Ar-BINMOL-derived salan-Cu(II) complex-catalyzed silyl-reformatsky reaction of aromatic aldehydes

Article abstract of DOI:10.1080/00397911.2014.920509

In this work, we have developed a facile protocol with salan-Cu system for the facile and selective synthesis of β-hydroxyesters via silyl-Reformatsky reaction with -silylester and aldehydes. The screening and optimization of reaction conditions led to the determination of a practical and efficient procedure in which the salan-Cu exhibited promising catalytic activity in dimethylsulfoxide, in which the silyl-Reformatsky reaction of aromatic aldehydes with -trimethylsilylmethylacetate gave the corresponding β-hydroxyester derivatives in excellent yields (up to 98%) under fluoride-free reaction conditions.

Full text of DOI:10.1080/00397911.2014.920509

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Ar-BINMOL–Derived Salan–Cu(II)
Complex–Catalyzed Silyl-Reformatsky
Reaction of Aromatic Aldehydes
Fei Li a , Wei Zhou a , Long-Sheng Zheng a , Li Li a , Zhan-Jiang Zheng
a & Li-Wen Xu a b
a Key Laboratory of Organosilicon Chemistry and Material Technology
of Ministry of Education, College of Material, Chemistry and
Chemical Engineering , Hangzhou Normal University , Hangzhou ,
China
b State Key Laboratory for Oxo Synthesis and Selective Oxidation ,
Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences , Lanzhou , China
Accepted author version posted online: 05 Jun 2014.Published
online: 28 Jul 2014.
To cite this article: Fei Li , Wei Zhou , Long-Sheng Zheng , Li Li , Zhan-Jiang Zheng & Li-Wen Xu
(2014) Ar-BINMOL–Derived Salan–Cu(II) Complex–Catalyzed Silyl-Reformatsky Reaction of Aromatic
Aldehydes, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 44:19, 2861-2869, DOI: 10.1080/00397911.2014.920509
To link to this article: http://dx.doi.org/10.1080/00397911.2014.920509
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Synthetic Communications1, 44: 2861–2869, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.920509
Ar-BINMOL–DERIVED SALAN–Cu(II) COMPLEX–
CATALYZED SILYL-REFORMATSKY REACTION
OF AROMATIC ALDEHYDES
Fei Li,1 Wei Zhou,1 Long-Sheng Zheng,1 Li Li,1
Zhan-Jiang Zheng,1 and Li-Wen Xu1,2
1Key Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education, College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou, China
2State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, China
GRAPHICAL ABSTRACT
Abstract In this work, we have developed a facile protocol with salan–Cu system for the
facile and selective synthesis of b-hydroxyesters via silyl-Reformatsky reaction with a-sily-
lester and aldehydes. The screening and optimization of reaction conditions led to the deter-
mination of a practical and efficient procedure in which the salan–Cu exhibited promising
catalytic activity in dimethylsulfoxide, in which the silyl-Reformatsky reaction of aromatic
aldehydes with a-trimethylsilylmethylacetate gave the corresponding b-hydroxyester deri-
vatives in excellent yields (up to 98%) under fluoride-free reaction conditions.
Keywords Ar-BINMOL; aromatic aldehydes; organosilicon chemistry; Reformatsky
reaction; salan ligand
Received April 1, 2014.
Address correspondence to Li-Wen Xu, Key Laboratory of Organosilicon Chemistry and Material
Technology of Ministry of Education, College of Material, Chemistry and Chemical Engineering,
Hangzhou Normal University, Hangzhou 310012, P. R. China. E-mail: liwenxu@hznu.edu.cn; licpxulw@
yahoo.com
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
2861

