K2S2O8-mediated regio- And stereo-selective thiocyanation of enamides with NH4SCN

Article abstract of DOI:10.1039/d1ob00156f

A direct and straightforward thiocyanation of enamides with NH₄SCN under metal-free conditions has been accomplished. A variety of (E)-β-thiocyanoenamides are readily produced in a regio- and stereo-selective manner. The protocol features mild reaction conditions, good functional group tolerance and operational simplicity. The potential utility of this strategy was further demonstrated by transformation of thiocyanate into thiotetrazole-containing compounds and a Pd-catalyzed cross-coupling reaction to afford six- or seven-membered sulfur-containing heterocycles. Mechanistic insights into the reaction indicate that the reaction may proceedviaa radical mechanism.

Full text of DOI:10.1039/d1ob00156f

Organic &
Biomolecular Chemistry
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K S O -mediated regio- and stereo-selective
2
2 8
thiocyanation of enamides with NH SCN†
4
Cite this: Org. Biomol. Chem., 2021,
9, 2512
1
a
a
a
a
a
Qingyun Gu,‡ Qiyang Wang,‡ Wenjing Dai, Xin Wang, Yingguo Ban,
b
a
a
a
a
Tianqing Liu, Yu Zhao, Yanan Zhang, Yong Ling * and Xiaobao Zeng
*
4
A direct and straightforward thiocyanation of enamides with NH SCN under metal-free conditions has
been accomplished. A variety of (E)-β-thiocyanoenamides are readily produced in a regio- and stereo-
selective manner. The protocol features mild reaction conditions, good functional group tolerance and
operational simplicity. The potential utility of this strategy was further demonstrated by transformation of
thiocyanate into thiotetrazole-containing compounds and a Pd-catalyzed cross-coupling reaction to
afford six- or seven-membered sulfur-containing heterocycles. Mechanistic insights into the reaction
indicate that the reaction may proceed via a radical mechanism.
Received 27th January 2021,
Accepted 25th February 2021
DOI: 10.1039/d1ob00156f
rsc.li/obc
and NH
4
SCN under transition-metal free conditions, leading
Introduction
to thiocyanated alkene derivatives in excellent yields
1
6a
Enamides are attractive and versatile building blocks in (Scheme 1a).
In the same year, Liu and coworkers demon-
modern synthetic chemistry since they can be readily strated an efficient construction of 2-iminothiazoles via the
converted to a wide range of pharmaceutically and biologi- cascade reaction of cyclic enaminones with KSCN
1
2
16b
cally important molecules and chiral amines. Thus, the (Scheme 1b).
Recently, the strategy was further applied to
2
direct β-C(sp )–H functionalization of enamides including ary- the solvent-dependent divergent synthesis of thiocyanated
lation, alkynylation, olefination, acylation, alkylation,
phosphorylation, sulfonylation, and others
achieved via transition-metal catalysis or radical coupling reac- presence of NXS/SCN (X = Cl or Br) all proceeded through the
tions in the past few decades. Despite the great progress nucleophilic substitution reaction mechanism of tri-substi-
achieved, the direct regio- and stereo-selective construction of tuted alkenes with thiocyanating reagents; additionally, the
3
4
5
6
7
enaminones and 2-aminothiazoles by Duan and coworkers
8
9
10
16c
has been (Scheme 1c). It should be noted that these protocols in the
−
β-thiocyanoenamides has remained unexplored.
generality of these protocols was investigated only with elec-
On the other hand, organic thiocyanates are versatile pre-
cursors, which can be easily transformed into valuable sulfur-
containing compounds such as thioethers, sulfides and sulfur-
1
1
containing heterocycles, and they are also prevalent in the
1
2
field of biomedical and organic chemistry. In this scenario,
much effort has been devoted to the development of more
1
3
simple and safe methods to access organic thiocyanates.
Although diverse strategies towards thiocyanation of aro-
1
4
15
matics and heterocycles have been developed, approaches
1
6
for the direct thiocyanation of alkenes are less studied.
In 2017, Wang and coworkers innovatively reported the
thiocyanation of ketene dithioacetals in the presence of NCS
a
School of Pharmacy, Nantong University, 19 Qixiu Road, Nantong, Jiangsu Province
2
26001, People’s Republic of China. E-mail: Lyyy111@sina.com, zengxb@ntu.edu.cn
b
NICM Health Research Institute, Western Sydney University, Sydney, NSW, Australia
†
Electronic supplementary information (ESI) available. CCDC 2014798. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1ob00156f
‡
These authors contributed equally to this paper.
