Copper-catalyzed oxidative esterification of ortho-formyl phenols without affecting labile formyl group

Article abstract of DOI:10.1016/j.tetlet.2015.06.024

A copper-catalyzed oxidative esterification of ortho-formyl phenols with aldehydes using TBHP as oxidant in water is developed. Besides aromatic and aliphatic aldehydes, benzylic alcohols are also suitable for this reaction. The sensitive formyl group rem

Full text of DOI:10.1016/j.tetlet.2015.06.024

Accepted Manuscript
Copper-catalyzed oxidative esterification of ortho-formyl phenols without af-
fecting labile formyl group
Yong Zheng, Wei-Bin Song, Li-Jiang Xuan
PII:
S0040-4039(15)01017-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.06.024
TETL 46416
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
10 March 2015
4 June 2015
9 June 2015
Please cite this article as: Zheng, Y., Song, W-B., Xuan, L-J., Copper-catalyzed oxidative esterification of ortho-
formyl phenols without affecting labile formyl group, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/
j.tetlet.2015.06.024
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Copper-catalyzed oxidative esterification of ortho-formyl phenols without
affecting labile formyl group
Yong Zheng, Wei-Bin Song, and Li-Jiang Xuan*
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica,
Chinese Academy of Sciences, 501 Haike Road, Zhangjiang Hi-Tech Park, Shanghai
201203, PR China
*Corresponding Author. Tel/fax: +86 21 20231968.
E-mail address: ljxuan@simm.ac.cn.
Abstract
1

A copper-catalyzed oxidative esterification of ortho-formyl phenols with aldehydes
using TBHP as oxidant in water is developed. Besides aromatic and aliphatic
aldehydes, benzylic alcohols are also suitable for this reaction. The sensitive formyl
group remains intact under oxidative conditions. This method provides an alternative
protocol for classical esterification reactions.
Key Words:
Copper-catalyzed, Esterification, Formyl, TBHP, Saliylaldehyde
2

Esters are not only prevalent structural motifs in natural products and
pharmaceutical drugs, but are also used as protecting groups in synthesis.1,2 As a
result, the synthesis of esters has attracted considerable interest and a number of
methods have been devised. Conventionally, ester groups were synthesized by the
nucleophilic addition of an alcohol to activated carboxylic acid derivatives such as
acid anhydrides or chlorides in the presence of a stoichiometric amount of bases.
Alternatively, transition-metal-catalyzed coupling reactions using aryl halides, CO,
and alcohols also offer an access to esters.3 However，these reactions required base,
air and moisture-sensitive reagants acyl halides, toxic CO gas, which limited
functional group tolerance. Although great progress has been achieved in this field,
there still remains significant challenges for synthesis of ester groups in both
atom-economic and mild conditions.
Direct C–H transformation has attracted continuous interest since early 1970s
because it avoids prefunctionalization of starting materials and can maximize atom
economy.4 Among the many C−H bond activation protocols, cross-dehydrogenative
−C,
coupling (CDC) has emerged as an attractive and useful method in formation of C
−O, C−N, and C−S bond.5, 6
C
Recently, NHC ligands have been utilized for oxidative esterifications of phenol
with aldehydes using Pd or Fe catalysts under base.7 However, this approach was not
suitable for ortho-substituted phenols.
Copper was found to be a cheap, and environmental friendly catalyst in many
CDC reactions.8 Recently, Cu-catalyzed directing-groups assisted CDC reaction under
oxidative conditions have provided an atom-economic route for C-O bond formation
of 2-carbonyl phenol and β-dicarbonyl compounds.9
Compared to ortho-keto phenol, the ortho-formyl phenol is considered to be
sensitive to oxidants and transition metals, and is easily oxidized into salicylic acid
under metal and oxidants.10 Thus, the formation of C-O bond of ortho-formyl phenol
under oxidative conditions while selectively keeping the formyl group intact is still a
challenging task. Chang reported a Fe-catalyzed oxidative coupling reaction between
salicylalhehydes and ether and a copper-catalyzed oxidative coupling of formamides
with salicylaldehydes (Scheme1, a-b).11a,11b Recently, Patel reported an esterification
reaction of ortho-formyl phenols with toluene derivatives without affecting the formyl
group (Scheme 1, c).11c However, this reaction required high temperature and long
reaction time, which may limit the functional tolerance. Furthermore, the use of
TBHP decane solution increased cost (TBHP decane solution is relatively expensive
than aq. TBHP). Thus, development of efficient approach for esterification of
2-formyl phenol, which preserves the formyl group under mild reaction conditions, is
highly desirable. In view of green chemistry, water as an ideal reaction solvent with
environmentally benign and nontoxic characters has been attracting considerable
attention. Besides that, water has been proven to affect the selectivity of lots of
organic reactions and to enhance the reaction rates. Herein, we reported Cu-catalyzed
oxidative esterification of 2-formyl phenols with aldehydes using cheap aq. TBHP as
oxidant in water (Scheme 1, d). Both aromatic and alphatic aldehydes undergo
esterification smoothly. Benzylic alcohols,12 which are environmentally friendly, low
3

