An expeditious synthesis of cyanohydrin trimethylsilyl ethers using tetraethylammonium 2-(carbamoyl)benzoate as a bifunctional organocatalyst

Article abstract of DOI:10.1016/j.tetlet.2009.04.090

Phthalimide and tetraethylammonium hydroxide react via an unusual pathway to afford tetraethylammonium 2-(carbamoyl)benzoate (TEACB) which is of interest as a bifunctional organocatalyst. TEACB (0.5 mol %) was found to catalyze the addition of trimethylsi

Full text of DOI:10.1016/j.tetlet.2009.04.090

Tetrahedron Letters 50 (2009) 4063–4066
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An expeditious synthesis of cyanohydrin trimethylsilyl ethers using
tetraethylammonium 2-(carbamoyl)benzoate as a bifunctional organocatalyst
,
*
Mohammad G. Dekamin , Solmaz Sagheb-Asl, M. Reza Naimi-Jamal
Department of Chemistry, Iran University of Science and Technology, Tehran 16846-13114, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Phthalimide and tetraethylammonium hydroxide react via an unusual pathway to afford tetraethylam-
monium 2-(carbamoyl)benzoate (TEACB) which is of interest as a bifunctional organocatalyst. TEACB
(0.5 mol %) was found to catalyze the addition of trimethylsilyl cyanide (TMSCN) to carbonyl compounds
under solvent-free conditions at room temperature with very short reaction times. A wide variety of alde-
hydes and ketones were transformed into the corresponding cyanohydrin trimethylsilyl ethers in high to
quantitative yields.
Received 27 February 2009
Revised 8 April 2009
Accepted 24 April 2009
Available online 3 May 2009
Keywords:
Cyanosilylation
Ó 2009 Elsevier Ltd. All rights reserved.
Bifunctional organocatalysis
Tetraethylammonium 2-
(carbamoyl)benzoate
Carbonyl compounds
Cyanohydrins
Cyanohydrins are highly versatile synthetic intermediates,
which can easily be converted into various important building
scale or asymmetric versions of the reaction, the ready availability
and low toxicity of the organocatalysts.3,12–20
blocks including
a
-hydroxy acids,
a
-amino acids,
a
-hydroxy alde-
In contrast to the traditional catalytic systems, organocatalytic
protocols for cyanosilylation of carbonyl compounds have mainly
demonstrated their advantages as Lewis basic catalysts;3,12–21
methods using Lewis acidic catalysts are scarce.22 More recently,
bifunctional organocatalysts have received attention for cyanosily-
lation of carbonyl compounds.23–25 It is noteworthy that most of
the above catalytic systems rely on the activation of only one of
the reacting species. Inspired by the multipoint binding sites of en-
zymes, attention has recently turned to the development of bi- and
poly-functional catalytic systems.10,22–26 All these developments
have advanced significantly the frontier of organocatalytic cyano-
hydrin synthesis. However, there is still room for improvement,
particularly with regard to reaction generality, catalyst simplicity,
safer solvents and especially reaction rate. In view of our contin-
uing interest in the use of organocatalysts for efficient cyanosilyla-
tion of carbonyl compounds,13,15 we herein disclose the first
application of tetraethylammonium 2-(carbamoyl)benzoate (TEA-
CB, 1) as an effective bifunctional organocatalyst for this transfor-
mation under solvent-free conditions (Scheme 1).
hydes or ketones, b-amino alcohols, and vicinal diols.1–5 They are
also components of commercially important compounds such as
the pyrethroid insecticides, cypermetrin and fluvaliate.5 However,
the preparation of cyanohydrins by the addition of highly toxic
HCN to carbonyl groups is not a straightforward process and the
procedure should be undertaken with caution. The other problem
which affects the yields of the products originates in the existence
of equilibrium conditions between the reactants and the prod-
ucts.3,5a To overcome these problems, the reaction of trimethylsilyl
cyanide (TMSCN)6 with carbonyl compounds in the presence of Le-
wis acid,7 Lewis base3,8 and double activating1c,9 or bifunctional10
catalytic systems has been described. Since the pioneering works,11
a plethora of catalysts have been reported in the literature for both
racemic and asymmetric addition of TMSCN to aldehydes and ke-
tones.7–10 The majority of these catalytic systems require metallic
Lewis acidic species7 which may contain a variety of ligands to en-
able enantioselective transfer of cyanide to carbonyls.1,4a,b,9,10 On
the other hand, the recent organocatalytic protocols are particu-
larly attractive because of the mildness of the reaction conditions,
operational simplicity, the potential for the development of large
Potassium phthalimide (PPI) has been traditionally used as a
suitable nucleophile in the Gabriel synthesis of primary amines.27
However, it has received less attention as a catalyst in organic
synthesis.28,29 In our previous works, PPI and the sodium salt of
saccharin combined with tetrabutylammonium iodide (TBAI) were
shown to be effective Lewis basic organocatalysts for the activation
of TMSCN in the cyanosilylation of carbonyl compounds15 or
* Corresponding author. Tel.: +98 21 7724 0284; fax: +98 21 7749 1204.
E-mail address: mdekamin@iust.ac.ir (M.G. Dekamin).
Previous name: Mohammad G. Dakamin.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.090

