Catalytic Intramolecular Aminofluorination, Oxyfluorination, and Carbofluorination with a Stable and Versatile Hypervalent Fluoroiodine Reagent

Article abstract of DOI:10.1002/anie.201503373

Application of a fluoroiodine analogue of the Togni reagent was studied in fluorocyclization reactions. In the presence of a transition-metal catalyst the applied fluoroiodine reagent can be used for aminofluorination, oxyfluorination, and carbofluorination reactions. The described procedure has a very broad synthetic scope for preparation of functionalized hetero- and isocyclic compounds having a tertiary fluorine substituent.

Full text of DOI:10.1002/anie.201503373

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201503373
German Edition: DOI: 10.1002/ange.201503373
Homogeneous Catalysis
Catalytic Intramolecular Aminofluorination, Oxyfluorination, and
Carbofluorination with a Stable and Versatile Hypervalent
Fluoroiodine Reagent**
Weiming Yuan and Kµlmµn J. Szabó*
Abstract: Application of a fluoroiodine analogue of the Togni
reagent was studied in fluorocyclization reactions. In the
presence of a transition-metal catalyst the applied fluoroiodine
reagent can be used for aminofluorination, oxyfluorination,
and carbofluorination reactions. The described procedure has
a very broad synthetic scope for preparation of functionalized
hetero- and isocyclic compounds having a tertiary fluorine
substituent.
dures are suitable for the synthesis of piperazine derivatives
(six-membered ring) but fluoro indoles/pyrroles and azepanes
have also been reported.[4,6] Fluorocyclization reactions for
the synthesis of oxygen-containing rings have also been an
attractive synthetic method for the introduction of the
fluorine functionality into organic molecules.[6c,m,7] A some-
what less developed but very challenging area involves
carbofluorination reactions for the synthesis of fluorine-
containing isocyclic compounds.[8] A limited number of
reports have also been published on intermolecular amino-
fluorination[9] and carbofluorination[10] reactions.
Relatively few studies have been reported for the
application of the same reagent under similar reaction
conditions to perform all three types of cyclization reactions.
There are a couple of reports for the application of intra-
molecular amino- and oxyfluorination methods based on
similar amine and alcohol precursors.[6c,m] However, as far as
we know all three types of fluorocyclizations (including even
carbofluorination) of similar substrates using the same
fluorination reagent have not been reported. The dominant
fluorination reagents in the above procedures are NFSI and
Selectfluor. The direct application of fluoroiodines is rather
limited, because of the low stability and high reactivity of
ArIF2 and related reagents.[6d,7h] However, a couple of
examples have been presented[4,6h–j] for using stable and
easily available acetyliodines [such as PhI(OAc)2, PhI(OPiv)2,
PhI(TFA)2] and PhIO in combination with various fluorine
sources for in situ generation of fluoroiodines.
As a concept-driven approach in our fluorine chemistry
program,[11] we decided to investigate the application of the
air-, moisture-, and thermostable fluoroiodine 1 (for structure
see Figure 1) as a reagent for fluorination reactions.[11a] This
reagent is a structural analogue of the Togni reagents,[2b,12]
which have been a very popular electrophilic trifluoromethy-
lating reagents in organic synthesis.[2b,d,e] We expected that
conceptual analogies between 1 and its CF3 analogue could be
exploited for the development of new catalytic fluorination
reactions. For example, catalytic oxytrifluoromethyla-
tion[11c,d,13] and aminotrifluoromethylation[14] with the Togni
reagent are well-known methods, and therefore it was
appealing to attempt oxyfluorination and aminofluorination
reactions with 1. Furthermore, unlike Ar-IF2 derivatives, 1 is
a stable and easily accessible reagent,[15] which is a potent
oxidant, a fluorine source, and a preformed base in one
reagent. A further interesting property is that 1 can be
prepared from its chloro analogue by addition of KF. Thus, it
satisfies an important criterion for late-stage electrophilic
fluorinating reagents, namely that 1 can be easily prepared
from simple anionic fluoride salts. This property can be very
O
rganofluorines are very important substances in the
pharmaceutical industry, agrochemistry, and medical diag-
nostics.[1] The widespread use of organofluorines (including
compounds with 18F isotopes) in life science related applica-
tions is due to their favorable biological, pharmacological,
and radiochemical properties.[1] Therefore, there is a large
demand for easy access to a broad variety of these species,
which has been one of the most important driving forces for
the development of new synthetic procedures for the
preparation of organofluoro compounds.[2]
Recently, the progress in this field was also advanced by
the appearance of new, safe, and easily accessible reagents
even for electrophilic fluorination reactions. Although excel-
lent electrophilic fluorination reagents have appeared pre-
viously, such as Selectfluor, NFSI, DAST, etc., the demand
from important application areas of organofluorines moti-
vates additional development of the reagents.[1–3] For exam-
ple, 18F-labelling-based methodologies (such as PET scan-
ning) requires the late-stage introduction of the fluorine to
the reagent and subsequently to the target molecule.[1d,2a,3]
Electrophilic fluorocyclization reactions represent a very
important methodology for the synthesis of heterocycles and
functionalized carbocyclic compounds. After the pionneering
studies of Liu and co-workers,[4] aminofluorination has
become a very important method for the synthesis of nitro-
gen-containing heterocycles.[5] Most of the published proce-
[*] Dr. W. Yuan, Prof. K. J. Szabó
Department of Organic Chemistry, Stockholm University (Sweden)
E-mail: kalman@organ.su.se
Homepage: http://www.organ.su.se/ks/
[**] Support by the Swedish Research Council and the Knut and Alice
Wallenbergs Foundation is greatfully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503373.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial License, which permits
use, distribution and reproduction in any medium, provided the
original work is properly cited and is not used for commercial
purposes.
