Chiral Palladacycle Catalysts Generated on a Single-Handed Helical Polymer Skeleton for Asymmetric Arylative Ring Opening of 1,4-Epoxy-1,4-dihydronaphthalene

Article abstract of DOI:10.1002/anie.201407358

Post-polymerization C-H activation of poly(quinoxaline-2,3-diyl)-based helically chiral phosphine ligands (PQXphos) with palladium(II) acetate afforded chiral phosphapalladacycles quantitatively. In situ generated palladacycles exhibited enantioselectivities up to 94 % ee in the palladium-catalyzed asymmetric ring-opening arylation of 1,4-epoxy-1,4-dihydronaphthalenes with arylboronic acids. Polymers with a twist: Poly(quinoxaline-2,3-diyl)-based helically chiral phosphine ligands (PQXphos) have been used to generate chiral phosphapalladacycles. The palladacycles resulted in enantioselectivities of up to 94 % ee in the palladium-catalyzed asymmetric ring-opening arylation of 1,4-epoxy-1,4-dihydronaphthalenes with arylboronic acids.

Full text of DOI:10.1002/anie.201407358

Angewandte
Chemie
DOI: 10.1002/anie.201407358
Helical Structures
Chiral Palladacycle Catalysts Generated on a Single-Handed Helical
Polymer Skeleton for Asymmetric Arylative Ring Opening of 1,4-
Epoxy-1,4-dihydronaphthalene**
Takeshi Yamamoto, Yuto Akai, and Michinori Suginome*
À
Abstract: Post-polymerization C H activation of poly(qui-
noxaline-2,3-diyl)-based helically chiral phosphine ligands
(PQXphos) with palladium(II) acetate afforded chiral phos-
phapalladacycles quantitatively. In situ generated palladacy-
cles exhibited enantioselectivities up to 94% ee in the palla-
dium-catalyzed asymmetric ring-opening arylation of 1,4-
epoxy-1,4-dihydronaphthalenes with arylboronic acids.
T
here has been increasing interest in metallacyclic catalysts,
including pincer-type complexes bearing carbon–transition-
metal bonds, as these commonly exhibit unique catalytic
activities and are more robust than ordinary transition-metal
catalysts bearing heteroatom-based ligands.[1] Palladacycle
catalysts are of particular interest because of the richness and
wide use of palladium-catalyzed reactions in synthetic organic
chemistry.[2] It is noteworthy that a palladacycle catalyst has
resulted in excellent turnover numbers in Suzuki–Miyaura
cross-coupling reactions.[3] While most palladacycle catalysts
are achiral and utilized in non-asymmetric catalytic reactions,
such as Heck-type and cross-coupling reactions,[4] some chiral
enantiopure palladacycle catalysts have been synthesized and
utilized in catalytic asymmetric reactions, including the
[3,3] sigmatropic rearrangement of allylic imidates and tri-
chloroacetimidates,[5] Miyaura–Hayashi-type conjugate ary-
lation,[6] hydrophosphination of a,b-unsaturated enones,[7]
and Lautens-type arylative ring opening of oxabicyclic
Figure 1. The poly(quinoxaline-2,3-diyl)-based helically chiral phosphine
ligand PQXphos.
various asymmetric reactions where palladium(0)-PQXphos
complexes are involved (Figure 1).[11,12] The enantioselectiv-
ities are particularly high, often exceeding those obtained
with low-molecular-weight chiral ligands. We expected that
the scaffold of PQXphos may also provide helical polymer-
based chiral palladacycle catalysts showing high enantiose-
lectivity. We report here that a palladacycle structure was
generated on a helical polymer scaffold by post-polymeri-
alkenes.[8,9] In the latter C C bond-forming reaction reported
by Hou and co-workers, enantioselectivities as high as 83% ee
have been obtained using a palladacycle catalyst derived from
optically pure 2-diphenylphosphino-1,1’-binaphthyl (H-
zation C H activation,[13] with the phosphorus pendant of the
À
À
PQXphos serving as a directing group. The polymer-based
palladacycle catalyst showed enantioselectivities up to
94% ee in the Lautens-type asymmetric arylative ring open-
ing of 1,4-epoxy-1,4-dihydronaphthalenes.
