Chromium mediated stereoselective synthesis of (Z)-1-fluoro-2-alkenyl alkyl and trialkylsilyl ethers from dibromofluoromethylcarbinyl ethers

Article abstract of DOI:10.1016/j.tetlet.2005.06.035

Reactions of 2,2-dibromo-2-fluoroethyl alkyl ethers and dibromofluoromethylcarbinyl silyl ethers with CrCl₂ and Mn powder provided 2-fluorovinyl alkyl ethers and 1-fluoro-2-alkenyl enol silyl ethers in high yields and in Z selective manner.

Full text of DOI:10.1016/j.tetlet.2005.06.035

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5257–5261
Chromium mediated stereoselective synthesis of
Z)-1-fluoro-2-alkenyl alkyl and trialkylsilyl ethers from
dibromofluoromethylcarbinyl ethers
(
Muga Nakagawa, Akio Saito, Aiko Soga, Natsuko Yamamoto and Takeo Taguchi*
Tokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
Received 24 May 2005; revised 9 June 2005; accepted 13 June 2005
2
Abstract—Reactions of 2,2-dibromo-2-fluoroethyl alkyl ethers and dibromofluoromethylcarbinyl silyl ethers with CrCl and Mn
powder provided 2-fluorovinyl alkyl ethers and 1-fluoro-2-alkenyl enol silyl ethers in high yields and in Z selective manner.
Ó 2005 Elsevier Ltd. All rights reserved.
Due to the importance of fluorinated olefins in synthetic
chemistry, medicinal chemistry and material science,
development of efficient synthetic methods forfluoir-
CrCl , Mn,
2
Br
Br
cat. NiCl (PPh )
H
2
3 2
OR
OR
THF, rt
F
1
–3
F
F
nated olefins has been extensively investigated.
For
1
2
example, fluorine-bearing alkenyl ether derivatives, in
particular enol silyl ethers, are used as versatile building
blocks in a variety of carbon–carbon bond forming reac-
tions. Among such molecules, although stereochemically
unilateral gem-difluorovinyl derivatives have been well
R1
R1
Br
Br
CrCl2, Mn
THF, rt
OSiR23
H
2
OSiR
3
F
3
4
4
–6
documented,
stereoselective preparation of mono-
fluorovinyl or monofluoroalkenyl ethers involving silyl
Scheme 1.
7
,8
derivatives is a remaining subject to be solved. To-
wards this subject, recent advances in chromium chemi-
stry attracted our attention to develop a new synthetic
of 2-fluorovinyl alkyl ethers 2 from 2,2-dibromo-2-flu-
oroethyl ether derivatives 1, and regio- and Z selective
preparation of enol silyl ethers of fluoromethyl ketone
from dibromofluoromethylcarbinyl silyl ethers 3
Scheme 1).
9
,10
method for monofluoroalkenyl ether derivatives.
In
particular, Mioskowski and co-workers demonstrated
that 2,2,2-trichloroethyl alkyl ethers can be converted
to the corresponding (Z)-2-chlorovinyl alkyl ethers upon
4
(
1
1
treating with CrCl2. When we started the chromium
chemistry to develop synthetic methods for fluorinated
olefinic compounds, there had been reported very few
examples on the reaction of dihalofluoromethyl deriva-
Preparative procedure for the starting ethers 1 and 3
is shown in Scheme 2. Etherification of 2,2-dibromo-2-
fluoroethanol 6a, obtained by LiBH reduction of the
4
1
5
1
2,13
ester 5 in the presence of trimethylborate, provided
the starting ethers 1. Due to the low nucleophilicity of
dibromofluoro alcohol 6a and its relatively labile nature
under the strong basic conditions, although reactive
benzylic, allylic and propargylic bromide gave the ether
compounds (1a–m except for 1g) in good yields using
phase-transfer catalyzed conditions (6% NaOH and
TBAI in CH Cl and H O at 0 °C to rt), alkyl (4-phen-
tives with CrCl2
and thus, we examined the CrCl2
mediated reactions using a variety type of dibromofluo-
romethyl and 1-bromo-1-fluoroalkene compounds to
1
4
accumulate basic knowledge on the reactivities. In this
letter, we report CrCl mediated Z selective preparation
2
2
2
2
Keywords: Chromium salt; Fluoroalkene; Fluoroenol ether; Dibromo-
fluoromethylcarbinol.
ylbutyl) derivative 1g was prepared in moderate yield
(55%) using NaH as a base and the triflate instead of
the bromide or iodide. On the other hand, silylation
*
Corresponding author. Tel./fax: +81 426 76 3257; e-mail:
taguchi@ps.toyaku.ac.jp
0
040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.06.035

