Internal chelation-guided regio- and stereoselective Pauson-Khand-type reaction by chiral Rhodium(I) catalysis

Article abstract of DOI:10.1002/asia.201100271

Touch, Pauson, engage: To explain the unique superior reactivity and stereoselectivity of O-tethered enynes in asymmetric Pauson-Khand type reaction (APKR) than their congeners, the internal-chelation to metal of oxygen atom of the substrate in the transi

Full text of DOI:10.1002/asia.201100271

DOI: 10.1002/asia.201100271
Internal Chelation-Guided Regio- and Stereoselective Pauson–Khand-Type
Reaction by Chiral Rhodium(I) Catalysis
Dong Eun Kim,[a] Sang Hee Park,[a] Yun Hee Choi,[a] Sueg-Geun Lee,[b] Dohyun Moon,[c]
Jungju Seo,[d] and Nakcheol Jeong*[a, d]
Dedicated to Professor Eun Lee on the occasion of his retirement and 65th birthday
Internal chelation-assisted transition-metal-catalyzed reac-
tions have attracted considerable interest[1,2] and can often
provide new opportunities for previously formidable reac-
ꢀ
tions. For example, C C bond activation followed by cleav-
age[3] and hydroacylation for the preparation of medium-
sized (larger than five-membered) heteroatom-containing
(O, S, N) rings,[4] have been realized using internally chelat-
ed rhodium(I) catalysts.
Scheme 1. Enantioselective Pauson–Khand-type reaction by cationic rho-
dium(I) catalysts.
Rhodium(I) catalysts might have potential chelation ef-
fects in the asymmetric Pauson–Khand reaction (APKR)
but the internal chelation-assisted rhodium(I)-catalyzed
PKR has not yet been addressed.[5] Herein, we report the
chelation-guided regio- and stereoselective Pauson–Khand-
type reaction by rhodium(I) catalysts. A review of previous
studies of rhodium(I)-catalyzed APKR (Scheme 1) revealed
a couple of unique features that suggest the intervention
role of oxygen atoms in the reaction.[6]
tivity in tetrahydrofuran than in toluene.[6i] The rationale
behind this is as follows. Portions of the proposed catalytic
species, which are in a dynamic equilibrium, are changeable
depending on the solvent (Scheme 2). A significant amount
of the catalytic species 3, which is more reactive than 4 and
First, the APKR by the cationic rhodium(I) catalyst was
much faster and provided significantly higher enantioselec-
[a] D. E. Kim, S. H. Park, Y. H. Choi, Prof. N. Jeong
Department of Chemistry
Korea University
Anamdong, Seongbuk-gu, Seoul, 136-701 (Korea)
Fax : (+82)2-3290-3121
Scheme 2. Dynamic equilibrium of possible catalytic species 3, 4, and 5.
E-mail: njeong@korea.ac.kr
[b] Dr. S.-G. Lee
5, is present in coordinating solvents, such as tetrahydrofur-
an, where competitive coordination between the solvent and
CO can take place even under the pressure of CO. In con-
trast, the least-active catalytic species 5 would be predomi-
nant in non-coordinating solvents such as toluene. There-
fore, the reaction was expedited in a coordinating solvent
bearing oxygen atoms.[7]
Second, the reactions with substrate 1-a containing an
oxygen tether are normally much faster, even at a substan-
tially lower reaction temperature than those with other con-
geners, 1-b, 1-c, and 1-d. The reactions proceeded to comple-
Korea Research Institute of Chemical Technology
P.O.Box 107, Yusung, Deajeon, 306-606 (Korea)
[c] Dr. D. Moon
Beamline Division, Pohang Accelerator Laboratory/POSTECH
San-31 Hyoja-Dong, Nam-Gu
Pohang, Kyung Buk 790-784 (Korea)
[d] Dr. J. Seo, Prof. N. Jeong
Korea Basic Science Institute
Seoul Center
Anamdong, Seongbuk-gu, Seoul, 136-713 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100271.
Chem. Asian J. 2011, 6, 2009 – 2014
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2009

COMMUNICATION
Table 1. Regio- and stereoselective enantioselective Pauson–Khand reac-
tion of 6.
tion smoothly at ambient temperature if the CO atmosphere
was kept low (Ar/CO=10:1, 1 atm).[6d] In addition, the
enantioselectivities for reactions with 1-a were uniformly
higher than those of the others.[5]
This observation was confirmed by the following competi-
tion experiment. When an equimolar mixture of 1-a, 1-b,
and 1-c in tetrahydrofuran was subjected to APKR with a
limited amount of catalyst, which was prepared in situ from
[{RhCl(CO)2}2] (3 mol%) and (R)-3,5-xylyl-binap, 2-a was
obtained predominantly in 86% yield and 87% ee (458C,
2 h).[6d] On the other hand, 2-b and 2-c were obtained in
marginal yields and significantly lower enantioselectivities:
15% yield and 68% ee for 2-b and 6% yield and 77% ee
for 2-c. (Scheme 3)
Entry
Substrates
t [h]
7 (yield/ee (%))
7–1
7–2
7--3
1
2
3
4
6-a
6-b
6-c
6-d
3
2.5
1.5
n.r.
