Metal-free cross-dehydrogenative coupling of aryl aldehydes to give symmetrical carboxylic anhydrides promoted by the TBHP/nBu4PBr system

Article abstract of DOI:10.1016/j.tetlet.2016.05.069

A novel, efficient, and metal-free dual C–O bond formation reaction for the synthesis of carboxylic anhydrides from aryl aldehydes via cross-dehydrogenative coupling is described. Heating a mixture of aromatic aldehydes and an aqueous solution of tert-butyl hydroperoxide as oxidant in the presence of catalytic nBu₄PBr in chlorobenzene at 80?°C for 3?h afforded the corresponding carboxylic anhydrides in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2016.05.069

Accepted Manuscript
Metal-free cross-dehydrogenative coupling of aryl aldehydes to give symmet-
rical carboxylic anhydrides promoted by the TBHP/nBu4PBr system
Mehdi Adib, Rahim Pashazadeh, Saideh Rajai-Daryasarei, Peiman Mirzaei
PII:
S0040-4039(16)30602-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.05.069
TETL 47688
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
29 February 2016
10 May 2016
18 May 2016
Please cite this article as: Adib, M., Pashazadeh, R., Rajai-Daryasarei, S., Mirzaei, P., Metal-free cross-
dehydrogenative coupling of aryl aldehydes to give symmetrical carboxylic anhydrides promoted by the TBHP/
nBu4PBr system, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.05.069
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Metal-free cross-dehydrogenative coupling of aryl aldehydes to give
symmetrical carboxylic anhydrides promoted by the TBHP/nBu4PBr
system
Mehdi Adib,a,* Rahim Pashazadeh,a Saideh Rajai-Daryasarei,a Peiman Mirzaeib
a School of Chemistry, College of Science, University of Tehran, PO Box 14155-6455,
Tehran, Iran
b Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Abstract- A novel, efficient and metal-free dual C−O bond formation reaction for the
synthesis of carboxylic anhydrides from aryl aldehydes via cross-dehydrogenative
coupling is described. Heating a mixture of aromatic aldehydes and an aqueous
solution of tert-butyl hydroperoxide as oxidant in the presence of catalytic nBu4PBr in
chlorobenzene at 80 °C for 3 h afforded the corresponding carboxylic anhydrides in
good to excellent yields.
Keywords: cross-dehydrogenative coupling, metal-free conditions, dual C−O bond
formation, nBu4PBr, tert-butyl hydroperoxide, carboxylic anhydrides, aldehydes
*Corresponding author. Tel./fax: +98(21)66495291; E-mail: madib@khayam.ut.ac.ir
1

The transition-metal-free C−H functionalization of organic substrates is a versatile
and powerful tool in current organic chemistry. An overwhelming amount of current
C-H activation methods use transition metals while the area of transition metal free C-
H activation remains relatively unexplored.1 Furthermore, the development of C−H
oxygenation of organic molecules using oxidants such as tert-butyl hydroperoxide
(TBHP), Oxone, O2, phthaloyl peroxide (PPO) is a significant challenge and remains
highly desirable,2 because the use of oxidants in the presence or absence of transition-
metals has gained a great deal of attention for the preparation of complex compounds
in synthetic organic chemistry. Amazing and unpredictable chemistry of these
compounds, has led to the researchers interest in the use of oxidants in their reactions.
On the other hand, functionalization of C−H bonds through cross-dehydrogenative
coupling (CDC) reactions has played a crucial role for the preparation of a wide
variety of organic compounds.3 Very recently, the application of TBHP as a powerful
oxidant in combination with Bu4NX catalysts has received attention for the formation
of C−C, C−N, C−O and C−S bonds.4
Carboxylic anhydrides are of great significance for the preparation of a variety of
carboxylic acid derivatives such as amides and esters5 as well as drug and peptide
synthesis.6
Up to now, a great deal of attention has been focused on finding new approaches for
the preparation of carboxylic anhydride derivatives.7 Xiao and co-workers have
reported a remarkable synthesis of carboxylic anhydrides via the Pd-catalyzed
carbonylation of aryl halides at atmospheric CO pressure8a (Scheme 1, route a).
Recently, Patel and co-workers reported a nano-CuO catalyzed CDC reaction for the
synthesis of symmetrical anhydrides from aldehydes using TBHP as an oxidant8b
(Scheme 1, route b). Very recently, Saberi and co-workers developed the oxidative
self-coupling of aldehydes in the presence of a CuCl2/TBHP system8c (Scheme 1,
route c). As part of our continuing effort to develop efficient methods for the
preparation of organic compounds from readily available starting materials,9 herein,
we report the preparation of symmetrical carboxylic anhydrides from inexpensive and
readily available aldehydes, promoted by a TBHP/Bu4PBr system without the aid of a
transition-metal catalyst (Scheme 1).
2

