Green Regio- and Enantioselective Aminolysis Catalyzed by Graphite and Graphene Oxide under Solvent-Free Conditions

Article abstract of DOI:10.1002/cctc.201600241

The ring-opening reactions of epoxides with amines were efficiently and regioselectively catalyzed by high-surface-area graphite and graphene oxide under metal-free and solvent-free conditions. For epoxides without aryl groups, catalytic activity was observed only for graphene oxide, and hence, the activity must have been due to its acidic groups. For styrene oxide, instead, graphite and graphene oxide exhibited rather similar catalytic activities, and hence, the activity was mainly due to activation of the electrophilic epoxide by π-stacking interactions with the graphitic π system. The described aminolysis procedure is green and cheap because the catalyst can be recovered and recycled without loss of efficiency. Moreover, these heterogeneous catalysts exert high stereoselective control in the presence of nonracemic epoxides and provide chiral β-amino alcohols with enantiomeric excess values up to 99 %.

Full text of DOI:10.1002/cctc.201600241

DOI: 10.1002/cctc.201600241
Communications
Green Regio- and Enantioselective Aminolysis Catalyzed
by Graphite and Graphene Oxide under Solvent-Free
Conditions
Maria Rosaria Acocella,* Luciana D’Urso, Mario Maggio, and Gaetano Guerra*[a]
The ring-opening reactions of epoxides with amines were effi-
ciently and regioselectively catalyzed by high-surface-area
graphite and graphene oxide under metal-free and solvent-
free conditions. For epoxides without aryl groups, catalytic ac-
tivity was observed only for graphene oxide, and hence, the
activity must have been due to its acidic groups. For styrene
oxide, instead, graphite and graphene oxide exhibited rather
similar catalytic activities, and hence, the activity was mainly
due to activation of the electrophilic epoxide by p-stacking in-
teractions with the graphitic p system. The described aminoly-
sis procedure is green and cheap because the catalyst can be
recovered and recycled without loss of efficiency. Moreover,
these heterogeneous catalysts exert high stereoselective con-
trol in the presence of nonracemic epoxides and provide chiral
b-amino alcohols with enantiomeric excess values up to 99%.
The use of heterogeneous catalyst as solids (e.g., silica gel,[29]
nanosilica,[30] functionalized mesoporous silica,[31–33] alumina
and/or modified alumina,[34–37] nanoalumino silicates,[38–42]
montmorillonite-K10 clay,[43] sulfated zirconia,[44,45] annotitani-
um dioxides,[46] heteropoly acids,[47] polyoxometalate inorganic
metal oxygen clusters,[48] Amberlyst-15,[49] nanocrystalline zirco-
silicate,[50] and iron oxides[51]) has tried to meet the need for
more sustainable protocols that assure good regioselective
control, often lost to competitive polymerization/isomerization
of the epoxides.
However, a metal-free, highly regioselective procedure is still
missing. One of the emerging promises in sustainable chemis-
try is carbon-based materials, which are already used in some
important synthetic reactions as efficient carbocatalysts.
In particular, the use of graphite oxide and exfoliated graph-
ite oxide as cheap and metal-free catalysts in oxidation reac-
tions,[52–55] Friedel–Crafts reactions,[56] aza-Michael additions,[57]
Mukaiyama–Michael additions,[58,59] polymerizations,[60] cross-
linking reactions,[61] and epoxide ring-opening reactions,[62,63] in
addition to some one-pot reactions,[64–66] have been successful-
ly recorded. Recently, we reported the first example of the
enantioselective Friedel–Crafts reaction catalyzed by graphene
oxide under solvent-free conditions; the reaction proceeded to
give the products in good yields with high enantioselectivities,
without side products, and with high regioselectivities.[63] The
possibility to control the stereochemical outcome of the reac-
tion in the presence of a carbon-based heterogeneous catalyst
represents an important goal for green synthetic approaches
that aim to prepare chiral molecules.
Epoxide ring-opening reactions are useful methods to provide
multifunctional compounds ready to be used as versatile inter-
mediates in total synthesis or as precursors of relevant mole-
cules. As three-membered heterocyclic rings, epoxides are
more reactive than ethers owing to ring strain and are suscep-
tible to attack by a range of nucleophiles, including nitrogen
(e.g., ammonia, amines, and azides), oxygen (e.g., water, alco-
hols, phenols, and acids), and sulfur (e.g., thiols) containing
compounds, and this leads to bifunctional molecules of great
industrial value.
