An efficient synthesis of organic carbonates using nanocrystalline magnesium oxide

Article abstract of DOI:10.1002/adsc.200600525

An efficient and selective synthesis of organic carbonates using nanocrystalline magnesium oxide has been realized by the direct condensation of alcohols and diethyl carbonate. The catalyst is quantitatively recovered by simple centrifugation and can be reused for four cycles with almost consistent activity.

Full text of DOI:10.1002/adsc.200600525

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DOI: 10.1002/adsc.200600525
An Efficient Synthesis of Organic Carbonates using Nanocrystal-
line Magnesium Oxide
a,
a
a
b
M. Lakshmi Kantam, * Ujjwal Pal, B. Sreedhar, and B. M. Choudary
a
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Phone: (+91)-40-2719-3510; fax: (+91)-40-2716-0921; e-mail: mlakshmi@iict.res.in
Ogene Systems (I) Pvt. Ltd., 11-6-56, GSR Estates, Moosapet, Hyderabad, India
b
Received: October 12, 2006; Revised: April 11, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: An efficient and selective synthesis of or-
ganic carbonates using nanocrystalline magnesium
oxide has been realized by the direct condensation
of alcohols and diethyl carbonate. The catalyst is
quantitatively recovered by simple centrifugation
and can be reused for four cycles with almost con-
sistent activity.
We herein report an efficient method for the syn-
thesis of unsymmetrical organic carbonates in quanti-
tative yields via direct condensation of various alco-
hols with diethyl carbonate in the presence of nano-
[
12a,b]
crystalline magnesium oxide (NAP-MgO).
Initially different nano metal oxides were screened
for the synthesis of organic ca rbonates using 1-phenyl-
ethanol and DEC as a model reaction (Scheme 1).
Among these metal oxides, NAP-MgO provided opti-
mum results (Table 1).
Keywords: chiral auxiliaries; direct condensation;
nanocrystalline magnesium oxide; selectivity; sol-
vent-free conditions; unsymmetrical organic carbo-
nates
[
13]
Later, various magnesium oxide crystals
[com-
2
À1
mercial MgO, CM-MgO (SSA: 30 m g ); conven-
tionally prepared MgO, NA-MgO (SSA: 250 m g ),
and aerogel-prepared MgO, NAP-MgO (SSA:
2
À1
2
À1
5
90 m g )] were screened. NAP-MgO shows higher
activity in terms of yields and selectivity compared to
NA-MgO and CM-MgO (Figure 1). To understand
the relationship between structure and reactivity in
Organic ac rbonates are important intermediates for
[
1,2]
[3]
the synthesis of fine chemicals,
pharmaceuticals,
[
4]
plasticizers, synthetic lubricants, monomers for or-
[
5,6]
ganicglasses, and solvents.
They are also used in
the field of solid phase chemistry as linkers and tag-
[7]
ging moieties. Most of the catalytic systems for the
synthesis of carbonates require the toxic and hazard-
[
1,2,8]
ous phosgene as reagent.
problem, several groups have used dimethyl carbon-
In order to address this
Scheme 1. Synthesis of organic carbonates using nanocrystal-
line magnesium oxide.
[
9]
ate (DMC) as a substituent for phosgene. The prep-
aration of organic carbonates has been recently re-
viewed by Shaik and Sivaram. Jung et al. reported
a mild and efficient preparation of alkyl carbonates
[a]
Table 1. Comparative study of different nano metal oxides.
[
9d]
[
9e–g]
on solid supports.
Heterogeneous basic ac talysts,
MCM-41-TBD or Mg-La metal oxide afforded un-
symmetrical alkyl carbonates from the reaction of di-
ethyl carbonate (DEC) with various alcohols/amines
[10]
at 1258C.
Some of reactions are time consuming
and the synthesized organics grafted on silica are not
easily regenerated. Nanocrystalline metal oxides find
excellent applications as active adsorbents for gases
and destruction of hazardous chemicals and as cata-
[a]
Conditions: 1-phenylethanol (2 mmol) and DEC
33 mmol, 4 mL) molar ratio (1: excess), catalyst
(0.125 g) at ca. 1258C.
(
[
11,12]
lysts for various organictransformations.
[b]
Determined by GC.
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M. Lakshmi Kantam et al.
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Table 2. Direct condensation of 1-phenylethanol and DEC
[a]
by different crystallites of MgO at ca. 1258C.
[
a]
Conditions: 1-phenylethanol (2 mmol), DEC (33 mmol,
mL) and catalyst (0.125 g).
Determined by GC.
The by-products were olefin, corresponding ether, dieth-
er and unidentified polymericprodu ct s.
