Organocatalytic enantioselective dehydrogenative α-alkylation of aldehydes with benzylic compounds

Article abstract of DOI:10.1002/cjoc.201200850

The highly enantioselective cross-dehydrogenative coupling of aldehydes with benzylic compounds has been developed as an efficient and rapid protocol for synthesis of enantioriched α-alkylated aldehydes.

Full text of DOI:10.1002/cjoc.201200850

NOTE
DOI: 10.1002/cjoc.201200850
Organocatalytic Enantioselective Dehydrogenative
α-Alkylation of Aldehydes with Benzylic Compounds
Huang, Fenb,c(黄芬)
Xu, Lubina(徐鲁斌)
Xiao, Jian*,a,c(肖建)
a College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao,
Shandong 266109, China
b School of Chemistry and Chemical Engineering, University of South China, Hengyang, Hunan 421001, China
c Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China
The highly enantioselective cross-dehydrogenative coupling of aldehydes with benzylic compounds has been
developed as an efficient and rapid protocol for synthesis of enantioriched α-alkylated aldehydes.
Keywords organocatalytic, alkylation, aldehyde, benzylic, coupling
Introduction
zylic compounds when the C—O bond is changed to a
C—H bond (Scheme 1). The obvious advantage is that
the desired enantioriched α-alkylated aldehydes can be
directly produced from aldehydes with benzylic com-
pounds through an oxidative C—H functionalization
process. Although this transformation has been recently
studied by Cozzi and coworkers,[9] their system has the
drawback of high catalyst loading (20 mol% MacMillan
catalyst), low temperature (－25 ℃), moderate enantio-
selectivity (10%—86% ee) as well as tedious operation.
Thus the required harsh conditions and unsatisfactory ee
value limited the overall sustainability and applicability.
Recently, anodic oxidation was applied to the enantio-
selective coupling of aldehydes and xanthene by use of
20—50 mol% of the MacMillan catalyst and the prod-
ucts were also afforded in low enantioselectivity
(10%—68% ee).[10] Despite the obviously green and
atom-economic advantage, this reaction still remains a
formidable challenge and the highly enantioselective
version is rare.[11] As a continuation of our interest in
developing highly enantioselective organocatalytic reac-
tions,[7,12] herein we report the highly enantioselective
In recent years, transition-metal catalyzed direct
cross-dehydrogenative coupling (CDC) of two C—H
bonds has attracted considerable attention owing to its
green, low-cost, and environmentally friendly nature in
efficient construction of C—C bonds.[1] In this context,
much effort has been directed toward the enantioselec-
tive version of CDC reactions. However, such asym-
metric reactions remain a great challenge given the ab-
sence of binding sites in hydrocarbon substrates and the
incompatibility of chiral catalysts with oxidants.[2] Thus
the enantioselective oxidative C—C bond formation is
still in its infancy and needs extensive exploration.
The direct, catalytic, and asymmetric functionaliza-
tion of aldehydes in high stereoselectivity represents a
remarkble challenge in asymmetric catalysis.[3] For in-
stance, alkylation of aldehydes at the α position using
the organometallic methods often required preformed
metal enolates and stoichiometric amounts of chiral
auxiliaries, where many undesirable side reactions may
occur.[4] Moreover, the catalytic enantioselective inter-
molecular α-alkylation of aldehydes has been regarded
as “the Holy Grail” reaction in asymmetric aminocata-
lysis.[5,6] We recently reported the highly enantioselec-
tive intermolecular α-alkylation of aldehydes with al-
cohols by a diarylprolinol silyl ether under Brønsted
acid, Lewis acid or acid-free conditions.[7] Encouraged
by this result and the recent reports on the highly enan-
tioselective β-functionalization of simple aldehydes,[8]
we envisage that coorporation of diarylprolinol silyl
ether catalyst with an suitable oxidant may offer an effi-
cient catalytic system to effect a highly enantioselective
cross-dehydrogenative coupling of aldehydes with ben-
Scheme 1 Enantioselective cross-dehydrogenative coupling of
aldehydes with benzylic compounds
O
O
R2
OH
R2
chiral catalyst
- H2O
H
H
+
+
*
H
H
R2
H
R2
Ref. [11]
R1
R1
90% 99% ee
O
O
R2
H
chiral catalyst
present work
R2
*
R2
R2
H
oxidant
- 2H
R1
R1
mostly >99% ee
*
E-mail: xj9698@163.com; Tel.: 0086-532-88030147
Received August 20, 2012; accepted September 25, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200850 or from the author.
Chin. J. Chem. 2012, 30, 2721—2725 © 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2721