I
+
H2O
R
NEt3, CO (1 atm)
Route a8a
Pd(OAc)2 (1 mol%)
DPPP (2 mol%)
DMF, 110 ºC, 6 h
This work:
Route b8b
O
O
O
O
tBuOOH (1.2 eq)
tBuOOH (2 eq)
Bu4PBr (30 mol%)
H
nano CuO (5 mol%)
H
O
R
R
R
R
PhCl, 80 ºC, 3 h
DCE, 120 ºC, 3-7 h
tBuOOH (1.5 eq)
CuCl2 (10 mol%)
CH3CN, 80 oC, 5 h Route c8c
O
H
R
Scheme 1. Some recent synthetic strategies for carboxylic anhydrides
To begin our study, we optimized the reaction conditions for the formation of benzoic
anhydride 2a from benzaldehyde 1a as a model substrate (Table 1). The effect of
various oxidants, reaction solvents, additives and reaction temperature were
examined. At first, the reaction was carried out in the presence of TBHP (1 eq.) as
oxidant and TBPB (15 mol%) as additive in chlorobenzene at ambient temperature.
These reaction conditions proved to be ineffective (Table 1, entry 1) however, when
the temperature was raised to 50 °C and 80 °C, the desired product 2a was obtained in
20% and 55% yields, respectively (entries 2,3). By increasing amount of TBHP to 1.2
eq., the yield improved to 70% (entry 4). When the reaction was performed with 30
mol% of TBPB, a maximum yield of 85% was observed (entry 5). We also tested a
higher reaction temperature of 100 °C and an increased amount of TBHP (1.5 eq.),
however the yield of 2a decreased to 72% and 78% respectively (entries 6 and 7).
Furthermore, increasing the reaction time to 6 h led to a lower yield (entry 8). When
the quantity of TBPB increased to 40 mol%, the yield of the product did not improve
(entry 9). Next, the effects of other additives such as TBAB, Aliquat 336, KI, I2 and
CuI was examined but the results were not acceptable (entries 10–14). Other oxidants
were also tested. Benzoyl peroxide (BPO) gave 2a in 65% yield (entry 15) while
K2S2O8 as well as (NH4)2S2O8 were ineffective (entries 16, 17). Upon the screening of
solvents, we found that PhCl was the most suitable. Carrying out the reaction in
CH3CN, DCE, PhCH3 and 1,4-dioxane resulted in decreased yields (entries 18–20,
23). In DMSO, H2O and DMSO/H2O the reaction did not proceed (entries 21, 22, 24).
3

Finally, in the absence of TBHP (entry 25) and TBPB (entry 26), formation of 2a was
not detected, which indicated that both TBHP and TBPB were crucial to the reaction.
Thus, the optimal conditions for the conversion of benzaldehyde 1a to benzoic
anhydride 2a were determined: TBHP (1.2 eq.) and TBPB (30 mol%), in
chlorobenzene at 80 °C for 3 h (Table 1, entry 5).10
Table 1. Condition screening for the CDC formation of benzoic anhydride 2a from benzaldehyde 1aa
O
O
O
Oxidant, additive
solvent
2 ×
Ph
H
Ph
O
Ph
1a
2a
Entry
Oxidant (eq.)b
TBHPd (1)
TBHP (1)
TBHP (1)
Additive (mol%)b
Solvent
Temp (ºC) Yield (%)c
1
2
3
4
5
6
7
TBPBe (15)
TBPB (15)
TBPB (15)
TBPB (15)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (40)
TBAB (30)
Aliquat 336h (30)
KI (30)
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
CH3CN
DCE
PhCH3
DMSO
H2O
25
NRf
20
50
80
80
80
100
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
55
70
85
72
78
65
85
70
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.5)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
BPOi (1.2)
K2S2O8 (1.2)
(NH4)2S2O8 (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
TBHP (1.2)
8g
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
35
trace
NR
NR
65
NR
NR
55
I2 (30)
CuI (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)
TBPB (30)

