Catalyst-free geminal aminofluorination of ortho-sulfonamide-tethered alkylidenecyclopropanes via a Wagner-Meerwein rearrangement

Article abstract of DOI:10.1039/C8CC05634J

A catalyst-free intramolecular geminal aminofluorination of ortho-sulfonamide-tethered alkylidenecyclopropanes has been developed. The reaction proceeded through two SET processes with Selectfluor to give a fluorinated cyclopropylcarbinyl cation and a further Wagner-Meerwein rearrangement to generate a cyclobutyl carbocation, which undergoes intramolecular nucleophilic capture by amide to forge fluorinated cyclobuta[b]indoline derivatives. A polycyclic multi-fluorinated byproduct was also formed through a Ritter-type reaction in some cases.

Full text of DOI:10.1039/C8CC05634J

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Catalyst-free Geminal Aminofluorination of ortho-
Sulfonamide-Tethered
Alkylidenecyclopropanes
via
Received 00th January 20xx,
Accepted 00th January 20xx
Wagner-Meerwein Rearrangement†
Xing Fan,a Qiang Wang,b Yin Wei*a and Min Shi*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
pyridinium polyhydrogen fluoride solution was used as the
fluorination source in the presence of N-bromosuccinimide (NBS) as
A catalyst-free intramolecular geminal aminofluorination
of ortho-sulfonamide-tethered alkylidenecyclopropanes has
been developed. The reaction proceeded through twice SET oxidant. After that, numerous studies were reported with different
processes with Selectfluor to give the fluorinated
cyclopropylcarbinyl cation and the further Wagner-Meerwein
rearrangement to generate cyclobutyl carbocation, which
undergoes intramolecular nucleophilic capture by amide to
fluorine sources, including the use of trifluoro(fluor-oxy)methane
(CF3OF) discovered by Wakselman's group3c and hypervalent iodine
reagents developed by the groups of Murphy,3d Szabó,3e and
Wang,3f respectively. Moreover, the use of Selectfluor, NFSI and
HF·Py was reported by the groups of Katzenellenbogen,3g Huang,3h
Szabó3i and Gouverneur's group3j (Scheme 1a, eq 1). In 2013, Chen
and co-workers demonstrated the first visible-light-promoted
selective C-H gem-difluorination of benzylic hydrogens using
Selectfluor as a fluorine source.4a One year later, Tang's group
reported a method to form difluoromethylated arenes by the
activation of benzylic C-H bonds utilizing AgNO3 as catalyst and
Selectfluor as a fluorination reagent.4b Subsequently, Huang and co-
workers conducted the first streamlined strategy for asymmetric α-
alkynylation-α-fluorination of ynones with the assistance of chiral
phosphoric acid and a recyclable chiral amine auxiliary using
Selectfluor as a fluorine source4c (Scheme 1a, eq 2). Besides, the
gem-fluorofunctionalization of alkenes has also emerged as an
attractive strategy in introducing fluorine atom into small
molecules.5 In 2014, Szabó's group reported that treatment of
hypervalent fluoroiodane with AgBF4 could induce geminal
difluorination of styrenes via phenonium ion intermediates.5a,b In
the next year, Oyamada's group developed difluorination of
styrenes with PhI(OCOCF3)2 and pyridine·HF under oxidation
conditions.5c Furthermore, with regard to unactivated amino
alkenes, Toste and co-workers achieved a palladium-catalyzed
enantioselective gem-fluoroarylation through an oxidative Heck
reaction using Selectfluor as the fluorination reagent5d (Scheme 1a,
eq 3).
forge fluorinated cyclobuta[b]indoline derivatives.
A
polycyclic multi-fluorinated byproduct was also formed
through a Ritter-type reaction in some cases.
