SO2F2-Mediated one-pot cascade process for transformation of aldehydes (RCHO) to cyanamides (RNHCN)

Article abstract of DOI:10.1039/d0ra02631j

A simple, mild and practical cascade process for the direct conversion of aldehydes to cyanamides was developed featuring a wide substrate scope and great functional group tolerability. This method allows for transformations of readily available, inexpensive, and abundant aldehydes to highly valuable cyanamides in a pot, atom, and step-economical manner with a green nitrogen source. This protocol will serve as a robust tool for the installation of the cyanamide moiety in various complicated molecules.

Full text of DOI:10.1039/d0ra02631j

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SO2F2-Mediated one-pot cascade process for
transformation of aldehydes (RCHO) to cyanamides
Cite this: RSC Adv., 2020, 10, 17288
(RNHCN)†
a
b
a
a
a
*
*
Yiyong Zhao, Junjie Wei, Shuting Ge, Guofu Zhang and Chengrong Ding
A simple, mild and practical cascade process for the direct conversion of aldehydes to cyanamides was
developed featuring a wide substrate scope and great functional group tolerability. This method allows
for transformations of readily available, inexpensive, and abundant aldehydes to highly valuable
cyanamides in a pot, atom, and step-economical manner with a green nitrogen source. This protocol
will serve as a robust tool for the installation of the cyanamide moiety in various complicated molecules.
Received 22nd March 2020
Accepted 21st April 2020
DOI: 10.1039/d0ra02631j
rsc.li/rsc-advances
As a class of multistep, one-pot processes without the separa- some of the transformations require harsh conditions or
tion of intermediates, the cascade (tandem or domino) reac- hazardous reagents. Recently, several new cyanide sources,
tions have been acknowledged as one of the most powerful tools including CuCN,11 AIBN,12 TMSCN,13 and imidazolium thiocy-
in modern chemistry with the features of atom-economy, saving anates,14 were employed in the direct N-cyanation of amines to
power and consumption, better resource management, easy synthesize cyanamides. As an alternative approach, the Tie-
purication and lowest waste generation while still providing mann rearrangement of amidoximes attracted chemists'
a higher yield than the traditional reactions.1 Therefore, interest in the synthesis of cyanamides.15 In 2014, Chien re-
designing controllable cascade reactions with excellent molec- ported that benzenesulfonyl chlorides (TsCl or o-NsCl)
ular efficiencies and high selectivity is a very challenging but promoted the Tiemann rearrangement of amidoximes to
rather highly desirable and strategic key element for modern generate the corresponding cyanamides.16 However, it was
synthetic and sustainable chemistry.2 Cyanamides represent the highly dependent on the electronic effect of the substrates and
core motif in biologically active molecules and have been widely required rigorous reaction conditions, pre-synthesis of
used in pharmaceuticals and functionalized materials.3 As substrates and redundant work-up.
a
reactive N–C–N building block, cyanamides are more
Recently, sulfuryl uoride (SO2F2),17 an inexpensive (about 1$
commonly used as a precursor in the synthesis of pharmaceu- per kg), abundant and relatively inert electrophile (stable up to
tically important N-containing heterocycles and N-alkyl or N- 400 ꢀC when dry) has attracted signicant attention to be used
aryl imides.4 Despite their versatile applications, only a limited for SuFEx click chemistry and other versatile manipulations.18
A
number of synthetic routes have been reported for cyanamides perusal of the literature revealed that the protons of phenolic
in the literature.5 The most frequently adopted method is the hydroxyls or oximes hydroxyls can activate the exchange of the
direct cyanation of amines using cyanogen halides,6 which is S–F bonds in SO2F2 for the S–O bonds to afford functional
overshadowed by its acute toxicity, unfavorable physical prop- products, and the uorosulfate functional group (–OSO2F) can
erties and sensitivity to moisture.7 Another straightforward be applied in a controllable and targeted manner for varied
approach is the direct alkylation of cyanamides; however, N,N- transformations.19 Most recently, our group reported a mild and
dialkylated cyanamides are usually obtained due to the robust method for efficiently converting aldoximes into the
competing alkylation of monoalkylated cyanamides.8 Other corresponding nitriles mediated by SO2F2/base in a green
approaches include the dehydrosulfurization of thiourea,9 the manner (Scheme 1, a).20a Subsequently, an efficient method for
dehydration of urea, and the conversion from isocyanides, the activation of the Beckmann rearrangement of ketoximes
isocyanates, or isothiocyanates.10 These above methods are into amides or lactams utilizing SO2F2 was developed in our
mutually complementary since they all originate from the cor- lab(Scheme 1, b).20b Coincidentally, we found that SO2F2 could
responding amines with multistep manipulations. In addition, also promote the Tiemann rearrangement of amidoximes which
were generated from corresponding nitriles to generate the
corresponding cyanamides in good to excellent yields (Scheme
1, c).20c Upon viewing the high value of cyanamide moieties, the
easy availability of aldehydes, and our continuous efforts on the
utilization of SO2F2 for chemical transformations of oximes
aCollege of Chemical Engineering, Zhejiang University of Technology, Hangzhou
310014, People's Republic of China. E-mail: gfzhang@zjut.edu.cn; dingcr@zjut.edu.cn
bZhejiang Emission Trading Center, Hangzhou 310014, People's Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra02631j
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Table 1 Optimization of reaction conditionsa
Base 1 (2.0
equiv.)
