Hydroarylation of 1-aryl-2-halogeno-3,3,3-trifluoropropenes in CF3SO3H: Regioselective approach to trifluoromethylated diarylethanes and ethenes

Article abstract of DOI:10.1016/j.tetlet.2016.01.099

The reaction of 2-halogeno-2-CF₃ substituted styrenes [ArCH=C(X)CF₃, X = F, Cl, Br] with the superacid TfOH resulted in 100% regioselective formation of the corresponding benzyl cations [ArHC⁺-CH(X)CF₃]. Subsequent reaction of these electrophilic species with arenes [Ar′H] afforded diastereomeric 1,1-diaryl-2-halogeno-3,3,3-trifluoro propanes [Ar(Ar′)CH-CH(X)CF₃] in high yields (up to 96%). The obtained halogenopropanes were easily transformed to the corresponding trifluoromethylated diarylethenes [Ar(Ar′)C=CCF₃] in yields of up to 95% by dehydrohalogenation using base. The elimination of chlorides and bromides using the KOH-ethanol system proceeds via an E2 elimination, whereas in the case of fluorides an E1cb elimination most probably takes place as the major reaction path.

Full text of DOI:10.1016/j.tetlet.2016.01.099

Accepted Manuscript
Hydroarylation of 1-Aryl-2-Halogeno-3,3,3-Trifluoropropenes in CF3SO3H.
Regioselective Approach to Trifluoromethylated Diarylethanes and Ethenes
Maria A. Sandzhieva, Dmitry S. Ryabukhin, Vasily M. Muzalevskiy, Elena V.
Grinenko, Valentine G. Nenajdenko, Aleksander V. Vasilyev
PII:
S0040-4039(16)30099-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.01.099
TETL 47263
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 January 2016
25 January 2016
27 January 2016
Please cite this article as: Sandzhieva, M.A., Ryabukhin, D.S., Muzalevskiy, V.M., Grinenko, E.V., Nenajdenko,
V.G., Vasilyev, A.V., Hydroarylation of 1-Aryl-2-Halogeno-3,3,3-Trifluoropropenes in CF3SO3H. Regioselective
Approach to Trifluoromethylated Diarylethanes and Ethenes, Tetrahedron Letters (2016), doi: http://dx.doi.org/
10.1016/j.tetlet.2016.01.099
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Hydroarylation of 1-Aryl-2-Halogeno-3,3,3-Trifluoropropenes in CF3SO3H.
Regioselective Approach to Trifluoromethylated Diarylethanes and Ethenes
Maria A. Sandzhieva,§ Dmitry S. Ryabukhin,§ # Vasily M. Muzalevskiy,† Elena V. Grinenko,§
Valentine G. Nenajdenko,† * Aleksander V. Vasilyev§ # *
§Department of Chemistry, Saint Petersburg State Forest Technical University, Institutsky per., 5,
Saint Petersburg , 194021, Russia
#Department of Organic Chemistry, Institute of Chemistry, Saint Petersburg State University,
Universitetskaya nab., 7/9, Saint Petersburg , 199034, Russia
†Department of Chemistry, Lomonosov Moscow State University, Vorobievy Gory, 1, Moscow,
119899, Russia
Corresponding Authors
* E-mail: nenajdenko@org.chem.msu.ru (Valentine G. Nenajdenko);
* E-mails: aleksvasil@mail.ru, a.vasilyev@spbu.ru (Aleksander V. Vasilyev)
Keywords: trifluoromethyl alkenes, superacids, carbocations, hydroarylation, dehydrohalogenation
Abstract: The reaction of 2-halogeno-2-CF3 substituted styrenes [ArCH=C(X)CF3, X = F, Cl, Br]
with the superacid TfOH resulted in 100% regioselective formation of the corresponding benzyl
cations [ArHC+–CH(X)CF3]. Subsequent reaction of these electrophilic species with arenes [Ar'H]
afforded diastereomeric 1,1-diaryl-2-halogeno-3,3,3-trifluoro propanes [Ar(Ar')CH–CH(X)CF3] in
high yields (up to 96%). The obtained halogenopropanes were easily transformed to the
corresponding trifluoromethylated diarylethenes [Ar(Ar')C=CCF3] in yields of up to 95% by
dehydrohalogenation using base. The elimination of chlorides and bromides using the KOH-ethanol
system proceeds via an E2 elimination, whereas in the case of fluorides an E1cb elimination most
probably takes place as the major reaction path.
Graphical abstract:
1

