A spirocyclic chiral borate for catalytic enantioselective Nozaki-Hiyama allylation of ketones

Article abstract of DOI:10.1002/adsc.200900595

A new class of chiral spirocyclic borate ligands 5a-d with a rigid borocycle has been developed. The catalyst formed from chromium(II)-5a promotes the highly efficient enantioselective Nozaki-Hiyama allylation of alkyl and aryl ketones using allyl bromide

Full text of DOI:10.1002/adsc.200900595

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DOI: 10.1002/adsc.200900595
A Spirocyclic Chiral Borate for Catalytic Enantioselective
Nozaki-Hiyama Allylation of Ketones
Xin-Ren Huang,a Chinpiao Chen,a,* Gene-Hsian Lee,b and Shie-Ming Pengb
a
Department of Chemistry, National Dong Hwa University, Soufeng, Hualien 974, Taiwan, Republic of China
Fax : (+886)-3-863-0475; e-mail: chinpiao@mail.ndhu.edu.tw
Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China
b
Received: August 25, 2009; Published online: November 24, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900595.
Abstract: A new class of chiral spirocyclic borate li-
gands 5a–d with a rigid borocycle has been devel-
oped. The catalyst formed from chromium(II)-5a
promotes the highly efficient enantioselective
Nozaki–Hiyama allylation of alkyl and aryl ketones
using allyl bromide. The scope of the present
method is shown to be wide, and it affords an effi-
cient access to chiral tertiary homoallylic alcohols.
of and permitted the design of chiral ligands for the
enantioselective variations.[6] Chiral homoallyl alco-
hols with high levels of enantioselectivity have been
synthesized by catalytic asymmetric addition of allyl-
chromium reagents to carbonyl compounds.[7,8]
Like other enantioselective allyl fragment additions,
the asymmetric Nozaki–Hiyama reaction has been
successful for aldehyde allylation with a wide range of
catalyst systems.[9,10] Recently, ketone allylations have
been explored.[11] The development of new, effective
chiral ligands for the Nozaki–Hiyama allylation of ke-
tones is extremely intricate and needs a wide screen-
ing of chiral ligands. Recently, Sigman et al. elucidat-
ed a chromium-catalyzed enantioselective ketone ally-
lation in which allyl bromides were used directly.[12]
Herein, we report the synthesis of a chiral spirocyclic
Keywords: aminoborates; bipyridyls; chromium cat-
alysts; Nozaki–Hiyama allylation; spirocyclic bo-
rates
The use of organochromium species as reagents borate ligand that is effective in the chromium-cata-
began in 1977 when Nozaki and Hiyama et al.[1] as lyzed enantioselective allylation of alkyl and aryl ke-
well as Heathcock et al.[2] recognized that these spe- tones.
cies mediated Barbier–Grignard carbonyl additions,
Chiral bipyridyl ligands derived from enantioen-
and so participated in carbon-carbon bond-forming riched pinene allows for a variety of structural varia-
reactions in organic solvents. Carbonyl allylations tions and affords moderate to high levels of enantise-
using allylchromium reagents (Nozaki–Hiyama reac- lectivities in different asymmetric transformations.[13]
tion) rank among the foremost methods for natural A chiral spirocyclic borate ligand which incorporates
product syntheses owing to the extreme chemoselec- a rigid framework in the bipyridyl moiety for NH al-
tivity and substrate compatibility.[3] Organochromium lylation was synthesized in a three-step process from
reagents are easily prepared in situ by the oxidative ketone 1[14] and amino alcohols 4a–d.[15] Barbier-type
addition of chromium(II) species to allyl, propargyl, allylation of ketone 1 stereoselectively formed homo-
aryl and vinyl halides or triflates (requiring catalytic allylic alcohol 2.[16] Ozonolysis followed by reductive
amounts of Ni(II) salt for sp2 carbon centers), and can amination of the hydroxy aldehyde 3 with amino alco-
be efficiently added to aldehydes to give the corre- hols 4a–d using sodium borohydride in acetic acid af-
sponding alcohols in favorable yields.[4] The original forded spirocyclic borates 5a–5d in 30–35% overall
Nozaki–Hiyama–Kishi (NHK) reaction relies on the yield (Scheme 1). For comparison, the borates were
use of stoichiometric quantities of chromium(II) chlo- converted to the corresponding boron-free amino al-
ride. The catalytic redox system described by Fꢀrstner cohols (6a–6d) by stirring in acidic methanol. The for-
et al. enabled the quantity of chromium salts to be re- mation of the borate ligands and their subsequent re-
duced, making these reactions more valuable and en- moval were followed by 11B NMR.