2862
F. LI ET AL.
INTRODUCTION
The Reformatsky reaction is one of the most important fundamental carbon–
carbon bond-forming reactions in organic chemistry because it affords synthetically
and biologically important b-hydroxyesters.[1–7] The silyl-Reformatsky reaction
with a-silylester and carbonyl compounds is an environmentally benign type of
Reformatsky reaction that avoid the use of stoichiometric metal, such as zinc and
other metal powder. Various fluoride reagents, such as tetrabutylammonium fluoride
(TBAF) and CsF, have been employed as a fluoride ion for desilyl activating the
a-silyl group to generate a naked carbanion, which then adds to the aldehydes.
However, in previous work, the desired product yield was generally only poor to
moderate (24–88% yield) and had poor selectivity under fluoride-promoted reaction
conditions because of the unavoidable formation of both b-hydroxyester and a,b-
unsaturated ester. Most of synthetic methods based on fluoride-promoted breaking
of Si-C bond also were deficient in substrate scope, which always led to limited
species of b-hydroxyesters.[3–7] Thus in this context, there is still a great need to
develop a mild and efficient method, which can be valuable alternative for the
silyl-Reformatsky reaction and facile synthesis of b-hydroxyester with good yields.
Herein, we reported the development of salan–Cu–catalyzed silyl-Reformatsky
reaction of aromatic aldehydes with a-trimethylsilylmethylacetate (Scheme 1).
Very recently, we have synthesized a sires of chiral Ar-BINMOL ligands that
derived from chiral BINOL (1,10-binaphthalene-2,20-diol) through [1,2]-Wittig
rearrangement.[8–10] The Ar-BINMOLs (1,10-binaphthalene-2-a-arylmethanol-20-ol)
have rigid structures with C2-axial and sp3 carbon-central chirality, and could act
as a supramolecular backbone due to the existence of hydrogen bonding and
aromatic interactions of rotatable aromatic rings. Thus, we expected a high potential
for the creation of a rotatable p wall in the designed salan ligand prepared from
Ar-BINMOL via key [1,2]-Wittig rearrangement and subsequent six-step transfor-
mations.[11,12] Interestingly, the Ar-BINMOL-derived salan ligand exhibited unusual
and unexpected catalytic efficiency in asymmetric Henry reaction of aldehydes in terms
of enantioselectivities and conversions. As shown in Scheme 2, the salan (1)–copper
complex exhibited good catalytic performance for nonsubstituted and halogen-
substituted aromatic aldehydes in terms of conversion and enantioselectivity. On the
Scheme 1. Salan–copper–catalyzed silyl-Reformatsky reaction.

COPPER-PROMOTED SILYL-REFORMATSKY REACTION
2863
Scheme 2. Enantioselective salan–Cu–catalyzed Henry reaction.
basis of our previous studies that the Ar-BINMOL-derived salan–copper–catalyzed
Henry reaction of aromatic aldehydes was successful,[11] we postulated that the
aromatic aldehydes could be also activated by salan (1)–copper complex in the
silyl-Reformatsky reaction of aromatic aldehydes with a-silylester.
RESULTS AND DISCUSSION
The initial studies of the salan (1)–copper–catalyzed silyl-Reformatsky reaction
of a-trimethylsilylmethylacetate (TMSMA, 5) to benzaldehyde focused on the
screening of different solvents and metal salts at room temperature. The results
from these investigations are presented in Table 1. With salan (1)–Cu as catalyst,
the solvent effect on the catalytic activity of salan (1)–copper was obvious and
the dimethylsulfoxide (DMSO) was found to the best media to promote silyl-
Reformatsky reaction with excellent yield (entry 6), whereas other solvents, such
as methanol, 1,4-dioxane, dimethylformamide (DMF), N-methylpyrrolidone
(NMP), and tetrahydrofuran (THF), resulted in no reaction or trace product (entries
1–5). Notably, only the salan ligand or Cu(OAc)2 in the absence of another coordi-
nator turned out to be poor catalyst in term of conversion (entries 7 and 8). Inspired
by these findings, we continued to investigate various copper salts, such as CuF2,
CuCl2, Cu(OTf)2, CuCl, CuI, and CuCN, in the model silyl-Reformatsky reaction
(entries 9–16). Surprisingly, only CuF2 was found to be the best catalyst in this reac-
tion (98% yield, entry 9), a result which is in accordance with the general observation
from salan (1)–Cu(OAc)2 system–catalyzed silyl-Reformatsky reaction. Interest-
ingly, CuF2 was also not effective for this addition in the absence of salan ligand
(entry 10). Comparably, the use of other general metal fluorides (KF, ZnF2) as cata-
lyst gave poor conversion under the present reaction conditions (entries 17 and 18).
Although AgOAc and HOAc were not successful in this reaction (entries 19 and 20),
the importance of copper and acetic ion was confirmed on the basis of these results.
Therefore, beside the salan (1)–CuF2 system, the fluoride-free salan (1)–
Cu(OAc)2 was observed to be a promising catalyst in the silyl-Reformatsky reaction