Scheme 1 Thiocyanation of alkenes or alkynes.
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tron-withdrawing carbonyl group substituted alkenes. It is
After screening various reaction conditions, including oxi-
worth mentioning that Majee and co-workers elegantly dants and solvents, we gratifyingly obtained the desired thio-
extended the substrate scope to unactivated alkenes and cyanated product 3aa in 87% yield by increasing the tempera-
alkynes in the presence of persulfate and thiocyanating ture to 50 °C (entry 9), while continuously increasing the temp-
reagents via a radical reaction mechanism to obtain various erature to 80 °C resulted in a similar yield (entry 10). Finally,
1
,2-dithiocyanated compounds under mild conditions in a by fine tuning the ratio of 1a : 2a (entries 11 and 12), the
1
6e
short reaction time (Scheme 1d).
optimal reaction conditions were determined as shown in
Considering the importance of organic thiocyanates, the entry 9.
exploration of an alternative method to construct such a desir-
With the optimal conditions established, we set out to
able framework in a regio- and stereo-selective manner is still investigate the generality of this reaction, and the results are
attractive. As part of our ongoing studies on the development summarized in Scheme 2. The protocol is applicable to a wide
1
7
of radical involving reactions, we report herein a direct and range of enamides to afford the desired products 3aa-y in mod-
straightforward thiocyanation of enamides with NH SCN erate to good yields. The reaction is not affected by the elec-
4
under mild reaction conditions to regio- and stereo-selectively tronic properties of the substituents; enamides bearing elec-
provide a diverse array of β-thiocyanoenamides in moderate to tron-donating groups (–Me, –OMe, –Ph) or electron-withdraw-
good yields.
3
ing groups (–CF , F, Cl, Br, I) at the para-position were toler-
ated to afford 3aa-i in good yields. Notably, the halide substitu-
ents are compatible in the reaction to give 3ag-i and 3al in
67–85% yields, which are amenable for further elaboration.
The reaction efficiency is not affected by the steric hindrance
of the substituents, to afford 3aj and 3ak in comparable yields
Results and discussion
We commenced our investigation by choosing 1a as the model to 3aa. Apart from enamides with a mono-substituent at the
substrate, NH
SCN (2a) as the thiocyanate source, and NBS as phenyl ring, enamides with di-substituents at the phenyl ring
an oxidant in CH
CN at r.t.; it was found that 3aa was obtained are also suitable substrates in the reaction to deliver the corres-
in 31% yield and 1a was recovered in 63% yield (Table 1, entry ponding products in good yields (3am, 3an).
). Then we examined several oxidants. To our delight, when
was used as the oxidant, the yield increased to 76%
entry 2); Na S O was more efficient than TBHP to afford 3aa
4
3
1
2 2 8
K S O
(
2
2 8
in 72% yield (entries 3 and 4), and the reaction could not be
carried out without an oxidant (entry 5). Among the solvents
tested, AcOH was found to be superior to other solvents such
as THF or toluene, and the yield of 3aa was increased to 81%
(entries 6–8).
a
Table 1 Optimization of reaction conditions
b
Entry
Oxidant
NBS
Solvent
Temp. (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
CH
CH
CH
CH
CH
3
CN
3
CN
3
CN
3
CN
3
CN
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
50
31
76
72
45
n.r.
81
35
65
87
86
85
80
K
Na
2
S
2
2
O
8
S
2
O
8
aq. TBHP
—
2
K S
2
K S
2
K S
2
K S
2
K S
2
K S
2
K S
O
2 8
O
2 8
O
2 8
O
2 8
O
2 8
O
2 8
O
2 8
AcOH
THF
Toluene
AcOH
AcOH
AcOH
AcOH
1
1
1
0
1
2
80
50
50
c
d
a
Scheme 2 Substrate scope of radical thiocyanation of enamides.
SCN
Reaction conditions: 1a (0.3 mmol), 2a (0.45 mmol), oxidant
(
0.45 mmol), solvent (2 mL) at the indicated temperature for 6 h. Reactions were carried out with enamides (1a-z) (0.3 mmol), NH
4
b
c
d
Isolated yield. 1a : 2a = 1 : 2. 1a : 2a = 1 : 1.
2 2 8
(1.5 equiv.) and K S O (1.5 equiv.) in HOAc (2 mL) at 50 °C for 6 h.