5258
M. Nakagawa et al. / Tetrahedron Letters 46 (2005) 5257–5261
O
R-Br
LiBH4, B(OMe)3
Et2O
Br
Br
NaOH, n-Bu N+ I-
Br
Br
Br
Br
4
OR
OEt
OH
CH2Cl2 - H2O
F
F
F
5
6a 96%
1
TBDPSCl
NaH
2
1a-1e R = ArCH 80-95%
1h-1k R = R CH=C(R )CH2 60-87%
1m R = Ph(CH2)2C CCH2 76%
NaH, ROTf
1
2
THF
THF
O(CH ) Ph
Br
Br
Br
Br
OTBDPS
93 %
2
4
F
F
3a
1g 55%
2
OSiR23
OH
R 3SiCl, Et3N
DMAP
FCBr3, Et2Zn
DMF
Br
Br
Br
Br
CHO
Ph
Ph
Ph
THF
F
F
6
b
3b-3d 80-95%
Scheme 2.
Table 1. Additive effect on reaction of 1a with CrCl
2
nyl ethers 2a–g in stereoselective manner (entries 1–7).
1
8
Existence of arylbromide and esterg ro up in the sub-
strate can be tolerated as in the cases of 1d and e (entries
CrCl2, Mn
Ni catalyst
Br
Br
H
O
O
4
and 5). On the other hand, with allylic and propargylic
F
THF, rt
F
1a
2a
ethers, this chromium mediated olefination reaction did
not always work well. For example, allylic ether having
vicinally disubstituted pattern 1h–i gave only trace
amount of 2-fluorovinyl ether compounds 2h–i, and in
the case of 1i cyclized tetrahydrofuran derivative 7 was
isolated in 63% yield (entries 8 and 9). As shown in
Scheme 3, formation of this tetrahydrofuran derivative
7 should involve the 5-exo addition reaction of the radi-
cal species 9 leading to the cyclized intermediate 10
prior to the second electron transfer reaction to the radi-
cal species 9 to form the carbenoid intermediate struc-
a
a
Entry
CrCl
equiv)
2
Mn
(equiv)
Ni
(equiv)
Time
(h)
2a
(%)
1a
(%)
(
1
2
4
3
2
2
3
0
2
3
3
2
—
—
21
26
6
41
32
29
15
0
b
b
d
d
d
3
0.3
27 (45)
59 (72)
76 (90)
c
4
5
0.1
3
0
c
0.1
2
0
a
b
c
Isolated yield.
NiCl was used.
NiCl (PPh was used.
Yield in parenthesis was determined by 300 MHz H NMR.
2
2
3 2
)
1
2a,19
d
1
turally similar to 11.
When the allylic etherhas
geminally disubstituted pattern as in the case of the sub-
strates 1j–k, such 5-exo cyclization can be clearly re-
tarded due to the sterically crowded reaction site in
0
the radical species 9 in Scheme 3, and thus the desired
1
6
reaction of these dibromofluoro alcohols 6a and b
fluorovinyl ether 2j–k possibly formed through the carb-
enoid intermediate 11 was obtained in moderate yields
(39% and 44% yield, respectively, entries 10 and 11).
Likewise, with propargyl derivative 1m, 5-exo cycliza-
tion was a major reaction pathway to give furan
compound 8 in 40% yield along with the isolation of
2-fluorovinyl ether 2m in 10% yield (entry 12, see also
Scheme 3). Similar furan formation through chromium
mediated radical cyclization of trichloro derivatives
smoothly proceeded using the corresponding silyl chlo-
ride and NaH for the primary alcohol 6a orte tr ia yr
amine forthe seconda yr alcohol 6b to give silyl ethers
3
a–d in high yields.
Reaction of 2,2-dibromo-2-fluoroethyl benzyl ether 1a
with CrCl was conducted underva ri ous conditions to
2
obtain 2-fluorovinyl ether 2a (Table 1). As shown in en-
tries 1 and 2, reaction of 1a with CrCl orC rC l and Mn
2
0
was reported by Mioskowski and co-workers.
2
2
powderin THF p ro ceeded ra therslowly. On the other
hand, as shown in entries 3–5, addition of a catalytic
amount of Ni was found to remarkably accelerate the
reaction and NiCl (PPh ) rather than NiCl was more
Chromium mediated reaction of dibromofluoromethyl-
carbinyl silyl ethers 3 also proceeded smoothly to pro-
vide (Z)-fluoroalkenyl enol silyl ethers 4 in high yield
and with complete stereoselectivity (Table 3). Thus,
reaction of trimethylsilyl (TMS) ether 3b with CrCl2
and Mn powderin THF smoothly p or ceeded to give
enol TMS ether 4b in excellent yield (93%), while in
the presence of Ni catalyst yield of 4b significantly
dropped to 35% since the enol silyl ether 4b thus formed
further reacted with Ni catalyst giving rise to a complex
mixture (entry 2 vs entry 3). As shown in Table 3, pres-
2
3 2
2
effective to give 2a in excellent yield (entry 5). Addition
of Mn powderwas sometimes effective to er duce the
17
amount of CrCl2. The stereoselectivity was also excel-
lent obtaining only Z isomer.
Next, we examined the fluorovinyl ether synthesis using
various 2,2-dibromo-2-fluoroethyl ethers (Table 2).
Under the similar reaction conditions (3 equiv CrCl2,
2
benzylic as well as alkyl ethers afforded (Z)-2-fluorovi-
equiv Mn powder, 0.1 equiv NiCl (PPPh ) , THF, rt)
ent CrCl –Mn powder mediated preparation of fluoro-
2
methyl ketone enol silyl ethers 4 is applicable to a
2
3 2

