Chiral lithium salts of phosphoric acids as Lewis acid-base conjugate catalysts for the enantioselective cyanosilylation of ketones

Article abstract of DOI:10.1002/adsc.200800314

The catalytic enantioselective cyanosilylation of aromatic ketones was developed by using chiral lithium salts of (R)-BINOL- or (S)-BINAM-derived phosphoric acid compounds. In the presence of 10 mol% of chiral conjugate lithium salts, the corresponding tertiary cyanohydrins were obtained in high yields with moderate to high enantio-selectivities. This is the first efficient example of asymmetric catalysis using lithium salts of synthetically useful chiral phosphoric acid compounds. A possible catalytic mechanism and transition states are also discussed as a preliminary working hypothesis.

Full text of DOI:10.1002/adsc.200800314

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DOI: 10.1002/adsc.200800314
Chiral Lithium Salts of Phosphoric Acids as Lewis Acid–Base
Conjugate Catalysts for the Enantioselective Cyanosilylation of
Ketones
Manabu Hatano,a Takumi Ikeno,a Tokihiko Matsumura,a Shinobu Torii,a
and Kazuaki Ishiharaa,*
a
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan
Fax : (+81)-52-789-3331 or (+81)-52-789-3222; e-mail: ishihara@cc.nagoya-u.ac.jp
Received: May 20, 2008; Published online: July 9, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The catalytic enantioselective cyanosilyla-
tion of aromatic ketones was developed by using
chiral lithium salts of (R)-BINOL- or (S)-BINAM-
derived phosphoric acid compounds. In the pres-
ence of 10 mol% of chiral conjugate lithium salts,
the corresponding tertiary cyanohydrins were ob-
tained in high yields with moderate to high enantio-
selectivities. This is the first efficient example of
asymmetric catalysis using lithium salts of syntheti-
cally useful chiral phosphoric acid compounds. A
possible catalytic mechanism and transition states
are also discussed as a preliminary working hypoth-
esis.
chiral lithium binaphtholate as a Lewis base catalyst
where intermediates of hypervalent silicate might be
responsible.[14] As an extension of this work, we also
reported a highly efficient secondary cyanohydrin syn-
thesis using chiral lithium binaphtholate aqua and/or
alcohol complexes as Lewis base catalysts [Eq. (1)].[15]
Keywords: asymmetric catalysis; cyanohydrins; cya-
nosilylation; ketones; lithium; phosphoric acids
Unfortunately, however, this catalytic system could
not be applied to the synthesis of tertiary cyanohy-
Over the past decade considerable attention has been drins from ketones: the reactions could scarcely pro-
centered on the catalytic enantioselective synthesis of ceed at À788C in low yields, but higher temperatures
optically active tertiary cyanohydrins from ketones of up to À408C gave racemates or quite low enan-
and trimethylsilyl cyanide (TMSCN) or hydrogen cya- tioenriched products in high yields. We assumed that
nide.[1,2] Optically active tertiary cyanohydrins are syn- this low catalytic activity was due to then inadequate
thetically important building blocks that can be trans- Lewis basicity of the lithium binaphtholate, and thus
formed into not only natural products but also biolog- stronger Lewis basicity would be needed to activate
ically and pharmaceutically active compounds.[3] How- TMSCN to react with inactive ketones.
ever, although several outstanding catalysts have been
To overcome this difficulty in catalytic enantiose-
developed for the cyanation of aldehydes, the catalyt- lective tertiary cyanohydrin synthesis, we designed
ic enantioselective cyanation of ketones is still limit- simple chiral lithium salts of phosphoric acids,[16]
ed.[1,2,4–13] This problem is partially due to the extreme- which were derived from 2,2’-binaphthol (BINOL)
ly low reactivity associated with conspicuous steric (Figure 1). Lithium phosphate complexes (Figure 1b)
and electronic constraint between ketones and nucle- as Lewis acid–base catalysts[17] should be more effi-
ophiles, and the difficulty of controlling the enantiofa- cient than lithium binaphtholate complexes (Fig-
cial stereoselectivity of ketones.
ure 1a) for three reasons. (1) Based on our recent de-
As a pioneering work, Kagan has developed the ad- velopments in the organozinc addition to aldehydes
dition of TMSCN to aldehydes catalyzed by a dry and ketones using 3,3’-diphosphoryl-BINOLs and l-
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Chiral Lithium Salts of Phosphoric Acids as Lewis Acid–Base Conjugate Catalysts
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First, the cyanosilylation of acetophenone (1a) with
TMSCN (1 equiv. each) was examined in the presence
of chiral phosphoric acid 3a and n-BuLi (10 mol%
each) (Table 1). The reactions proceeded, albeit
slowly, at room temperature in dichloromethane or
THF (entries 1 and 2), while the desired product 2a
was obtained with low enantioselectivity in moderate
yields even at À408C in Et2O or toluene (entries 3
and 4).[25] The yield of 2a was improved to 82% with
36% ee under higher concentration (1.0M) conditions
in toluene (entry 5). As a lithium source in place of n-
BuLi, LiO-i-Pr was not effective (entry 6),[26] and
other alkaline metal salts such as NaO-t-Bu and KO-
Figure1. Design of chiral lithium phosphate complexes as
Lewis acid-base conjugate catalysts.