Introducing a fluorine atom into organic scaffolds can significantly
improve their physical and chemical properties such as higher
thermal stability, higher stability to metabolic degradation,
increased lipophilicity and lower polarity as well as weaker
intermolecular interactions, which are of great significance in
pharmaceuticals,1a agrochemicals,1b material sciences,1d and
biochemical imaging.1c Therefore, the synthetic methods on
fluorofunctionalizations of small molecules to access various
substances containing fluorine atom have attracted a substantial
amount of attention from organic chemists. Thus far, although
great progress has been made in the fluorofunctionalization of
small molecules at vicinal carbon atoms,2 introducing a fluorine
atom together with another functional group at the geminal carbon
atom remains challenging.3-5
Among
the
reported
examples
in
the
geminal
fluorofunctionalization, the use of diazo compounds as substrates
has been extensively investigated so far.3 Pioneering studies on
hydrofluorination and halofluorination of diazoalkanes and
diazoketones were provided by Olah and co-workers,3a,b in which
Our
group
has
reported
a
fluorination
of
a.State Key Laboratory of Organometallic Chemistry, Center for Excellence in
Molecular Synthesis, University of Chinese Academy of Sciences, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road,
Shanghai 200032, China. weiyin@mail.sioc.ac.cn. mshi@mail.sioc.ac.cn.
b.Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School
of Chemistry & Molecular Engineering, East China University of Science and
Technology, Meilong Road No. 130, Shanghai 200237, China
methylenecyclopropanes (MCPs) and alkylidenecyclopropanes
(ACPs) with NFSI in 2009, affording ring-opened fluorination
products in good yields.6a Later, Ji's group also developed a similar
fluorination process with NFSI or Selectfluor (Scheme 1b, eq 4).6b
These two reports indicated that the ring-opening of MCPs or ACPs
can also provide driving force for introducing fluorine atom into
†
Electronic Supplementary Information (ESI) available: Experimental
procedures, characterization data of new compounds, and CCDC 1474587,
1838252, and 1504263. See DOI: 10.1039/b000000x
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small molecules. More recently, we also presented
a
Pd(II)- the better result8c (Table 1, entry 19) (see Supporting Information
catalyzed gem-oxyamination of urea-tethered ACPs through a ring for more details).
expansion of ACP along with the nucleophilic attack of nitrogen
Table 1. Optimization of Conditionsa,b,c,d
.
atom onto the in situ generated palladium carbenoid species, giving
the corresponding cyclobuta[b]indolines6c (Scheme 1b, eq 5).
Herein, we wish to report
aminofluorination of
a
novel strategy for the gem-
ortho-sulfonamide-tethered
alkylidenecyclopropanes (ACPs) upon treatment with Selectfluor
under catalyst-free condition (Scheme 1c, this work).
With the optimized conditions in hand, we next investigated the
substrate scope and its limitations, and the results are summarized
in Scheme 3. Overall, most of the reactions proceeded smoothly
under the optimal conditions, giving the desired products in
moderate to good yields except for substrates 1f and 1u. At first,
substrates 1b to 1e, having different sulfonyl protecting groups,
were examined, yielding the desired products 3b to 3e in 32-69%
yields. Substrate 1f, bearing a methyl group, did not give the
desired product under the optimal conditions. In all these cases, 3a
was obtained in higher yield. Therefore, tosyl group was used as the
protecting group to the other substrates. Substrates 1g-1i with
different alkyl substituents afforded the corresponding products 3g-
3i in good yields ranging from 71-90% yields. Introducing halogen
atoms on the aromatic ring produced the corresponding products
3j-3q in moderate to high yields ranging from 39%-98%. It should be
noted that the yield of 3 decreased along with the increasing in the
carbon number of alkyl chain in the cases of substrates 1j vs 1k, 1l
vs 1m vs 1n and 1o vs 1p vs 1q. Moreover, phenyl substituent is also
tolerated, furnishing the desired products 3r-3t in moderate yields.
