Niobium fluoride (NbF5): A highly efficient catalyst for solvent-free cyanosilylation of aldehydes

Article abstract of DOI:10.1055/s-2006-958942

An efficient method for the addition of trimethylsilylcyanide (TMSCN) to aldehydes using dispersed NbF₅ as the catalyst is described. Cyano transfer from TMSCN to aldehydes occurs within 10 minutes at room temperature in the presence of 0.5 mol

Full text of DOI:10.1055/s-2006-958942

PAPER
215
Niobium Fluoride (NbF5): A Highly Efficient Catalyst for Solvent-Free
Cyanosilylation of Aldehydes
T
he Cyanosily
u
lation of
A
ld
n
ehydes usin
g
Department of Chemistry, Inha University, Incheon 402-751, South Korea
Fax +82(32)8675604; E-mail: sungsoo@inha.ac.kr
Received 11 September 2006; revised 10 October 2006
‘sustainable chemistry’.15 One of the major goals is to fa-
cilitate easy separation of the final reaction mixture. This
feature can lead to improved processing steps, better pro-
cess economics and also environmentally friendly indus-
Abstract: An efficient method for the addition of trimethylsilylcy-
anide (TMSCN) to aldehydes using dispersed NbF5 as the catalyst
is described. Cyano transfer from TMSCN to aldehydes occurs
within 10 minutes at room temperature in the presence of 0.5 mol%
of NbF5 under solvent-free conditions giving cyanohydrins in ex- trial manufacturing.
cellent yields of over 95%. These conditions are extremely mild,
simple and tolerate various functional groups.
Numerous reactions have been carried out in solvents in-
cluding dichloromethane, acetonitrile and tetrahydrofu-
ran. However, due to the current challenges in developing
solvent-free and environmentally-benign synthetic sys-
tems, we would like to report a novel method for the cya-
nation of aldehydes using TMSCN in the presence of
dispersed NbF5 under solvent-free conditions.
Key words: aldehydes, catalysis, green chemistry, cyanohydrins,
solvent-free
The catalytic addition reaction of trimethylsilylcyanide to
carbonyl compounds is an area of great interest due to the
synthetic versatility of cyanohydrins. These can be elabo-
rated into a variety of useful synthetic building blocks
such as a-hydroxy acids, a-hydroxy aldehydes, 1,2-diols,
and a-amino alcohols.2 A variety of catalysts can be
used3,4 including Lewis acids, Lewis bases, metal alkox-
ides, bifunctional catalysts and inorganic salts. Many met-
al complexes have been employed as Lewis acids for the
addition of hydrogen cyanide (HCN) or TMSCN to alde-
hydes and ketones including magnesium, zirconium, tita-
nium, aluminium, yttrium, lanthanum, samarium,
vanadium and gadolinium complexes containing mono-
or polydentate ligands.5 Metal halides, such as AlCl3,
BiBr3, InX3, LnCl3 (where Ln = La, Ce, Sm), MgBr2,
SnCl4, TiCl4, and ZnI2 are known to be Lewis acidic cata-
lysts used in the cyanosilylation of aldehydes.6 Lithium
chloride acts as an active and simple catalyst in the cy-
anosilylation of aldehydes and ketones.7
Niobium(V) has strong oxophilicity and can promote
Lewis acid mediated reactions such as the Diels–Alder re-
action, allylation of aldehydes and others.16 We have used
these properties of NbF5 in the silylcyanation of aldehydes
and the results are described here.
The catalytic activity of NbF5 was tested using benzalde-
hyde as a model substrate. NbF5 exhibited excellent activ-
ity under solvent-free conditions. When a mixture of
benzaldehyde (1 mmol) and TMSCN (1.2 equiv) was
treated with various amounts of NbF5 (0.1–10 mol%) at
room temperature, the cyanide addition occurred in rela-
tively short reaction times (Table 1, entries 1–5). The
amount of catalyst appeared to be an important parameter
in the reduction of reaction time. The reaction took place
more rapidly with larger quantities of catalyst (Table 1,
entry 1 and 2). Using 0.5 mol% of NbF5 proved to be the
optimal amount and gave excellent yields (96%) at room
temperature under solvent-free conditions (Table 1, entry
4). A further reduction in catalytic loading meant that
longer reaction times were needed. However, the NbF5-
cataylzed system is still an excellent method in terms of
reaction time as an effective reaction can take place using
only 0.1 mol% of catalyst in 25 minutes and 93% yield.
Other recently reported cyanosilylation reactions under
solvent-free conditions took four hours and gave a 99%
yield.9 The catalytic efficiency decreased when the reac-
tion was conducted in organic solvents (Table 1, entries
6–8).
P(RNCH2CH2)N, a non-ionic strong base has been used
as a catalyst in the cyanosilylation of aldehydes and ke-
tones.8 Tetramethylguanidine was successfully employed
as an effective catalyst in the cyanosilylation of ketones.9
Very recently, N-heterocyclic carbenes (NHC) were re-
ported in the activation of TMSCN.10,11 In recent years,
several optically active catalysts used in the asymmetric
synthesis of cyanohydrins have been reported in the liter-
ature.12 Recently we have reported the cyanosilylation of
aldehydes and ketones utilizing several chiral13 and
achiral14 catalysts.
These results clearly indicate that low catalyst loading and
solvent-free conditions are optimal in the activation of
TMSCN with NbF5. 20 mol% of BiCl3,3m 5 mol% of
Cu(OTf)23d or 20 mol% of N-methylmorpholine N-oxide
(NMO)14c was required in the presence of organic solvents
to promote complete conversion of benzaldehyde to the
corresponding trimethylsilylated cyanohydrin.
The heterogenization of inorganic reagents and catalysts
in organic reactions is a very important area in ‘green’ or
SYNTHESIS 2007, No. 2, pp 0215–0218
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x
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x
x
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0
0
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Advanced online publication: 14.12.2006
DOI: 10.1055/s-2006-958942; Art ID: F14706SS
© Georg Thieme Verlag Stuttgart · New York

