An improved synthesis of fluorinated imidoylsilanes

Article abstract of DOI:10.1055/s-2006-942361

A practical and efficient method for the preparation of fluorinated imidoylsilanes is described. The key step involves finely controlled activation of C-Cl and C-Br bonds in fluorinated imidoyl chlorides and bromides, respectively. Georg Thieme Verlag Stu

Full text of DOI:10.1055/s-2006-942361

1836
PAPER
An Improved Synthesis of Fluorinated Imidoylsilanes
An
Improved
S
yn
h
thesis of Flu
i
orinate
e
d ImidoylsilanesAkamatsu, Atsushi Yamauchi, Takeshi Kobayashi, Yu Ozeki, Jun Takagi, Hideki Amii,* Kenji Uneyama*
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushimanaka 3-1-1, Okayama 700-8530, Japan
Fax +81(86)2518021; E-mail: uneyamak@cc.okayama-u.ac.jp
Received 19 December 2005; revised 16 January 2006
lithium species with chlorotrimethylsilane by iodine–lith-
Abstract: A practical and efficient method for the preparation of
ium exchange of the corresponding trifluoroacetimidoyl
iodides and (b) the reaction of trifluoroacetimidoyl chlo-
fluorinated imidoylsilanes is described. The key step involves finely
controlled activation of C–Cl and C–Br bonds in fluorinated imi-
rides with silyl cuprate reagents (Scheme 2). The limita-
tion of the former method (via iodine–lithium exchange)
is the range of applicable substrates. In general, imidoyl
lithium species are not thermally stable at higher temper-
atures due to the existence of an equilibrium between an
imidoyl anion and an aminocarbene species.8 This proto-
col worked well only for trifluoroacetimidoyl iodides en-
dowed with bulky aromatic substituents such as a 2,6-
dimethylphenyl (Xy) group on the nitrogen atoms of the
imine moieties, which prevents the isomerization of imi-
doyl lithium agents to the corresponding carbenes.
doyl chlorides and bromides, respectively.
Key words: magnesium, fluorine, imines, umpolung, imidoyl-
silane
Fluorinated acyl anions and their equivalents are highly
useful synthons for the nucleophilic introduction of fluor-
inated acyl groups into organic molecules.1–6 Fluorinated
acylsilanes 13,4 and imidoylsilanes 27 have been regarded
as versatile intermediates in the synthesis of fluorinated
ketones and their derivatives (Figure 1).
N-Xy
N-Xy
N-Xy
O
N-Ar
Me3SiCl
BuLi
THF
–78 °C
CF3
SiMe3
2a (84%)
CF3
CF3
I
CF3
Li
Rf
SiR3
Rf
SiR3
5a
6
1
2
N-PMP
SiMe3
Me3SiLi
CuCN
N-PMP
SiMe2SiMe3
Figure 1
N-PMP
+
CF3
CF3
Cl
THF, HMPA
Previously, we demonstrated several examples of trifluo-
roacetimidoylsilanes 2 acting as useful carbanion sourc-
es.7 Upon treatment with TBAF, imidoylsilanes 2
underwent C–C bond-forming reactions with carbon elec-
trophiles such as aldehydes, ketones, and chloroformate to
afford functionalized imines 4 in good yields (Scheme 1).
A key feature of this methodology is the mildness of the
reaction conditions (room temperature), which was attrib-
uted to the thermal stability of pentavalent silicates 3,
whereas the corresponding imidoyl lithium agents require
low temperature conditions (below –60 °C) due to their
poor thermal stability.
2b
7
5b
2b: 7 = 89:11 (total yield: 75%)
Scheme 2
Moreover, the latter method (nucleophilic silylation) re-
quired significant amounts of toxic HMPA as an additive
to give trimethylsilyl lithium; in addition, small amounts
of the byproduct disilane 7 were observed. Therefore, an
efficient method for the preparation of fluorinated imi-
doylsilanes is required. Herein, we present Mg-promoted
selective activation of imidoyl carbon–halogen bonds,9,10
which provides a practical method to prepare fluorinated
imidoylsilanes 2 (Scheme 3).
N-Ar
SiMe3
N-Ar
N-Ar
SiMe3
Electrophile
TBAF
r.t.
CF3
CF3
E
4
Mg
R3SiCl
CF3
N-Ar
SiR3
N-Ar
F
3
2a (Ar = 2,6-Me2C6H3)
2b (Ar = 4-MeOC6H4)
Rf
Electrophile:
Rf
X
THF
aldehydes, ketones, ClCO2Et
5
2
Scheme 1
Scheme 3
A Mg/TMSCl system11,12 is considered to be useful for the
reductive dehalogenation of fluorinated imidoyl halides.
