Cationic Chiral Pd-Catalyzed “Acetylenic” Diels–Alder Reaction: Computational Analysis of Reversal in Enantioselectivity

Article abstract of DOI:10.1002/asia.201801035

The highly enantioselective Diels–Alder reaction of acetylenic dienophiles is shown to be effectively catalyzed by cationic chiral palladium complexes. Not only the degree but also the sense of enantioselectivity critically depends on the steric demand of ligands. Computational analyses indicate that the steric demand does not affect the endo/exo-selectivity but the enantioface selectivity of dienes.

Full text of DOI:10.1002/asia.201801035

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Accepted Article
Title: Cationic Chiral Pd-Catalyzed "Acetylenic" Diels-Alder Reaction:
Computational Analysis of Reversal in Enantioselectivity
Authors: Koichi Mikami and Kazuya Honda
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COMMUNICATION
Cationic Chiral Pd-Catalyzed “Acetylenic” Diels-Alder Reaction:
Computational Analysis of Reversal in Enantioselectivity
[
a]
[a]
[a]
[a]
Kazuya Honda, Yoshihiro Hayashi, Susumu Kawauchi, and Koichi Mikami*
Abstract: Highly enantioselective Diels-Alder reaction of acetylenic
dienophiles is shown to be effectively catalyzed by cationic chiral
palladium complexes. Not only the degree but also the sense of
enantioselectivity critically depends on the steric demand of ligands.
Computational analyses indicate that the steric demand affects not
the endo/exo-selectivity but the enantioface selectivity of dienes.
Chiral Lewis acid-catalyzed Diels-Alder (DA) reaction is one of
the most effective methods of constructing chiral cyclic building
blocks[ because of the high atom efficiency originated from direct
and double C-C bonds construction. Simultaneously, two
stereogenic centers are formed in a single reaction. In recent
years, numerous studies are reported[1,2] on various Lewis acid
catalysts (Ti, Cu, B, Fe, etc.) for the DA reactions. However, the
1]
Scheme 1. Cationic chiral palladium-catalyzed enantioselective “acetylenic”
Diels-Alder reaction.
olefinic enone-type dienophiles were generally used and
[
3]
“
acetylenic” counterpart was extremely limited, even though
substituted 1,4-cyclohexadiene products are of highly synthetic
demand.[ It is widely recognized that the development of highly
enantioselective DA reaction of acetylenic dienophile would be
rather difficult than the olefinic counterpart.[3a] The difficulty stems
from the distal catalyst coordination from the reaction center
because of the linear nature of acetylenic dienophile. For this
reason, the existing Lewis acid catalysts showed insufficient steric
effects in acetylenic DA reaction.[3a] The drastic decrease in yield
was observed in all precedent reports with B or Cu catalysts with
the less reactive terminal alkyl or aryl acetylenic ynone
dienophiles.
4]
Initially, silyl-substituted ynone was executed as comparatively
reactive dienophile (1b) with 1,3-cyclohexadiene (2) in the
presence of 5 mol% Pd(II) complex at -40 ˚C for 13 h (Table 1).
The reaction smoothly proceeded to give the cycloadducts (4b) in
moderate to good yields. In the case of (S)-BINAP-Pd(SbF
6 2
) ,
(
1R, 4S)-product was obtained in 68% yield and 84% ee (entry 1).
In order to improve the enantioselectivity, more bulky (S)-DM-
SEGPHOS was used to give, however, the decreased degree of
enantioselectivity (entry 2). In contrast, (S)-DTBM-BINAP and -
SEGPHOS, the most bulky phosphine ligands, gave up to the
highest yield and enantioselectivity (89% ee and 78% yield) (entry
We have reported a variety of enantioselective C-C bond forming
reactions of 1,2-dicarbonyl compounds catalyzed by cationic
palladium complexes.[5] The catalysts have very stable square-
3
and 4). Surprisingly, inversion of the sense of enantioselectivity
(
(
1S, 4R) was observed in contrast to the (1R, 4S)-selectivity with
S)-BINAP and (S)-DM-SEGPHOS.
2
planar structure by virtue of its large energy gap between dz
2
2
orbital and dx -y orbital of the Pd center, in contrast to Cu(II)
Lewis acid catalysts;[6] Bulky ligands were reported to change the
Cu(II) structures. By virtue of the structural stability of the Pd
catalysts, we report here the development of the highly
enantioselective DA reaction of acetylenic dienophiles catalyzed
by the Pd(II) complexes with diphosphine ligands bulky enough
for distal control over acetylenic reaction centers (Scheme 1).
