Asymmetric binary-acid catalysis with InBr3 in the inverse-electron-demanding hetero-diels-alder reaction of mono- and bis-substituted cyclopentadienes: Remote fluoro-effect on stereocontrol

Article abstract of DOI:10.1002/chem.201103340

Remote F effect: Unprecedented hetero-Diels-Alder reactions of mono- and bis-substituted cyclopentadienes have been realized by an asymmetric binary-acid catalyst that synergistically combines a chiral phosphoric acid 1h/InBr ₃ with good perise

Full text of DOI:10.1002/chem.201103340

COMMUNICATION
DOI: 10.1002/chem.201103340
Asymmetric Binary-Acid Catalysis with InBr3 in the Inverse-Electron-
Demanding Hetero-Diels–Alder Reaction of Mono- and Bis-Substituted
Cyclopentadienes: Remote Fluoro-Effect on Stereocontrol
Jian Lv, Long Zhang, Shenshen Hu, Jin-Pei Cheng, and Sanzhong Luo*[a]
The inverse-electron-demanding hetero-Diels–Alder reac-
tion of a,b-unsaturated-carbonyl compounds and alkenes is
one of the versatile approaches for the synthesis of dihydro-
pyran derivatives.[1,2] Currently, several catalytic enantiose-
lective processes have been developed,[3] however, with one
single exception: cyclopentadiene, CP; this reaction has
been limited to electron-rich alkenes such as enol ethers[3a,b]
and enamines.[4] To enable the reaction with less reactive al-
kenes would significantly expand the synthetic utility. In this
regard, 1,3-dienes, particularly cyclopentadienes, are appeal-
ing substrates due to their wide utilization as building blocks
in organic synthesis. Apart from the issues of stereoselective
control, the reaction of CP has the additional periselectivity
problem in differentiating the two bifurcating Diels–Alder
and hetero-Diels–Alder pathways. It even becomes more
challenging in the context with substituted CPs in which se-
rious issues of regioselective control are raised, a situation
that arises from, on one hand, the labile nature of substitu-
ents on CP ring due to the facile [1,5]-sigmatropic rear-
rangement and, on the other hand, the difficulties with sepa-
rations of CP regioisomers as well as their regioisomeric ad-
ducts. Recently, Yamamoto reported the first example of
regio- and enantioselective Diels–Alder reactions of mono-
substituted CPs.[5] There is strikingly no report on the use of
bis-substituted CPs in asymmetric Diels–Alder and hetero-
Diels–Alder reactions. We herein present the first catalytic
regio-, peri- and stereoselective hetero-Diels–Alder reaction
of mono- and bis-substituted CPs. This reaction is made pos-
sible by an asymmetric binary-acid strategy that synergisti-
cally integrates a chiral Brønsted acid and a Lewis acid,
leading to mutually enhanced acidity with concomitant gen-
eration of multiacidic centers for synergistic catalysis.[6,7]
Taking advantage of this intriguing feature as well as the
combinatorial flexibility, we examined this binary-acid catal-
ysis in the inverse-electron-demanding hetero-Diels–Alder
reactions of CPs.
Our investigation started with a model cycloaddition be-
tween cyclopentadiene 2a and b,g-unsaturated-a-ketoester
3a, for which the parent chiral phosphoric acid 1a is totally
inert (Table 1, entry 1). To our delight, the combined use of
1a with a Lewis acid led to an active reaction with marked
enantioselectivity and the hetero-Diels–Alder pathways are
generally favored under these conditions. A survey of differ-
ent Lewis acids was then followed and the best results were
again obtained with InBr3 in terms of both reactivity and
stereoselectivity (Table 1, entry 7), whereas other Lewis
acids resulted in either low activity or poor enantioselectivi-
ty (Table 1, entries 2–8).
The impact of the ratio of two acids was also examined.
Whereas the activity was maintained at a similar level by in-
creasing the ratio of 1a/InBr3 from 1:1 to 2:1, the enantiose-
lectivity was considerably improved when using a higher
ratio of 1a/InBr3 (2:1) (Table 1, entry 7 vs. 9). Further in-
creasing the ratio to 3:1 did not lead to further improvement
in activity (Table 1, entry 10). ESI-MS studies indicated that
when the ratio was increased from 1:1 to 2:1 or higher, the
metal would completely assemble with the free phosphoric
acid; no free indium species is detected under a higher ratio
(>2:1).[6b] Both the 2:1 and 1:1 complex are present in solu-
tion together with free phosphoric acid regardless of the
molar ratio of 1a/InBr3.[6b] These results indicate that a free
phosphoric acid such as 1a performs well as a chiral ligand
in non-protic media, albeit with weak coordinating proper-
ties; the catalysis with free InBr3 would slightly favor the
DA pathway rather than HDA pathway (Table 1, entry 7 vs.