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8533

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useful for 18F-labelling studies, as for example the synthesis of
PET ligands.[3] So far relatively few studies have appeared on
the application of 1 as a fluorinating reagent. This small
number of reports is probably because its first synthesis was
reported just a couple of years ago by Legault and PrØvost.[15a]
Stuart and co-workers[15c,16] reported that 1 reacts with 1,3-
diketoesters and 1,3-diketones in the presence of TREAT-HF
to give mono- or difluoro products. In addition, recently, we
have shown[11a] that 1 can be employed for the difluorination
of styrene derivatives.
We selected the fluorocyclization of the alkene tosylamide
2a by 1 as a model reaction (Table 1). Several metal catalysts,
such as Cu, Pd, Ag, and Zn (tetrafluoroborate salt), proved to
be useful catalysts for obtaining 3a with a tertiary fluoride
substituent (entries 1–4 and 6). CuCl was inefficient (entry 5)
and CH2Cl2 was the best solvent for the reaction (entries 6–9).
We could not observe any reaction, when either 1 (entry 10)
or the metal catalyst (entry 11) was omitted. The best results
were obtained with the commercially available Zn(BF4)2
catalyst (5 mol%), which contains crystal water (entry 6).
This trace amount of water did not affect (at least) the
aminocyclization reaction.
We have now found that 1 is an excellent and versatile
reagent for fluorocyclization reactions in the presence of
metal catalysts (Figure 1). Furthermore, 1 is suitable for
With the optimization results in hand, we studied the
synthetic scope of the aminocyclization reaction in detail
(Table 2). The reaction of 2 with 1 in the presence of Zn(BF4)2
proceeds under mild reaction conditions at room temperature
without any additives. The typical reaction time was 3 hours.
The reaction was very clean, thus affording high yields for
different substituent patterns. The dimethyl-substituted 2a
(entry 1) reacted about as fast as its diphenyl analogue 2b
(entry 2), thus affording the tertiary fluoro piperidines 3a and
3b, respectively. The tosyl group could be replaced with mesyl
(entry 3) or nosyl (entry 4) without affecting the reaction rate
or lowering the yield. The cyclohexyl derivative 2e gave the
spiro-piperidine derivative 3e in high yield (entry 5). The
reaction proceeds well without a methyl substituent on the
alkene group (2 f), thus affording the secondary fluoro
piperidine derivative 3 f (entry 6). Conversely, either an
ethyl (2g) or pentyl group (2h), instead of methyl, on the
alkene gave the crowded tertiary fluorines 3g and 3h,
respectively (entries 7 and 8). The achievement of high
stereoselectivity for tertiary fluorides is usually challenging.
Cyclization of 2i to 3i (entry 9) proceeds with a poor
stereoselectivity (d.r. = 2:1), which might be due to the
relatively remote stereocenters. Gratifyingly, cyclization of
2j (in which the stereocenters are in adjacent position)
afforded 3j (entry 10) in high stereoselectivity (d.r. > 10:1).
The double bond in 2j is at an internal position, thus
indicating that the fluorocyclization does not necessarily
require terminal alkenes. However, the aminocyclization was
slower for 2j (9 h) than for terminal alkenes (typically 3 h).
When we employed 2k (the Z isomer of 2j) the reaction
resulted in 3k (the diastereomeric form of 3j) with high
diastereoselectivity (entry 11). The cyclization of 2k had to be
performed at 408C (instead of RT), as this substrate is
probably more sterically hindered than 2j. We have also
studied the cyclization of tosyl amide alkenes with different
tethers (entries 12–14) to explore the possibility for variation
of the ring size. The compound 2l (entry 12) gave the pyrrole
derivative 3l, and proceeded more slowly than the reaction of
2m, thus leading to the piperidine derivative 3m (entry 13).