Huang, Xu & Xiao
NOTE
cross-dehydrogenative coupling of aldehydes with ben-
zylic compounds by cooperative catalysis of diaryl-
prolinol silyl ether with an oxidant, leading to a variety
of enantioriched α-alkylated aldehydes in high optical
purity at room temperature under very mild conditions.
3H); 13C NMR (CDCl3, 100 MHz) δ: 153.4, 153.1,
129.7, 128.75, 127.7, 127.5, 125.1, 123.3, 122.9, 122.5,
116.3, 116.2, 64.9, 45.1, 40.2, 12.0; HRMS (ESI) calcd
for C16H17O2 241.1229 [M＋H]＋, found 241.1230 [M＋
H]＋.
Experimental
Results and Discussion
Unless otherwise noted, commercial reagents were
used as received and all reactions were carried out di-
rectly in air atmosphere. Flash chromatography was
performed using Merck silica gel 60 with freshly dis-
tilled solvents. Columns were typically packed as slurry
and equilibrated with the appropriate solvent system
prior to use. Infrared spectra were recorded on a
Bio-Rad FTS 165 FTIR spectrometer. The oil samples
were examined under neat conditons. High Resolution
Mass (HRMS) spectra were obtained using Finnigan
MAT95XP GC/HRMS (Thermo Electron Corporation).
Proton nuclear magnetic resonance spectra (1H NMR)
were recorded on a Bruker Avance DPX 300 and
Bruker AMX 400 spectrophotometer (CDCl3 as solvent).
Enantioselectivities were determined by HPLC analysis
employing a Daicel Chiracel column at 25 ℃. Optical
rotation was measured using a JASCO P-1030 Po-
larimeter equipped with a sodium vapor lamp at 589 nm.
Concentration is denoted as c and was calculated as
grams per deciliters (g/100 mL) using chloroform as
solvent. Absolute configuration of the products was de-
termined by comparison with known compounds.
The reaction of butyraldehyde 1a and xanthene 2a
was selected as the model reaction to define the suitable
oxidant in the presence of 10 mol% diarylprolinol silyl
ether I.[13] Our preliminary results showed that the com-
monly used metal oxidants such as Fe(III) and Cu(II)
salts as well as IBX cannot afford satisfactory results
(Table 1, Entries 1—4). When O2 was employed as oxi-
dant, only trace of product was found in the presence of
catalyst I, however, catalyst II can afford 58% yield,
abeit no enantioselectivity was observed (Table 1, En-
tries 5—6). Inspired by the recent success on DDQ
(2,3-dichloro-5,6-dicyano-1,4-benzoquinone) as an
oxidant in implementing oxidative cross coupling reac-
tions,[14] we tried DDQ as oxidant to perform this reac-
tion. To our delight, this reaction proceeded smoothly to
give the desired product 3a in moderate yield and mod-
erate ee in CH2Cl2 (Table 1, Entry 7). This encouraged
us to investigate the other solvents. Gratifyingly, 42%
yield and 94% ee was given when nitromethane was
used as solvent (Table 1, Entry 8). The best result was
obtained in CHCl3, affording 62% yield and 92% ee
(Table 1, Entry 9). However, other solvents such as tol-
uene and THF were not effective and low chemical