8). In addition, the free phosphoric acid 1a was found to be
superior to its alkaline salts such as 1c and 1d (with re-
versed peri-selectivity, Table 1, entries 12 and 13) or pyridi-
nium salt 1b (no activity, Table 1, entry 11) in terms of both
activity and selectivity. These observations, together with
the dramatic effect of metal Lewis acids on both activity
and stereoselectivity (e.g., Table 1, entry 7 vs. entries 3 and
4), highlight the synergistic combinations of free phosphoric
acid and Lewis acid for effective catalysis, and the role of
free phosphoric acid (e.g., 1a) as a dual ligand and acid is
thus verified.
[a] Dr. J. Lv, Dr. L. Zhang, Dr. S. Hu, Prof. Dr. J.-P. Cheng,
Prof. Dr. S. Luo
Beijing National Laboratory for Molecular Sciences (BNLMS)
CAS Key Laboratory of Molecular Recognition and Function
Institute of Chemistry, the Chinese Academy of Sciences
Beijing 100190 (P.R. China)
Fax : (+86)10-62554449
With the Lewis acid InBr3, we next set out to further im-
prove the stereoselectivity by examining different free phos-
phoric acids. Interestingly, steric tuning, a strategy widely
E-mail: luosz@iccas.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103340.
Chem. Eur. J. 2012, 18, 799 – 803
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
799

Table 1. Selected screening and optimization results.[a]
Entry
1/Lewis acid
1a/none
Yield
[%][b]
NR[d]
28
99
51
40
99
99
99
4a/5a
ee/4a [%][c]
ee/5a [%][c]/d.r.
Entry
1/Lewis acid
Yield
[%][b]
4a/5a
ee/4a [%][c]
ee/5a [%][c]/d.r.
1
2
3
4
5
6
7
–
–
28
À79
À39
30
57
80
–
56
–
28/90:10
À79/90:10
À40/90:10
30/85:15
57/90:10
80/90:10
–
10[f]
11
12
13
14
1a/InBr3
1b/InBr3
1c/InBr3
1d/InBr3
1e/InBr3
1 f/InBr3
1g/InBr3
1h/InBr3
1h/InBr3
99
72/28
–
80
–
80
76
89
4
81
99
99
80/90:10
–
1a/Mg
1a/Sc(OTf)3
1a/Cu(OTf)2
1a/Bi(OTf)3
1a/In(OTf)3
A
67/33
79/21
70/30
68/32
72/28
72/28
40/60
70/30
NR[d]
99
N
25/75
43/57
75/25
75/25
71/29
81/19
81/19
82/90:10
76/90:10
89/90:10
5/90:10
85/90:10
98/99:1
98/98:2
E
99
99
52
99
99
99
R
A
15
16
1a/InBr3
None/InBr3
1a/InBr3
8
17
9[e]
99
56/90:10
18[g]
[a] General conditions: 2a (0.625 mmol), 3a (0.125 mmol), 1 (5 mol%), Lewis acid (2.5 mol%), and 4ꢁ molecular sieves (MS) (20 mg) at À708C in
CH2Cl2, 4 h; except for entries 1 and 2, 4–6, and 8, 24 h and for entry 18, 12 h. [b] Isolated product yield. [c] Enantioselectivities were determined by
HPLC analysis. [d] NR: No reaction. [e] 1a (2.5 mol%). [f] 1a (7.5 mol%). [g] 1h (2 mol%) and InBr3 (1 mol%) in CH2Cl2 (0.5 mL) for 12 h.
applied in the catalysis with phosphoric acids,[8] worked
against us. For example, though moderate improvement of
Table 2. DA and HDA reaction between CP 2 and b,g-unsaturated-a-ke-
toesters 3.[a]
stereoselectivity could be obtained by switching the remote
phenyl ring to a naphthyl group (Table 1, entry 7 vs. 14), the
incorporation of a larger 9-anthracyl moiety led to dramatic
depletion of both activity and stereoselectivity (Table 1,
entry 15, for more examples see the Supporting Informa-
tion), suggesting that steric interaction is not the determin-
ing factor for stereocontrol in our binary-acid catalysis.
