Organic base-catalysed solvent-tuned chemoselective carbotrifluoromethylation and oxytrifluoromethylation of unactivated alkenes

Article abstract of DOI:10.1039/c6cc00364h

An unprecedented and efficient organic base-catalysed highly chemoselective carbo- and oxytrifluoromethylation of unactivated alkenes with Togni's reagent was developed. The switchable chemoselectivity was tuned by simply changing the organic base catalyst and solvent. Mechanistic studies indicated that a radical cyclization pathway for carbotrifluoromethylation in DMSO and a carbocation pathway for oxytrifluoromethylation in DCE were probably involved.

Full text of DOI:10.1039/c6cc00364h

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Organic base-catalysed solvent-tuned chemoselective carbotrifluoro-
methylation and oxytrifluoromethylation of unactivated alkenes
Ning-Yuan Yang,‡ Zhong-Liang Li,‡ Liu Ye, Bin Tan and Xin-Yuan Liu*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An unprecedented and efficient organic base-catalysed highly directly with the radical partner, followed by an SET oxidation
chemoselective carbo- and oxytrifluoromethylation of unactivated (Scheme 1b);7b, 7f (3) It formed an alkylcopper(III) species with the
alkenes with Togni’s reagent was developed. The switchable corresponding nucleophiles, followed by a reductive elimination
chemoselectivity was tuned by simply changing the organic base (Scheme 1c).8d In spite of the blooming development of
catalyst and solvent. Mechanistic studies indicated that radical methodology in trifluoromethylation of alkenes, the reaction
cyclization pathway for carbotrifluoromethylaton in DMSO and mechanism is still not well-documented. Therefore, the detailed
carbocation pathway for oxytrifluoromethylation in DCE were mechanistic study for radical trifluoromethylation-triggered
probably invovled.
difunctionalization of alkenes is highly urgent and will provide
instructive insight for switching the reaction selectivity with
different nucleophiles via simply altering reaction conditions.
Controlling selectivity in radical reaction is a fundamental challenge
due to the intrinsic high reactivity and instability of radical
intermediate.1 Although the recently developed mild conditions
allow for the highly stereoselective accumulation of desired
product,1-2 there is still a demanding on development of general
strategy to tune the radical reaction selectivity. The strategy for
realization of reaction chemodivergence in a controlled version is a
long-standing goal and represents
a powerful and promising
synthetic tool in diversity-oriented synthetic chemistry since it can
enable the assembly of various molecules from identical reactants.
Thus, tuning the chemoselectivity of a reaction in a facial and
predictable manner attracts more and more attention.
Scheme 1 Proposed reaction mechanisms for radical trifluoromethylation of alkenes
Compared with the widely-developed transition-metal
catalysed radical trifluoromethylation of alkenes, metal-free
methods, which has a huge advantage due to its capacity of
tolerating many different coordinating functional groups that would
otherwise form a strong coordination with metal catalysis to
impede the reaction efficiency,10 remains widely unexplored.7l,8b
More recently, our research group has developed organic base-
initiated radical trifluoromethylation of alkenes to trigger remote C-
H bond functionalization of amide.13a,i In this context, the choice of
solvent always has a significant impact on reaction efficiency in
radical trifluoromethylation reactions.7-11 For example, DMSO, as a
polar solvent, was widely studied in radical reactions12 and
supposed to promote radical reaction presumably because the
radical intermediate formed a cluster with DMSO, thus stabilizing
the radical-solvent adduct via delocalization of radical to further
tune the redox potential of the oxidant.12b,c Considering the
possibility of control of radical selectivity through solvent-binding
effect12 and in our continued efforts to develop
trifluoromethylation of alkenes,13 we envisioned that substrates I
bearing both carbon reactive center and oxygen reactive center
Over the past decades, owing to the importance of
trifluoromethylated compounds in pharmaceutical and agriculture,3
installing CF3 group into organic molecules has been a hot topic.4 In
this context, the difunctionalization of unactivated alkenes,5 such as
aminotrifluoromethylation,6,13c,d carbotrifluoromethylation7 and
oxytrifluoromethylation,8 represents an appealing strategy to
introduce various functional groups into alkenes. With regard to
reaction mechanism, there are three possible pathways commonly
presented in the literature (Scheme 1). All of them were initiated by
the reaction of CF3 radical with alkene I to generate α-CF3 alkyl
radical II, followed by the below different pathways: (1) It was firstly
oxidized to carbocation III and subsequently trapped by
nucleophiles to afford the product V (Scheme 1a);9 (2) It coupled
Department of Chemistry, South University of Science and Technology of China,
Shenzhen, 518055, China. email: liuxy3@sustc.edu.cn
‡These authors contributed equally to this work.