4064
M. G. Dekamin et al. / Tetrahedron Letters 50 (2009) 4063–4066
Table 1
Et4N+
O
O
Optimization of the TEACB catalyst loading for cyanosilylation of 4-
chlorobenzaldehydea
O
H
H
OSiMe3
N
1 (0.5 mol %)
O
OTMS
CN
O
TEACB
CN
H
H
1.2 equiv. TMSCN
solvent-free, r.t.
R'
R
R
1.2 equiv. TMSCN
solvent-free, r.t.
R'
Cl
Cl
Scheme 1. Cyanosilylation of carbonyl compounds catalyzed by TEACB.
Entry
Mol %
Time (min)
Conversion (%)
TONb
TOFc
1
2
3
1.0
0.5
0.25
0.5
0.5
2.5
0.5
—
12
15
45
30
45
30
120
120
95
98
93
98
97
96
0
95
475
784
496
392
259
76.8
0
cyclotrimerization of isocyanates.29,30 In continuation of our inter-
est to develop more effective phthalimide-based nucleophilic
organocatalysts, we decided to investigate the possibility of using
tetraethylammonium phthalimide (TEAPI, 4) for the addition of
TMSCN to carbonyl compounds. Since phthalimide (PI, 2) has a
pKa of 8.3, it reacts with different alkali metal hydroxides in EtOH
to give the corresponding phthalimide salts.27 Despite this well-
known chemistry, it was observed that TEACB 1 was produced
exclusively upon reaction of 2 and tetraethylammonium hydroxide
(TEAOH, 3) in EtOH. Similar results were observed in other solvents
such as water and toluene. However, the yield of 1 was signifi-
cantly improved in water (Scheme 2).31
A systematic study on the cyanosilylation of 4-chlorobenzalde-
hyde, as a model compound, was carried out using different
amounts of the bifunctional organocatalyst 1 at room temperature
and the results are summarized in Table 1. It was found that the
best result in terms of turnover number (TON) and turnover fre-
196
372
196
194
38.4
0
4d
5e
6f,15
7g
8
0
0
0
a
TMSCN (1.2 mmol) was added to
a
mixture of 4-chlorobenzaldehyde
(1.0 mmol) under solvent-free conditions at room temperature.
b
Turnover number.
Turnover frequency.
Potassium benzoate and PPI were used as catalysts, respectively (0.5 mol % of
c
d,e
TEAI was used as cocatalyst).
f
PPI as catalyst.
Benzamide as catalyst.
g
sulfinate (45 min at room temperature)13c were employed to
promote cyanosilylation of 4-chlorobenzaldehyde to the corre-
sponding trimethylsilylated cyanohydrin with yields of 80%, 98%
and 98%, respectively.
Encouraged by these results, aromatic, heteroaromatic, conju-
gated and aliphatic aldehydes or ketones were subjected to
cyanosilylation under the optimized reaction conditions (TEACB;
0.5 mol %, 1.2 equiv of TMSCN, 25 °C, solvent-free conditions).32
Table 2 shows the scope of the reaction using a number of rep-
resentative carbonyl compounds wherein high to quantitative
yields of cyanohydrin trimethylsilyl ethers 5 were obtained with-
in short reaction times in all the cases studied (Table 2, entries
1–20).
No by-products such as products of benzoin condensation or
desilylation were observed.17 This protocol tolerated acid-sensitive
substrates such as furfural, thiophene-2-carbaldehyde and cinna-
maldehyde which can decompose or polymerize under acidic con-
ditions (Table 2, entries 10–12).7e Notably, the cyanosilylation of
(E)-cinnamaldehyde afforded the corresponding 1,2-addition prod-
uct, exclusively (entry 12). Furthermore, carbonyl compounds
quency (TOF) could be achieved with
a catalyst loading of
0.5 mol % (Table 1, entry 2).32 Furthermore, the reaction of benzoic
acid and TEAOH gave a viscous liquid and no crystals were ob-
tained. Hence, the catalytic activity of combined potassium benzo-
ate and tetraethylammonium iodide (TEAI) was examined under
similar reaction conditions to 1. On the other hand, combination
of PPI and TEAI required longer reaction times for complete conver-
sion of 4-chlorobenzaldehyde under similar reaction conditions
(entries 4 and 5). Interestingly, no reaction was observed when
using benzamide or in the absence of TEACB under similar reaction
conditions (entries 7 and 8). Therefore, the low catalyst loading re-
quired for this transformation underscores the extraordinary cata-
lytic activity of the bifunctional organocatalyst 1 in activating both
silicon-based reagents such as TMSCN33,34 and carbonyl com-
pounds.10f,23,24 For example, with the previously introduced organ-
ocatalysts, 5 mol % of trisaminophosphine in THF (0.5 h at room
temperature),17 5 mol % of N-heterocyclic carbenes (NHCs) in THF
(2 h at room temperature),18a or 2.5 mol % of potassium p-toluene-
O
+
Et4N
N
O
O
4
_
+
Et4N OH
N
H
+
O
O
+
Et4N
O
2
3
NH2
O
1
Scheme 2. Unusual reaction pathway of phthalimide and tetraethylammonium hydroxide.