4
[
[
b]
c]
Figure 1. Direct condensation of 1-phenylethanol and DEC
using NAP-MgO at ca. 1258C, determined by GC analysis. sirable by-products along with carbonate product and
Conditions: 1-phenylethanol (2 mmol), DEC (33 mmol, CM-MgO gives very low conversion. The results were
4
mL) and catalyst (0.125 g).
checked by gas chromatography with a Shimadzu
GC-2010 gas chromatograph equipped with a ZB5 ca-
pillary column (Figure 1). The activity of NAP-MgO
the synthesis of organic ac rbonates, it is better to
know the structure and nature of the reactive sites of in the direct condensation of 1-phenylethanol with
[10a]
NAP-MgO. The CM-MgO samples are generally large DEC was compared with those of MCM-41-TBD,
[10c]
[
10b]
cubic crystals, the NA-MgO samples are thin hexago- supported fluorides,
nal platelets, about 150 nm long and 10 nm thick and the results are presented in Table 3.
having large exposed areas of the (100) crystal face. The reaction works efficiently with a variety of al-
and Mg/La mixed oxides
NAP-MgO has a three-dimensional polyhedral struc- cohols including aromatic, cyclic, heterocyclic or ali-
ture, having high surface concentrations of edge/ phatic co mpounds (Table 4). PhenolicOH and amine
corner sites and various exposed crystal planes (such groups remain inert under similar reaction conditions.
as 002, 001, and 111), which leads to inherently high The organic carbonates obtained as cholesteric liquid
surface reactivity per unit area. Thus, NAP-MgO crystals from cholesterol (Table 4, entry 6) find appli-
indeed displayed the highest activity and selectivity cation in electro-optics.
compared to those of NA-MgO and CM-MgO. Be-
To widen the scope, NAP-MgO was tested in the
2+
sides this, NAP-MgO has a Lewis acid site Mg , tandem synthesis of the oxazolidine moiety
1
2À
À
16
Lewis basicsites O and O , lattice bound and isolat- (Scheme 2) at ca. 1258C.
ed Brønsted hydroxy groups, and anionicand ca tionic We have recently reported asymmetric epoxidation
[14]
[12a,b]
vacancies. The direct condensation of various alco- and Henry reactions using NAP-MgO.
hols with DEC is known to be driven by base cata- highly promising strategy for the syntheis of various
Herein a
2À
lysts, and accordingly, the surface OH and O func-
tions of these oxide crystals are expected to trigger
Table 3. Comparing the activity of NAP-MgO with other
these reactions. To examine the role of OH groups, heterogeneous catalysts in direct condensation of 1-phenyle-
[
15]
[a]
the Sil-NA-MgO and Sil-NAP-MgO devoid of free thanol with DEC at ca. 1258C.
OH groups, were tested in these reactions (Table 2).
The silylated NAP-MgO and NA-MgO samples are
prepared by the addition of MgO (500 mg) and me-
thoxytrimethylsilane (300 mg) in dry toluene (20 mL)
under reflux for 7 hours, after which the reaction mix-
ture was allowed to cool and centrifuged to obtain si-
lylated MgO, which was washed several times with n-
[15]
pentane.
These results indicate that Brønsted hy-
droxy groups add to the activity in the reactions,
[a]
Conditions: 1-phenylethanol (2 mmol), DEC (33 mmol,
2À
À
which is largely driven by Lewis basic O and O
sites. A comparison of the different MgO samples
Table 2) shows that NAP-MgO reacts efficiently with
a higher reaction rate, while NA-MgO affords unde-
4
mL) and catalyst (entry 1, 0.125 g; entries 2–4, 0.100 g).
[
[
b]
c]
Isolated yields.
Benzyl alcohol (10 mmol), DEC (20 mL) and catalyst
(0.100 g).
(
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An Efficient Synthesis of Organic Carbonates
COMMUNICATIONS
[a]
Table 4. Synthesis of organic carbonates using NAP-MgO as catalyst at ca. 1258C in solvent-free system.
[
[
[
[
a]
b]
c]
d]
Conditions: substrate and DEC molar ratio (1:excess), NAP-MgO (0.125 g).
Determined by GC.
Isolated yields.
th
4
recycle (i.e. fifth cycle).
chiral auxiliaries by NAP-MgO is described high resolution narrow scan for Mg 2p in the fresh
Scheme 3). NAP-MgO exhibits one line at 48.7 eV (Figure 2).
The [a] value of the carbonate obtained from the The XPS spectrum of the 1-phenylethanol-treated
(
D
chiral auxiliaries (R)-(À)-2-amino-3-methyl-1-butanol NAP-MgO for the Mg 2p exhibits two lines at 48.7
(
a) is found to be À188 (c=6, C H OH), and that and 50.7 eV, which can be attributed to magnesium in
2
5
from (S)-(À)-2-amino-3-phenyl-1-propanol (b) is NAP-MgO and Ph-CH(Me)-O-MgO, respectively
found to be À628, (c=1, CHCl ), therefore it can be (Figure 2). This provides evidence that in the pro-
3
2À
À
concluded that the amino alcohol is neither racemized posed mechanism, O /O of NAP-MgO abstracts an
nor inverted but retains its stereochemistry after the acidic proton of the 1-phenylethanol, giving a Ph-
À
+
reaction.