In particular, epoxy ring-opening reactions with amines are
well documented,[1–49] and with the choice of specific physical
parameters (e.g., heating,[1,2] microwave,[3–5] and ultrasound[5]),
the use of polar reaction media (e.g., ionic liquids,[6] fluoro al-
cohols,[7] or water under different pH conditions[8,9]), or the use
of catalysts or activators as homogeneous catalysts (e.g.,
Brønsted acids and bases[6,9–14] and several metal salts and/or
complexes[15–28]), good results have been achieved, although
most suffer from poor regioselectivity, high temperature, and/
or stoichiometric amounts of the catalyst and the use of
excess amounts of reagents.
This aspect is particularly interesting considering the wide
presence of chiral b-amino alcohol units in numerous natural
products and bioactive molecules and the important role that
they play in organic chemistry and related research fields.[67]
Chiral b-amino alcohols have been widely used as chiral li-
gands,[68] organocatalysts,[69] and versatile intermediates in the
synthesis of various medicines[70] and unnatural amino acids.[71]
Consequently, numerous methods for the preparation of opti-
cally active b-amino alcohols have been reported,[72] including
the asymmetric reduction of amino ketones,[73] the ring open-
ing of enantioenriched epoxides,[74] and the Sharpless asym-
metric aminohydroxylation of alkenes.[75] To the best of our
knowledge, aminolysis catalyzed by carbon-based materials
has not been previously reported.
[a] Dr. M. R. Acocella, L. D’Urso, M. Maggio, Prof. G. Guerra
Department of Chemistry and Biology
University of Salerno
Via Giovanni Paolo II, 132 Fisciano, SA (Italy)
E-mail: macocella@unisa.it
All of the reported procedures for the opening of epoxide
rings are based on basic or acidic functionalities (as Lewis or
Brønsted acids) that are able to activate the epoxy ring and to
facilitate the ring-opening reaction. Herein, we show the first
gguerra@unisa.it
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600241.
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examples of the opening of epoxy rings by amines catalyzed
by graphite as well as graphene oxide under solvent-free con-
ditions with high regioselective and enantioselective control.
The X-ray diffraction patterns of the graphite-based catalysts
used in this study are reported in Figure 1.
Scheme 1. Ring-opening reaction of styrene oxide (1) with benzylamine (2)
under solvent-free conditions.
Table 1. Catalyst screening for the ring opening of styrene oxide (1) with
benzylamine (2) (see Scheme 1).
Entry
Catalyst (wt%)[a]
t [h]
Yield [%][b]
Ratio (3/4)[c]
1
2
3
4
5
6
–
24
24
48
72
24
48
trace
55
97
95
54
–
GO (3)
GO (3)
GO (1)
G (3)
G (3)
96:4
80:20
72:28
94:6
75:25
94
[a] The wt% was calculated with respect to the entire reaction mixture.
[b] All yields refer to the chromatographically isolated products. [c] The
ratio 3/4 was evaluated by analysis of the crude material by 1H NMR spec-
troscopy.
Figure 1. X-ray diffraction pattern (CuKa) of carbon-based nanomaterials:
a) High-surface-area graphite (G); b) graphite oxide; c) graphene oxide ob-
tained by ball milling.
performed. The reaction did not proceed at all and gave only
a trace amount of the product (Table 1, entry 1), which thus
showed the need for activation for the ring-opening reaction.
Data relative to the catalytic activity of GO and G are collected
in Table 1 (entries 2–6).
The used high-surface-area graphite (G, with a negligible
oxygen content, Figure 1a) shows an interlayer distance of
d=0.339 nm and a high shape anisotropy (D /D =3.1, where
Surprisingly, irrespective of the absence of acidic groups, the
catalytic activity of the graphite sample for the aminolysis reac-
tion was only slightly lower than that of GO, that is, a carbon
material characterized by OH and COOH functional groups
(Table 1, entries 2 and 5).
k
?
D and D are the correlation lengths parallel and perpendicu-
k
?
lar to the graphitic plane, respectively).[76] The X-ray diffraction
pattern of the derived graphite oxide by Hummers oxidation
(with an oxygen content of 32 wt%, excluding water) shows
an increase in the interlayer distance from d=0.339 to 0.84 nm
(Figure 1b). The correlation length perpendicular to the layers
(as evaluated for the first 00L reflection) decreases from 9.8 to
4.2 nm, whereas the in-plane correlation length (as evaluated
for the 100 reflection) remains almost unchanged
For both catalysts, by prolonging the reaction time to 48 h,
very good efficiency was obtained, although with a reduction
in regioselectivity, possibly as a result of a competitive auto-
catalytic pathway for the aminolysis reaction (Table 1, entries 3
and 6). In fact, it is possible that by extending the reaction
time, the formed amino alcohol could activate the epoxide
ring-opening reaction through hydrogen bonding. This com-
petitive activation could promote the formation of a carbocat-
ion intermediate and significantly reduce the regioselective
control (for further details, see the Supporting Information).
