A new method for the preparation of 2,3-diaryl-1,1-difluoro-1,3-butadienes

Article abstract of DOI:10.1081/SCC-100104825

Reactions of 1-trifluoromethyl-1,2-diarylvinyl sulfones 3 with aryllithium afforded aryl substituted adduct 4. Bromination of 4 with NBS, followed by debromofluorination with Mg, provided 2,3-diaryl-1,1-difluoro-1,3-butadienes (6) in good yields.

Full text of DOI:10.1081/SCC-100104825

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
A NEW METHOD FOR THE PREPARATION OF 2,3-
DIARYL-1,1-DIFLUORO-1,3-BUTADIENES
In Howa Jeong , Young Sam Park , Min Wook Chung & Bum Tae Kim
To cite this article: In Howa Jeong , Young Sam Park , Min Wook Chung & Bum Tae Kim (2001)
A NEW METHOD FOR THE PREPARATION OF 2,3-DIARYL-1,1-DIFLUORO-1,3-BUTADIENES,
Synthetic Communications, 31:15, 2261-2270, DOI: 10.1081/SCC-100104825
To link to this article: http://dx.doi.org/10.1081/SCC-100104825
Published online: 09 Nov 2006.
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Date: 10 December 2015, At: 19:29

SYNTHETIC COMMUNICATIONS, 31(15), 2261–2270 (2001)
A NEW METHOD FOR THE PREPARATION
OF 2,3-DIARYL-1,1-DIFLUORO-1,3-
BUTADIENES
1
In Howa Jeong,1, Young SamPark, Min Wook Chung,1
*
2
and BumTae Kim
1Department of Chemistry, Yonsei University,
Wonju 220-710, S. Korea
2Korea Research Institute of Chemical Technology,
Daejeon 305-606, S. Korea
ABSTRACT
Reactions of 1-trifluoromethyl-1,2-diarylvinyl sulfones 3 with
aryllithium afforded aryl substituted adduct 4. Bromination of
4 with NBS, followed by debromofluorination with Mg, pro-
vided 2,3-diaryl-1,1-difluoro-1,3-butadienes (6) in good yields.
1,3-Butadienes are important compounds in organic synthesis. For
example, Diels-Alder reactions of these compounds with dienophiles are
widely used for the preparation of various six-membered rings.1,2 The
electrocyclic reaction of 1,3-butadienes under the photochemical condition
is a useful synthetic method for the preparation of cyclobutene ring systems.3
Although various synthetic methods for the preparation of nonfluorinated
1,3-butadienes and their application have been well established,4–7 the
* Corresponding author.
2261
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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JEONG ET AL.
methods for the synthesis of fluorinated 1,3-butadienes, especially
1,1-difluoro-1,3-butadienes, have been quite limited in previous
literatures.8–12 The reaction of commercially available 4-bromo-1,1,2-tri-
fluoro-1-butene with potassium hydroxide in the presence of phase-transfer
catalyst afforded 1,1,2-trifluoro-1,3-butadiene.8 The versatile transforma-
tions of this compound are very useful synthetic tools to give a variety of
functionalized monofluoroalkene compounds.8,13 4-Phenyl-1,1-difluoro-
1,3-butadiene was obtained in low yield from the reaction of
3,3-difluoroallyltriphenylphosphonium bromide with benzaldehyde in the
presence of potassium carbonate.9 4-Phenyl-1,1-difluoro-1,3-butadience
can be also prepared from the reaction of 1,3-dibromo-1,1-difluoro-
compounds, which were synthesized by adding dibromodifluoromethane to
olefins, with DBU.10 Reaction of 4-phenyl-1,1-difluoro-1,3-butadiene with
4-phenyl-1,2,4-triazoline-3,5-dione provided cycloadduct.10 1,1-Difluoro-
2-triphenylsiloxy-1,3-butadiene was also prepared from the reaction of
trifluoroacetyltriphenylsilanewithvinylmagnesiumbromide.11Thiscompound
also undergoes Diels-Alder reaction with various dienophiles.12 However,
several previous methods for the synthesis of 1,1-difluoro-1,3-butadienes
can not be applied to prepare 2,3-diaryl-1,1-difluoro-1,3-butadienes. In this
communication we will describe a new method for the preparation of
2,3-diaryl-1,1-difluoro-1,3-butadienes.
1,1,1-Trifluoro-2-aryl-3-phenylsulfonyl-2-butenes (3)(E:Z ¼ 45:55) can
be easily prepared from the oxidation of 1,1,1-trifluoro-2-aryl-3-phenylthio-
2-butenes (2), which were synthesized from the reaction of 3,3-bis(phenyl-
thio)-1,1,1,2,2-pentafluorobutane (1)14 with 2.5 equiv. of substituted
phenyllithium compounds.
The reaction pathway for the formation of 2 seems likely that the initial
reaction of 1 with phenyllithium compounds via attack of sulfur atom pro-
vides the carbanions bearing a pentafluoroethyl group, which quickly under-
goes b-defluorination to give the b-fluoro-b-trifluoromethylvinyl sulfides.
These compounds are so reactive that they quickly undergo addition-elim-
ination reaction with phenyllithium compounds presented in solution as soon