† Electronic Supplementary Information (ESI) available: [1H NMR, 13C NMR, HRMS
(ESI) and NMR spectrum are included]. See DOI: 10.1039/x0xx00000x
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might chemoselectively undergo carbotrifluoromethylation via a recently
developed
organic
base-catalysed
radical
radical pathway or oxytrifluoromethylation via
a
carbocation trifluoromethylation of alkenes,13i we next focused on organic base-
pathway, respectively, by simply tuning the catalyst and solvent catalysed trifluoromethylation of alkenes. Several types of bases
system (Scheme 2). Herein, we disclose an unprecedented solvent- including secondary and tertiary amines with ethyl acetate (EtOAc)
tuned chemodivergent realization of carbo- and oxy- as the solvent at 80 °C were tested, while all of them favoured the
trifluoromethylation of unactivated alkenes catalysed by organic formation of 4a resulting from oxytrifluoromethylation, and the
base to afford trifluoromethylated tetralins and 4,5-dihydrofurans desired product 3a was obtained as the minor product (entries 4-7).
in
a predictable and mechanistic controlled manner. These To our surprise, DMSO, which is believed to stabilize the radical
structural motifs are promising components of various biologically intermediate well,12c reversed the product distribution completely,
active medicinal compounds.3 Also reported here are our implying that the choice of solvent can significantly control the
mechanistic
studies
of
such
organic
base-catalysed chemoselectivity of the reaction (entry 11). Other solvents, such as
trifluoromethylation of alkenes employing the control experiments, THF, dioxane and chloroform, proved no better than DMSO (entries
KIE study as well as the Hammett experiment.
8-10). Lowering the temperature to 50 °C enhanced the yield of 3a
to 79% and meanwhile 4a was obtained in only 1% yield
determined by 19F NMR spectroscopy (entry 12). Replacing 2a with
2b or reducing the catalytic amount of DBN did not give a better
result (entries 13-15). Finally we identified the optimal condition as
following: Reaction of 1a (1.0 equiv) and 2a (2.0 equiv) catalysed by
DBN (20 mol%) with DMSO as solvent for 10 h (entry 12) delivered
carbotrifluoromethylation product 3a in 79% NMR yield.
Scheme 2 Substrate design and chemodivergent transformation
Table 1 Screening of reaction conditions for carbotrifluoromethylation.a
Table 2 Substrate scope with carbotrifluoromethylation reaction.a, b
Entry
1
Cat. (X equiv)
CuI (0.1)
Solvent
Dioxane
T (°C)
80
Y(%) of 3a (4a)b
13 (69)
2
3c
4
n-Bu4NI (0.3)
PhI(OAc)2 (2.0)
Pyrrolidine (0.2)
DIPEA
CH3CN
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
THF
80
50
80
80
80
80
80
80
23(24)
trace
15 (61)
7 (70)
5
6
7
DMAP (0.2)
DBN (0.2)
20 (78)
28 (46)
29 (33)
17 (80)
8
DBN (0.2)
9
DBN (0.2)
Dioxane
a Reaction conditions: 1 (0.20 mmol), 2a (0.40 mmol), DBN (0.04 mmol), DMSO (2 mL)
at 50 °C for 10 h. b Isolated yield based on 1. c EtOAc as solvent. d at 80 °C. e at 100 °C.
10
11
DBN (0.2)
DBN (0.2)
DBN (0.2)
DBN (0.2)
DBN (0.15)
DBN (0.10)
CHCl3
DMSO
DMSO
DMSO
DMSO
DMSO
80
80
50
50
50
50
18 (40)
41(5)
12
79 (1)
28 (8)
78 (7)
58 (5)
With the optimized reaction conditions in hand, we next turned
our attention to investigate the generality of the protocol.