93/89
86/88
96/88
–
0
0
0
–
3/n.d.[a]
6/n.d.[a]
0
–
Scheme 3. Competition experiment illustrating the relative reactivity of
the various substrates.
[a] n.d.=not determined.
This apparent superior reactivity and stereoselectivity of
substrate 1-a in the APKR is perplexing. Nevertheless, a
mechanism that accounts for the unique features of 1-a can
be proposed, the key feature of which is that the oxygen
atom on the substrate participates in the transition state
(Scheme 4). A faster reaction rate might be due to the en-
hanced binding of the enyne in 1-a to the metal, which is
presumably initiated by the pre-chelation of an oxygen
atom. The higher enantioselectivity might be due to the
rigid conformation of the transition state, which results from
the chelation shown in Scheme 4a.
rates can be compared. Table 1 lists the reaction condi-
tions[6d] and a summary of the results.
When 6-a was subjected to APKR, as described in the re-
action Scheme in Table 1, 7–1-a was obtained as the major
product (93% yield) with excellent enantioselectivity (89%
ee). A trace amount of 7–3-a (<3%) was also obtained
(Table 1, entry 1) but 7–2-a was not observed. This product
distribution appears confusing but can be explained. 7–1-a
and 7–2-a were assumed to be formed initially in a ratio of
93:3. As soon as 7–2-a was formed, the remaining oxygen-
tethered enyne on 7–2-a underwent a second PKR to pro-
vide compound 7–3-a. On the other hand, the malonate-
tethered enyne remaining on 7–1-a was reluctant to undergo
a second PKR. This is because the malonate-tethered enyne
is less prone to PKR than the oxygen-tethered enyne. As a
result, 7–a-2 was completely consumed under the given reac-
tion conditions. Therefore, the observed overall product dis-
tribution of 7–1-a and 7–3-a could be considered to be that
of 7–1-a and 7–2-a in the first APKR. The regioselectivity is
extremely high and the enantioselectivity was also excellent.
In a reaction with substrate 6-b, 7–1-b was still the major
product (86%, 88% ee) but it was also contaminated by 7–
3-b (6%), which was presumably obtained from 7-b-2
(Table 1, entry 2).
Scheme 4. Two binding modes of the enyne to the metal: a) chelation-as-
sisted, b) normal.
Substrates 6, from which the role of the oxygen atom in
the APKR can be tested more directly, were then consid-
ered. Each substrate has a pair of enyne groups on opposite
sides of a central phenyl ring, where one of them has a fixed
oxygen tether. Therefore, direct intramolecular competition
between oxygen and the other tethers is possible and their
relative effects on yields and enantioselectivities can be
compared. The reactions were intentionally halted at the
halfway point, found by monitoring the reaction with TLC,
so that the relative effects of such tethers on the reaction
These first two examples strongly suggest the special fea-
ture of oxygen tethering during APKR, but the role of the
oxygen is not fully understood. Because the reaction rate
could be affected by many factors other than chelation by
oxygen atoms, for example, the electronic properties of
enynes and the steric bulkiness around the tether, several
more tests are needed.
2010
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Chem. Asian J. 2011, 6, 2009 – 2014

Chelation-Guided Regio- and Stereoselective Pauson–Khand-Type Reaction
Table 2. Regio- and stereoselective kinetic resolution of 8 by the asym-
metric PKR catalyst.
More convincing evidence was obtained with substrate 6-
c, in which two appendages on the central phenyl ring had
almost identical steric bulkiness and the only difference be-
tween them was the presence of a coordinating oxygen
atom. When 6-c was subject to APKR, 7–1-c was formed ex-
clusively (96% yield, 88% ee; in Table 1, entry 3). This evi-
dence supports the proposed chelation. Nonetheless, it still
did not exclude the possibility that the electronic character
of the two enynes, which were differentiated by different
tethering atoms, could affect the reactivity of the enynes.