M. G. Dekamin et al. / Tetrahedron Letters 50 (2009) 4063–4066
4065
Table 2
bearing electron-withdrawing groups such as NO2, CN or F (Table
2, entries 3–5, 16 and 18), were more active than those with elec-
tron-donating groups (entries 7–11). It is of particular interest to
note that cyanosilylation of relatively unreactive ketones such as
acetophenone, benzophenone or their derivatives (entries 15–18)
was readily achieved under the optimized reaction conditions.16a
The reaction of acyclic and cyclic aliphatic carbonyl compounds
afforded the corresponding products in very good yields in longer
times compared with aromatic and heterocyclic counterparts (en-
tries 13, 14, 19 and 20). In general, the reaction conditions are mild
and the catalyst could be separated easily from the reaction mix-
ture by aqueous extraction. Hence, separation of the catalyst by
aqueous extraction prior to purification of the product could lead
to the simplification of the process.
Organocatalytic cyanosilylation of various carbonyl compounds
O
Et4N+
O
NH2
OSiMe3
CN
1
O
O
R
R'
R
1.2 equiv. TMSCN
solvent-free, r.t.
R'
5
Entry
Carbonyl compound
Time (min)
Conversion (%)
R
R0
According to the results obtained, a double activating mecha-
nism for the cyanosilylation of carbonyl compounds catalyzed by
TEACB can be proposed (Scheme 3). Whereas the carboxylate moi-
ety in TEACB reacts initially with the silicon atom of TMSCN to ex-
pand its coordination sphere and giving pentacoordinated
intermediate 6,33,34 hydrogen bonding between the carbamoyl
moiety of the organocatalyst and the carbonyl group (intermediate
7)23,24 facilitates subsequent addition of cyanide to the carbonyl
compounds to produce the desired product 5 via intermediate
8.10f It may be that the presence of electron-withdrawing groups
on the phenyl ring results in shorter reaction times compared with
electron-donating groups. On the other hand, steric hindrance and
competitor methoxy groups disrupting formation of hydrogen
bonds during the reaction of 2-methoxybenzaldehyde (entry 9)
may be the reason that this substrate requires a longer reaction
time compared to its 4-isomer (entry 8). It is noteworthy that this
substrate required shorter reaction times using Lewis basic organ-
ocatalysts introduced by our research group.13,15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
4-ClC6H4
4-BrC6H4
4-NO2C6H4
3-NO2C6H4
4-CNC6H4
C6H5
4-MeC6H4
4-MeOC6H4
2-MeOC6H4
2-Furyl
2-Thienyl
(E)-Ph–CH@CH
Ph(CH2)2
CH3(CH2)6
C6H5
H
H
H
H
H
H
H
H
H
H
H
H
H
H
15
25
5
10
10
20
30
25
40
45
100
45
60
90
120
120
240
180
150
120
98
>99
99
99
96
92
98
93
99
>99
92
97
>99
98
70
>99
83
>99
95
Me
Me
Ph
Ph
Me
4-NO2C6H4
C6H5
4-FC6H4
CH3(CH2)4
Cyclohexanone
90
_
O
Me
Me
O
R
R'
CN
Et4N+
Si
O
N
H
H
Me
O
CN
Me3Si
(6)
Et4N+
O
_
Me
Me
O
O
O
N
H
H
CN
Me
Si
Et4N+
H
O
N
H
(1)
O
O
OSiMe3
CN
R'
(7)
R
R
_
Me
O
R'
Me
O
O
(5)
Si
Me
O
R
+
Et4N
NH2
R' CN
(8)
Scheme 3. Proposed mechanism for the cyanosilylations of carbonyl compounds by TEACB.