2864
F. LI ET AL.
Table 1. Evaluation of salan (1)–copper complex as catalyst for the synthesis of b-hydroxyester in the
silyl-Reformatsky reaction of benzaldehyde with a-trimethylsilylmethylacetatea
Entry
Metal salt
Cu(OAc)2
Ligand
Solvent
MeOH
Yield (%)b
1
2
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Tracec
Trace
Trace
Trace
Trace
91
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
1,4-Dioxane
DMF
THF
3
4
5
NMP
6
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
d
7
—
Salan (1)
Salan (1)
Trace
Trace
98
e
8
—
CuF2
9
d
—
10
11
12
13
14
15
16
17
18
19
20
CuF2
CuCl
Trace
Trace
Trace
Trace
Trace
Trace
Trace
25
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Salan (1)
Salan (1)
CuI
CuCN
CuCl2
Cu(CH3CN)4PF6
Cu(OTf)2
ZnF2
KF
AgOAc
HOAcf
37
Trace
Trace
aAll reactions were carried out with 5 mol% of catalyst and ligand at room temperature for 48 h.
bThe yield was isolated by flash column.
cNo reaction or only trace product was detected (<5% yield).
dNo addition of salan (1) ligand.
eNo addition of metal salt or other catalyst.
fThe use of 10 mol% of acetic acid.
of a-trimethylsilylmethylacetate. Surprisingly, it is noteworthy that the reaction
results in the desired methyl 3-hydroxy-3-phenylpropanoate (6a) without any side
products, such as b-siloxylester fmethyl 3-phenyl-3-(trimethylsilyloxy)propanoateg
or a,b-unsaturated ester (methyl cinnamate), which is different from previous
findings.[1–7] Although almost no enantioselectivity of the present silyl-Reformatsky
reaction was determined by high-performance liquid chromatography (HPLC), it was
concluded that the salan–copper was an efficient catalyst or promoter to accelerate
the silyl-Reformatsky reaction for the clean and chemoselective synthesis of b-hydro-
xyesters under the reaction conditions.
With the optimized conditions in hand, the catalytic activity of salan (1)–
copper generated in situ from salan (1) and Cu(OAc)2 was then investigated in the
silyl-Reformatsky reaction of various aromatic aldehydes. As shown in Scheme 3,
the salan–copper catalyst was also applicable to the silyl-Reformatsky reaction of
various aromatic aldehydes that were converted into corresponding b-hydroxyesters

