Copper-Catalyzed Diastereoselective Synthesis of Trifluoromethylated Tetrahydrofurans

Article abstract of DOI:10.1002/adsc.201501166

The copper-catalyzed intramolecular diastereoselective trifluoromethylcycloetherification of homoallylic alcohols with Togni's reagent as trifluoromethylating reagent was realized under mild conditions. Various trifluoromethylated tetrahydrofurans were synthesized in moderate to good yields. Moreover, a wide range of common functional groups was tolerated.

Full text of DOI:10.1002/adsc.201501166

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DOI: 10.1002/adsc.201501166
Copper-Catalyzed Diastereoselective Synthesis of Trifluoromethy-
lated Tetrahydrofurans
Yanan Wang,a Min Jiang,a and Jin-Tao Liua,*
a
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Science, 345
Lingling Road, Shanghai 200032, Peopleꢀs Republic of China
Fax: (+86)-21-6416-6128; e-mail: jtliu@sioc.ac.cn (homepage: http://liujintao.sioc.ac.cn)
Received: December 25, 2015; Revised: February 18, 2016; Published online: March 31, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501166.
Abstract: The copper-catalyzed intramolecular dia- Moreover, a wide range of common functional
stereoselective trifluoromethylcycloetherification of groups was tolerated.
homoallylic alcohols with Togniꢀs reagent as
trifluoromethylating reagent was realized under mild Keywords: copper; homoallylic alcohols; Togniꢀs re-
conditions. Various trifluoromethylated tetrahydro- agent;
trifluoromethylated
tetrahydrofurans;
furans were synthesized in moderate to good yields. trifluoromethylcycloetherification
Introduction
ly studied. For example, the Buchwald[7] and Sode-
oka[8] groups have successfully developed the intramo-
Introducing a trifluoromethyl group into organic com- lecular oxytrifluoromethylation of alkenes and amino-
pounds can significantly improve their hydrophobicity, trifluoromethylation of alkenes, respectively. Liang[9]
metabolic activity, electronegativity and bioavailabil- has also reported a method for the trifluoromethyla-
ity. Thus, many molecules bearing trifluoromethyl tion of alkenes involving oximes. However, the intra-
groups, especially heterocycles, have been applied in molecular trifluorooxylation of styrenes has been
the field of agrochemicals and pharmaceuticals.[1] On rarely studied. As part of an ongoing program for the
the other hand, substituted tetrahydrofurans are key synthesis of trifluoromethylated blocks,[10] we herein
structural blocks in a large group of bioactive natural report
the
copper-catalyzed
diastereoselective
products.[2] Recently, several effective methods for the trifluoromethylcycloetherification of homoallylic alco-
syntheses of tetrahydrofurans have been reported, in- hols (Scheme 1).
cluding the transition metal- or Lewis acid-promoted
ones.[3] To date, only few reactions for trifluorome-
thylcycloetherification have been reported,[4] although Results and Discussion
the halocycloetherification reactions have made sig-
nificant progress.[5]
In our preliminary experiments, 4-phenylbut-3-en-1-ol
Nowadays, an immense number of efficient meth- (1a, E:Z=3:1) was chosen as the substrate. The
ods for the bifunctionalization of alkenes to synthe- model reaction was carried out with 1.0 equivalent of
size some valuable trifluoromethylated building Togniꢀs reagent I (2), 1.5 equivalents of 1a and
blocks have been developed.[6] The intramolecular tri- 20 mol% of CuTc in dichloromethane (DCM) at
fluoromethylcyclizations of alkenes using an oxygen 808C in a sealed tube. Gratifyingly, the corresponding
or nitrogen atom as nucleophile have been extensive- tetrahydrofuran 3a was obtained in 75% yield with
excellent diastereoselectivity (Table 1, entry 1). The
structure of the major product was established to
have the syn-configuration by NOE experiments. In-
spired by the result, we screened different conditions
to find a suitable protocol for the selective formation
of tetrahydrofurans. Firstly, we focused on the catalyst
effects. Copper is essential for the reaction as no reac-
tion was detected in the absence of copper salt
Scheme 1. Trifluoromethylcycloetherification of homoallylic
alcohols.
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Cu-Catalyzed Diastereoselective Synthesis of Trifluoromethylated Tetrahydrofurans
Table 1. Optimization of the reaction conditions.[a]
acetonitrile (CH3CN) could not improve the yield of
3a (entries 13–15). DCM was the best solvent for this
reaction among the solvents tested. Carrying out the
reaction in DCM at 608C gave syn-isomer 3a in 62%
yield (entry 16). When 2.0 equiv. of 1a were used, 3a
was still obtained in 77% yield. Using 1.0 equiv. of 1a
only gave a 56% yield of 3a (entries 17 and 18). At-
tempts to use phenanthroline or triphenylphosphine
(PPh3) as additive did not further improve the yield
(entries 19 and 20). The reaction did not occur in the
presence of 1.0 equiv. of Cs2CO3 (entry 21). Under
the optimal conditions, the reaction time could be
shortened to 3 h (entry 22).
With the optimized reaction conditions in hand, we
studied the scope of homoallylic alcohols for this
copper-catalyzed trifluoromethylcycloetherification.