valine-derived phosphoramide as chiral ligands,[18] the t-Bu were also not effective (entries 7 and 8).
conjugate P=O moiety with Lewis acidic metal cen- Next, we examined the effect of 3,3’-disubstitution
ters can act as a Lewis base to activate nucleophiles in the (R)-BINOL-derived phosphoric acids (3)
such as TMSCN.[19,20] In general, the P=O moiety in (Table 2). The enantioselectivities of (S)-2a gradually
phosphoric acids [R2P(=O)OH] shows a relatively increased with the use of compounds 3b (R=Me)
high basicity[21] and, moreover, this basicity should in- and 3c (R=Br) (entries 2 and 3). Interestingly, com-
À À
crease through the ionic conjugation of Li O P=O in pound 3d (R=Ph) dramatically improved the enan-
the corresponding lithium salts [R2P(=O)OLi]. (2) tioselectivity of (R)-2a up to 86% ee with a change in
Since silicon compounds tend to form highly Lewis the absolute stereochemistry (entry 4). To achieve
acidic complexes with five or six valences,[22] ketones higher enantioselectivity, we examined other candi-
can be doubly activated by Si and Li centers. A fa- dates with large aromatic groups. However, 3e (R=1-
vored six-membered chelation ring may be formed in Naph), 3f (R=3,5-Me2C6H3), 3g (R=3,5-Mes2C6H3),
a lithium phosphate, in contrast to a less-favored four- 3h (R=4-PhC6H4), and 3i (R=2-i-PrOC6H4) gave
membered chelation ring in a lithium binaphtholate. (R)-2a with low enantioselectivities (0–31% ee) (en-
(3) Based on recent developments in the chemistry of tries 5–9).[27]
chiral phosphoric acids by Alper, Inanaga, Akiyama,
Under the optimized reaction conditions using 3d,
Terada, and others, it should be possible to design the cyanosilylation of other ketones was examined
BINOL-derived phosphoric acids with 3,3’-disubstitu- (Table 3). Aromatic ketones with an electron-with-
tion in the binaphthyl skeleton.[23,24]
drawing group gave the corresponding (R)-products
Table1. Enantioselective cyanosilylation of acetophenone (1a) catalyzed by an alkaline metal salt of chiral phosphoric acid
3a.
Entry
Metal source
Solvent
Temperature
Time [h]
Yield [%]
ee [%] (Configuration)
1
2
3
n-BuLi
n-BuLi
n-BuLi
n-BuLi
CH2Cl2
THF
Et2O
toluene
toluene
toluene
toluene
toluene
r.t.
r.t.
24
12
9
12
12
12
15
24
<3
54
45
48
82
49
13
28
–
10 (S)
26 (S)
31 (S)
36 (S)
13 (S)
26 (R)
7 (R)
À408C
À408C
À408C
À408C
08C
4
5[a]
6
n-BuLi
LiO-i-Pr
NaO-t-Bu
KO-t-Bu
7[b]
8[b]
r.t.
[a]
Concentration was 1.0M.
20 mol% of catalysts were used. t-BuOH was removed during the catalyst preparation.
[b]
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Table2. Effect of 3,3’-disubstitution of (R)-BINOL-derived
(2b–d) in high yields with moderate to good enantio-
selectivities (65–75% ee) (entries 2–4). For the rela-
tively less-reactive 2-acetonaphthone and o-, m-, and
p-methoxyacetophenone, the products were obtained
with up to 63% ee (entries 5–8). Unfortunately, ali-
phatic ketones such as benzylacetone gave almost rac-
emic products in low yields (entry 9).
chiral phosphoric acid.[a]
By taking advantage of the axially chiral binaphthyl
skeleton, we also prepared novel (S)-2,2’-binaphthyl-
diamine [(S)-BINAM]-derived phosphoric acid 4. Cy-
anosilylation of 1a using 10 mol% of the lithium salt
of (S)-4 proceeded in toluene at À408C for 6 h, and
(R)-2a was obtained in 83% yield with 68% ee [Eq.
(2)].
Entry
3 (R)
Yield [%]
ee [%]
(Configuration)
1
2
3
4
5
6
7
8
9
3a (H)
3b (Br)
3c (Me)
3d (Ph)
3e (1-Naph)
3f (3,5-Me2C6H3)
3g (3,5-Mes2C6H3)
3h (4-PhC6H4)
3i (2-i-PrOC6H4)
82
95
99
96
30
78
86
73
69
36 (S)
40 (S)
54 (S)
86 (R)
0
11 (R)
29 (R)
9 (R)
31 (R)
[a]
Reactions were examined at 1.0M toluene.
Finally, we examined the mechanistic aspects. Al-
though further investigation is necessary to obtain a
clear understanding, a postulated catalytic cycle is
shown in Figure 2 as a working model. An in situ-pre-
pared chiral lithium phosphate complex 5 activates
TMSCN and then ketone 1 to give Lewis acidic hy-
pervalent silicon intermediates 6 and 7.[14] An activat-
ed cyanide attacks a carbonyl moiety through a six-
membered cyclic transition state. In the transition
Table3. Catalytic enantioselective cyanosilylation of ketones
using lithium salt of 3d.
Entry 1 (R)
Prod
ACHTREUNG
AHCTREUNG
1[a]
2
1a (Ph)
2a
2b
2c
2d
96
99
99
95
59
91
94
96
38
86 (R)
75 (R)
65
68 (R)
32
37 (R)
63 (R)
55 (R)
2 (R)
1b (2-ClC6H4)
1c (3-ClC6H4)
1d (4-ClC6H4)
1e (2-MeOC6H4) 2e
1f (3-MeOC6H4) 2f
1g (4-MeOC6H4) 2g
3
4[a]
5[b]
6
7[a]
8
1h (2-Naph)
1i (Ph(CH2)2)
2h
2i
9
ACHTREUNG
[a]
[b]
Reaction time was 12 h.
Reaction was examined at 08C for 24 h.
Figure2. Postulated catalytic cycle with a Lewis acid–base
conjugate catalyst.
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Chiral Lithium Salts of Phosphoric Acids as Lewis Acid–Base Conjugate Catalysts
COMMUNICATIONS
Schlenk tube under a nitrogen atmosphere and dissolved in
dry toluene (0.75 mL). To the solution was added n-BuLi
(1.60M in hexane, 46.9 mL, 0.075 mmol, 10 mol%) at À788C
and the solution was stirred at that temperature for 30 min.
Acetophenone (1a, 90.1 mg, 0.75 mmol) was added and the
mixture was stirred at À788C for 30 min. Trimethylsilyl cya-
nide (0.10 mL, 0.75 mmol) was added dropwise at À788C,
and then the mixture was stirred at À408C for 12 h. The re-
sulting mixture was quenched with water at À408C. The or-
ganic layer was extracted with ethyl acetate, washed with
brine, dried over MgSO4, and evaporated under reduced
pressure. The crude product was purified by neutral silica
gel column chromatography (hexane/AcOEt=20/1) to give
the desired product (2a) as a colorless oil; yield: 157.8 mg
(96% yield). The enantiomeric purity was determined by
chiral GC (g-TA): 86% ee, (R).
Acknowledgements
Financial support for this project was provided by the JSPS.
KAKENHI (20245022), Grant-in-Aid for Young Scientists B
of MEXT (19750072), the G-COE Program of MEXT, and
Toray Science Foundation.
Figure3. Possible transition states.
states, the si-face attack via TS-1, which leads to (R)-
products, should be favored exclusively without con-
spicuous steric repulsion between the substrate and a
Ph group of 3d (Figure 3, TS-1 vs. TS-2). Eventually,
catalyst 5 would be regenerated via intermediate 8 ac-
companied by the release of product 2. In the case of
a chiral lithium salt of 4, similar transition states TS-3
and TS-4 were assumed (Figure 3). However, steric
repulsion would be observed even in the favored TS-
3, and therefore a slight decrease in enantioselectivity
was observed when 4 was used [Eq. (2)].
In summary, we have developed a catalytic enantio-
selective cyanosilylation of aromatic ketones using
chiral lithium salts of (R)-BINOL- or (S)-BINAM-de-
rived phosphoric acid compounds. In the presence of
10 mol% of chiral lithium catalyst, the corresponding
tertiary cyanohydrins were obtained in high yields with
moderate to high enantioselectivities (up to 86% ee).
This is the first successful example of asymmetric catal-
ysis using lithium salts of synthetically useful chiral
phosphoric acid compounds. Further studies on de-
tailed mechanistic aspects and the application of this
methodology to other catalyses are now underway.
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[26] The 2-propanol generated in situ from 3a and LiO-i-Pr
should change the aggregation of the catalyst and thus
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should be involved.
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Adv. Synth. Catal. 2008, 350, 1776 – 1780