Cascade Reaction of Propargyl Amines with AgSCF3, as Well as One-Pot Reaction of Propargyl Amines, AgSCF3, and Di- tert-butyl Peroxide: Access to Allenyl Thiocyanates and Allenyl Trifluoromethylthioethers

Article abstract of DOI:10.1021/acs.orglett.8b01181

An efficient cascade reaction of propargyl amines with AgSCF₃ and KBr is developed, affording allenyl thiocyanates at room temperature in high yields. This transformation proceeds via the in situ formation of isothiocyanate intermediates, followed by a [3,3]-sigmatropic rearrangement. The resulting allenyl thiocyanates bearing 3-(electro-donating phenyl) substitutions without isolation can then be reacted with di-tert-butyl peroxide and AgSCF₃ under reflux to generate novel allenyl trifluoromethylthioether compounds in moderate to good yields via a "one-pot" three-step process.

Full text of DOI:10.1021/acs.orglett.8b01181

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cascade Reaction of Propargyl Amines with AgSCF3, as Well as One-
Pot Reaction of Propargyl Amines, AgSCF3, and Di-tert-butyl
Peroxide: Access to Allenyl Thiocyanates and Allenyl
Trifluoromethylthioethers
Long Zhen,† Kun Yuan,† Xiu-yan Li,‡ Chenyun Zhang,† Jun Yang,† Hui Fan,† and Liqin Jiang*,†
†Shanghai Engineering Research Center of Molecular Therapeutics and New Drug Development, School of Chemistry and Molecular,
East China Normal University, 500 Dongchuan Road, Shanghai 200241, China
‡Shanghai Key Lab for Ecological Process and Eco-Restoration, School of Ecological and Environmental Sciences, East China Normal
University, 500 Dongchuan Road, Shanghai 200241, China
S
* Supporting Information
ABSTRACT: An efficient cascade reaction of propargyl amines with
AgSCF3 and KBr is developed, affording allenyl thiocyanates at room
temperature in high yields. This transformation proceeds via the in
situ formation of isothiocyanate intermediates, followed by a [3,3]-
sigmatropic rearrangement. The resulting allenyl thiocyanates bearing
3-(electro-donating phenyl) substitutions without isolation can then
be reacted with di-tert-butyl peroxide and AgSCF3 under reflux to
generate novel allenyl trifluoromethylthioether compounds in
moderate to good yields via a “one-pot” three-step process.
Scheme 1. Different Methods for Isothiocyanates and Allenyl
Thiocyanates, as Well as for Trifluoromethylthiolation
llenes have been found in natural products and have been
1,2
Aincorporated into drugs and materials.
The last two
decades have witnessed an increasing popularity of allenes as
building blocks for a variety of chemical transformations.2
Organic thiocyanates also exhibit biological and pharmaceutical
activities,3 and they are important precursors for the synthesis
of various sulfur-containing compounds, including hetero-
cycles.4 Many methods have been developed for the
construction of organic thiocyanates.5 However, access to
allenyl thiocyanate derivatives is very limited.6 In 2001, Banert
and co-workers reported that propargyl isothiocyanates 3 can
be prepared from propargyl amines 1 and thiophosgene or via a
multistep sequence from propargylic electrophiles 2 (Scheme
1a).6a The equilibration between isolated isothiocyanate 3 and
allenyl thiocyanates 4 was established. When R3 was an aryl,
only one example of 3-phenyl allenyl thiocyanate 4a was
reported (Scheme 1a).6a However, trisubstituted allenyl
thiocyanates (4) with R3 as an aryl have never been synthesized.
In addition, thiophosgene is highly toxic, making it challenging
to safely handle and store, and its high electrophilic property is
also disadvantageous to the functional group tolerance and
selectivity. Thus, a safe, convenient, efficient method to access
to a variety of allenyl thiocyanates is still in high demand.
