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Trifluoromethylated Allyl Alcohols. Acid Promoted Reactions with
Arenes and Unusual “Dimerization”
Anna N. Kazakova,† Roman O. Iakovenko,† Vasiliy M. Muzalevskiy,‡ Irina A. Boyarskaya,† Margarita
S. Avdontceva, † Galina L. Starova, † Aleksander V. Vasilyev,*†,§ Valentine G. Nenajdenko*‡
†
Department of Chemistry, Saint-Petersburg State University, 198504 Saint-Petersburg, Petrodvorets, Universitetsky pr., 26,
Russia; ‡ Department of Chemistry, Lomonosov Moscow State University, 119899 Moscow, Russia; §Department of Organic
Chemistry, Saint Petersburg State Forest Technical University, Institutsky per., 5, Saint Petersburg, 194021, Russia.
Supporting Information Placeholder
H
R
H
62-90%
Ar
H
OH
OH
CF3
H
FeCl3
20 C, 1h
Ar
Ar'
Ar'H
CF3
FeCl3
20 C, 1h
Ph
CF3
48-75%
CF3
ABSTRACT: Unusual “dimerization” of CF3-allyl alcohols ArCH=CHCH(OH)CF3 under action of anhydrous FeCl3 was found to
give fluorinated indanes in 62-90% yields via an intermediate formation of allyl cations. Reactions of CF3-allyl alcohols with arenes
ArH led to CF3-alkenes Ar(Ar)CHCH=CHCF3 in 48-75% yields. The mechanisms of the transformations are discussed.
Fluorinated organic compounds have a great theoretical and
practical value for chemistry, biology, medicine, and material
science.1,2 Incorporation of fluorinated moiety into the molecule often changes such important parameters as lipophilicity,
metabolic activity, and bioavailability. Electron-withdrawing
character of fluorinated groups is another advantage, allowing
to control fluorinated compounds transformations very effectively. CF3-substituted carbocations are very promising but
still rare type of fluorinated species exhibited high electrophilicity and selectivity.3
The present work is a continuation of our investigations on
electrophilic activation of alkenes4 and alkynes.5 Reactions of
CF3-substituted allyl alcohols 1 promoted by Bronsted or Lewis acids were investigated (Scheme 1, нет ее). Dehydroxylation of 1 can lead to allyl cation having resonance forms A and
B. However, due to strong electron-withdrawing character of
CF3-group form A is destabilized significantly. As a result,
one can expect a very selective reaction of such cations by
position 4. To confirm this hypothesis we performed DFT
calculations for the CF3-allyl cations bearing various substituents in arene ring and the CH3-substituted allyl cation C1 (Table 1).6 The global electrophilicity indexes ω (14.1-16.6 eV)
for the CF3-cations A1-A5 are significantly higher than ω value (13.4 eV) for the cation C1, therefore cations A1-A5 are
much more electrophilic. Due to electron withdrawing effect
of CF3-group cation A1 has a greater charge on carbon C4 and
lower charge on C2, in comparison with C1 having similar
charges for both C2 and C4. As a result calculations predict
highly selective reactions for trifluoromethylated allyl cations
A1-A5.
One of the most important transformations of substituted allyl cations7 is their participation in new carbon-carbon bond
forming reactions by interaction with arenes,8 heteroarenes,9
alkenes,10 alkynes,11 or carbonyl compounds.12 To our surprise, reactions of trifluoromethylated allyl cations with Cnucleophiles have not been described yet.
Table 1. Formation of CF3-allyl cations and selected characteristics of the cations A1-A5, and C1
Bronsted or
OH Lewis acids
Ar
CF3
1
Ar
A
4
CF
3 2 1 3
Ar
CF3
Ph
4
B
2
3
C1
CH3
1
A-1 Ph; A-2 4-MeC6H4; A-3 4-ClC6H4;A-4 3-MeC6H4; A-5 4-MeOC6H4
Cation
EHOMO,
eV
ELUMO,
eV
ω,a
eV
1
C-1
-11.54
-7.97
2
A-1
-12.18
-8.75
3
A-2
-11.89
4
A-3
-11.88
5
A-4
6
A-5
entry
aGlobal
q(C2),b
e
q(C4),b
e
13.4
0.10
0.08
16.0
-0.05
0.09
-8.43
14.9
-0.07
0.08
-8.66
16.4
-0.06
0.08
-11.71
-8.60
16.6
-0.06
0.10
-11.88
-8.01
14.1
-0.09
0.04
electrophilicity index.
bNatural
charges.
To start our investigation, a series of CF3-allyl alcohols
were prepared by reduction of α,β-unsaturated trifluoromethylketones13 using literature procedures.14 At first we studied the reaction of 1a with benzene under treatment with various Brönsted and Lewis acids (Table 2). Indeed, the expected
alkene 2a was obtained through an intermediate formation of
the corresponding cation B (Scheme 1, нет ее) in protic acid
(H2SO4, CF3CO2H is too weak) and superacids (FSO3H,
TfOH) (Entries 1-4). The reaction is highly diastereoselective
to form only E-isomer of 2a. Moreover, unusual “dimeric”
indane derivative 3 was isolated as a mixture of diastereomers.