yields were obtained (Table 1, Entries 10—11). The
yield becomes lower when the amount of DDQ was in-
creased to 1.5 equiv. (Table 1, Entry 12). Performing
this reaction at 0 ℃ also resulted in lower yield (Table
1, Entry 13). Addition of an acid to the mixture was not
beneficial for this reaction (Table 1, Entry 14). In sharp
contrast, catalyst II without any trifluoromethyl group
only gave 27% yield and 57% ee, indicative of the
importance of the trifluoromethyl group in catalyst I in
terms of reactivity and enantioselectivity (Table 1, Entry
15). In addition, when the MacMillan catalyst III was
used, 55% yield and 70% ee was obtained (Table 1, En-
try 16).
A typical reaction procedure is as follows: To a 5
mL vial equipped with a magnetic stirring bar, were
added CHCl3 (2 mL) and benzylic compound 2a (0.2
mmol). Then catalyst I (0.02 mmol) and DDQ (0.24
mmol) were added before addition of aldehyde 1 (0.8
mmol). The resulting solution was stirred at room tem-
perature for 12 h. Then the mixture was filtered through
celite to remove any precipitate. The filtration was
evacuated and the resulting mixture was dissolved in
EtOH (5 mL). NaBH4 was then cautiously added to the
solution. The mixture was stirred at room temperature
for 0.5 h and was quenched with 1 mol/L HCl. The or-
ganic phase was separated and the aqueous solution was
extracted with ethyl acetate (10 mL×3). The combined
organic phases were washed with brine and dried over
anhydrous MgSO4. The solvent was removed under re-
duced pressure and the resulting oil was purified by
preparative chromatography or column chromatography
(hexane/ethyl acetate＝4∶1) to afford the desired prod-
uct 3. The enantiomeric excess was determined by
HPLC using chiral AD-H or AS-H columns. The abso-
lute configuration of the products was determined by
optical rotation in comparison with the literature re-
ported values.[7,9,10] (R)-2-(9H-xanthen-9-yl)propan-1-
With the optimized conditions in hand, we next ex-
amined the substrate scope using various aldehydes and
xanthene 2a (Table 2, Entries 1—8). Satisfyingly, in all
cases the desired products were obtained in moderate
yields with good to excellent enantioselectivities (Table
2, Entries 1—8), making this methodology highly valu-
able considering the biochemical and pharmaceutical
importance of xanthene derivatives.[15] Propionaldehyde
reacted to give 52% yield and 88% ee (Table 2, Entry 1).
Bulky aldehydes such as isovaleraldehyde is reactive
enough to provide product 3h, albeit with lower yield
and lower enantioselectivity (Table 2, Entry 8). For sub-
strate 9H-thioxanthene 2b, lower enantioselectivities
were observed, propionaldehyde and butylaldehyde
1
ol (3a): yellow oil; H NMR (CDCl3, 500 MHz) δ:
7.26—7.22 (m, 4H), 7.10—7.07 (m, 4H), 4.22 (d, J＝
4.5 Hz, 1H), 3.53—3.46 (m, 1H), 3.45—3.43 (m, 1H),
2.01—1.98 (m, 1H), 1.25 (br s , 1H), 0.64 (d, J＝7.0 Hz,
2722
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 2721—2725