Base 2 (2.0
equiv.)
Entry
Yieldb (2a, %)
1
2
3
4
5
6
7
8
K2CO3
Na2CO3
KHCO3
NaHCO3
Na3PO4
DBU
DIPEA
Et3N
Et3N
Et3N
DBU
DIPEA
Et3N
Et3N
K2CO3
Na2CO3
KHCO3
NaHCO3
Na3PO4
DBU
DIPEA
Et3N
DBU
DIPEA
Et3N
Et3N
Et3N
Et3N
5
<1
<1
<1
7
40
68
94
47
75
86
89
90
78
51
9
10
11
12
13c
14d
15e
Et3N
Et3N
a
Reaction conditions: benzaldehyde 1a (1.0 mmol), 50 wt% NH2OH (1.2
mmol, 1.2 equiv.), CH3CN (10 mL), reux, 2.0 h; then Base 1 (2.0 mmol,
2.0 equiv.), and SO2F2 balloon, r.t., 30 min; then 50 wt% NH2OH (1.5
mmol, 1.5 equiv.), reux, 3.0 h; then the mixture was concentrated,
base 2 (2.0 mmol, 2.0 equiv.), CH2Cl2 (10 mL), and SO2F2 balloon, r.t.,
Scheme 1 Our works on transforming of oximes mediated by SO2F2.
(aldoximes, ketoximes, and amidoximes),20 we proposed a one-
pot process for direct conversion of aldehydes to cyanamides
through a cascade sequence following similar mechanism as
our cascade nitrile synthesis process. We envisioned that in
common polar solvent acetonitrile (CH3CN), aldehydes 1 would
react with NH2OH to provide the aldoxime intermediate A aer
dehydration, and the aldoxime will further react with SO2F2 to
generate the corresponding sulfonyl ester B, and with the
assistance of the base, the following b-elimination of the
precursor sulfonyl ester B would generate the desired carbon–
nitrogen triple bonds of nitriles C. Subsequently, the nitriles are
transformed to the amidoxime intermediate D reacting with
NH2OH through a nucleophilic addition and dehydration
process; then the amidoxime was deprotonated with SO2F2
under the promotion of the base to form the corresponding
sulfonyl ester E, and the N–O bond cleavage occurred with
concomitant R group migration over to the C–N bond to furnish
the N-substituted cyanamides 2 (Scheme 1, d).
We conducted our initial study with benzaldehyde 1a as the
model substrate to examine the feasibility of the proposed
transformation. Accordingly, aer screening a large variety of
conditions as shown in Table 1. Considering of inorganic bases
have signicant advantages over their organic counterparts,21
inorganic bases, including K2CO3, Na2CO3, KHCO3, NaHCO3
and Na3PO4, were rstly screened (entries 1–5). Although inor-
ganic bases were more advantageous than organic bases in
Qin's oxidation system,19 we were disappointed to nd that the
use of inorganic bases provided only a trace amount of the
desired product 2a. It is worth noting that the use of 1,8-dia-
zabicyclo[5.4.0]undec-7-ene (DBU) and N,N-diisopropylethyl-
amine (DIPEA) provided 40% and 68% isolated yields of the
b
c
2.0 h. Isolated yields. NH2OH.HCl (1.2 mmol, 1.2 equiv.) and Base
1 (1.5 mmol, 1.5 equiv.) were used to replace 50 wt% NH2OH in the
d
rst step of one-pot process. NH2OH.HCl (1.5 mmol, 1.5 equiv.) and
Base 2 (2.0 mmol, 2.0 equiv.) were used to replace 50 wt% NH2OH in
e
the third step of one-pot process. The reaction mixture wasn't
concentrated, and carried out in CH3CN in the fourth step of one-pot
process.
desired product, respectively, while the use of triethylamine
(Et3N) assisted the reaction more efficiently to generate the
desired product 2a in a great isolated yield of 94% (entries 6–8).
