The first asymmetric ring-expansion carbonylation of meso-epoxides

Article abstract of DOI:10.1039/c2cc35596e

The first asymmetric ring-expansion carbonylation of meso-epoxides to β-lactones is reported. Two structurally diverse chiral Cr(iii) chloro complexes in conjunction with Co₂(CO)₈ were shown to be competent catalytic systems for this

Full text of DOI:10.1039/c2cc35596e

View Online / Journal Homepage
Dynamic Article Links
ChemComm
Cite this: DOI: 10.1039/c2cc35596e
www.rsc.org/chemcomm
COMMUNICATION
The first asymmetric ring-expansion carbonylation of meso-epoxidesw
Prasad Ganji and Hasim Ibrahim*
Received 1st August 2012, Accepted 22nd August 2012
DOI: 10.1039/c2cc35596e
8
c–h
The first asymmetric ring-expansion carbonylation of meso-epoxides
to b-lactones is reported. Two structurally diverse chiral Cr(III)
chloro complexes in conjunction with Co (CO) were shown to be
reported seminal systems of Coates et al.
With this study as
our basis, and taking into account that Cr-derived systems are
8
d
the most active, we sought to explore a stereoselective
variant of this transformation employing chiral Cr(III) chloro
complexes, and disclose herein our preliminary investigations.
Chiral Cr(III) chloro complexes 1 and 3 were chosen for
evaluation in the asymmetric carbonylation of meso-epoxides.
Our choice of the former was guided by the fact that Jacobsen’s
catalyst, (R,R)-(salen)CrCl 1, and related Schiff base catalysts
have been shown to induce high levels of stereocontrol in the
2
8
competent catalytic systems for this transformation, displaying
significant levels of asymmetric induction of up to 56% ee.
b-Lactones represent an important class of biologically active,
naturally occurring compounds with broad utility in organic
1
synthesis and as precursors for polymeric materials. The first
direct enantioselective synthesis of b-lactones was described by
Wynberg and Staring employing a cinchona alkaloid catalysed
asymmetric intermolecular [2+2] cycloaddition between
12
asymmetric ring-opening of meso-epoxides. The D
Cr(III) chloro complex 3 derived from Halterman’s porphyrin,
which has been shown to induce high levels of stereocontrol in
4
-symmetric
13
2
ketene and activated aldehydes. Over the past 30 years, this
1
4
the asymmetric hetero Diels–Alder reaction, was chosen as a
pioneering study inspired intensive research into this reaction
resulting in various innovative modifications and ever more
widening substrate scope. At present, the chiral Lewis base
15
chiral analogue of (TPP)CrCl.
3
4
5
(
LB*), chiral Lewis acid (LA*) or cooperative LB*–LA
catalysed intermolecular [2+2] cycloaddition between ketenes
mostly generated in situ) and carbonyl compounds delivers
(
b-lactones with various substitution patterns in high enantio-
and diastereoselectivities. An elegant intramolecular variant
6
of this reaction has also been developed leading to bi- and
7
tricyclic b-lactones in high enantioselectivities.
The ring-expansion carbonylation of epoxides to b-lactones
is an attractive alternative to the above strategy as epoxides
8
are more readily available than ketenes. However, with the
exception of two reports on the carbonylative asymmetric
9
kinetic resolution of trans-2-butene oxide by Coates et al.
1
0
and propylene oxide by Bildstein et al., the asymmetric
carbonylation of epoxides remains unexplored. Surprisingly,
to the best of our knowledge, there has been no report on the
asymmetric carbonylation of meso-epoxides in spite of the
clear synthetic benefits of such a transformation.
2
,3-Benzyloxymethyl ethylene oxide was selected as the test
substrate as the enantiomeric excess of the resulting b-lactone
product could be determined directly by chiral HPLC analysis.