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2,3,-DIARYL-1,1-DIFLUORO-1,3-BUTADIENES
2263
as they were formed. The treatment of 3 with another 1.5 equiv. of substituted
phenyllithium compounds resulted in the formation of E and Z isomeric
mixture (E:Z ¼ 35:65) of 2,3-diaryl-1,1,1-trifluoro-2-butenes (4) in good
yields. Assignment of isomers for 4 was based on the chemical shift of CH3
in each isomer. We observed that the chemical shift (d ¼ 1.84 ppm) of CH3
which is arranged to the same side of CF3 group is lower than the chemical
shift (d ¼ 2.35 ppm) of CH3 which is arranged to the opposite side of CF3
group.15 The results for the formation of 4 are summarized in Table 1.
Allylic bromination of 4 was carried out with N-bromosuccinimide
(NBS). Several solvents such as CCl4, CH2Cl2, CH3CN and benzene were
examined in this reaction. The optimized condition was established by
refluxing of 4 with NBS (3–5 equiv.) in CH3CN for 12–24 h. Therefore,
the reaction of 4a with NBS (3 equiv.) in CH3CN at reflux temperature
for 24 h afforded allylic bromide 5a (E:Z ¼ 60:40) in 78% yield. The similar
treatment of 4d and 4e with NBS resulted in the fomation of allylic bromides
5d and 5e in 71% and 73% yields, respectively. When 4b was reacted with
NBS (4 equiv.) in CH3CN for 12 h, however, allylic bromide 5b was
obtained in only 40% yield. The major side product was benzylic
Table 1. Preparation of 2,3-Diaryl-1,1,1-trifluoro-2-butenes (4)
Compound No.
X
Y
4, Yield(%)a,b
4a
4b
4c
4d
4e
4f
H
H
H
H
Cl
CH3
CH3
CH3O
CH3O
CH3O
H
CH3
OCH3
Cl
H
H
CH3
H
CH3
OCH3
80
71
68
65
82
76
81
77
82
83
4g
4h
4i
4j

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2264
JEONG ET AL.
brominated compound 5k. The similar results were obtained from the reac-
tions of 4f, 4g and 4i with NBS, in which allylic bromides 5f, 5g and 5i were
formed in 41%, 57% and 34% yields, respectively. The reactions of 4c and
4h with NBS also provided allylic bromides 5c and 5h in relatively low
yields, in which alkoxy brominated compounds 5l and 5m were formed
as a major side product, respectively. To make matters worse, the treatment
of 4j with NBS resulted in the formation of a messy reaction mixture.
The results for the formation of 5 are summarized in Table 2.
Table 2. Preparation of 2,3-Diaryl-4-bromo-1,1,1-trifluoro-2-butenes (5)
Compound No.
X
Y
5, Yield(%)a,b
5a
5b
5c
5d
5e
5f
H
H
H
H
Cl
CH3
CH3
CH3O
CH3O
CH3O
H
CH3
OCH3
Cl
H
H
CH3
H
CH3
OCH3
78
40
42
71
73
41
57
40
34
5g
5h
5i
c
5j
–