Substrates containing electron-withdrawing group or electron-
donating group on para position of phenyl ring underwent the
carbotrifluoromethylation smoothly to afford the desired products
3b-3f in moderate to good yields (52-72%). Notably, for
monosubstituted alkenes 1g-1o, the reaction showed good
compatibility. Substrates with several substituents on para position
of phenyl ring gave 3g-3l in good yields (70-73%). In addition to the
symmetric diketone substrates, reaction of the unsymmetric
substrates, as exemplified by 1,3-diketone substrate (1m) and β-
keto ester (1n), worked under the standard conditions albeit with a
lower yield (26% and 36%, respectively). The desired product can be
accessed from other types of substrates. For example, linear ketone
substrate 1o delivered the desired product in 34% yield and the
previous reported trifluoromethylated oxindole skeleton 3p13a can
also be generated from the corresponding substrate in 54% yield.
13d
14e
15f
a Unless otherwise noted, the reaction was conducted with 1a (0.1 mmol), Togni’s ester
2a (0.2 mmol), and base (0.02 mmol) in 1.0 mL solvent for 10 h. Determined by 19F
b
c
NMR spectroscopy using PhCF3 as an internal standard. TMSCF3 (4.0 equiv) as the
source of CF3, PhI(OAc)2 as the oxidant, KF (4.0 equiv) was added. d 2b (0.2 mmol) was
e
f
used and reaction time is 36 h. Reaction time is 24 h. Reaction time is 36 h. DIPEA:
diisopropylethylamine. DMAP: N,N-dimethylaniline. DBN: 1,5-Diazabicyclo[4.3.0]non-5-
ene.
To validate our hypothesis, substrate 1a bearing
diphenylketone group was selected as the model substrate with
Togni’s reagent 2a14 as the CF3 radical source to optimize the
reaction conditions for carbotrifluoromethylation of alkenes (Table
1). Several reported protocols, with Cu(I) salt7l or n-Bu4NI15 as the
catalysts to initiate 2a, or with PhI(OAc)2 as the oxidant to initiate
TMSCF313a were firstly screened. To our disappointment, the results
were not satisfactory and both carbotrifluoromethylation product
3a and oxytrifluoromethylation product 4a were obtained with poor
selectivity or yield (entries 1-3). Inspired by the success of our
We next focused on optimization of the reaction conditions for
oxytrifluoromethylation. As mentioned before, the best ratio of
4a/3a was observed in EtOAc with DIPEA as catalyst. Encouraged by
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this result, we further screened solvent effect, the ratio of 2a as
To further demonstrate the role of solvent in tuning the
well as the CF3 source, and finally identified the following protocol reaction pathway, the electronic effect of substituents on phenyl
as optimal: reaction of 1a and 2a with the molar ratio of 1.0:2.0 in ring of substrates was investigated under the standard reaction
the presence of DIPEA (20 mol%) with DCE as the solvent at 80 °C conditions (Figure 1). For DMSO-tuned carbotrifluoromethylation to
for 10 h, 4a was obtained in 85% yield (See Table S1 in SI).
Table 3 Substrate scope with oxytrifluoromethylation reaction.a, b
generate 3, substrates 1h, 1i and 1j with electron-withdrawing
group exhibited a much faster reaction rate than 1k and 1l with
electron-donating group, presenting a large positive σ value (2.73)
(Figure 1A).16 The result argued against the involvement of the
carbocation intermediate in carbotrifluoromethylation and thus
demonstrated that a radical cyclization pathway might be involved
in DMSO for carbotrifluoromethylation.16 In sharp contrast,
substrate 1b, 1c and 1d with electron-withdrawing group reacted
slightly slower than 1e and 1f with electron-donating group in DCE,
generating a negative σ value (-0.94) (Figure 1B), which implied that
an oxygen-centred nucleophilic attack to the carbocation center
might occur in the cyclization step and thus
a carbocation
intermediate might be involved before the cyclization step for
oxytrifluoromethylation.16b
a
Reaction conditions: 1 (0.20 mmol), Togni’s reagent 2a (0.40 mmol), DIPEA (0.04
b
c
mmol), DCE (1.0 mL) at 80 °C for 10 h. Isolated yield based on 1. Ratio of 4a/3a in
parenthesis.