50
50
NR
NR
trace
NR
NR
NR
1,4-Dioxane
DMSO:H2O (1:1)
PhCl
PhCl

TBHP (1.2)
aReaction conditions: 1a (0.5 mmol), solvent (0.75 mL), 3 h; bWith respect to 1a; cIsolated yields; dt-
BuOOH in H2O (70 wt%); eTetrabutylphosphonium bromide; fNR = no reaction; gReaction time of 6 h;
hTricaprylmethylammonium chloride; iBenzoyl peroxide.
After optimization of the reaction conditions, a series of aromatic anhydrides 2a–t
were prepared from various aromatic aldehydes 1a–t (Table 2). Electron-donating
substituents, such as CH3 and methoxy, regardless of their positions on the aryl ring,
gave the corresponding anhydrides 2b–j in 60–86 % yields (entries 2–10). Aromatic
aldehydes with electron-withdrawing substituents such as NO2 and halogen groups
4

were ineffective in this reaction and the corresponding anhydrides 2k–o were not
detected in the reaction mixtures (entries 11–15). Naphthalene carbaldehydes (1p and
1q) were also suitable for this CDC reaction and afforded 1-naphthoic- and 2-
naphthoic anhydrides 2p and 2q in 70%, 72% yields, respectively (entries 16,17).
Cinnamaldehyde was also subjected to the reaction which gave anhydride 2r in 51%
yield (entry 18). Furthermore, heteroaromatic aldehydes such as furan-2-carbaldehyde
and thiophene-2-carbaldehyde were also examined affording anhydrides 2s and 2t in
76% and 82% yields, respectively (entries 19 and 20).10
Table 2. Substrate scope for the synthesis of carboxylic
anhydrides 2a–ta
O
O
O
tBuOOH (1.2 eq)
2 ×
Ar
H
Ar
O
Ar
TBPB, PhCl, 80 °C, 3 h
1a-t
2a-t
Entry
1
Ar
Ph
Yield (%)b
1
2
85
1a
2a
2
3
4
5
6
7
8
2-CH3C6H4
3-CH3C6H4
4-CH3C6H4
2,5-(CH3)2C6H3
2-CH3OC6H4
3-CH3OC6H4
4-CH3OC6H4
2,5-(CH3O)2C6H3
3,4-(CH3O)2C6H3
2-ClC6H4
4-ClC6H4
4-FC6H4
4-BrC6H4
4-O2NC6H4
1-Naphthyl
2-Naphthyl
trans-PhCH=CH−
2-Furyl
78
80
78
86
73
76
70
1b
1c
1d
1e
1f
1g
1h
1i
2b
2c
2d
2e
2f
2g
2h
2i
9
60
63
10
11
12
13
14
15
16
17
18
19
20
1j
1k
1l
2j
2k
2l
NR
NR
NR
NR
NR
70
72
51
76
82
1m
1n
1o
1p
1q
1r
1s
1t
2m
2n
2o
2p
2q
2r
2s
2t
2-Thienyl
aReaction conditions: aryl aldehyde (1, 0.5 mmol), TBHP (0.6
mmol), TBPB (30 mol%), PhCl (0.75 mL), 80 °C, 3 h.
bIsolated yields.
A reasonable mechanism for the formation of benzoic anhydride 2a is provided in
Scheme 2. Initially, TBPB reacts with TBHP to give tert-butoxy A and tert-
butylperoxy B radicals.4b,8b,11 Next, the tert-butoxy radical A removes a hydrogen
atom from benzaldehyde 1a to form benzoyl radical C, which can be trapped by tert-
5

butylperoxy radical B through a C−O bond formation to give tert-butyl
peroxybenzoate8b,11g D. The latter may undergo O−O bond cleavage to release
benzoyloxy radical E and return tert-butoxy radical A to the reaction cycle. Finally,
benzoyloxy radical E and benzoyl radical C are coupled through a C−O bond
formation to afford benzoic anhydride 2a.
_
+
Bu4P Br
.
tBuO-OH
tBuO2
B
H2O
+
_
OH
_
.
tBuO
A
tBuO-OH
OH
+
+
.
+
Bu4P Br
.
tBuO2
B
O
O
O
O
Ph
tBu
.
D
Ph
C
O
.
tBuO Ph
H
+
tBuOH
O
O
O
1a
A
.
.
Ph
O
Ph
O
2a
Ph
O
tBuO
E
A
tBuOH
Ph
H
1a
Scheme 2. Proposed reaction mechanism for the cross-dehydrogenative self-coupling
of benzaldehyde 1a and formation of benzoic anhydride 2a promoted by
nBu4PBr/TBHP system
To provide evidence for the proposed radical mechanism of the CDC reaction, the
reaction of benzaldehyde 1a under the optimized reaction conditions was carried out
in the presence of the radical trap TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl. In
this reaction, 2,2,6,6-tetramethylpiperidino benzoate 3 was isolated in 95% yield and
the desired anhydride 2a was not detected (Scheme 3). This observation strengthens
presence of a radical intermediate in the examined CDC reaction.
tBuOOH (1.2 eq.)
O
O
O
O
TEMPO (1.2 eq.)
N
Ph
H
Ph
O
Ph
O
Ph
TBPB, PhCl, 80 °C, 3 h
3
1a
2a
95%
not detected
Scheme 3. Trapping of benzoyl radical with TEMPO
6

