Oxidative C-Se coupling of formamides and diselenides by using aqueous tert-butyl hydroperoxide: A convenient synthesis of selenocarbamates

Article abstract of DOI:10.1002/ejoc.201301248

An oxidative coupling reaction between formamides and diselenides under metal-free conditions is described. The C-Se bond formation occurred exclusively at the carbonyl carbon by using aqueous tert-butyl hydroperoxide and 4 A molecular sieves and the coupled products, selenocarbamates, were obtained in moderate-to-good yields. A general oxidative coupling reaction for the formation of C-Se bonds by using formamides and diselenides is demonstrated. This strategy provides a selective synthesis of selenocarbamates, a class of synthetically useful and biologically active organoselenium compounds under metal-free conditions. Copyright

Full text of DOI:10.1002/ejoc.201301248

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301248
Oxidative C–Se Coupling of Formamides and Diselenides by Using Aqueous
tert-Butyl Hydroperoxide: A Convenient Synthesis of Selenocarbamates
Pushpinder Singh,[a] Aanchal Batra,[a] Paramjit Singh,[a] Amarjit Kaur,[a] and
Kamal Nain Singh*[a]
Keywords: Oxidation / Coupling reactions / Reaction mechanisms / Selenium / C–H activation
An oxidative coupling reaction between formamides and di-
selenides under metal-free conditions is described. The
C–Se bond formation occurred exclusively at the carbonyl
carbon by using aqueous tert-butyl hydroperoxide and 4 Å
molecular sieves and the coupled products, selenocarb-
amates, were obtained in moderate-to-good yields.
Selenocarbamates, a group of organoselenium com-
pounds, act as precursors for α-alkylidene-β/δ-lactams ex-
hibiting antibiotic properties.[9] The antiviral effects of com-
pounds containing this framework have also been
studied.[10] N-Substituted Se-phenylselenocarbamates are
useful precursors for the generation of carbamoyl radicals
and other synthetic transformations.[11] Selenocarbamates
can be used as protected selenols and smoothly deprotected
under alkaline conditions.[12] Traditionally, selenocarb-
amates have been prepared from aromatic isocyanates and
haloalkanes by using LiAlHSeH as a selenating agent[12b]
or from aryl halides by lithium/halogen exchange followed
by selenium metal insertion and quenching with N,N-dialk-
ylcarbamoyl chloride.[12b,12c]
Dimethylformamide is normally used as a solvent,[13] but
is also considered as a source of CO, Me2N, Me2NCO and
oxygen.[14] However, the direct C–H activation of form-
amides has also been reported.[15–18] In these oxidative reac-
tions, hydrogen abstraction can occur from two different
sites: the formyl C–H or the C–H α to the nitrogen atom.
tert-Butyl hydroperoxide (TBHP)/Cu-mediated direct amid-
ation of β-keto esters and β-dicarbonyl phenols with form-
amides has recently been achieved and occurs at the C–H
centre of the formyl moiety.[16] The metal-free reaction of
formamides with phthalimides and decarboxylative C–H
acyloxylation of DMF have been reported to occur regiose-
lectively at the C–H centre α to the nitrogen atom.[18] Xiang
and co-workers reported that in the direct oxidative thiol-
ation of DMF with diphenyl disulfide, the products corre-
sponding to hydrogen abstraction from the formyl C–H and
the C–H α to the nitrogen atom are formed in approxi-
mately equal amounts.[18a] However, by using thiophenol as
a coupling partner along with Cu(OAc)2/TBHP, the re-
gioselective formation of thiocarbamate was observed.[18b]
It may be noted that the reaction of simple phenol with
DMF in the presence of CuCl/TBHP resulted in no product
formation.[16b]
Introduction
Direct C–H functionalization of heteroatom-containing
compounds by cross-dehydrogenative coupling (CDC) is
one of the most efficient routes for C–C bond formation
and has been used for the synthesis of versatile building
blocks and biologically active natural products.[1] The ad-
vantage of using non-functionalized substrates makes this
procedure more effective with wider applicability.[2] In the
recent past, several efficient methods for C–H activation α
to nitrogen and oxygen atoms have been developed under
both transition-metal (e.g., Cu, Fe and Ru)-catalysed and
metal-free conditions.[3,4] Many reports also describe the
formation of C–N, C–P, C–O and C–S bonds by using CDC
procedures.[5] However, C–Se bond formation by direct C–
H selenylation has received very little attention and is lim-
ited to metal-catalysed reactions of electron-rich arene or
indole C–H bonds with diaryl selenides.[6] Other metal-cata-
lysed reactions of diselenides, selenols or selenohalides with
substrates such as alkyl halides, alkynes, organoboranes and
organosilanes have also been explored in the synthesis of
organoselenides of biological and pharmaceutical impor-
tance and with applications in materials science.[7] However,
the metal-catalysed reactions are generally accompanied by
toxic metal impurities along with pharmaceutically impor-
tant final products[8a–8c] and the mechanistic pathways are
usually complicated.[8d] Thus, from the perspective of de-
veloping an efficient and greener methodology by using
simple reaction conditions, a metal-free approach to direct
C–Se bond formation would be an attractive strategy.
[a] Department of Chemistry and Centre of Advanced Studies in
Chemistry, Panjab University,
Chandigarh 160014, India
E-mail: kns@pu.ac.in
http://chemistry.puchd.ac.in/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301248.
7688
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 7688–7692

