Development of (Trifluoromethyl)zinc Reagent as Trifluoromethyl Anion and Difluorocarbene Sources

Article abstract of DOI:10.1021/acs.orglett.5b02439

The trifluoromethylation of carbonyl compounds is accomplished by the stable (trifluoromethyl)zinc reagent generated and then isolated from CF₃I and ZnEt₂, which can be utilized as a trifluoromethyl anion source (CF₃^-). The reaction proceeds smoothly with diamine as a ligand and ammonium salt as an initiator, providing the corresponding trifluoromethylated alcohol products. Moreover, the (trifluoromethyl)zinc reagent can also be employed as a difluorocarbene source (:CF₂) not only for gem-difluoroolefination of carbonyl compounds with phosphine but also for gem-difluorocyclization of alkenes or alkynes via the thermal decomposition, respectively.

Full text of DOI:10.1021/acs.orglett.5b02439

Letter
pubs.acs.org/OrgLett
Development of (Trifluoromethyl)zinc Reagent as Trifluoromethyl
Anion and Difluorocarbene Sources
Kohsuke Aikawa, Wataru Toya, Yuzo Nakamura, and Koichi Mikami*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama,
Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: The trifluoromethylation of carbonyl com-
pounds is accomplished by the stable (trifluoromethyl)zinc
reagent generated and then isolated from CF3I and ZnEt2,
−
which can be utilized as a trifluoromethyl anion source (CF3 ).
The reaction proceeds smoothly with diamine as a ligand and
ammonium salt as an initiator, providing the corresponding
trifluoromethylated alcohol products. Moreover, the (trifluor-
omethyl)zinc reagent can also be employed as a difluor-
ocarbene source (:CF2) not only for gem-difluoroolefination of carbonyl compounds with phosphine but also for gem-
difluorocyclization of alkenes or alkynes via the thermal decomposition, respectively.
he introduction of fluorine atoms into small organic
molecules is a powerful strategy to increase the binding
respectively. Accordingly, the development of a reaction with the
zinc-based reagent remains far behind compared to the silicon-
based counterpart. We have recently succeeded in the aromatic
trifluoromethylation catalyzed by copper iodide with the stable
bis(trifluoromethyl)zinc reagent 1a generated and then isolated
from CF3I, ZnEt2, and DMPU.8−10 Encouraged by this reagent,
we continued to explore the potential of the zinc reagent 1
through application to a variety of reactions. Herein, we describe
the trifluoromethylation of carbonyl compounds4,11 with the
(trifluoromethyl)zinc reagent bearing diamine ligand, which can
be utilized as a trifluoromethyl anion source (CF3−).
Furthermore, the zinc reagent as a difluorocarbene source
(:CF2) is applicable not only to gem-difluoroolefination of
carbonyl compounds12,13 with phosphine but also to gem-
difluorocyclization of alkenes and alkynes12b,14,15 via thermal
decomposition, respectively.
We commenced our studies by examining the trifluoromethy-
lation reaction of 1-naphthaldehyde 2a with the zinc reagent 1 in
DMF at 40 °C (Scheme 1). No product could be observed when
Zn(CF3)2(DMPU)2 1a was employed. The reagent Zn(CF3)IL2
(L = DMF, DMPU),8b which can be generated and isolated by
the reaction of zinc dust with CF3I, was also totally inactive to 2a.
To our delight, after treatment of Zn(CF3)2(TMEDA)8a bearing
diamine ligand, the structure was unambiguously confirmed by
X-ray analysis and the desired product 3a was obtained in 72%
yield after 24 h. A variety of N,N ligands such as 1,10-
phenanthoroline, 2,2′-bipyridine and other ethylenediamine
derivatives were investigated, but TMEDA was found to lead
to the highest yield of 3a. Significantly, monitoring the change of
conversion in reaction time by 19F NMR analysis clarified that an
induction period of at least 3 h exists on this reaction system.
