Chiral (salen)CoIII catalyst for the synthesis of cyclic carbonates

Article abstract of DOI:10.1039/b401543f

A catalyst system comprised of a Co^III(salen) complex and aLewis base is investigated for the reaction of CO₂ and a variety of epoxides to form cyclic carbonates. Application of this catalyst system in the kinetic resolution of propylene oxide is also discussed.

Full text of DOI:10.1039/b401543f

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III
Chiral (salen)Co catalyst for the synthesis of cyclic carbonates†
Robert L. Paddock and SonBinh T. Nguyen*
Department of Chemistry and Institute for Environmental Catalysis, Northwestern University, 2145 Sheridan
Rd., Evanston, IL 60208, USA. E-mail: stn@northwestern.edu; Fax: 847-491-7713; Tel: 847-467-3347
Received (in Corvallis, OR, USA) 2nd February 2004, Accepted 26th March 2004
First published as an Advance Article on the web 15th June 2004
A catalyst system comprised of a CoIII(salen) complex and a
Lewis base is investigated for the reaction of CO and a variety
of epoxides to form cyclic carbonates. Application of this
catalyst system in the kinetic resolution of propylene oxide is
also discussed.
(Table 1, entry 3). Several LBs were tested as co-catalyst for
reaction 1, with an increase in Lewis basicity resulting in a
corresponding increase in activity (Table 1, entries 1, 4–6). The
number of equivalents of DMAP, the best LB tested, were varied
relative to catalyst 1 and found to be optimal at 2, the TOF was
halved when employing 1 equiv. and decreased steadily above 2
equiv. (Table 1, entries 1, 7–9).
2
The development of synthetic processes which incorporate CO
remains a very important challenge, both in the context of carbon-
management and from a practical standpoint: CO is an ideal C
feedstock in that it is safe, inexpensive and abundant. Although the
coupling of CO and epoxides to form cyclic carbonates and
2
2
1
2
1
polycarbonates has been known for over 50 years, the past decade
has seen a resurgence of interest in the development of well-defined
and selective catalyst systems for these reactions.
2
The 1/DMAP catalyst system was determined to effectively
convert a variety of monosubstituted terminal epoxides to the
corresponding cyclic carbonates in near quantitative yield (Table
III
Recently we reported a highly efficient Cr (salen)/DMAP
catalyst system for the coupling of CO
cyclic carbonate in high yield under mild reaction conditions.
2
and epoxides to provide
2b
2
). This includes epoxides which possess aliphatic, aromatic,
III
Jacobsen et al. have demonstrated that in addition to Cr (salen)
electron-withdrawing and -donating substituents. The catalyst
system was also active towards the conversion of a 1,1-dis-
ubstituted epoxide, though as anticipated this required longer
reaction times. An internal epoxide, diastereomerically pure cis-
III
complexes, Co (salen) complexes also function as catalysts for the
3
ring-opening of epoxides with a variety of nucleophiles. Thus we
began to explore the possibility of employing Co macrocycles as
III
4
catalysts for the coupling of CO
2
and epoxides. Coates and
2
,3-epoxybutane, was also employed as a substrate and was
5
III
coworkers’ recent communication regarding the use of Co (sa-
len) complexes for polycarbonate synthesis prompted us to disclose
our results in evaluating these complexes as catalysts, in conjunc-
successfully converted to the corresponding cyclic carbonate with
an overall retention of configuration (93 : 7 cis : trans, Table 2
footnote d). In the presence of the 1/DMAP catalyst system,
2
tion with a Lewis base (LB), for the reaction of CO and epoxides
reaction of enantiomerically pure (R)-propylene oxide with CO
2
to yield cyclic carbonates.
yielded enantiomerically pure propylene carbonate with complete
retention of the original stereochemistry.
III
The Co (salen) 1/DMAP catalyst system was found to be highly
2
active for the reaction of CO and propylene oxide to yield
propylene carbonate (eqn. 1), with a turn-over frequency (TOF) that
is comparable to the most active catalyst reported to date (Table 1,
Table 2 Substrate scope for the 1/DMAP catalyst systema
2f
entry 1). We found 1 to be much more active than the analogous
Yieldb
Substrate
Time (h)
Product
II
Co (salen) complex which demonstrated only limited activity
6
(
Table 1, entry 2). The LB co-catalyst is required for high activity;
1
1
.5
100%
in its absence catalyst 1 yielded only a trace amount of the product
Table 1 Activity of the 1/Lewis base catalyst system in the coupling of CO
and propylene oxide
2
100%c
95%
a
LB
(# equiv.)
