Rhodium-catalyzed addition of aryldifluoromethylsilanes to N-sulfonylaldimines

Article abstract of DOI:10.1016/j.tetlet.2004.04.152

The addition of aryldifluoromethylsilanes to N-sulfonylaldimines was found to be catalyzed by a rhodium complex, [Rh(cod)(MeCN)₂]BF₄, in the presence of potassium fluoride to give the corresponding arylated N-sulfonylamines in good yield. The reaction mechanism would involve the generation of a fluoride-coordinated arylsilicate and the transmetalation between the arylsilicate and the rhodium complex to give the arylrhodium species as a key intermediate.

Full text of DOI:10.1016/j.tetlet.2004.04.152

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4855–4857
Rhodium-catalyzed addition of aryldifluoromethylsilanes
to N-sulfonylaldimines
*
*
Shuichi Oi, Mitsutoshi Moro, Takanori Kawanishi and Yoshio Inoue
Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
Received 15 April 2004; revised 22 April 2004; accepted 26 April 2004
Available online 18 May 2004
Abstract—The addition of aryldifluoromethylsilanes to N-sulfonylaldimines was found to be catalyzed by a rhodium complex,
[
2 4
Rh(cod)(MeCN) ]BF , in the presence of potassium fluoride to give the corresponding arylated N-sulfonylamines in good yield.
The reaction mechanism would involve the generation of a fluoride-coordinated arylsilicate and the transmetalation between the
arylsilicate and the rhodium complex to give the arylrhodium species as a key intermediate.
Ó 2004 Elsevier Ltd. All rights reserved.
Transition metal-catalyzed transformations using orga-
nometallic reagents are of great importance in modern
organic chemistry. One of the recent topics of such
N-sulfonylaldimines catalyzed by rhodium complexes to
give arylated N-sulfonylamines (Eq. 1).
1
transformations is the rhodium-catalyzed addition of
the organometallic reagents to the carbon–heteroatom
double bonds such as aldehydes or imines as well as the
R
H
Ar
R
cat. Rh, KF
ArSiF2Me
+
ð1Þ
N
HN
2
Ts
Ts
1
,4-addition to a,b-unsaturated carbonyl compounds.
These reactions commonly involve the formation or-
ganorhodium complexes as active species by the trans-
metalation with the organometallic reagents. In most
cases, they occur chemo- and regioselectively rendering
them useful in organic synthesis.
When the reaction of phenyldifluoromethylsilane (1a)
with N-tosylbenzylideneamine (2a) was carried out in
the presence of 2 mol % of [Rh(cod)(MeCN)
3
2 4
]BF and
equiv of potassium fluoride in THF at 60 °C for 20 h,
Rhodium-catalyzed addition of organometallic reagents
to imines is a useful method to synthesize amines. The
N-tosyldiphenylmethylamine (3a) was obtained in a
good isolated yield of 88% (Table 1, entry 1). In the
reaction of 1a with 2a, absence of KF markedly
decreased the yield to 11% (entry 2), while changing the
solvent to DMF increased the yield to 99% even in the
3
additions of aryl- or alkenyltin reagents, arylboron
4
5
reagents, and alkenylzirconium reagents to aldimines
were reported. Organosilicon compounds are very
attractive reagents due to their low cost, low toxicity, and
tolerance to a variety of functional groups, however,
reaction ofthose with aldimines has not yet been reported.
We previously reported that aryldifluoromethylsilanes
smoothly added to aldehydes in the presence of a catalytic
amount of rhodium complex and fluoride salt, giving
7
shorter reaction time of 5 h (entry 3). The reaction of
phenyltrimethylsilane (1b) and phenylfluorodimethyl-
silane (1c) in THF did not take place (entries 4 and 5),
and phenyltrifluorosilane (1d) gave the product 3a in a
low yield of 22% (entry 6). The tendency of the reac-
tivities of these phenylsilanes was similar to those in the
6
6
secondary alcohols in good yield. In this letter, we
report the addition of the aryldifluoromethylsilanes to
reaction with aldehydes.
The reactions of 2- and 4-methoxy-N-tosylbenzylidene-
amine (2b and 2c) with phenyldifluoromethylsilane (1a)
also proceeded smoothly in DMF, affording the corre-
sponding N-tosyldiarylmethylamines (3b and 3c)
in good isolated yields of 90% and 87%, respectively
Keywords: Arylation; Aldimines; Rhodium catalyst; Arylsilanes.
*
Corresponding authors. Tel.: +81-222175876; fax: +81-222175873;
e-mail: oishu@aporg.che.tohoku.ac.jp
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.04.152