Table 1. Pd(II)-catalyzed “Acetylenic” DA reactions
Entry[a]
L
Yield [%][b]
ee [%][c]
(1R,4S)
1
2
3
4
(S)-BINAP
68
54
55
78
84
46
60
89
(S)-DM-BINAP
(S)-DTBM-BINAP
(S)-DTBM-SEGPHOS
(1R,4S)
(1S,4R)
(1S,4R)
[
a]
Prof. Dr. K. Mikami, Dr. K. Honda, Dr. Y. Hayashi, Prof. Dr. S.
Kawauchi
Department of Applied Chemistry
Tokyo Institute of Technology
Tokyo 152-8552, Japan
[
a] Reaction conditions: 1b (0.1 mmol), 2 (0.2 mmol) in CH
2
Cl
2
(1 mL) [b]
Isolated yield [c] Enantiomeric excess and absolute configuration were de-
termined via transformation (see Supporting Information)
E-mail: mikami.k.ab@m.titech.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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Table 3. Diels-Alder reaction with cyclopentadiene
With the DTBM-catalyst, various terminal substituted ynones
were evaluated as dienophiles (Table 2). The bulky tert-
butyldimethylsilyl substituted ynone gave excellent yield but
decreased enantioselectivity (entry 2). Even with the less reactive
alkyl-substituted ynones, the reactions smoothly proceeded in
high yields and enantioselectivities, in sharp contrast to the
precedent reports.[3] Remarkably, the scope of dienophiles
includes not only primary alkyl-substituted ynone (entry 3) but also
secondary alkyl-substituted ones (entry 4, 5). However, tertiary
alkyl-substituted ynone has very low reactivity because of its
strongly electron donating character and large steric hindrance
Entry[a]
R
Temp [°C]
-78→-40[c]
-78
Yield [%][b]
80
ee [%]
1
2
3
TMS (1b)
n-Bu (1f)
c-Hex (1h)
91
mixture[d]
(
entry 6). Aryl-substituted ynones were also employable, while
higher catalyst loading was necessary (entry 7, 8).
-40
62
88
[
a] Reaction conditions: 1b, 1e-k (0.1 mmol), 3 (0.2 mmol) in CH
2
Cl
2
/Toluene
Table 2. Scope of terminal substituted acetylenic dienophiles
= 1:1 (1 mL) [b] Isolated yield [c] React for 2 h at -78 °C and then warm up
to -40 °C [d] Complex mixture containing the [4+2] and [2+2] product
Reversal in the sense of asymmetric induction is intriguing in
view of the acetylenic DA mechanism. The inversion of the sense
of enantioselectiviy was observed by changing the least bulky (S)-
BINAP to the most bulky (S)-DTBM-SEGPHOS in spite of the
same (S)-axial backbone. We have reported the enantiocontrol
process of Pd(II) Lewis acid catalyzed 1,2-/1,4-addition reactions.
The α-ketoester substrates coordinate to the Pd(II) center with
square planar five-membered ring chelate and equatorial aryl
groups on phosphine ligands block nucleophilic approaches
(Figrue 1);[5e] The (S)-ligands block the bottom side of α-ketoester
moiety and, hence, the diene should approach from the top side.
In this mechanism, the absolute configuration of products was
controlled solely by the axial chirality; Not only (S)-BINAP but also
Entry[a]
R
Temp [°C]
Yield [%][b]
ee [%]
74[c]
48[c]
86
1
2
TMS (1b)
TBS (1e)
n-Bu (1f)
c-Pr (1g)
c-Hex (1h)
t-Bu (1i)
Ph (1j)
-40
0
68
96
3
0
96
4
0
80
84
5
0
87
78
(
S)-DTBM-BINAP and -SEGPHOS should give the same sense
6
r.t.
r.t.
r.t.
trace
73
-
of enantiomers.
7[e][f]
8[e][f]
82
p-NO
2
-Ph (1k)
78
86
[
[
a] Reaction conditions: 1b, 1e-k (0.1 mmol), 2 (0.2 mmol) in CH
b] Isolated yield [c] Enantiomeric excess and absolute configuration were
2 2
Cl (1 mL)
de-termined via transformation (see Supporting Information) [e] 10 mol% of
Pd-catalyst was used [f] 25 %v CH Cl solution of 2 was added dropwise
2
2
with microfeeder over 2 h to the solution of the catalyst and 1j-k.
Cyclopentadiene was also employable under 10 mol% BINAP-
Figure 1. Transition states in 1,2-additions by (S)-BINAPs-Pd(II) catalysts.