T
affinity to molecular receptors in the development of novel
pharmaceuticals and agrochemicals.1 Indeed, the modern
pharmaceutical and agrochemical industry critically depends on
the recent progress of organic fluorine chemistry. In the past
decades, considerable effort has been devoted to the develop-
ment of efficient nucleophilic, electrophilic, and radical
approaches to introduce the trifluoromethyl (CF3) group.2
Particularly, the (trifluoromethyl)metal species (MCF3) have
played an important role in the development of these methods.3
However, the main obstacle is that the (trifluoromethyl)metal
species with lithium and magnesium are labile even at very low
temperatures and consequently decompose via α-fluorine
elimination to produce metal fluoride and singlet difluorocar-
bene. Therefore, the Ruppert−Prakash reagent (Me3SiCF3),
which can be stabilized through an Si−CF3 bond, is commonly
utilized as the most convenient trifluoromethylating reagent.4 It
has also been recognized that the (trifluoromethyl)zinc reagent
can be applied to the trifluoromethylations. The (trifluorometh-
yl)zinc reagent (Zn(CF3)I) generated in situ from trifluor-
omethyl iodide (CF3I) and zinc dust is reported to serve for a
variety of trifluoromethylations.5 However, the reproducibility of
this method in which ultrasonic irradiation is required during the
course of the preparation and reaction remains problematic.
Recently, it has been demonstrated that the (trifluoromethyl)
zinc reagent prepared in situ from TMP2Zn and fluoroform
(CHF3) is efficient in the trifluoromethylation of aryl iodide
catalyzed by copper chloride.6 It has also been reported that the
combination of trifluoromethyl bromide (CF3Br) and zinc dust
in DMF can lead to the (trifluoromethyl)zinc reagent (Zn-
(CF3)Br·2DMF) as the stable solid, which is applicable to the
copper-mediated trifluoromethylation of aryl iodide.7 However,
these methods suffer from intrinsic disadvantages such as the use
of laborious preparation of TMP2Zn and ozone-depleting CF3Br,
Received: August 24, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02439
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Letter
a
Scheme 1. Nucleophilic CF3 Addition to Aldehyde
a
Conditions: 2a (0.1 mmol) and 1 (0.2 mmol) in DMF (1.0 mL) at
b
c
40 °C for 24 h. Reaction using 1b. Yield was determined by 19F
NMR analysis using benzotrifluoride (BTF) as an internal standard.
On the basis of the result observed above, we investigated the
effect of additive as an initiator (Table 1). Thus, 0.2 equiv of KO-
a
Table 1. Effect of Activator in CF3 Addition to Aldehyde
Figure 1. Scope and limitation of carbonyl compounds. (a) Conditions:
2 (0.1 mmol), n-Bu4NOAc (0.02 mmol), and 1b (0.2 mmol) in DMF
(1.0 mL) at 40 °C for 3 h. Yield was determined by 19F NMR analysis
using benzotrifluoride (BTF) as an internal standard. (b) Reaction time
was 6 h. (c) CH2Cl2 instead of DMF was used and reaction time was 12
h. (d) Conditions: 2m−p (0.1 mmol), n-Bu4NOAc (0.04 mmol), and 1b
(0.4 mmol) in DMF (1.0 mL) at 50 °C for 12 h.
b
entry
additive
conditions
yield (%)
1
2
3
4
5
6
7
KO-t-Bu
KF
DMF, 40 °C, 6 h
DMF, 40 °C, 6 h
DMF, 40 °C, 6 h
DMF, 40 °C, 6 h
DMF, 40 °C, 3 h
CH2Cl2, 40 °C, 3 h
DMF, rt, 24 h
85
47
37
55
92
amount of the product. Benzophenone derivatives and chalcone
were trifluoromethylated smoothly to give the desired products
3m−o in good yields, while a larger amount of n-Bu4NOAc and
1b was needed.
n-Bu4NBr
Me4NF
n-Bu4NOAc
n-Bu4NOAc
n-Bu4NOAc
c
51 (96)
On the basis of our results and previous reports4 involved in
trifluoromethylations of carbonyl compounds using the
Ruppert−Prakash reagent, the reaction is likely to proceed
through an autocatalytic process (Scheme 2). The reaction
50
a
Conditions: 2a (0.1 mmol) and 1b (0.2 mmol) in solvent (1.0 mL).
b
Yield was determined by 19F NMR analysis using benzotrifluoride
c
(BTF) as an internal standard. Reaction time was 6 h.
Scheme 2. Plausible Reaction Mechanism
t-Bu or KF was found to facilitate the reaction without an
induction period to give the alcohol 3a in 85% and 47% yields,
respectively, even after 6 h (entries 1 and 2). Various ammonium
salts can also be applied to the reaction as initiators, and
especially n-Bu4NOAc gave higher yields after 3 h (entries 5 vs
1−4). DMF is often selected as an effective solvent for the
trifluoromethylation reaction using trifluoromethylating re-
agents3c,4 because the trifluoromethyl anion can be trapped by
DMF to provide the reservoir of trifluoromethylating hemi-
aminolate species.16 Accordingly, whether noncoordinating
solvents can be exploited or not for the reaction was examined.