By reducing the amount of catalyst to 1 wt% (Table 1,
entry 4), a high yield was also reached, although with a longer
reaction time and with rather low regioselectivity.
(D ꢀ30 nm), which thus leads to an increase in the shape ani-
k
sotropy up to D /D =7. Ball-milling treatment of this graphite
k
?
oxide sample leaves the oxygen content essentially unaltered
(31.4 wt%, excluding water), but it largely changes the X-ray
diffraction pattern, and complete disappearance of the 001 re-
flection is replaced by a broad halo centered at d=0.37 nm
(Figure 1c). This indicates complete loss of crystalline order
perpendicular to the graphite oxide layers, that is, the forma-
tion of an uncorrelated graphene oxide layer. As a conse-
quence, the powder with the pattern shown in Figure 1c will
hereafter be named graphene oxide (GO). This GO sample ex-
hibits acidic functionalities, which leads to pH 2.75 for an
Because all reported procedures to activate the epoxide
ring-opening reaction involve basic or acidic functionalities
(Lewis and Brønsted acids), the similar catalytic activities of the
graphite and graphene oxide powders were certainly unex-
pected. This result can possibly be rationalized by p-stacking
interactions between the styrene oxide and the graphitic sur-
face, as already suggested, also on the basis of DFT calcula-
tions, for the Mukaiyama–Michael[58] and Friedel–Crafts reac-
tions.[63] These p-stacking interactions would be able to acti-
vate the ring-opening reaction and favor nucleophilic attack of
nitrogen to generate the desired product.
aqueous suspension of GO at a concentration of 5 mgmLÀ1
.
The powders characterized in Figure 1 were used as possible
catalysts for the ring-opening reaction of styrene oxide (1)
with benzylamine (2) (Scheme 1) as a representative example
(see Table 1).
To verify a possible background reaction, an experiment in
the absence of a catalyst, under solvent-free conditions, was
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To assess the scope of the procedure, a variety of alkyla-
mines and arylamines were treated with styrene oxide (1). Al-
though GO and G presented similar catalytic activities, for the
following study we mainly used GO, which for the aminolysis
reaction depicted in Table 1 led to slightly higher yields and re-
gioselectivities (Table 1, entries 3 and 6).
Notably, and as already reported,[8,77] even though morpho-
line is a secondary amine, better regioselectivity can be ob-
tained with graphene oxide, which gives 99% selectivity in
favor of product 3c.
The results obtained with secondary amines are particularly
interesting in that the regioselectivities are higher than those
obtained for the corresponding reactions performed in the
presence of other solid acid catalysts.[29,30]
As reported in Table 2, the reaction proceeded to give the
products in very good yields with excellent regioselectivities
for most amines. Notably, in most cases (Table 2, entries 2 and
3) the regioselectivity was >90, whereas only in the presence
of an aromatic residue on the alkylamine (Table 2, entry 1) did
it go down to 80:20 in favor of the SN2-type product. This
result can be rationalized by hypothesizing that a p-stacking
interaction between benzylamine and the graphitic surface
could result in a decrease in the rate reaction and favor the
less regioselective competitive autocatalytic pathway.
Moving to arylamines, an inversion of regioselectivity was
observed (Table 2, entries 5–7). This result was not surprising
because of the reduced nucleophilicity of aniline relative to
that of alkylamines. In this instance, the electrophile has to be
sufficiently activated throughout partial carbocation formation,
which is not involved in isomerization or polymerization of sty-
rene oxide but is ready to react with arylamines in the reaction
mixture.
Interesting results were also obtained with secondary
amines. In this case, it is believed that steric hindrance of the
amines slows down nucleophilic attack and favors the compet-
itive regioselective pathway to afford 3/4 in product ratios of
90:10 and 85:15 for pyrrolidine and piperidine, respectively
(Table 1, entries 2 and 4).
Other electrophiles were finally tested, and the SN2-type
products were delivered in good yields with good regioselec-
tivities (Table 2, entries 8 and 9). Notably, whereas the yield
was quantitative for bisphenol A diglycidyl ether (Table 2,
entry 8), in the presence of a nonaromatic epoxide such as epi-
chlorohydrin, the reaction proceeded with less efficiency. In
Table 2. Ring opening of epoxides with different amines.