6 2
Pd(SbF ) catalyst in dichrolomethane/toluene mixted solvent
(
Table 3). The Lewis acidity of Pd-catalyst was tuned by
coordination of toluene[7a] and, hence, the catalyst deactivation[7b]
by products could be controlled. Trimethylsilyl substituted ynones
gave high enantioselectivity (entry 1). With n-butyl substituted
ynone dienophile, the reaction became complex to give both [4+2]
and [2+2] adducts (entry 2). More bulky cyclohexyl substituted
ynone gave the [4+2] cycloadduct selectively (entry 3).
The inversion might be superficially explainable in terms of the
change in tetrahedral/square planar structure of the Pd center due
to steric repulsion of bulky substituent on the ligands in a similar
manner to tert-butyl- or phenyl-bisoxazoline-Cu(II) catalysts.[
6]
[
9a]
However, the X-ray structures of both DM-SEGPHOS-Pd(OTf)
2
[
9b]
and DTBM-SEGPHOS-Pd(OTf)
2
have the same square planar
; Drastic change in these
coordination structure as BINAP-PdCl
Pd structures was not observed.
2
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In the DA reaction with ynone dienophiles, both enantioface
selectivity and endo/exo selectivity affect the absolute
configuration of cycloadducts, different from the enone systems
to give the endo diastereomers. The Pd(II) catalysts with (S)-
ligands which could block the bottom side of α-ketoesters (Figure
In these exo selective reaction systems, the sense of
enantioselectivity was controlled by the enantioface of dienes.
The energy differences were calculated between top and bottom
side TSs with (S)-BINAP and (S)-DTBM-SEGPHOS ligands
(Table 5); In the case with (S)-BINAP, the diene would approach
from bottom side (+0.5 kcal/mol), while with (S)-DTBM-
SEGPHOS, the diene should approach from top side (-1.2
kcal/mo) (Table 5). The dienes could approach from either top or
bottom side of ynone because the reaction center of the
acetylenic DA reaction is far distal from the Lewis acid catalysts.
This mechanism is characteristics in the ynone cycloaddition.
1) should give (1R,4S)-products from endo-TS and (1S,4R)-
product from exo-TS (Figure 2); The ynone DA reactions could
have endo selectivity to give (1R,4S)-products and that the
inversion would occur only when very bulky DTBM-BINAP and -
SEGPHOS ligands are used. However, the DA reactions of
ynones are normally recognized to show exo selectivity because
of unfavorable secondary orbital interactions in contrast to the
secondary endo interactions of enones.[3c,e]
Table 5. Calculated energies of top and bottom side-TSs
Figure 2. Transition states of acetylenic Diels-Alder reaction.
DFT-calculations were thus employed to evaluate the transition
states in these reactions. All the calculations were performed with
ωB97XD[10a]/6-31G(d)/IEF-PCM[10b]/CH
2 2
Cl //B3LYP/6-31G(d)
conditions and LANL2DZ ECP basis set was used for Pd atom.
Gaussian09 rev. B01 was used as the calculation software. First,
the energy differences of endo- and exo-TSs were calculated with
various ligands (Table 4). The results show that energy levels of
exo-TSs were consistently lower than those of endo-TSs (ca. 2~5
kcal/mol) regardless of steric hindrance of ligands. Therefore,
these systems had exo selectivity as generally observed in ynone
[
a] ωB97XD/6-31G(d)/IEF-PCM/CH2Cl2//B3LYP/6-31G(d) conditions and
LANL2DZ ECP basis set was used for Pd atom. Difference of Interaction
energy between reactant fragments at top and bottom side-TS geometry
shows similar energy (+0.4) in BINAP systems by ωB97XD.
These DFT calculations along with experimental results show
that bottom side TS is favored with (S)-BINAP-Pd catalyst and
that top side TS is favorable with sterically demanding (S)-DTBM
counterpart. (S)-DTBM-SEGPHOS could effectively control the
distal reaction center of ynones by blocking the bottom side of
dienophile. In contrast, (S)-BINAP ligand is not so effective in
steric shielding to block the bottom side of dienohiles, wherein
CH- attractions are in turn operative between equatorial phenyl
group of BINAP and allylic hydrogens in diene (Figure 3); The
bottom side-TS in BINAP system is thus stabilized and the
inversion of the sense of enantioselectivity is observed.
systems.[3c,e]
Table 4. Calculated energies of endo/exo-TSs.