The structure of 3t was confirmed by X-ray diffraction (see
Supporting Information). However, for these substrates bearing
electron-rich aromatic rings, the reactions did not proceed
effectively as those of electron-poor ones. For example, in the case
of substrate 1u bearing a methoxyl group at the benzene ring, the
desired product was observed in less than 5% yield under identical
conditions although the starting materials were completely
consumed. For substrate 1v, the reaction gave the desired product
3v in 36% yield within 5 h. Furthermore, in the case of substrate 1w,
difluorinated product 3w' was yielded in 23% yield along with the
desired product 3w in 6% yield under the standard reaction
conditions, suggesting that strong oxidation conditions are not
beneficial to the electron-rich substrates. The structure of 3w' was
Scheme 1. Strategies for gem-Fluorofunctionalization Reactions and Fluorination and
Acetoxylation of MCPs or ACPs and This Work.
Scheme 2. Our Strategies for gem-Aminofluorination of ortho-Sulfonamide-tethered
Alkylidenecyclopropanes.
To the best of our knowledge, gem-aminofluorination is a
much less explored area in synthetic organofluorine
chemistry.3i,k
We
envisaged
that
fluorinated
cyclopropylcarbinyl cation formed from the oxidation of ACPs
with Selectfluor could go through a Wagner-Meerwein
rearrangement along with the intramolecular nucleophilic
capture by the amide in sequence, affording the corresponding
fluorinated cyclobuta[b]indoline derivatives7 (Scheme 2).
We initiated the investigation by examining the gem-
aminofluorination of 1a with Selectfluor 2a and found that using 3.0
equivalents of 2a in CH3CN afforded the desired product 3a in 68%
yield (Table 1, entry 1). The use of Selectfluor 2b gave 3a in 41%
yield (Table 1, entry 2). The examination of other fluorination
reagents 2c-2f revealed that they are totally ineffective in this
transformation (Table 1, entry 3). Carrying out the reaction in
MeNO2 gave 3a in 32% yield (Table 1, entry 4) and the other
solvents are not suitable for this reaction (Table 1, entry 5). To
improve the yield of 3a, we next examined additives such as InF3,8a
bases1k and acids8b and realized that adding 10 equivalents of acetic
acid in CH3CN produced 3a in 89% NMR yield and 81% isolated yield
under otherwise identical conditions (Table 1, entries 6-18). Further
increasing the amount of acetic acid to 100 equivalents did not give
2 | J. Name., 2012, 00, 1-3
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confirmed by X-ray diffraction (see Scheme 4 and Supporting Information for more information). Interestingly, the addition of
Information for more information). Substrates 1x and 1y having acetic acid could slightly raise the yield of desired product 3ab from
cyclopentyl and cyclohexyl groups also afforded the desired 42% to 45% but impair the formation of 4ab (entries 2 and 3).
products 3x and 3y in 54% and 45% yields, respectively.
Additionally, tosylamide-tethered alkylidenecyclobutane 1z was
compatible in this reaction, giving the corresponding product 3z in
87% yield.
Scheme 5. Substrate Scope of the Reactiona,b,c,d
.
To obtain more insights into the reaction mechanism, the
reaction of 1a with Selectfluor 2a was conducted in the presence of
2,2,6,6-tetramethylpiperidinooxy (TEMPO) under the standard
conditions. We found that the use of catalytic amount of TEMPO
(0.2 eq) reduced the yield of 3a from 81% to 77% along with the
recovery of 1a in 11%, while, the use of 3.0 eq of TEMPO prohibited
the reaction proceeding and 1a was recovered in 95% yield along
with the decomposition of Selectfluor, which has been confirmed
by HRMS (see Supporting Information at S96). In addition, the
reaction proceeded very well under darkness, suggesting that
photo-irradiation does not accelerate the reaction proceeding (see
Supporting Information for more details).[12]
Scheme 3. Substrate Scope for Synthesis of 3a,b,c
.