For the initial experiment, 5.0 mol% of 1 and 5.0 mol% of
Co (CO) were used as co-catalysts in THF under our previously
We recently reported an efficient catalytic system for the
2
8
11
optimised conditions (500 psi of CO, 70 1C, 16 h). We were
pleased to find that this catalytic system proved active delivering
the trans-b-lactone product in 65% conversion and in 54%
isolated yield. Moreover, chiral HPLC analysis indicated
that this desymmetrative carbonylation had occurred with 12%
enantiomeric excess (Table 1, entry 1).
carbonylation of epoxides comprised of commercially available
11
(
TPP)CrCl (TPP = tetraphenylporphyrinato) and Co (CO) .
2 8
This offers a distinct practical advantage over the previously
Centre for Synthesis and Chemical Biology, School of Chemistry and
Chemical Biology, University College Dublin, Belfield, Dublin 4,
Ireland. E-mail: hasim.ibrahim@ucd.ie; Fax: +353 1 7161178;
Tel: +353 1 7162978
w Electronic supplementary information (ESI) available: Experimental
procedures and analytical data including HPLC traces, as well as the
crystal structure of the cyclooctene derived b-hydroxy benzylamide.
CCDC 895104. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc35596e
Encouraged by this result, we embarked on a screen of the
reaction parameters. Switching to DME as the solvent gave
comparable conversion and an increased ee of 19% (Table 1,
entry 2). No reaction occurred when toluene or acetonitrile
was used as the solvent and DME was adopted as the solvent of
choice for all subsequent reactions. To investigate if reducing
This journal is c The Royal Society of Chemistry 2012
Chem. Commun.

View Online
Table 1 Preliminary assessment of L*CrX 1–3 in the asymmetric
ring-expansion carbonylation
Table
carbonylation of meso-epoxides
2
2 8
L*CrCl–Co (CO) -catalysed asymmetric ring-expansion
a
a
b
c
Conv. Yield ee
d
L*CrCl
Entry (mol %)
Co
(mol %)
2
(CO)
8
b-Lactone
(%)
(%)
(%)
1
2
1 (5.0)
3 (0.5)
5.0
86
82
+6
+2
b
c
L*CrX Co-source
Entry (cat.) (cat.)
Temp/ Time/ Conv
h
ee
(%)
Solv. 1C
(%)
0.75
Z 98 85
1
2
3
4
5
6
7
8
1
1
1
1
1
2
2
3
Co
Co
Co
Co
2
2
2
2
(CO)
(CO)
(CO)
(CO)
8
8
8
8
THF 70
DME 70
DME 45
DME rt
DME 70
DME 70
DME 70
DME 70
16
16
48
16
16
16
48
16
65 (54) 12
62 (49) 19
41 (33) 12
3
4
1 (5.0)
3 (0.5)
5.0
Z 98 83
Z 98 94
+11
12
23 (15)
nd
9
Na[Co(CO)
Co (CO)
Na[Co(CO)
4
]
]
0.75
ꢀ6
2
8
Trace nd
84 (56) 15
62 (56) 13
4
d
d
2 8
Co (CO)
5
6
1 (5.0)
3 (0.5)
5.0
Z 98 86
Z 98 71
+13
+11
a
Reaction conditions: epoxide (1.0 mmol), solvent (1.5 mL), CO
b
500 psi), L*CrX (5.0 mol%), Co-source (5.0 mol%). Determined
(
by H NMR spectroscopy of the crude material; yield of isolated
1
0.75
c
b-lactone is given in parentheses. Determined by chiral HPLC
analysis. 3 (0.5 mol%), Co (CO) (0.75 mol%).
d
2
8
7
8
1 (5.0)
3 (0.5)
5.0
Z 98 91
Z 98 81
ꢀ4
the reaction temperature had a beneficial effect on the enantio-
selectivity, we conducted the carbonylation at 45 1C and at
room temperature and found a decrease in both conversion
and enantioselectivity (Table 1, entries 3 and 4).
0.75
ꢀ16
9
1
1 (5.0)
3 (0.5)
5.0
Z 98 89
Z 98 83
ꢀ40
ꢀ33
In an attempt to explore if an in situ generated catalyst from
0
0.75
1
and Na[Co(CO) ] could deliver better results, equimolar
4
amounts of 1 and Na[Co(CO) ] were subjected to the reaction
4
conditions. In this case, a low conversion and a lower ee of 9%
were observed (Table 1, entry 5).