A Convenient Synthesis of Selenocarbamates
As a result of our continuing interest in developing new Table 1. Optimization of reaction conditions.[a]
reactions of synthetic utility,[19] we have investigated
whether an oxidative coupling reaction between form-
amides and diselenides can be used to form a new C–Se
bond (Scheme 1). Furthermore, the regioselectivity of such
a reaction would be of interest not only vis-a-vis the corre-
Entry
Oxidant
(amount [equiv.])
Additive/catalyst
Time Temp. Yield
sponding reactions of disulfides, but also because seleno-
carbamates, one of the possible products, are of consider-
able importance. This report describes our findings in this
context.
[h]
[°C]
[%][b]
1
2
3
4
5
6
7
8
aq. TBHP (4)
aq. TBHP (4)
aq. TBHP (4)
aq. TBHP (4)
aq. TBHP (4)
TBHP (4)
aq. TBHP (4)
aq. TBHP (4)
aq. TBHP (4)
aq. TBHP (4)
aq. TBHP (4)
aq. TBHP (4)
aq. TBHP (2)
aq. TBHP (6)
–
4 Å MS
4 Å MS
4 Å MS
12
12
5
100
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
27
68
41
51
60
50
20
27
34
38
37
8
48
44
n.r[e]
n.r
51
47
67
4 Å MS
7
–
12
12
12
12
12
12
12
12
12
12
12
12
12
12
15
4 Å MS
4 Å MS/CuI
4 Å MS/CuBr
4 Å MS/Cu(OAc)2
Cu(OAc)2
4 Å MS
9
10
11[c]
12[d]
13
14
15
16
17
18[f]
19
Scheme 1. Metal-free coupling of formamides and diselenides.
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
Results and Discussion
DDQ (4)
DTBP (4)
aq. TBHP (4)
aq. TBHP (4)
Our initial studies focused on the coupling of N,N-di-
methylformamide (DMF, 1a) and diphenyl diselenide (2a).
The reaction between DMF (80 equiv., also used as a sol-
vent) and 2a (1 equiv.) was performed in the presence of
4 Å molecular sieves at 100 °C for 12 h under nitrogen by
using 4 equiv. of aq. TBHP (70%) as oxidant. After workup
and purification by column chromatography, the product
3a was obtained in 27% yield (Table 1, entry 1) along with
unreacted 2a (45%). No other new product was detected by
TLC analysis. Raising the reaction temperature to 120 °C
improved the yield of 3a to 68% (Table 1, entry 2). Low
yields of 3a were obtained upon decreasing the length of
the reaction (Table 1, entries 3 and 4), but a longer reaction
time did not improve the yield either (Table 1, entry 19).
The reaction occurred even in the absence of molecular si-
eves (Table 1, entry 5), however, the positive role of molecu-