Therefore, the reaction with substrate 6-e containing only
oxygen tethers was tested. One of the enyne appendages has
two methyl groups on the carbon atom next to the oxygen
tether. As a result, two opposite effects were expected to be
in competition. The Thorpe–Ingold effect would encourage
the formation of 7–2-e. On the other hand, steric interfer-
ence of the coordination of the oxygen atom to the metal
caused by the two geminal methyl groups would discourage
the formation of 7–2-e. In practice, 7–1-e was formed exclu-
sively (91%, 88% ee; Scheme 5).
Entry
Substrate
t [h]
8 and 9 (yield/ee [%])
9–1
9–2
9–3
8
1
2
8-a
8-b
1.5
2
20/91
23/74
29/81
22/52
0
0
46/n.d.[a]
45/n.d.[a]
[a] See reference [11].
An expansion of this reaction
was examined using dienynes 8-
a and 8-b,[10] as a couple of
issues are associated with these
substrates (Table 2). First, the
regioselectivity dependency on
the tether atom needs to be
checked as a primary concern.
Second, because substrates
8
are racemic mixtures, kinetic
resolution would be observed in
the APKR. The reaction with
8-a, which was stopped half-
way to completion, provided
PKR products as a mixture of
diastereomers, 9–1-a and 9–2-a
in 20% and 29% yield, respec-
tively (49% combined yield)
Scheme 5. Regio- and stereoselective PKR of 6-e.
These complete discriminations exhibited by the reactions
with 6-e and 6-c showed that the internal chelation was a
more-decisive factor in determining the regioselectivity.
Chelation by atoms other than oxygen is also intriguing.
Therefore, APKR with substrate 6-d was attempted with the
expectation of reversed regioselectivity because a sulfur
atom is a stronger binder than oxygen. However, the cata-
lyst was completely deactivated, and the reaction did not
proceed. Strong chelation of the sulphur atom rendered the
sulfur-chelated metallic species unable to act as the reacting
intermediate. This poisonous chelation by sulfur is contra-
dictory to the reported hydroacylation reactions that use
similar rhodium(I) catalysts,[4a,8] where a sulfur atom also
led a chelation-controlled reaction similar to an oxygen
atom. This observation is consistent with the poisonous in-
terruption of the PKR by cobalt carbonyls.[9] These observa-
tions suggest that the chelation control of the APKR must
be controlled delicately.
with significant stereoselectivity (91% ee for 9–1-a and 81%
ee for 9–2-a). Interestingly, there was no trace of the regio-
isomer, 9--3-a.
In addition, a recovered ent-8-a, which was presumably
enriched by a specific enantiomer, was obtained in 46%
yield, but its optical purity was not determined because of
its cryptochiral feature.[11]
The more-congested substrate 8-b also exhibited a similar
pattern under the same reaction conditions. 9–1-b (23%,
74% ee) and 9–2-b (22%, 52% ee) were obtained together
with 8 (45% yield). As expected from the structure, the dia-
stereoselectivity in the reaction with 8-b was not significant,
because there was no significant size-difference between the
methyl and the alkyl groups.
Finally, the desymmetrization of dieneyne 10 was rein-
vestigated (Table 3).[12] Substrate 10-a, which contains an
oxygen tether, provided 11–1-a as the sole product (91%
yield) with perfect diastereoselectivity as well as excellent
enantioselectivity (93% ee). On the other hand, substrate
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N. Jeong et al.
COMMUNICATION
Table 3. Desymmetrization of 10 by asymmetric PKR catalyst.
atom) and the bridgehead ring carbon was replaced with a
vinyl group. Note that the N-tosyl side-chain was placed on
the same face of hydrogen at C*, as in the crystal structure
of 13.[14]
Of the two competing transition states, TS-1-a and TS-2-a,
leading to products 11-1-a and 11-2-a, TS-1-a is favored over
TS-2-a because it can avoid steric congestion (Scheme 7a).
Internal chelation by oxygen would make the transition
state more tightly organized and thus eliminate its flexibility.
The steric congestion would be exaggerated significantly,
and thus the diastereoselectivity and enantioselectivity of
the reaction were improved substantially.
On the other hand, TS-1-b, leading to 11–1-b, was favored
over TS-2-b, leading to 11–2-b, because it can avoid the un-
desired steric congestion between the N-tosyl group and the
vinyl group. In addition, the transition states lacking chela-
tion appeared to be responsible for the reversal of the dia-
stereoselectivity. In addition, the diastereoselectivity and
enantioselectivity of 11-b were not as high as those for 11-a
because the transition states were bound relatively loosely.
Entry
Substrate
T [8C]
t [h]
11 (yield/ee [%])
11–1
11–2
1
2
3
10-a
10-b
10-c
23
50
23–50
3
2.5
n.r.