4066
M. G. Dekamin et al. / Tetrahedron Letters 50 (2009) 4063–4066
K.; Masumoto, S.; Kanai, M.; Curran, D. P.; Shibasaki, M. Tetrahedron Lett. 2002,
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582; (b) Dekamin, M. G.; Javanshir, S.; Naimi-Jamal, M. R.; Hekmatshoar, R.;
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oyl)benzoate (TEACB), as a new bifunctional organocatalyst, was
demonstrated for the clean and rapid cyanosilylation of a wide
range of carbonyl compounds. The mild reaction conditions, low
catalyst loading, excellent functional group tolerance, high to
quantitative yield, chemical stability and the simple preparation
of the catalyst and its removal from reaction mixture illustrate
the attractive features of this protocol. Studies are in progress to
develop the catalytic scope of TEACB to other synthetically impor-
tant reactions as well as the reusability of the catalyst.
15. Dekamin, M. G.; Karimi, Z. J. Organomet. Chem. 2009, 694, 1789.
16. (a) Raj, I. V. P.; Suryavanshi, G.; Sudalai, A. Tetrahedron Lett. 2007, 48, 7211; (b)
Denmark, S. E.; Chung, W. J. Org. Chem. 2006, 71, 4002; (c) Tian, S. K.; Hong, R.;
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Acknowledgements
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We are grateful for the financial support from the Research
Council of Iran University of Science and Technology (IUST), Iran
(Grant No. 160/5295). We also thank Professor Issa Yavari (Tarbiat
Modarres University, Tehran, Iran) for helpful suggestions.
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31. Preparation of tetraethylammonium 2-(carbamoyl)benzoate (TEACB, 1): To
25 mL round-bottomed flask equipped with a magnetic stirrer and a condenser
were added phthalimide (6.80 mmol, 1.00 g) and tetraethylammonium
a
2
hydroxide 3 (6.80 mmol, 20% w/w in water, d = 1.01 g/mL, 5.0 mL). The mixture
was stirred at room temperature for 5 min. To this was added 5 mL of distilled
water and the mixture was refluxed for 4 h and then allowed to cool. The
solvent was evaporated and the residue was kept at 0–4 °C for 1 h to afford
pure TEACB (1) in quantitative yield. The white crystals were collected and
dried under reduced pressure. Mp 86–88 °C; IR (KBr):
m
3561–3208 (br s, N–H),
3067, 2988, 2953, 1666, 1580, 1550, 1487, 1400, 1173, 1002 cmꢀ1
;
1H NMR
(500 MHz, CDCl3): d 0.98–1.01 (t, J = 7.20 Hz, 12H), 2.97–3.01 (t, J = 7.20 Hz,
8H), 6.15 (br s, s, 1H), 7.05–7.08 (t, J = 7.50 Hz, 1H), 7.16–7.19 (t, J = 7.40 Hz,
1H), 7.34–7.36 (d, J = 7.50 Hz, 1H), 7.70–7.72 (d, J = 7.75 Hz, 1H), 10.05 (br s, s,
1H), ppm; 13C NMR (125 MHz, CDCl3): d 7.6, 52.4, 126.7, 128.3, 128.4, 129.6,
130.6, 143.5, 170.8, 174.9 ppm. Anal. Calcd for C16H26N2O3: C, 65.28; H, 8.90; N,
9.52. Found: C, 65.02; H, 8.75; N, 9.82.
32. General procedure for cyanosilylation of carbonyl compounds: TMSCN (1.2 mmol,
0.15 mL) was added to a mixture of 1.0 mmol of carbonyl compound and
TEACB (0.005 mmol, 1.5 mg). The resulting mixture was stirred at room
temperature for time indicated in Table 2. The reaction was monitored by TLC.
After completion, the reaction mixture was quenched with water (1.0 mL) and
the organic materials were extracted with EtOAc (2 ꢁ 1.5 mL). The organic
phase was washed with brine followed by water (1.5 mL) and dried over
MgSO4. The solvent was evaporated to afford the desired products which in
some cases were essentially pure cyanohydrin TMS ethers. Further purification
of the products could be performed by silica gel column chromatography
(EtOAc–hexane, 1:10). The isolated yields were in good agreement with those
obtained by GC analysis.
10. (a) Hamashima, Y.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122, 7412;
(b) Hamashima, Y.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2001, 42, 691; (c)
Yabu, K.; Masumoto, S.; Yamasaki, S.; Hamashima, Y.; Kanai, M.; Du, W.;
Curran, D. P.; Shibasaki, M. J. Am. Chem. Soc. 2001, 123, 9908; (d) Masumoto, S.;
Suzuki, M.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2002, 43, 8647; (e) Yabu,
33. For a review on selective catalysis using the hard-soft acid and base (HSAB)
principle, see: Woodward, S. Tetrahedron 2002, 58, 1017.
34. Orito, Y.; Nakajima, M. Synthesis 2006, 1391.