Palladium(II)-catalyzed 1,4-addition of arylboronic acids to β-arylenones for enantioselective synthesis of 4-aryl-4H-chromenes

Article abstract of DOI:10.1002/adsc.200600622

The enantioselective 1,4-addition of arylboronic acids to β-arylenones to give β-diaryl ketones was carried out at 0-25 °C in the presence of a dicationic palladium(II) catalyst, [Pd(S,S-chiraphos)(PhCN) ₂](SbF₆)₂. Additio

Full text of DOI:10.1002/adsc.200600622

FULL PAPERS
DOI: 10.1002/adsc.200600622
Palladium(II)-Catalyzed 1,4-Additionof Arylboronic Acids to
b-Arylenones for Enantioselective Synthesis of
4-Aryl-4H-chromenes
Takashi Nishikata,a Yasunori Yamamoto,b and Norio Miyaurab,*
a
Innovation Plaza Hokkaido (Japan) Science and Technology Agency, Sapporo 060-0819, Japan
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628,
b
Japan
Fax : (+81)-11-706-6561; e-mail: miyaura@org-mc.eng.hokudai.ac.jp
Received: December 3, 2006; Revised: April 11, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The enantioselective 1,4-addition of aryl- 0.05 mol%. The protocol provided a simple access to
boronicaicds to
b-arylenones to give b-diaryl ke- 4-aryl-4H-chromenes. Optically active chromenes
tones was carried out at 0–258C in the presence of a were synthesized with up to 99% ee via dehydration
dicationic palladium(II) catalyst, [Pd(S,S-chiraphos)- of the 1,4-adducts between arylboronic acids and b-
ACHTREUNG(PhCN)2]ACHTREUNG(SbF6)2. Addition of a silver salt such as (2-hydroxyaryl)-a,b-unsaturated ketones.
silver tetrafluoroborate [AgBF4] or silver hexafluor-
oantimonate [AgSbF6] (5–10 mol%) was effective to Keywords: asymmetriccatalysts; asymmetricsynthe-
achieve high enantioselectivities at low temperatures sis; boron; chromenes; conjugate addition; palladi-
(92–99% ee) and to reduce the catalyst loading to um
Introduction
availability of a number of functionalized derivatives,
a simple extension of the protocol to boronic acids
b-Diaryl carbonyl compounds are synthetically useful suffered from lower yields than those obtained with
as intermediates in the syntheses of natural products [ArBF3]K.[16,17] In this paper, we report the efficiency
and drug candidates such as a muscarinic receptor an- of silver salts for the palladium(II)-catalyzed 1,4-addi-
tagonist (tolterodine),[1] a selective inhibitor of phos- tion of arylboronicaicds ( 2) to b-arylenones (1) to
phodiesterase[2] and endothelin receptor antago- give b-diaryl carbonyl compounds (4) (Scheme 1).
nists.[3,4] For the preparation of such diarylmethine Addition of a silver salt such as AgBF4 or AgSbF6 (5–
stereogeniccenters at the b-carbon of carbonyl com- 10 mol%) was found to be effective to achieve high
pounds, a flexible approach for introducing two differ-
ent aryl fragments is the 1,4-addition of arylmetal re-
agents to a,b-unsaturated carbonyl compounds.[5–8]
For example, Cacchi reported the palladium(II)-cata-
lyzed 1,4-addition of (2-hydroxyaryl)mercury chlor-
ides to b-arylenones to give 4-aryl-4H-chromenes,[9]
which were recently identified as potent apoptosis in-
ducers.[10,11] An enantioselective version for the syn-
thesis of b-diaryl ketones was recently accomplished
utilizing rhodium(I) complexes of chiral dienes[12] or
chiraphos[4,13] for the 1,4-addition of arylboronic
acids.[7] We have also shown that a dicationic pallad-
ium(II)-chiraphos complex is efficient for the analo-
gous asymmetric1,4-addition of [ArBF 3]K, ArSiF3
and Ar3Bi to b-arylenones at a temperature lower
than 08C.[14,15] Although arylboronicaicds are better
reagents than [ArBF3]K and ArSiF3 because of the Scheme 1. Palladium-catalyzed 1,4-addition to b-arylenones.