À
MOP) generated by intramolecular stoichiometric C H
activation.[9c] However, in other attempted asymmetric reac-
tions, relatively low ee values have been obtained using chiral
palladacycle catalysts.[10]
An initial trial of the asymmetric ring-opening arylation of
1,4-epoxy-1,4-dihydronaphthalene (1a) with 4-methylphenyl-
boronic acid (2a) was performed using Pd(OAc)2 and (P)-
(R)-PQXphos (L1), which bears diphenylphosphino groups
(Table 1). After stirring a mixture of L1, Pd(OAc)2, and
Cs2CO3 in chloroform for 5 min, 1a and 2a were added. The
reaction proceeded at room temperature to form 2-(4-
methylphenyl)-1,2-dihydro-1-naphthol (3aa) in 92% yield
and 86% ee (entry 1). Other palladium precursors were also
tested. The reaction with Pd(TFA)2 proceeded relatively
slowly, with the product formed with 88% ee (entry 2), while
[PdCl2(CH3CN)2] led to a product with 80% ee (entry 3). It is
noteworthy that [{PdCl(h3-C3H5)}2] did not act as a catalyst,
while [Pd2(dba)3] provided a high yield of 3aa but low
enantioselectivity (entries 4 and 5). As Pd(OAc)2, Pd(TFA)2,
and [PdCl2(CH3CN)2] are widely recognized as palladacycle
precursors,[2] we assume that palladacycles generated in situ
were acting as efficient chiral catalysts in these reactions.
In recent years, we have developed helical polyquinoxa-
line-based chiral monophosphine ligands PQXphos for use in
[*] Dr. T. Yamamoto, Y. Akai, Prof. Dr. M. Suginome
Department of Synthetic Chemistry and Biological Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: suginome@sbchem.kyoto-u.ac.jp
Prof. Dr. M. Suginome
JST, CREST
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
[**] Financial support for this research was provided by the Japan
Science and Technology Corporation (CREST, “Development of
High-performance Nanostructures for Process Integration” Area).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407358.
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Communications
Table 1: Asymmetric ring-opening reaction of 1,4-epoxy-1,4-dihydro-
of PQXphos (Table 2). To ensure the generation of pallada-
cycle, L1 and Pd(OAc)2 were stirred in toluene with Cs2CO3
at 608C for 2 h prior to the addition of 1a and 2a. This
procedure for catalyst generation increased the enantioselec-
tivity slightly (entry 1; 88% ee). Although PQXphos bearing
a 2-naphthyl group (L2) or 3,5-dimethylphenyl group (L3)
showed moderate enantioselectivities, the cyclohexyl deriva-
tive L4 formed the product with only 10% ee (entries 2–4).
We also prepared PQXphos derivatives L5–L7 bearing
electron-donating and electron-withdrawing groups, although
all showed lower enantioselectivities than L1 (entries 5–7).
Further optimization of the reaction conditions was
performed using L1 as the ligand of choice. The use of
toluene or THF as a solvent resulted in lower enantioselec-
tivities (Table 2, entries 8 and 9). The high catalytic activity of
the palladacycle facilitated reduction of the catalyst loading to
1.0 mol% with no decline in the enantioselectivity (entry 10).
The use of KF instead of Cs2CO3 decreased the reaction rate,
and slightly increased the enantioselectivity (entry 11,
90% ee).
naphthalene in the presence of PQXphos.[a]
Entry
Pd precursor
Yield [%][b]
ee [%][c]
1
2
3
4
5
Pd(OAc)2
Pd(TFA)2
[PdCl2(CH3CN)2]
[{PdCl(h3-C3H5)}2]
[Pd2(dba)3]
92
91
94
<5
98
86
88
80
nd
19
[a] 1a (0.24 mmol), 2a (0.20 mmol), Cs2CO3 (0.10 mmol), Pd precursor
(4.0 mmol Pd), and ligand (4.8 mmol P) were stirred in CHCl3 (800 mL)
and H2O (80 mL) at room temperature. [b] Yield of isolated product.