The trifluoromethylthio (SCF3) group possesses remarkable
electro-withdrawing character and extremely high lipophilicity.7
Much effort has been devoted to incorporating the SCF3 group
into organic molecules8 for the improvement of membrane
permeability and absorption rate.7 However, to the best of our
knowledge, the SCF3 group has never been introduced into
allenyl scaffolds. It is well-known that there is an equilibrium
between trifluoromethanethiolate with carbonothioic difluoride
and fluoride.9 In 2015, the Qing group successfully applied this
reactivity to the direct dehydroxytrifluoromethylthiolation of
alcohols with AgSCF3 (see Scheme 1b).8a Recently, the same
transformation and several other elegant transformations were
Received: April 13, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01181
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
realized using trifluoromethanesulfonamides,10a (Me4N)-
SCF3,10b,c (bpy)CuSCF3,10d and sulfuration of difluorocarbene
with elemental sulfur.10e,f For example, the Schoenebeck group
reported the synthesis of isothiocyanates from anilines and
aliphatic amines with (Me4N)SCF3 under Et3N,10c but never
reported the reactivity of propargyl amines. Herein, we report
that propargyl amines react with carbonothioic difluoride
derived from AgSCF3 in the presence of KBr at room
temperature to give propargyl isothiocyanate intermediates,
which undergo in situ [3,3]-sigmatropic rearrangement to
generate allenyl thiocyanates. Allenyl thiocyanates bearing 3-
(electron-donating phenyl) substitutions without isolation
could then be converted, in the presence of di-tert-butyl
peroxide (DTBP) and AgSCF3, to the corresponding allenyl
trifluoromethylthioethers in moderate to good yields via a “one-
pot” three-step process. Other allenyl thiocyanates could be
transformed to allenyl trifluoromethylthioethers via treatment
with Me3SiCF3 and TBAF in THF (see Scheme 1c).
inexpensive halide salts. The addition of 1.5 equiv of KBr and
NaBr afforded the desired allenyl thiocyanate 4b in 82% and
64% yields, respectively (Table 1, entries 8 and 9). However, n-
Bu4NI (Table 1, entry 10),8a n-Bu4NBr, NH4Br, and KI were all
less effective than KBr (see the SI). Acetonitrile also proved to
be the best solvent for the transformation (Table 1, entries 11
and 12). Thus, our optimized conditions employ 1.0 equiv of
propargyl amine 1b, 1.5 equiv of AgSCF3, and 1.5 equiv of KBr
in air at room temperature in acetonitrile over 1 h, affording
trisubstituted allene thiocyanate 4b in 82% yield.
With the optimized conditions in hand, the substrate scope
of the cascade reaction was investigated (see Scheme 2). First,
Scheme 2. Scope of the Cascade Reaction of Propargyl
Amine 1 with AgSCF3 under KBr for Allenyl Cyanate 4
ab
,
AgSCF3 is a readily prepared,8i stable, and easily handled
solid. We began our study into the reactivity of propargyl
amines with AgSCF3 by evaluating the model reaction between
propargyl amine 1b and 1.5 equiv of AgSCF3 in the presence of
an additive (see Table 1). Performing the reaction in
Table 1. Optimization of Reaction Conditions of Propargyl
a
Amine 1b and AgSCF3 for Allenyl Thiocyanate 4b
b
entry
additive
additive, X (equiv)
solvent
yield (%)
1
2
K2CO3
Et3N
1.5
2.5
1.5
2.5
0.5
0.5
0.75
1.5
1.5
1.5
1.5
1.5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
toluene
12
<3
0
3
Cs2CO3
NaHCO3
FeCl3
FeBr3
FeBr2
KBr
4
8
5
50
49
81
82
64
45
29
30
6
7
8
9
NaBr
10
11
12
n-Bu4Nl
KBr
KBr
a
Conditions: 0.1 mmol of 1b, 0.15 mmol of AgSCF3, and X equiv of
additive react at air for 12 h (entries 1−4) or for 1 h (entries 5−12).
b
Isolated yield.
a
Conditions: 0.1 mmol of 1, 0.15 mmol of AgSCF3, and 0.15 mmol of
6a
b
c
acetonitrile with K2CO3 as an additive at room temperature
KBr react at air for 1−12 h. Isolated yield. Scale = 1 mmol.