Adv. Synth. Catal. 2007, 349, 1759 – 1764
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Takashi Nishikata et al.
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yields and high enantioselectivities (in the range of the trifluoroborate salt resulted in the best yield
95–99% ee) and to reduce the catalyst loading to less (62%) with 0.05 mol% catalyst loading under condi-
than 0.05 mol%. The protocol provides the first cata- tions optimized for these arylmetal reagents[14] (en-
lyticmethod for enantioseletcive synthesis of 4-aryl-
tries 1–4). The corresponding reaction of boronic acid
4H-chromenes[18] via dehydration of the 1,4-adducts was not at a practical level (16%, entry 4), but the
between arylboronicaidcs and
enones.
b-(2-hydroxyaryl)- turnover number exceeded 1,800 when 10 mol% of
AgBF4 was added (entries 5 and 6). Although this
effect is not significant for p-tolylboronicacids, the re-
actions resulted in a TON of 1,376 in the absence of
AgBF4 and of 1,460 in the presence of AgBF4 (en-
tries 7 and 8). The performance of AgBF4 for cyclic
Results and Discussion
Effects of Arylmetal Reagents and Silver Salts on
Turnover Number of Catalysts
enones was also investigated by using 2-cyclohexe-
none as a substrate (entries 9–11). [Pd(S,S-dipamp)-
ACHRTE(UNG PhCN)2]ACHTRE(UGN SbF6)2 was used in place of 3 since it result-
In a previous study on the palladium-catalyzed 1,4-ad- ed in higher enantioselectivities (93–95% ee) than did
dition of arylboron compounds to enones at 08C, the 3 (<10% ee) for the 1,4-addition of [ArBF3]K,
use of arylboronicacids suffered from low yields due
ArSiF3 and Ar3Bi (Ar=3-methoxyphenyl) to 2-cyclo-
to slow transmetalation to the palladium catalyst.[16] hexenone (entries 9–11).[14] In the presence of AgBF4,
Since it has been shown that silver salts exert a re- catalyst loading can be reduced to 0.01 mol% to
markable accelerating effect on transmetalation be- result in a TON of 9,900 for phenylboronicacid and
tween organoboronicacids and RPd(II)X in palladi-
of 9,800 for 3-methoxyphenylboronicacid (entries 9–
um-catalyzed cross-coupling reactions,[19,20] and on the 11). It was interesting that these high turnover num-
insertion step of Heck coupling,[21,22] the effects of ar- bers of the catalyst were easily achieved using only a
ylmetal reagents (Ar-[m]) and AgBF4 on yields, enan- 20% excess of arylboronic acids toward the enones,
tioselectivities and turnover number of the catalyst whereas more than a 50% excess of arylboronic acids
were investigated in the presence of [Pd(S,S- was required to achieve a TON of less than 100 with
chiraphos)
tries 1–8) and [Pd(S,S-dipamp)ACTHER(UGN PhCN)2]ACHTRE(UGN SbF6)2 for 2- five-fold excess of arylboronic acids to achieve a
(SbF6)2 (3) for acyclic enones (en- the corresponding rhodium-chiraphos catalyst[7] and a
ACHTREUNG(PhCN)2]ACHTREUGN
cyclohexenone (entries 9–11). The yields, enantiose- TON of 13,200 with the Rh-Digm-BINAP catalyst.[23]
lectivities and turnover numbers of the catalyst Such an effect of silver salts can be attributed to a
(TON) at 208C are shown in Table 1. Among the role in the generation of a nitrile-free palladium(II)
three 3-methoxyphenyl derivatives of silicon, bismuth, species such as [Pd(chiraphos)
potassium trifluoroborate and boronicacid, the use of transmetalation via ligand exchange between 3 and
(solvent)2]2+ active for
ACHTREUNG
Table 1. Effects of electrophiles and AgBF4 on turnover number of the catalyst (TON)[a]
Entry
1
Electrophile Ar-[m] Catalyst Additive
Acetone/H2O
[mL]
Time Yield
[h]
ee
TON
[mol%] (equivs.)