The lower yields of electron-rich substrates 3u-3w and the
formation of difluorinated product 3w' warned us that the strong
oxidation property of Selecfluor may be harmful to this reaction.
Therefore, we reduced the amount of Selectfluor in the reaction
with 1w and the results are summarized in Scheme 4. The use of 1.0
eq of Selecfluor 2a afforded the desired product 3w in 45% yield
along with 3w' in 9% yield and 25% of substrate 1w. Using 3.0 eq of
2a gave the desired product 3w in 6% yield along with 23% of 3w'
within 6 h. Product 3w decomposed and the yield of difluorinated
product 3w' decreased after prolonging the reaction time to 24 h.
Thus, it is obvious that both electron-rich substrates and products
were instable under the oxidation conditions.[9]
Scheme 6. Control Experiments and Proposed Mechanism.
A plausible mechanism for this geminal aminofluorination of
functionalized ACPs is outlined in Scheme 6. The reaction is initiated
by Selectfluor 2a through twice SET processes[12b,c] to generate the
Scheme 4. The Strong Oxidation Property of Selectfluor 2aa,b
.
To clarify the reaction mechanism and identify the intermediacy
of fluorinated cyclopropylcarbinyl cation,[10] we performed the
further exploration using two aromatic rings substituted substrates
such as 1aa-1ac, which would enable to stabilize the in situ
generated benzyl carbocation (Scheme 5).[10e] The crude products
have been checked by 1H and 19F NMR spectroscopy, and we
identified a new product for the use of substrates 1ab and 1ac. The
structure of this new fluorinated product 4ac was determined by X-
ray diffraction,[6ab,10,11] suggesting the existence of fluorinated
fluorinated cyclopropylcarbinyl cation I, which undergoes
a
Wagner-Meerwein rearrangement to forge intermediate II along
with a cascade ring closure process by the intramolecular capture of
the amide to afford the product 3 (blue frame). Alternatively, the
intermediate I can undertake a Ritter-type reaction process to give
intermediate III,[6a,13] which generates a benzylic cation IV upon
treating with Selectfluor. Trapping the formed cation by the
acetamido group and addition of the sulfonamide group on the
formed ring along with fluorination of nitrogen atom[14] afford the
cyclopropylcarbinyl cation (see Scheme
6 and Supporting
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Catal., 2016,
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multi-fluorinated polycyclic product 4 (green frame).
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Gouverneur, Org. Lett., 2014, 16, 6004; (k) J. Huang, L. Y. Li,
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In summary, we have disclosed a novel intramolecular cascade
gem-aminofluorination
of
ortho-sulfonamide-tethered
alkylidenecyclopropanes, proceeding through twice SET processes
to form the fluorinated cyclopropylcarbinyl cation using Selectfluor
as a fluorination reagent under catalyst-free conditions. The further
Wagner-Meerwein rearrangement gives a cyclobutyl carbocation,
which can be captured by the amide in an intramolecular manner to
produce a variety of fluorinated cyclobuta[b]indolines in moderate
to good yields. The mechanistic investigations suggested that this
reaction proceeds through twice SET processes and the side
reaction could afford the Ritter-type polycyclic multi-fluorinated
product. Further investigations on the application of this
methodology to synthesize more interesting fluorinated
compounds are underway in our laboratory.
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Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgment
We are grateful for the financial support from the National Basic
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A
catalyst-free intramolecular geminal aminofluorination of
Catalyst-free Geminal Aminofluorination of ortho-
Sulfonamide-Tethered Alkylidenecyclopropanes via
Wagner-Meerwein Rearrangement
ortho-sulfonamide-tethered alkylidenecyclopropanes has been
developed to forge fluorinated cyclobuta[b]indoline derivatives.
A polycyclic multi-fluorinated byproduct was also formed
through a Ritter-type reaction in some cases.
Xing Fan,a Qiang Wang,b Yin Wei*a and Min Shi*a,b
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