After trial and error, it was found that stereoselectivity en-
hancement could only be achieved by remote substituent
tuning of the 4’-phenyl moiety, which turns out to be the es-
sential structural unit for stereocontrol (Table 1, entries 14–
17, see below for a discussion). An optimal phosphoric acid
bearing a pentafluorophenyl group, PentaF-1h, was eventu-
ally reached through electronic tuning of the remote phenyl
group. This binary-acid catalyst PentaF-1h/InBr3 demon-
strated good periselectivity (4a/5a=81:19), so far the best
periselectivity that has been achieved for a HDA pathway,[9]
and excellent stereoselectivity (99% enantiomeric excess
(ee) for 4a, endo/exo=99:1, 98% ee for 5a; Table 1,
entry 17). In addition, the catalyst loading can be reduced
from 2.5 mol% to 1 mol% while maintaining a similar per-
formance (Table 1, entry 18).
Entry
R1
R2
Yield
[%][b]
4/5
ee/4
ee/5
[%][c]
[%][c]/d.r.
a
b
c
d
e
f
g
h
i
Ph
Me
Me
Me
Me
Me
Me
Me
iPr
Et
99
99
99
99
99
88
99
99
99
81/19
81/19
78/22
83/17
81/19
87/13
80/20
80/20
79/21
99
98
99
98
99
98/98:2
98/99:1
98/98:2
98/96:4
98/97:3
99/99:1
>99/99:1
95/98:2
99/98:2
4-ClC6H4
3,4-Cl2C6H3
2-FC6H4
4-MeC6H4
4-MeOC6H4
4-PhC6H4
Ph
99
>99
>99
99
3,4-Cl2C6H3
[a] General conditions: 2 (0.625 mmol), 3 (0.125 mmol), 1h (2 mol%),
InBr3 (1.0 mol%) and 4ꢁ MS (20 mg) at À708C in CH2Cl2, 12 h; for en-
tries e–g, 48 h. [b] Isolated product yield. [c] Enantioselectivities were de-
termined by HPLC analysis.
tioselectivity for both 4 (>98% ee) and 5 (95–99% ee,
>96:4 endo/exo).
The catalytic scope of the obtained optimal binary-acid
PentaF-1h/InBr3 was next explored in the reactions of cyclo-
pentadienes, with a focus on those of mono- and bis-substi-
tuted CPs. As shown in Table 2, a variety of b,g- unsaturated
a-ketoesters can be applied in the reactions with cyclopenta-
diene to give the desired HDA products 4a–i and DA prod-
ucts 5a–i in good yields (4/5 up to 87:13) and excellent enan-
Experiments were then conducted to explore the reac-
tions with substituted CPs in the presence of 1h/InBr3. The
inseparable regioisomeric mixtures of substituted CPs (two
regioisomers for mono-substituted CPs and more than four
isomers for bis-substituted CPs) were directly employed
(Table 3 and Table 4). Remarkably, the reactions occur re-
gioselectively with one isomer among their respective mix-
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Asymmetric Binary-Acid Catalysis
COMMUNICATION
Table 3. DA and HDA reaction between mono-substituted cyclopentadienes 6 and
b,g-unsaturated a-ketoesters 3.[a]
four consecutive carbon stereogenic centers with
excellent enantioselectivity (Scheme 1).
In an effort to unveil the origin of the unexpected
remote substituent effect on stereocontrol
(Table 1), a systematic substituent mapping has
been carried out on the far 4’-phenyl moiety of
chiral phosphoric acid 1 (Table 5). A survey of the
data in Table 5 revealed several interesting observa-
tions: 1) there seems no uniform substituent trend
since the obtained stereoselectivity do not correlate
well with any single substituent parameter such as
sp or sm, polarizability, surface area, quadrupole
moment and dipole moment (see the Supporting In-
formation for details); instead 2) substituent effect
on stereocontrol is found to be highly position sen-
sitive and a significantly beneficial stereoeffect is
observed with ortho-substitution on the remote 4’-
phenyl ring (Table 5, entry 3 vs. 2; entry 8 vs. en-
tries 6 and 7), and 3) dramatic remote fluoro effect
on stereocontrol is also noted and a single ortho-
fluoro atom (see compound 1n) is suffice to pro-
vide comparable stereocontrol with that of the opti-
mal PentaF-1h (Table 1, entries 8 vs. 9, see the Sup-
porting Information for more substrates test with
1n),[12] indicating a distinctive stereocontrol mode
that is undisclosed with chiral phosphoric acid cata-
lysis.[5a,13]
Entry
a
Product
b
c
d
e
Based on the determined absolute configurations
of (4S,4aR, 7aS)-4a and (2S,3R)-5a, a tentative
transition state[14] (consistent with Houkꢂs
[a] General conditions:
(1.0 mol%) and 4ꢁ M.S. (20 mg) at À708C in CH2Cl2, 1 h.