[%][b] [%][c]
Ar= R=
1
2
Ph
Ph
Me
Me
3-MeOC6H4SiF3
(3-MeOC6H4)3Bi
0.05
0.05
ZnF2 (1.0)
Cu(BF4)2
MeOH/H2O[d]
MeOH/H2O[d]
21
21
10
30
-
-
200
600
AHCTREUNG
(0.75)
none
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Ph
[3-MeOC6H4BF3]K
0.05
MeOH/H2O[d]
21
62
16
90
55
69
73
66
99
98
-
1,240
320
3-MeOC6H4B(OH)2 0.05
3-MeOC6H4B(OH)2 0.05
3-MeOC6H4B(OH)2 0.01
4-MeC6H4B(OH)2
4-MeC6H4B(OH)2
none
acetone/H2O[d] 21
acetone/H2O[d] 21
acetone/H2O[d] 96
acetone/H2O[e] 21
acetone/H2O[e] 21
acetone/H2O[e] 48
94
94
94
95
95
89
89
91
AgBF4 (0.1)
AgBF4 (0.1)
none
AgBF4 (0.1)
none
1,806
5,500
1,376
1,460
6,620
9,900
9,804
0.05
0.05
0.01
0.01
Ph
9[f]
10[f]
11[f]
2-cyclohexenone C6H5B(OH)2
2-cyclohexenone C6H5B(OH)2
2-cyclohexenone 3-MeOC6H4B(OH)2 0.01
AgBF4 (0.05) acetone/H2O[e] 48
AgBF4 (0.05) acetone/H2O[e] 48
[a]
A mixture of enone (1, 5 mmol) and Ar-[m] (6 mmol) in aqueous solvent was stirred at 208C in the presence of [Pd(S,S-
chiraphos)(PhCN)2](SbF6)2 (3) and an additive (if used), unless otherwise noted.
Isolated yields by chromatography.
Enantiomer excess determined by HPLC on chiral stationary columns.
In MeOH/H2O (10 mL/1 mL) or acetone/H2O (10 mL/1 mL).
In acetone/H2O (2.5 mL/0.25 mL).
A
ACHTREUNG
[b]
[c]
[d]
[e]
[f]
[Pd(S,S-dipamp)ACHTREUNG(PhCN)2]ACHTRE(UGN SbF6)2 was used in place of 3.
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Enantioselective Synthesis of 4-Aryl-4H-chromenes
FULL PAPERS
AgX,[14c] recycle of the catalyst via oxidation of palla- are comparable to those previously achieved by
dium(0) species generated by double transmetalation [ArBF3]K at À5 to +58C and 6–12% higher than
and reductive elimination of two aryl groups of the those provided by arylboronicaicds and a rhodiu-
[14c,24]
boronicaicds,
or activation of enones for inser- m(I)-chiraphos catalyst at 208C (entries 2, 4 and 10).
À
tion into the C Pd bond by chelation with the double The absolute configurations of most products are not
bond or carbonyl group of the enones,[25] although no known, but the formation of an S-product from the
mechanistic information is currently available.
