Copper-catalyzed trifluoromethylation of organic zinc reagents with an electrophilic trifluoromethylating reagent

Article abstract of DOI:10.1039/c5ra02279g

A copper-catalyzed trifluoromethylation of aryl, vinyl and alkyl zinc reagents with Togni's reagent was described. Mechanistic studies indicated that the aryl group is initially transferred from the zinc reagent to hypervalent iodine to form a tri-substituted hypervalent iodine intermediate. Consequent reductive-elimination via a concerted bond-forming step and/or radical pathway from this intermediate generates the trifluoromethylated arenes.

Full text of DOI:10.1039/c5ra02279g

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Copper-catalyzed trifluoromethylation of organic
zinc reagents with an electrophilic
trifluoromethylating reagent†
Cite this: RSC Adv., 2015, 5, 24783
a
b
a
Received 5th February 2015
Accepted 2nd March 2015
*
Chang-Sheng Wang, Haoyang Wang and Cheng Yao
DOI: 10.1039/c5ra02279g
www.rsc.org/advances
A copper-catalyzed trifluoromethylation of aryl, vinyl and alkyl zinc since zinc reagent has been demonstrated to be an effective
reagents with Togni's reagent was described. Mechanistic studies coupling partner in a number of transition metal-catalyzed
indicated that the aryl group is initially transferred from the zinc cross-coupling reactions.6 Herein, we disclose
a
copper-
reagent to hypervalent iodine to form a tri-substituted hypervalent catalyzed triuoromethylation of aryl, vinyl and alkyl zinc
iodine intermediate. Consequent reductive-elimination via reagents using Togni's reagent 2 as the triuoromethyl source.
concerted bond-forming step and/or radical pathway from this Mechanistic studies indicated that the aryl group from the zinc
a
intermediate generates the trifluoromethylated arenes.
reagent is initially transferred to hypervalent iodine to form a
tri-substituted hypervalent iodine intermediate. Consequent
reductive-elimination via a concerted bond-forming step and/or
triuoromethyl radical pathway from this intermediate gener-
ates the triuoromethylated arenes.
We began our study with the reaction of 4-biphenyl zinc
reagent 3 with Togni's reagent 1 (ref. 7 and 8) in the presence of
a catalytic amount of CuI/1,10-phenanthroline (Phen) in
different solvents.
It is widely recognized and accepted that the introduction of
uorine into small molecules will bring signicant benecial
effects such as improved pharmacokinetics, increased binding
selectivity and enhanced metabolic stability.1 Thus, incorporation
of uorine into drug-like molecules has become an indispensible
strategy for medicinal chemists in drug design and a daily routine
practice for the discovery of new lead compounds.2 Among many
uorinated functional groups, the triuoromethyl group (CF3–) is
highly valuable because it might improve the drug's cellular
membrane permeability, and thus its bioavailability.3 As a result,
in the past ve years, tremendous efforts have been directed
towards the development of efficient methods for the formation
of C–CF3 bonds. Consequently, signicant advances have been
achieved in transition metal-catalyzed direct triuoromethylation
of aryl halides, aryl boronic acids or arenes.4,5
In 2011, Shen's group reported a copper-catalyzed tri-
uoromethylation of aryl boronic acids using Togni's reagent 1
as the triuoromethyl source.4 The reaction was conducted
under mild conditions and was compatible with a variety of
functional groups. Inspired by this investigation, we envisaged
that an efficient triuoromethylation method would be devel-
oped if a zinc reagent could be employed as the nucleophile
To our surprise, the formation of triuoromethylated
biphenyl was observed in less than 10% yield in solvents such as
THF, diethyl ether, diglyme, dioxane, 1,2-dichloroethane or
DMF (Table 1, entries 1–6). Interestingly, when Togni's reagent
2 was used, reaction of 4-biphenyl zinc reagent 3 occurred
smoothly at room temperature to give the desired product in
13% yield aer 12 h at room temperature (Table 1, entry 7). The
yield was improved to 42% when the reaction was conducted in
the absence of 1,10-phenanthroline (Table 1, entry 8). The effect
of the copper salts was then investigated. Nevertheless, the yield
of the desired product did not increased signicantly when
other copper salts such as CuCN, CuCl, CuBr, Cu(MeCN)4PF6 or
Cu(OAc)2 were used (Table 1, entries 9–13). Since it was
observed that side products such as triuoromethane and
iodotriuoromethane were formed in the reaction mixture, it is
likely only part of the Togni's reagent was converted to the nal
product. We then studied the effect of the amount of the Togni's
reagent. The yield was improved to 50% and 61%, respectively,
when 2.0 or 3.0 equivalents of reagent 2 was used (Table 1,
entries 14–15). Furthermore, when 30 mol% of CuI was used,
the yield was further increased to 90% (Table 1, entry 17).