vironmentally benign.[5] The catalytic modification of
this process has significantly improved the application the spiroborate ligand, a complete set of stereoiso-
In order to study the stereochemical diversity of
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Scheme 1. Synthesis of chiral spiroborate ligands. Reactions conditions: (a) allyl bromide, Zn, NH4OAc, THF (82%); (b) O3,
CH2Cl2/MeOH, À78 8C, DMS (71%); (c) amino alcohol 4, THF, NaBH4, AcOH (45–60%); (d) MeOH, reflux.
mers was synthesized starting from the antipode of tetracyclic skeleton with a rigid spirocyclic borate and
ketone 1 and both the enantiomers of amino alcohol unambiguously establish the absolute configuration of
4a (Figure 2). Interestingly the ligands 5a-1 and 5a-2 5c and 5c-3 as SRRS and RSSR, respectively
showed no borate formation in the structure as a (Figure 1).
result of unfavorable disposition of groups. X-ray
Catalytic evaluation of the Cr(II)-ligand 5a in the
crystallographic structures on the other hand reveal a allylation of 1-(3-methoxyphenyl)-ethanone with allyl
Figure 1. X-ray crystallographic structures of spiroborate 5c (CCDC 713511) and enantiomer 5c-3 (CCDC 713512).
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A Spirocyclic Chiral Borate for Catalytic Enantioselective Nozaki-Hiyama Allylation
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steric influence of the chiral ligands 5b–d and 6b–d,
respectively, on the asymmetric induction of the ally-
lation of 1-(3-methoxyphenyl)-ethanone display a sim-
ilar trend with the ligand containing isopropyl group
in the amino alcohol moiety affording the highest se-
lectivity. A pentacyclic ligand, expected to be formed
from ligand 5d with a proline module deviated from
the usual trend, affording (S)-alcohol in 49% ee.
The catalyst system was optimized using ligand 5a
with a Cr(II) complex formed in situ in the presence
of various bases, allyl halides and solvents. Of the
three tested allyl halides that comprised CrY3, the
allyl bromide and CrBr3 combination afforded the
best enantioselectivity. Similarly, THF at room tem-
perature in the presence of triethylamine represented
optimized conditions for the reaction (Table 2).
The extent of enantioselection for different ketones
using Cr(II)-5a was explored under optimized reac-
tion conditions (Table 3). Aryl ketones were excellent
Figure 2. Stereochemical diversity of chiral spiroborate 5a.
bromide following reported conditions formed (R)-2- substrates for the transformation, as evidenced by a
(3-methoxyphenyl)-pent-4-en-2-ol with excellent 74% yield and a 97% ee for the allylation of 1-(2-bro-
enantioselectivity (93%) and in good isolated yield mophenyl)-ethanone (Table 3, entry 6), and excep-
(85%). A similar level of asymmetric induction was tional functionality tolerance as observed with halide-
found for the antipode 5a-3. Diasteromeric ligands containing compounds. The nature and position of the
5a-1 and 5a-2 with no rigid framework and presuma- substituent on the aryl ring had drastic effect on the
bly a different mode of coordination to the chromium enantioselective outcome of the reaction (Table 3,
center lowered the enantiomeric excess without any entry 1–13). The substrate scope demonstrated no
change in stereochemistry of the product (Table 1).
straight correlation between the electronic nature of
The corresponding boron-free ligand 6a under iden- aryl group and enantioselectivity, although high yields
tical conditions formed the (S)-tertiary alcohol in of homoallylic alcohol was obtained in all the cases.
