High-valent titanium(IV)salophen: An efficient catalyst for rapid conversion of aldehydes to 1,1-diacetates with acetic anhydride

Article abstract of DOI:10.1016/j.ica.2012.02.008

In the present work, a mild and highly efficient method for protection of aldehydes with acetic anhydride in the presence of high-valent titanium (IV) salophen trifluoromethanesulfonate, [Ti^IV(salophen)(OTf) ₂], at room temperature is reported. Under these conditions, different aldehydes bearing electron-withdrawing and electron-donating substituents were reacted with acetic anhydride and the corresponding 1,1-diacetates were obtained in good to excellent yields. The results showed that the yields were higher for aldehydes bearing electron-withdrawing substituents such as nitro, chloro, bromo and 3-methoxy, while aldehydes containing electron donating groups such as 4-methoxy and 4-methyl produced the lower yields. The catalyst was reused several times without loss of its catalytic activity.

Full text of DOI:10.1016/j.ica.2012.02.008

Inorganica Chimica Acta 388 (2012) 102–105
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Note
High-valent titanium(IV)salophen: An efficient catalyst for rapid conversion
of aldehydes to 1,1-diacetates with acetic anhydride
Maryam Yadegari a, Majid Moghadam b, , Shahram Tangestaninejad b, Valiollah Mirkhani b,
⇑
Iraj Mohammadpoor-Baltork b
a Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
b Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-7344, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work, a mild and highly efficient method for protection of aldehydes with acetic anhydride
in the presence of high-valent titanium (IV) salophen trifluoromethanesulfonate, [TiIV(salophen)(OTf)2],
at room temperature is reported. Under these conditions, different aldehydes bearing electron-withdraw-
ing and electron-donating substituents were reacted with acetic anhydride and the corresponding 1,1-
diacetates were obtained in good to excellent yields. The results showed that the yields were higher
for aldehydes bearing electron-withdrawing substituents such as nitro, chloro, bromo and 3-methoxy,
while aldehydes containing electron donating groups such as 4-methoxy and 4-methyl produced the
lower yields. The catalyst was reused several times without loss of its catalytic activity.
Ó 2012 Elsevier B.V. All rights reserved.
Received 24 February 2011
Received in revised form 27 November 2011
Accepted 7 February 2012
Available online 15 February 2012
Keywords:
Titanium Schiff base
Protection
Acetic anhydride
1,1-Diacetate
Aldehyde
1. Introduction
modimethylsulfonium bromide [29], solid lithium perchlorate
[30], [bmim]BF4 [31], [Hmim]HSO4 [32] and zeolite Y [33] which
Protection of functional groups plays an important role in mul-
ti-step synthesis of natural products. However, selective protection
and deprotection of carbonyl groups are substantial steps in syn-
thetic organic chemistry [1]. Acylals (1,1-diacetates) are appropri-
ate candidates to this aim due to their stability in basic and neutral
reaction media as well as in aqueous acids [2]. Meanwhile, gem-
are also efficient for this conversion. However, many of these meth-
odologies have drawbacks involve strongly acidic conditions, corro-
sive reagents, long reaction times, high temperature and high
toxicity.
Schiff base complexes of Mn, Fe, Ru, Cr, Co and V have found
many applications in organic chemistry [34]. These compounds
have been used as catalyst in the oxidation of alkenes and alkanes,
sulfides, amines and alcohols. Titanium Schiff bases have been used
as catalysts for polymerization of ethylene and propene [35], regio-
and stereoselective epoxidation of allylic alcohols [36], asymmetric
ring-opening of epoxides by dithiophosphorus acid [37], enantiose-
lective catalytic ring-opening of epoxides with carboxylic acids [38],
efficient kinetic resolution of terminal epoxides by means of cata-
lytic hydrolysis [39], enantioselective trimethylsilyl cyanation of
aldehyde [40,41], oxidation of sulfides to sulfoxides with hydrogen
peroxide [42], enantioselective ring-opening of meso-epoxides with
ArSH [43], asymmetric alkynylation of aldehydes [44] and enantio-
selective Pinacol coupling of aryl aldehydes [45].