When imidoyl chloride 5b was treated with an excess
amount of magnesium metal (4 equiv) and TMSCl (4
equiv) in THF at 0 °C, selective formation of bis(si-
lyl)enamine 812 was observed and the desired imidoylsi-
lane 2b was not detected (Table 1, entry 1).
Despite their usefulness, there have been only two meth-
ods described for the preparation of trifluoroacetimidoyl-
silanes 2: (a) the reaction of the trifluoroacetimidoyl
SYNTHESIS 2006, No. 11, pp 1836–1840
x
x.
x
x
.
2
0
0
6
Advanced online publication: 05.05.2006
DOI: 10.1055/s-2006-942361; Art ID: F21505SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
An Improved Synthesis of Fluorinated Imidoylsilanes
1837
A plausible mechanism for the formation of difluoro-
enamine 8 is shown in Scheme 4. Initially, the reductive
cleavage of the C–Cl bond of the imidoyl chloride 5b took
place to generate imidoyl magnesium species 9, which re-
acted with TMSCl to provide the imidoylsilane 2b. Sub-
sequent two-electron reduction of the resultant
imidoylsilane 2b afforded defluorinated product 8.
N-PMP
SiMe3
N-PMP
N-PMP
Mg
Me3SiCl
CF3
CF3
MgCl
CF3
Cl
C–Cl bond
cleavage
2b
9
5b
Me3Si
PMP
Me3Si
PMP
Mg
Me3SiCl
N
N
F
F2C
F
SiMe3
SiMe3
Focusing on the selective synthesis of 2b, further reduc-
tion involving C–F bond cleavage should be avoided.
Upon treatment with a reduced amount of magnesium
metal (1 equiv), the formation of imidoylsilane 2b was ob-
served (Table 1, entry 2). Conducting the reaction at a low
C–F bond
cleavage
Mg
F
10
8
Scheme 4
temperature (–40 °C) for 30 minutes, further reduction entry 3). However, the corresponding bromide 5d¢
was suppressed and the desired imidoylsilane 2b was ob- smoothly underwent selective monosilylation at –60 °C to
tained selectively (Table 1, entry 3), however, the conver- provide pentafluoroethyl imidoylsilane 2d in 96% yield
sion of the starting imidoyl chloride 5b was low. In order (Table 2, entry 4). On the whole, while the desired reac-
to increase the chemical yield of 2b under low tempera- tions proceeded sluggishly under low temperature condi-
ture conditions, a technique to activate the surface of me- tions (below –50 °C), both remarkable enhancement in
tallic magnesium would be important. When the reaction the reactivity and high reproducibility were obtained
was carried out at –40 °C in the presence of a catalytic when a catalytic amount of ethyl bromide was employed
amount of ethyl bromide,13 the chemical conversion of (Table 2, entries 1, 6, and 7).
imidoyl chloride 5b was dramatically improved to pro-
vide 2b and 8 in 52% and 25% yields, respectively
Furthermore, synthesis of imidoylsilanes with other alkyl-
silyl groups was examined. As shown in Scheme 5, chlo-
(Table 1, entry 5). At lower temperatures (–75 °C), over-
rotriethylsilane (TESCl) provided the corresponding
reduction of imidoylsilane 2b was prevented, resulting in
TES-derivative 2g in 63% yield upon reacting with tri-
the selective formation of 2b in 73% isolated yield
fluoroacetimidoyl bromide 5b¢ (Rf = CF3, X = Br). How-
(Table 1, entry 6).
ever, the use of more bulky silanes such as TBDMSCl,
Other examples of the selective synthesis of imidoylsi- TBDMS-OTf, and TIPSCl were unsuccessful. The
lanes 2 are given in Table 2. Difluoromethyl, perfluoro- present silylation of the imidoyl magnesium seems to be
alkyl and pentafluoroethyl imidoylsilanes 2 were obtained affected by the steric bulkiness of trialkylsilyl reagents.
in moderate to good yields (Table 2, entries 2–7). For the
synthesis of perfluoroalkylated imidoylsilanes 2d and 2e,
the use of the corresponding imidoyl bromides (5d¢ and
5e) as substrates was important (Table 2, entries 3–6).
When imidoyl chloride 5d was employed, the desired im-
idoylsilane 2d was produced in only 40% yield (Table 2,
In summary, we have described a convenient synthesis of
fluorinated imidoylsilanes, which are the stable versions
of fluorinated acyl anion equivalents. Finely controlled re-
ductive cleavage of carbon–halogen bonds expands the
scope of these building blocks in synthetic organic chem-
istry.