M. Nakagawa et al. / Tetrahedron Letters 46 (2005) 5257–5261
5259
a
Table 2. Synthesis of (Z)-2-fluorovinyl ethers 2
Br
Br
CrCl2 , Mn, NiCl2(PPh3)2
THF, rt
H
OR
OR
F
F
1
2
b
Yield (%)
Entry
1
Time (h)
2
1
2
3
4
5
1a
1b
1c
1d
1e
R = H
R = OMe
R = Me
2
1
2
2a
2b
2c
2d
2e
76
98
56
47
60
Br2FC
O
O
F
R
R
R = Br1
R = AcOCH
2
6
7
Br2FC
Br2FC
c
6
O
O
F
F
O
O
1f
2f
75
Ph
Ph
O
O
O
7
1g
2
2g
66
4
Br FC
O
2
F
F
F
F
d
R
8
2
Br FC
O
O
O
R
1h
1i
R = Ph
4
4
4
6
2h
2i
d
9
e
R = PhCH
R = Ph
2
CH
CH
2
2
—
R
d
R
1
0
1
2
Br FC
1j
1k
2j
2k
39
44
d
1
R = PhCH
2
d
2
Br2FC
O
f
1
1m
6
2m
10
Ph
Ph
a
b
c
d
e
f
3
equiv of CrCl
2
, 2 equiv of Mn and 0.1 equiv of NiCl
2
(PPh
3
)
2
were used, unless otherwise stated.
Isolated yield.
6
4
equiv of CrCl
equiv of CrCl
2
, 4 equiv of Mn and 0.1 equiv of NiCl
and 4 equiv of Mn were used.
2
(PPh
3
)
2
were used.
2
Cyclized product 7 was obtained in 63% yield.
Furan 8 was obtained in 40% yield.
Ph
Ph
O
O
Br
F
7
8
CrIII
F
Br
Br
Br
CrCl2
CrCl2
Br
O
R1
O
R1
R1
2
(
R = H)
R2
F
F
H
F
O
1
9
10
Br
O
2
(
R = alkyl)
R2
Br
F
9
'
R1
R1
CrCl2
O
7
Br
Br
F
O
X2Cr
O
CH
2
2
R
O
F
R2
F
R2
O
8
2j,k
11
1
0'
Scheme 3.
variety type of substrates with respect to the substituents
on both fluorovinyl and silyl parts. Steric effect of silyl
group on the reaction rate was obvious, that is, reaction
period was 2 h for TMS ether 3b, 6 h fort ir ethylsilyl
yield. Furthermore, reaction proceeded in a completely
regio- and stereoselective manner to give Z isomer.
Although Takai reported that elimination of trimethylsil-
yloxy group is a major pathway in the reaction of tri-
(
(
TES) ether 3c, and 13 h for tert-butyldimethylsilyl
TBDMS) ether 3d, respectively (entries 2, 4 and 5).
chloromethylcarbinyl TMS ether with CrCl pretreated
2
2
1
with DMF in THF orwith C Cr l
in DMF, under
2
Regarding the substituent on fluorovinyl part, both
alkyl and hydrogen derivatives can be prepared in good
the similar conditions dibromofluoro derivative 3b gave
the enol silyl ether 4b as a main product.

