Iron-catalyzed silylcyanation of aldehydes and ketones with silyl cyanide prepared from silane and acetonitrile

Article abstract of DOI:10.1246/cl.2005.1054

Silylcyanation of aldehyde or ketone was performed in a solution involving Et₃SiCN produced by photoreaction of Et₃SiH with MeCN in the presence of a catalytic amount of Cp(CO)₂FeMe (Cp = C ₅H₅). The iron complex derived from Cp(CO)₂FeMe serves as a catalyst both for C-C bond cleavage of MeCN and concomitant Si-C bond formation and for Si-C bond cleavage in silylcyanation. The catalytic reaction mechanism has been discussed. Copyright

Full text of DOI:10.1246/cl.2005.1054

1054
Chemistry Letters Vol.34, No.7 (2005)
Iron-catalyzed Silylcyanation of Aldehydes and Ketones
with Silyl Cyanide Prepared from Silane and Acetonitrile
Masumi Itazaki and Hiroshi Nakazawaꢀ
Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585
(Received May 6, 2005; CL-050586)
Silylcyanation of aldehyde or ketone was performed in a
solution involving Et3SiCN produced by photoreaction of
Et3SiH with MeCN in the presence of a catalytic amount of
Cp(CO)2FeMe (Cp ¼ C5H5). The iron complex derived from
Cp(CO)2FeMe serves as a catalyst both for C–C bond cleavage
of MeCN and concomitant Si–C bond formation and for Si–C
bond cleavage in silylcyanation. The catalytic reaction mecha-
nism has been discussed.
Table 1. Silylcyanation of aldehydes and ketones with
Et3SiCNa
Yield/%b
88
Entry
1
Time/h
3
Substrate
O
Product
OSiEt3
H
CN
H
O
O
OSiEt3
2
3
90
18
95
93
H
CN
H
H
OSiEt3
H
CN
Cyanohydrin, RR0C(OH)CN, which is widely used in organ-
ic synthesis,1 has been prepared mainly by hydrocyanation of
aldehyde or ketone with hydrogen cyanide. Use of hydrogen
cyanide is environmentally harmful because of its toxicity. Silyl
cyanide has attracted considerable attention as an alternative CN
source because it reacts with aldehyde and/or ketone to give the
corresponding cyanohydrin silyl ether, which is readily conver-
tible into cyanohydrin by hydrolysis (Scheme 1).
O
OSiEt3
4
5
6
24
168
4
51
69
85
CN
Et3SiO
CN
O
Et3SiO
CN
O
O
O
O
Et3SiO
CN
OSiR''3
90
O
7
8
96
96
R''3SiCN
O
Et3SiO
CN
C
R
R'
R
CN
R'
O
NC OSiEt3
OH
C
20
56
HCN
H2O
O
O
R
CN
Et3SiO
OSiEt3
CN NC
R'
cyanohydrin
NC OSiEt3
NC OSiEt3
Scheme 1. Cyanohydrin formation.
19
59
O
O
This silylcyanation takes place under mild conditions using
some iron catalysts such as [CpꢀFe(C4H4N)] (Cpꢀ ¼ C5Me5) or
their derivatives.2 Without a catalyst the reaction undergoes very
slowly (a half-life time of ca. 38 h).2 Silyl cyanide is a useful CN
source, but one of salient drawbacks is the use of HCN in its
preparation. If silyl cyanide could be synthesized without
HCN, silylcyanation of aldehyde2,3 and ketone3a–3c,3g,4 would
become more environmentally acceptable. Recently we found
a new synthetic route for preparation of silyl cyanide from silane
and acetonitrile, promoted by Cp(CO)2FeMe.5 Herein we report
silylcyanation reaction of aldehydes and ketones with silyl
cyanide, which is prepared in situ from Et3SiH and MeCN with
an iron catalysis (Scheme 2).
9
96
O
Et3SiO CN
aReaction conditions: triethylsilane (5.00 mmol), acetonitrile
(50.00 mmol), Cp(CO)2FeMe (0.20 mmol), aldehyde (4.00
mmol) or ketone (2.00 mmol), THF (0.70 mL), room tempera-
b
ture, Isolated yield based on the substrate.
0.80 mL), MeCN (50.00 mmol, 2.61 mL), and Cp(CO)2FeMe
(0.20 mmol, 38.4 mg). The reaction mixture was stirred for 3 h
at room temperature. After workup, C4H9CH(OSiEt3)(CN)
was isolated in 88% yield based on valeraldehyde (Table 1,
Entry 1). Table 1 presents the results of silylcyanation of several
aldehydes and ketones. Both aliphatic and aromatic aldehydes
are silylcyanated in excellent yields (Entries 1–3). The ꢀ,ꢁ-un-
saturated aldehyde is selectively silylcyanated at the carbonyl
position; no 1,4-addition product was produced (Entry 2). It is
noteworthy that ketones are convertible into silylcyanated
compounds in our system (Entries 4–9), but Fu reported that ali-
phatic ketones do not react with Et3SiCN (catalyst: Bu3SnCN).3g
With ꢀ,ꢁ-diketones, a singly silylcyanated product was obtained
as the main product (85% isolated yield) after the reaction for 4 h
(Entry 6). In contrast, reaction for 96 h gave a doubly silylcya-
