Direct preparation of trifluoromethylindium reagents from trifluoromethyl iodide: Effective trifluoromethylation and perfluoroalkylation reagents

Article abstract of DOI:10.1002/ejoc.201201116

The direct synthesis of trifluoromethylindium reagents prepared from trifluoromethyl iodide by using just one equivalent each of indium, lithium chloride or sodium iodide, and ferrocene is reported. The weak In···F interaction is suitable to prepare stable but nucleophilic perfluoroalkyl reagents, which were successfully employed in the perfluoroalkylation of aldehydes. Notably, the nucleophilic trifluoromethylation of various aldehydes took place with trifluoromethyl iodide by utilizing just one equivalent each of indium, lithium chloride, and ferrocene.

Full text of DOI:10.1002/ejoc.201201116

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201116
Direct Preparation of Trifluoromethylindium Reagents from Trifluoromethyl
Iodide: Effective Trifluoromethylation and Perfluoroalkylation Reagents
Motohiro Fujiu,[a] Yuzo Nakamura,[a] Hiroki Serizawa,[a] Kohsuke Aikawa,[a]
Shigekazu Ito,[a] and Koichi Mikami*[a]
Keywords: Indium / Fluorine / Ferrocene / Lithium / Alkylation
The direct synthesis of trifluoromethylindium reagents pre-
pared from trifluoromethyl iodide by using just one equiva-
lent each of indium, lithium chloride or sodium iodide, and
ferrocene is reported. The weak In···F interaction is suitable
to prepare stable but nucleophilic perfluoroalkyl reagents,
which were successfully employed in the perfluoroalkylation
of aldehydes. Notably, the nucleophilic trifluoromethylation
of various aldehydes took place with trifluoromethyl iodide
by utilizing just one equivalent each of indium, lithium chlor-
ide, and ferrocene.
prepared but transmetalated from trifluoromethylcadmium
or -mercury complexes (Equation 1). Furthermore, there
has been no report on the carbonyl addition reaction of
trifluoromethylindium complexes[8,9] even with aldehydes.
Introduction
Organofluorine compounds have attracted much interest
because of their significant applications in medicinal and
material sciences.[1] Trifluoromethylated compounds are ef-
fective analogues of hydrocarbons that can contain remark-
able physiological activities and physical properties.[2] How-
ever, nucleophilic trifluoromethylation by using trifluoro-
methyl organometallics such as lithium and magnesium
Grignard reagents cannot be employed for the synthesis of
trifluoromethylated compounds. These trifluoromethyl–
metal reagents are generally recognized as being too un-
stable and difficult to prepare because of facile α-fluoride
(M–F) elimination.[3] The corresponding zinc reagents that
can be used for carbonyl addition have been prepared by
oxidative addition of trifluoromethyl iodide and bromide.
The preparation of these trifluoromethylzinc reagents, how-
ever, requires special ultrasonic irradiation[4] and pressur-
ized reaction conditions;[5] moreover, trifluoromethyl brom-
ide is now prohibited to produce and use because of the
ozone depletion effect.[6] Trifluoromethylsilane, the Rup-
pert–Prakash reagent,[7] is an effective trifluoromethylation
reagent, but it is prepared from trifluoromethyl halides. Di-
agonal to zinc in the periodic table, indium can be found.