5260
M. Nakagawa et al. / Tetrahedron Letters 46 (2005) 5257–5261
a
Table 3. Synthesis of (Z)-fluoroalkenyl silyl enol ethers 4
OSiR23
OSiR23
R1
CrCl2 , Mn
THF, rt
Br
Br
F
R1
4
F
3
b
Yield (%)
Entry
1
3
Time (h)
12
4
OTBDPS
Br FC
2
OTBDPS
OSiR23
3a
4a
68
F
F
OSiR2
3
2
3
2
3b
3b
3c
3d
R Si ¼ TMS
2
2
4b
4b
4c
4d
93
35
91
68
c
3
Br2FC
Ph
2
3
Ph
R Si ¼ TMS
2
3
4
5
R Si ¼ TES
6
2
3
7c
R Si ¼ TBDMS
13
OSiMe3
OSiMe3
6
3e
3
4e
70
OBn
F
OBn
Br2FC
a
b
c
4 equiv of CrCl
Isolated yield.
2
and 4 equiv of Mn were used, unless otherwise stated.
(PPh was added.
0.1 equiv of NiCl
2
3 2
)
III
3. (a) Burton, D. J.; Yang, Z.-Y.; Qiu, W. Chem. Rev. 1996,
96, 1641; (b) Xu, J.; Burton, D. J. Org. Lett. 2002, 4, 831;
(c) Okada, M.; Nakamura, Y.; Sato, A.; Horikawa, H.;
Saito, A.; Taguchi, T. Tetrahedron Lett. 2002, 43, 5845; (d)
Nakamura, Y.; Okada, M.; Sato, A.; Horikawa, H.;
Koura, M.; Saito, A.; Taguchi, T. Tetrahedron 2005, 61,
2
CrCl2
Cr
Br2CF
OR
Br
OR
F
1, 3
12
α-elimination
H
5
741; (e) Sano, S.; Saito, K.; Nagao, Y. Tetrahedron Lett.
CrIII
OR
2003, 44, 3987; (f) Lei, X.; Dutheuil, G.; Pannecoucke, X.;
Quirion, J.-C. Org. Lett. 2004, 6, 2101.
H-1,2 shift
F
OR
F
Br
H
2, 4
4. Typical examples of difluoro enol ethers, (a) Yamanaka,
M.; Ishihara, T.; Ando, T. Tetrahedron Lett. 1983, 24, 507;
13
(
(
b) Qiu, Z.-M.; Burton, D. J. J. Org. Chem. 1995, 60, 5570;
c) Jin, F.-Q.; Xu, Y.-Y.; Huang, W.-Y. J. Chem. Soc.,
Scheme 4.
Perkin Trans. 1 1993, 795; (d) Kodama, Y.; Yamane, H.;
Okumura, M.; Shiro, M.; Taguchi, T. Tetrahedron 1995,
It would be likely that the present reaction involves
carbene mechanism for the fluorovinyl ether formation.
Thus, the carbenoid intermediate 12 is formed by the
reaction of dibromide with two equimolar amount of
4
5, 12217; (e) Kodama, Y.; Okumura, M.; Yanabu, N.;
Taguchi, T. Tetrahedron Lett. 1996, 37, 1061; (f) Amii, H.;
Kobayashi, T.; Hanamoto, Y.; Uneyama, K. Chem.
Commun. 1999, 1323; (g) Berber, H.; Brigaud, T.; Lefeb-
vre, O.; Plantier-Royon, R.; Portella, C. Chem. Eur. J.
2001, 7, 903; (h) Kobayashi, T.; Tanaka, H.; Amii, H.;
Uneyama, K. Tetrahedron 2003, 59, 1547; (i) Uneyama,
K.; Tanaka, H.; Kobayashi, S.; Shioyama, M.; Amii, H.
Org. Lett. 2004, 6, 2733.
. Typical examples of difluoro ketene silyl acetal, (a)
Kitagawa, O.; Taguchi, T.; Kobayashi, Y. Tetrahedron
Lett. 1988, 29, 1803; (b) Taguchi, T.; Kitagawa, O.; Suda,
Y.; Okawa, S.; Hashimoto, A.; Iitaka, Y.; Kobayashi, Y.
Tetrahedron Lett. 1988, 29, 5291; (c) Iseki, K.; Kuroki, Y.;
Asada, D.; Kobayashi, Y. Tetrahedron Lett. 1997, 38,
CrCl and then a-elimination and simultaneous hydro-
2
gen-1,2-shift occurs to give the fluorovinyl ether 2 or 4
1b,22,23
1
(
Scheme 4).
In summary, a high yield and Z selective formation of 2-
fluorovinyl alkyl ethers and 1-fluoro-2-alkenyl enolsilyl
ethers was achieved through chromium mediated alke-
nylation reaction of 2,2-dibromo-2-fluoroethyl ethers
and dibromofluoromethylcarbinyl silyl ethers.
5
1
447.
References and notes
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8
9
1
1
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9
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1
3. Recent reports on chromium mediated reaction using
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1
2
Recently we reported CrCl mediated fluoroalkenylation
reaction of aldehyde using 1-bromo-1-fluoroalkenes.