It is not surprising to nd that xing Et3N as base 1 and
switching base 2 to DBU and DIPEA, or xing Et3N as base 2 and
switching base 1 to DBU and DIPEA caused varying degrees
decreased yields of the product 2a (entries 9–12).20 Hence,
further studies were carried out by using Et3N as base since it
provided the best yield of 2a to 94% isolated yield (entry 8).
Although solid NH2OH$HCl is easier to operate and more
inexpensive that 50 wt% NH2OH (aqueous solution), the
moderate decreased yields of 2a were occurred when using
NH2OH$HCl instead of 50 wt% NH2OH as the nitrogen source
(entries 13, 14). Subsequently, in order to simplify the opera-
tion, the reaction mixture wasn't concentrated and carried out
in CH3CN in the fourth step of one-pot process, however, effi-
ciency was sharply decreased (entry 15). Therefore, the condi-
tions of entry 8 were chosen as the standard procedure for the
examinations of functional group tolerability and substrate
scope.
Next, we evaluated the substrate scopes, functional group
compatibility and limitation of the one-pot cascade process
(Table 2). In most cases, the corresponding aldehydes,
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Table 2 Scope of this SO2F2-promoted one-pot cascade process.a,b
Scheme 2 The gram-scale preparation and further transformations of
N-phenylcyanamide 2a.
Since the resulting product N-phenylcyanamide 2a is widely
applied as an estimable building block in the direct and effi-
cient synthesis of many bioactive molecules, this protocol is
particularly useful. Examples include tetrazolamine 3,22 urea
4,23 amide 5,24 guanidine 6,25 2-aminoquinazolin-4-one 7,26 and
aminobenzonitrile 8.27 These representative transformations
clearly demonstrate the versatility of cyanamides in organic
chemistry.
a
Conclusions
Reaction conditions: aldehyde 1 (1.0 mmol), 50 wt% NH2OH (1.2
mmol, 1.2 equiv.), CH3CN (10 mL), reux, 2.0 h; then Et3N (2.0 mmol,
2.0 equiv.), and SO2F2 balloon, r.t., 30 min; then 50 wt% NH2OH (1.5
mmol, 1.5 equiv.), reux, 3.0 h; then the mixture was concentrated,
Et3N (2.0 mmol, 2.0 equiv.), CH2Cl2 (10 mL), and SO2F2 balloon, r.t.,
2.0 h. Isolated yields. Stirring 6.0 h for the fourth step of one-pot
process.
In conclusion, we disclosed a new one-pot cascade process
which allowed transformation of a broad range of inexpensive,
easily accessible and abundant aldehydes into cyanamides with
green nitrogen source. This reported SO2F2-modiated reaction
proceeded with the features of mild condition, high efficiency,
wide scope, and excellent functional group compatibility.
Moreover, gram-scale reaction was performed to demonstrate
the applicability of cyanamides, which could be efficiently
converted to various structures.
b
c
including aromatic and aliphatic substituted aldehydes, were
successfully furnished in moderate to great yields (2a–2y). Both
electron-donating (2b–2e) and electron-withdrawing groups (2f–
2i) were all well tolerated under the standard reaction condi-
tions. Notably, the satisfactory results showed that the position
of the substituents on the aryl rings exhibited insignicant
inuence on the efficiency (2b, 2g, 2h vs. 2k–2m vs. 2n–2p).
Furthermore, multi-functionalized benzylic aldehydes (2q, 2r)
and naphthalene aldehydes (2s, 2t) were also successfully con-
verted into their corresponding cyanamide products in
moderated to great yields. Excitingly, a set of heterocyclic
benzylic aldehydes (2u, 2v) were successfully converted into
their corresponding cyanamides in acceptable yields. Besides,
the phenylpropiolaldehyde 1w was also converted to give the
nal product at a 70% isolated yield (2w). For aliphatic moieties,
the representative aldehydes (2x–2z) were also successfully
transformed into their corresponding cyanamides with
moderate yields.
Conflicts of interest
There are no conicts to declare.
Notes and references
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