Carefully monitoring of the reaction showed that the reaction
was completed within 20 min at room temperature (Table 1,
aCollege of Science, Nanjing Tech University, 30 Puzhu South Road, Nanjing 211816,
China. E-mail: yaochengnjut@126.com; yaocheng@njtech.edu.cn
bShanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai 200032, China
† Electronic supplementary information (ESI) available. CCDC 1047558. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra02279g
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Table
1
Optimization of conditions for copper-catalyzed tri- Table
2
Scope of copper-catalyzed trifluoromethylation of zinc
fluoromethylation of aryl zinc reagenta
reagents with Togni's reagenta,b
Entry CF3 reagent CuX
Ligand Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Cul
Cul
Cul
Cul
Cul
Cul
Cul
Cul
Phen
Phen
Phen
Phen
Phen
Phen
Phen
—
—
—
—
—
—
—
—
—
—
—
—
—
THF
Et2O
Diglyme
1,4-Dioxane
ClCH2CH2Cl
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
—
—
—
—
—
—
13
42
42
24
36
35
9
CuCN
CuCl
CuBr
Cu(MeCN)4PF6
Cu(OAc)2
Cul
Cul
Cul
Cul
Cul
10
11
12
13
14
15
16
17
18
19
20
—
50c
61d
72e
90f
90g (64)
90h
65i
Cul
Cul
DMF
DMF
a
a
Reaction conditions: 4-biphenyl zinc reagent (0.1 mmol), Togni's
reagent (0.1 mmol), CuX (10 mol%), ligand (20 mol%), solvent
Reaction conditions: zinc reagent (0.5 mmol), Togni's reagent
(1.5 mmol), CuI (30 mol%), DMF (5.0 mL) at room temperature for
b
b
c
20 min. Isolated yields. Yields were determined by 19F NMR
analysis of the crude product using p-uorotoluene as an internal
standard.
(1.0 mL) at room temperature for 12 h. Yields were determined by
19F NMR analysis of the crude product using p-uorotoluene as an
c
internal standard.
2.0 equivalents of reagent
2
was used.
d
e
3.0 equivalents of reagent 2 was used. 3.0 equivalents of reagent 2
f
and 20 mol% of CuI were used. 3.0 equivalents of reagent 2 and
g
30 mol% of CuI were used. Reaction was conducted for 20 min;
h
i
isolated in parentheses. Reaction was conducted at 0 ꢀC. Reaction
Typically, reaction of Togni's reagent 2 was proposed to
proceed via a pathway involving an in situ generated CF3 radical
species.8 To probe if the copper-catalyzed triuoromethylation
of zinc reagents proceeds via a radical mechanism, we con-
ducted two sets of different experiments. In the rst experi-
ment, 1.5 equivalents of TEMPO, a radical trap, was added to
the reaction mixture of 4-biphenyl zinc reagent with Togni's
reagent 2 in the presence of 30 mol% CuI. The reaction was
monitored by 19F NMR spectroscopy and it showed the forma-
tion of triuoromethylated TEMPO in 80% yield and the tri-
uoromethylated biphenyl in 16% yield, respectively (eqn (1)).
This result indicated that the copper-catalyzed tri-
uoromethylation reaction was only partially inhibited by
TEMPO. In the second experiment, we synthesized a zinc
reagent 5 to probe if the radical is indeed involved under the
was conducted at 40 ꢀC.
entry 18). Higher temperature was detrimental to the reaction.
The yield dropped to 65% when the reaction was conducted at
40 C (Table 1, entry 20).
ꢀ
With a relied procedure in hand, we then examined the
scope of the copper-catalyzed triuoromethylation of the zinc
reagents, and the results were summarized in Table 2. In
general, reactions of both electron-rich and electron-poor aryl
zinc reagents occurred in good yields (Table 2, 4a–f). Heteroaryl
zinc reagents such as pyridyl and quinolinyl zinc reagents also
reacted under the optimized conditions to give the corre-
sponding triuoromethylated heteroarenes in good yields
(Table 2, 4g–i). Likewise, reaction of vinyl zinc reagent 3j also
afforded the triuoromethylated alkene in 57% yield (Table 2
and 4j). Similarly, alkyl zinc reagents could also be tri-
uoromethylated under the optimized reaction conditions
(Table 1, entry 4k–m). One advantage of using zinc reagents is
its tolerance of functional groups. As illustrated in Table 2,
various functional groups such as ester, cyano, protected ketone
and amide were compatible.
standard
reaction
conditions.
Both
directed
tri-
uoromethylated product 6a and cyclized triuoromethylated
products 6b were observed aer 20 min at room temperature in
36% and 29% yields, respectively (eqn (2)). These results
suggest that the copper-catalyzed triuoromethylation of aryl
zinc reagent might proceed via two parallel mechanisms: one
involved a radical pathway and the other involved a non-radical
pathway.
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the DMF solution of the reaction mixture was treated with 1.0
equivalent of 4-biphenyl zinc reagent. Aer 20 min, the peak at
ꢁ30.5 ppm disappeared and a new peak at ꢁ62.6 ppm which
corresponds to the triuoromethylated biphenyl formed in 47%
yield, as determined by 19F NMR spectroscopy (Fig. 1).