lar sieves as a weak base prompted us to use them under
optimized conditions.[18a] The use of 4 equiv. of aq. TBHP
gave a good yield of the product, but a lower amount of
oxidant reduced the yield (Table 1, entry 13). The use of 6 m
TBHP in decane also gave a moderate yield (Table 1, en-
try 6). In the presence of a catalyst such as CuI, CuBr or
Cu(OAc)2, the product 3a was obtained in a low yield
(Table 1, entries 7–10). No reaction was observed in the ab-
sence of TBHP (Table 1, entry 15), and the use of 2,3-
dichloro-5,6-dicyanobenzoquinone (DDQ), another oxi-
dant, failed to afford the desired product (Table 1, en-
try 16), but in the presence of di-tert-butyl peroxide
(DTBP), 3a was obtained in 51% yield (Table 1, entry 17).
Reducing the amount of DMF in the reaction (Table 1, en-
tries 11 and 12) or increasing the amount of aq. TBHP
(Table 1, entry 14) also resulted in a lower yield of the prod-
uct 3a. When the reaction was performed in air, the product
yield decreased to 47% (Table 1, entry 18). Thus, the best
results were obtained by using aq. TBHP (4 equiv.) and 4 Å
molecular sieves at 120 °C for 12 h (Table 1, entry 2) under
nitrogen.
[a] Reaction conditions: DMF (80 equiv.), 2a (1 equiv.), 4 Å molec-
ular sieves (MS) (0.15 g). [b] Isolated yield. [c] DMF (50 equiv.). [d]
DMF (20 equiv.). [e] No reaction. [f] In air.
The scope of this metal-free oxidative coupling reaction
was examined by using various formamides and diselenide
substrates under the optimized conditions. All the form-
Table 2. Oxidative coupling reactions of formamides and diselen-
ides.[a]
Entry
1
2
Product Yield
[%][b]
1
2
3
4
5
6
7
8
1a (R = Me)
1b (R = Et)
1c (R = Bu)
1d (R = iPr)
2a (Ar = C6H5)
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
68
80
60
70
68
64
50
62
75
67
62
65
67
80
63
31
47
74
67
2a
2a
2a
1a
1b
1c
1d
1a
1b
1c
1d
1a
1b
1c
1a
1b
1a
1b
2b (Ar = 4-CH3C6H4)
2b
2b
2b
9
2c (Ar = 4-CH3OC6H4)
10
11
12
13
14
15
16
17
18
19
2c
2c
2c
2d (Ar = 1-naphthyl)
2d
2d
2e (Ar = 2-CH3O-1-naphthyl)
2e
2f (Ar = CH2C6H5)
2f
[a] Reaction conditions: 1 (80 equiv.), 2 (1 equiv.), 4 Å MS (0.1 g/
0.2 mmol), aq. TBHP (4 equiv.). [b] Isolated yield.
Eur. J. Org. Chem. 2013, 7688–7692
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7689