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2,3,-DIARYL-1,1-DIFLUORO-1,3-BUTADIENES
2265
First of all, we examined the debromofluorination of 5 with metals,
such as Zn, activated Zn, Zn(Cu) and Mg, to give 1,1-difluorodienes 6. The
use of Zn, activated Zn or Zn(Cu) in this reaction did not afford a desired
product 6, but the reaction of 5a with Mg (1.5 equiv.) in the presence of
catalytic amount of I2 provided 2,3-diphenyl-1,1-difluoro-1,3-butadiene (6a)
in 81% yield. Therefore, we decided to use Mg (1.5 equiv.) and I2 (cat.) in
this type of reaction. When 5b–i was treated with Mg (1.5 equiv.) in the
presence of catalytic amount of I2, 1,1-difluorodiene 6b–i was synthesized in
42% to 74% yields. The reaction pathway for the formation of 6 seems
likely that the initial reaction of 5 with Mg affords oxidative insertion inter-
mediate between vinyl carbon and bromine bond, which quickly undergoes
d-defluorination to give 2,3-diaryl-1,1-difluoro-1,3-butadienes (6) via six-
membered ring transition state [I]. The results of the elimination reaction
of 5 with Mg are summarized in Table 3.
In conclusion, the present method provides a new and general pro-
cedure for the preparation of various 2,3-diaryl-1,1-difluoro-1,3-butadienes
which can not be prepared from the several previous methods. Diels-Alder
reactions of 6 with dienophiles are now in progress.
EXPERIMENTAL
Synthesis of 3,3-Bis(phenylthio)-1,1,1,2,2-pentafluorobutane (1)
A 500 mL three-neck round bottom flask equipped with a septum,
a solid addition tube filled with AlCl3 (6.09 g, 0.05 mol), a magnetic
stir bar and a nitrogen tee connected to a source of argon, was charged
with 3,3,4,4,4-pentafluoro-2-butanone (8.1 g, 0.05 mol), thiophenol (11.0 g,
0.1 mol) and 300 mL of dry CH2Cl2. The reaction mixture was cooled
to À78ꢀC and AlCl3 was added in several portions via a solid addition
tube. After stirring at À78ꢀC for 20 h, the reaction mixture was quenched
with water at À78ꢀC. The mixture was poured into 200 mL of H2O, extracted
with CH2Cl2 (300 mL Â 2). After washing with saturated NaCl water
solution, CH2Cl2 layer was dried with anhydrous Na2CO3. Column
chromatography (n-hexane) afforded 15.5 g (85% yield) of 3,3-bis
(phenylthio)-1,1,1,2,2-pentafluorobutane (1). 1: oil; 1H NMR(CDCl
)
3
d 7.68–7.25 (m, 10H), 1.34 (s, 3H); 19F NMR(CDCl ) d À109.86 (s, 2F),
3
À75.68 (s, 3F); MS, m/z (relative intensity) 364 (Mþ, 3), 255 (100), 215
(33), 177 (10), 165 (9), 134 (10), 109 (61), 77 (13), 65 (10); IR(neat)
3062, 3003, 2944, 1474, 1440, 1315, 1215, 1184, 1148, 1069, 991, 751,
691 cm–1.

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2266
JEONG ET AL.
Table 3. Preparation of 2,3-Diaryl-1,1-difluoro-1,3-butadienes (6)
Compound No.
X
Y
6, Yield(%)a
6a
6b
6c
6d
6e
6f
H
H
H
H
Cl
CH3
CH3
CH3O
CH3O
H
CH3
OCH3
Cl
H
H
CH3
H
CH3
81
60
65
74
72
58
42
61
45
6g
6h
6i
Synthesis of 1,1,1-Trifluoro-2-phenyl-3-phenylthio-2-butene (2)
A 250 mL two-neck flask equipped with a septum, a magnetic stir bar
and a nitrogen tee connected to a source of argon, was charged with
3,3-bis(phenylthio)-1,1,1,2,2-pentafluorobutane (3.64 g, 0.01 mol) and 50 mL
of dry THF. The reaction mixture was cooled to À78ꢀC and phenyllithium
(1.8 M solution, 11.2 mL) was added dropwise at À78ꢀC, followed by slow
warming to ambient temperature. The reaction mixture was quenched with
water (50 mL) and extracted with ether (50 mL Â 2). After the ether layer was

