N-Heterocyclic carbene catalyzed oxidative stannylation of aldehydes: A facile entry to organotin(IV) carboxylates

Article abstract of DOI:10.1016/j.tetlet.2013.03.050

A simple protocol is described for the oxidative transformation of aldehydes to the corresponding organotin(IV) carboxylates in high yields (up to 90%) that utilizes atmospheric O₂ as the sole oxidant, N-heterocyclic carbene as catalyst (at 10 mol %), and tributyl tin chloride as stannylating agent. The uniqueness of the reaction lies in the direct conversion of aldehydes to the corresponding organotin(IV) carboxylates via stannylation of carboxylic acids, generated from the reaction of a Breslow intermediate with O₂.

Full text of DOI:10.1016/j.tetlet.2013.03.050

Tetrahedron Letters 54 (2013) 2679–2681
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
N-Heterocyclic carbene catalyzed oxidative stannylation of aldehydes:
a facile entry to organotin(IV) carboxylates
⇑
Rambabu N. Reddi, Pushpa V. Malekar, Arumugam Sudalai
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pashan Road, Pune 411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple protocol is described for the oxidative transformation of aldehydes to the corresponding organo-
tin(IV) carboxylates in high yields (up to 90%) that utilizes atmospheric O2 as the sole oxidant, N-hetero-
cyclic carbene as catalyst (at 10 mol %), and tributyl tin chloride as stannylating agent. The uniqueness of
the reaction lies in the direct conversion of aldehydes to the corresponding organotin(IV) carboxylates via
stannylation of carboxylic acids, generated from the reaction of a Breslow intermediate with O2.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 10 January 2013
Revised 8 March 2013
Accepted 12 March 2013
Available online 21 March 2013
Keywords:
Carboxylic acid stannanes
N-Heterocyclic carbene
Oxidative stannylation
Oxygen
Tributyltin chloride
R1
R2
Organotin(IV) carboxylates have attracted considerable atten-
tion owing to the enormous variety of discrete or polymeric struc-
tural topologies1 and their unexpected properties as potential
pharmaceuticals such as anti-tumor,2 antimicrobial,3 anti-fungal4,
and cytotoxic activity.5 They are also used as fungicides,6 pesti-
cides,7 antifouling coating materials,8 preservatives for wood,9 aca-
iPr
HO
S
N
N
N
N
Ar
S
Cl
Ar
Bn
ClO4
Cl
iPr
1d
1 = R2 = Me
Ar = Mes
1c
1a
R
ricides10
,
and homogeneous catalysts11 in industry. In the
1e Ar = 2,6 - iPr2C6H3
1b R1 , R2 = cyclohexyl
literature, their synthesis has been reported from the correspond-
ing carboxylic acids12 and their salts.13 However, these methods
suffer from disadvantages such as harsh reaction conditions, met-
als as by-products and often giving low yields.
Ph
Ph
N
N
Mes
N
Ph
N
N
N
Cl
ClO4
In recent years, many NHC-catalyzed reactions have been re-
ported for C–C, C–N, and C–O bond-forming reactions.14 In partic-
ular, the direct esterification of aldehydes was well-documented
with alcohols as nucleophiles under different oxidative condi-
tions.15 However, these reactions employ both boronic acids16
and alkyl halides17 under aerobic conditions. Recently, we reported
a useful preparative procedure for the oxidative esterification18 of
aldehydes under aerobic oxidative conditions with alcohols. Thus,
it is of interest to explore NHC catalyzed oxidative metallation of
aldehydes with metal halides as electrophiles. In this Letter, we
wish to report a new efficient and practical method for the one-
step conversion of aldehydes directly into the corresponding
organotin(IV) carboxylates using N-heterocyclic carbenes as cata-
lysts (Scheme 1).
1g
1f
Figure 1. NHC precatalysts screened for oxidative stannylation.
NHC cat.(10 mol%)
nBu3SnCl (1.2 equiv)
ArCO2SnnBu3
Ar-CHO
THF, DBU (1.2 equiv)
25°C,16h, O2
3a-p
2a-p
Scheme 1. NHC-catalyzed oxidative stannylation of aromatic aldehydes.
Initially, we have chosen 4-nitrobenzaldehyde 2g as a test sub-
strate for the oxidative stannylation using tributyltin chloride
(1.2 equiv) as tin source, NHC precatalyst 1a (10 mol %), and DBU
⇑
Corresponding author. Tel.: +91 20 25902174; fax: +91 20 25902676.
E-mail address: a.sudalai@ncl.res.in (A. Sudalai).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.050