4856
S. Oi et al. / Tetrahedron Letters 45 (2004) 4855–4857
a
Table 1. Rhodium-catalyzed addition of arylsilanes 1 to N-sulfonylaldimines 2
b
Entry
Arylsilane 1
Aldimine 2
Solvent
Temp (°C)/time (h)
Product
Yield (%)
Ph
N
(
2a)
1
PhSiF
1a
2
Me (1a)
Ts
THF
THF
DMF
THF
THF
THF
60/20
60/20
60/5
3a
3a
3a
88
11
99
0
c
2
2a
2a
2a
2a
2a
3
4
5
6
1a
PhSiMe
PhSiFMe
3
(1b)
(1c)
(1d)
60/20
60/20
60/20
2
0
PhSiF
3
3a
3b
22
MeO
7
8
1a
(2b)
DMF
DMF
60/5
60/5
90
87
N
Ts
OMe
N
1a
(2c)
3c
Ts
Me2N
9
1a
1a
(2d)
DMF
THF
60/20
60/48
3d
3d
30
66
N
Ts
10
2d
Cl
1
1
2
1a
1a
(2e)
DMF
THF
60/20
60/5
3e
3e
55
96
N
Ts
1
2e
Cl
N
1
3
4
1a
1a
(2f)
DMF
THF
60/20
60/48
3f
3f
18
42
Ts
1
2f
O2N
1
5
6
1a
1a
(2g)
DMF
THF
60/20
60/20
3g
3g
55
93
N
Ts
1
2g
1
7
8
1a
N
(2h)
DMF
DMF
60/20
60/20
3h
3b
73
98
O
Ts
1
MeO
SiF Me (1b)
2a
2
19
Cl
SiF Me (1c)
2a
DMF
60/20
3e
76
2
a
b
c
Common reaction conditions: 1 (1.25 mmol), 2(0.5 mmol), [Rh(cod)(MeCN)
2 4 2
]BF (0.01 mmol), KF (1.5 mmol), solvent (1.0 mL), N atmosphere.
Isolated yield.
The reaction was carried out in the absence of KF.
(
entries 7 and 8). In the case of 4-dimethylamino-N-
The mechanism of the present reaction would involve
the transmetalation between the arylsilicon compounds
and the rhodium complex to give the arylrhodium
complex as a key intermediate. In addition, a fluoride-
coordinated arylsilicate would be an active species for
the transmetalation with the rhodium complex, from the
result that the addition of potassium fluoride is neces-
sary for the reaction to proceed smoothly. The pre-
sumed reaction pathway is shown in Scheme 1. As the
initial step, the transmetalation of the cationic rhodium
complex with the silicate 4 generated from 1 and
potassium fluoride produces the arylrhodium interme-
diate 5. Addition of 5 to the aldimine affords the rho-
dium amide complex 6, which then reacts with 4 to
regenerate 5 and the aminosilicate 7. The silicate 7 or its
equilibrium partner of silylamine 8 is easily hydrolyzed
during aqueous work-up to give the product 3.
tosylbenzylideneamine (2d), the yield of the reaction in
DMF was low (entry 9), however, the reaction in THF
gave the better yield of 66% (entry 10). Such solvent
effect was also observed in the reaction of N-tosylbenz-
ylideneamine bearing chloro or nitro group. The reac-
tion of 4-chloro- and 4-nitro-N-tosylbenzylideneamine
(
2e and 2g) with 1a in THF gave the corresponding
adducts 3e and 3g in good yields of 96% and 93%,
respectively (entries 12 and 16), whereas the reactions in
DMF gave the products in moderate yield of 55% in
both cases (entries 11 and 15). Although the reaction of
2
-chloro-N-tosylbenzylideneamine (2f) was sluggish, the
reaction in THF also gave the better yield of 42% (entry
4). N-Tosylfurylideneamine (2h) reacted well with 1a in
1
DMF giving the product 3h in 73% yield (entry 17). 4-
Methoxy- and 4-chlorophenyldifluoromethylsilane (1b
and 1c) also reacted well with 2a in DMF affording the
corresponding adducts 3b and 3e in good isolated yields
of 98% and 76%, respectively (entries 18 and 19).
In conclusion, the addition of aryldifluoromethylsilanes
to N-sulfonylaldimines was achieved by use of the