for 12 h provided the desired allenyl thiocyanate 4b in only
12% yield (entry 1). Other bases, including Et3N,10c Cs2CO3,
and NaHCO3 under otherwise identical conditions, gave
uniformly poor results (Table 1, entries 2−4). Next, Lewis
acid additives such as FeCl3 were explored, and it was found
that the addition of 0.5 equiv of FeCl3 led to 50% yield of the
desired allenyl thiocyanate 4b in 1 h (entry 5). Increasing or
decreasing the amount of FeCl3 both led to a decreased yield of
4b (see the Supporting Information (SI) for details). Replacing
FeCl3 with FeBr3 provided similar results (Table 1, entry 6). To
our delight, it was observed that the use of 0.75 equiv of FeBr2
led to 4b in 81% yield (Table 1, entry 7). Hypothesizing that
the iron salts simply served as a source of halide to remove the
Ag+ ion of AgSCF3, we next investigated the addition of
we investigated the effect of substituent R1 on the terminal
alkyne of the propargyl amine 1. The propargyl amine bearing a
benzyloxymethyl group (R1 = benzyloxymethyl, R2 = p-
methoxyphenyl) afforded the corresponding allenyl thiocyanate
product 4c in 90% yield. Simple alkyl groups are also tolerated
(R1 = n-butyl, R2 = p-methoxyphenyl), and the corresponding
allenyl thiocyanate 4d was obtained in 77% yield. The substrate
bearing a bulky tert-butyl group on the triple bond also
proceeded smoothly to furnish 4e in 84% yield. However, the
propargyl amine bearing a phenyl group on the terminal alkyne
(R1 = phenyl, R2 = p-methoxyphenyl) gave no desired allenyl
thiocyanate product product, and the use of 3 equiv of AgSCF3
B
DOI: 10.1021/acs.orglett.8b01181
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Organic Letters
Letter
afforded 76% of propargyl trifluoromethylthioether 5a′. Next,
we turned our attention to the substituent R2 at the 1-position
of propargyl amine 1. It was found that various aryls bearing
electron-donating (1b−1l), neutral (1m−1p), or electron-
withdrawing (1q−1t) substituents at the para-, meta-, or ortho-
positions served as R2, consistently furnishing the allenyl
thiocyanate products (4b−4t) in high yields. A variety of
functional groups including methoxy (4b−4j), benzyloxy (4k),
[1,3]dioxolyl (4l), methyl (4m), chloro (4q−4s), and bromo
(4t) were well-tolerated. Chlorine and bromine groups could
serve as highly valuable synthetic handles for further
functionalization. In the case of 4u, which bears both
electron-donating and electron-withdrawing substituents on
the phenyl ring, a slightly decreased yield of 68% was observed.
When R2 was alkyl, such as 2-phenyl substituted ethyl (1v), the
propargyl isothiocyanate 3v was obtained in 96% yield.
We then sought to develop conditions to convert our allenyl
thiocyanate products into trifluoromethylthioethers (5). The
use of Prakash’s reagent (Me3SiCF3) and Cs2CO3 in
acetonitrile11a failed to convert allenyl thiocyanates 4 to allenyl
trifluoromethylthioethers 5. However, we found that subjecting
the isolated allenyl thiocyanates 4n and 4q, which bear neutral
or electron-deficient aryl substituents, to the conditions
reported by Langlois (2.0 equiv of Me3SiCF3 and 0.2 equiv
of TBAF in THF at 0 °C to room temperature (rt)),11b the
corresponding allenyl trifluoromethylthioethers 5a and 5b
could be obtained in moderate yields (Scheme 3, condition
optimized conditions, 1.5 equiv of AgSCF3, and 2.0 equiv of
oxidant was added to the crude reaction solution and the
mixture was refluxed in the presence of an oxidant. We found
that the addition of Na2S2O8, K2S2O8, PhI(OAc)2, or TBHP as
oxidant gave none of the desired allenyl trifluoromethylth-
ioether 5c. To our delight, however, when di-tert-butyl peroxide
(DTBP) was used as an oxidant, 5c was obtained in 50% yield.
After further optimization of the amounts of DTBP and
AgSCF3, as well as temperature (details in the SI), we found
that 2.5 equiv of AgSCF3 and 3.0 equiv of DTBP were optimal,
and that 57% yield of 5c could be obtained after 12 h at 80 °C
through this “one-pot” three-step process (Scheme 3, condition
b).
Under the optimized conditions, the substrate scope of this
“one-pot” three-step process was investigated, as shown in
Scheme 3. Other 1-(p-methoxy phenyl)-substituted propargyl
amines also afforded the corresponding allenyl trifluorome-
thylthioethers 5d−5f in moderate to good yields. The 1-(o-
methoxy phenyl) substituted substrates containing either alkyl
or ether groups on the terminal alkyne gave the desired
products 5g and 5h in 62% and 57% yields, respectively. The 1-
(p-benzyloxy phenyl) substituted propargyl amine furnished the
desired allenyl trifluoromethylthioether 5i in 78% yield.
Product 5j containing a dioxolane functional group also
obtained in 52% yield. The structure of 5c was unambiguously
confirmed by single-crystal X-ray analysis (shown in the SI).
To gain insight into the mechanism for formation of
trifluoromethylthioethers 5, the control reaction without
DTBP was performed, affording the mixture of 5c and 5b′ in
a ratio of 2:1 in 48% yield (Figure 1). We rationalized that,
Scheme 3. Scope of the “One-Pot” Reaction for
b
Trifluoromethylthioethers 5
Figure 1. Control reaction without DTBP.