However, the yields for the two processes were about the
same. Azepane with a tertiary fluoro substituent (3n) also
formed easily and in high yield from 2n. In this process
formation of smaller ring sizes (such as 3l–n) was not
observed.
Figure 1. Catalytic fluorocyclization reactions in this study. Ts=
4-toluenesulfonyl.
amino-, oxy-, and carbofluorination of analogue substrates
under very similar reaction conditions. The reagent 1 could be
easily used for the synthesis of tertiary fluorides, which is
often challenging with the commonly used reagents because
of steric issues.
Table 1: Development of the catalytic aminofluorination reaction.[a]
Entry
Catalyst
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
[Cu(MeCN)4]BF4
[Pd(MeCN)4](BF4)2
[Ag(MeCN)4]BF4
AgBF4
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
PhCH3
CH3CN
CH2Cl2
CH2Cl2
65
50
46
58
0
CuCl
Zn(BF4)2·xH2O
Zn(BF4)2·xH2O
Zn(BF4)2·xH2O
Zn(BF4)2·xH2O
Zn(BF4)2·xH2O
–
75
40
63
<5
0
9
10[c]
11
0
[a] Reaction conditions: 0.1 mmol of 2a, Zn(BF4)2·XH2O (5 mol%) and
1 (1.1 equiv) was reacted in CH2Cl2 (0.5 mL) at RT. [b] Yield of the
isolated product. [c] Fluoroiodine 1 was not added. THF=tetrahydro-
furan.
Subsequently, we studied the possibility for extending the
fluorocyclization reaction to the synthesis of oxygen-contain-
ing heterocycles and isocyclic compounds (Table 3). The
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Table 2: Catalytic aminofluorination with 1.[a]
Table 3: Fluorocyclization by oxy- and carbofluorination using 1.[a]
Entry
Substrate
t [h]
Product
Yield [%][b]
75
Entry Substrate
Method t [h] Product
Yield [%][b]
1
A
A
2
1
62
1
3
2
3
65
60
2
3
4
3
3
3
73
82
84
A
B
B
B
B
1
8
8
8
4
4
55
60
76
5
6
7
3
3
3
84
71
72
62
5
6[c]
8
9
4
3
60
7
(2:1)[d]
62
(2:1)[c]
[a] Method A: 0.3 mmol of 4, Zn(BF4)2·xH2O (5 mol%) and 1 (1.1 equiv)
was reacted in CH2Cl2 (0.5 mL) at RT; Method B: 0.3 mmol of 6,
[Cu(MeCN)4]BF4 (10 mol%) and 1 (1.5 equiv) was reacted in CH2Cl2
(0.5 mL) at 408C. [b] Yield of the isolated product. [c] The reaction was
performed at RT. [d] The ratio of diastereomers was determined by
19F NMR analysis of the crude reaction mixture.
63
10[d]
9
5
(>10:1)[c]
65
11[e]
(>10:1)[c]
were shorter (typically 1–2 h) for the oxyfluorination process
than for the aminofluorination reaction (typically 3 h). The
reaction was suitable for formation of both six- and seven-
membered heterocycles (entries 1–3). In these processes only
tertiary fluorides were formed. The regioselectivity was the
same as for the aminofluorination, thus affording the endo-
cyclized pyrane (entries 1 and 2) and oxepane (entry 3)
products exclusively. To our delight, the cyclization could be
extended to carbofluorination reactions as well (entries 4–7).
Zn(BF4)2 was not suitable as a catalyst, probably because it
contained crystal water. However, [Cu(MeCN)4]BF4
(10 mol%) proved to be an excellent catalyst for carbocyc-
lization of the alkenyl malonate derivatives 6a–d. The process
results exclusively in cyclopentane derivatives having a ter-
tiary fluorine substituent (7a–d). The reaction requires longer
reaction times (typically 8 h) than the cyclization of the amino
and hydroxy analogues (typically 1–3 h). The longer reaction
time is apparently required because of the poor nucleophi-
licity of the malonate carbon atom in 6 compared to the
nitrogen and oxygen atoms in 2 and 4, respectively. The ethyl
malonate 6b gave a somewhat higher yield than the methyl
malonate 6a (entries 4 and 5). The reaction proceeds
12
13
14
6
3
4
70
73
70
[a] 0.3 mmol of 2, Zn(BF4)2·xH2O (5 mol%) and 1 (1.1 equiv) was
reacted in CH2Cl2 (0.5 mL) at RT. [b] Yield of isolated product. [c] The
ratio of diastereomers was determined by 19F NMR analysis of the crude
reaction mixture. [d] Zn(BF4)2·xH2O (10 mol%) in CH2Cl2 (0.3 mL) was
used. [e] The reaction was performed at 408C. Ms=methanesulfonyl,
Ns=4-nitrobenzenesulfonyl.