Table 1 Mg-Promoted Reductive Silylation of Imidoyl Chloride 5b
Me3Si
F
PMP
N-PMP
SiMe3
N
Mg
Me3SiCl
N-PMP
+
CF3
SiMe3
CF3
Cl
THF
F
2b
8
5b
Entry
Mg (equiv)
TMSCl (equiv)
Additive
Conditions
Yield (%)a
2b
8
1
2
3
4
5
6
4
4
4
4
4
4
2
–
0 °C, 30 min
0 °C, 30 min
–40 °C, 30 min
–75 °C, 30 min
–40 °C, 2.5 h
–75 °C, 2.5 h
0
85
20
–
1
–
12
2
–
37
3
–
5b recovered
3
EtBr (0.05 equiv)b
EtBr (0.05 equiv)b
52
25
–
3.5
73c
a Determined by 19F NMR spectroscopy.
b A catalytic amount of EtBr (0.05 equiv) was added prior to each reaction.
c Isolated yield.
Synthesis 2006, No. 11, 1836–1840 © Thieme Stuttgart · New York

1838
C. Akamatsu et al.
PAPER
Table 2 Synthesis of Imidoylsilanes 2
Mg
Me3SiCl
N-PMP
N-PMP
Rf
SiMe3
2b–f
Rf
X
5b–f
THF
Entry
5
Mg
Me3SiCl
(equiv)
Conditions
2
Yield (%)a
Rf
X
(equiv)
1b
2
CF3
Cl
Cl
Cl
Br
Br
Br
Cl
5b
3.5
8
2
4
2
2
2
2
8
–75 °C, 2.5 h
0 °C, 1 h
2b
2c
2d
2d
2e
2e
2f
73
77
40
96
5c
CF2H
C2F5
C2F5
C3F7
C3F7
C6F5
5c
5d
5d¢
5e
5e
5f
3
4
–75 °C, 1 h
–60 °C, 2.5 h
–60 °C, 11 h
–60 °C, 5 h
–70 °C, 1.5 h
4
4
5
4
6b
7b
4
63
79
2
a Isolated yields.
b Reaction was performed in the presence of EtBr (cat.).
c Most of the starting material 5e remained.
19F NMR (188 MHz, CDCl3): d = 93.1 (s, 3 F).
GC-MS: m/z (%) = 275 (20) [M+], 206 (100), 133 (8), 73 (72).
Mg (3.5 equiv)
Et3SiCl (2.5 equiv)
N-PMP
N-PMP
CF3
SiEt3
CF3
Br
THF
–75 °C, 10 h
Anal. Calcd for C12H16F3NOSi: C, 52.35; H, 5.86; N, 5.09. Found:
C, 52.55; H, 5.87; N, 5.44.
5b'
2g (63%)
Scheme 5
N-(4-Methoxyphenyl)-N-[2,2-difluoro-1-(trimethylsilyl)eth-
ylidene]amine (2c)
To a suspension of Mg (293 mg, 12.0 mmol) in freshly distilled
THF (15 mL) were added TMSCl (0.77 mL, 6.0 mmol) and 5c (311
mg, 1.4 mmol) at 0 °C under an argon atmosphere. The reaction
mixture was stirred at 0 °C for 1 h. Then, the majority of the THF
was removed under reduced pressure; filtration of the residue
through a pad of Celite followed by Kugelrohr distillation afforded
2c as a white solid (277 mg, 77%); bp 90 °C (13 mmHg); mixture
of E/Z isomers.
1H and 19F NMR spectra were recorded at 200 and 188 MHz, re-
spectively, in CDCl3. The chemical shifts are reported in ppm rela-
1
tive to CHCl3 (7.26 ppm for H NMR) and C6F6 (0 ppm for 19F
NMR). Coupling constants are reported in Hz. All air- and/or water-
sensitive reactions were carried out under an argon atmosphere with
anhydrous, freshly distilled solvents using standard syringe/cannu-
la/septa techniques. THF was distilled from Na-benzophenone
ketyl. All other reagents and solvents were employed without fur-
ther purification. Column chromatography was carried out with
Merck silica gel (Kieselgel 60, 230–400 mesh). IR spectra were re-
corded on a Hitachi 270-30 spectrometer. A Perkin-Elmer Series II
CHNS/O Analyzer 2400 was employed for elemental analyses. GC/
MS analyses were performed on a Hewlett-Packard HP5971A. The
starting imidoyl halides 5 were prepared from the corresponding
fluorinated carboxylic acid, p-anisidine, PPh3, Et3N, and CX4
(X = Cl, Br).14
IR (KBr): 2968, 1504 cm–1.