As shown in Table 2, a range of homoallylic alcohols
1 including aromatic alcohols and aliphatic alcohols
could react with Togniꢀs reagent 2 to give products 3
with excellent diastereoselectivities. Both electron-
rich and electron-deficient alcohols are compatible
with the reaction conditions. The substrates bearing
methoxy, methyl or aryl substituents gave the expect-
ed products 3b–3f in moderate to good yields. Sub-
strates bearing functional groups, such as amine and
hydroxy, were also tolerated in this cyclization reac-
tion to give the corresponding products 3g–3i in excel-
lent yields. Halide-substituted substrates also reacted
well to give the cyclic products 3j–3l in moderate
yields. However, the reaction of 3,4,5-trifluorophenyl-
substituted homoallylic alcohol afforded only 47%
yield of 3m, probablely due to the electron-withdraw-
ing effect of the fluorine atoms. The substrates con-
taining an electron-deficient substituent such as tri-
fluoromethyl, nitro or cyano afforded the correspond-
ing cyclic products 3n–3p in moderate yields. Homoal-
lylic alcohols containing other substituents, such as
naphthyl and heteroaromatic rings, could also react
with 2 to give the corresponding products 3q and 3r,
albeit the lower yield. When an alkyl substituent was
used, the reactions could also occur to give the corre-
sponding cyclic products 3s–3u, but the yields were
somewhat lower and the diastereoselectivity was
Entry Catalyst
Solvent Additive Yield [%, syn:anti][b]
1
2
3
4
5
6
7
CuTc
–
CuCl
CuCN
CuCl2
DCM
DCM
DCM
DCM
DCM
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
75, >95:5
NR[c]
51, >95:5
71, >95:5
48, >95:5
66, >95:5
37, >95:5
77, >95:5
50, >95:5
49, >95:5
34, >95:5
54, >95:5
61,>95:5
61, >95:5
71, >95:5
62, >95:5
77,>95:5
56, >95:5
51, >95:5
67, >95:5
Cu(OAc)2 DCM
Cu(OTf)2 DCM
8[d]
9[e]
10[d]
11[d]
12[d]
13[d]
14[d]
15[d]
CuTc
CuTc
CuTc
CuTc
CuTc
CuTc
CuTc
CuTc
DCM
DCM
MeOH
DMA
DMSO
DCE
acetone
CH3CN
DCM
DCM
DCM
DCM
DCM
DCM
DCM
16[d,f] CuTc
17[d,g] CuTc
18[d,h] CuTc
–
–
19[d]
20[d]
CuTc
CuTc
Phen
PPh3
21[d,i] CuTc
Cs2CO3 NR[c]
22[j]
CuTc
–
75, >95:5
[a]
Reaction conditions: 1a (0.15 mmol),
2
(0.10 mmol),
copper catalyst (0.02 mmol), solvent (1.0 mL), 808C, 8 h,
under N2.
[b]
Determined by 19F NMR spectroscopy using hexafluoro-
benzene as internal standard.
NR=no reaction.
10 mol% of CuTc was used.
5 mol% of CuTc was used.
The temperature was 608C.
2.0 equiv. of 1a were used.
1.0 equiv. of 1a was used.
[c]
[d]
[e]
[f]
[g]
[h]
[i]
1.0 equiv. of Cs2CO3 was used.
The reaction time was 3 h.
[j]
(entry 2). However, other copper catalysts, such as poor. It is worth mentioning that the reaction of 1v
CuCl, CuCN, CuCl2, Cu(OAc)2 and Cu(OTf)2, afford- containing two phenyl groups took place successfully
ed low yields of 3a, albeit with high selectivity for the to give the expected product 3v in good yield (75%).
syn-isomer (entries 3–7). Further investigation To further explore the scope of this reaction, we also
showed that the catalyst loading could be lowered tested the reaction of other related compounds. Un-
down to 10 mol% without any erosion in yield and se- fortunately, the reaction of (E:Z)-5-phenylpent-4-en-
lectivity (entry 8), whereas a lower yield was obtained 1-ol (1w) afforded the desired product 3w in less than
with 5 mol% of CuTc (entry 9). Then the solvent 5% yield with the recovery of an amount of starting
effect was studied using 10 mol% of CuTc as the cata- material 1w. In the case of (E:Z)-6-phenylhex-5-en-1-
lyst. Various solvents were screened to examine the ol (1x), the reaction did not occur at all. The reasons
feasibility of the reaction. It was found that lower are not clear at present. However, the expected prod-
yields were obtained with methanol (MeOH), dime- uct 3y could be obtained in 50% yield from the reac-
thylacetamide (DMA) or dimethyl sulfoxide (DMSO) tion of amine substrate 1y with excellent diastereose-
(entries 10–12). Dichloroethane (DCE), acetone and lectivity.
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Yanan Wang et al.
Table 2. Scope of the trifluoromethylcycloetherification.[a]
[a]
Reaction conditions: 1 (0.15 mmol), 2a (0.1 mmol), CuTc (0.01 mmol), DCM (1.0 mL), 808C, 3 h, under N2, isolated yield
for syn-isomer (total yields for products 3 determined by 19F NMR spectroscopy using hexafluorobenzene as internal
standard in the parentheses).