Entry
1
Epoxide
RNH2
t [h]
Yield [%][a]
97
Ratio (3/4)[b]
2a
2b
2c
2d
2e
48
80:20
2
3
4
5
16
16
16
16
91
82
94
95
90:10 (3)
>99 (3)
85:15 (3)
4:96
6
7
2 f
16
16
96
97
5:95 (4)
>99 (4)
2g
8
9
2h
2i
24
2
98
85
>99 (3)
>99 (3)
1
[a] All yields refer to the chromatographically isolated products. [b] The ratio 3/4 ratio was evaluated by analysis of the crude material by H NMR spectros-
copy. Compound numbers in parentheses refer to the major regioisomer formed.
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that case, no p-stacking interaction can be involved, and only
the OH and COOH groups of graphene oxide can activate the
ring-opening reaction. For the sake of comparison, it is worth
adding that the same experiment performed in the presence
of G proceeded with the same efficiency as that of the uncata-
lyzed reaction (45% yield after 2 h), which shows the inability
of graphite to activate epoxide ring opening with an alkyl
substrate.
Table 4. Enantioselective aminolysis with different amines (see
Scheme 2).
Entry
1
RNH2
t [h] Yield [%][a] ee [%][b] Ratio (3/4)[c]
2a
48
97
91
91
90
80:20
2
3
2b 16
90:10 (3)
As a heterogeneous catalyst, the reusability of GO was inves-
tigated in the addition of aniline (2e) to styrene oxide (1) as
a model reaction. The solid GO, recovered after extraction from
the aqueous solution and dried at 608C overnight, was used
without any further treatment. The reaction conditions (RT and
16 h) were kept the same in all cycles. As shown in Table 3,
both the yield of the reaction and the regioselectivity re-
mained almost unchanged after five recycling steps.
2c
16
82
94
95
>99
80
>99 (3)
85:15 (3)
4:96 (4)
4
5
2d 16
2e
2 f
16
16
>99
6
7
97
95
95
5:95 (4)
>99 (4)
Table 3. Recycling test with styrene oxide (1) and aniline (2e).
2g 16
>99
[a] All the yields refer to the chromatographically isolated products.
[b] The ee was evaluated by HPLC analysis and refers to the major regio-
isomer. The absolute configuration was determined by comparison with
those reported in the literature. [c] The ratio 3/4 was evaluated by analy-
sis of the crude material by H NMR spectroscopy. Compound numbers in
parentheses refer to the major regioisomer formed.
Run
Yield [%][a]
Regioselectivity (3/4)[b]
1
2
3
4
5
95
95
96
94
95
4:96
4:96
5:95
5:95
4:96
1
[a] All the yields refer to the chromatographically isolated products.
[b] The ratio 3/4 was evaluated by analysis of the crude material by
1H NMR spectroscopy.
a typical product of an SN1-type reaction was provided, the in-
version of configuration that was detected is derived from an
SN2-type reaction. It is reasonable to assume that acidic activa-
tion and p-stacking interactions of the graphene oxide surface
lead to an incipient carbocation that quickly reacts with the ar-
ylamine without involvement in the competitive racemization
pathway.
Excellent results were obtained by performing the stereose-
lective version of the optimized protocol (Scheme 2). In partic-
ular, we performed the ring-opening reaction of (S)-styrene
oxide with the amines examined under the best reaction con-
ditions.
The slight decrease in the enantioselectivity was observed
for 4-chloroaniline, and this can be attributed to a slight reduc-
tion in nucleophilicity owing to the presence of the chlorine
substituent, whereas an excellent result was obtained for 4-
methylaniline, for which, on the contrary, very high enantiose-
lectivity was achieved.
Although GO proved to be the best catalyst for the symmet-
ric and asymmetric ring-opening reaction, very impressive re-
sults were obtained in the presence of G with (S)-styrene
oxide. In particular, aniline and piperidine, an arylamine and al-
kylamine, respectively, were considered for the enantioselec-
tive reaction with (S)-styrene oxide.
Scheme 2. Enantioselective aminolysis with different amines.
As reported in Table 4, high enantioselectivities were ob-
tained with all the amines. The absolute configuration was de-
termined by comparison with reported HPLC data.[51]
As reported in Scheme 3, excellent results were obtained for
both nucleophiles. As for GO, with piperidine as the alkyla-
mine, the reaction proceeded and gave the SN2-type product
with good regioselectivity (85:15) and good enantioselectivity
for the major product (80%ee). In the presence of an aryla-
mine such as aniline, major product 4e was obtained with ex-
cellent enantioselectivity.
Notably, as for the regioselective control, reduced enantiose-
lectivities for benzylamine, pyrrolidine, and piperidine (Table 4,
entries 2 and 4) were detected, possibly due to the competi-
tive autocatalytic pathway that is responsible for racemization
of (S)-styrene oxide during the course of the reaction.
