Copper-catalyzed selective radical-radical cross-coupling for C-S bond formation: An access to α-alkylthionitriles

Article abstract of DOI:10.1039/c8cc02371a

A new protocol for C-S bond formation was developed by selective cross-coupling between a thiyl radical and an isobutyronitrile radical. Using this strategy, a series of valuable α-alkylthionitrile derivatives were synthesized from basic starting material

Full text of DOI:10.1039/c8cc02371a

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Copper-Catalyzed Selective Radical-Radical Cross-coupling for C-S
Bond Formation: An Access to α-alkylthionitriles
Received 00th January 20xx,
Accepted 00th January 20xx
Caihong Zheng,a,c Fangling Lu,a,c Hanwei Lu,a Jie Xin,b Yi Deng,b Dali Yang,b Shengchun Wang,b
Zhiliang Huangb,* ,Meng Gaoa,* ,and Aiwen Leia,b,
*
DOI: 10.1039/x0xx00000x
Dedicated to Professor Xiyan Lu on the occasion of his 90th birthday
www.rsc.org/
A new protocol for C-S bond formation was developed by selective center.
cross-coupling between thiyl radical and isobutyronitrile radical.
Based on our previous research, stabilizing one of the
With this strategy,
a series of valuable α-alkylthionitriles transient radicals to a “persistent” radical is essential for
derivatives were synthesized from basic starting materials. selective transient radical cross-coupling reaction.8 Meanwhile,
Preliminary mechanistic investigation was performed by EPR and there are some clues in the literatures regarding the
XAFS, revealing that the transient thiyl radical could be stabilized stabilization of transient thiyl radical by copper salts.9 Herein, a
by copper catalyst to a persistent one. Therefore, on the basis of copper-catalyzed strategy was proposed for this thiyl
persistent radical effect, selective radical-radical cross-coupling radical/isobutyronitrile radical cross-coupling reaction. As
between the thiyl radical and isobutyronitrile radical was achieved shown in Scheme 1, once the reactivity of thiyl radical could be
successfully in this work.
tuned by Cu(I), the desired radical-radical cross-coupling will
occur to form a new C-S bond with high selectivity.
Direct cross-coupling of two different radicals to form C−C
and C-X (X = O, N, etc.) bonds is of great significance, and
represent
a powerful approach for synthetic organic
chemistry.1 Generally, in order to achieve the high selectivity,
a persistent radical and a transient radical should be engaged
for this coupling reaction obey the persistent radical effect.2
However, to the best of our knowledge, most of the in-situ
generated radicals in nature are transient species, while only a
few persistent radicals have been reported.3 As a result, the
existing radical-radical cross-coupling reactions, which could
be effectively applied for chemical bond construction, are
extremely limited.4 To overcome this disadvantage, recently
our group demonstrated a selective transient radical cross-
coupling reaction by in-situ tuning the reactivity of radicals.5
With this strategy, cross-coupling reactions of transient C-
centered radical with X-centered radical (X = C, N, S, etc.) are
realized in a highly selective manner.6 Although some progress
has been made to date, direct cross-coupling of two transient
radicals still remains largely unexplored.7 In this regard, here
we would like to communicate the first cross-coupling reaction
of a transient thiyl radical with a transient tertiary-carbon
radical towards the construction of a new quaternary carbon
Scheme 1. Proposal for selective cross-coupling of thyil radical with
isobutyronitrile radical.
To commence our study, the reaction between p-
toluenethiol (1a) and Cu(OTf)2 was firstly investigated by X-ray
absorption fine structure (XAFS) spectroscopy. As shown in
Figure 1(A), upon addition of 1a, Cu(II) is immediately reduced
to Cu(I) as evidenced by the disappearance of the Cu(II) pre-
edge at 8977.4 eV and a shift in the edge-energy position from
8988.3 eV to 8980.4 eV, implying that 1a was oxidized via a
single electron transfer.10 Meanwhile, the coordinated O-atom
of Cu(OTf)2 was replaced by S-atom during the reaction, since
a longer Cu-S bond scattering was observed for the reduced Cu
(Figure 1(B)). These results suggested that a p-toluenethiyl
radical should be formed via single electron transfer oxidation
of 1a in the presence of Cu(OTf)2, and it might be stabilized by
coordinating to the in-situ generated Cu(I). This conclusion was
further supported by the controlled oxidation experiment. As
expected, the S-atom coordinated Cu(I) could not be oxidized
by air, even it was stirred for 1 h at 80 oC, which was evidenced
by the identical XANES and EXAFS spectra (Figure 2). This
experimental phenomenon is consistent well with the
a.