In conclusion, we have disclosed a practical, and metal-free oxidative cross-
dehydrogenative self-coupling of aldehydes. This CDC coupling for the preparation
of carboxylic anhydrides through dual C−O bond formation was promoted by a
nBu4PBr/TBHP system using TBHP for oxygen atom transfer. This novel, efficient
and economical approach was carried out using the inexpensive TBHP/TBPB system,
under mild and neutral reaction conditions, with moderate temperatures, short times
and without the aid of a transition metal. To the best of our knowledge this is the first
report on the use of tetraalkylphosphonium halides as a catalyst in CDC reactions.
Acknowledgement
This research was supported by the Research Council of the University of Tehran.
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Tajbakhsh, M.; Amanlou, M. Helv. Chim. Acta 2015, 98, 1079–1086; Adib, M.;
Sheikhi, E.; Azimzadeh, M. Tetrahedron Lett. 2015, 56, 1933–1936; Adib, M.;
Sheikhi, E.; Haghshenas, P.; Rajai-Daryasarei, S.; Bijanzadeh, H. R.; Zhu, L. G.
Tetrahedron Lett. 2014, 55, 4983–4986; Adib, M.; Sheikhi, E.; Rezaei, N.;
Bijanzadeh, H. R.; Mirzaei, P. Synlett 2014, 25, 1331–1334.
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10. General procedure for the synthesis of benzoic anhydride (2a). In a 5 mL
sealed tube, TBPB (30 mol%, 0.051 g), and TBHP (70 wt% in H2O, 0.6 mmol,
0.078 g) were added to a solution of benzaldehyde 1a (0.5 mmol, 0.053 g), in
chlorobenzene (0.75 mL). The resultant mixture was heated at 80 °C for 3 h. After
reaction completion as indicated by TLC, the reaction mixture was cooled to room
temperature and saturated NaHCO3 (2 mL) was added. The product was extracted
with EtOAc (2  3 mL). The combined organic extract was dried over Na2SO4 and
the solvent removed under reduced pressure. The residue was purified by column
chromatography using n-hexane-EtOAc (15:1) as eluent to afford 2a as a white
solid: 1H NMR (400 MHz, CDCl3): δ = 7.52 (dd, J = 8.1, 7.8 Hz, 4H), 7.65 (t, J =
7.8 Hz, 2H), 8.15 (d, J = 7.8 Hz, 4H). 13C NMR (100.6 MHz, CDCl3): δ = 128.9,
129.0, 130.5, 134.6, 162.4.
11. (a) Gan, L.; Huang, S.; Zhang, X.; Zhang, A.; Cheng, B.; Cheng, H.; Li, X.;
Shang, G. J. Am. Chem. Soc. 2002, 124, 13384–13385; (b) Jones, C. M.; Burkitt,
M. J. J. Am. Chem. Soc. 2003, 125, 6946–6954; (c) Uyanik, M. Okamoto, H.
Yasui, T. Ishihara, K. Science 2010, 328, 1376; (d) Feng, J.; Liang, S.; Chen, S.
Y.; Zhang, J.; Fu, S. S.; Yu, X. Q. Adv. Synth. Catal. 2012, 354, 1287–1292; (e)
Liu, Z.; Zhang, J.; Chen, S.; Shi, E.; Xu, Y.; Wan, X. Angew. Chem. Int. Ed. 2012,
51, 3231–3235; (f) Li, H.; Xie, J.; Xue, Q.; Cheng, Y.; Zhu, C. Tetrahedron Lett.
2012, 53, 6479–6482; (g) Rout, S. K.; Guin, S.; Ali, W.; Gogoi, A.; Patel, B. K.
Org. Lett. 2014, 16, 3086–3089.
10

Metal free cross-dehydrogenative coupling of aryl aldehydes
to give symmetrical carboxylic anhydrides promoted by the
TBHP/nBu4PBr system
Mehdi Adib,* Rahim Pashazadeh, Saideh Rajai-Daryasarei, Peiman Mirzaei
O
O
O
tBuOOH
2 ×
Ar
H
Ar
O
Ar
TBPB, PhCl, 80 °C, 3 h
15 examples
51 86% yields
Ar = C6H5, 2-CH3C6H4, 3-CH3C6H4, 4-CH3C6H4,
2,5-(CH3)2C6H3, 2-CH3OC6H4, 3-CH3OC6H4,
4-CH3OC6H4, 2,5-(CH3O)2C6H3,
3,4-(CH3O)2C6H3, 1-naphthyl, 2-naphthyl,
trans-PhCH=CH-, 2-furyl, 2-thienyl
11

Research highlights
 Dual C−O bond formation via cross-dehydrogenative coupling reaction
 An efficient synthesis of symmetrical anhydrides under metal-free conditions
 TBHP/nBu4PBr as a novel and efficient catalytic system
12