We speculated that the unknown species at ꢁ30.5 ppm
might be [CF3Cu]. To probe if this is the case, we independently
prepared [CF3Cu] by a method reported by Grushin.9 Interest-
ingly, stoichiometric reactions of independently prepared
[CF3Cu] with 1.0 equivalent of 4-biphenyl zinc reagent 3 in DMF
in the presence of oxidants such as air, Ag2CO3, K2S2O8 or AgOTf
did not generate the coupled product. When PhI(OAc)2 was
used as the oxidant, the triuoromethylated biphenyl was
formed in 20% yield. These experiments suggest that the
unknown species at ꢁ30.5 ppm might not be [CF3Cu].
(1)
(2)
To gain more insight about the mechanism of the reaction,
we conducted a stoichiometric reaction of CuI with Togni's
reagent 2 (eqn (3)). A clear blue solution was formed within
1.0 min at room temperature upon mixing 19 mg of CuI with
3.0 equivalents of Togni's reagent 2 in 1.0 mL of DMF. The
reaction was monitored by 19F NMR spectroscopy and it showed
the formation of CF3I (resonated at d ¼ ꢁ10.5 ppm in 19F NMR),
CF3H (resonated at d ¼ ꢁ79.5 ppm in 19F NMR), unreacted
reagent 2 and an unknown species 7 (resonated at d ¼
ꢁ30.5 ppm in 19F NMR).
Togni reported that reaction of Togni's reagent 2 with
Zn(NTf)2 resulted in the formation of [Zn(2)2(NTf)2]+, which
showed a shi of the CF3 signal of the hypervalent iodine
species from ꢁ33.0 ppm to ꢁ26.9 ppm in the 19F NMR spec-
troscopy. We think that a similar complex might be formed
when Togni's reagent 2 was reacted with CuI in DMF. To gure
out structural information of this transient reactive species at
ꢁ30.5 ppm, we carefully monitored the diluted reaction mixture
of CuI with Togni's reagent 2 in DMF by electrospray ionization
tandem Mass spectroscopy (ESI-MS). The typical isotopic
distributions of the ion species containing copper helped us the
screen the reactive Cu-complexes from the ESI-MS spectra at
different time.10 The ESI-MS experimental results revealed that
aer adding CuI, Togni's reagent 2 to DMF solution, the major
peak in the ESI-MS spectrum was the signal of Cu(II) with
2-iodobenzoic acid, which remained inert through the ESI-MS
experiments. Interestingly, a new signal with a m/z ¼ 451.6,
which was assigned as the signal of [2$Cu(I)$DMF]+, appeared
two minutes aer addition of CuI, Togni's reagent 2 to DMF
solution. This peak slowly increased and nally disappeared
aer nearly 2.5 h (Fig. 2). These results indicated that this
species might be the active transient species that was observed
in the 19F NMR spectroscopy, which is consistent with Togni's
observation for the formation of [Zn(2)2(NTf)2]+.
(3)
Addition of 10 mL of diethyl ether precipitated a blue solid
which was characterized to be bimetallic copper(II) species 8, as
determined by X-ray diffraction of its single crystals. This is
consistent with previously reported observation by Xi and
coworkers.8 Aer the diethyl ether was removed under vacuum,
Based on these preliminary results, we propose the reaction
mechanism for copper-catalyzed triuoromethylation of
organic zinc reagent with Togni's reagent 2, as outlined in
Fig. 3. Reaction of CuI with Togni's reagent 2 formed an inter-
mediate 7 and bimetallic copper(II) species 8. Transmetallation
of the aryl group from organic zinc reagent to intermediate 7
generated intermediate 9. Directed reductive-elimination from
this hypervalent iodine species affords the triuoromethylated
arene (pathway A). Alternatively, intermediate 9 may undergo a
homolytic C–I bond scission to form a CF3c, which will recom-
bine with aryl group to form triuoromethylated arene (pathway
B) or a hemolytic C–I bond scission to form a Rc, which then
recombine with the triuoromethyl group to form the nal
product (pathway B0). However, the detailed mechanism for the
formation of the nal product is more complicated and more
experiments are required to elucidate this process.
Fig. 1 19F NMR spectroscopy for reaction of CuI with 3.0 equivalents
of Togni's reagent 2 in DMF at room temperature for 5 min; 19F NMR
spectroscopy for reaction of the DMF solution of above solution after
removing the copper(II) salts with aryl zinc reagent 2 after 20 min at
room temperature.
In summary, a copper-catalyzed efficient triuoromethylation
of organic zinc reagents is developed. The reaction conditions
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Fig. 2 The expanded ESI-MS spectra of: (a) the diluted reaction solution at reaction time 10 min; (b) the diluted reaction solution after 3 hour, in
positive ion mode.
Acknowledgements
The authors gratefully acknowledge Prof. Dr Qilong-Shen of Key
Laboratory of Organouorine Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences for his fruitful
discussions and the SIOC for nancial support.
Notes and references
´
´
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3
Proposed
mechanism
for
copper-catalyzed
tri-
fluoromethylation of organic zinc reagents with Togni's reagent 2.
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