Trifluoromethylindium complexes have been analyzed by
19F NMR spectroscopy, even at room temperature,[8] and
hence, these compounds should be stable species. However,
trifluoromethylindium complexes have not been directly
Herein, we report the direct synthesis of trifluoromethyl-
indium reagents prepared from trifluoromethyl iodide by
using just one equivalent each of indium, lithium chlor-
ide[10] or sodium iodide,[9,11] and ferrocene, and the carb-
onyl addition reactions of these reagents were also studied
(Scheme 1). On the basis of our own DFT calculations on
the global minimum configuration of the putative trifluoro-
methylindium complexes (Scheme 1) at the MP2(full)/
DGDZVP level in acetonitrile (Supporting Information),
we confirmed that essentially no In···F interaction in the
trifluoromethylindium complexes was present,[12] which is
in stark contrast to its lithium counterpart (Supporting In-
formation): Kvícala has already reported that the localized
ˇ
[a] Department of Applied Chemistry,
Graduate School of Science and Engineering,
Tokyo Institute of Technology,
Tokyo 152-8552, Japan
Fax: +81-357342776
E-mail: mikami.k.ab@m.titech.ac.jp
Scheme 1. Direct synthesis and carbonyl addition reactions of the
trifluoromethylindium reagent. Possible CF3–In species were opti-
mized at the MP2(full)/DGDZVP (solvent = acetonitrile) level.
Homepage: http://www.apc.titech.ac.jp/~mikami/index-j.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201116.
Eur. J. Org. Chem. 2012, 7043–7047
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M. Fujiu, Y. Nakamura, H. Serizawa, K. Aikawa, S. Ito, K. Mikami
SHORT COMMUNICATION
structure of CF3Li has a bridged three-membered C–– tension of the best metal salts LiCl and NaI. One to two
F···Li+ structure that has a highly twisted form that leads equivalents of LiCl and NaI thus provide the best yields of
to α-elimination of LiF.[12,13]
2a. The observed effects of LiCl and NaI can be rational-
ized in a manner similar to that observed for indium-medi-
ated allylation: NaI[9,11] and LiCl[10] have been employed to
promote the allylation reaction, wherein NaI or LiCl can
(1) decomplex the dimeric structure of the perfluoroalkylin-
dium complexes in the present case and (2) increase the nu-
cleophilicity of the perfluoroalkylindium complexes
through formation of the “ate”-type complex with NaI or
LiCl.
Solvent effects were next examined (Table 2). DMF was
again proven to be the best solvent (Table 2, entry 1). Other
solvents such as NMP, THF, and acetonitrile provided
carbonyl addition product 2a, albeit in lower yields
(Table 2, entries 2–4). In DMSO, no reaction proceeded
(Table 2, entry 5). Organoindium reagents are generally
stable in water, and their reactions are often executed in
water or in an aqueous co-solvent system.[15] An aqueous
co-solvent was thus examined; however, no product was ob-
served (Table 2, Entry 6).
Results and Discussion
The preparation of the perfluoroalkylindium reagent was
first executed with and without lithium chloride (LiCl) in
the context of the indium-mediated perfluoroalkylation re-
action of benzaldehyde (Table 1), according to Knochel’s
reports on the lithium chloride effect on the preparation
of non-fluorinated alkyl indium reagents.[10] In THF, the
carbonyl addition reaction took place only in the presence
of LiCl to give product 2a, whereas no addition product
was observed in the absence of LiCl (Table 1, entries 1, 2).
A change in the solvent from THF to DMF (Table 1, en-
tries 2, 3) and the use of other lithium salts such as LiBr
and LiI also provided the carbonyl addition product in high
yields (Table 1, entries 3, 9, 10). Metal and non-metal fluo-
rides including Li+, Na+, K+, Cs+, and nBu4N+ salts gave
relatively low yields of carbonyl addition product 2a
(Table 1, entries 4–8). Among the lithium salts employed,
LiCl was found to be the best to promote the carbonyl ad-
dition reaction, and addition product 2a could be obtained
in 60% yield (Table 1, entry 3). Reactions performed in the
presence of NaI[9,11] and KI,[14] which have also been re-
ported to promote indium-mediated carbonyl addition re-
actions, provided lower yields than that obtained with one
equivalent of LiCl (Table 1, entries 3, 11,12). However, two
equivalents of NaI provided a good yield of product 2a that
was as high as that obtained with LiCl (Table 1, entry 3 vs.
entry 13). A variety of metal and non-metal salts including
ZnCl2 and nBu4N+Br– also showed acceleration effects
(Table 1, entries 14,15), which can be recognized as an ex-
Table 2. Solvent effects on the perfluoroalkylindium reagent.