toxicities, stable, commercially available, and easy to handle compared to aldehydes,
are also suitable for this reaction.
Scheme 1 Formyl-group assisted oxidative C-O bond formation reactions
We initially investigated the reaction between salicylaldehyde 1 with
benzaldehyde 2 in the presence of 10 % mol Cu(OAc)2 and 3 equiv of TBHP
(tert-butyl hydroperoxide) in DMSO at 80 ℃ for 2h (Table1, entry 1). To our delight,
the ester 3 was obtained in 59 % yield. Other copper catalysts, such as CuCl2·2H2O,
CuCl, CuBr, and CuI, failed to give any improvement (entries 2-5). TBHP as an
oxidant was found to be superior over the other oxidants screened, such as DTBP, O2,
H2O2, and mCPBA (entries 6-9). When H2O was selected as solvent, the yield
increased to 65 % (entry 10). No significant increase in the yield of 3 was observed
when extending or shorting reaction time (entries 11 and 12). Other solvents, such as
PhCl, toluene, and DCE, were proved to be less efficient than H2O (entries 13-15).
Higher or lower temperatures seemed to be inappropriate for the reaction (entries 16
and 17). Furthermore, the reaction did not proceed without the use of Cu catalyst or
TBHP (entries 18 and 19).
Table1. Optimization of reaction conditions.a
4

Entry Catalyst
Oxidant
(equiv.)
Solvent Yieldb
(%)
1
Cu(OAc)2
TBHP (3)
DMSO 59
DMSO 27
DMSO 25
DMSO 29
DMSO 24
DMSO trace
2
CuCl2·2H2O TBHP (3)
3
CuCl
TBHP (3)
TBHP (3)
TBHP (3)
DTBP (3)
O2
4
CuBr
5
CuI
6
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
7
DMSO
0
8
H2O2 (3)
mCPBA (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
DMSO trace
9
DMSO
H2O
H2O
0
10
11c
12d
13
14
15
16e
17f
18
19
65
61
58
62
H2O
PhCl
Toluene 16
DCE
H2O
H2O
H2O
H2O
27
36
55
0
Cu(OAc)2
0
aConditions: 1a (0.2 mmol), 2a (0.3 mmol), catalyst (10 % mol),
oxidant, solvent (2 ml), 80 ℃, 2h.
bIsolated yield.
c0.5 h.
d5 h.
e60 ℃.
f100 ℃.
With the optimized conditions in hand, we next explored the benzaldehyde scope
of this oxidative esterification reaction. As showed in Table 2, benzaldehydes with
electron-donating groups such as methoxy, methyl, tert-butyl, acetylamino, and
diethylamino reacted with salicylaldehyde smoothly in good to moderate yield (3b, 3c,
3f-3h, 3k, and 3l). While weak electron-withdrawing groups (F, Cl, Br) substituted
benzaldehydes gave the moderate yields (3d, 3e, 3i, 3j, and 3m). Ortho-, meta-, and
para-substituted benzaldehydes could all proceed in this reaction. Slightly lower
yields were obtained in the cases of ortho-substituted benzaldehyde in comparison to
their para or meta analogues, possibly due to steric hindrance (3i and 3j). Heteroaryl
aldehydes, such as 3-pyridinecarboxaldehyde, 2-thiophenaldehyde, and
2-furanaldehyde, reacting with salicylaldehyde could also afforded the desired
products in 60 %, 73 %, and 50% yields, respectively (3n-3p). Only trace of product
was observed when salicylaldehyde reacted with benzaldehyde substituted with NO2
group (3s). Besides aromatic aldehydes, aliphatic aldehydes could also react with
salicylaldehyde under the optimized conditions, giving the desired products in
5

moderate yields (3q and 3r).
Next, a series of substituted salicylaldehydes were tried for this esterification
reaction (Table 3). Both electron-rich (OCH3, CH3, and Et2N) and electron-deficient
(F, Cl, Br) salicylaldehydes could proceed smoothly and afford the corresponding
acylated products in moderate to good yields (Table 3, 4a-4h). The 3-substituted
salicylaldehydes gave slightly poorer yields of the corresponding products possibly
due to their steric hindrance (4e and 4g). Furthermore, 2-hydroxy-1-naphthaldehyde
provided the expected product in 66 % yield (4i). The strong electron-withdrawing
group, such as NO2, obviously restrained the reaction and afforded trace of the desired
product under the standard reaction conditions (4j).
Table 2. Esterification of salicylaldehyde (1) with substituted aldehydes(2). a,b
aReaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), Cu(OAc)2 (10 % mol), TBHP (3
equiv), H2O (2 ml), 80 °C, 2h.
bIsolated yield.
6