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2,3,-DIARYL-1,1-DIFLUORO-1,3-BUTADIENES
2267
dried with anhydrous MgSO4, column chromatography (n-hexane) afforded
2.23 g (76% yield) of (E ) and (Z) isomeric mixture (E:Z ¼ 45:55) of 1,1,1-
trifluoro-2-phenyl-3-phenylthio-2-butene (2). 2: oil; 1H NMR(CDCl 3) d
7.54–7.14 (m, 10H), 2.09 (q, J ¼ 2.4 Hz, 3H, Z isomer), 1.67 (q, J ¼ 2.0 Hz,
3H, E isomer); 19F NMR(CDCl 3) d À56.05 (s, 3F, Z isomer), À56.58 (s, 3F, E
isomer); MS, m/z (relative intensity) 294 (Mþ, 100), 279 (10), 225 (17), 165
(25), 115 (45), 77 (31), 59 (43); IR(neat) 3062, 3026, 2927, 1607, 1475, 1441,
1310, 1214, 1170, 1115, 1016, 758, 707 cm–1.
Synthesis of 1,1,1-Trifluoro-2-phenyl-3-phenylsulfonyl-2-butene (3)
To a mixture of 1,1,1-trifluoro-2-phenyl-3-phenylthio-2-butene (2.0 g,
6.8 mmol) and 30 mL of CH2Cl2 was added m-chloroperbenzoic acid (50%
technical grade, 27.2 mmol) at 0ꢀC and then the mixture was stirred at
ambient temperature for 12 h. After quenching the reaction mixture with
saturated NaHCO3 solution and 10% NaHSO3, the mixture was extracted
with CH2Cl2 (20 mL Â 2) and then dried over anhydrous Na2CO3. After
evaporation of solvent, the crude product was recrystallized (n-hexane:
methylene chloride ¼ 1:1) to provide 1.73 g (78% yield) of (E) and (Z) iso-
meric mixture (E:Z ¼ 45:55) of 1,1,1-trifluoro-2-phenyl-3-phenylsulfonyl-
2-butene (3). 3: mp 112–114ꢀC; H NMR(CDCl ) d 7.98–6.89 (m, 10H),
1
3
2.44 (q, J ¼ 2.3 Hz, 3H, Z isomer), 1.81 (q, J ¼ 1.6 Hz, 3H, E isomer);
19F NMR(CDCl ) d À54.04 (s, 3F, E isomer), À59.45 (s, 3F, Z isomer);
3
MS, m/z (relative intensity) 326 (Mþ, 24), 307 (9), 201 (100), 165 (75), 145
(35), 125 (60), 115 (90), 77 (62), 51 (42); IR(KBr) 3067, 2963, 1799, 1548,
1447, 1335, 1161, 1011, 764 cm–1.
Synthesis of 1,1,1-Trifluoro-2,3-diphenyl-2-butene (4a)
To a mixture of 1,1,1-trifluoro-2-phenyl-3-phenylsulfonyl-2-butene
(0.652 g, 2.0 mmol) and 5 mL of ether was added phenyllithium (1.8 M sol-
ution, 7.0 mmol) at room temperature and then the mixture was stirred at
room temperature for 1 h. After quenching the reaction mixture with satu-
rated NaCl solution, the mixture was extracted with ether (10 mL Â 2) and
then dried over anhydrous MgSO4. After evaporation of solvent, the crude
product was chromatographed (n-hexane) to provide 0.419 g (80% yield) of
(E ) and (Z) isomeric mixture (E:Z ¼ 35:65) of 1,1,1-trifluoro-2,3-diphenyl-
2-butene (4a). 4a: oil; 1H NMR(CDCl 3) d 7.51–6.88 (m, 10H), 2.35
(q, J ¼ 2.8 Hz, 3H, Z isomer), 1.84 (q, J ¼ 2.2 Hz, 3H, E isomer); 19F
NMR(CDCl ) d À56.28 (s, 3F, E isomer), À56.68 (s, 3F, Z isomer); MS,
3