216
S. S. Kim, G. Rajagopal
PAPER
a
Table 2 Cyanosilylation of Aldehydes Using NbF5
Table 1 Cyanosilylation of Benzaldehyde under Various Condi-
tions
OSiMe3
O
NbF5
H
+ Me3SiCN
Entry
Catalyst
(mol%)
Solvent
Time (min) Yield (%)
CN
solvent-free, r.t.
R
H
R
Entries
1
Substrate
Time (min) Yield (%)b
1
2
3
4
5
6
7
8
10
5
–
–
–
–
–
5
5
98
98
96
96
93
95
87
80
10
3 h
24 h
3 h
4 h
30
96c
9517
753g
813d
999
O
H
1
10
10
25
20
45
90
0.5
0.1
0.5
0.5
0.5
1003n
2
3
10
12 h
96
O
a
803d
CH2Cl2
THFa
H
Me
a
CHCl3
10
30
4 h
24
1 h
95
O
6914
9517
853g
947
a 1 mmol of benzaldehyde was used in these studies.
H
MeO
The scope of the NbF5 method was examined using a
number of representative aldehydes with 0.5 mol% cata-
lyst loading (Table 2). Aromatic, aliphatic, cyclic and het-
4
10
98
97
O
H
erocyclic
aldehydes
underwent
NbF5-catalyzed
cyanosilylation in excellent yields. Unsubstituted and
substituted benzaldehydes (Table 2, entries 1–5) under-
went very smooth silylcyanation in over 95% yield. Naph-
thaldehyde (Table 2, entry 6) gave the corresponding
silylethers in excellent yield. As a comparison, methyl-
triphenyl phosphoniumiodide,3g LiClO4,3n and NHC11-
catalyzed cyanosilylation of naphthaldehyde required
longer reaction times (30 min–24 h) as well as higher cat-
alyst loading (1–100 mol%). The nature of the substitu-
ents on the aromatic ring seems to have no major effect on
the reaction time and yields. The branched and cyclic ali-
phatic aldehydes (Table 2, entries 7–10) were converted
into the corresponding cyanohydrin silylethers in excel-
lent yield.
5
6
10
O
CHO
10
30
24 h
98
H
O
9414
863g
7
10
30
97
(Me)2-CH-CHO
CHO
903n
8
9
10
97
10
2 h
96
H
O
907
O
F
Me3Si
R
H
F
+
Me3SiCN
10
11
10
95
H
O
CN
1
10
96
24 h
3 h
6 h
45
973g
753d
8818
943n
OSiMe3F
OSiMe3
CHO
O
H
R
+
F
H
CN
CN
R
2
3
12
10
4.5 h
96
CHO
9017
Scheme 1
a 0.5 mol% NbF5 used.
b Isolated yield.
Acid-sensitive aldehydes, such as 2-furfuraldehyde, react
smoothly without any decomposition or polymerization
under the reaction conditions (Table 2, entry 11). Further-
c Reproducibility is good.
Synthesis 2007, No. 2, 215–218 © Thieme Stuttgart · New York