Elecron-deficient complexes of Fe, Cr and Sn have been used
as mild Lewis acids in organic transfonations [46–62]. In this
paper, we report a highly efficient method for protection of alde-
hydes with Ac2O catalyzed by titanium(IV) salophen trifluoro-
methanesulfonate, [TiIV(salophen)(OTf)2] at room temperature
(Scheme 1).
diacetates derived from
a,b-unsaturated aldehydes are useful as
dienes for Diels–Alder cycloaddition reactions. Moreover, acylals
are used as cross linking reagents for cellulose in cotton [3].
Several methods have been used for the synthesis of 1,1-diace-
tates from strong acids including sulfuric acid [4], methane sulfuric
acid [5], sulfamic acid [6], Lewis acids such as lithium bromide [7],
aluminum chloride [8], anhydrous ferrous sulfate [9], PCl3 [10],
FeCl3 [11], NBS [12], Nafion-H [13], sulfated zirconia [14], montmo-
rillonite clay [15], expansive graphite [16], aluminum dodecatung-
stophosphate [17], Well–Dawson acid (H6P2W18O62Á24H2O) [18]
zeolite HSZ-360 [19], Cu(OTf)2 [20], Sc(OTf)3 [21], Bi(OTf)3 [22],
Zn(BF4)2 [23], Bi(NO3)3Á5H2O [24] and ZrCl4 [25], P2O5/Al2O3 [26],
dodecamolybdophoshporic acid [27], SO42À/SnO2 [28], bro-
⇑
Corresponding author. Tel.: +98 311 7932712; fax: +98 311 6689732.
E-mail addresses: moghadamm@sci.ui.ac.ir, majidmoghadamz@yahoo.com
(M. Moghadam).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.02.008

M. Yadegari et al. / Inorganica Chimica Acta 388 (2012) 102–105
103
2.2.6. 3-NO2(C6H4)CH(OAc)2 (Table 4, entry 6)
O
OAc
OAc
IR (KBr) cmÀ1
:
m
= 3094, 2359, 1762, 1534, 1351, 1230, 1201,
1010, 912, 819, 740. 1H NMR (400 MHz, CDCl3): d = 8.41 (s, 1H),
8.27 (d, J = 7 Hz, 1H), 7.85 (d, J = 7 Hz, 1H), 7.75 (s, 1H), 7.61 (t,
J = 7.5 Hz, 1H), 2.2 (s, 6H) ppm.
[Ti(salophen)(OTf)2]
Ac2O, CH3CN
H
R
R
Scheme 1. Conversion of aldehydes to 1,1-diacetates with Ac2O catalyzed by
[TiIV(salophen)(OTf)2].
2.2.7. 3-MeO(C6H4)CH(OAc)2 (Table 4, entry 7)
IR (KBr) cmÀ1
: m = 3030, 2945, 2800, 1755 (s), 1610, 1500, 1380,
1220 (s), 1205, 1015, 790, 720. 1H NMR (400 MHz, CDCl3) d = 7.55
(s, 1H), 7.30 (t, J = 7.5 Hz, 1H), 7.15–7.00 (m, 2 H), 6.80 (d, J = 7.5 Hz,
1H), 3.70 (s, 3H), 2.00 (s, 6H) ppm.
2. Experimental
Chemicals were purchased from Merck or Fluka chemical com-
panies. FT-IR spectra were obtained with potassium bromide pel-
lets in the range 400–4000 cmÀ1 with a Nicolet Impact 400D
spectrometer. Gas chromatography experiments (GC) were per-
formed with a Shimadzu GC-16A instrument using a 2 m column
packed with silicon DC-200 or Carbowax 20m. In the GC experi-
ments, n-decane was used as an internal standard. 1H NMR spectra
were recorded on a Bruker-Avance AQS 400 MHz spectrometer.
The [TiIV(salophen)(OTf)2] catalyst was prepared according to the
literature [63].
2.2.8. 2-MeO(C6H4)CH(OAc)2 (Table 4, entry 8)
IR (KBr) cmÀ1
: m = 3016, 2971, 1763, 1606, 1498, 1375, 1240,
1202, 1024, 951, 758. 1H NMR (400 MHz, CDCl3): d = 8.02 (s, 1H),
7.49 (d, J = 8.4 Hz, 2H), 7.36 (t, J = 7.6 Hz, 1H), 7.00 (t, J = 7.6 Hz,
1H), 6.91 (d, J = 8.4 Hz, 1H), 3.83 (s, 3H), 2.11 (s, 6H) ppm.