Table 2. Acid promoted reaction of 1a with benzene
H
Ph
1a
CF3
Acid
OH +
H
CF3
CF3 +
Ph
E-2a
H
3a
entry
CF3
H
OH
H
CF3
Yield, %
1a:PhH:acid
ratio
T,
°C
time,
h
2a
TfOHa
1:3:50
-35
1
33
2
TfOH
1:15:50
20
1
oligomers
3
FSO3Hb
1:3:86
-75
1
24
H2SO4
1:16:268
20
1
22
CF3CO2H
1: 50: 5
20
1
no reaction
6
FeCl3
1: 50:1
20
1
65
-
7
FeCl3a
1: 1.1:1
20
1
32
40
8
AlCl3
1: 50:1
20
1
21
-
9
AlBr3
1: 50:2
20
1
oligomers
10
BF3×Et2O
1: 50:1
20
72
32
11
BBr3
1: 50:1
20
1
oligomers
12
TiCl4c
1:50:1
20
1
30
13
GaCl3
1:50:1
20
1
no reaction
20
1
time,
h
FeCl3a
1:1
20
1
62 / 1:1
2
FeCl3a
1:0.5
20
1
60 / 1.2:1
3
BF3×Et2Oa
1:1
20
48
45 / 1.2:1
4
TiCl4a,b
1:1
20
1
11 / 1:2.7
5
CF3SO3H
1:50
-35
1
30b не надо /
1.1:1
6
H2SO4
1:268
20
1
19 / 1.4:1
1
3a+a' (%)
/a:a' ratio
-
5
1:50:1
T,
°C
Acid
22
4
InCl3
1a:acid
ratio
entry
3a+a'
1
14
Table 3. Transformation of 1a into 3a,a'
3a' CF3
Reaction conditions
Acid
H Ph
H
Ph
H
Ph
OH
22). Next, the reaction was optimized for the synthesis of indanes 3a,a' (Table 3). For this purpose alcohol 1a was treated
with some acids without benzene as nucleophile. Again the
best results were obtained for FeCl3 (Entries 1-2). It is enough
to use 0.5 equivalent of FeCl3 to conduct the reaction (Entry
2), because the hydroxyl group of the second molecule 1a is
unaffected. Other catalysts used (Entries 3-6, Table 3) were
found less effective. In most cases the ratio of isomers 3a:3a'
is roughly 1:1, except the reaction with TiCl4 (Entry 4).
38
44
СН2Cl2
aCo-solvent
bE-4a
obtained as a major product in 51%
yield.
Scheme 2. Dimerization of alcohols 1b,c
OH
CF3
R
1b,c
FeCl3(0.5 eq.), CH2Cl2
H
R
CuBr2
1:50:1
20
1
no reaction
16
SnCl4
1:50:1
20
1
no reaction
17
ZnCl2
1:50:1
20
1
no reaction
18
ZnBr2
1:50:1
20
1
no reaction
19
Sc(OTf)3
1:50:1
20
1
no reaction
20
In(OTf)3
1:50:1
20
1
no reaction
21
Fe(OTf)2
1:50:1
20
1
no reaction
22
Cu(OTf)2
1:50:1
20
1
no reaction
aCo-solvent СН Cl . bCo-solvent SO . cThe main reaction
2 2
2
product (yield 45%) is the compound E-4a PhCH=CHCH(Cl)CF3
The formed dimer is a result of domino reaction of two
molecules of 1a to form two new C-C bonds. Among the Lewis acids (Entries 6-22) the highest yield of the compound 2a
was achieved for anhydrous FeCl3. In this case alcohol 1a
gave E-2a as the only reaction product with excess of benzene
(Entry 6). Reaction of equimolar amounts of 1a and benzene
afforded preferentially 3a,a' (Entry 7). Other Lewis acids tested were found less effective to give lower yields of 2a. Moreover, in most cases reaction did not taken place (Entries 13-
H
H
no reaction
15
R
R
20 °C, 1 h
H
3b,c
OH
CF3
R
H
+
H
H
b R=Me, 84% (1:1.2)
CF3 c R=Cl, 90% (1:5)
OH
H
CF3
3b',c'
CF3
Thus, the anhydrous FeCl3 was found to be very effective
Lewis acid to achieve the transformations of CF3-substituted
allyl alcohols 1 to the corresponding dimers 3 at rt for just 1
hour. It is worth mentioning that these reactions need at least
50 mole % of FeCl3, rather than catalytic one. The use of
smaller charges of FeCl3 (1-10%) decreases the yields dramatically. Having found optimized conditions we synthesized
dimeric indanes 3 from other allyl alcohols 1 (Scheme 2).
To determine the stereochemical structures of the isomers
3a-c, and 3a'-c' we have separated them by preparative TLC.
