10.1002/adsc.201801695
Advanced Synthesis & Catalysis
Scheme 4). Additionally, when dibenzyl disulfide 1a
was heated with NMP in DMSO, depending on the
presence of elemental sulfur, we obtained different
results. In the absence of sulfur, 1a was recovered
unchanged. On the other hand, the presence of sulfur
was found to be of vital importance in breaking S-S
bond of 1a, leading to hexabenzylidyne tetrasulfide 8
as described in our previous work.[10]
by a cyclizable group such as OH, NH2 or NHPh, the
corresponding benzoxazoles or benzimidazoles 7 were
obtained in excellent yields and only a very small
amount of elemental sulfur (1-5 mol %) was required
as a result of the autocatalytic nature of the reaction.
We hope that this preliminary result will pave the way
for the development of other sulfur-catalyzed
processes leading to useful products that are difficult
or impossible to obtain otherwise.
These results showed clearly that a competent
thioacylation agent should have an S-S bond at the
benzylic position.
Based on this information on the reaction involving
elemental sulfur and previous literature report, we
proposed a mechanism starting with activation of
elemental sulfur with aliphatic amines to provide
zwitterion A (Scheme 5).
Reaction of this sulfur species, which is at the same
time nucleophilic and oxidizing, with dibenzyl
disulfide 1a, would result in an oxidative
fragmentation of 1a to yield two molecules of
thiobenzaldehyde C (eq. 3). Addition of amine 2 to
thiobenzaldehyde C would lead to -aminated
benzenethiol E, which was next oxidized to thioamide
3 by either DMSO or elemental sulfur. The recycling
of elemental sulfur could be achieved by the oxidation
of dihydrogen sulfide or polysulfide D according to eq.
4.
Experimental Section
General procedure
A mixture of dibenzyl disulfide 1 (0.5 mmol), aliphatic
amine 2 (1 mmol), elemental sulfur (0.25 mmol, 8 mg) and
DMSO (3 mmol, 0.2 mL) was heated under an argon
atmosphere in a 7-mL test tube at 80 °C for 16 h. In case of
thioamides 5 or aza-heterocycle 7, the reaction required N-
methylpiperidine (1 mmol) and heating at 100 °C for 16 h.
The reaction mixture was purified by column
chromatography on silica gel (heptane:EtOAc or
CH2Cl2:EtOAc).
For characterization, and copies of NMR spectra of all
compounds, see Supporting Information.
Acknowledgements
We thank ICSN-CNRS for financial support and Dr. A. Marinetti
(ICSN-CNRS) for her helpful support.
References
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Scheme 5. Proposed mechanism
[5] For selected examples of the syntheses of thioamides
from elemental sulfur, see: a) T. B. Nguyen, M. Q. Tran,
P. Retailleau, A. Almourabit, Org. Lett. 2014, 16, 310;
b) T. B. Nguyen, L. Ermolenko, A. Almourabit, Org.
Lett. 2012, 14, 4274; c) K. Xu, Z. Li, F. Cheng, Z. Zuo,
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In conclusion, we have reported in this manuscript
an interesting catalytic activity of elemental sulfur in
promoting oxidative condensation of dibenzyl
disulfides 1 with a wide range of nitrogen
nucleophiles, including aliphatic amines 2 and anilines
4 to provide the corresponding thiobenzamides 3 and
5. Reductive precursor of dibenzyl disulfides 1, i.e.
phenylmethanethiols 8 could also be used for this kind
of tramsformation. In case of anilines 6 o-substituted
[6] K. Ajzert, K.Takacs, Liebigs Ann. Chem. 1987, 1061.
4
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