2.2.9. 4-MeO(C6H4)CH(OAc)2 (Table 4, entry 9)
IR (KBr) cmÀ1
: m = 3035, 2942, 2805, 1758 (s), 1607, 1502, 1380,
1213, 1203, 1015, 790. 1H NMR (400 MHz, CDCl3): d = 2.11 (s, 6H),
3.81 (s, 3H), 6.95 (d, J = 8.5 Hz, 2H), 7.45 (d, J = 8.5 Hz, 2H), 7.56 (s,
1H) ppm.
2.1. General procedure for preparation of 1,1-diacetates in the
presence of [TiIV(salophen)(OTf)2]
2.2.10. 4-Me(C6H4)CH(OAc)2 (Table 4, entry 10)
In a around bottom flask equipped with a magnetic stirrer, to a
solution of aldehyde (1 mmol), acetic anhydride (4 mmol) in
CH2Cl2 (0.5 mL), catalyst (12 mg, 1.8 mol%) was added and the mix-
ture was stirred at room temperature for an appropriate time. The
progress of the reaction was monitored by GC or TLC (ethyl ace-
tate/ n-hexane 7: 1). After the reaction was completed, saturated
NaHCO3 (5 mL) was added and the catalyst was filtered. The prod-
uct was extracted with diethyl ether (2 Â 15 mL) and the etherates
were dried over Na2SO4. The solvent was evaporated to give the
corresponding 1,1-diacetate.
IR (KBr) cmÀ1
: m = 3033, 2360, 1771, 1367, 1241, 1206, 1068,
1006, 959, 816. 1H NMR (400 MHz, CDCl3): d = 7.62 (s, 1H), 7.42
(d, J = 8.4 Hz, 2H), 7.22 (d, J = 8.4 Hz, 2H), 2.4 (s, 3H), 2.15 (s, 6H)
ppm.
2.2.11. 2-C10H7CH(OAc)2 (Table 4, entry 11)
IR (KBr) cmÀ1 = 1748, 1507, 1370, 1237, 1009. 1H NMR
: m
(400 MHz, CDCl3), d = 2.10 (s, 6H), 7.50–7.62 (m, 4H), 7.74–7.75
(m, 2H), 8.02 (s, 1H), 8.27–8.29 (m, 1H) ppm.
2.2. Spectral data
2.2.12. CH3(CH2)5CH(OAc)2 (Table 4, entry 12)
IR (Neat) cmÀ1: 2930, 2863, 1762 (CO), 1465, 1378, 1250, 1214,
1112, 1015, 968. 1H NMR (400 MHz, CDCl3): d = 0.98 (t, 3H,
2.2.1. 4-Br(C6H4)CH(OAc)2 (Table 4, entry 1)
IR (KBr) cmÀ1
: m = 3100, 2992, 2930, 1743, 1590, 1482, 1370,
1230, 1210, 1060, 1010, 968, 940, 820, 717. 1H NMR (400 MHz,
CDCl3): d = 2.10 (s, 6H, 2ÂCOCH3), 7.39 (d, 2H, J = 8.4 Hz, ArH),
7.53 (d, 2H, J = 8.8 Hz, ArH), 7.61 (s, 1H, CH(OAc)2) ppm.
Table 1
The effect of solvent on the protection of 4-chlorobenzaldehyde with acetic
anhydride.a
Entry
Solvent
Time (min)
Yield (%)b
1
2
3
4
5
CH2Cl2
CH3CN
CH3OH
EtOAc
1
1
1
1
1
95
87
40
31
23
2.2.2. 4-Cl(C6H4)CH(OAc)2 (Table 4, entry 2)
IR (KBr) cmÀ1
: m = 3090, 2990, 2920 1760, 1738, 1592, 1490,
1376, 1242, 1200, 1084, 1060, 1010, 993, 970, 938, 910, 840, 820,
608, 540. 1H NMR (400 MHz, CDCl3): d = 7.64 (s, 1H), 7.44 (d,
J = 7.2 Hz, 2H), 7.38 (d, J = 7.2 Hz, 2H), 2.11 (s, 6H) ppm.
n-Hexane
a
Reaction conditions: 4-chlorobenzaldehyde (1 mmol), acetic anhydride (4 mmol),
catalyst (1.8 mol%), solvent (0.5 mL).
b
2.2.3. 2,4-Cl2(C6H3)CH(OAc)2 (Table 4, entry 3)
Isolated yield.