59% ee and 70% isolated yield. Consequently the Notably, similarly high levels of reactivity and enan-
chirality of the bipyridyl moiety alongside the pres- tioselectivity were observed for aliphatic ketones
ence or absence of borate ring governs the stereo- under identical reaction conditions (Table 3, entries 16
chemical outcome of the allylation reaction. The and 17). Modifying the aryl moiety or the methyl
group of aryl methyl ketones resulted only in a very
slight decrease in the asymmetric induction validating
a broad substrate scope with the present catalyst
system (Table 3, entries 14, 15, 18 and 19).
Table 1. Allyl bromide addition to 3-methoxyacetophenone
in the presence of chiral spiroborates 5a–d and boron-free li-
gands 6a–d.
In summary, the chiral spiroborate ligand,
(S,R,R,S)-5a, was demonstrated to promote efficient
and highly enantioselective catalytic Nozaki–Hiyama
reactions of alkyl and aryl ketones. The rigid frame-
work of the ligand is essential for high stereoselection
(up to 97% ee) and interestingly induced the opposite
sense of enantioselectivity after the removal of borate
ring. Further studies will be directed at the generality
and functional group compatibility of this catalytic
carbon-carbon bond forming process.
Entry
Ligand
Yield [%]
ee [%]
Configuration[a]
1
2
3
4
5
6
7
8
6a
5a
5a-1
5a-2
5a-3
5b
6b
5c
6c
5d
70
85
80
82
86
77
78
80
79
77
85
59
93
33
33
89
69
3
75
3
49
10
S
R
R
S
S
R
R
R
R
S
Experimental Section
9
10
11
Synthesis of 5a
6d
R
To a solution of compound 3 (1.00 g, 3.2 mmol) in THF
(20 mL) was added (S)-2-amino-3-methyl-1,1-diphenylbu-
tan-1-ol (820 mg, 3.2 mmol), and the mixture was stirred at
[a]
The ee% was determined on a Chiracel OD-H HPLC
column.
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Table 2. Optimization of reaction conditions for Nozaki–Hiyama allylation of 3-methoxyacetophenone.
Entry
X
Y
Base
T [8C]
Solvent
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
8
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Br
I
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
K2CO3
DIPEA
tBuOK
Pyridine
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
5
CH3CN
Deithyl ether
CH2Cl2
Toluene
Benzene
THF
DMF
Hexanes
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
80
–
33
–
55
68
65
83
75
–
63
81
72
83
56
85
59
75
77
80
61
75
84
63
61
19
29
91
3
–
9
93
93
9
91
47
87
57
73
3
91
81
71
93
71
10
11
12
13
14
15
16
17
18
19
20
21
22
r.t.
60
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
Cl
Br
I
THF
[a]
Isolated yields.
[b]
Enantiomeric ratio determined by HPLC equipped with a chiral stationary phase.
room temperature for 2 h. The solvent was removed under
reduced pressure, and glacial acetic acid (20 mL) was added
into the resultant residue. The reaction solution was cooled
in an ice-bath, and sodium borohydride (1.20 g, 32.0 mmol)
was added. After being stirred at room temperature for 2 h,
a saturated sodium bicarbonate solution was added into the
reaction solution until it became basic, which was then ex-
tracted with ethyl acetate. The combined extracts were
dried over anhydrous magnesium sulfate, filtered and the
solvent was removed under reduced pressure to yield a
crude product which was purified by flash column chroma-
tography on silica gel using ethyl acetate-hexane (1:4, 1:2)
as the mobile phase to afford the pure compound 5a; yield:
1.00 g (1.8 mmol, 56%); mp 2238C; [a]2D5: +308 (c 1.00,
1576, 1432, 1383, 1242, 1164, 1098, 1082, 1049, 1027, 846,
783, 746 cmÀ1; MS-FAB: m/z=556 ([MÀH2O]+, 100), 512.4
(23); HR-MS: m/z=556.3141 (M+ÀOH), calcd. for
C36H40BN3O3 (M+): 573.3163.