Consequently, it was found that the reaction proceeded
smoothly even in dichloromethane to give the product in 96%
yield, while the reactivity was slightly decreased compared to
DMF (entries 5 vs. 6). The reaction took place even at room
temperature, in spite of moderate yield (entry 7).
With the optimized reaction conditions in hand, various
aldehydes and ketones 2 were employed for the reaction, giving
the corresponding alcohols 3 in good to excellent yields (Figure
1). Aromatic aldehydes bearing the electron-donating and
-withdrawing para-substituents resulted in high yields of the
products 3b−f. Interestingly, the reaction of aldehyde with the
electron-donating para-substituent showed a faster reaction rate
than with the electron-withdrawing one (3d vs 3e). The reaction
of aromatic aldehydes with ortho-substituents led to slightly
lower yields (3h−j) compared with para- and meta-substituents.
Although cinnamaldehyde also served as an acceptable substrate
for the reaction, nonanal with an α-proton provided only a trace
process would start from the generation of alkoxide A bearing
tetrabutylammonium cation and zinc species B with acetate,
involving activation of n-Bu4NOAc as an initiator to zinc reagent
1b. Subsequently, the trifluoromethyl anion can be transferred
into carbonyl compound 2 via a putative zincate C generated by
the reaction of zinc reagent 1b with alkoxide A. As a result,
alkoxide A is regenerated to autocatalyze the following reaction.
Finally, zinc alkoxide D obtained simultaneously is converted to
the desired alcohol product 3 via protonation by water.
With the aim of enhancing the utility of the isolated zinc
reagent, we next explored gem-difluoroolefination of carbonyl
compounds employing the zinc reagent as a difluorocarbene
B
DOI: 10.1021/acs.orglett.5b02439
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Letter
source. The reaction in THF by treatment of the zinc reagent 1a
and aldehyde 2a in the presence of phosphine, which is necessary
to generate in situ the (difluoromethylene)phosphonium ylide
(CF2 = PR3) as an active species, proceeded smoothly to provide
the difluoroolefinated product 4a (Scheme 3). However, the
a
Scheme 3. Difluoroolefination to Aromatic Aldehyde
a
Conditions: 2a (0.1 mmol), 1a (0.12 mmol), and phosphine (0.24
b
mmol) in THF (1.0 mL) at 70 °C for 12 h. Yield was determined by
19F NMR analysis using benzotrifluoride (BTF) as an internal
standard.
Figure 2. Scope and limitation of carbonyl compounds. (a) Conditions:
2 (0.1 mmol), 1a (0.12 mmol), and triphenylphosphine (0.24 mmol) in
THF (1.0 mL) at 70 °C for 12 h. Yield was determined by 19F NMR
analysis using benzotrifluoride (BTF) as an internal standard. (b) DMF
instead of THF was used as solvent. (c) At 80 °C for 3 h. (d) Conditions:
2 (0.1 mmol), 1a (0.3 mmol), and P-n-Bu3 (0.3 mmol) in DMF (1.0
mL) at 110 °C for 3 h. Zinc reagent 1a was added dropwise over 1 h.
reaction employing 1b instead of 1a did not occur because the
ligand exchange between monodentate phosphine and TMEDA
is inefficient for generating the phosphonium ylide, in sharp
contrast to the case of DMPU. Regarding the effect of phosphine,
PPh3 and P(p-MeOPh)3, as a type of triarylphosphine, were
more suitable for the present reaction than more electron-rich
analogues such as P(NMe2)3 and PCy3, affording the product 4a
quantitatively. It was also found that the reactions with electron-
poor or bulky triarylphosphines such as P(p-CF3Ph)3 and P(o-
tol)3 resulted in almost complete recovery of substrate.