IR (KBr) cmÀ1 = 1763, 1541, 1473, 1233, 1199, 1012. 1H NMR
: m
(CDCl3, 400 MHz), d = 2.16 (s, 6H), 7.32–7.34 (m, 1H), 7.45–7.53 (m,
1H), 7.67 (s, 1H), 7.89–7.93 (m, 1H) ppm.
Table 2
Optimization of the catalyst amount in the protection of 4-chlorobenzaldehyde with
acetic anhydride.a
2.2.4. 2-Cl(C6H4)CH(OAc)2 (Table 4, entry 4)
IR (KBr) cmÀ1
: m = 3060, 3020, 1760(s), 1605, 1480, 1210(s),
Entry
Catalyst amount (mol%)
Time (min)
Yield (%)b
1010, 780, 620. 1H NMR (400 MHz, CDCl3): d = 7.78 (s, 1H), 7.36–
1
2
3
4
5
6
0.0
0.5
1.0
1.5
1.8
2.5
1
1
1
1
1
1
13
36
54
82
95
95
7.12 (m, 4H), 1.94 (s, 6H) ppm.
2.2.5. 2-NO2(C6H4)CH(OAc)2 (Table 4, entry 5)
IR (KBr) cmÀ1
: m = 3100, 3050, 1760, 1740, 1587, 1454, 1430,
1370, 1350, 1232, 1198, 1095, 1060, 1010, 970, 938, 908, 786,
744. 1H NMR (400 MHz, CDCl3): d = 2.15 (s, 6H, 2 Â COCH3),
7.57–7.61 (m, 1H, ArH), 7.67–7.74 (m, 2H, ArH) 8.05 (dd, 1J = 0.8,
3J = 7.6 Hz, 1H, ArH), 8.20 [s, 1H, CH(OAc)2] ppm.
a
Reaction conditions: 4-chlorobenzaldehyde (1 mmol), acetic anhydride (4 mmol),
CH2Cl2 (0.5 mL), catalyst.
b
Isolated yield.

104
M. Yadegari et al. / Inorganica Chimica Acta 388 (2012) 102–105
Table 3
3. Results and discussion
The effect of axial ligands on the catalytic activity of [Ti(salophen)X2] in the
protection of 4-chlorobenzaldehyde with acetic anhydride.a
First, in order to find the reaction media, the reaction of 4-chlo-
robenzaldehyde with acetic anhydride was performed in different
solvents. Among the CH2Cl2, CH3CN, CH3OH, EtOAc and n-hexane,
dichloromethane and acetonitrile showed higher yield. Since, the
catalyst is insoluble in CH2Cl2, the catalyst reusability is simpler in
this solvent and therefore, dichloromethane was used as solvent
(Table 1). Then, the amount of catalyst was optimized in the same
reaction. As can be seen in Table 2, the highest yield was obtained
in the presence of 1.8 mol% of [TiIV(salophen)(OTf)2]. In order to
show the effect of OTf groups (axial ligands) on the catalytic activity
of [TiIV(salophen)(OTf)2], the catalytic activity of [TiIV(salophen)Cl2],
[TiIV(salophen)(OPh)2] and [TiIV(salophen)(OTf)2] was investigated
Entry
Catalyst
Time (min)
Yield (%)b
1
2
3
[TiIV(salophen)(OTf)2]
[TiIV(salophen)Cl2]
1
1
1
95
46
29
[TiIV(salophen)(OPh)2]
a
Reaction conditions: 4-chlorobenzaldehyde (1 mmol), acetic anhydride (4 mmol),
CH2Cl2 (0.5 mL), catalyst.
b
Isolated yield.
J = 6.8 Hz, CH3), 1.22–1.40 (m, 8H, CH2), 1.66–1.80 (m, 2H, CH2),
2.07 (s, 6H, 2ÂCOCH3), 6.77 (t, 1H, CH(OAc)2) ppm.