(S,S)-chiraphos complex was established by the specif-
icrotation reported for ( S)-3-(3-methoxyphenyl)-1,3-
diphenylpropan-1-one {[a]D: +7.18 (c 0.71, CHCl3)}[14]
(entry 10). Coordination of an enone to the
[Rh(Ph)(S,S-chiraphos)]+ or [Pd(Ph)(S,S-chiraphos)]+
intermediate and the conformation of the chiraphos
ligand at this stereodetermining insertion stage were
Asymmetric 1,4-Additionof Arylboronic acids to
b-Aryl Ketones
The results of the asymmetric1,4-addition of arylbor-
onicaicds to the representative b-arylenones in the calculated by DFT computations.[14] In both the rho-
presence of 3 (1 mol%) and a silver salt (if used, 5–20 dium(I) and palladium(II) intermediates, a phenyl
mol% of AgBF4 or AgSbF6) are shown in Table 2. group on the metal and one of the axial phenyl
Although most reactions were completed at 08C in groups on the phosphine atom constitute a planar
the presence of 5–10 mol% of AgBF4 or AgSbF6, two free space for coordination of an enone to the metal
arylboronic acids, the 3-chloro (entry 1) and the 4- center, and an up-right quadrant is blocked by one of
acetyl derivatives (entry 6), were used at 258C since the equatorial phenyl groups of the (S,S)-chiraphos
the reactions were very slow at 08C even in the pres- ligand, suggestive of si-coordination of enones giving
ence of a silver salt. Such an effect of the substituents S products and high performance of chiraphos in rec-
might be due to the coordination ability of these sub- ognition of planar enones such as b-aryl ketones.
stituents to the cationic palladium(II) catalyst.
Indeed, a thiomethoxy group in the boronicaicd
(entry 5) and a nitro group in the substrate (entry 13) Synthesis of 4-Aryl-4H-chromenes
strongly retarded the reaction. Most of the reactions
were completed within 21 h at room temperature with In a previous study on the synthesis of 4-aryl-4H-
enantioselectivities in a range of 92–99% ee which chromenes, Cacchi and co-workers used a combina-
Table 2. Asymmetricaddition of arylboronicacids ( 2) to b-aryl enones (1).[a]
Entry
1
2
Additive
(equivs.)
Product Temp.
Yield
[%][b]
ee
[%][c] [%][d]
Ref.[4] ee
˚
[C]
Ar=
R=
FG=
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Me
Me
n-C4H9
2-C3H7
cyclo-C6H11
Ph
3-Cl
none
4a
4b
4c
4d
4e
4f
4g
4h
25
0
0
90
96
75
77
<10
95
66
80
93
86
91
73
44
75
99
93
3-MeO
4-MeO
3,4-O2CH2
4-MeS
4-COMe
3-MeO
3-MeO
3-MeO
3-MeO
4-Me
AgBF4 (0.1)
AgBF4 (0.1)
none
AgBF4 (0.1)
none
AgBF4 (0.1)
AgBF4 (0.1)
AgSbF6 (0.05) 4i
AgBF4 (0.1)
none
AgSbF6 (0.1) 4l
AgSbF6 (0.2) 4m
AgBF4 (0.1)
95
94
95
-
93
99
95
95
83
89
[e,f]
0
25
25
0
0
0
0
0
0
0
9
10
11
12
13
14
15
4j
4k
97 (S) 86 (S)
Ph
95
95
92
99
96
4-MeOC6H4
4-NO2C6H4
Ph
3-MeO
3-MeO
3-MeO
3-MeO
Ph
4-MeOC6H4
2-naphthyl
4n
0
0
Me
AgBF4 (0.05) 4o
[a]
A mixture of enone (1, 5 mmol), ArB(OH)2 (6 mmol) in acetone-H2O (10 mL/1 mL) was stirred for 21 h in the presence
of a palladium catalyst (3, 1 mol%) and an additive (if used).
Isolated yields by chromatography.
Enantiomer excess determined by HPLC on a chiral stationary column.
A mixture of 1 (1 mmol), ArB(OH)2 (1.5 mmol) and base (1 mmol) in dioxane-H2O (6/1) was stirred at 208C for 6 h in
[b]
[c]
[d]
the presence of [RhACHTRE(UNG nbd)2]BF4 (3 mol%) and (S,S)-chiraphos (3.3 mol%).
A methylenedioxy group.
The boronicacid (2 equivs.) was used.
[e]
[f]
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Takashi Nishikata et al.