National Research Center for Carbohydrate Synthesis, Jiangxi Normal University,
Nanchang 330022, Jiangxi, P. R. China.
b.The Institute for Advanced Studies (IAS), College of Chemistry and Molecular
Sciences, Wuhan University, Wuhan 430072, Hubei, P. R. China.
c. C. Zheng and F. Lu contributed equally.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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assumption that the Cu(I) is coordinated by p-toluenethiyl blocks in synthetic chemistry.11 Therefore, this novel oxidative
radical, and its oxidation potential was increased afterwards.
radical cross-coupling reaction towards the synthesis of α-
alkylthionitriles derivatives is valuable, which led us to make a
further investigation.
After many efforts, the combination of 1a (0.3 mmol), 2a
(0.45 mmol), Cu(OTf)2 (5 mol%) and 1, 10-phenanthroline (20
o
mol%) in dichloroethane (DCE) at 80 C was found to be the
best reaction conditions for this transformation (Table 1, entry
4). Other copper salts, such as CuI, CuBr and CuF2 were also
tested, but lower yields were obtained (Table 1, entries 1–3).
In the absence of copper, no desired product was obtained,
indicating that copper salt is essential for the reaction (Table 1,
entry 8). Several additives were also examined. L-proline and
dppe could provide the desired product, albeit in lower yield
(Table 1, entries 5–6). Solvent has a great influence on the
yield, since THF and MeCN only afforded moderate yields
(Table 1, entries 10-11), and trace amount of product 3a is
obtained when toluene was used (Table 1, entry 9).
Replacement of the air atmosphere with N2 reduced the yield
distinctly (Table 1, entry 12).
Table 1. Impact of reaction parameters on the oxidative radical cross coupling.a
entry
1
2
3
4
5
6
7
8
[Cu]
CuI
CuBr
additives
Phen
Phen
Phen
Phen
dppe
L-proline
-
Phen
Phen
Phen
Phen
Phen
solvent
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Tol
yield (%)b
11
17
70
85
32
21
37
0
trace
52
58
12
Figure 1. (A) XANES spectra of Cu(OTf)2, the mixture of Cu(OTf)2 and 1a in DMF, the
inset shows the first derivatives of the XANES spectra. (B) The corresponding k2-
weighted R-space EXAFS spectra.
CuF2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
-
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
9
10
11
12c
THF
MeCN
DCE
[a] The reaction was carried out on a scale of 0.3 mmol of 1a and 0.45 mmol of
2a in the presence of copper salts and ligand in 3 mL of solvent under air for 12 h.
[b] Isolated yield. [c] The reaction was carried out under N2.
With the optimized conditions in hand, various
thiophenols
access the corresponding products 3 (Scheme 2). The reaction
was readily extended to variety of aryl-substituted
thiophenols. For example, Me-, Et-, isopropyl-, tert-butyl-,
On the basis of above results, a transient isobutyronitrile OMe-, Cl-, Br- and F-substituted thiophenols were well
radical (generated from AIBN) was then added to capture the tolerated in this reaction (3a 3m). Steric effects had little
conceivable Cu(I)-coordinated p-toluenethiyl radical. To our influence, since thiophenols with substituents at ortho-, meta-,
delight, the corresponding 2-methyl-2-(p- or para- position could react smoothly to afford the desired
1 were reacted with different AIBN analogues 2 to
Figure 2. XANES spectra of the DMF solution of Cu(OTf)2 with 1a under N2 or
exposing to air, and the inset shows the corresponding k2-weighted R-space
EXAFS spectra.
a
-
tolylthio)propaneitrile product was indeed obtained (Table 1, products in moderate to good yields. Moreover, heteroaryl
entry 9). Meanwhile, no homocoupling product from the thiophenol were also found to be suitable reaction partners.
transient isobutyronitrile radical was observed in this reaction. Various heteroaryl thiols, including those with 1-methyl-1H-
It is well-known that the α-alkylthionitriles represent an imidazole, 1-methyl-1H-tetrazole, pyrazine, benzo[d]thiazole
important class of nitriles, which are not only found in several and benzo[d]oxazole groups, could react with AIBN
biologically active compounds, but also versatile building successfully to afford the corresponding products in good
2 | J. Name., 2012, 00, 1-3
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yields (3n
were also compatible with this reaction under the optimized the desired cross-coupling product 3a in a highly selective
conditions to give the desired products in good yields (3s 3t). manner. Meanwhile, Cu(I) is released, and will be oxidized by
Several AIBN analogues were also tested to react with 1a air to regenerate Cu(II). It is worth noting that there might be a
-3r). Naphthalene-2-thiol and phenylmethanethiol the other in-situ generated isobutyronitrile radical, affording
-
,
which could give the corresponding product 3u and 3v in 54- interaction between Cu center and the -CN group of
97% yields, respectively. Meanwhile, several potential isobutyronitrile radical, which could bring the radicals close to
applications of the obtained
α
examined as shown in scheme 3. The results showed that
-alkylthionitrile have been each other and afford the vital intermediate II
.