Entry
Solvent
Yield [%]
1
DMF
NMP
THF
acetonitrile
DMSO
58
20
31
37
0
2
3[a]
4
5
6
DMF/H2O (6:1)
0
[a] Instead of NaI, LiCl (1.0 equiv.) was added.
Table 1. Direct synthesis and carbonyl addition of the perfluoroalk-
ylindium reagent.
Under the optimized reaction conditions for perfluoro-
alkylation [In (1 equiv.), LiCl (1 equiv.) or NaI (2 equiv.), in
DMF], the trifluoromethylation reaction was studied
(Table 3). In the case of CF3I, no trifluoromethylation pro-
ceeded, whereas CF3Br gave 45% yield of product 3a
(Table 3, entries 1, 2). In the case where two equivalents of
Entry
Salt
Solvent
Yield [%]
1
2
3
4
5
6
7
8
–
LiCl
LiCl
LiF
NaF
THF
THF
0
Table 3. Effect of trifluoromethyl halides.
31
60
39
40
42
37
0
58
57
42
48
58
31
20
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
KF
CsF
nBu4N+F–
LiBr
Entry
Salt
CF3X
Benzaldehyde [m] Yield [%]
9
10
11
12
13[a]
14
15
LiI
KI
NaI
NaI
1
LiCl
LiCl
NaI
NaI
NaI
NaI
CF3I
1.0
1.0
1.0
2.0
4.0
0.2
0
2
CF3Br
CF3Br
CF3Br
CF3Br
CF3Br
45
60
56
57
3
3[a]
4
ZnCl2
5
6
nBu4N+Br–
[a] 2.0 equiv. of NaI was used.
[a] 2.0 equiv. of NaI was used.
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Preparation of Trifluoromethylindium Reagents from CF3I
NaI were used, the yield increased up to 60% (Table 3, en- drawing perfluoroalkyl group in sharp contrast to the elec-
try 3). Even with one equivalent of NaI, an increase in the tronic repulsion of the lone pairs of electrons on the trifluo-
concentration of 1a in DMF to 2 m led to an increase in romethyl radical anion (Figure 1).[1]
the yield up to 56% (Table 3, entry 3). However, a further
increase in the concentration of 1a to 4 m did not improve
the yield (Table 3, Entry 5). On the contrary, a decrease in
the concentration of 1a to 0.2 m resulted in a drastic de-
crease in the yield to 3% (Table 3, Entry 6).
Under the optimized conditions, there was no significant
difference between iodide and bromide reagents in the per-
fluoroalkylation reaction. Interestingly, perfluoroalkyl iod-
Figure 1. The significant difference between RfCF2I and CF3I in
ide gave a higher chemical yield than the corresponding
perfluoroalkyl bromide (Equation 2). In contrast, trifluoro-
methylation with CF3I gave no adduct even in the presence
of LiCl or NaI (1–2 equiv.) in DMF, but CF3Br gave up to
60% yield of the trifluoromethylation product in the same
solvent.
the preparation of the perfluoroalkylindium reagents.
With the success in the preparation of the trifluoro-
methylindium reagent from CF3Br and the mechanistic ra-
tionale on the difference between CF3Br and CF3I[18]
(Scheme 2), we challenged the direct generation of the tri-
fluoromethylindium reagent from CF3I. In0 initiates the
SET process to give InI and the radical anion CF3I–·. The
crucial stage is the second SET process from InI to the radi-
cal anion of CF3I–· to give the trifluoromethylindium(II)
species. This InII species is unstable and easily dispropor-
tionates into InI and InIII (A). CF3I further oxidatively adds
to CF3InI eventually to provide the other type of CF3–InIII
reagent (B). CF3InI2 (A) and (CF3)2InI (B) were detected
at δ = –49.0 and –48.0 ppm, respectively, in the 19F NMR
spectrum in the presence of LiCl [cf. CF3InCl2 and (CF3)2-
InCl at δ = –47.3 and –46.2 ppm, respectively[8]].
