Iodine-catalyzed guanylation of amines with N, N ′-di-Boc-thiourea

Article abstract of DOI:10.1039/c9ob02014d

Herein, we report that iodine-catalyzed guanylation of primary amines can be accomplished with N,N′-di-Boc-thiourea and TBHP to afford the corresponding guanidines in 40-99% yields. Oxidation of the HI byproduct by TBHP eliminates the need for an extra base to prevent the protonation of substrates and makes the reaction especially useful for both electronically and sterically deactivated primary anilines.

Full text of DOI:10.1039/c9ob02014d

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Iodine-catalyzed guanylation of amines with
N,N’-di-Boc-thiourea†
Cite this: Org. Biomol. Chem., 2019,
17, 9280
Hao-Jie Rong,a,b Cui-Feng Yang,a Tao Chen,a Ze-Gang Xu,a Tian-Duo Su,a
Received 15th September 2019,
Accepted 14th October 2019
Yong-Qiang Wangb and Bin-Ke Ning
*
a,c
DOI: 10.1039/c9ob02014d
rsc.li/obc
Herein, we report that iodine-catalyzed guanylation of primary an extra base (usually NEt3) is needed to prevent the protona-
amines can be accomplished with N,N’-di-Boc-thiourea and TBHP tion of the amine. This strategy has been widely used for the
to afford the corresponding guanidines in 40–99% yields. late-stage functionalization of natural products and
Oxidation of the HI byproduct by TBHP eliminates the need for an pharmaceuticals.33–36 However, the catalytic guanylation of
extra base to prevent the protonation of substrates and makes the amines with N,N′-di-Boc-thiourea has not previously been
reaction especially useful for both electronically and sterically de- reported.
activated primary anilines.
Catalytic syntheses of guanidines with protected thiourea
compounds often produce metal sulfides,19,25,37–41 and there-
fore an excess of a desulfurization reagent is required.
Recently, we discovered that I2 could mediate the guanylation
of amines with N,N′-di-Boc-thiourea.42 Considering that the
HI43–50 produced as a byproduct could be reoxidized to I2 with
a suitable oxidant, we speculated that the I2-catalyzed guanyla-
tion of amines with N,N′-di-Boc-thiourea would be possible. In
addition, the oxidation of the HI would eliminate the need for
an extra base to prevent the protonation of the amine sub-
strates (Scheme 1).
To explore this possibility, we carried out reactions of
4-methoxyaniline as a model substrate for the I2-catalyzed gua-
nylation of amines with N,N′-di-Boc-thiourea (Table 1). After
carefully screening various catalysts, oxidants, and solvents, we
found that with I2 as the catalyst, TBHP as the oxidant, and
toluene as the solvent, the desired guanidine 3a could be
Guanidine moieties are widespread in natural products,
pharmaceuticals, sweeteners, and explosives.1–6 Examples of
guanidine-bearing compounds include peramine, a feeding
deterrent that has been isolated from several plants;1 zanami-
vir, a potent neuraminidase inhibitor;7,8 rosuvastatin, which is
used to treat high cholesterol and cardiovascular disease;9 and
nitroguanidine, a commonly used explosive propellant (Fig. 1).
In addition, guanidines are organic superbases that have been
shown to catalyze base-mediated reactions,10–12 and the incor-
poration of a chiral auxiliary into the nitrogen atom makes
guanidines useful for asymmetric reactions.13,14 Owing to the
versatility of guanidines, numerous methods have been devel-
oped for their synthesis,1,15,16 the most direct and effective
being the reaction of amines with guanylating agents.1,15–17
Guanylating agents can be thiourea, isothiourea, carbodi-
imide, and pyrazole-1-carboximidamide; among these, N,N′-di-
Boc-thiourea is one of the most commonly used agents
because it is highly reactive and because subsequent removal
of the Boc group is easy. A stoichiometric amount of a desul-
furization reagent is usually necessary to promote the direct
guanylation of amines with N,N′-di-Boc-thiourea because
otherwise the desulfurization reaction is slow.18 Reagents used
for this purpose include metal salts (e.g., Hg19–23 and Cu24–27),
Mukaiyama’s reagent,28 and iodine reagents.29–32 In addition,
aModern Chemistry Research Institute of Xi’an, Xi’an 710065, China
bDepartment of Chemistry & Materials Science, Northwest University, Xi’an 710069,
China
cState Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an 710065, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9ob02014d
Fig. 1 Representative compounds with guanidine moieties.
9280 | Org. Biomol. Chem., 2019, 17, 9280–9283
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Table 2 Iodine-catalyzed guanylation of aminesa
Scheme 1 Guanylation of amines using N,N’-di-Boc-thiourea.
Table 1 Optimization of guanylation conditionsa
Entry Catalyst
Oxidant (equiv.) Solvent Yieldb (%)
