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- carbon by using aqueous tert-butyl hydroperoxide and 4 Å
selenides under metal-free conditions is described. The molecular sieves and the coupled products, selenocarb-
C–Se bond formation occurred exclusively at the carbonyl amates, were obtained in moderate-to-good yields.

Selenocarbamates, a group of organoselenium com-

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

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-
sponding reactions of disulfides, but also because seleno- Entry Oxidant Additive/catalyst Time Temp. Yield
(amount [equiv.]) [h] [°C] [%][b]
carbamates, one of the possible products, are of consider-
able importance. This report describes our findings in this 1 aq. TBHP (4) 4 Å MS 12 100 27
context. 2 aq. TBHP (4) 4 Å MS 12 120 68
3 aq. TBHP (4) 4 Å MS 5 120 41
4 aq. TBHP (4) 4 Å MS 7 120 51
5 aq. TBHP (4) – 12 120 60
6 TBHP (4) 4 Å MS 12 120 50
7 aq. TBHP (4) 4 Å MS/CuI 12 120 20
8 aq. TBHP (4) 4 Å MS/CuBr 12 120 27
9 aq. TBHP (4) 4 Å MS/Cu(OAc)2 12 120 34
Scheme 1. Metal-free coupling of formamides and diselenides. 10 aq. TBHP (4) Cu(OAc)2 12 120 38
11[c] aq. TBHP (4) 4 Å MS 12 120 37
12[d] aq. TBHP (4) 4 Å MS 12 120 8
13 aq. TBHP (2) 4 Å MS 12 120 48
14 aq. TBHP (6) 4 Å MS 12 120 44
Results and Discussion 15 – 4 Å MS 12 120 n.r[e]
16 DDQ (4) 4 Å MS 12 120 n.r
Our initial studies focused on the coupling of N,N-di- 17 DTBP (4) 4 Å MS 12 120 51
methylformamide (DMF, 1a) and diphenyl diselenide (2a). 18[f] aq. TBHP (4) 4 Å MS 12 120 47
19 aq. TBHP (4) 4 Å MS 15 120 67
The reaction between DMF (80 equiv., also used as a sol-
vent) and 2a (1 equiv.) was performed in the presence of [a] Reaction conditions: DMF (80 equiv.), 2a (1 equiv.), 4 Å molec-
4 Å molecular sieves at 100 °C for 12 h under nitrogen by ular sieves (MS) (0.15 g). [b] Isolated yield. [c] DMF (50 equiv.). [d]
DMF (20 equiv.). [e] No reaction. [f] In air.
using 4 equiv. of aq. TBHP (70 %) as oxidant. After workup
and purification by column chromatography, the product
The scope of this metal-free oxidative coupling reaction
3a was obtained in 27 % yield (Table 1, entry 1) along with
was examined by using various formamides and diselenide
unreacted 2a (45 %). No other new product was detected by
substrates under the optimized conditions. All the form-
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 Table 2. Oxidative coupling reactions of formamides and diselen-
ides.[a]
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 Entry 1 2 Product Yield
gave a good yield of the product, but a lower amount of [%][b]
oxidant reduced the yield (Table 1, entry 13). The use of 6 m 1 1a (R = Me) 2a (Ar = C6H5) 3a 68
TBHP in decane also gave a moderate yield (Table 1, en- 2 1b (R = Et) 2a 3b 80
try 6). In the presence of a catalyst such as CuI, CuBr or 3 1c (R = Bu) 2a 3c 60
Cu(OAc)2, the product 3a was obtained in a low yield 4 1d (R = iPr) 2a 3d 70
5 1a 2b (Ar = 4-CH3C6H4) 3e 68
(Table 1, entries 7–10). No reaction was observed in the ab- 6 1b 2b 3f 64
sence of TBHP (Table 1, entry 15), and the use of 2,3- 7 1c 2b 3g 50
dichloro-5,6-dicyanobenzoquinone (DDQ), another oxi- 8 1d 2b 3h 62
dant, failed to afford the desired product (Table 1, en- 9 1a 2c (Ar = 4-CH3OC6H4) 3i 75
10 1b 2c 3j 67
try 16), but in the presence of di-tert-butyl peroxide
11 1c 2c 3k 62
(DTBP), 3a was obtained in 51 % yield (Table 1, entry 17). 12 1d 2c 3l 65
Reducing the amount of DMF in the reaction (Table 1, en- 13 1a 2d (Ar = 1-naphthyl) 3m 67
tries 11 and 12) or increasing the amount of aq. TBHP 14 1b 2d 3n 80
(Table 1, entry 14) also resulted in a lower yield of the prod- 15 1c 2d 3o 63
16 1a 2e (Ar = 2-CH3O-1-naphthyl) 3p 31
uct 3a. When the reaction was performed in air, the product 17 1b 2e 3q 47
yield decreased to 47 % (Table 1, entry 18). Thus, the best 18 1a 2f (Ar = CH2C6H5) 3r 74
results were obtained by using aq. TBHP (4 equiv.) and 4 Å 19 1b 2f 3s 67
molecular sieves at 120 °C for 12 h (Table 1, entry 2) under [a] Reaction conditions: 1 (80 equiv.), 2 (1 equiv.), 4 Å MS (0.1 g/
nitrogen. 0.2 mmol), aq. TBHP (4 equiv.). [b] Isolated yield.

Eur. J. Org. Chem. 2013, 7688–7692 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org 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/ Scheme 2. Tentative mechanism for the coupling reaction between
0.2 mmol), aq. TBHP (4 equiv.). [b] Isolated yield. [c] No reaction. formamides and diselenides.

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A Convenient Synthesis of Selenocarbamates

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Eur. J. Org. Chem. 2013, 7688–7692 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org 7691
 P. Singh, A. Batra, P. Singh, A. Kaur, K. N. Singh
SHORT COMMUNICATION
[16] a) G. S. Kumar, C. U. Maheswari, R. A. Kumar, M. L. Kan- [19] a) K. N. Singh, P. Singh, P. Singh, Y. Maheshwary, S. V. Kessar,
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Kumar, R. A. Kumar, N. V. Reddy, K. R. Reddy, Eur. J. Org. [20] The reaction of N,N-dimethylformamide (1a) with PhSeH un-
Chem. 2013, 1218–1222. der the optimized conditions given in Table 1, entry 2 afforded
[17] a) Z. Lao, W. Zhong, Q. Lou, Z. Li, X. Meng, Org. Biomol. the product 3a in 36 % yield, which is roughly half that ob-
Chem. 2012, 10, 7869–7871; b) D. Li, M. Wang, J. Liu, Q. served with PhSeSePh (68 %). This experiment lends support
Zhao, L. Wang, Chem. Commun. 2013, 49, 3640–3642. to the proposed mechanism in Scheme 2 in which intermediate
[18] a) R. Tang, Y. Xie, Y. Xie, J. Xiang, J. Li, Chem. Commun. formation of PhSeH is postulated. We are grateful to a referee
2011, 47, 12867–12869; b) Y.-q. Yuan, S.-r. Guo, J.-n. Xiang, for suggesting this experiment.
Synlett 2013, 24, 443–448. Received: August 20, 2013
Published Online: October 29, 2013

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