CH(Me)-O , which forms a complex with the unsatu-
The NAP-MgO was reused for four cycles with con- rated Mg site (Lewis acid-type) of NAP-MgO
sistent activity after activating the used catalyst under (Scheme 4). 1-Phenylethanol-treated NAP-MgO sam-
a nitrogen atmosphere at 2508C for 1 h (Table 4, ples were subjected to TGA-DTA-MS to detect the
entry 1).
evolved gas fragments as a function of temperature.
We prepared surface transient organometallic inter- The observed m/z values of the fragments for Ph-
À
mediates by treating NAP-MgO with 1-phenylethanol CH(Me)-O -NAP-MgO are 90, 91, 104 and 122 amu
and characterized them by XPS and TGA-MS. XPS that correspond to C H , C H , C H and C H O, re-
7
6
7
7
8
8
8
10
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M. Lakshmi Kantam et al.
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2
À
À
Lewis base sites (O /O ) of the catalyst activate 1-
2+
+
phenylethanol, and the Lewis acid sites (Mg /Mg )
activate the carbonyls of DEC (Scheme 4). There-
fore, the DEC bound NAP-MgO directs the delivery
[
18]
+
of Ph-CH(Me)-O stereoselectively to the Mg (Lewis
acid)-activated carbonyl of DEC via oxygen coordina-
tion to afford the desired product.
In conclusion, the synthesis of organic carbonates is
achieved by using a recyclable heterogeneous catalyst,
nanocrystalline magnesium oxide with excellent yields
and selectivities.
Scheme 2. Tandem synthesis of the oxazoldine moiety using
NAP-MgO.
Experimental Section
The activation of the solid catalyst (fresh catalyst) was car-
ried out at 4508C for 4 h.
General Procedure for the Synthesis of
Unsymmetrical Organic Carbonates
DEC (33 mmol, 4 mL), alcohol (2 mmol) (the ratio of DEC/
alcohol=16.5:1) and catalyst (0.125 g) were charged into a
oven-dried, nitrogen-flushed 25-mL flask, equipped with a
magnetic stirring bar and a reflux condenser. The reaction
mixture was stirred under a nitrogen atmosphere at 1258C
until the completion of the reaction. After completion of
the reaction, (monitored by TLC, and GC), the reaction
mixture was centrifuged to separate the catalyst and washed
several times with ether. The combined organic layers were
dried over Na SO and the solvent was removed under re-
Scheme 3. Synthesis of chiral auxiliaries using NAP-MgO.
spectively. The major fragment, m/z=91 amu ob-
served corresponds to the benzyl radical cation of the
surface transient organometallic intermediate (see
Supporting Information, Fig. 3) further supports the
proposed mechanism.
The synthesis of unsymmetrical organic carbonates
proceeds via dual activation of both substrates (nucle- duced pressure. The crude product was purified by column
2
4
ophiles and electrophiles) by NAP-MgO. Thus, the chromatography (silica gel: 100–200 mesh) using a mixture
Figure 2. XPS high resolution narrow scans for Mg 2p of (a) NAP-MgO and of (b) 1-phenylethanol-treated NAP-MgO.
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An Efficient Synthesis of Organic Carbonates
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Scheme 4. Proposed mechanism for the synthesis of unsymmetrical organic carbonates.
of hexane:ethyl acetate, 80:20, as eluent to give the corre-
sponding organic ca rbonate.
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6, 951; e) S.-I. Kim, F. Chu, E. E. Dueno, K. W. Jung,
Reuse of the Catalysts
J. Org. Chem. 1999, 64, 4578; f) Chu. F, E. E. Dueno,
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Salvatore, V. L. Flander, D. Ha, K. W. Jung, Org. Lett.
After completion of the reaction, the catalyst was recovered
by centrifugation and activated under a nitrogen flow for
2
000, 2, 2797.
1
h at 2508C for further reuse. NAP-MgO showed consistent
activity for five cycles under the same reaction conditions.
Table 4, entry 1), The reusability of NAP-MgO catalyst is
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(
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Acknowledgements
2
005, 46, 1369.
[
12] a) B. M. Choudary, M. L. Kantam, K. V. S. Ranganath,
We wish to thank the CSIR forfinancial suppo tr underthe
Task Force Project CMM-0005. U. Pal thanks Council of Sci-
entific and Industrial Research, India, for research fellowship.
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Scale Materials Inc., Manhattan, Kansas, USA.
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1675