91/93
16/41
–
0
80/89
–
n.r.=no reaction.
10-b, bearing a N-para-toluenesulfonyl (tosyl) tether, also af-
forded the PKR product quantitatively (96% combined
yield). But, the diastereoselectivity with this substrate was
slightly inferior to the previous case, and provided a mixture
of diastereomers, 11–1-b and 11–2-b, with a strong favor of
the other diastereomer 11–2-b and reasonably high enantio-
selectivity (80% yield, 89%
ee). As anticipated from the
result with 6d, 10-c was poison-
ous to the catalyst.
Reversal in the diastereose-
lectivity depending on the
nature of the tether was quite
interesting. This result might
also be explained by a chela-
tion-controlled
whose proposed
mechanism,
transition
states are given in Scheme 6c.
The single-crystal structures
of 12 and 13, which were de-
rived from compounds (S)-2-a
and (S)-2-b obtained from the
APKR using (S)-binap, were
determined by X-ray diffraction
(Scheme 6a and b).[13] The
structures revealed that the ab-
solute configurations of C* of
both products were invariably
(S) if the binap-based ligand
employed had an (S) configura-
tion.
A plausible transition state in
the stereoselectivity-determin-
ing step was proposed, as
shown in Scheme 6c, and used
to explain the diastereoselectiv-
ity in the APKR of 10. These
transition states were obtained
by modifying the crystal struc-
tures, where one of the hydro-
gen atoms on the carbon atom
Scheme 6. Proposed structures of the transition state in a stereoselectivity-determining step. a) Chemical struc-
ture of 12 and 13 derived from 2-a and 2-b, respectively. b) ORTEP of 12 and 13. c) Proposed transition states
between a tethering group (or for 2-a and 2-b.
2012
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Chem. Asian J. 2011, 6, 2009 – 2014

Chelation-Guided Regio- and Stereoselective Pauson–Khand-Type Reaction
Scheme 7. Explanation of the diastereoselectivities in APKRs of 10-a and 10-b, based on the proposed transition states.
The role of an extra vinyl group in the transition state was
also examined. If this group competed with the oxygen
atom to participate in the reaction pathway, there must be a
profound change in the proposed transition states. To deter-
mine whether any competition was occurring, substrates 14-
a and 14-b, in which the vinyl group was replaced with an
ethyl group, were utilized (Scheme 8).
Besides the absence of an extra coordinating group, kinet-
ic resolution also needed to be accounted for because sub-
strates 14-a and 14-b were present as a racemic mixture.
Nonetheless, only an analysis of the diastereoselectivity of
the APKR products would be sufficient to explain the role
of the extra vinyl group in the transition state.
b were not associated with the transition state and diastereo-
selectivity. Only chelation by an oxygen atom appeared to
be responsible for the reversal of diastereoselectivity.
In conclusion, the participation of oxygen atoms of the
substrates into the transition state of APKR was demon-
strated. A competition experiment using substrates 6 and 8
exhibited near-complete regioselectivities together with high
enantioselectivity.
A combined effort for analysis of single-crystal structures
of 12 and 13 and an examination of the diastereoselectivity-
dependency on the tethers in the desymmetrization of 10 al-
lowed us to determine the tran-
sition states of APKR more
Focusing on the diastereoselectivity of the PKR products,
the diastereoselectivity in the reaction with 14-a and 14-b re-
mained unchanged from those with 10-a and 10-b, regardless
of the coordinating ability of the side-chain. 14-a produced a
single diastereomer 15–1-a (44%, 80% ee) exclusively, 14-b
yielded 15–2-b as the major product (45%, 41% ee) togeth-
er with 15–1-b as the minor product (5%; Table 4). These
results strongly suggest that the vinyl groups in 10-a and 10-
precisely.
Among the various possible
coordination groups, oxygen
was found to be the best and
most well-balanced in APKR
employing the cationic rhodiu-
m(I) catalysts to give distinctive
chelation effects. Sulfur deterio-
Scheme 8. An alternative tran-
sition state for 10-b.
rated the catalysts because of its strong binding. The vinyl
group was not strong enough to compete with oxygen.
Direct evidence for our postulation, such as direct meas-
urements of chemical shift of substrate-bound 103RhI, will be
pursued in future studies.
Table 4. APKR of 14: a kinetic resolution.