11
1 (5.0)
3 (2.0)
5.0
3.0
Z 98 78
Z 98 83
+45
+41
In order to investigate if an analogous of complex 1 with a
weakly coordinating counterion would deliver higher enantio-
1
2
selectivities, [(R,R)-(salen)Cr]BF
4
2 was synthesised and subjected
to the reaction conditions. Conducting the reaction with
Co (CO) as the cobalt source gave only traces of the product
lactone, clearly indicating that the chloro ligand in 1 is
16
2
8
1
1
3
4
1 (5.0)
3 (0.5)
5.0
Z 98 93
Z 98 86
ꢀ56
ꢀ33
1
7
necessary for the disproportionation of Co (CO) . However,
2
8
0.75
the 2–Na[Co(CO) ] combination proved active affording the
4
product in good conversion after 48 h and in comparable
enantioselectivity (Table 1, entry 7 vs. entry 2).
g
1
5
6
1 (5.0)
3 (2.0)
5.0
3.0
54
75
47
56
+31
The catalyst generated from 0.5 mol% of the chiral
[
(+)-porphyrin]CrCl 3 and 0.75 mol% of Co
similar high activity to that generated from equimolar amounts
of (TPP)CrCl and Co (CO) delivering a 62% conversion to the
2 8
(CO) showed a
g
1
ꢀ31
2
8
a
Reaction conditions: epoxide (1.0 mmol), DME (1.5 mL, 0.67 M),
b
CO (500 psi), 70 1C, 16 h. Determined by H NMR spectroscopy of
product lactone with an ee of 13% (Table 1, entry 8).
Based on these results, we decided to evaluate the perfor-
mance of catalysts generated from both, the 1–Co (CO) and
1
c
d
the crude material. Yield of isolated b-lactone. Determined by
e
2
8
chiral HPLC analysis of the b-hydroxy benzylamides derivative.
Absolute configuration was assigned based on ref. 7e. Absolute
f
the 3–Co (CO) systems for substrate screening according to
2
8
reaction conditions in entries 2 and 8 of Table 1. Bicyclic
configuration is predicted based on analogy to that determined for
g
the cyclopentane derived b-lactone. Determined directly by chiral
HPLC analysis.
epoxides that readily underwent ring-expansion carbonylation
1
1
were chosen as substrates. z
A good conversion with both catalytic systems was obtained
with cyclooctene oxide as the substrate, however with a low ee
of the lactone product (Table 2, entries 1 and 2). Higher ee’s of
enantioselectivity was obtained in the case of cycloheptane
oxide with 1 affording the bicyclic b-lactone product in 4% ee,
and in contrast to 16% ee when using complex 3 (Table 2,
entries 7 and 8).
1
1% and 6% were obtained with cyclooctane oxide as the
substrate using complexes 1 and 3, respectively (Table 2,
entries 3 and 4). The enantiomeric excess remained low at 13
and 11% when moving to the 12-membered ring derived
epoxide (Table 2, entries 5 and 6). A somewhat comparable
We were delighted to see a marked increase in asymmetric
induction with cyclopentane and 2,5-dihydrofuran derived
epoxides which, as reported previously, gave the expected
Chem. Commun.
This journal is c The Royal Society of Chemistry 2012

View Online
1
1
cis-b-lactone products. Cyclopentane oxide was carbony-
lated with 40% and 33% ee using 1 and 3, respectively (entries
4 (a) S. G. Nelson, T. J. Peelen and Z. Wan, J. Am. Chem. Soc., 1999,
21, 9742–9743; (b) Y. Tamai, H. Yoshiwara, M. Someya,
J. Fukumoto and S. J. Miyano, J. Chem. Soc., Chem. Commun.,
994, 2281–2282.
1
9
and 10). Higher enantioselectivities of 45% and 41% ee were
1
achieved with the diester substituted cyclopentane oxide. In
the latter case, a higher loading of 2.0 mol% was necessary
for complete conversion. Interestingly, the sense of chiral
induction was reversed when compared with cyclopentane
oxide (entries 11 and 12 vs. entries 9 and 10).