PAPER
Cyanosilylation of Aldehydes Using Niobium Fluoride
217
2-(4-Methoxyphenyl)-2-(trimethylsilyloxy)acetonitrile(Table2,
Entry 3)
more, it should be noted that the branched aliphatic alde-
hyde, citral, was transformed into the corresponding
silylether in excellent yield (Table 2, entry 12).
1H NMR (200 MHz, CDCl3): d = 0.21 (s, 9 H), 3.82 (s, 3 H), 5.43
(s, 1 H), 6.94 (d, J = 8.8 Hz, 2 H), 7.38 (d, J = 8.8 Hz, 2 H).
NbF5 is superior in the activation of TMSCN when com-
pared with other recently reported achiral catalytic sys-
tems used in the silylcyanation of aldehydes. The present
system has given excellent yields with comparatively
short reaction times. It is also worthwhile to note that the
addition reaction of TMSCN to aldehydes was achieved
without any additive. As shown in Table 2, our catalytic
system requires considerably less catalyst loading when
compared to previous studies.3d,g,n,9,7,14,17
13C NMR (50 MHz, CDCl3): d = –0.23, 55.34, 63.34, 114.25,
119.32, 127.93, 128.46, 160.33.
2-(4-tert-Butylphenyl)-2-(trimethylsilyloxy)acetonitrile
(Table 2, Entry 4)
1H NMR (200 MHz, CDCl3): d = 0.24 (s, 9 H), 1.32 (s, 9 H), 5.46
(s, 1 H), 7.29–7.42 (m, 4 H).
13C NMR (50 MHz, CDCl3): d = –0.39, 31.10, 34.52, 63.32, 119.18,
125.73, 126.04, 133.19, 152.37.
We have started to test the cyanosilylation of ketones us-
2-(4-Phenoxyphenyl)-2-(trimethylsilyloxy)acetonitrile (Table 2,
ing NbF5. Using acetophenone as the substrate, the cy- Entry 5)
1H NMR (200 MHz, CDCl3): d = 0.22 (s, 9 H), 5.46 (s, 1 H), 6.99–
anosilylation reaction took 20 minutes and gave a 96%
yield. The difference in structure between aldehydes and
ketones appeared to lengthen the reaction time from 10
minutes (aldehydes) to 20 minutes (acetophenone). Fur-
ther results for the cyanosilylation of ketones will be pub-
lished later.
7.13 (m, 5 H), 7.31–7.44 (m, 4 H).
13C NMR (50 MHz, CDCl3): d = –0.26, 63.18, 117.50, 118.75,
119.48, 120.42, 124.04, 124.94, 28.02, 128.41, 129.92, 130.73,
131.98, 156.38, 158.84.
HRMS (EI): m/z calcd for C17H19NO2Si: 297.1185; found:
297.1181.
A possible mechanism for the reaction is as follows. The
formation of hypervalent silicate 1 is formed from the ad-
dition of F– ions to TMSCN. 1 then reacts with the alde-
hyde to generate complex 2 which upon fragmentation
provides the corresponding silylether 3 and F– ions
(Scheme 1).
2-(Naphthalen-1-yl)-2-(trimethylsilyloxy)acetonitrile (Table 2,
Entry 6)
1H NMR (200 MHz, CDCl3): d = 0.23 (s, 9 H), 6.05 (s, 1 H), 7.45–
7.70 (m, 3 H), 7.85–7.95 (m, 3 H), 8.23 (d, 1 H).
13C NMR (50 MHz, CDCl3): d = 0.29, 63.4, 118.45, 122.37, 125.62,
125.01, 126.3, 128.3, 131.01, 133.45, 136.12.
In summary, we have described the use of a readily avail-
able metal salt (NbF5) that effectively promotes the cy-
anosilylation of aldehydes with low and dispersed
catalytic loading under mild conditions. This NbF5-cata-
lyzed reaction may contribute to the development of green
chemistry because of its solvent free-conditions. Further-
3-Methyl-2-(trimethylsilyloxy)butanenitrile (Table 2, Entry 7)