−
without DTBP, the attack of SCF3 at the C1 position of the
allene in 4b was sterically hindered through the SN2
mechanism, thus leading to attack at the C3 position to form
5b′. We believe that the addition of DTBP with AgSCF3 to the
allene 4b leads to the formation of an allenyl carbocation after
loss of the thiocyanate group. Therefore, ESI-MS analysis of the
“one-pot” model reaction mixture was performed and a peak
corresponding to this proposed carbocation intermediate was
observed in the mass spectrum (see the SI). In addition, the
“one-pot” reaction performed in the presence of 3.0 equiv of
TEMPO gave 5c in 37% yield. Isothiocyanate intermediate 3q
could also be isolated and characterized (see the SI).
Based on the above experimental results, a plausible
mechanism is proposed in Scheme 4. AgSCF3 and KBr react
to give KSCF3 and AgBr as a precipitate, and the coordination
of Ag+ with the amine of the propargyl amine 1 is avoided (eq 1
in Scheme 4). An equilibrium between potassium trifluor-
omethanethiolate with carbonothioic difluoride and potassium
fluoride is established (eq 2 in Scheme 4). The propargyl amine
1 reacts with the in-situ-generated carbonothioic difluoride to
form the propargyl isothiocyanate 3, which is subsequently
transformed to the corresponding allenyl thiocyanate 4 through
[3,3] sigmatropic rearrangement (eq 3 in Scheme 4). After
b
Conditions: 0.1 mmol of isolated 4n or 4q, 0.2 mmol of Me3SiCF3,
and 0.02 equiv of TBAF in THF reacted at 0 °C for 5 min, then at rt
for 3 h; isolated yield. Conditions: 0.1 mmol of 1, 0.15 mmol of
a
AgSCF3, and 0.15 mmol of KBr reacted at air for 1−12 h, 0.25 mmol
of AgSCF3 and 0.3 mmol of DTBP were added and refluxed for 1−12
c
h; isolated yield. Scale = 1 mmol.
a). However, these reported conditions failed to deliver the
allenyl trifluoromethylthioether 5c from the corresponding
allenyl thiocyanates 4b, which bears a strong electron-donating
group on the 3-aryl substituent.
Our next goal was to develop a method for preparing allenyl
trifluoromethylthioethers bearing electron-rich aryl substituents
at the 3-position. After 4b was formed under the above
C
DOI: 10.1021/acs.orglett.8b01181
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, U.K.; fax: +44 1223 336033.
Scheme 4. Proposed Mechanisms for Formation of Allenyl
Thiocyanate 4 and Allenyl Trifluoromethylthioether 5
AUTHOR INFORMATION
Corresponding Author
■
*E-mail addresses: lqjiang@sat.ecnu.edu.cn, jiangliqin_777@
163.com.
ORCID
Liqin Jiang: 0000-0002-9751-7510
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the Science and
Technology Commission of Shanghai Municipality (No.
15ZR1410500) and the sponsorship from the National Natural
Science Foundation of China (No. 21402052).
adding AgSCF3 and DTBP, the Ag+ ion is oxidized to Ag2+ ion
with DTBP (eq 4 in Scheme 4). AgII facilitates abstraction of
the −SCN anion, resulting in the formation of allenyl
carbocation 7 (eq 5 in Scheme 4). The −SCF3 then reacts
with allenyl carbocation 7 to afford the desired allenyl
trifluorothioether 5 (eq 6 in Scheme 4). We could not rule
out that AgII −SCN might transform to Ag+ and SCN radical,
which might terminate the radical translation with another SCN
radical or OtBu radical.
In conclusion, a safe, convenient, and highly efficient cascade
reaction of propargyl amines, AgSCF3, and KBr was developed,
affording a series of trisubstituted allenes bearing a thiocyanate
functional group. This transformation proceeds at room
temperature and gives the allene products in high yields. The
reaction proceeds through in situ formation of isothiocyanate
intermediates, followed by a [3,3] sigmatropic rearrangement.
In addition, a “one-pot” reaction of propargyl amines bearing
electro-rich aryl substituents at 1-position with AgSCF3, KBr,
and di-tert-butyl peroxide afforded trisubstituted allenes bearing
a trifluoromethylthio group in moderate to good yields. The
proposed allenyl carbocation intermediate was detected by ESI-
MS to rationalize the high regioselectivity. Allenyl thiocyanates
bearing electron-neutral and electron-poor phenyl substituents
at the 3-position could also be transformed to the
corresponding allenyl trifluoromethylthioethers through treat-
ment with Me3SiCF3 and TBAF in THF. This report represents
the first synthesis of allenyl trifluoromethylthioether com-
pounds.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01181.
■
S
Experimental procedures and spectroscopic character-
ization data, 1H and 13C NMR spectra of the new
compounds (PDF)
Accession Codes
CCDC 1588662 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
D
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