2680
R. N. Reddi et al. / Tetrahedron Letters 54 (2013) 2679–2681
Table 1
Table 2
Oxidative stannylaltion of 4-nitrobenzaldehyde: optimization studiesa
Oxidative stannylaltion of aromatic aldehydes: substrate scopea
O
1a
(10 mol%)
Cat:
O
NHC catalyst (10 mol%)
nBu3SnCl (1.2 equiv)
Ar-CO2nSnBu3
nBu3SnCl (1.2 eqiv)
n
Ar-CHO
OSnBu3
H
THF, DBU (1.2 equiv),
25°C, 16h
2a-p
3a-p
solvent, base, 25°C,
16h
O2N
O2N
3g
2g
Entry
Substrate, Ar (2a–p)
Product (3a–p)
Yieldb (%)
Entry
1
Catalyst
Solvent
Base
Yield of 3gb (%)
1
2
3
4
5
6
7
8
Benzaldehyde
m-Tolualdehyde
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
40
76
79
81
74
71
90
85
70
87
82
87
1a
CH3CN
THF
DMF
CH2Cl2
THF
THF
THF
THF
THF
THF
THF
THF
THF
DBU
DBU
DBU
DBU
K2CO3
KOtBu
NaH
DBU
DBU
DBU
DBU
DBU
DBU
55
90
35
5
40
35
20
41
75
40
33
54
20
4-OMe-benzaldehyde
3,4-(–OCH2O–)benzaldehyde
Salicylaldehyde
3-NH2-benzaldehyde
4-NO2-benzaldehyde
2-NO2-benzaldehyde
4-Br-benzaldehyde
4-CN-benzaldehyde
3-Pyridine carboxaldehyde
Furfural
2
1a
9
3
4
5
6
7
8
1b
1c
1d
1e
1f
10
11
12
13
14
15
16
Cinnamaldehyde
22 (84)c
79c
72c
74c
4-Br-cinnamaldehyde
3,4-(OMe)2-cinnamaldehyde
1-Br-3,4-dihydronaphthaldehyde
1g
a
Reaction conditions: 4-nitrobenzaldehyde (1 equiv), n-tributyltin chloride
(1.2 equiv), NHC catalyst (10 mol %), base (1.2 equiv), under O2 atmosphere; 25 °C,
a
Reaction conditions: aldehyde (5 mmol), n-tributyltin chloride (6 mmol), NHC
16 h.
b
catalyst 1a (10 mol %), DBU (6 mmol), under O2 atmosphere; 25 °C, 16 h.
Isolated yields after column chromatographic purification.
b
Isolated yield after column chromatographic purification.
NHC catalyst 1d was used.
c
(1.2 equiv) as base in CH3CN, which produced tri-n-butyltin 4-
nitrobenzoate 3g in 55% yield (Table 1, entry 1). We conducted sev-
eral experiments to improve the yield and found that THF was the
best choice (90%) while other solvents (DMF, CH2Cl2) gave only
moderate yield of 3g. Use of other inorganic bases (K2CO3, KOBu,t
and NaH) resulted in a sluggish reaction with poor yields. DBU
was thus chosen as the base for further optimization (Table 1, entry
2). Among the NHC precatalysts18,20 screened (Fig. 1), thiazolium
(1b and 1c)-based precatalysts afforded 3g in 41% and 75% yields,
respectively while the imidazolium (1d and 1e) and triazolium (1f
and 1g)-based precatalysts also gave only moderate yields (54%).
With the optimized conditions in hand, we examined the sub-
strate scope of the reaction. Various aldehydes were then subjected
to oxidative stannylation; the results of which are presented in
Table 2. Aromatic aldehydes, having electron-withdrawing and
releasing groups at various positions on the aromatic ring were
reacted to give the corresponding organotin(IV) carboxylates
(Table 2) in excellent yields. Also, heteroaromatic aldehydes
underwent this oxidative stannylation efficiently to give the corre-
sponding tin(IV) carboxylates in 82% and 87% yields, respectively.
R1
HO
Ar
S
N
O2
R2
R3
Ar-CHO
I
R1
S
R1
R2
H
O
O
O
Ar
S
N
N
R3
R2
R3
II
Ar-CHO
O
O
nBu3SnCl
n
Ar
OSnBu3
Ar
OH
2
III
Scheme 2. Proposed mechanistic pathway for tributyltin(IV) carboxylates.
In the case of
a,b-unsaturated aldehydes, the oxidative stannyla-
another molecule of aldehyde to afford carboxylic acid in situ; the
stannylation of which with tri-n-butyltin chloride produces tri-n-
butyltin carboxylates.
tion under the above optimized conditions gave 3m (entry 13, Ta-
ble 2) in low yield (22%). However, replacing 1a with 1d gave the
corresponding
a,b-unsaturated acid stannane in 84% yield. Thus,
In conclusion, we have developed an efficient catalytic pro-
cess19 for the preparation of organotin(IV) carboxylates21,22 using
N-heterocyclic carbene catalyzed oxidative metallation of alde-
hydes under aerobic conditions. This methodology illustrates the
use of metal electrophiles in NHC catalyzed reactions.
with 1d as precatalyst,
a
,b-unsaturated aldehydes (entries 14–
16) underwent this reaction successfully to give excellent yields
of the corresponding organotin(IV) carboxylates. The structure of
tri-n-butyltin carboxylates 3a–p was specifically ascertained on
the basis of spectroscopic techniques. Their carbonyl carbons have
shown typical signals in the range d 169–172 in their 13C NMR
spectra. Strong characteristic IR absorptions in the range 1600–
1640 cmÀ1 for all the tri-n-butyltin carboxylates confirmed the
presence of carboxylate functionality. Their structures were further
supported by their high resolution mass spectra.
Acknowledgments
R.B.R. and P.M. thank the CSIR, New Delhi for the award of re-
search fellowship and DST, (No. SR/S1/OC-67/2010) New Delhi,
for financial support. The authors are thankful to Dr. V. V. Ranade,
Head, Chem. Engg. & Process Development., for his constant sup-
port and encouragement.
Based on literature reports,17 a probable mechanistic pathway
is shown in Scheme 2. The Breslow intermediate I, upon reaction
with O2 gives the peroxy anion II,17a which then reacts with