Organofluorine compounds are of a great importance for chemistry, biology, medicine,
nanotechnology, and material science. The trifluoromethylated fragment is a valuable structural
unit, which is often present in pharmaceuticals, agrochemicals, and functionalized organic materials
(Figure 1).1
Figure 1. Practically valuable molecules containing a CF3-alkene group.
Several approaches have been developed for the synthesis of CF3-alkenes.2 Such alkenes are
involved in various useful transformations: reactions with nucleophiles,3 aryl halides,4 silanes,5
organoboron derivatives,6 organolithium compounds,7 enamines,8 enolates,9 and terminal alkynes,10
oxidative cyclization with aldehydes,11 as well as other reactions.12 Trifluoromethylated alkenes can
be used for the synthesis of polymers.13 However, there are only two literature examples of
superacidic activation of CF3-substituted alkenes.14 To the best of our knowledge such reactions
with trifluoromethylated alkenes bearing additional halogens are unknown. The presence of a
halogen atom (F, Cl, Br) in the structure can be important for cation stabilization via formation of
cyclic halonium cations. Such species are well recognized classical intermediates of many
electrophilic reactions. The stability of halonium ions is decreased from heavy to light halogens.
Iodonium and bromonium salts can be isolated, but the first evidence of fluoronium cations in
solution was only demonstrated in 2013.15
Scheme 1. Protonation of alkenes 1 followed by the Friedel-Crafts alkylation of arenes
2

Based on our recent works on the transformations of CF3-substituted alkynes,16 carbonyl
compounds,17 and allyl alcohols18 under the action of Brønsted or Lewis (super)acids, we extended
this study to the reaction of trifluoromethylated styrenes bearing an additional halogen atom with
various arenes under superelectrophilic activation. Due to the acceptor character of the CF3 group,
protonation of alkene 1 should proceed with 100% regioselectively at the C2 carbon to afford benzyl
cation A, which may also exist as halonium ion B (Scheme 1). These species may participate in the
Friedel-Crafts alkylation of arenes leading to trifluoromethylated diarylethane 2, as products of the
hydroarylation of the C=C bond of starting alkene 1. The reaction of halonium ion B with
nucleophiles (arenes) should be stereoselective, while contrary to that, a loss of stereoselectivity
should be observed in the reaction of open cation A because nucleophilic attack can proceed from
both sides of such a carbocation.
Starting 1-aryl-2-halogeno-3,3,3-trifluoro propenes 1a-l (see Tables 1, 2) were synthesized by
catalytic olefination from the corresponding arylaldehydes and 1,1,1-trifluoro-2,2,2-
trihalogenoethanes.2a-c
We investigated the activation of alkenes 1 bearing various substituents on the aromatic ring
and different halogen substituents at the double bond with Brønsted acids in the presence of arenes.
First, we tested the reaction of 1-phenyl-2-chloro-3,3,3-trifluoropropene 1a. This alkene did not
react with benzene using H2SO4 or CF3CO2H and only proceeded in the stronger trifluoromethane
sulfonic acid CF3SO3H (TfOH) affording 2a as the product of hydrophenylation of 1a in 86% yield
(Table 1, entry 1). Similar reactions were observed for other arenes including toluene, isomeric
xylenes, pseudocumene, anisole, veratrole, and even the less nucleophilic 1,2-dichlorobenzene
(entries 2-9). The reaction with TfOH is general and gave the target compounds 2 in high yields.
Table 1. Hydroarylation of CF3-alkene 1a in TfOH and dehydrochlorination of compounds 2a-j
using the KOH-ethanol system
ratio of D1/D2 for
2 and Z/E for 3
entry
R–ArHa
R (in 2, 3)
2 (yield, %) 3 (yield, %)
1
2
3
4
5
benzeneb
toluenec
o-xylenec
m-xylenec
p-xylenec
H
2a (86)
2b (91)
2c (91)
2d (66)
2e (89)
3a (89)
3b (72)
3c (87)
3d (85)
3e (79)
–
4-Me
37 : 63
53 : 47
29: 71
3,4-Me2
2,4-Me2
2,5-Me2
67: 33
3