The reason why a significant difference was observed be-
tween CF3Br and CF3I can be exemplified (Scheme 2):[16]
The initiation step is the single-electron transfer (SET) from
In0 to CF3X. The second stage of the reaction involves sub-
sequent SET from InI to the resultant radical anion of
CF3X–· to give the trifluoromethylindium(II) species. How-
ever, iodide, which has a higher leaving ability than brom-
ide, was easily eliminated from the radical anion intermedi-
ate of CF3I–· prior to the second SET, thus resulting in the
The rate-limiting step would be the second SET from InI
to InII, which is relatively unfavorable compared with In0
to InI stage (Scheme 3). Knochel reported in the prepara-
tion of non-fluorinated alkylindium reagents in which the
SET process from InI to InII can be accelerated by using
LiCl as an additive.[10] Recently, Loh further reported a sig-
nificant effect of CuCl on the generation of non-fluorinated
alkyl indium reagents.[19]
·
formation of the trifluoromethyl radical (CF3 ) to end up
with the formation of the Wurtz coupling product,[17] as
observed by the signal in the 19F NMR spectrum at δ =
–85 ppm (C2F6).
Scheme 2. The significant difference between CF3Br and CF3I in
the preparation of the trifluoromethylindium reagent.
Scheme 3. Sequential single-electron transfer in the preparation of
the trifluoromethylindium reagents.
The question remained as to why the perfluoroalkyl iod-
The metal complexes were further scrutinized as a SET
ide provided the active perfluoroalkylindium complex, reagent (Table 4). However, CuCl gave trifluoromethylation
whereas trifluoromethyl iodide did not. The reason is the product 3a in only 21% yield (Table 4, entry 2). In the case
difference in stability between the trifluoromethyl radical of CuCl2, the adduct was not obtained (Table 4, entry 3).
anion of CF3X–· and the perfluoroalkyl radical anion The use of AgI[20] led to an increase in the yield of the
RfX–·, which is stabilized by the strongly electron-with- product up to 40% (Table 4, entry 6). Further investigation
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M. Fujiu, Y. Nakamura, H. Serizawa, K. Aikawa, S. Ito, K. Mikami
SHORT COMMUNICATION
with the use of ferrocene gave a maximum yield of 54% (Table 5, entries 2, 3). α,β-Unsaturated cinnamylaldehyde
(Table 4, entry 7), which is as high as the yields obtained in provided the product in 50% yield (Table 5, entry 4), and
perfluoroalkylation reactions with RfI or trifluoromethyl- the aliphatic aldehyde nonanal provided 43% yield of the
ation reactions with CF3Br. In the presence of CuI[21] and desired product (Table 5, entry 5).
powdered Cu0, the trifluoromethylation product was ob-
tained, although in lower yields (Table 4, entries 4, 5). We
Conclusions
confirmed that in the absence of the indium metal, lithium
chloride salt, and/or ferrocene, the perfluoroalkylation reac-
tion with trifluoromethyl iodide did not proceed to give any
trifluoromethylation product (Table 4, entries 1, 8, 9).
We have thus developed the direct synthesis of trifluoro-
methyl- and perfluoroalkylindium reagents prepared from
trifluoromethyl iodide and perfluoroalkyl halides by using
just one equivalent each of indium, lithium chloride or so-
dium iodide, and ferrocene. Preparation of the reagent was
found to be particularly effective with the use of trifluoro-
methyl iodide. The carbonyl addition reactions of these new
reagents were studied, and the corresponding trifluoro-
methylated alcohols were obtained in good yields.
Table 4. The effect of the single-electron transfer reagent.