P. Singh, A. Batra, P. Singh, A. Kaur, K. N. Singh
SHORT COMMUNICATION
Table 3. Study of the substrate scope in the coupling reaction be- amides, namely N,N-dimethyl-, N,N-diethyl-, N,N-dibutyl-
tween formamides and deselenides.[a]
and N,N-diisopropylformamide (1a–1d), reacted with the
diaryl diselenides to give the coupled products in moderate-
to-good yields (Table 2). Diselenides bearing electron-do-
nating groups on the phenyl ring (2b, 2c) smoothly afforded
the corresponding coupled products 3e–3l (Table 2, en-
tries 5–12). Di-1-naphthyl diselenide (2d) also gave the
products 3m–3o in good yields (Table 2, entries 13–15).
However, bis(2-methoxy-1-naphthyl) diselenide (2e) gave
lower yields (Table 2, entries 16 and 17), which may be due
to steric factors. The use of dibenzyl diselenide (2f) as a
coupling partner also gave the products 3r and 3s in yields
of 74 and 67%, respectively (Table 2, entries 18 and 19). In
addition, no other new products were detected in these re-
actions.
To test the generality of this procedure, we also evaluated
the reaction with cyclic formamides 1e–1g. The correspond-
ing selenocarbamates were obtained in yields of 51–71%
(Table 3, entries 1–6). However, N-methylformamide (1h)
and N-phenylformamide (1i) failed to give the coupled
product (Table 3, entries 7 and 8). In addition, N,N-dimeth-
ylacetamide (1j) also failed to react (Table 3, entry 9).
Therefore it can be inferred that the product corresponding
to C–H abstraction α to the nitrogen atom is not formed
even when formyl C–H abstraction is blocked.
It was proposed earlier that the reaction of formamide
with different coupling partners such as azole, β-keto esters
and thiol in the presence of oxidants like TBHP and DTBP
occurs regioselectively by formyl hydrogen abstraction and
proceeds through a radical pathway.[16–18] In this case also,
when the radical scavenger TEMPO (4 equiv.) was added to
the reaction mixture of 1a and 2a under the optimized reac-
tion conditions (Table 1, entry 2), the yield of product 3a
was reduced dramatically (7%), which suggests the involve-
ment of a radicaloid species. Therefore we have proposed a
plausible mechanism for this reaction (Scheme 2). Hydro-
gen radical abstraction from formamide 1a by TBHP gives
intermediate A, which reacts with PhSeSePh to give the
coupled product 3a and selenyl radical B. The intermediate
B either reacts with 1a to give PhSeH and intermediate A
or it directly reacts with the initially formed intermediate A
to give the desired product. In the presence of TBHP,
PhSeH is oxidized to PhSeSePh to complete the cycle.[20]
[a] Reaction conditions: 1 (80 equiv.), 2 (1 equiv.), 4 Å MS (0.1 g/
0.2 mmol), aq. TBHP (4 equiv.). [b] Isolated yield. [c] No reaction.
Scheme 2. Tentative mechanism for the coupling reaction between
formamides and diselenides.
7690
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A Convenient Synthesis of Selenocarbamates
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Conclusions
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An efficient, direct metal-free coupling between form-
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affords selenocarbamates. Further studies of the scope and
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Experimental Section
General Procedure for the Coupling Reactions of 1a and 2a: Com-
pound 1a (2 mL, 25.6 mmol), 2a (100 mg, 0.32 mmol), 4 Å MS
(150 mg) and 70% aq. TBHP (0.2 mL, 1.28 mmol) were placed in
a 10 mL flame-dried two-necked round-bottomed flask equipped
with condenser and stirring bar under nitrogen. The reaction mix-
ture was stirred at 120 °C for 12 h. The resulting reaction mixture
was dissolved in ethyl acetate (20 mL) and washed with water (2ϫ
10 mL) and brine (2ϫ 10 mL). Evaporation of the solvent and pu-
rification of the residue by flash column chromatography over silica
gel (230–400 mesh) by using ethyl acetate/hexane as eluent afforded
the product 3a (49.5 mg, 68%).
[8]
[9]
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization data, and the 1H,
13C and 77Se NMR spectra.
Acknowledgments
A. B. and P. S. acknowledge the University Grants Commission
(UGC) and the Council of Scientific and Industrial Research
(CSIR), New Delhi, respectively, for providing research fellowships.
The NMR and mass spectrometer facility of SAIF, Panjab Univer-
sity, Chandigarh is gratefully acknowledged.
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Eur. J. Org. Chem. 2013, 7688–7692
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7691

P. Singh, A. Batra, P. Singh, A. Kaur, K. N. Singh
SHORT COMMUNICATION
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[20] The reaction of N,N-dimethylformamide (1a) with PhSeH un-
der the optimized conditions given in Table 1, entry 2 afforded
the product 3a in 36% yield, which is roughly half that ob-
served with PhSeSePh (68%). This experiment lends support
to the proposed mechanism in Scheme 2 in which intermediate
formation of PhSeH is postulated. We are grateful to a referee
for suggesting this experiment.
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Received: August 20, 2013
Published Online: October 29, 2013
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