2,4,5-Me3
2,3,5-Me3
3,4-Cl2
2f (73)
2g (14)
2h (22)
2i (70)
2j (43)
3f (74)
3g (15)
3h (89)
3i (87)
3j (86)
53: 47
50: 50
83 :17
42: 58
40: 60
6
pseudocumenec
b
7
8
9
1,2-Cl2C6H4
anisolec
4-MeO
veratrolec
3,4-(MeO)2
aMolar ratio 1a : arene 1: 5, for benzene 1 : 17. b r.t., 1 h. c 0 C, 2 h.
Lower yields were observed in the case of 1,2-dichlorobenzene (entry 7) due to significant
1
polymerization of the starting alkene 1 according to H NMR data. The reaction was also highly
regioselective in terms of arene selectivity. The carbocation formed attacks only the para-position
of the substituted arenes, demonstrating the high steric demand of the electrophilic species formed
(see, for instance, Table 1, entries 2, 3, 8). Also quite important was the stereochemical result of the
reaction. The starting alkene 1a had a 83 : 17 Z/E ratio. However, in all cases we observed the
formation of a mixture of two diastereomers of 2b-j (D1 and D2) in variable ratios ranging from 50
: 50 (1:1) up to 83:17 (5:1). These diastereomeric products have different (R)-, (S)- configurations
of atoms C1 and C2: D1 (1RS, 2RS) and D2 (1SR, 2RS) (Table 1, entries 2-9). In all 1H, 13C, and 19F
NMR spectra two sets of separate signals for each diastereomer 2b-j were observed (see ESI). This
data shows that the participation of chlorine in the stabilization of the formed cation is either not
observed or only takes place to a very low extent. Therefore, the most probable carbocationic
intermediate formed under the reaction conditions exists in the open form as the A type species
(Scheme 1).
To elucidate the exact structure of the diastereomers we carried out dehydrochlorination of
compounds 2 to give alkenes 3 by heating at reflux in KOH/ethanol for 15 h (Table 1). The Z/E-
ratios of the formed alkenes 3 was the same as that of the D1/D2 ratios of their precursors 2.
According to these stereochemical results we can conclude that the transformation of 2 into 3
proceeds as an E2 elimination. Z-Alkenes 3 were formed from diastereomers D1 (1RS,2RS) and E-
isomers 3 were obtained from diastereomers D2 (1SR,2RS), as depicted in Scheme 2 with the use of
Newman projections. The configuration of the Z- and E-isomers of 3 can be easily elucidated using
1H NMR spectra. The E-isomers of 3 have the vinyl proton signal low field shifted compared to the
vinyl proton of the Z-isomer, as previously reported.16 Based on these spectral data, we were able to
determine the Z/E-stereochemistry of the formed alkenes 3 and elucidate the structures of the
diastereomeric precursors, D1 and D2, of 2.
4

Scheme 2. Newman projections of 2 and transformation of 2 into 3.
Next, the reactions of a series of alkenes 1b-l containing various aryl substituents and
halogens (X = F, Cl, Br) with benzene in TfOH were studied (Table 2). We found that the
transformation was general. Analogous to the reactions of 1a (Table 1), superacidic activation of
other trifluoromethylated alkenes 1 led to formation of trifluoromethylated diarylethanes 2 in high
yield (Table 2). Chloro- (X = Cl) 2b-l and bromo- (X = Br) 2m-o derivatives were converted into
the corresponding alkenes 3 (Table 2, entries 1-7) by treatment with KOH in ethanol. This allowed
the determination of the structures of diastereomers of compounds 2b,i,k-o. Dehydrobromination
proceeded smoothly even at r.t. (Table 2, entries 5-7). Dehydrofluorination required harsher
conditions (KOH, reflux, ethanol, 20 h), and in the case of compound 2s, gave alkene 3i with a Z/E-
ratio of 1:1, despite the initial ratio of diastereomers for 2s being 1 : 2 (Table 2, entry 11). This
meant that, due to the strong electron withdrawing properties of the CF3 and F substituents,
elimination of HF from 2s, most probably, proceeded in accordance with an E1cb mechanism,
rather than an E2 mechanism (contrary to the chloro- and bromo- compounds 2, X = Cl, Br). We
found that dehydrofluorination of 2p-s could be carried out stereoselectively using t-BuOK in THF,
affording alkenes 3a,b,i,l in good yields with Z/E-ratios corresponding to that of D1/D2 for 2p-s
(Table 2, entries 8-11).
Table 2. Hydrophenylation of CF3-alkenes 1b-l in TfOH and the dehydrohalogenation of
compounds 2b,i,k-s
ratio of D1/D2 for
entry alkene (Z/E-ratio)a R in Ar
1b (83 : 17)b
3-Me
X
2 (yield, %) 3 (yield, %)
2 and Z/E for 3
1
Cl 2k (78)
3k (85)e
77 : 23
5