1H NMR (200 MHz, CDCl3): d = 0.09 (s, 9 H), 3.80 (s, 3 H), 5.93
(t, JHF = 57.1 Hz, 1 H), 6.69 (d, J = 8.5 Hz, 2 H), 6.86 (d, J = 8.5
Hz, 2 H).
19F NMR (188 MHz, CDCl3): d = 46.9 (d, JHF = 57.1 Hz, 2 F, major
isomer), 47.8 (d, JHF = 57.1 Hz, 2 F, minor isomer).
GC-MS: m/z (%) = 257 (7) [M+], 206 (64), 73 (100).
Anal. Calcd for C12H17F2NOSi: C, 56.00; H, 6.66; N, 5.44. Found:
C, 56.35; H, 6.90; N, 5.46.
N-(4-Methoxyphenyl)-N-[2,2,2-trifluoro-1-(trimethylsilyl)eth-
ylidene]amine (2b)
N-(4-Methoxyphenyl)-N-[2,2,3,3,3-pentafluoro-1-(trimethyl-
silyl)propylidene]amine (2d)
To a suspension of Mg (173 mg, 7.1 mmol) and EtBr (8 mL, 0.1
mmol) in freshly distilled THF (15 mL) were added TMSCl (0.54
mL, 4.2 mmol) and 5b (505 mg, 2.1 mmol) at –75 °C under an argon
atmosphere. The reaction mixture was stirred at –75 °C for 2.5 h. A
solution of Et3N (10%) in hexane (10 mL) was added to the residue.
Then, the mixture was decanted to remove residual Mg, the organic
layer was washed with ice-cold H2O (20 mL), and dried over
Na2SO4. Column chromatography (hexane–EtOAc, 40:1) afforded
2b (410.5 mg, 73%) as a pale-yellow oil.
IR (neat): 2968, 1506 cm–1.
1H NMR (200 MHz, CDCl3): d = 0.06 (s, 9 H), 3.81 (s, 3 H), 6.71
(d, J = 9.0 Hz, 2 H), 6.87 (d, J = 9.0 Hz, 2 H).
To a suspension of Mg (51 mg, 2.1 mmol) in freshly distilled THF
(6 mL) were added TMSCl (0.15 mL, 1.2 mmol) and 5d¢ (165 mg,
0.50 mmol) at –60 °C under an argon atmosphere. The reaction
mixture was stirred at –60 °C for 2.5 h. A solution of Et3N in hexane
(10%; 10 mL) was added to the residue. The mixture was decanted
to remove the residual Mg, the organic layer was washed with ice-
cold H2O (10 mL), and dried over Na2SO4. Column chromatogra-
phy (hexane–EtOAc, 50:1) afforded 2d (155 mg, 96%) as a yellow
oil.
IR (neat): 2964, 1504 cm–1.
Synthesis 2006, No. 11, 1836–1840 © Thieme Stuttgart · New York

PAPER
An Improved Synthesis of Fluorinated Imidoylsilanes
1839
1H NMR (200 MHz, CDCl3): d = 0.05 (s, 9 H), 3.80 (s, 3 H), 6.66
(d, J = 9.0 Hz, 2 H), 6.86 (d, J = 9.0 Hz, 2 H).
19F NMR (188 MHz, CDCl3): d = 50.7 (s, 2 F), 81.8 (s, 3 F).
19F NMR (282 MHz, CDCl3): d = 93.1 (s, 3 F).
GC-MS: m/z (%) = 317 (10) [M+], 248 (100), 115 (39), 87 (97), 77
(32), 59 (40).
GC-MS: m/z (%) = 325 (5) [M+], 252 (2), 206 (57), 107 (3), 92 (10),
73 (100).
Anal. Calcd for C15H22F3NOSi: C, 56.76; H, 6.99; N, 4.41. Found:
C, 56.74; H, 7.18; N, 4.04.
Anal. Calcd for C13H16F5NOSi: C, 47.99; H, 4.96; N, 4.31. Found:
C, 48.22; H, 5.31; N, 4.61.
Acknowledgment
N-[2,2,3,3,4,4,4-Heptafluoro-1-(trimethylsilyl)butylidene]-N-
(4-methoxyphenyl)amine (2e)
This work has been supported by the Ministry of Education, Cul-
ture, Sports, Science and Technology of Japan [Grant-in-Aid for
Scientific Research B (No. 13555254), Grant-in-Aid for Scientific
Research C (No. 17550104), Grant-in-Aid for Scientific Research
on Priority Areas ‘Reaction Control of Dynamic Complexes’ (No.