Synthesis of 6a
To a solution of 5a (57.4 mg, 0.1 mmol) in methanol (5 mL)
was added 1N HCl (5 mL), and stirred at room temperature
for 2 h. Then methanol was removed, and a saturated
sodium bicarbonate solution (5 mL) was added to the resi-
due until it became basic. The resulting aqueous solution
was extracted by diethyl ether (5 mL ꢂ 3), and the com-
bined extracts were dried over anhydrous magnesium sul-
fate. The solvent was removed under reduced pressure to
yield a crude product which was purified by flash column
chromatography on silica gel using ethyl acetate-hexane
(1:4, 1:2) as the mobile phase to afford the pure compound
1
CH2Cl2). H NMR (300 MHz, CDCl3): d=8.64–8.6 2 (d, J=
6.0 Hz, 1H), 8.40–8.38 (d, J=9.0 Hz, 1H), 8.14–8.11 (d, J=
9.0 Hz, 1H), 8.68–8.65 (t, J=9.0 Hz, 1H), 7.68–7.65 (d, J=
9.0 Hz, 1H), 7.28 (m, 2H), 7.23–7.07 (m, 5H), 3.97 (s, 1H),
3.76 (s, 1H), 3.10 (s, 1H), 2.87–2.83 (m, 1H), 2.67–2.65 (t,
J=6.0 Hz, 1H), 2.65–2.63 (m, 1H), 2.55–2.47 (m, 1H), 2.19–
2.04 (m, 1H), 1.69–1.68 (d, J=3.0 Hz, 1H), 1.31–1.28 (m,
2H), 1.26 (s, 3H), 1.10–1.07 (d, J=9.0 Hz, 3H), 0.95–0.92
(d, J=9.0 Hz, 3H), 0.72 (s, 3H); 13C NMR (125.7 MHz,
CDCl3): d=160.5, 149.2, 148.6, 145.6, 142.2, 137.7, 134.5,
127.6, 126.8, 126.5, 126.0, 125,9, 123.4, 121.2, 120.1, 84.0,
76.6, 71.6, 50,0, 47.3, 46.2, 44.3, 42.3, 34.3, 32.5, 28.7, 26.6,
23.8, 26.6, 23.8, 21.8, 15.6; IR (KBr): n=3353, 3063, 2938,
6a; yield: 50.0 mg (0.09 mmol, 90%); mp: 90–928C [a]D23:0
:
+3.08 (c 1.00, CHCl3). 1H NMR (400 MHz, CDCl3): d=
8.65–8.64 (d, J=4.0 Hz, 1H), 8.46–8.44 (d, J=8.0 Hz, 1H),
8.17–8.15 (d, J=8.0 Hz, 1H), 7.83–7.80 (t, J=6.0 Hz, 1H),
7.69–7.67 (d, J=8.0 Hz, 2H), 7.56–7.54 (d, J=8.0 Hz, 2H),
7.36–7.34 (d, J=8.0 Hz, 1H), 7.31–7.26 (m, 5H), 7.18–7.14
(m, 2H), 2.78 (s, 1H), 2.74–2.72 (t, J=4.0 Hz, 1H), 2.67–
2.62 (m, 2H), 2.29–2.20 (m, 1H), 2.17–2.14 (t, J=6.0 Hz,
1H), 2.09–2.04 (m, 1H), 1.79–1.69 (m, 1H), 1.44 (s, 3H),
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A Spirocyclic Chiral Borate for Catalytic Enantioselective Nozaki-Hiyama Allylation
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Table 3. Yields and enantioselectivities for the asymmetric allylation of alkyl and aryl ketones with Cr(II)-5a.