Rather satisfactory results were obtained with arylamines, by
which high enantioselectivities were achieved. Even through
Notably, although in the absence of acidic functional
groups, the reaction proceeded with excellent enantioselectivi-
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Sigma–Aldrich and were used without further purification. TLC was
performed on silica gel 60 F254 0.25 mm glass plates (Merck) and
non-flash chromatography was performed on silica gel (0.063–
0.200 mm) (Merck).
Synthetic procedures
Preparation of graphite oxide
Graphite oxide samples were prepared by Hummers’ method from
graphite samples. Sulfuric acid (120 mL) and sodium nitrate (2.5 g)
were introduced into a 2000 mL, three-necked, round-bottomed
flask immersed in an ice bath; then, graphite (5 g) was added
under an atmosphere of nitrogen with magnetic stirring. After ob-
taining a uniform dispersion of the graphite powder, potassium
permanganate (15 g) was added very slowly to minimize the risk of
explosion. The mixture was then heated to 358C and stirred for
24 h. The resulting dark green slurry was first poured into deion-
ized water (6 L) and then centrifuged at 10000 rpm for 15 min
with a Hermle Z 323K centrifuge. The isolated GO powder was ex-
tensively washed with 5 wt% HCl aqueous solution (100 mL) and
subsequently with deionized water. Finally, it was dried at 608C for
12 h. The residual sulfur content was less than 0.2%.
Scheme 3. Enantioselective ring opening reaction catalyzed by G.
ties for both the alkylamine and the arylamine. This behavior
can be explained by assuming that styrene oxide can be suffi-
ciently activated by p-stacking interactions with the graphitic
surface and becomes susceptible to nucleophilic attack, re-
gardless of whether it is an aliphatic or aromatic amine, where-
as the nature of the nucleophile determines the type of prod-
uct, SN1 or SN2, coming from the reaction.
In conclusion, the opening of epoxy rings by amines was ef-
ficiently catalyzed under solvent-free and metal-free conditions
with high regioselectivity, not only by graphene oxide but also
by high-surface-area graphite. The observed catalytic activity is
not surprising for graphene oxide, which has many acidic
groups. In fact, most catalysts described in the literature for
this kind of reaction are based on Lewis or Brønsted acids.
Rather surprising instead is the very similar catalytic activity
(both with aliphatic and aromatic amines) of high-surface-area
graphite, for which the content of atoms different from carbon
is lower than 0.1%
Exfoliation of graphite oxide by ball milling
Graphite oxide powders were introduced in 125 mL ceramic jars
(inner diameter of 75 mm) together with stainless steel balls
(10 mm in diameter) and were dry milled in a planetary ball mill
(FRISCHT) for 20 min with a milling speed of 200 rpm and a ball-to-
powder mass ratio of 10:1.
General procedure for the epoxy ring-opening reaction with
amines
These results indicate that for both graphite and graphene
oxide their extended p systems activate styrene oxide through
p-stacking interactions. The presence of large amounts of OH
and COOH groups on the graphene oxide surface only slightly
increases the yields and regioselectivities of the studied ring-
opening reactions. In the presence of a nonaromatic epoxide
such as epichlorohydrin, on the contrary, only graphene oxide
is catalytically activity, whereas graphite completely loses his
activity. Hence, for nonaromatic epoxides the catalytic activity
only comes from the acidic groups on the graphene oxide
layers.
Styrene oxide (1; 120.2 mg, 1.0 mmol) and benzylamine (131 mL,
128.6 mg, 1.2 mmol) were added to a vial containing the GO cata-
lyst (3 wt% with respect to the mixture) at room temperature. The
mixture was stirred at the same temperature for the time indicat-
ed. The mixture was extracted with EtOAc, and the combined or-
ganic phase was dried (MgSO4) and concentrated. The residue was
purified by column chromatography (silica gel, petroleum ether/
EtOAc, gradient) to obtain the pure product.
Acknowledgements
These heterogeneous catalysts can be recovered and recy-
cled without loss of efficiency or regioselectivity, and thus this
method is a cheap and ecofriendly procedure.
Ministero dell’ Istruzione, dell’ Università e della Ricerca and
(INSTM) are gratefully acknowledged.
Moreover, these graphite-based catalysts for ring-opening
reactions of nonracemic styrene oxide lead to high enantiose-
lectivities (up to 99%) with all studied amines.
Keywords: aminolysis · enantioselectivity · graphene oxide ·
graphite · green chemistry
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Published online on May 17, 2016
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