16033621)]. We also thank the SC-NMR laboratory of Okayama
University for 19F NMR analysis.
To a suspension of Mg (44 mg, 1.8 mmol) and EtBr (2 mL, 0.026
mmol) in freshly distilled THF (5 mL) were added TMSCl (0.13 mL,
1.0 mmol) and 5e (192 mg, 0.50 mmol) at –60 °C under an argon
atmosphere. The reaction mixture was stirred at –60 °C for 5 h. A
solution of Et3N in hexane (10%; 10 mL) was added to the residue.
The mixture was decanted to remove the residual Mg, the organic
layer was washed with ice-cold H2O (10 mL), and dried over
Na2SO4. Column chromatography (hexane–EtOAc, 50:1) afforded
2e (118 mg, 63%) as a yellow oil.
IR (neat): 2968, 1504 cm–1.
1H NMR (200 MHz, CDCl3): d = 0.06 (s, 9 H), 3.81 (s, 3 H), 6.66
(d, J = 8.9 Hz, 2 H), 6.87 (d, J = 8.9 Hz, 2 H).
References
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19F NMR (188 MHz, CDCl3): d = 37.5 (s, 2 F), 53.5 (s, 2 F), 82.1 (s,
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GC-MS: m/z (%) = 375 (2) [M+], 206 (59), 133 (4), 73 (100).
Anal. Calcd for C14H16F7NOSi: C, 44.80; H, 4.30; N, 3.73. Found:
C, 44.73; H, 4.44; N, 3.96.
N-(4-Methoxyphenyl)-N-[pentafluorophenyl(trimethyl-
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To a suspension of Mg (99 mg, 4.1 mmol) and EtBr (8 mL, 0.1 mmol)
in freshly distilled THF (16 mL) were added TMSCl (2.1 mL, 16
mmol) and 5e (672 mg, 2.0 mmol) at –70 °C under an argon atmo-
sphere. The reaction mixture was stirred at 0 °C for 1.5 h. The ma-
jority of the THF was removed under reduced pressure; filtration of
the residue through a pad of Celite followed by Kugelrohr distilla-
tion afforded 2e as a pale-yellow solid (532 mg, 79%): bp 140 °C
(13.5 mmHg).
IR (KBr): 2968, 1502 cm–1.
1H NMR (200 MHz, CDCl3): d = 0.26 (s, 9 H), 3.73 (s, 3 H), 6.60
(d, J = 9.0 Hz, 2 H), 6.72 (d, J = 9.0 Hz, 2 H).
19F NMR (188 MHz, CDCl3): d = 0.79–1.06 (m, 2F), 7.93 (t,
J = 21.1 Hz, 1 F), 22.7 (d, J = 17.3 Hz, 2 F).
GC-MS: m/z (%) = 373 (15) [M+], 300 (22), 206 (26), 73 (100).
Anal. Calcd for C17H16F5NOSi: C, 54.68; H, 4.32; N, 3.75. Found:
C, 54.32; H, 4.15; N. 3.64.
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N-(4-Methoxyphenyl)-N-[2,2,2-trifluoro-1-(triethylsilyl)eth-
ylidene]amine (2g)
To a suspension of Mg (169 mg, 7.0 mmol) in freshly distilled THF
(4 mL) were added TMSCl (0.591 mL, 4.0 mmol) and 5b¢ (562 mg,
2.0 mmol) at –75 °C under an argon atmosphere. The reaction mix-
ture was stirred at –75 °C for 10 h. A solution of Et3N in hexane
(10%; 5 mL) was added to the residue. The mixture was decanted to
remove residual Mg, the organic layer was washed with ice-cold
H2O (10 mL), and dried over Na2SO4. Purification by Kugelrohr
distillation afforded 2g (247 mg, 63%) as a pale-yellow oil; bp
150 °C (3 mmHg).
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IR (neat): 2964, 1502 cm–1.
1H NMR (300 MHz, CDCl3): d = 0.53 (q, J = 8.1 Hz, 6 H), 0.87 (t,
J = 8.1 Hz, 9 H), 3.82 (s, 3 H), 6.71 (d, J = 9.0 Hz, 2 H), 6.86 (d,
J = 9.0 Hz, 2 H).
Synthesis 2006, No. 11, 1836–1840 © Thieme Stuttgart · New York

1840
C. Akamatsu et al.
PAPER
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(11) (a) Amii, H.; Kobayashi, T.; Hatamoto, Y.; Uneyama, K.
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(8) A carbene-type intermediate has been proposed for benzoyl
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