TOF
(h )
1.5
.5
TONb
21 c
Entry
1
Lewis base
1
100%
DMAP
DMAP
None
Pyridine
N-Me-imidazole
Triethylamine
DMAP
DMAP
DMAP
2
2
0
2
2
2
1
4
8
400
147
20
452
422
395
452
497
464
1200
16
d
2
3
4
5
6
7
8
9
a
0.5
25
506
99
603
993
697
1.5
6
98%
1
94%
Reaction conditions: catalyst 1 (0.066 mol%), propylene oxide (3.5 mL,
b
95%d,e
1500 equiv.), CO
2
(300 psig), CH
2
Cl
2
(0.5 mL), 100 °C. mol of propylene
12
c
carbonate produced per mol of catalyst. mol of propylene carbonate
produced per mol of catalyst per hour. Co (salen) as catalyst.
d
II
a
Reaction conditions: 1 (1 mol%), DMAP (2 mol%), epoxide (1 mL, 100
equiv.), CO
2
(300 psig), CH
2
Cl
2
(0.5 mL), 100 °C. b Yields were
1
determined by comparing the ratio of product to substrate in the H NMR
†
Electronic supplementary information (ESI) available: general experi-
spectrum of an aliquot of the crude reaction mixture. c 1 (0.4 mol%). d The
mental procedures and analytical data for new compounds. See http://
www.rsc.org/suppdata/cc/b4/b401543f/
e
substrate was diastereomerically pure. Product cis : trans ratio (93 : 7).
1
622
C h e m . C o m m u n . , 2 0 0 4 , 1 6 2 2 – 1 6 2 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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The formation of enantiomerically pure cyclic carbonates from
enantiomerically pure epoxides and CO has been demon-
2
strated.2 However, employing chiral Co (salen) complexes as
catalysts offers the potential for kinetic resolution of racemic
epoxides, providing both enantiomerically enriched epoxide and
cyclic carbonate as products. We investigated this possibility and
the initial results for the kinetic resolution of propylene oxide are
presented in Table 3. Under the previously optimized reaction
conditions, the 1/DMAP catalyst system demonstrated a selectivity
b,7
III
8
factor, s, of 1.8 (Table 3, entry 2). When the corresponding
Co (salen) complex was employed as catalyst a selectivity of 1.1
II
was attained (Table 3, entry 1).
Decreasing the temperature had a notable effect, increasing the
selectivity to 3 at r.t. (Table 3, entries 3 and 4). However, at these
lower temperatures the TOF decreased and a mixture of propylene
carbonate and poly(propylene carbonate) was formed. Heating this
product mixture in vacuo resulted in complete degradation of the
Scheme 1 Proposed mechanism for CO
2
/epoxide coupling.
III
of a LB, Co (salen) complexes are inactive for CO
2
/epoxide
coupling at pressures below 600 psig, perhaps limited by the CO
2
5
insertion step. However, in our work, the presence of a LB allows
for the coupling of CO /epoxides at 300 psig of CO . Thus, the LB
seems to have enabled the rate-limiting CO insertion step. That the
9
polymer to the cyclic carbonate product without a change in
enantioselectivity.10
2
2
2
To further improve enantioselectivity, (R)-(+)-4-dimethylamino-
pyridinyl(pentaphenylcyclopentadienyl)iron, DMAP*, a planar
chiral DMAP which has recently been successfully employed as a
LB may have more than one role also explains why 2 equiv. of
DMAP are optimal. One equiv. may coordinate to the LA,
facilitating CO insertion leaving the other free to act as a
2
nucleophile to ring open the epoxide. Excess DMAP can inhibit
activation of the epoxide substrate by competing for coordination to
the Co complex and therefore leads to a decrease in TOF (Table 1,
cf. entries 1, 8–9).
In conclusion, we have demonstrated that Co (salen) com-
plexes, in conjunction with a LB co-catalyst, are effective and
2
versatile catalysts for the coupling of CO and epoxides, including
mono- and di-substituted terminal epoxides as well as internal
epoxides. In addition, initial investigations into the kinetic
1
1
catalyst in a number of enantioselective reactions, was investi-
gated as a co-catalyst. The use of DMAP* resulted in an increase in
selectivity to 4.8, as well as a 10-fold increase in TOF at r.t. This
increase in activity allowed us to investigate the reaction at 3 °C
which improved the selectivity further to 5.6 (Table 3, entries 5 and
III
6
).