2
1c (86 : 14)c
1d (75 : 25)d
1e (91 : 9)c
1f (89 : 11)b
1g (75 : 25)b
1h (87: 13)c
1i (97 : 3)b
1j (97 : 3)c
1k (97 : 3)d
1l (97 : 3)b
4-Me
4-Cl
Cl 2b (96)
Cl 2l (91)
Cl 2i (91)
Br 2m (88)
Br 2n (76)
Br 2o (54)
3b (87 e
3l (85) e
3i (94) e
3a (89) f
3l (95) f
3b (87) f
3a (90) g
3b (88) g
3l (92) g
3i (93) g
71 : 29
75 :25
83 : 17
–
3
4
4-MeO
H
5
6
4-Cl
67 : 33
53 : 47
–
7
4-MeO
H
8
F
F
F
F
2p (78)
2q (67)
2r (92)
2s (90)
9
4-Me
4-Cl
59: 41
56 : 44
33 : 67
10
11
4-MeO
aMolar ratio 1a : arene 1: 5. br.t., 1 h. c-10 C, 0.5 h. d60 C, 1 h.
eKOH, EtOH, reflux, 15 h. fKOH, EtOH, r.t., 20 h. gt-BuOK, THF, r.t. (or reflux), 2 d.
Some features of this hydroarylation reaction should be noted. The reaction is very sensitive
to the nature of the halogen atoms on the double bond and substituents on the aromatic ring. For
example, alkenes 1d and 1k (with X = Cl and F) bearing an acceptor chlorine atom in the para-
position of the aryl ring are hardly protonated using TfOH. These alkenes 1d and 1k interact with
benzene only at an elevated temperature of 60 °C (Table 2, entries 3, 10). Other alkenes 1
containing electron donating substituents react smoothly at -10 °C or r.t. (Tables 1, 2).
Polymethylated arenes such as mesitylene and durene do not participate due to the sensitivity of this
reaction to steric hindrance from the two ortho-methyl groups in these arenes. Also, the key point of
this reaction is the preservation of the C2–X (X = F, Cl, Br) bond in compounds 2 under superacidic
conditions. Usually this kind of carbon-halogen bond is easily cleaved in superacids,19 however no
cleavage of this bond is observed for substrates 2 due to the electron withdrawing influence of the
neighboring CF3 group.
In many cases the hydroarylation proceeds rather stereoselectively with predominant
formation of one of the diastereomers D1 or D2 (see ratios of D1/D2 in Table 1, entries 2, 4, 5, 7,
and Table 2, entries 1-4, 6, 11). The diastereomeric ratio most probably depends on steric factors.
Despite not regulating the stereoselectivity of the reaction, data on the diastereomeric content of
compounds 2 sheds light on the reactivity of cations A and B (Scheme 1). Thus, in the case of
fluoroalkenes 1i-l (X = F) the stereoselectivity is poor and alkenes 1j-l with a Z/E- ratio of 97 : 3 led
to compounds 2q-s with D1/D2 ratios ranging from 33-59 : 67-41 (Table 2, entries 9-11). Such a
loss of stereoselectivity reveals that in the case of the fluoroderivatives a cyclic fluoronium ion
cannot be postulated, and thus the diastereocontrol could only come from a cation of type A. For
6

brominated (X = Br) and chlorinated (X = Cl) substrates 1 the formation of halonium ions B is more
probable.
It should be noted that alkenes 3 are structurally close to Panomifene, which is a well known
anticancer drug (Figure 1). Therefore, the synthesis of such alkenes is of significant theoretical and
practical importance.20
In conclusion, we have developed simple and highly efficient method for hydroarylation of
double bond of 2-halogeno-2-CF3 styrenes by their reactions with arenes in TfOH leading to 1,1-
diaryl-2-halogeno-3,3,3-trifluoro propanes. The latter compounds can be transformed into 1,1-
diaryl-3,3,3-trifluoro propenes by dehydrohalogenation.
Supporting Information
Experimental details, compound characterization, and 1H, 13C, 19F NMR spectra.
Acknowledgment
This work was supported by Saint Petersburg State University, Saint Petersburg, Russia (grant no.
12.38.195.2014) (superacidic activation, substance isolation and identification), RFBR (grant no.
15-33-50678), and Russian Science Foundation [grant no. 14-13-00083 for V.M. and V.N.
(synthesis of starting compounds, part of experiments and discussion)].
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