Reaction time was 6 h.
Catalyst loading was 20 mol% and reaction time was 20 h.
Reaction time was 12 h.
[b]
[c]
[d]
Furthermore, the cyclization of E-3-phenylprop-2- presence of 1.5 equiv. of LiCl using acetone as sol-
en-1-ol (1z) was investigated under the standard con- vent, 19F NMR analysis showed that the trifluorome-
ditions. To our surprise, no expected cyclic product thylcycloetherification reaction was inhibited, and the
was obtained. Only 14% of E-4 was observed and chloride trapping product 5 was obtained in 31%
most of the 1z was recovered. However, the cycliza- yield (Scheme 3).
tion of terminal alkene 1aa worked well to give prod-
uct 3aa in 69% isolated yield (Scheme 2).
Next, the reaction of Z-1a and CuTc in the absence
of 2 in DCM was conducted, and E-1a was not detect-
In order to gain insight into the reaction mecha- ed in the reaction system, indicating that the transfor-
nism, we performed the reaction of 1a and 2 in the mation of Z-1a to E-1a was not involved in this reac-
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Cu-Catalyzed Diastereoselective Synthesis of Trifluoromethylated Tetrahydrofurans
carbon-carbon double bond in homoallylic alcohols
1 to produce the corresponding intermediates B-1 and
B-2, and regeneration of the copper(I) species at the
same time. The transformation between B-2 and B-
À
1 is feasible via the rotation of a C C single bond.
Then intramolecular cyclization of intermediates B-
1 and B-2 takes place via transition states C-1 and C-
2 to give syn-3 and anti-3, respectively. According to
the experimental results, the cyclization of intermedi-
ate B-1 with the syn-configuration is more favored
and syn-products were formed predominantly, proba-
bly because the di-pseudo-equatorial transition state
C-1 is more stable than C-2.[11] When R is an aryl
group, the cations B are more stable and easy to form
due to the stabilization effect of the aromatic ring.
Therefore, aromatic alcohols gave better results in
terms of yield and diastereoselectivity.
Scheme 2. Reactions of E-1z and 1aa under the standard
conditions.
Conclusions
In summary, we have developed a facile method for
the diastereoselective trifluoromethylcycloetherifica-
tion of homoallylic alcohols. The reaction tolerates
a wide range of functional groups and provides an ef-
ficient protocol for the synthesis of various trifluoro-
methylated tetrahydrofuran derivatives that may be
used as important intermediates in organic synthesis.
The plausible mechanisms are also proposed on the
basis of experimental results. Further research on the
Scheme 3. Reaction of 1a and 2 in the presence of LiCl.
tion. When the reaction of Z-1a and 2 in the presence application of this reaction is currently underway in
of CuTc was quenched after 20 min, only Z-1a and 3a our laboratory.
1
were observed by H NMR analysis.
Based on the above experiments, a plausible mech-
anism is proposed as outlined in Scheme 4. Initially,
the interaction of the Togni I reagent and Cu catalyst
gives the iodine(III) species A, which reacts with
Experimental Section
Typical Procedure for the Synthesis of Homoallylic
Alcohols
Under an argon atmosphere, a solution of triphenylphos-
phine (9.5 g, 36.2 mmol) and 3-bromopropan-1-ol (5.0 g,
36.0 mmol) in p-xylene (30 mL) was stirred under reflux for
6 h. Then the reaction mixture was cooled to room tempera-
ture and ether (50 mL) was added. The solids were collected
by filtration and dried under vacuum to afford 3-(triphenyl-
phosphonium)propan-1-ol bromide as a white solid which
was used in the next step without further purification.
Then 5.25 mmol (876.8 mg) of lithium bis(trimethylsilyl)-
amide were added dropwise to a suspension of 2.25 mmol
(900.0 mg) of (3-propan-1-ol)triphenylphosphonium bro-
mide in 5 mL of tetrahydrofuran at À208C. The solution
was stirred at À208C for 1 hour and 1.88 mmol of aldehyde
were added dropwise. After 2 h, the mixture was warmed to
room temperature and stirred for another 12 h, and then sa-
turated aqueous NH4Cl solution was added. The organic
layer was dried over MgSO4 and concentrated under re-
Scheme 4. Proposed mechanism for the trifluoromethylcy-
duced pressure. The residue was purified by flash column
cloetherification.
chromatography (EtOAc/Petroleum ether=1/5).
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General Procedure for the Trifluoromethylcyclo-
etherification Reaction
To a reactor charged with Togni I (0.1 mmol, 33 mg), CuTc
(0.01 mmol, 1.9 mg), and dichloromethane (1.0 mL) was
added the homoallylic alcohol (0.15 mmol) under an N2 at-
mosphere. The mixture was stirred at 808C. After the com-
pletion of the reaction, the solvent was removed under re-
duced pressure and the residue was purified by column
chromatography to give product 3 (dichloromethane/petro-
leum ether).
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (No. 21572257, 21421002 and 21502213) is
gratefully acknowledged.
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