Under the optimized conditions employing cheaper PPh3,
aliphatic aldehyde 2j also underwent the reaction, but the yield of
the desired product 4j was low (20%). In contrast to the reaction
of 2a, the phosphonium salt E13c formed by protonation of the
phosphonium ylide was confirmed in 67% yield by 19F NMR
analysis (Scheme 4). The replacement of a solvent from THF to
DMF is thus found to improve the yield of product 4a to 77%,
along with only a trace amount of E.
alkynes 5 (Scheme 5). After surveying a wide range of solvents
and additives as activators of 1a, we found that the reactions
a
Scheme 5. Difluorocyclopropanation and -cyclopropenation
a
Scheme 4. Difluoroolefination to Aliphatic Aldehyde
a
Conditions: 5 (0.1 mmol) and 1a (0.2 mmol) in toluene (1.0 mL) at
a
Yields were determined by 19F NMR analysis using benzotrifluoride
80 °C for 12 h. Yield was determined by 19F NMR analysis using
benzotrifluoride (BTF) as an internal standard. Use of PPh3 (0.2
(BTF) as an internal standard.
b
mmol).
According to this protocol, a variety of aldehydes were
difluoroolefinated affording the corresponding products 4 in
good to excellent yields (Figure 2). Benzaldehyde derivatives
with electron-donating and -withdrawing para-substituents
showed relatively high reactivity in excellent yields (4a−d).
The reactions with aromatic aldehydes bearing ortho-substituents
also took place (4f−h), while the reactivity was decreased owing
to steric hindrance. It was demonstrated that fluoroalkylated
ketones were applicable to the reaction in 86% yield (4k). The
reactions with ketones resulted in the formation of the
corresponding products (4l−o) due to improvement of the
reaction conditions, while yields were decreased.
occurred efficiently without any additives in toluene at 80 °C,
providing the difluorocyclopropanated and -cyclopropenated
products 6. In the case of cyclopropanation, styrene derivatives
and electron-rich tri- and tetrasubstituted alkynes could be
exploited for the reaction (6a−f). Furthermore, cyclopropena-
tion with terminal alkynes proceeded to give the corresponding
products in moderate to good yields (6g−i). Addition of PPh3
was found to improve the yield in 6i because the ylide CF2PR3
generated in situ can be utilized as the reservoir of
difluorocarbene.13b
In summary, we have succeeded in the trifluoromethylation
reaction of carbonyl compounds by using the stable bis-
(trifluoromethyl)zinc reagent, which can be utilized as a
Finally, it was further clarified that the zinc reagent 1a was
successfully applied to gem-difluorocyclization of alkenes and
C
DOI: 10.1021/acs.orglett.5b02439
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Organic Letters
Letter
−
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trifluoromethyl anion source (CF3 ). The reaction proceeds
smoothly with TMEDA as a ligand and ammonium salt as an
initiator to provide the corresponding secondary and tertiary
alcohol products bearing a trifluoromethyl substituent. Addi-
tionally, the zinc reagent can be applied as a difluorocarbene
source (:CF2) not only for gem-difluoroolefination of carbonyl
compounds with (difluoromethylene)phosphonium ylide gen-
erated by addition of phosphine but also to gem-difluorocycliza-
tion of alkenes and alkynes via the thermal decomposition of the
zinc reagent, respectively. Development of catalytic asymmetric
reactions with the (trifluoromethyl)zinc reagent is ongoing in
our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02439.
Experimental procedures and compound characterization
data (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: mikami.k.ab@m.titech.ac.jp.
Notes
The authors declare no competing financial interest.
(13) For recent examples of gem-difluoroolefinations, see: (a) Zheng,
J.; Cai, J.; Lin, J.-H.; Guo, Y.; Xiao, J.-C. Chem. Commun. 2013, 49, 7513.
(b) Zheng, J.; Lin, J.-H.; Cai, J.; Xiao, J.-C. Chem. - Eur. J. 2013, 19,
15261. (c) Wang, F.; Li, L.; Ni, C.; Hu, J. Beilstein J. Org. Chem. 2014, 10,
344. (d) Li, Q.; Lin, J.-H.; Deng, Z.-Y.; Zheng, J.; Cai, J.; Xiao, J.-C. J.
Fluorine Chem. 2014, 163, 38.
ACKNOWLEDGMENTS
■
This research was supported by Japan Science and Technology
Agency (JST) (ACT-C: Advanced Catalytic Transformation
Program for Carbon Utilization) and the Noguchi Institute. We
thank TOSOH F-TECH, Inc., for the gift of CF3I.
(14) For reviews, see: (a) Dolbier, W. R., Jr.; Battiste, M. A. Chem. Rev.
́
2003, 103, 1071. (b) Fedorynski, M. Chem. Rev. 2003, 103, 1099.
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