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2268
JEONG ET AL.
m/z (relative intensity) 262 (Mþ, 96), 193 (53), 184 (98), 178 (28), 115 (34),
103 (100), 91 (17), 77 (48), 51 (12); IR(neat) 3060, 3027, 2957, 2925, 2854,
1638, 1492, 1443, 1331, 1264, 1208, 1162, 1116, 1029, 761, 699 cm–1.
Synthesis of 4-Bromo-1,1,1-trifluoro-2,3-diphenyl-2-butene (5a)
A mixture of 1,1,1-trifluoro-2,3-diphenyl-2-butene (0.393 g, 1.5 mmol),
N-bromosuccinimide (0.68 g, 4.5 mmol), AIBN (cat.) and 5 mL of CH3CN
was heated to reflux for 24 h. After cooling to room temperature and then
evaporating solvent, the crude mixture was chromatographed on SiO2 to
give 0.40 g (78% yield) of (E ) and (Z) isomeric mixture (E:Z ¼ 60:40) of
4-bromo-1,1,1-trifluoro-2,3-diphenyl-2-butene(5a).5a:oil;1HNMR(CDCl3)
d 7.58–7.05 (m, 10H), 4.51 (s, 2H, E isomer), 3.91 (s, 2H, Z isomer);
19F NMR(CDCl ) d À57.03 (s, 3F, Z isomer), À57.45 (s, 3F, E isomer);
3
MS, m/z (relative intensity) 342 (Mþ, Br-81, 16), 340 (Mþ, Br-79, 16), 261
(48), 221 (22), 191 (34), 183 (100), 133 (15), 115 (8), 77 (8), 51 (8); IR(neat)
3059, 3025, 2927, 1491, 1444, 1329, 1269, 1206, 1168, 1117, 898, 762,
698 cm–1.
Synthesis of 1,1-Difluoro-2,3-diphenyl-1,3-butadiene (6a)
A mixture of 4-bromo-1,1,1-trifluoro-2,3-diphenyl-2-butene (0.408 g,
1.2 mmol), Mg (0.044 g, 1.8 mmol), iodine(cat.) and 5 mL of ether was
heated to reflux for 1 h. After cooling to room temperature and quenching
the reaction mixture with 10% HCl solution, the mixture was extracted with
ether (10 mL Â 2) and then dried over anhydrous MgSO4. After evaporation
of solvent, the crude product was chromatographed (n-hexane) to provide
0.235 g (81% yield) of 1,1-difluoro-2,3-diphenyl-1,3-butadiene (6a). 6a: oil;
1H NMR(CDCl 3) d 7.48–7.18 (m, 10H), 5.88 (s, 1H), 5.40 (s, 1H); 19F NMR
(CDCl3) d À84.60 (d, J ¼ 29.5 Hz, 1F), À88.34 (d, J ¼ 29.6 Hz, 1F); MS, m/z
(relative intensity) 242 (Mþ, 100), 221 (71), 191 (18), 178 (21), 164 (23), 127
(59), 115 (19), 91 (14), 77 (21), 51 (16); IR(neat) 3058, 3027, 2942, 1707,
1495, 1445, 1264, 1234, 1035, 967, 912, 763, 696 cm–1.
ACKNOWLEDGMENT
This work was supported by Korea Research Foundation Grant
(KRF-1999-015-DP0218).

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REFERENCES
2269
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15. Authetic samples of (E ) and (Z )-1,1,1-trifluoro-2,3-diphenyl-2-bute-
nes(4a) were prepared from the cross-coupling reaction of separable
(E ) and (Z)-2-tributylstannyl-1,1,1-trifluoro-3-phenyl-2-butenes(7)
with iodobenzene in the presence of (PPh3)4Pd (10 mol%) and CuI
(10 mol%). (E ) and (Z)-2-tributylstannyl-1,1,1-trifluoro-3-phenyl-
2-butenes (7) were figured out via occurrence of cyclization
of (E ) and (Z)-2-trifluoromethyl-1,3-diphenyl-2-buten-1-ols (8) which
can be prepared from the cross-coupling reaction of each (E ) and

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JEONG ET AL.
(Z)-2-tributylstannyl-1,1,1-trifluoro-3-phenyl-2-butenes(7)withbenzoyl
chloride, followed by reduction with LiAlH4.16 The (E ) isomer of 8
underwent Friedel-Craft’s type of cyclization to give indene derivative
in the presence of AlCl3, but the (Z) isomer of 8 did not undergo
cyclization.
16. Park, Y.S. Thesis, Yonsei University, 2000.
Received in Japan July 18, 2000

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