6 (0.625 mmol), 3 (0.125 mmol), 1h (2 mol%), InBr3
tures of regioisomers. In the case of mono-substituted CPs,
3-substituted CPs are generally preferred except in one case
(with a steric demanding cyclohexyl-substituted CP (Table 3,
entry e)) in which the reactions favored either HDA prod-
ucts or DA products depending on the substitution
(Table 3). For bis-substituted CPs, the reactions proceeded
exclusively with 1,3-bis-subsituted CPs to give hetero-Diels–
Alder adducts as single stereoisomers and substituents in-
cluding benzyl, alkyl, and allyl groups are well accommodat-
ed in the reactions (Table 4).[10] In both cases, the corre-
sponding adducts were obtained in high yields with excellent
diastereoselectivity and enantioselectivity (Tables 3 and
4).[11] Heteroaromatic and alkyl-substituted ketoesters are
equally applied to the current reactions, though the latter re-
action is less selective (Table 4, entries 10 and 11). To the
best of our knowledge, the current results represent the first
examples on using substituted CPs in hetero-Diels–Alder re-
actions.
Scheme 1. The reactions with other 1,3-dienes.
Other 1,3-dienes have also been examined in the current
reactions (Scheme 1). Accordingly, cyclohexa-1,3-diene and
acyclic 1,3-dienes such as isoprene and 2,3-dimethylbuta-1,3-
diene can be applied to give DA or HDA adducts in high
yields and with excellent enantioselectivity. Notably, trans,
trans-1,4- diphenyl-1,3-butadiene is also a workable sub-
strate, affording a single cyclohexane product 14 bearing
model),[15] was proposed to account for the regio- and ste-
reoselectivity (Figure 1). Accordingly, the reactions occur
through bidentate activation of an a-ketoester by the cata-
lytic active complex formed from chiral phosphoric acid
(e.g., 1h or 1n and InBr3 (1:1)).[16] Approach of CP from the
Chem. Eur. J. 2012, 18, 799 – 803
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801

S. Luo et al.
Table 4. DA and HDA reaction between mono-substituted cyclopenta-
Table 5. Remote substituent effect of chiral phosphoric acid.[a]
dienes 6 and b,g-unsaturated a-ketoesters 3.[a]
Entry
R
Yield
[%]
4a/5a
ee 4a
[%]
ee 5a
[%]
1
2
1a
1g
99
99
72/28
71/29
80
81
80 (90:10)
85 (90:10)
Entry
1
Entry
2
3
1i
89
75/25
93
92 (92:8)
4
5
6
1j
1k
1l
90
10
98
55/45
55/45
70/30
79
78
86
94 (87:13)
69 (80:20)
87 (90:10)
3
5
7
4
6
8
7
8
1m
1n
99
60/40
91
91 (87:13)
99
75/25
98
96 (94:6)
9
1h
99
81/19
99
99 (98:2)
[a] General conditions: 2a (0.625 mmol), 3a (0.125 mmol), 1 (5.0 mol%),
Lewis acid (2.5 mol%) and 4ꢁ M.S. (20 mg) at À708C in CH2Cl2 for 4 h.
9
10
11
[a] General conditions: 9 (0.625 mmol), 3 (0.125 mmol), 1h (2 mol%),
InBr3 (1.0 mol%) and 4ꢁ M.S. (20 mg) at À708C in CH2Cl2, 1 h.