[c] Determined by supercritical fluid chromatography (SFC) analysis on
a chiral stationary phase. TFA=trifluoroacetate, dba=dibenzylidene-
acetone.
The generation of palladacycles on PQXphos was con-
firmed by comparison with identified low-molecular-weight
palladacycles.[14] According to the procedure developed by
Hou and co-workers,[8c] the H-MOP-based palladacycle 5 was
prepared by stirring a solution of H-MOP and Pd(OAc)2 in
benzene at 508C for 2 h (Scheme 1).
Ring-opening arylation also proceeded with phenylbor-
onic acid (2b) and its derivatives, including boronic esters and
trifluoroborate (Table 3). The reaction with phenylboronic
acid (2b) afforded 3ab with 90% ee (entry 1). The use of KF
The palladacycle showed a complex
NMR spectrum, presumably because
of equilibrium between monomeric
and dimeric forms. We found that
addition of diethylamine led to disso-
ciation to the monomeric form, thus
enabling identification by 31P NMR
spectroscopy, which showed a signal
at d = 32.5 ppm. This signal is assign-
able to the monomeric palladacycle
coordinated by diethylamine.[15] palla-
dacycle 8 was obtained by using qui-
noxaline-based triarylphosphine 7 as
a
model of a phosphine unit of
PQXphos. By using the same proce-
dure, palladacycle 8 was confirmed to
form an air- and moisture-stable palla-
dacycle-amine complex 9, which
showed a signal at d = 32.9 ppm in
the 31P NMR spectrum. PQXphos-
based palladacycle 10 was also
obtained as an insoluble material by
stirring a solution of L1 and Pd(OAc)2
in benzene at 608C for 2 h. Deaggre-
gation of the palladacycle by the
addition of diethylamine made it solu-
ble in benzene, and an amine complex
11 was formed. A shift in the 31P NMR
signal from d = À12.4 to 31.1 ppm
indicated quantitative conversion of
the phosphine units on PQXphos into
palladacycles.
Based on these results, we screened
substituents on the phosphorus atom Scheme 1. Synthesis of palladacycles.
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Table 2: Asymmetric ring-opening reaction of 1,4-epoxy-1,4-dihydro-
chiral palladacycle acts as an efficient catalyst in the ring-
naphthalene in the presence of PQXphos derivatives.[a]
opening desymmetrization of 1a with phenylboronic acids
and has higher enantioselectivity than the palladacycle
catalyst system developed by Hou and co-workers
(79% ee),[8c] the platinum catalyst system developed by
Yang and co-workers (84% ee),[8e] and the rhodium catalyst
system developed by Lautens et al. (92% ee).[8a] Although
neopentylglycol ester 12 led to the product with 80% ee
(entries 4 and 5), readily available pinacol ester 13 and
trifluoroborate 14 afforded products with enantioselectiveties
as high as that with phenylboronic acid (entries 6 and 8,
respectively). Lower reactivities of 13 and 14 relative to that
of 2b were observed when KF was used as a base, with only
a trace amount of product being formed after 120 h.
Entry
Ligand
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L7
L1
L1
L1
L1
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
toluene
THF
96
87
94
65
88
63
89
89
90
89
97
88
52
50
10
47
30
63
76
79
88
90
Under the optimized conditions, substrates for the reac-
tion were investigated at 08C (Table 4). In some cases the use
Table 4: Asymmetric ring-opening arylation of oxabicyclic alkenes using
PQXphos-based palladacycles.[a]
9
10[d]
11[d,e]
CHCl3
CHCl3
[a] 1a (0.24 mmol), boronic acid (0.20 mmol), Cs2CO3 (0.10 mmol), Pd
precursor (4.0 mmol Pd), and ligand (4.8 mmol P) were stirred with
solvent (0.40 mL) at room temperature. [b] Yield of isolated product.