Entry
Ligand
ΔGendo-ΔGexo [kcal/mol]
1
2
3
4
5
6
No catalyst
+4.9
+3.0
+1.9
+2.5
+3.3
+4.5
(CH
2 2 2
PMe )
BINAP
Tol-BINAP
DM-BINAP
DTB-BINAP
[
a] ωB97XD/6-31G(d)/IEF-PCM/CH
2
Cl
2
//B3LYP/6-31G(d) conditions and
LANL2DZ ECP basis set was used for Pd atom
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In conclusion, the Pd(II) Lewis acid catalysts exhibited high
catalytic activity in the acetylenic DA reaction. The cycloadducts
were thus obtained in high yields and enantioselectivities in a wide
variety of ynones such as terminal silyl, alkyl and aryl substituted
ones. The enantioselectivity was strongly dependent on the steric
effects of aryl groups on phosphine ligands. The inversion in the
sense of enantioselectivity was observed between (S)-BINAP-
Pd(II) and bulky (S)-DTBM-SEGPHOS-Pd(II) systems.
Top
Bottom
The DFT calculations and experimental results on steric effects
of ester moieties indicate that the inversion of enantioselectivity is
not caused by the inversion of endo/exo-selectivity but that of
enantioface selectivity.
Figure 3. CH- attractions in the bottom TS (right) of (S)-BINAP-Pd system
In proof of our mechanistic rationale, the steric effects of the
ester groups were validated. If the reaction proceeded through
proposed mechanism, the steric effects of ester moieties should
directly affect the degree of enantioselectivity. When (S)-DTBM-
SEGPHOS was used, the steric hindrance of the ester moiety
should further retard the bottom side approach of dienes. As the
results, the enantioselectivity would increse. In contrast, when
Experimental Section
Typical Procedure for Diels-Alder Reaction: To a solution of Pd cat.
2 2 6
(0.005 mmol) in CH Cl (1.0 mL) was added AgSbF (3.8 mg, 0.011 mmol)
at room temperature under argon atmosphere. After stirring for 30 min,
ynone (0.1 mmol) and diene (0.2 mmol) were added to the mixture at
reaction temperature. After stirring for 13 hours, the reaction mixture was
directly loaded on the short pad of silica-gel (hexane/AcOEt = 3/1) and
evaporated under reduced pressure. The crude product was purified by
silica-gel column chromatography.
(
S)-BINAP was used, equatorial phenyl group allow repulsion with
the ester moiety should cancel the favorable bottom side
approach; the enantioselectivity was expected to decrease.
The experimental results were shown in Table 6. The
enantioselectivities by (S)-BINAP ligand decreased with increase
in steric effects of the ester moieties.
In contrast, the
enantioselectivity by (S)-DTBM-SEGPHOS remarkably improved
with increase in the steric bulk of esters. These results support
the proposed inversion mechanism in the sense of
enantioselectivity.
Acknowledgements
This research was supported by Japan Science and Technol-ogy
Agency (JST) (ACT-C: Creation of Advanced Catalytic Trans-
formation for the Sustainable Manufacturing at Low Energy, Low
Environmental Load). The numerical calculations were carried out
on the TSUBAME 3.0 supercomputer at the Tokyo Institute of
Technology, Tokyo, Japan, and on the supercomputer at the
Research Center for Computational Science, Okazaki, Japan.
Table 6. Scope of terminal substituted acetylenic dienophiles
Keywords: Asymmetric catalysis • Cycloaddition• Alkynes•
Palladium • Computational chemistry
Entry[a]
R
L
Yield [%][b]
ee [%]
(1R,4S)
1
2
Me (1a)
Et (1b)
88
68
67
66
95
78
93
74
80
74
57
17
90
89
95
99
[
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(1R,4S)
(1R,4S)
(1R,4S)
(1S,4R)
(1S,4R)
(1S,4R)
(1S,4R)
(
S)-BINAP
(b) Narasaka, K.; Iwasawa, N.; Inoue, M.; Yamada, T.; Nakashima, M.;
3
i-Pr (1c)
t-Bu (1d)
Me (1a)
Et (1b)
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N.; Zhang, H. Y.; J. Am. Chem. Soc. 1991, 113, 728.
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S)-DTBM-
SEGPHOS
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7[e][f]
8[e][f]
i-Pr (1c)
t-Bu (1d)
2
[
[
3]
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(
[
a] Reaction conditions: 1a-d (0.1 mmol), 2 (0.2 mmol) in CH
2
Cl
2
(1 mL) [b]
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Isolated yield [c] Enantiomeric excess and absolute configuration were de-
termined via transformation (see Supporting Information)
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Kazuya Honda, Koichi Mikami*
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Title
Cationic Chiral Pd-Catalyzed
“
Acetylenic” Diels-Alder Reaction:
Computational Analysis of Reversal
in Enantioselectivity
Highly enantioselective Diels-Alder reaction of acetylenic dienophiles is shown to be
effectively catalyzed by cationic chiral palladium complexes. Not only the degree
but also the sense of enantioselectivity critically depends on the steric demand of
ligands. Computational analyses indicate that the steric demand affects not the
endo/exo-selectivity but the enantioface selectivity of dienes.
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