1H NMR (200 MHz, CDCl3): d = 0.2 (s, 9 H), 0.88–1.05 (m, 6 H),
1.94–1.96 (m, 1 H), 4.16 (d, 1 H).
13C NMR (50 MHz, CDCl3): d = 0.34, 17.68, 33.92, 67.28, 119.94.
more, the procedure tolerates a wide variety of substrates. 2-(Trimethylsilyloxy)pent-3-enenitrile (Table 2, Entry 8)
1H NMR (200 MHz, CDCl3): d = 0.20 (s, 9 H), 1.79 (d, J = 2.4 Hz,
Efforts to extend NbF5 catalysis to other organic transfor-
mations are ongoing. Further investigations to clarify the
reaction mechanism and to recover and reuse the catalyst
are in progress.
3 H), 4.90 (d, J = 8.4 Hz, 1 H), 5.51–5.62 (m, 1 H), 5.93–6.04 (m, 1
H).
13C NMR (50 MHz, CDCl3): d = –0.39, 17.16, 61.91, 118.55,
126.06, 130.98.
Cyclohexyl(trimethylsilyloxy)acetonitrile (Table 2, Entry 9)
1H NMR (200 MHz, CDCl3): d = 0.26 (s, 9 H), 1.18–1.29 (m, 5 H),
Cyanosilylation of Aldehydes; Typical Procedure
A mixture of benzaldehyde (2 mmol), dispersed NbF5 (1 mol%) and
TMSCN (2.4 equiv) was stirred for 10 min at r.t. in a 10 mL round
bottom flask. Then 0.5 mL of CH2Cl2 was added and the mixture
was stirred for less than 10 min. Purification by silica gel flash col-
umn chromatography (hexanes–EtOAc, 9:1) gave the desired 2-
phenyl-2-(trimethylsilyloxy)acetonitrile as a colorless oil (96%).
1.68–1.88 (m, 6 H), 4.15 (d, 1 H).
13C NMR (50 MHz, CDCl3): d = 0.335, 25.47, 26.09, 28.15, 28.21,
42.98, 66.55, 119.39.
Cyclohex-3-enyl(trimethylsilyloxy)acetonitrile (Table 2, Entry
10)
1H NMR (200 MHz, CDCl3): d = 0.25 (s, 9 H), 5.52 (s, 1 H), 7.42–
7.47 (m, 5 H).
1H NMR (200 MHz, CDCl3): d = 0.23 (s, 9 H), 1.60–2.12 (m, 7 H),
4.27 (m, 1 H), 5.70 (s, 2 H).
13C NMR (50 MHz, CDCl3): d = –0.33, 63.59, 119.12, 126.29,
128.87, 129.27, 136.18.
13C NMR (50 MHz, CDCl3): d = 0.67, 23.72, 24.26, 26.48, 65.59,
119.07, 124.72, 126.75.
2-(4-Methylphenyl)-2-(trimethylsilyloxy)acetonitrile (Table 2,
Entry 2)
2-Furanyl(trimethylsilyloxy)acetonitrile (Table 2, Entry 11)
1H NMR (200 MHz, CDCl3): d = 0.22 (s, 9 H), 5.55 (s, 1 H), 6.41–
6.43 (m, 1 H), 6.55–6.57 (m, 1 H), 7.28–7.48 (m, 1 H).
13C NMR (50 MHz, CDCl3): d = –0.43, 57.41, 109.70, 110.78,
117.11, 143.84, 148.20.
1H NMR (200 MHz, CDCl3): d = 0.22 (s, 9 H), 2.35 (s, 3 H), 5.45
(s, 1 H), 7.21 (d, J = 7.8 Hz, 2 H), 7.37 (d, J = 7.8 Hz 2 H).
13C NMR (50 MHz, CDCl3): d = –0.25, 55.78, 63.87, 114.66,
119.47, 127.88, 128.77, 160.23.
HRMS (EI): m/z calcd for C9H13NO2Si: 195.0715; found: 195.0712.
Synthesis 2007, No. 2, 215–218 © Thieme Stuttgart · New York

218
S. S. Kim, G. Rajagopal
PAPER
4,8-Dimethyl-2-(trimethylsilyloxy)nona-3,7-dienenitrile (Table
2, Entry 12)
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1H NMR (200 MHz, CDCl3): d = 0.22 (s, 9 H), 1.62 (s, 3 H), 1.70–
1.80 (m, 6 H), 2.10–2.12 (m, 4 H), 5.09–5.13 (m, 2 H), 5.31–5.33
(m, 1 H).
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39.13, 58.42, 119.50, 120.59, 123.14, 133.04, 142.82.
HRMS (EI): m/z calcd for C14H25NOSi: 251.1705; found: 251.1713.
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Synthesis 2007, No. 2, 215–218 © Thieme Stuttgart · New York