To expand the application of this methodology, the substrate
scope was further examined employing the optimal conditions for
oxytrifluoromethylation (Table 3). Firstly, the electronic effect on
phenyl ring was investigated and the result showed that substrates
bearing both para-electron-withdrawing and electron-donating
groups afforded the 4,5-dihydrofurans 4b-4f in good yields (68-82%)
with excellent chemoselectivity. Notably, geminal-substituted
substrates 1q and 1r also generated products 4q and 4r with 75%
and 76% yields and excellent regioselectivity (>20:1). The phenyl
substituents (1s-1x) can also be tolerated to provide the desired
products in moderate to good yields (38-73%).
Figure 1 Hammett Study (A) using σm for carbotrifluoromethylation in DMSO; (B)
using σp for oxytrifluoromethylation in DCE.
Scheme 4 A proposed mechanism for solvent-tuned trifluoromethylation of alkenes
On the basis of the above control experiments and mechanistic
studies, a proposed mechanism for these reactions is proposed as
depicted in Scheme 4. The amine and Togni’s reagent 2a easily form
an EDA complex, which provides CF3 radical and amino radical
cation VI after electron transfer.13i, 17 The CF3 radical attacks alkene
1a to generate a nascent α-CF3-alkyl radical intermediate VI. With
DMSO as the solvent, intermediate VII could be stabilized through
solvation to elongate its lifetime,12c allowing the radical cyclization
on aryl ring to give rise to VIII, and the subsequent SET process from
2a and deprotonation step furnish the final product 3a.1d,e
However, in the presence of DCE, intermediate VII is readily
oxidized to cation IX by 2a, which is prone to be attacked by enol
rather than aryl ring to deliver the product 4a after deprotonation.
Another possibility is that the reductive potential of 2a differed in
different solvents. In DCE or other solvents it is high enough to
Scheme 3 Control and KIE experiments
To gain insight into the reaction mechanism, control
experiments were performed. Treatment of 1a and 2a under
standard conditions for carbo- and oxytrifluoromethylation was
completely inhibited upon addition of
2
equiv of 1,4-
dinitrobenzene, suggesting that a radical pathway is likely involved
under the current system (Scheme 3, eqs. 1 and 2). Furthermore,
the intermolecular KIE experiment was conducted, and the small
kH/kD (1.15:1) indicated that C-H bond cleavage was not involved in
the rate-determining step in carbotrifluoromethylation (eq 3).7b
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Miyazaki and M. Sodeoka, Angew. Chem., Int. Ed., 2013, 52
,
oxidize VII to IX, while in DMSO, it fails to oxidize VII and the radical
pathway is more favourable to afford 3a.12b On the other hand, it
cannot be ruled out that amino radical cation VI takes part in the
SET process instead of Togni’s reagent 2a in both pathways.
7841; (c) S. Kawamura, H. Egami and M. Sodeoka, J. Am.
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7
In summary, we have successfully developed the
unprecedented
chemodivergent
trifluoromethylation
transformations of alkenes concurrently bearing different reactive
sites under metal free protocol, which allowed for a rapid and
diverse accumulation of trifluoromethylated tetralins and 4,5-
dihydrofurans, simply by adjusting the catalyst and reaction solvent.
The detailed mechanistic studies in the selected solvent indicated
that a radical pathway in DMSO for carbotrifluoromethylation and a
carbocation pathway in DCE for oxytrifluoromethylation might be
involved. The high chemoselectivity renders this methodology a
powerful tool in radical transformations and is guidable for other
radical involved chemodivergent synthetic methodology.
8
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Acknowledgements
Financial support from the National Natural Science Foundation of China
(Nos. 215722096, 21302088), Shenzhen overseas high level talents
innovation plan of technical innovation project (KQCX20150331101823702),
Shenzhen special funds for the development of biomedicine, Internet, new
energy, and new material industries (JCYJ20150430160022517) and South
University of Science and Technology of China (FRG-SUSTC1501A-16) is
greatly appreciated.
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