FULL PAPERS
tion of (2-hydroxyaryl)mercury chlorides and b-aryl-
enones due to the easy availability of these nucleo-
philes via mercuration of phenols.[9] However, the
presence of such an ortho functionality in the arylbor-
onic acids significantly slows down the reaction rates
and reduces the enantioselectivities for the present
asymmetric1,4-addition. Thus,
b-(2-hydroxyaryl)-
enones (5) were chosen as substrates for arylboronic
acids (2). The reaction provided chromanols (7) as a
mixture of cis and trans isomers, which then trans-
formed to single isomers, chromenes (8), according to
the procedures reported by Cacchi (Scheme 2).
The effect of an ortho functionality on yields and
enantioselectivities is shown in Scheme 3. Additions
of phenylboronicacid to the 2-benzyloxy derivatives
9a and 9b smoothly took place at 08C in the presence
of 1 mol% of a palladium(II)-chiraphos catalyst (3)
with 96% ee and 98% ee, respectively. On the other
hand, unprotected 5a provided a 1:13 mixture of 6a
and chromanol (7a), which was then converted into
Scheme 2. Synthesis of optically active 2-methyl-4-aryl-4H-
the single 4-phenyl-4H-chromene (8a) via acid-cata- chromenes 8 (for substituents R1, R2, R3 and FG, see
Table 3).
lyzed dehydration. The enantioselectivity of 10a
(96% ee) was the same as that of 8a (96% ee). Thus,
the presence of a free hydroxy group did not have an
affect on the selectivity by its chelation with the cat-
ionicpalladium metal center.
The mode of face selection was the same as that
previously reported for the 1,4-addition of arylboronic
acids to enones catalyzed by 3.[14] Addition of phenyl-
boronicacid to enone 5b provided a 2:1 mixture of 6g
and 7g (Scheme 4), which was then subjected to
Baeyer–Villiger oxidation to give phenylchromanone
(11) with 93% yield and 93% ee. Recrystalization
from hexane/ether afforded pure 11 {99.6% ee, [a]D:
À41.98} in 70% yield (Scheme 4). Thus, the formation
of the R-product from the (S,S)-chiraphos complex
was well established by the specific rotation reported
for (R)-4-phenylchroman-2-one {[a]D: À45.18 (c 0.98,
CHCl3)}.[26]
The 1,4-addition of the representative arylboronic
acids to b-(2-hydroxyaryl)enones (5) catalyzed by the
palladium(II)-(S,S)-chiraphos complex (3) is shown in
Table 3. Phenyl- and 4-tolylboroniciadcs were
smoothly added to enones at 08C without any difficul-
ty (entries 1, 5 and 7–9), but reactions of arylboronic
acids possessing oxygen functionalities such as alkoxy,
methylenedioxy and acetyl groups were carried out in
the presence of 10 mol% of AgBF4 since these reac-
tions were very slow in the absence of a silver salt
(entries 2–4 and 6). The chromanols (7) thus obtained
Scheme 3. 1,4-Addition to protected and unprotected b-(2-
hydroxyaryl)enones.
as a mixture of 6 and 7 were converted to 4-aryl-4H- Conclusions
chromenes (8) in high yields by acid-catalyzed dehy-
dration. The enantioselectivities were in a range of Our strategy based on an asymmetric1,4-addition of
95–99% ee. Among them, phenyl ketone (5b) resulted arylboronicaidcs to enones represents a novel
in a higher selectivity (99% ee) than did the methyl method for the enantiocontrolled installation of a ste-
ketones (5a and 5c, 95–98% ee).