Entry
Additive
Yield [%]
1
2
3
4
5
6
–
0
21
trace
26
16
40
54
0
Experimental Section
CuCl
CuCl2
CuI
General Procedure for the In-Mediated Perfluoroalkylation: LiCl
(8.5 mg, 0.20 mmol) was placed in a test tube and dried under high
vacuum. In powder (23.0 mg, 0.20 mmol, –100 mesh, 99.99%, Ald-
rich) and DMF (0.2 mL) were added. The In was activated by the
addition of a catalytic amount of 1,2-dibromoethane (ca. 10 mol-
%) and TMSCl (ca. 10 mol-%), and the mixture was then heated
for a few minutes. To the heterogeneous solution was added benzal-
dehyde (1a; 20 μL, 0.20 mmol) and then perfluorohexyl iodide
(43 μL, 0.20 mmol). After 2 h, LiCl and other inorganic salts were
removed by filtration of the residue through a short silica gel col-
umn. After removal of the solvent under reduced pressure, the resi-
due was purified by silica gel column chromatography (hexane/
EtOAc = 20:1 to 10:1) to give 2,2,3,3,4,4,5,5,6,6,7,7,7-tride-
cafluoro-1-phenylheptan-1-ol (2a; 51.1 mg, 60%).
Cu0
AgI
7
ferrocene
ferrocene
ferrocene
8[a]
9[b]
0
[a] Without LiCl. [b] Without In.
Under the optimized reaction conditions, the trifluoro-
methylation reaction with CF3I gave products 3 in up to
54% yield (Table 5). Interestingly, aromatic 1- and 2-naph-
thylaldehyde gave the trifluoromethylation product in 54%
yield, irrespective of the change in their steric environment
General Procedure for the In-Mediated Trifluoromethylation with
CF3Br: LiCl (8.5 mg, 0.20 mmol) was placed in test tube and dried
under high vacuum. In powder (23.0 mg, 0.20 mmol, –100 mesh,
99.99%, Aldrich) and DMF (0.2 mL) were added. The In was acti-
vated by the addition of a catalytic amount of 1,2-dibromoethane
(ca. 10 mol-%) and TMSCl (ca. 10 mol-%), and the mixture was
then heated for a few minutes. To the heterogeneous solution was
added 1a (20 μL, 0.20 mmol) and then trifluoromethyl bromide (ca.
0.15 g, b.p. –58 °C) at –78 °C and the argon balloon was removed
to prevent trifluoromethyl bromide from vaporizing. After 2 h,
LiCl and other inorganic salts were removed by filtration of the
residue through a short silica gel column. After removal of the
solvent under reduced pressure, the residue was purified by silica
gel column chromatography (hexane/EtOAc = 20:1 to 10:1) to give
2,2,2-trifluoro-1-phenylethanol (3a; 15.9 mg, 45%).
Table 5. Carbonyl addition reaction of the trifluoromethylindium
reagent to aldehydes.
General Procedure for the In-Mediated Trifluoromethylation with
CF3I: LiCl (8.5 mg, 0.20 mmol) was placed in test tube and dried
under high vacuum. In powder (without activation, 23.0 mg,
0.20 mmol, –100 mesh, 99.99%, Aldrich), ferrocene (37.2 mg,
0.20 mmol), and DMF (0.2 mL) were added (if In were activated,
the reaction would fail). To the heterogeneous solution was added
1a (20 μL, 0.20 mmol) and then trifluoromethyl iodide (ca. 0.2 g,
sufficiently dissolved to the solution). After 2 h, LiCl and the other
inorganic salts were removed by filtration of the residue through a
short silica gel column. After removal of the solvent under reduced
pressure, the residue was purified by silica gel column chromatog-
raphy (hexane/EtOAc = 20:1 to 10:1) to give 3a (19.0 mg, 54%).
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Preparation of Trifluoromethylindium Reagents from CF3I
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Received: August 16, 2012
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Published Online: November 9, 2012
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7047