S. Oi et al. / Tetrahedron Letters 45 (2004) 4855–4857
4857
KF
Ar SiMeF2
Ar SiMeF3 K+
1
-KF
4
+
Rh BF4-
R
Ar
R
Ar
R
Ar
2
H O
KF
MeSiF3 + KBF4
R
TsN
TsN
NHTs
-KF
SiMeF3 K+
SiMeF2
H
8
7
Rh Ar
3
5
NTs
2
R
Ar
Ar SiMeF3 K+
TsN
4
Rh
6
Scheme 1. Presumed reaction mechanism.
rhodium complex as the catalyst and potassium fluoride
as the activator for the aryldifluoromethylsilanes. This
process provides a new synthetic route to a wide range
of arylamines from arylsilanes, which have advantages
such as low cost, low toxicity, and tolerance to a variety
of functional groups. Further investigation involving an
asymmetric version of the reaction is now underway.
Hayashi, T.; Ishigedani, M. Tetrahedron 2001, 57, 2589;
d) Oi, S.; Moro, M.; Fukuhara, H.; Kawanishi, T.; Inoue,
Y. Tetrahedron 2003, 59, 4351.
(
4
. (a) Ueda, M.; Miyaura, N. J. Organomet. Chem. 2000,
5
1
95, 31; (b) Ueda, M.; Saito, A.; Miyaura, N. Synlett 2000,
637.
5
6
. Kakuuchi, A.; Taguchi, T.; Hanzawa, Y. Tetrahedron
Lett. 2003, 44, 923.
. Oi, S.; Moro, M.; Inoue, Y. Organometallics 2001, 20,
1036.
7. A following procedure is representative. A mixture of N-
tosylaldimines (0.5 mmol), aryldifluoromethylsilanes (1.25
References and notes
1
2
. Knight, D. W. In Comprehensive Organic Synthesis; Trost,
B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 3,
pp 481–520.
. Reviews: (a) Hayashi, T. Synlett 2001, 879; (b) Fagnou,
K.; Lautens, M. Chem. Rev. 2003, 103, 169; (c) Hayashi,
T.; Yamasaki, K. Chem. Rev. 2003, 103, 2829; (d) Oi, S.;
Inoue, Y. J. Synth. Org. Chem. Jpn. 2003, 61, 572.
. (a) Oi, S.; Moro, M.; Fukuhara, H.; Kawanishi, T.; Inoue,
Y. Tetrahedron Lett. 1999, 40, 9259; (b) Hayashi, T.;
Ishigedani, M. J. Am. Chem. Soc. 2000, 122, 976; (c)
mmol), KF (87 mg, 1.5 mmol), and [Rh(cod)(MeCN)
2
]-
(3.8 mg, 0.01 mmol) in 1 mL of dried THF or DMF
atmosphere in a Schlenk
3
d
BF
was stirred at 60 °C under N
4
2
tube. The reaction was quenched by adding a small
amount of water and then stirred for 1 h. After the solvent
was evaporated in vacuo, the residue was purified by silica
3
gel chromatography eluting with hexane/CHCl /AcOEt
3
(10:11:1) to give the products 3. All products were
identified by comparing the spectroscopic data with those
3
d
reported before.