Figure 1. Proposed transition-state model for the stereocontrol (I) and re-
gioselectivity control/substitution bias (II).
more accessible upper space by the aid of attractive p-inter-
actions between fluorobenzene and CP and subsequent [a,
b]- and [b, c]-endo additions lead to the HDA and DA
adduct, respectively (Figure 1, I). Recently, single ortho-
fluoro substitution has been shown to contribute significant-
ly in aromatic stacking of a RNA-protein complex[17] and
this precedent together with the well-known propensity of
the pentafluorophenyl ring to participating in p–p stack-
ing,[18] are supportive of an attractive p–p interaction with
CPs for highly effective stereocontrol in our binary-acid cat-
alyst.[19] The observed substituent bias as well as the ob-
tained regioselectivity can be explained by considering the
intrinsic steric effect on the 2- and 4-positions of CP ring in
the proposed endo-transition state as a result of maximum
orbital overlap requirement in the typical HDA pathway
(Figure 1, II).[15]
In conclusion, we have developed the first peri-, regio-
and enantioselective HDA reaction of mono- and bis-substi-
tuted cyclopentadienes catalyzed by a unique binary-acid
1h/InBr3. A salient feature of the current reactions is as-
cribed to the identification of an extremely active binary-
acid catalyst 1n or 1h/InBr3 that enables unprecedentedly
802
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Asymmetric Binary-Acid Catalysis
COMMUNICATION
[7] For a review on combined chiral phosphoric acid and metal cataly-
sis, see: M. Rueping, R. M. Koenigs, I. Atodiresei, Chem. Eur. J.
2010, 16, 9350.
[8] a) T. Akiyama, Chem. Rev. 2007, 107, 5744–5758; b) M. Terada,
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[9] For comparison, Ref. [3e] and [3j] reported periselectivity 66 :34
and 60: 40 favoring 4a, respectively.
[10] As monitored by NMR and TCL, the reaction of 9h and 9i are
slightly slower than other bis-substituted CPs. The reaction time was
set at 30 min for the convenience of manipulation.
the reactions of substituted CPs and acyclic 1,3-dienes in
good yields and with high enantioselectivity. In addition, we
also disclose dramatic remote substituent effect, particularly
the ortho-fluoro effect, on stereocontrol likely through p-in-
teraction between fluorinated benzene ring and 1,3-dienes.
This distinctive stereocontrol mode has not been reported in
the catalysis with chiral phosphoric acid. Further work to
elucidate the nature of this fluoro-effect is currently under-
way in our laboratory.
[11] A larger scale (1 mmol) reaction has also been tried to give the de-
sired product 10b (0.42 g, 90% yield) with>99% ee.
[12] 1n also performs comparably in the reactions with other substrates.
[13] For examples of the fluoro-effect on stereocontrol, see: a) S.-s. Jew,
M.-S. Yoo, B.-S. Jeong, I. Y. Park, H.-g. Park, Org. Lett. 2002, 4,
4245–4248; b) T. Ooi, M. Takeuchi, M. Kameda, K. Maruoka, J.
Am. Chem. Soc. 2000, 122, 5228–5229; c) K. Ishihara, D. Nakashi-
ma, Y. Hiraiwa, H. Yamamoto, J. Am. Chem. Soc. 2003, 125, 24–25.
Acknowledgements
The project was supported by the Natural Science Founda-
tion
(NSFC
20972163,
21025208)
and
MOST
(2011CB808600). J. L. thanks the China Postdoctoral Sci-
ence Foundation for support.
1
[14] The relative configurations of 7 and 10 were established by H NMR
analysis according to Ref. [3e], [3f] and A. Martel, S. Leconte, G.
Dujardin, E. Brown, V. Maisonneuve, R. Retoux, Eur. J. Org. Chem.
3
2002, 514: The lower Jb–c coupling constants and the existence of a
a
c
4J coupling between the vinylic proton H and H strongly suggests a
c
pseudo-equatorial position for H, which rules out a trans-ring junc-
Keywords: asymmetric catalysis · fluorine · hetero-Diels–
tion and thus implies a normal endo cycloaddition. The absolute
configuration of 4a was determined by comparison of optical rota-
tion with the reported one ([a]2D5 = +191.1 (c=1.3, CHCl3) (reported
in Ref. [3e]: [a]2D5 =À226.5 (c=1.2, CHCl3)). By assuming similar
transition states, absolute configurations of 7a and 10a were as-
signed by analogy.
Alder reaction
·
indium bromide
·
substituted
cyclopentadienes · regioselectivity
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Received: October 24, 2011
Published online: December 14, 2011
Chem. Eur. J. 2012, 18, 799 – 803
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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