Entry
Yield [%]
ee [%][a]
73 (R)
[a] (T8C, c CHCl3)
+34.1 (27.2, 0.86)
+27.8 (25.0, 0.69)
1
2
79
82
57 (R)[c]
3
82
41 (na)
+12.3 (27.2, 0.9)
4
5
83
79
93 (na)
55 (R)
+44.8 (27.2, 1.02)
+43.9 (27.2, 1.23)
6
74
97 (na)[b]
+23.0 (30.4, 1.27)
7
87
82
82
73
70
75
73
89 (R)
+34.1 (30.4, 1.03)
+35.6 (30.4, 1.48)
+28.8 (30.4, 1.43)
+41.6 (30.4, 1.18)
À38.0 (24.6, 0.25)
+12.2 (24.7, 0.45)
+9.4 (24.8, 0.50)
8
81 (R)
9
81 (na)[b]
82 (R)[c]
93 (S)
10
11
12
13
37 (na)
27 (na)
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Table 3. (Continued)
Entry
Yield [%]
75
ee [%][a]
73 (R)[b]
[a] (T8C, c CHCl3)
+21.2 (28.3, 0.52)
14
15
76
60 (R)[b]
+13.3 (28.6, 0.61)
16
17
18
68
75
77
75 (S)
73 (S)
77 (R)
À22.0 (24.6, 0.25)
À10.0 (24.8, 0.26)
+41.4 (24.6, 0.7)
19
82
79 (R)
+43.5 (24.6, 1.22)
[a]
The ee % was determined with a Chiracel OD-H HPLC column and the absolute configuration was assigned based on a
comparison with the literature data.
The ee % was determined by converting the homoallyl alcohols to diols.
The ee % was determined using a Chiracel OJ HPLC column.
[b]
[c]
1.07–1.06 (d, J=4.0 Hz, 3H), 0.78–0.77 (d, J=4.0 Hz, 3H),
analysis using a chiral column (Chiralcel OD-H or OJ
column, flow rate 0.25 mLminÀ1).
0.68 (s, 3H); MS-FAB: m/z=547.00 ([M]+, 29.86), HR-MS:
m/z=547.3196, calcd. for C36H41N3O2 (M+): 547.3199.
Procedure for Catalytic Enantioselective Nozaki–
Hiyama Allylation
Acknowledgements
Anhydrous THF (2 mL) was added to CrBr3 (7.3 mg, 25.0
mmol) and Mn (42.0 mg, 750 mmol) in an inert atmosphere
box, and the mixture was stirred for 1 h at room tempera-
ture. After ligand 5a (14.0 mg, 24.4 mmol) and anhydrous
NEt3 (7.0 mL, 5.0 mg, 50 mmol) had been added, the suspen-
sion was stirred at room temperature for 1 hour. Finally,
allyl bromide was added (65 mL, 90.9 mg, 750 mmol). After
1 h at room temperature, ketone (0.5 mmol) and Me3SiCl
(93 mL, 81.5 mg, 750 mmol) were added, and the suspension
was stirred at room temperature for 20 h. Saturated aqueous
NaHCO3 was added. Following filtration and evaporation,
We thank Dr. Sankareswaran Srimurugan for his helpful dis-
cussions (Postdoctoral fellowship awarded by National Sci-
ence Council of the Republic of China, under Contract NSC
96-2811M-259-001).The authors thank Ms. L. M. Hsu, at the
Instruments Center, National Chung Hsing University, for
her help in obtaining HR-MS, and the National Science
Council of the Republic of China, for financially supporting
this research under Contract NSC 97-2113M-259-002-MY3.
the aqueous phase was extracted with Et2O. After evapora- References
tion of the combined organic phases, the residue was dis-
solved in THF (2 mL). A TBAF solution (1.5 mL, 1.5 mmol,
1M in THF) was added and the mixture was stirred until de-
silylation was complete (as verified by TLC). Saturated
aqueous NaHCO3 was added, and the solution was extracted
using CH2Cl2, and the combined organic phases were dried
over Na2SO4. Evaporation of the solvent and flash chroma-
tography (hexane/EtOAc 4:1) gave (R)-1-phenyl-3-buten-1-
ol as a pale-yellow oil; yield: 26.7 mg (180 mmol, 72%, 90%
ee). The enantiomeric excess was determined by HPLC
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A Spirocyclic Chiral Borate for Catalytic Enantioselective Nozaki-Hiyama Allylation
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