Unfortunately these compounds were appeared to be oils, unsuitable for X-ray analysis. First proof for the structures of the
compounds 3 has been obtained by NOESY experiments. The
observed correlations clearly confirmed the stereochemical
structure of the compounds 3a-c, 3a'-c' (Fig. 1). To have final
proof, the oily indanes 3a,b, and 3a',b' were converted into
the solid para-nitrobenzoates 5a,b and 5a',b' under very mild
conditions (Scheme 3). The X-ray structures of the compounds
2
5b, and 5b' confirmed NOESY data and unambiguously indicated the stereochemistry of 3a,b and 3a',b' (Fig. 2).
H
R
Ar
H
CF3
H
H
H
3a-c
H
R
OH
Ar
H
OH
H
CF3
H
H
H
H
CF3
3a'-c'
CF3
cationic center C4 attacks the double bond of the starting alcohol 1 to form benzyl cation, which reacts intramolecularly
with arene fragment to cyclize finally to indane. Two new C-C
bond and 4 stereocenters are formed and the reaction is rather
stereoselective because only two diastereomers are formed out
of 8 possible. It is worth noting that configuration at the atom
C4 is the same for all the diastereomers 3a-c, and 3a'-c'. It
reveals that interaction between the cation B and initial 1 proceeds in stereoselective way. We found that 50% of FeCl 3 do
catalyze the dimerization, therefore it is reasonable to propose
that coordination on the metal play important role for this process.
Scheme 4. Possible mechanism of dimerization
OH
Figure 1. Selected NOESY correlations for 3a-c, and 3a'-c', (blue
- H-H correlations, green - H-F correlations)
Ar
1
FeCl3
Ar 4'
CF3 - FeCl (OH)3
B
CF3
FeCl3
CF3
Ar
Scheme 3. Derivatization of indanes 3a,b, and 3a',b'
H
R
R
NO2
H
H
R = H (a)
Me (b)
OH
CF3
H
Cl
H O
H
CF3
CF3
R
Ar
H
5a (72%), b (88%)
NEt3, CH2Cl2
20 °C, 24 h
3a,b
CF3 3a',b'
H
NO2
Ar
H
O
O
CF3
CF3 D2
-H+
3a-c
H Ar
R
OX
CF3
H
H
CF3
H
-H+
H
H
Ar
H OX
R
CF3
D1
X= H or FeCl3OH
O
CF3
1
OX
H
CF3
FeCl3
O
O
H
O
Ar
Ar
H
Ar
H
R
H
OH
Ar
H
OX
H
CF3
H
H
CF3
3a'-c'
CF3
H
5a' (86%), b' (95%)
NO2
Then we have carried out FeCl3 promoted reactions of the
alcohol 1a with selected arenes, leading to alkenes 2b-f in the
yields 48-75% (Table 4). This reaction is 100% diastereoselective to form E-isomers 2b,c,f, only (configuration was confirmed by NOESY). In the case of anisole mixture of ortho
and para isomers are formed in 72% total yield. Both indanes
and trifluoromethylated alkenes have a great practical value.
Thus, Panomifene, bearing trifluoromethylated alkene moiety,
is the well-known antiestrogenic drug in the therapy of breast
cancer and for the treatment of menstrual disorders.15
Table 4. FeCl3 promoted reactions of 1a with arenes
OH
Ph
CF3
+
Figure 2. X-Ray structures of 5b and 5b'
The mechanism of the reaction has also been contemplated.
The most probable way of the compounds 3a-c, 3a’-c’ formation is shown in Scheme 4. At the first stage of the reaction
cationic intermediate is formed by action of acid. Then the
Ph
20 °C, 1h
1a
entry
Ar
FeCl3(1eq.), CH2Cl2
ArH
CF3
E-2b-f
ArH
Product
Ar
Yield, %
1
o-xylene
2b
3,4-diMeC6H3
75
2
m-xylene
2c
2,4-diMeC6H3
56
3
anisol
2d+2ea
MeOC6H4
72
4
veratrol
2f
3,4-diMeOC6H3
48
a Two regioisomers 2d (2-MeOC H , 14%) and 2e (46 4
MeOC6H4, 58%).
3
In conclusion, we have found that the CF3-substituted allyl
alcohols undergo electrophilic “dimerization” under the action
of anhydrous iron trichloride leading to indane derivatives.
FeCl3 promoted reactions of the CF3-substituted allyl alcohols
with arenes give 100% stereoselectively the corresponding
CF3-alkenes as E-isomers. This is the first example of the
paticipation of CF3-allyl cations in new carbon-carbon bond
forming reactions. Other reactions of CF3-substituted allyl
alcohols are under the current investigation.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and spectral data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* E-mail: [email protected]
* E-mail: [email protected]
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by SPBU (grants no. 12.50.1558.2013,
and no. 12.38.195.2014) and RSCF (grant no 10 14-13-00083).
NMR, HRMS, and X-ray studies were performed at Center for
Magnetic Resonance, Center for Chemical Analysis and Materials
Research, and Research Center for X-ray Diffraction Studies of
Saint Petersburg State University.
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5