5
(a) M. A. Calter, O. A. Tretyak and C. Flaschenriem, Org. Lett.,
2005, 7, 1809–1812; (b) C. Zhu, X. Shen and S. G. Nelson, J. Am.
Chem. Soc., 2004, 126, 5352–5353.
For an overview, see: D. H. Paull, A. Weatherwax and T. Lectka,
Tetrahedron, 2009, 65, 6771–6803.
6
7 (a) H. Nguyen, S. Oh, H. Henry-Riyad, D. Sepulveda and
D. Romo, Org. Synth., 2011, 88, 121–137; (b) C. A. Leverett,
V. C. Purohit and D. Romo, Angew. Chem., Int. Ed., 2010, 49,
The carbonylation of 2,5-dihydrofuran derived epoxide
1
8
using catalyst 1 gave 56% ee, the highest enantioselectivity
of the presented study, while catalyst 3 gave an enantiomeric
excess of 33%, and equal to the value obtained with cyclo-
pentane oxide (Table 2, entry 14 vs. entry 10).
9
479–9483; (c) K. A. Morris, K. M. Arendt, S. H. Oh and
D. Romo, Org. Lett., 2010, 12, 3764–3767; (d) S. H. Oh,
G. S. Cortez and D. Romo, J. Org. Chem., 2005, 70, 2835–2838;
(e) G. S. Cortez, R. L. Tennyson and D. Romo, J. Am. Chem. Soc.,
2001, 123, 7945–7946.
1
1
The carbonylation of the slow-reacting 1-tosyl-2,3,6,7-
tetrahydro-1H-azepine derived epoxide using catalyst 1 gave
a conversion of 54% and an isolated yield of 47%. The
enantiomeric excess of the lactone product was determined
to be 31% by chiral HPLC analysis. A higher loading of
8
(a) J. T. Lee, P. J. Thomas and H. Alper, J. Org. Chem., 2001, 66,
5424–5426; (b) Q. Chen, M. Mulzer, P. Shi, P. J. Beuning,
G. W. Coates and G. A. O’Doherty, Org. Lett., 2011, 13,
6
592–6595; (c) J. W. Kramer, D. S. Treitler and G. W. Coates,
Org. Synth, 2009, 86, 287–297; (d) J. W. Kramer, E. B. Lobkovsky
and G. W. Coates, Org. Lett., 2006, 8, 3709–3712; (e) J. A. R.
Schmidt, E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc.,
2
.0 mol% was required to achieve a good conversion with
2
005, 127, 11426–11435; (f) J. A. R. Schmidt, V. Mahadevan,
Y. D. Y. L. Getzler and G. W. Coates, Org. Lett., 2004, 6, 373–376;
g) V. Mahadevan, Y. D. Y. L. Getzler and G. W. Coates, Angew.
catalyst 3 and the corresponding b-lactone was isolated in
5
6% yield and in an enantiomeric excess of 31%.
(
In conclusion, we show for the first time that the asymmetric
Chem., Int. Ed., 2002, 41, 2781–2784; (h) Y. D. Y. L. Getzler,
V. Mahadevan, E. B. Lobkovsky and G. W. Coates, J. Am. Chem.
Soc., 2002, 124, 1174–1175.
cis-2,3-Dimethyloxetan-2-one was obtained in 44% ee at 49%
conv.; for details, see: Y. D. Y. L. Getzler, V. Mahadevan,
E. B. Lobkovsky and G. W. Coates, Pure Appl. Chem., 2004, 76,
ring-expansion carbonylation of meso-epoxides to b-lactones
is feasible. The active catalysts for this transformation were
generated in situ from air stable components consisting of
Jacobsen’s catalyst 1 or the D -symmetric 3 and Co (CO) .
9
4
2
8
5
57–564.
0 b-Butyrolactone was obtained in 19% ee; for details, see:
H. Wolfle, H. Kopacka, K. Wurst, P. Preishuber-Pflugl and
B. Bildstein, J. Organomet. Chem., 2009, 694, 2493–2512.
11 P. Ganji, D. J. Doyle and H. Ibrahim, Org. Lett., 2011, 13,
142–3145.