1
2
3
4
5
6
7
8
9
NIS (10 mmol%)
I2(10 mmol%)
Bu4NI (10 mmol%) TBHP (1.5)
I2(10 mmol%)
I2(10 mmol%)
I2 (10 mmol%)
I2(5 mmol%)
I2(20 mmol%)
I2 (10 mmol%)
TBHP (1.5)
TBHP (1.5)
PhMe
PhMe
PhMe
PhMe
EtOAc
EtOH
PhMe
PhMe
PhMe
64
82
47
58
75
68
79
83
70
a Conditions: To a solution of 1 (0.2 mmol) and 2 (0.24 mmol) in
PhMe (2 mL) at rt were added I2 (0.02 mmol) and 70% aq. TBHP
(0.3 mmol), in this order. b Isolated yield.
H2O2 (1.5)
TBHP (1.5)
TBHP (1.5)
TBHP (1.5)
TBHP (1.5)
TBHP (1.2)
thiourea led to a slow generation of I2 and the active inter-
mediate which make our strategy work well with these sub-
a Conditions: To a solution of 1a (0.2 mmol) and 2 (0.24 mmol) in
solvent (2 mL) at rt were added catalyst and oxidant, in this order. strates. Benzyl amines with different substituents (2j–2m) can
b Isolated yield.
afford the desired guanidines in 80–90% yields. When electro-
nically or sterically deactivated primary alkyl amines (2n–2t)
were used as the substrates, the desired guanidines can also
obtained in 82% yield (entry 2). With the optimal conditions
in hand, we carried out the reactions of primary anilines eroaromatic amines such as 3-aminopyridine and 4-aminopyri-
be obtained in 40–61% yields. However, when it comes to het-
(2a–2i), benzyl amines (2j–2m), and alkyl amines (2n–2t) to
explore the substrate scope of the reaction (Table 2). Most of
dine, none of the desired guanidines were detected. To eluci-
date the mechanism of the reaction, we conducted a series of
the tested substrates afforded the corresponding guanidines in control experiments (Scheme 2). Specifically, the reaction of 1a
moderate to excellent yields (40–99%). Notably, the method
worked well for electronically or sterically deactivated primary
anilines, which often give only moderate yields of the corres-
ponding guanidines when previously reported methods are
used.25,28,29,51–53 Under our conditions, such substrates gave
the desired products (3e–3i) in 95–99% yields. We propose the
following explanation for this difference in reaction outcome.
When the guanylation reactions of electronically or sterically
deactivated primary anilines with N,N′-di-Boc-thiourea are
carried out in the presence of an excess of a desulfurization
reagent, the active intermediate is rapidly generated; but the
addition of this intermediate to the carbodiimide is slow, and
the byproducts are therefore produced. In contrast, in our I2-
catalyzed guanylation reaction, the desulfurization reagent (I2)
was regenerated by the oxidation of the byproduct HI, and the
formation of the active intermediate therefore depended on
the nucleophilic addition of the amine to N,N′-di-Boc-thiourea.
When electronically or sterically deactivated primary anilines
were used, the slow addition of these anilines to N,N′-di-Boc- Scheme 2 Control experiments.
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Scheme 3 Proposed mechanism.
and 2 in toluene in the absence of I2 and TBHP generated only
a trace amount of the guanidine product after 2 h (Scheme 2a).
When TBHP, but not I2, was present as a desulfurization
reagent, the desired guanidine product was isolated in 35%
yield (Scheme 2b). The product was possibly generated by the
substitution of an amine to aminoiminomethanesulfonic and
-sulfinic acids which were formed by the oxidation of the
thiourea.54 These experiments demonstrated that both I2 and
TBHP were essential. Radical trapping experiments using
TEMPO as a scavenger show that none of the trapping
products were detected which indicate that the ionic pathway
is more feasible (Scheme 2c).
On the basis of our control experiments and iodine-
mediated desulfurization process of thiourea,32,53,55–59 we
propose the reaction pathway outlined in Scheme 3.
The reaction begins with the I2-mediated desulfurization of
N,N′-di-Boc-thiourea to give S and active carbodiimide, and the
resulting activated carbodiimide is attacked in situ by the free
amine substrate to give the corresponding guanidine. The HI
generated in the desulfurization step is oxidized by TBHP to
regenerate I2.
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Conclusions
In conclusion, we have developed a protocol for the I2-cata-
lyzed guanylation of primary amines with N,N′-di-Boc-thiourea
in the presence of TBHP. Oxidation of the HI byproduct by
TBHP eliminates the need for an extra base to prevent protona-
tion of the free amine. Because I2 and the active intermediate
are generated depending on the nucleophilic addition of the
amine to N,N′-di-Boc-thiourea, this strategy works well for elec-
tronically or sterically deactivated primary anilines.
Conflicts of interest
There are no conflicts to declare.
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