[c] Determined by SFC analysis on a chiral stationary phase. [d] Pd
precursor (2.0 mmol Pd), ligand (2.4 mmol P). [e] KF used instead of
Cs2CO3.
Entry 1 (R1, R2)
2 (Ar)
Product, 3 Yield ee
[%][d] [%][c]
1
2
1a (H, H)
1a
1a
1a
1a
1a
1a
1a
2a (4-MeC6H4)
2c (4-MeOC6H4)
2d (4-FC6H4)
3aa
3ac
3ad
93
87
84
81
68
88
86
75
88
79
24
94
89
90
91
89
90
84
80
87
94
53
3[d]
4
Table 3: Scope of the boronic acid derivatives.[a]
2e (4-CO2EtC6H4) 3ae
5
2 f (3-MeOC6H4)
2g (3-ClC6H4)
2h (2-MeC6H4)
2i (2-ClC6H4)
2b (Ph)
3af
6[d]
7
3ag
3ah
3ai
3bb
3cb
3db
8[d]
9
1b (Me, H)
1c (F, H)
1d (H, MeO) 2b
10
11
2b
[a] Pd(OAc)2 (2.0 mmol), PQXphos (2.4 mmol P), and base (0.1 mmol)
were heated with toluene at 608C for 2 h. After evaporation,
1 (0.24 mmol), boronic acid 2 (0.20 mmol), CHCl3 (800 mL), and H2O
(80 mL) were added at 08C. [b] Yield of isolated product. [c] Determined
by SFC analysis on a chiral stationary phase. [d] Cs2CO3 used instead of
KF.
Entry
PhB(FG)
base
T [8C]
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
2b
2b
2b
12
12
13
13
14
14
Cs2CO3
KF
KF
Cs2CO3
KF
Cs2CO3
KF
RT
RT
0
0
0
0
0
0
0
12
12
36
36
97
87
97
90
87
80
trace
82
trace
90
91
94
78
of Cs2CO3 instead of KF increased the enantioselectivity. In
the case of 4- or 3-substituted phenylboronic acids, both
electron-donating and electron-withdrawing groups led to
products with high enantioselectivities (entries 1–6). The use
of 2-substituted phenylboronic acids 2h and 2i afforded
products with slightly decreased enantioselectivities (entries 7
and 8), although the selectivities were much higher than those
obtained using a low-molecular-weight palladacycle (3ah,
30% ee)[8c] and platinum catalysts (3ah, 11% ee; 3ai,
49% ee).[8e] Derivatives of 1,4-epoxy-1,4-dihydronaphthalene
bearing substituents at the 6,7-positions were applicable for
the reaction (entries 9 and 10), although the presence of 5,8-
dimethoxy groups lowered the reactivity and led to product
with moderate enantioselectivity (entry 11).
96
80
120
120
120
120
93
nd[d]
92
Cs2CO3
KF
nd[d]
[a] Pd(OAc)2 (2.0 mmol), PQXphos (2.4 mmol P), and base (0.1 mmol)
were heated with toluene at 608C for 2 h. After evaporation of the
solvent, 1a (0.24 mmol), PhB(FG) (0.20 mmol), CHCl3 (800 mL), and
H2O (80 mL) were added at 08C. [b] Yield of isolated product.
[c] Determined by SFC analysis on a chiral stationary phase. [d] Not
determined.
instead of Cs2CO3 decreased the reaction rate and increased
the enantioselectivity (entry 2). The ee value of the product
was improved (94%) by lowering the reaction temperature to
08C (entry 3). This result indicates that the PQXphos-based
In conclusion, we identified new chiral palladacycle
catalysts generated on a helical scaffold which had high
enantioselectivities in the asymmetric arylative ring opening
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Received: July 18, 2014
Published online: September 22, 2014
Keywords: asymmetric synthesis · helical structures ·
.
ligand design · palladacycles · polymer catalyst
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