Organocatalytic Enantioselective Dehydrogenative α-Alkylation of Aldehydes
Table 1 Optimizations for enantioselective cross-dehydrogen-
A plausible catalytic cycle is given in Scheme 2.
Condensation of aldehyde 1 with catalyst I gives
(E)-enamine 4. Concurrently, the benzylic compound 2
was oxidized in situ by DDQ to form carbocation 5.
Subsequent nucleophilic attack on the carbocation 5 by
enamine 4 from the Si face affords the intermediate 6
since the Re face was efficiently shielded by the bulky
silyl ether group. Hydrolysis of 6 afforded product 3
with the regeneration of catalyst I.
ative coupling of aldehyde 1a with 2a
F3C
CF3
Me
O
Me
Me
Me
N
OTMS
OTMS
N
N
N
H
H
CF3
H
Ph
F3C
+
III
I
II
Scheme
2
Plausible catalytic cycle for enantioselective
O
cross-dehydrogenative coupling of aldehydes with benzylic
compounds
10 mol% catalyst
NaBH4
EtOH
H
1.2 equiv. oxidant
solvent
O
2a
1a
R1
CHO
OH
R2
R2
R
N
H
3
O
H2O
I
O
3a
H
R1
1
Entrya
Oxidant
CAN
FeCl3
Solvent Catalyst
Yieldb/% eec/%
+
R
N
R
N
R1
R2
1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeNO2
CHCl3
Toluene
THF
I
I
Trace
Trace
Trace
Trace
Trace
58
—
—
—
—
—
0
Si face attack
4
2
R1
R2
6
H
3
Cu(OAc)2
IBX
I
R = -CAr2OSiMe3
Ar = 3,5-(CF3)2-C6H3
+
DDQ
R2
R2
R2
R2
4
I
5
2
5d
6d
7
O2
I
O2
II
I
Table 2 Enantioselective cross-dehydrogenative coupling of
aldehydes with benzylic compounds
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
50
72
94
92
—
—
—
—
—
57
70
8
I
42
O
10 mol% I
R1
R2
9
I
62
H
NaBH4
EtOH
H
OH
+
H
R2
R2
10
11
12e
13f
14g
15
16
I
18
1.2 equiv. DDQ
CHCl3 or MeNO2
R1
R2
I
17
1
2
3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
I
51
Entrya
Substrate
Product
Yieldb/% eec/%
I
29
I
23
H
OH
II
27
III
55
1
52
57
51
88
92
a Reactions were performed with 1a (0.8 mmol), 2a (0.2 mmol)
and I (0.02 mmol) in 2 mL solvent at room temperature. b Isolated
yield by column chromatography. c ee was determined by chiral
HPLC on a chiral stationary phase. 20 mol% catalyst and 20
mol% TfOH was used. 1.5 equiv. DDQ was used. 0 ℃ for 12
h. g 10 ml% 4-NO2C6H5CO2H was added.
O
O
2a
3a
d
OH
e
f
2
3
2a
2a
O
reacted to afford products in 77% ee and 74% ee, re-
spectively (Table 2, Entries 9—10). This cooperative
system could be extended to substrate 1,3,5-cyclohep-
tatriene 2c, where good yields and high enantioselectiv-
ities still could be obtained (58% yield and 91% ee for
butylaldehyde, 47% yield and 90% ee for valeraldehyde)
(Table 2, Entries 11—12). The absolute configuration of
the products 3 was determined by optical rotation in
comparison with the reported values.[7,9,10]
3b
OH
94
O
3c
Chin. J. Chem. 2012, 30, 2721—2725
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
2723

Huang, Xu & Xiao
NOTE
Conclusions
Continued
Entrya
Substrate
Product
Yieldb/% eec/%
In summary, we have developed the highly enantio-
selective cross-dehydrogenative coupling of aldehydes
with benzylic compounds catalyzed by diarylprolinol
silyl ether. DDQ proved to be an efficient and compati-
ble oxidant. This method provides an efficient and rapid
protocol for synthesis of enantioriched α-alkylated al-
dehydes in high optical purity, and many of the desired
α-alkylated aldehydes were obtained in good to high
enantioselectivities. Additionally, the simple room tem-
perature operations and use of non-metal oxidant should
make this methodology attractive for synthesis of opti-
cally pure α-alkylated aldehydes.
OH
4
5
6
7
47
42
44
57
95
92
92
83
2a
O
3d
OH
5
2a
2a
2a
O
3e
OH
OH
6
Acknowledgement
We are grateful to the National Natural Science
Foundation of China (No. 21102142) and Qingdao Ag-
ricultural University for financial support. We thank
Prof. Xingwei Li for the generous help.
O
3f
Ph
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3g
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