O
OSiEt3
cat. Cp(CO)2FeMe
R
R'
Et3SiH + MeCN
Et3SiCN
C
R
hν
CN
R'
Scheme 2.
In a typical reaction, valeraldehyde (4.00 mmol, 425 mL)
was added to a solution of Et3SiCN obtained by photolysis of
a THF solution (0.70 mL) containing Et3SiH (5.00 mmol,
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.7 (2005)
1055
A or B or C or D
Fe
Fe
Fe
Fe
C
C
C
C
SiEt3
SiEt3
SiEt3
SiEt2
CH2
O
O
O
O
N
C
L
H
OSiEt3
C
O
C
C
N
Fe
H2
L = MeCN, THF
C
SiEt3
C
SiEt3
SiEt3
R
C
O
CN
R
C
R'
A
B
C
D
R'
Figure 1. Plausible structures for the complex in the reaction
mixture.
nated product in 90% yield (Entry 7). The 1H and 13C{1H} NMR
spectra revealed the formation of two diastereomers at 1.5:1
ratio. With ꢀ,ꢂ-diketones and ꢀ,ꢃ-diketones, both singly and
doubly silylcyanated products were formed even after 4 h, in-
creasing in amount with time with keeping the relative ratio,
and finally (after 96 h) isolated with the latter predominating
(yield: 20:56% for Entry 8, 19:59% for Entry 9). This is the
first report of double silylcyanation for diketones catalyzed by
a transition metal complex.4b
Fe
Fe
SiEt3
SiEt3
O
O
SiEt3
O
O
Et3SiCN
C
C
C
N
R
R'
R
R'
Scheme 3. Plausible catalytic cycle of silylcyanation.
In summary, we found an unprecedented reaction system in
which cyanohydrin silyl ether is prepared from silane, acetoni-
trile, and aldehyde or ketone. In this system, an iron compound,
presumably Cp(CO)Fe(SiEt3), catalyzes both formation of silyl
cyanide from silane and acetonitrile involving C–CN bond
cleavage and C–Si bond formation and for silylcyanation of
aldehyde and ketone involving C–Si bond cleavage.
We infer that some compounds existing in our system work
as catalysts. It is highly likely that a catalyst working for conver-
sion of silane and acetonitrile into silylcyanide serves also as a
catalyst for silylcyanation of aldehyde and ketone. Our previous
report proposed Cp(CO)Fe(SiEt3) as an actual catalyst for for-
mation of silyl cyanide.5 The following attempts were conducted
to pursue an actual catalyst. A solution of Et3SiH (5.00 mmol,
0.80 mL), acetonitrile (50.00 mmol, 2.61 mL), and Cp(CO)2-
FeMe (0.50 mmol, 96.0 mg) in THF (0.7 mL) was photolyzed
for 12 h and volatile compounds were removed under reduced
pressure. The 1H and 13C{1H} NMR spectra of the residue
showed no signals attributable to Cp(CO)2FeMe, and showed
two signals attributable to Cp ring (1H NMR (400 MHz, C6D6):
ꢃ ¼ 4:42, 4.48; 13C{1H} NMR (100.4 MHz, C6D6): ꢃ 82.17 and
83.68). Exactly the same spectra were obtained when Cp(CO)2-
Fe(SiMe3) was used in place of Cp(CO)2FeMe. Addition of
valeraldehyde to the above reaction mixture causes silylcyana-
tion of the aldehyde group, giving C4H9CH(OSiEt3)(CN).
Figure 1 displays the structures A–D which are possible for
the complexes found in the reaction mixture (A: a solvated form,
B: C–H agostic interactive form). Et3SiNC contained in D is
formed by the reaction of Et3SiH with MeCN in the presence
of Fe catalyst. Scheme 3 depicts a proposed catalytic cycle for
silylcyanation. The cycle is initiated by coordination of aldehyde
or ketone through its carbonyl oxygen to the 16e species,
Cp(CO)Fe(SiEt3). Then, silyl cyanide comes near the complex
to interact with OꢁꢁꢁSi and, concomitantly, with the carbonyl car-
bonꢁꢁꢁC in silyl cyanide, followed by completion of silylcyana-
tion and its dissociation from the coordination sphere to regener-
ate Cp(CO)Fe(SiEt3). Recently, Shibasaki et al.4e and Feng
et al.4h proposed a similar mechanism, but in these studies silyl
isocyanide, instead of silyl cyanide, comes near the coordinated
ketone. The possibility can not be ruled out in our case because
silyl cyanide is reported to be in equilibrium with silyl isocy-
anide.6
This work was supported by a Grant-in-Aid (No. 15205010),
by a Grant-in-Aid for Science Research on Priority Areas
(No. 16033250, Reaction Control of Dynamic Complexes) from
the Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan, by the Sasagawa Scientific Research Grant from the
Japan Science Society, and by the Yamada Science Foundation.
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Published on the web (Advance View) June 25, 2005; DOI 10.1246/cl.2005.1054