alkenyl alcohols 4a–c (entries 1–3) could also be cyclized with
1 and Zn(BF4)2 under identical reaction conditions to those
used for the amino derivatives 2a–n. In fact the reaction times
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smoothly, even for the densely substituted substrate 6c, which
gave a cyclopentane derivative 7c having three quaternary
carbon atoms. The substrate 6d underwent facile cyclization
to give 7d with poor stereoselectivity (entry 7). The poor
stereoselectivity can be explained by the remote position of
the stereocenters and presence of the bulky carboxylate
groups.
the catalyst, the alkoxide in 1 may directly deprotonate the
substrate (Table 1, entry 1 and Table 3 entries 4–7). Alterna-
tively, the nucleophilic attack by the fluoride may follow the
deprotonation of the nitrogen atom. The high basicity of the
alkoxide moiety in 10 is probably one of the main reasons for
the broad synthetic scope of the reactions. In particular, the
successful achievement of the carbocyclization of 6a–d may
be due to the presence of a highly efficient internal base.
Furthermore, the alkoxide moiety is very bulky, therefore it
cannot compete with the tosyl amide (alcohol moiety in 4 or
the malonate in 6) in the nucleophilic attack of the
iodocyclopropylium cation in 9. Formation of this bulky
alkoxide is an inherent property of 1 and its activation
mechanism. When other hypervalent iodines are used for
fluorocyclizations, the internally formed base is often acetoxy
or a halogenide ion, which are usually less efficient for
deprotonation than the tertiary alkoxide ion generated from
1.
Based on the above results we propose a mechanism for
the fluorocyclization reactions in Figure 2. For the clarity the
In summary, we have shown that 1 is an efficient reagent
for fluorocyclization-based aminofluorination, oxyfluorina-
tion, and carbofluorination. The processes are suitable for the
synthesis of hetero- and isocycles having tertiary and secon-
dary fluorine substituents. By using this method functional-
ized pyrrole, piperidine, azepane, pyrane, oxepane, and
cyclopentane derivatives can be synthesized. The reaction
requires activation of 1 by transition metals, such as Zn, Cu,
Ag, or Pd. The broad synthetic scope of the reaction probably
also depends on the in situ generation of an alkoxide base,
which ensures an efficient deprotonation in the nucleophilic
step of the process.
Figure 2. Proposed mechanism of the fluorocyclization reactions exem-
plified by the reaction of 2m to afford 3m (Table 2, entry 13).
Experimental Section
In a typical procedure (Method A) 0.3 mmol of either 2 or 4,
Zn(BF4)2·xH2O (5 mol%), and 1 (1.1 equiv) were stirred in CH2Cl2
(0.5 mL) at RT for three hours. Then the solvent was removed and the
product was purified by chromatography. The carbocyclization of the
malonate derivatives (6) was performed in a similar way (Method B),
except that 0.3 mmol of 6, [Cu(MeCN)4]BF4 (10 mol%), and
1 (1.5 equiv) were reacted at 408C for 8 h.
mechanism is given for the cyclization of 2m to 3m, which
probably can be extended to the other substrates as well. As
mentioned above the reactions cannot be performed without
a metal catalyst (Table 1, entry 11). Therefore, we suggest that
the metal catalyst, such as Zn(BF4)2, activates 1 to form 8. A
similar type of activation of the CF3 analogue using Zn salts
was reported by Togni and co-workers.[17] In the activated
complex 8 the hypervalent iodine has a low-lying empty
orbital (formed by cleavage of the iodine–oxygen bond),
which is easily accessible for the p electrons in the double
bond of 2m. Thus electrophilic addition of 8 to 2m leads to
the formation of the iodonium ion 9.[11a] Formation of a similar
type of iodonium ion has been invoked for reactions of Ar-IF2
reagents and its analogues.[6h,7h] The next step is probably
a nucleophilic attack at the least hindered corner of the
iodocyclopropylium cation in 9 to give 10. This step proceeds
with very high regioselectivity, as all the presented reactions
give only one regioisomer of the cyclic product. The next step
can be nucleophilic displacement of the hypervalent iodine by
the fluoride ion to give a tertiary fluoride functionality. The
alkoxide moiety in 10 probably deprotonates the nitrogen
atom to give the product 3m, 11, and regenerates the catalyst.
In the case of using Zn(BF4)2·xH2O as the catalyst, the water
may mediate the proton transfer from the substrate to the
alkoxide moiety. However, when [Cu(MeCN)4]BF4 is used as
Keywords: copper · cyclizations · fluorine · heterocycles ·
homogeneous catalysis
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Angew. Chem. 2015, 127, 8653–8657
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Received: April 14, 2015
Published online: June 9, 2015
Angew. Chem. Int. Ed. 2015, 54, 8533 –8537
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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