Table 4
Conversion of aldehydes to 1,1-diactetates with acetic anhydride catalyzed by [TiIV(salophen)(OTf)2] at room temperature.a
Entry
1
Aldehyde
1,1-Diacetate
Time (min)
1
Yield (%)b
90
OAc
Br
CHO
CHO
CHO
Br
OAc
OAc
2
3
1
1
95
96
Cl
Cl
Cl
Cl
OAc
OAc
OAc
Cl
Cl
OAc
4
5
1
3
97
93
CHO
OAc
Cl
Cl
OAc
OAc
CHO
NO2
NO2
6
1
94
OAc
CHO
OAc
O2N
O2N
7
8
4
1
97
90
OAc
OAc
CHO
MeO
MeO
OAc
OAc
CHO
OMe
OMe
9
10
11
9
3
1
70
75
75
OAc
OAc
MeO
Me
CHO
MeO
Me
OAc
CHO
CHO
OAc
OAc
OAc
12
13
15
15
30
OAc
OAc
CHO
N.R.
AcO OAc
O
a
Reaction conditions: aldehyde (1 mmol), acetic anhydride (4 mmol), catalyst (1.8 mol%), CH2Cl2 (0.5 mL).
Isolated yield.
b

M. Yadegari et al. / Inorganica Chimica Acta 388 (2012) 102–105
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these catalysts is in the following order: [TiIV(salophen)(OTf)2]
(95%) > [TiIV(salophen)Cl2] (46%) > [TiIV(salophen)(OPh)2] (29%)
(Table 3). The results clearly show that introducing the OTf groups
on the Ti(salophen) increases the electron- deficiency of the
catalyst which in turn increases the catalytic activity of [TiIV(salo-
phen)(OTf)2] in the protection of aldehydes with Ac2O. The amount
of acetic anhydride was also optimized and the best result was
obtained with a 3:1 M ratio of acetic anhydride to aldehyde.
Under the optimized reaction conditions, a wide range of
aromatic aldehydes bearing electron- donating and electron-
withdrawing groups were reacted with acetic anhydride in the pres-
ence of [TiIV(salophen)(OTf)2] at room temperature and the corre-
sponding 1,1-diacetates were obtained in good to excellent yields
(70–97%) in 1–9 min. The results are summarized in Table 4. The
electronic properties of the substituents on the aromatic aldehydes
have a significant influence on the reaction time and yield. Benzal-
dehyde and the aldehydes bearing electron-withdrawing groups
such as nitro, chloro, bromo and 3-methoxy [64], afforded the corre-
sponding products in high yields (Table 4, entries 1–7). By contrast,
aldehydes containing electron-donating groups such as 4-methoxy
and 4-methyl groups gave the corresponding 1,1-diacetates in lower
yield and longer reaction times (Table 4, entries 9 and 10). This is
attributed to the reduced electrophilicity of the aldehyde group as
a result of the electron-rich nature of the phenyl ring to which the
aldehyde is attached. The product yield for reaction of 2-methoxy-
benzaldehyde with Ac2O was 90% after 1 min (Table 4, entry 8). It
seems that this is due to the steric hindrance between methoxy
group at the ortho position and aldehyde group, in which the meth-
oxy group is not coplanar with the aldehyde group and therefore,
cannot act as an entirely electron-donating group via resonance.
When, an aliphatic aldehyde such as heptanal was used, only 30%
of the corresponding 1,1-diacetate was produced after 15 min (Ta-
ble 4, entry 12). While, in the reaction of acetophenone (as a ketone)
with acetic anhydride, no product was detected in the reaction mix-
ture (Table 4, entry 13).
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Shaibani, J. Mol. Catal. A: Chem. 219 (2004) 73.
In summary, in this paper we reported a mild and efficient pro-
tocol for the protection of aldehydes by their conversion to 1,1-
diacetates. This catalytic system showed a good catalytic activity
in these reactions. Other advantages of this catalyst are easy prep-
aration of the catalyst, short reaction times, high yields and reus-
ability of the catalyst.
Acknowledgement
[55] M. Moghadam, S. Tangestaninejad, V. Mirkhani, R. Shaibani, Tetrahedron 60
(2004) 6105.
The partial support of this work by Islamic Azad University is
gratefully acknowledged.
[56] S. Gharaati, M. Moghadam, S. Tangestaninejad, V. Mirkhani, I.
Mohammadpoor-Baltork, F. Kosari, Inorg. Chim. Acta 363 (2010) 1995.
[57] M. Moghadam, S. Tangestaninejad, V. Mirkhani, I. Mohammadpoor-Baltork, S.
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[58] M. Moghadam, S. Tangestaninejad, V. Mirkhani, I. Mohammadpoor-Baltork,
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