Table 3. Esterification of substituted sacylialdehydes (1) with aldehydes (2).a,b
CHO
CHO
CHO
Cu(OAc)2
R
R
+
R'
TBHP
H2O
80 °C, 2h
O
OH
O
1
2
R'
4
Me
Br
CHO
O
Cl
CHO
CHO
O
F
CHO
O
O
O
O
O
O
4a, 71%
4b
4c
, 58%
4d
, 60%
, 55%
CHO
CHO
Cl
CHO
O
MeO
O
O
OMe
Cl
O
O
O
OMe
OMe
OMe
4e, 55%
4f, 69%
4g, 57%
CHO
O
O2N
CHO
O
CHO
O
N
O
O
O
OMe
4j, trace
4h, 71%
4i, 66%
aReaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), Cu(OAc)2 (10 % mol), TBHP (3
equiv), H2O (2 ml), 80 °C, 2h.
bIsolated yield.
Considering the stability and the availability of alcohols, we performed this
esterification reaction between salicylaldehyde and benzilic alcohol under the
standard conditions. To our delight, both electron-rich group (OMe) and
electron-deficient groups (Cl, Br) proceeded well giving the esters in 54-72% yields
(Scheme 2).
Scheme 2. Esterification of sacylialdehyde (1) with benzylic alcohols.
According to literature, formyl group transformed into amide under TBAI and aq.
7

TBHP using DMF as nitrogen source.14 Benzaldehyde reacting with DMF under
TBAI and TBHP, the amide 5 was obtained in good yield. When salicylaldehyde as
substrate using Cu(OAc)2 as catalyst and TBHP as oxidant, cabamate 6 was formed in
93% yield. 3a reacting with N-formylmorpholine under TBAI and TBHP resulted in 7
in 56% yield, which has the similar structures with trypase inhibitor A (Scheme 3).15
Scheme 3. Benzaldehydes, salicylaldehyde and 3a reacting with DMF or N-
formylmorpholine.
2-acyl benzaldehydes would be further transformed into different useful products,
such as coumarin 8 and benzofuran 9 (Scheme 4).15,16
Scheme 4. Synthesis of coumarin and benzofuran from 2-acyl benzaldehydes.
On basis of previous literature results and our experiments, a plausible mechanism
of this esterificaton reaction is proposed, as shown in Scheme 5. When the reaction
underwent in the presence of radical scavenger TEMPO (1.5 equiv), only trace of
product was formed, suggesting a possible radical pathway. TBHP generates t-BuO•
in the presence of Copper, which abstracts an H atom from benzaldehyde (benzylic
alcohol is oxidized to benzaldehyde by TBHP) to give the acyl radical.17 Copper
forms a coordinating complex A with formyl group. This hypothesis was supported
by failure to obtain ester when simple phenol reacting with benzaldehyde under the
standard condition, which indicated that directing group (formyl) is essential for the
esterification reaction. The proposed complex, reacts with acyl radical to form the
8

complex B, which can undergo reductive elimination to afford the product and Cu(I)
catalyst. Finally, Cu (I) is oxidized to Cu (II) by TBHP, which maintains the catalytic
cycle.
OH
Ph
O
Standard conditions
PhCHO
+
O
5%
CHO
CHO
Standard conditions
TEMPO (1.5 equiv)
O
OH
O
Ph
Trace
Cu
t-BuOOH
t-BuO
O
O
t-BuO
t-BuOOH
H
OH
t-BuOH
t-BuOH
H
O
Cu
CHO
OH
O
A
OAc
H
O
Cu(OAc)2
Cu
O
O
OAc
B
t-BuOOH
CuOAc
H
O
O
O
Scheme 5. Proposed reaction mechanism
In summary, we have developed a copper-catalyzed oxidative esterification of
ortho-formyl phenols with aldehydes using TBHP as oxidant. Both aromatic and
aliphatic aldehydes proceed well giving the products in moderate to good yields.
Benzylic alcohols are also suitable for this reaction. The labile formyl group remains
intact under oxidative conditions, and can be further transformed into various useful
products.
Acknowledgements
We are grateful to the National Science & Technology Major Project “Key New Drug
Creation and Manufacturing Program”, China (Number: 2009ZX09301-001).
Supplementary data
Supplementary data associated with this article can be found, in the online version, at
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Graphical abstract
13