COPPER-PROMOTED SILYL-REFORMATSKY REACTION
2865
Scheme 3. Salan (1)–Cu–catalyzed silyl-Reformatsky reaction of various aromatic aldehydes.
in good to excellent yields (75–97% yields for 6a–n). Similar to the Henry reaction of
aromatic aldehydes, a variety of aromatic aldehydes bearing halogen groups, such
as fluoride, chloride, and bromide, at p-, m-, and o-positions afforded excellent
yields (up to 97%). Sterically hindered aldehydes, such as 2-bromo-5-methoxy-
benzaldehyde, 2-bromo-4-fluorobenzaldehyde, and dichloro-benzaldehydes, were
also resulted in good yields of the desired products (6j, 6k, 6m, and 6n). 2-Naphthal-
dehyde also tolerated our reaction conditions to the formation of methyl 3-hydroxy-
3-(naphthalen-2-yl)propanoate (6l) in 75% yield. As expectedly, the salan–copper
catalyst was not effective in the silyl-Reformatsky reaction of aromatic aldehydes
with hydroxyl, methyl, methoxy, and nitro-groups (6o–w) because almost no
reaction occurred for these substrates. On the basis of these experimental results,
the salan–copper catalyst still exhibited substrate-sensitive features in the silyl-
Reformatsky reaction, which is different from previous reports on catalytic silyl-
Reformatsky reaction of aromatic aldehydes that impacted by electronic and steric
factors of substrates.[1b] It is worth noting that the selectivity to b-hydroxyesters is
excellent because there are no by-products (b-siloxylester or a,b-unsaturated ester)
in this reaction.[13] In addition, the synthetic method described in this reaction is also

2866
F. LI ET AL.
Scheme 4. Proposed mechanism for salan–copper complex–catalyzed silyl-Reformatsky reaction.
a very mild and effective procedure for the high-yielding synthesis of various aldol
products (b-hydroxyesters). Though no enantioselectivity of this silyl-Reformatsky
reaction was observed, it is surprising that the catalytic role of the salan–copper
complex was realized on the silyl-Reformatsky reaction for the first time. The present
study seems to be the first and the best attempt to use fluoride-free metal complex
as the catalyst for the desilylation and corresponding nucleophilic addition of
a-trimethylsilylmethylacetate to aromatic aldehydes.
On the basis of these experimental results, a possible mechanistic procedure for
the salan–copper-catalyzed silyl-Reformatsky reaction is shown in Scheme 4. As
shown in Scheme 4, in the first step, reaction of acetic anion (OAc-) and a-trimethyl-
silylmethylacetate afforded the anionic pentacoordinate intermediate (S-a) because
the free acetic anion derived from salan ligand and Cu(OAc)2 was an effective
nucleophile to attack silicon center. The pentacoordinate intermediate was unstable
in generally because of its high reactivity to form possible desilylated nucleophile
S-b.[14–16] Simultaneously, the solvent effect of DMSO played an important role in
the formation of key intermediates S-b and S-c, which accelerated the addition of
intermediate S-b to aromatic aldehyde.[17] At the same time, similar to the previous
report on Henry reaction,[11] the interaction of aromatic aldehyde and salan–
copper catalyst activated the electrophile. Also inspired by previous findings on
salan–copper–catalyzed Henry reactions, we can conclude that the presence of
crowded phenyl groups in the salan ligand led to the activation of aldehydes
is largely impacted by the salan–copper complex. Although cooperative effect of
noncovalent interactions between aromatic aldehydes and salan–copper catalyst,
such as hydrogen bonding, aromatic interactions, and metal coordination, on
the catalytic activity of salan–copper cannot rationalized at present,[18,19] the
enhanced activity of salan ligand with aromatic interactions can be identified
in this reaction, consistent with previous reports on the aromatic interactions
accelerating transformations.[20–28]

COPPER-PROMOTED SILYL-REFORMATSKY REACTION
2867
CONCLUSIONS
In summary, we have developed a novel protocol for the facile and selective
synthesis of b-hydroxyesters via silyl-Reformatsky reaction with a-silylester and
aldehydes. The screening and optimization of reaction conditions led to the determi-
nation of a practical and efficient procedure that the salan–Cu exhibited promising
catalytic activity in DMSO, in which the silyl-Reformatsky reaction of aromatic
aldehydes with a-trimethylsilylmethylacetate (TMSMA) gave the corresponding
b-hydroxyester derivatives in excellent yields (up to 98%) under mild reaction
conditions. In terms of the catalytic efficiency of salan–Cu system in DMSO, the silyl-
Reformatsky reaction of a-trimethylsilylmethylacetate could occur in the absence
of fluoride reagent. Although the detailed mechanism of salan–Cu–catalyzed silyl-
Reformatsky reaction in DMSO is not clear at present, further efforts will be devoted
to develop a chiral salan–copper asymmetric silyl-Reformatsky reaction.
EXPERIMENTAL
All reagents and solvents were used directly without purification. Flash column
chromatography was performed over silica (200–300 mesh). 1H NMR and 13C NMR
spectra were recorded at 400 and 100 MHz, respectively, on an Avance (Brucker)
400-MHz nuclear magnetic resonance spectrometer, and were referenced to the inter-
nal solvent signals. Thin-layer chromatography (TLC) was performed using silica gel
F254 TLC plates and visualized with ultraviolet light. HPLC was carried out with
a Waters 2695 Millennium system equipped with a photodiode array detector.
Electron impact (EI) and chemical ionization (CI) mass spectra were performed
on a Trace DSQ GC=MS spectrometer. Data are reported in the form of m=z.
The electrospray ionization–mass spectrographic (ESI-MS) analysis of the samples
was performed on an LCQ advantage mass spectrometer (ThermoFisher Company,
USA), equipped with an ESI ion source in the positive ionization mode, with data
acquisition using the Xcalibur software (Version 1.4).
General Procedure for the Silyl-Reformatsky Reaction
A flame-dried flask under argon was charged with salan ligand 1 (47.8 mg,
0.05 mmol), Cu(OAc)2 (10.0 mg, 0.05 mmol), and DMSO at room temperature. After
the mixture was stirred for 1 h, methyl (trimethylsilyl)acetate (1.1 mmol) and
aldehyde (1 mmol) were added successively to the solution, and the mixture was
stirred at ambient temperature for 48 h. The reaction mixture was diluted with water
and then extracted with dicholormethane (DCM). The organic layer were combined,
washed with brine, dried over anhydrous Na2SO4, and then concentrated under
reduced pressure. The crude residue was purified by column chromatography. All
the products are known[1–7,29–41] and confirmed by GC-MS, NMR, IR, and HRMS
(see Supporting Information). For example, methyl 3-hydroxy-3-phenylpropanoate
(6a): 1H NMR (400 MHz, CDCl3) d ¼ 7.28–7.35 (m, J ¼ 8.4 Hz, 5H, and HAr),
5.12 (q, J ¼ 9.2 Hz, H, and CH-OH), 3.70 (s, 3H, and -OCH3), 2.72 (m, J ¼ 8.0 Hz,
2H, and -CH2COOMe); 13C NMR (100 MHz, CDCl3) d ¼ 172.3 (COOMe), 142.6
(1-CAr), 128.6 (3-CAr), 127.8 (4-CAr), 125.7 (2-CAr), 70.3 (CHOH), 60.9

2868
F. LI ET AL.
(-CH2COOMe), 43.4 (-OCH3). IR (neat, cmꢀ1): 3461, 3063, 3031, 2953, 1732, 1454,
1438, 1361, 1269,1200, 1083, 1061, 1025, 988, 916,762, 701 cmꢀ1. Exact mass calcd.
for C10H13O3[M þH]þ, 181.0865. Found: 181.0850.
ACKNOWLEDGMENTS
X. L. W. thanks F. E. Zhu and C. Q. Sheng for their help.
FUNDING
This project was supported partially by the National Natural Science Founder
of China (Nos. 21173064 and 51303043), Zhejiang Provincial Natural Science Foun-
dation of China (LR14B030001 and LQ12B02004), and Program for Excellent
Young Teachers in Hangzhou Normal University (HNUEYT, JTAS 2011-01-014).
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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COPPER-PROMOTED SILYL-REFORMATSKY REACTION
2869
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