α
-
alkylthionitrile could be further transformed into useful
carboxylic acids, amides and ketones.12
R2
CN
R3
5 mol% Cu(OTf)2
20 mol% Phen
R2
R3
S
R2
R1
R1-SH
+
NC
N N
CN
DCE, 80 oC, 12h
R3
3
1
2
R= 3a p-Me 85%
R= 3b o-Me 77%
R= 3d p-OCH3 72%
R= 3e m-OCH3 72%
R= 3f p-CH3CH2 83%
R= 3g p-iPr 85%
CN
CN
R
S
S
3a, 85%
3c, 65%
R= 3h p-tBu 95%
N
N
3i p-F 50%
N
R=
CN
CN
N
R= 3j o-F 90%
R= 3k o-Cl 78%
R= 3l p-Cl 71%
R= 3m p-Br 84%
N
S
Scheme 3. Reaction conditions: [a] 3a (0.5 mmol), H2SO4 (98% in H2O, 4 mL),
H2O (4 mL), 120 °C, 1.5 h. [b] 3a (0.5 mmol), phenylboronic acid (0.6 mmol),
CuBr2 (5 mol%), potassium tert-Butoxide (1.5 mmol), tert-Butanol (3 mL), 20
h, 30 °C . [c] 3a (0.5 mmol), NaOH (10 mol%), NH3•H2O (1 mL), 100 °C, 12 h.
[d] 3a (1 mmol), chlorophenylmagnesium (2 mol in 1 L dry THF, 1 mL), CuI (5
mol%), 66 °C, 4 h.
N
S
3n, 89%
3o, 90%
CN
CN
N
S
N
O
CN
S
S
N
N
S
3q, 81%
3r, 52%
3p 94%
CN
S
CN
CN
S
S
3u, 97%
3s, 78%
3t, 81%
CN
S
3v, 54%
Scheme 2. Reactions of alkynes
conditions: The reactions were carried out with
1
and secondary phosphine oxides
2. Reaction
1 (0.3 mmol), 2 (0.45 mmol),
Cu(OTf)2 (5 mol%), phen (20 mol%) and DCE (3 mL), 80 °C, 12 h, isolated yield
Figure 3. The electron paramagnetic resonance (EPR) spectra (X band, 9.4 GHz,
room temperature) of the reaction mixture of 1a, phen and Cu(OTf)2 in DMF at
80 °C.
As a follow-up study, electron paramagnetic resonance
(EPR) experiments were performed to get some insights of the
reaction mechanism. With the combination of 1a, Cu(OTf)2 and
1, 10-phen under air in DMF at 80 °C, a distinct EPR signal was
detected in the presence of spin trap 5,5-dimethyl-1-pyrroline
N-oxide (DMPO) (Figure 3). The hyperfine coupling constants
are αN = 12.3 G and αH = 14.3 G. This signal was assigned to a
DMPO-p-toluenethiyl radical adduct by comparison with a
synthesizedsample.13 This result suggested that p-toluenethiyl
radical from the single electron transfer oxidation of 1a was
formed under the standard reaction conditions.
Conclusions
In summary, a novel copper-catalyzed selective oxidative
radical/radical cross-coupling has been demonstrated. This
strategy offered a new protocol for C-S bond formation by
direct cross-coupling of thiyl radical with isobutyronitrile
radical, and provided an practical approach for the synthesis of
valuable α-alkylthionitriles derivatives from basic starting
materials. Preliminary mechanistic investigation suggested
that the transient thiyl radical could be stabilized by copper
catalyst to a persistent one. It is the key process for this
selective radical/radical cross-coupling. The detailed
mechanism is currently under investigation in our laboratory
and will be reported in due course.
Consequently, according to above results and previous
studies,14 a putative mechanism of this direct oxidative radical
cross-coupling is outlined in Scheme 4. Initially, 4-
methylbenzenethiol is oxidized by Cu(II) to release the
thiophenol radical
transient radical
I
, which will coordinate to Cu(I). Since the
I
was stabilized by Cu(I), it could couple with
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Scheme 4. Proposed mechanism.
This work was supported by the National Natural Science
Foundation of China (21702081, 21702150), China
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(BX201600114,
2016M602340), and Jiangxi Provincial Education Department
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Corresponding Author
*zlhuang@whu.edu.cn; drmenggao@163.com;
aiwenlei@whu.edu.cn
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