2 (a) E. N. Jacobsen, Acc. Chem. Res., 2000, 33, 421–431;
b) B. D. Brandes and E. N. Jacobsen, Synlett, 2001, 1013–1015;
(c) C. Schneider, Synthesis, 2006, 3919–3944.
13 R. L. Halterman and S. T. Jan, J. Org. Chem., 1991, 56,
253–5254.
4 A. Berkessel, E. Ertu
48, 223–228.
Although enantioselectivities achieved in this study are moderate,
we anticipate that a substantial improvement might be possible
when evaluating rationally selected examples from the large
number of literature-known chiral Cr(III) complexes.
1
¨
¨
3
This work was supported by a UCD Ad Astra Scholarship
to P.G., UCD’s School of Chemistry and Chemical Biology
and Science Foundation Ireland (07/RFP/CHEF394). We
1
(
¨
thank Dr Helge Muller-Bunz for X-ray structure analysis.
H.I. is indebted to Prof. Donal O’Shea for helpful suggestions
on the manuscript.
5
1
1
¨
rk and C. Laporte, Adv. Synth. Catal., 2006,
3
5 The anti-dimethanoanthracene framework required for the synthesis
of the (+)-porphyrinato ligand in 3 was prepared according to an
efficient method developed within our laboratory. For synthetic
details, see: P. Ganji and H. Ibrahim, J. Org. Chem., 2012, 77,
Notes and references
z b-Lactone products without a UV active chromophore were derivatised
by the reaction with benzylamine to the corresponding b-hydroxy
benzylamides, the enantiomeric excess of which was determined by chiral
HPLC analysis (see ESIw). X-Ray analysis of the cyclooctene derived
b-hydroxy benzylamide confirmed the expected trans-stereochemistry
5
11–518.
1
1
6 S. A. Kozmin, T. Iwama, Y. Huang and V. H. Rawal, J. Am.
Chem. Soc., 2002, 124, 4628–4641.
7 (a) P. S. Braterman, B. S. Walker and T. H. Robertson, J. Chem.
Soc., Chem. Commun., 1977, 651–652; (b) G. Fachinetti,
T. Funaioli and M. Marcucci, J. Organomet. Chem., 1988, 353,
393–404; (c) P. S. Braterman and A. E. Leslie, J. Organomet.
Chem., 1981, 214, C45–C49.
(see ESIw).
1
For reviews on b-lactones, see: (a) A. Pommier and J. M. Pons,
Synthesis, 1993, 441–459; (b) H. W. Yang and D. Romo, Tetrahedron,
1
4
999, 55, 6403–6434; (c) C. Schneider, Angew. Chem., Int. Ed., 2002,
1, 744–746; (d) H.-M. Muller and D. Seebach, Angew. Chem., Int.
¨
2 8
18 The effect of the relative stoichiometry of 1 and Co (CO) on the
enantioselectivity was studied using the 2,5-dihydrofuran derived
epoxide as the substrate. Under otherwise identical conditions to
those in entry 13 of Table 2, decreasing the loading of 1 to 1.0 mol%
gave a full conversion to the lactone (89% yield) with a comparable
enantioselectivity of 53% ee. Increasing the loading of 1 to
10.0 mol% resulted in a diminished selectivity of 35% ee (Z 98%,
82% yield). These experiments indicate that an equimolar ratio
Ed., 1993, 32, 477–502.
(a) H. Wynberg and E. G. J. Staring, J. Am. Chem. Soc., 1982, 104,
1
2
3
66–168; (b) H. Wynberg and E. G. J. Staring, J. Org. Chem., 1985,
0, 1977–1979.
5
(a) M. Mondal, A. A. Ibrahim, K. A. Wheeler and N. J. Kerrigan,
Org. Lett., 2010, 12, 1664–1667; (b) L. He, H. Lv, Y.-R. Zhang and
S. Ye, J. Org. Chem., 2008, 73, 8101–8103; (c) J. E. Wilson and
G. C. Fu, Angew. Chem., Int. Ed., 2004, 43, 6358–6360.
2 8
or an excess of Co (CO) to 1 is necessary to minimise non-
enantioselective background reactions.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun.