Synergistic effect of achiral quaternary ammonium salt on asymmetric additions of diethylzinc to aldehydes

Article abstract of DOI:10.1016/j.tetlet.2013.03.066

Our investigation has shown that achiral quaternary ammonium salt has a significant synergistic effect on the asymmetric additions of diethylzinc to aldehydes catalyzed by chiral phosphoramide-Zn(II) complex. The addition of 10 mol % NBu₄X can dramatically reduce the loading amount of chiral ligand without sacrificing the excellent reactivity and enantioselectivity of the asymmetric reaction.

Full text of DOI:10.1016/j.tetlet.2013.03.066

Tetrahedron Letters 54 (2013) 2722–2725
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synergistic effect of achiral quaternary ammonium salt on asymmetric
additions of diethylzinc to aldehydes
⇑
Hua Zong, Huayin Huang, Guangling Bian, Ling Song
The State Key Lab of Structural Chemistry, The Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Our investigation has shown that achiral quaternary ammonium salt has a significant synergistic effect
on the asymmetric additions of diethylzinc to aldehydes catalyzed by chiral phosphoramide–Zn(II) com-
Received 11 January 2013
Revised 7 March 2013
Accepted 15 March 2013
Available online 23 March 2013
plex. The addition of 10 mol % NBu
out sacrificing the excellent reactivity and enantioselectivity of the asymmetric reaction.
Ó 2013 Elsevier Ltd. All rights reserved.
4
X can dramatically reduce the loading amount of chiral ligand with-
Keywords:
Synergistic effect
Quaternary ammonium salt
Asymmetric addition
Introduction
(<1 mol %) are still rare.10 Thus, in terms of catalyst loading, signif-
icant progress is still desirable.
The demand of using asymmetric catalysis in the production of
chiral compounds has been driving the development of excellent
chiral catalytic systems to be the focus of intensive research in
Successful examples have been reported using quaternary
ammonium salts as stoichiometric or catalytic additives in asym-
1
1
metric reactions which involve Pd, Ru, Ir etc. but no report of using
an achiral quaternary ammonium salt as an additive in the asym-
metric addition of organozinc to a carbonyl group has been found de-
spite the fact that it has been used to promote the non-asymmetric
1
asymmetric chemistry. Enantioselective catalytic transformations
which involve carbon–carbon bond formation are probably one of
the most attractive asymmetric reactions. Concerning this subject,
the asymmetric additions of organometallic reagents, particularly
organozinc reagent, to carbonyl groups have been extensively
1
2
addition of organozinc to aldehyde. We have been interested in
investigating the effects of achiral quaternary ammonium salts on
2
13
studied. Various chiral catalysts have been developed which lead
the addition reactions of carbonyl compounds. Herein, we report
to alcohol products with excellent enantiomeric excesses in high
yields for the asymmetric additions of organozinc reagents to alde-
hydes and ketones. Although chiral catalyst is the key factor for an
a highly efficient catalytic system for the asymmetric additions of
dialkylzinc to aldehydes using an achiral quaternary ammonium salt
and a chiral phosphoramide as cocatalysts. The significant synergis-
tic effect of achiral quaternary ammonium salt and chiral phospho-
ramide results in the dramatic decrease of the loading amount of
the chiral catalyst needed for the asymmetric reaction to achieve
high efficiency.
3
efficient asymmetric reaction, additive is also important for the
catalytic turnover and enantioselectivity. Many achiral additives,
4
5
such as metal Lewis acids (Al, Ti, Cu, Ba etc.), polyethers (DiMPEG
6
7
etc.), alcohols (MeOH, iPrOH etc.), borate ester, phosphine
8
9
amine, and phosphine oxides etc. have also been reported to im-
prove the reactivity and enantioselectivity of asymmetric additions
of organozinc to carbonyl groups. However, for most of these
developed chiral catalytic systems, high loading amounts of chiral
catalysts over 1 mol %, normally in the range of 10–20 mol %, are
required to achieve synthetically useful results. Highly efficient
catalytic systems for the asymmetric additions of organozinc with
carbonyl groups giving excellent yields and enantioselectivities of
addition products with very low chiral catalyst loadings
Results and discussion
In our previous study, we developed a new chiral 1,2-amino
phosphoramide L1 derived from (1R,2R)-1,2-cyclohexyl diamine
(Scheme 1), which catalyzed the asymmetric addition of diethyl-
zinc to benzylaldehyde to afford 99% yield and 94% ee value of
addition product 2a, but at least 10 mol % of L1 was needed for
the reaction to achieve such high efficiency.
1
4
Reducing the chiral catalyst loading amount dramatically de-
creased both the yield and ee value of 2a. However, we have found
that this chiral phosphoramide catalyzed asymmetric reaction can
be benefited by adding achiral quaternary ammonium salts. As can
⇑
Corresponding author. Tel.: +86 591 83720913; fax: +86 591 83722697.
E-mail address: songling@fjirsm.ac.cn (L. Song).
0
040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.066

H. Zong et al. / Tetrahedron Letters 54 (2013) 2722–2725
2723
Table 2
Effects of different achiral quaternary ammonium salts on the asymmetric catalytic
addition of ethylzinc to benzylaldehyde
O
a
Ph P NH HN i-Pr
2
L1 (1.0 mol%)
O
L1
OH
Additive (y mol%)
+
Et Zn
2
Ph
H
hexane: toluene = 1:1
1
a
Ph
Scheme 1. The chiral ligand L1 derived from (1R,2R)-1,2-cyclohexyl diamine.
2.0 equiv
o
0
C ~ r.t., 6 h
1
.0 equiv
2a
Entry
Additive
None
y (mol %)
Yieldb (%)
eec (%)
Table 1
The effects of NBu
1
2
3
4
5
6
7
8
9
None
10
10
10
10
10
10
10
10
10
10
10
5
52
86
77
96
96
97
96
95
62
93
62
23
87
97
92
88
84
78
58
87
92
92
90
92
94
94
89
92
<1
28
94
88
76
59
47
16
Br on the asymmetric additions of diethylzinc to benzylaldehydesa
4
3 4
N(CH ) Cl
L1 (x mol%)
C
18
H37N(CH
3 3
) Cl
O
NEt
NEt
4
Cl
Bu NBr (10 mol%)
OH
4
+
Et Zn
3
BnCl
Ph
H
2
hexane: toluene = 1:1
1
a
2.0 equiv
o
Ph
NBu
NBu
NBu
NBu
4
Cl
Br
I
0
C ~ r.t., 6 h
1
.0 equiv
2a
4
4
4
Entry
x (mol %)
With Bu
4
NBr (without)
eec (%)
ClO
4
1
1
0
1
Bu
ZnCl
MgBr
4
PBr
Yieldb (%)
2
1
2
3
4
5
6
7
8
10
99 (99)
98 (81)
98 (65)
96 (52)
95 (32)
92 (27)
84, 90d (18)
40 (16)
96 (94)
95 (92)
95 (72)
94 (58)
93 (38)
93 (34)
12
13
14
15
16
17
18
2
5.0
3.0
1.0
0.5
0.3
0.1
NBu
NBu
NBu
NBu
NBu
NBu
4
4
4
4
4
4
Br
Br
Br
Br
Br
Br
20
30
50
70
100
90, 92d (30)
76 (11)
0.05
a
b
c
The reactions were carried out at 1.0 mmol scales and repeated three times.
Isolated yields.
Determined by chiral GC analysis.
a
The reactions were carried out at 1.0 mmol scales and repeated three times for
each.
b
Isolated yields.
Determined by chiral GC analysis.
Reaction time is 24 h.
c
d
slightly higher yield and ee value. In contrast, 2a was obtained in
dramatically lower yield along with decreased enantioselectivity
when perchlorate anion was the counter anion (Table 2, entry 9).
Interestingly, phosphonium salt also shows the similar positive ef-
fect as ammonium salt. Using Bu PBr as the additive gave 2a with
93% yield and 92% ee values (Table 2, entry 10). It is noteworthy
that using a metal Lewis acid as the additive has negative effect
for the asymmetric addition reaction. The addition of 10 mol %
ZnCl or MgBr dramatically decreased the ee value of the desired
be seen from viewing the data in Table 1, the synergistic effect of
Bu NBr on this asymmetric reaction increases significantly with
the decrease of the loading amount of L1. In the presence of
0 mol % L1, the ee value of 2a was improved slightly from 94%
without Bu NBr to 96% with Bu NBr (Table 1, entry 1). However,
4
4
1
4
4
compared with only 32% yield and 38% ee value of 2a obtained
in the presence of 0.5 mol % L1 alone, the yield and ee value of
2
2
2
1
a were dramatically enhanced to 95% and 93%, respectively, using
0 mol % Bu NBr as an additive (Table 1, entry 5), which is compa-
product 2a with only low to medium yield (Table 2, entries 11 and
4
12). Further investigation of the loading amount of Bu NBr in the
4
rable with the use of 10 mol % L1 in reaction. Further decreasing
the loading amount of L1 to 0.1 mol % still gave us 87% yield and
reaction showed that changing the amount of Bu NBr from
10 mol % to 5 mol % decreased the yield of 2a from 96% to 87%
4
9
0% ee value of 2a for 6 h and 90% yield and 92% ee value of 2a
for 24 h when 10 mol % NBu Br was present, while 0.1 mol % L1
used alone only afforded 18% yield and 30% ee value of 2a (Table
, entry 7). Even for only 0.05 mol % of L1 used in the asymmetric
but still gave excellent enantioselectivity of 94% (Table 2, entry
13). Owing to the background reaction caused by Bu NBr,
4
12
4
increasing the amount of Bu NBr resulted in the decrease of the
4
1
activity and enantioselectivity of the asymmetric addition reaction
reaction, the synergistic effect of Bu
ough as shown in Table 1, entry 8.
4
NBr was still significant en-
(Table 2, entries 14–18).
Because halide effect in metal catalyzed reactions has been well
recognized by the coordination of halide anion to metal,1
3b,15
we
Encouraged by our initial study, we next evaluated the effec-
tiveness of various achiral quaternary ammonium salts on the
asymmetric addition of diethylzinc to benzylaldehyde catalyzed
by 1.0 mol % L1 (Table 2). As shown in Table 2, entries 2–6, the al-
kyl chain length plays an important role for the effect of a tetraal-
proposed a plausible transition state for NBu X/chiral L1 co-cata-
lyzed asymmetric addition reaction on the basis of Ishihara’s con-
4
jugate Lewis acid–Lewis base double activated transition state
1
6
mode (Scheme 2). Without NBu X, the asymmetric reaction is
4
kylammonium salt. (CH
3
)
4
NCl containing the shortest alkyl chain
catalyzed by chiral L1–zinc(II) complex via transition state A. The
has the least effect for the asymmetric addition. The less solubility
in hydrocarbon solvent, which results in a dilute solution of chlo-
ride anions, and the less steric hindrance of the short alkyl chain
chiral L1–zinc(II) complex serves as a Lewis acid while concomi-
tantly increasing the nucleophilicity of diethylzinc by P@O moiety
of L1 and Re-face attack leading to (R)-product should be highly
preferred without steric repulsion between the i-Pr group of chiral
of (CH
yield and 87% ee value of 2a. However, tetraalkylammonium salts
with a medium alkyl chain length, such as Et NCl, Et NBnCl, and
NCl gave excellent yields and enantioselectivities of 2a and
NCl is the best one among them. In contrast, the bulky long ali-
37N(CH Cl slowed down the addition
3 4 3 4
) NCl might be the reasons for (CH ) NCl to give only 86%
1
6
metallic complex and the R group of aldehyde. In the presence of
4
3
NBu X, halide anion can coordinate with diethylzinc to form com-
4
1
2b,17
Bu
Bu
4
plex.
Therefore, the nucleophilicity of diethylzinc is increased
4
by not only P@O moiety of L1, but also the coordinated halide an-
ion. Thus, transition states B and C are possible. As shown in Table
2, entries 5–7, the differences in product yields and enantioselec-
tivities with different halide anions (Cl, Br and I) are slight. Further-
more, the differences in product enantioselectivities are also not
obvious between perchlorate anion and halide anions, but the dif-
phatic carbon chain of C18
H
3 3
)
reaction providing 2a with 77% yield only. No significant differ-
ences in product yields and enantioselectivities were observed in
the presence of tetraalkylammonium salts with different halide
4
ions (Table 2, entries 6–9), but the reaction with Bu NBr gave a

2
724
H. Zong et al. / Tetrahedron Letters 54 (2013) 2722–2725
4
Scheme 2. The proposed transition states for NBu X/chiral L1 co-catalyzed asymmetric addition of diethylzinc to aldehyde.
Conclusion
We have demonstrated that the significant synergistic effect of
achiral quaternary ammonium salt on chiral phosphoramide cata-
lyzed asymmetric additions of diethylzinc to aldehydes. Various
Table 3
4
Enantioselective addition of diethylzinc to aldehydes employing L1 and Bu NBr
L1 (0.5 mol%)
O
Bu NBr (10 mol%)
OH
4
+
2
Et Zn
.0 equiv
2
R
H
hexane: toluene = 1:1
R
o
achiral quaternary ammonium salts are effective, but Bu
4
NBr is
1
.0 equiv
0
C ~ r.t., 6 h
2a-j
1a-j
the best. In the presence of 10 mol % Bu NBr, only 0.5 mol % chiral
4
phosphoramide is enough for the asymmetric reaction to achieve
the same high efficiency as 10 mol % chiral phosphoramide used
alone.
Entry
Product, R =
With Bu
4
NBr (without)
eeb (%)
Yielda (%)
1
2
3
4
5
6
7
8
9
Ph
2a
2b
2c
2d
2e
2f
2g
2h
2i
95 (32)
89 (29)
98 (25)
99 (19)
99 (31)
99 (55)
94 (10)
94 (23)
98 (23)
79 (31)
93 (38)
91 (77)
91 (60)
90 (9)
96 (77)
93 (81)
90 (21)
89 (67)
93 (55)
94 (89)
4-MeO-C
4-Cl-C
4-CF -C
2-Me-C
6 4
H
H
4
Experimental
6
3
6
H
4
6
H
4
General procedure for enantioselective additions of diethylzinc
to aldehydes
2- MeO-C
1-Naph
6 4
H
2-Naph
2-Thienyl
Cyclohexyl
Method A (without additive)
1
0
2j
a
Isolated yields.
Determined by chiral GC analysis.
O
L1 (10 mol%)
b
OH
+
Et Zn
2
hexane: toluene = 1:1
R
H
2.0 equiv
o
R
0
C ~ r.t., 6 h
1
.0 equiv
ferences in product yields are significant (Table 2, entries 5–8).
These results indicate that the halide anion (Cl, Br and I) coordi-
nated to diethylzinc does not further bridge the chiral zinc center
to cause the change of the Zn(II) coordination environment in the
To a solution of chiral ligand L1 (35.6 mg, 0.10 mmol, 10 mol %)
in dry toluene (2.0 mL), Et Zn (1.0 M in hexane, 2.0 mL, 2.0 mmol,
.0 equiv) was added dropwise at 0 °C via a syringe under nitrogen
atmosphere. After being stirred for 30 min, aldehyde (1a, 106 mg,
01.4 L, 1.0 mmol) was added dropwise to the solution at 0 °C
2
2
4
chiral L1–zinc(II) complex. Therefore, the addition of NBu X can
improve the catalytic turnover by enhancing the ethyl group trans-
fer from diethylzinc to aldehyde, but has little influence on the
enantioselectivity of alcohol product, which should be determined
only by the chiral ligand L1. Thus, the transition state B is excluded
and the transition state C is preferred. Additional evidence for the
role of halide anion in the asymmetric reaction can be given by the
fact that the synergistic effect of achiral quaternary ammonium
salt is weak with high loading amount of chiral L1, but is improved
dramatically by lowering the catalyst loading amount (Table 1).
The general applicability of this catalytic system, probed by uti-
lizing a variety of aldehydes was demonstrated by the data dis-
1
l
via a syringe. The mixture was stirred for 6 h at room temperature
and quenched by addition of aqueous HCl (1.0 M, 5 mL). Extraction
with EtOAc (10 mL Â 3) gave combined organic layers that were
4
washed with brine (10 mL), dried over MgSO , and concentrated
in vacuo to give a residue that was subjected to silica gel column
chromatography (EtOAc/hexane = 1/10), which afforded the corre-
sponding enantio-enriched secondary alcohol (2a) as a colorless
oil. The ee value of the secondary alcohol was determined by Chiral
GC. The absolute configuration of the alcohol was assigned as (R)
by comparison of the optical rotation with reported data.
4
played in Table 3. Thus, by using 10 mol % NBu Br and 0.5 mol %
L1 as co-catalysts, asymmetric addition of diethylzinc to diverse
aldehydes (aryl or alkyl, electron withdrawing or donating group
substituted, and steric hindered) occurred in excellent yields and
ee values. In contrast, the reactions carried out in the absence of
Method B (with additive)
L1 (0.5 mol%)
4
O
Bu NBr (10 mol%)
OH
Bu
yields and ee values. Particularly, for aromatic aldehyde 1d contain-
ing an electron-withdrawing group CF at 4-position of the aromatic
ring, the synergistic effect of Bu NBr was very obvious. In the ab-
sence of Bu NBr, very poor yield and ee value of 2d were obtained,
but the yield and enantioselectivity of this process jumped to 99%
and 90%, respectively, when Bu NBr was present (Table 3, entry 3).
4
NBr had only normally poor to moderate efficiencies for both
+
Et2Zn
.0 equiv
R
H
hexane: toluene = 1:1
2
R
o
1
.0 equiv
0
C ~ r.t., 6 h
3
4
4
To a well-dried Schlenk tube charged with Bu
4
NBr (32.2 mg,
4
0.10 mmol, 10 mol %), solution of chiral ligand L1 (1 mL,
a

H. Zong et al. / Tetrahedron Letters 54 (2013) 2722–2725
2725
À2
0
.5 mol %, 0.5 Â 10 M in toluene) and dry toluene (1.0 mL) was
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