III
Interestingly, changing the isomer of catalyst to (S,S)-Co (sa-
len) while using the same isomer of DMAP* did not change the
TOF or selectivity factor but merely provided the opposite
enantiomer of product. Thus, it appears that the chiral nature of the
co-catalyst is not a significant contributor to the overall enantiose-
lectivity. To test this conjecture, we employed N,N-4-dimethylami-
noquinoline (DMAQ) as a co-catalyst and indeed obtained a
selectivity similar to that obtained with the DMAP* co-catalyst,
albeit with a slight decrease in rate, supporting our hypothesis that
the chirality of the LB does not contribute significantly to the
improved selectivity. N,N-9-Dimethylaminoacridine (DMAA) also
yields a similar selectivity but a slower rate (Table 3, cf. entries 5,
2
resolution of epoxides with CO have shown encouraging selectiv-
ities. We have demonstrated that the steric bulk and electronic
properties of the co-catalyst can play a significant role in
determining the system’s selectivity in addition to the activity.
Notes and references
1 A.-A. G. Shaikh and S. Sivaram, Chem. Rev., 1996, 96, 951–976.
2 For a general review see: (a) D. J. Darensbourg and M. W. Holtcamp,
Coord. Chem. Rev., 1996, 153, 155–174; . For specific recent examples
see: (b) R. L. Paddock and S. T. Nguyen, J. Am. Chem. Soc., 2001, 123,
1498–11499; (c) V. Calo, A. Nacci, A. Monopoli and A. Fanizzi, Org.
Lett., 2002, 4, 2561–2563; (d) J. Huang and M. Shi, J. Org. Chem.,
003, 68, 6705–6709; (e) Y.-M. Shen, W.-L. Duan and M. Shi, J. Org.
Chem., 2003, 68, 1559–1562; (f) F. Li, C. Xia, L. Xu, W. Sun and G.
Chen, Chem. Commun., 2003, 2042–2043.
3 E. N. Jacobsen, Acc. Chem. Res., 2000, 33, 421–431.
4 R. L. Paddock, Y. Hiyama, J. M. McKay and S. T. Nguyen, Tetrahedron
Lett., 2004, 45, 2023–2026.
Z. Qin, C. M. Thomas, S. Lee and G. W. Coates, Angew. Chem., Int. Ed.,
003, 42, 5484–5487.
6 Shen and coworkers have also reported that a Co (salen) complex
catalyzed the formation of propylene carbonate. See ref. 2e.
7 T. Yano, H. Matsui, T. Koike, H. Ishiguro, H. Fujihara, M. Yoshihara
and T. Maeshima, Chem. Commun., 1997, 1129–1130.
7–8), suggesting that the steric bulk and the electron donating
ability of the LB are the important factors in determining activity
and selectivity.
The possible role of the LB co-catalyst in dual-site LA/LB (LA
1
=
Lewis acid) catalyst systems for the coupling of CO
2
and
2
2
e,12
epoxides has been investigated.
These reports have proposed
that the LB acts as a nucleophile to ring-open the LA-activated
epoxide. In our system, the LB may also serve an important role in
2
facilitating the insertion of CO into the ring-opened intermediate
(Scheme 1). This is supported by the observation that in the absence
5
2
and propylene oxidea
II
Table 3 Enantioselective reaction of CO
2
Temp.
(°C)
Time
(h)
TOF
(h )
2
1
sc
Entry
Lewis base
8
The selectivity factor, s, was determined by using the conversion, C, and
ee of the propylene carbonate product in the equation s = ln[1 2 C(1 +
ee)]/ln[1 2 C(1 2 ee)] as described by S. H. Wilen and E. L. Eliel, in
Stereochemistry of Organic Compounds; John Wiley and Sons, Inc.:
New York, 1994, p. 396.
1b
2
3
4
5
6
7
8
a
DMAP
DMAP
DMAP
DMAP
DMAP*
DMAP*
DMAQ
DMAA
100
100
50
r.t.
r.t.
3
9
0.33
8
48
4.5
50
15
37
16
1200
65
9
115
10
1.1
1.8
2.8
3.0
4.8
5.6
4.6
4.7
9 W. J. Kruper and D. D. Dellar, J. Org. Chem., 1995, 60, 725–727.
10 The ee of the propylene carbonate was monitored at various stages of
degradation and did not change. In addition, the product mixture from
r.t.
r.t.
47
13
2
the reaction of CO with pure (S)-propylene oxide at 50 °C was
degraded and complete retention of stereochemistry was observed.
Reaction conditions: catalyst 1 (0.066 mol%), Lewis base (0.132 mol%),
propylene oxide (3.5 mL, 1500 equiv.), CO (300 psig), CH Cl (0.5 mL).
1
1 G. C. Fu, Acc. Chem. Res., 2000, 33, 412–420.
2
2
2
b
II
c
12 H. S. Kim, J. J. Kim, B. G. Lee, O. S. Jung, H. G. Jang and S. O. Kang,
Co (salen) as catalyst. See ref. 8.
Angew. Chem., Int. Ed., 2000, 39, 4096–4098.
C h e m . C o m m u n . , 2 0 0 4 , 1 6 2 2 – 1 6 2 3
1623