Experimental Section
[{Rh(CO)2Cl}2] (2.6 mg, 0.007 mmol, 5 mol%) and (R)-xylyl-binap
(10.0 mg, 0.014 mmol, 10 mol%) were placed in THF (1 mL) and the
mixture was stirred for 30 min at 208C under an atmospheric pressure of
argon. A solution of AgOTf (4.2 mg, 0.016 mmol, 12 mol%) in THF
(1 mL) was added, and the resultant reaction mixture was stirred for an-
other 30 min at 208C. The argon atmosphere was replaced with CO in
argon (1:10, 1 atm), and then a solution of 6-e (40 mg, 0.136 mmol) in
THF (1 mL) was introduced. The reaction mixture was stirred at 358C.
After completion of the reaction, a gaseous mixture was released in the
Entry[a]
Substrate
T [8C]
t [h]
15 and 14 (yield/ee [%])
15–1
44/80
<5/n.d.
15–2
14
1
2
14-a
14-b
23
50
3
2.5
–
48/82
40/67
45/41
n.d.=not determined.
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N. Jeong et al.
COMMUNICATION
hood. The crude reaction mixture was concentrated in vacuo, and then
the residue was purified by column chromatography on silica gel using n-
hexane/ethyl acetate as an eluent to afford 7-1-e (39.6 mg, 91% yield) as
a colorless oil.
[6] a) D. E. Kim, V. Ratovelomanana-Vidal, N. Jeong, Adv. Synth.
Catal. 2010, 352, 2032–2040; b) D. E. Kim, J. S. Kwak, I. S. Kim, N.
Jeong, Adv. Synth. Catal. 2009, 351, 97–102; c) D. E. Kim, B. H.
˘
Lee, M. Rajagopalasarma, J.-P. Genet, V. Ratovelomanana-Vidal, N.
1H NMR (300 MHz, CDCl3): d=1.57 (s, 6H), 2.33 (dd, J=17.8, 3.3 Hz,
1H), 2.84 (dd, J=17.8, 6.1 Hz, 1H), 3.21 (dd, J=11.3, 7.2 Hz, 1H), 3.27–
3.40 (m, 1H), 4.17 (d, J=5.5 Hz, 2H), 4.37 (dd, J=7.3, 7.1 Hz, 1H), 4.57
(d, J=16.5 Hz, 1H), 4.93 (d, J=16.5 Hz, 1H), 5.16 (d, J=10.2 Hz, 1H),
5.31 (d, J=17.0 Hz, 1H), 5.92–6.04 (m, 1H), 7.44 (d, J=8.3 Hz, 2H),
7.48 ppm (d, J=8.3 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=29.1, 40.5,
43.7, 65.8, 66.6, 71.0, 71.6, 84.1, 93.0, 116.7, 123. 4, 128.0, 130.6, 132.1,
134.2, 135.7, 178.2, 206.8 ppm; IR (KBr): n˜ =1710 cmꢀ1; HRMS (FAB+)
m/z [M+H]+ calcd for C21H22O3, 323.1647; found, 323.1646; ½aꢁ2D5 =+76.0
(c=0.015, CH2Cl2).
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˘
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2007, 349, 1999–2006; f) D. E. Kim, C. Choi, I. S. Kim, S. Jeulin, V.
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The ee value was determined to be 88% by HPLC analysis using a chiral
column (DAICEL CHIRALPAK OD-H, nHex/IPA=3:1, flow
1.0 mLminꢀ1, detection at 254 nm). Retention time: 9.63 min (major) and
11.23 min (minor).
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Willis, J. Am. Chem. Soc. 2008, 130, 17232–17233.
[9] a) A. Stumpf, N. Jeong, S. Hwang, Synlett 1997, 205–207. Arrested
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Acknowledgements
This work was supported by the National Research Foundation of Korea
(NRF) grant (KRF-2007–313-C00414 and 2011-0016303) funded by the
MEST.
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[11] The term “cryptochirality” was introduced by K. Mislow et al. to de-
scribe chiral molecules that have non-measurable optical rotations:
K. Mislow, P. Bickart, Isr. J. Chem. 1976/1977, 15, 1. As noticed from
the structure of compound 8, it was a typical cryptochiral molecule
because of the lack of difference between a propyl and an allyl side-
chain. Thus we have not measured the optical purity of the recov-
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[12] N. Jeong, D. H. Kim, J. H. Choi, Chem. Commun. 2004, 1134–1135.
[13] The preparation of 12 and 13 are given in the Supporting Informa-
tion. CCDC 816561 (12) and CCDC 816560 (13) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. The determination
of the absolute configuration of the stereocenters can be found in
the Supporting Information. The chiral auxiliary, N-(2-nitrophenyl)-
proline (2-NPP), was prepared according to the literature procedure.
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Keywords: chelation effects · enantioselectivity · kinetic
resolution · Pauson–Khand reaction · rhodium
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Received: March 16, 2011
Published online: June 15, 2011
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