next, we determined the scope of sulfenylating reagents
2a-2e that could be employed. Several N-thiosuccinimides or
N-thiophthalimides were synthesized according to literature
procedures for similar reagents (see SI for details). We first
looked at azide containing N-thiosuccinimide 2b, which proved
an effective sulfenylation reagent with catalyst 6, yielding 16
and 17 in good yields. The ability to facilely incorporate azides
onto N-heterocycles should find utility in chemical biology, as
azides are a common tool that allow for the incorporation of
diverse moieties through the Huisgen cycloaddition. Cysteine
derived 2c was also amenable to this chemistry, with 6
effecting the sulfenylation of Boc-Trp-OH with 2C to give 18 in
good yield with no observed deprotection.
We were also able to achieve trifluoromethyl thiolation28 in
excellent yields (19) on the hour time scale. The ability to
easily install the S-CF3 group using 6 may find use in a
medicinal chemistry setting as the S-CF3 group is commonly
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CF3
H
S
DOI: 10.1039/C6CC09998J
O
Ph
N
O
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2
Cl
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NH
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HN
HN
H
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NH
CF3
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Activated sulfenium
Figure 2. A proposed mechanism for the catalytic activation of N-thiosuccinimides by
conjugate Lewis Base-Brønsted acid catalysis.
6
7
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8
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operates through a mechanism in which the carboxylic acid
activates the N-thiosuccinimide via protonation and the
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(figure 2A) that functions as a more electrophilic sulfenium
source.. While thioureas are also known to function as
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31G(d)) to better understand the observed differences in
reactivity between catalysts 5 and 6 (Figure S1).. In less
reactive 5 these studies predicted the complete deprotonation
of N-2 (proton in blue,) resulting in a neutral intermediate. On
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equatorial and there is no such cleft, resulting in a steric
interaction forcing the succinimide away from the carboxylate,
lessening any H-bonding (O-O bond distance of 3.98 Å).
In summary we have developed a mild catalytic system to
sulfenylate electron rich heterocycles including peptides and
biologically relevant small molecules. The mildness of this
chemistry coupled with the versatility of the groups that can
be incorporated via sulfur is expected to render this chemistry
broadly useful, particularly in chemical biology.
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Notes and references
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