R. N. Reddi et al. / Tetrahedron Letters 54 (2013) 2679–2681
16. Meng, J.; Gao, M.; Wei, Y.; Zhang, W. Chem. Asian J. 2012, 7, 872.
2681
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21. Tri-n-butyltin(IV) furan-2-carboxylate (3l): Yield: 87%, colorless liquid; IR
(CHCl3, cmÀ1): 1011, 1364, 1389, 1409, 1549, 1579, 1600, 2853, 2921, 2954; 1
H
NMR (200 MHz, CDCl3):
d 7.53 (s, 1H), 7.11 (d, J = 3.4 Hz, 1H), 6.48 (dd,
J = 3.3 Hz, J = 1.6 Hz, 1H), 1.85–1.58 (m, 6H), 1.48–1.25 (m, 12H), 0.92 (t,
J = 7.1 Hz, 9H); 13C NMR (50 MHz, CDCl3): d 163.2, 146.3, 145.3, 117.0, 111.5,
27.8, 27.0, 16.8 and 13.6; Analysis: C17H30O3Sn requires C, 50.90; H, 7.54.
Found: C, 50.78; H, 7.39; MS (ESI): [M+Na]+ calcd for C17H30NaO3Sn: 425.11;
found: 425.44.
22. Tri-n-butyltin(IV) 3,4-dimethoxy-trans-cinnamate (3o): Yield: 72%, colorless
solid, mp: 224–225 °C; IR (CDCl3, cmÀ1); 2954, 2922, 1639, 1513, 1262, 1139,
1025. 1H NMR (200 MHz, CDCl3): d 7.62 (d, J = 15.9 Hz, 1H), 7.07 (m, 2H), 6.83
(d, J = 8.6 Hz, 1H), 6.32 (d, J = 15.9 Hz, 1H), 3.90 (s, 6H), 1.83–1.62 (m, 6H),
1.36–1.28 (m, 12H), 0.93 (t, J = 7.2 Hz, 9H). 13C NMR (50 MHz, CDCl3): d 172.3,
151.1, 149.2, 145.2, 127.5, 122.7, 110.9, 109.6, 55.8, 27.8, 27.0, 16.5 and 13.7;
Analysis: C23H38O4Sn requires C, 55.55; H, 7.70. Found: C, 55.77; H, 7.48. MS
(ESI): [M+Na]+ calcd for C23H38NaO4Sn: 521.16; found: 521.25.