reogenicecnter in 4-arylhcromanols and 4-aryl-4
chromenes. The successful incorporation of an aryl
H-
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Enantioselective Synthesis of 4-Aryl-4H-chromenes
FULL PAPERS
Table 3. Synthesis of 4-aryl-4H-chromenes via asymmetrical 1,4-addition of arylboronic acids.[a]
Entry
5
ArB(OH)2 2 Additive [mol%]
FG=
6 and 7
8
[a]D
R1
R2
R3
Yield [%] of 6/7[b]
Yield [%] (ee%[b,c]
1
2
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
Me
H
Me 5a H
none
99 (1/13)
96 (1/13)
96 (1/13)
99 (1/16)
99 (1/13)
99 (1/16)
99 (2/1)
6a/7a 90 (96)
8a 184 (c 0.25)
8b 145 (c 0.70)
8c 182 (c 0.67)
8d 185 (c 0.72)
8e 120 (c0.74)
8f 263 (c 0.72)
8g 31 (c 0.62)
8h 172 (c 0.53)
8i 156 (c 0.76)
Me 5a 4-OMe
Me 5a 3-MeO
Me 5a 3,4-O2CH2
Me 5a 4-Me
Me 5a 4-COMe
Ph 5b H
AgBF4 (10)
AgBF4 (10)
AgBF4 (10)
none
AgBF4 (10)
none
6b/7b 90 (97)
6c/7c 94 (97)
6d/7d 89 (98)
6e/7e 94 (97)
6f/7f 90 (96)
6g/7g 92 (99)
6h/7h 94 (95)
6i/7i 90 (nd)[f]
3
[e]
4[d]
5
6
7
8
9
OMe Me 5c
H
none
none
99 (1/16)
94 (1/99)
t-Bu t-Bu Me 5d H
[a]
A mixture of enone (1 mmol), arylboronic acid (1.2 mmol), palladium catalyst 3 (1 mol%) and additive (if used) in ace-
tone-H2O (10/1, 2.2 mL) was stirred for 21 h at 08C.
Isolated yields by chromatography.
Enantiomer excess determined by HPLC on a chiral stationary column.
ArB(OH)2 (2 equivs.).
[b]
[c]
[d]
[e]
[f]
A methylenedioxy group.
An inseparable mixture by chiral columns.
corresponding 1,4-adducts 4. The enantiomer excess was de-
termined by chiral HPLC analysis.
General Procedure for Synthesis of Chromenes 8
A chromanol (7, 0.5 mmol), 4 Š molecular sieves (1 g), p-
toluenesulfonicaicd (0.05 mmol) and toluene (4 mL) were
added to a flask under nitrogen. The mixture was then
heated to reflux. After being stirred for 3 h, chromatography
on silica gel gave a corresponding chromene 8.
Characterization
Compounds 4b,[14] 4c,[27] 4g,[14c] 4j,[14c] 4k,[27] and 4o[14c] were
previously known. Characterization data for the new com-
pounds 4a, 4d, 4f, 4h, 4i, 4l, 4m, 4n, 7a, 7b, 7c, 7d, 7e, 7f, 7g
(as mixture with 6g), 7h, 7i, 8a, 8b, 8c, 8d, 8e, 8f, 8g, 8h, 8i,
10a and 10b are given in the Supporting Information.
Scheme 4. Conversion to 4-phenylchroman-2-one.
substituent at C-4 would make the method suitable
for generating a library of optically pure 4-arylchro-
mene analogues.
Acknowledgements
This work was supported by Grant-in-Aid for Science Re-
search on Priority Areas (No. 18064001, Synergy Effects for
Creation of Functional Molecules) from Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan, and
Grant-in-Aid from Innovation Plaza Hokkaido in Japan Sci-
ence and Technology Agency.
Experimental Section
General Remarks
All experiments were carried out under a nitrogen atmos-
phere. HPLC analysis was directly performed with a chiral
stationary phase column, Chiralcel OD-H, AD, AD-H, OJ-
H, and OB-H purchased from Dicel Co., Ltd.
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General Procedure for Asymmetric 1,4-Addition
ArB(OH)2 (1.2 mmol), acetone (2 mL), AgBF4 (0.1 mmol, if
used), enone (1 mmol) and water (0.2 mL) were added to a
flask under nitrogen. [Pd(S,S-chiraphos)ACHTER(NGU PhCN)2]ACHTRE(UGN SbF6)2
(0.001 mmol) was then added at 08C. After being stirred for
21 h at 08C or 258C, chromatography on silica gel gave the
[14c]
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Takashi Nishikata et al.
FULL PAPERS
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Adv. Synth. Catal. 2007, 349, 1759 – 1764