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With the results obtained above, the substituents tethering which was in stark contrast to the previous report where the
to the nitrogen atom are also examined. Changing benzyl-pro- six-membered ring as a single product was obtained through
1
g,6g,7f,h
tected enamides to alkyl-, allyl- or propargyl-protected enam- the Heck reaction (Scheme 3c).
Furthermore, the hydro-
ides afforded the target products in comparable yields (3ao-q). lysis of 3aa was easily achieved to obtain α-thiocyanoketone 9
Notably, excellent regio-selectivity is observed in our reaction in good yield, which is a versatile synthon that could be trans-
to furnish 3ap and 3aq as mono-thiocyanated products even formed to various sulfur-containing bioactive scaffolds
1
8
with an increasing amount of NH
tion time. In addition, substituents on the phenyl ring of the
4
SCN and a prolonged reac- (Scheme 3d).
To probe the mechanism of the developed protocol, control
benzyl group are also compatible to afford 3ar-t in acceptable experiments were performed in the presence of a radical sca-
yields. The structure of 3ar was unambiguously confirmed by venger (Scheme 4). It was found that the corresponding
X-ray single crystal diffraction (CCDC 2014798†). When repla- product 3ab was obtained in trace or 23% yield upon the
cing the –Me group with –Et and –Pr, 3au and 3av are obtained addition of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) or 2,6-
in good yields. Additionally, naphthyl- and thienyl-substituted di-tert-butyl-4-methylphenol (BHT) (Scheme 4a); the results
enamides also participate in the reaction to deliver 3aw and showed that a radical process may be involved in this reaction.
3
ax in 77% and 83% yield, respectively. To our delight, 3ay is The radical intermediate could further be trapped by 1,1-
formed in 86% yield using KSeCN as the selenium source. diphenylethylene (DPE) to afford the adduct of DPE and SCN
Unfortunately, when aliphatic enamides were subjected to the radical 10 in 31% yield (Scheme 4b).
standard conditions, no reaction occurred (3az, 4aa). Based on
the X-ray structure determination for compound 3ar, the reports,
Based on the results obtained above and previous
3
–10
a plausible reaction mechanism was proposed as
stereochemistry of the other compounds in Scheme 2 is tenta- shown in Scheme 5; initially, the reaction of NH SCN with
4
•
tively assigned as E. The excellent stereo-selectivity is likely
attributed to the steric repulsion between the SCN group and ated SCN radical undergoes an addition process with enam-
protected amide group. ides (1b) to afford intermediate A, which was further oxidized
2 2 8
K S O could generate a SCN radical ( SCN); the in situ gener-
To further expand the application of this protocol, thiotetra- to yield a cationic intermediate B. We believe that intermediate
zole 4 was prepared in 76% yield via the cycloaddition reaction B could possibly go through either path I or path II to afford
of 3ab with NaN3 (Scheme 3a). Next, a Pd-catalyzed intra- 3ab as E isomer. As depicted in path I, product 3ab could be
molecular cross-coupling cyclization reaction was successfully formed through the deprotonation of intermediate B. The
conducted to obtain benzo[b]thiophene 5 in 45% yield stereo-selectivity could be attributed to the steric repulsion
(Scheme 3b). Notably, we are surprised to find that the between the protected amide group and the SCN group, result-
expected product 8 was not detected in the Pd-catalyzed cross- ing in the sterically more favourable E-type products. While a
coupling reaction of 3ar and 3as, instead, the seven-membered more convincible pathway was also proposed as path II,
rings 6 and 7 were obtained in 36% and 33% yield, respect- carbon cationic species B would resonate to iminium ion C,
ively, probably through the initial isomerization of 3ar and which could undergo [1,5]-σ rearrangement to furnish inter-
3
as, followed by the Pd-catalyzed C–S bond formation reaction mediate D, followed by loss of the proton to give 3ab as an E
(see the ESI† for details about the plausible mechanism), isomer.
Experimental section
General procedure for the syntheses of 3aa-y
A mixture of enamides (1a-y) (0.3 mmol), NH SCN (34.2 mg,
4
2 2 8
0.45 mmol, 1.5 equiv.) and K S O (121.5 mg, 0.45 mmol, 1.5
equiv.) in HOAc (2 mL) was stirred at 50 °C for 6 h (monitored
Scheme 3 Synthetic applications.
Scheme 4 Preliminary mechanistic studies.
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Innovation Program, and the Start-Up Funding for Research of
Nantong University (03081231).
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