Transition-Metal-Free Coupling of Polyfluorinated Arenes and Functionalized, Masked Aryl Nucleophiles

Article abstract of DOI:10.1002/chem.202101731

A chemoselective C(sp²)?C(sp²) coupling of sufficiently electron-deficient fluorinated arenes and functionalized N-aryl-N’-silyldiazenes as masked aryl nucleophiles is reported. The fluoride-promoted transformation involves the in situ generation of the aryl nucleophile decorated with various sensitive functional groups followed by a stepwise nucleophilic aromatic substitution (S_NAr). These reactions typically proceed at room temperature within minutes. This catalytic process allows for the functionalization of both coupling partners, furnishing highly fluorinated biaryls in good yields.

Full text of DOI:10.1002/chem.202101731

Accepted Article
Accepted Article
Title: Transition-Metal-Free Coupling of Polyfluorinated Arenes and
Functionalized, Masked Aryl Nucleophiles
Authors: Lucie Finck and Martin Oestreich
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To be cited as: Chem. Eur. J. 10.1002/chem.202101731
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01/2020

10.1002/chem.202101731
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Transition-Metal-Free Coupling of Polyfluorinated Arenes and
Functionalized, Masked Aryl Nucleophiles
Lucie Finck and Martin Oestreich*
Abstract: A chemoselective C(sp2)–C(sp2) coupling of sufficiently
electron-deficient fluorinated arenes and functionalized N-aryl-N’-
silyldiazenes as masked aryl nucleophiles is reported. The fluoride-
promoted transformation involves the in-situ generation of the aryl
nucleophile decorated with various sensitive functional groups
followed by a stepwise nucleophilic aromatic substitution (SNAr).
These reactions typically proceed at room temperature within
minutes. This catalytic process allows for the functionalization of
both coupling partners, furnishing highly fluorinated biaryls in good
yields.
studying a new class of kinetically stable and easily accessible
silicon-based aryl pronucleophiles: N-aryl-N’-silyldiazenes.[13]
Upon initiation with a silaphilic Lewis base in catalytic amounts,
highly reactive aryl alkali metal reagents are generated in situ.[14]
In this context, we considered that these versatile
pronucleophiles would be a suitable alternative to displace the
strong C(sp2)–F bond in sufficiently electron-deficient
fluoroarenes through a stepwise SNAr (Scheme 1, bottom).[15]
We herein report a transition-metal-free coupling reaction of
functionalized aryl nucleophiles and polyfluoroarenes through
the intermediacy of stabilized Meisenheimer-type intermediates
(Scheme 1, gray box).
Polyfluorinated biaryls have been identified as versatile and
robust building blocks possessing interesting properties relevant
for agrochemicals, pharmaceuticals, and materials science.[1,2]
Hence, the development of viable synthetic strategies for their
preparation through C(sp2)–C(sp2) bond-forming reactions
continues to attract significant interest.[3] Typical approaches to
access this motif engage transition-metal catalysts (Scheme 1,
top left).[4–7] Among them, conventional cross-coupling[4] and
oxidative
prefunctionalization of
direct
arylation[5]
coupling partner while cross-
generally
require
the
a
dehydrogenative coupling[6] involves non-prefunctionalized
arenes. These transformations normally operate at elevated
temperature and require long reaction times. As a result, the
chemoselectivity of these methods can be deteriorated, and
sensitive functional groups poorly tolerated, especially on the
electron-deficient aromatic ring. Also, transition-metal-free
approaches have been far less explored.[8–11] Radical couplings
have been identified as a valuable alternative (Scheme 1, top
right).[8] Recently, König and co-workers disclosed
a
photocatalytic process involving formal C–H bond activation that
proceeds at 40 °C yet requiring 72 h.[9] While the polyfluorinated
arylbromide as the other component is readily available, a large
excess (20 equiv) was necessary and the regio- and
chemoselectivity were moderate.
Scheme 1. Synthetic methodologies for the preparation of polyfluorinated
biaryls through C(sp2)–C(sp2) bond-forming reactions. Ar = (hetero)aryl group,
ArF = polyfluorinated (hetero)aryl group, TM = transition metal, X and Y =
(pro)nucleophile or (pseudo)halogen or H, FG = functional group, R = alkyl or
aryl group, n = 4–9.
Alternatively, these synthetically useful biaryls can be
prepared by SNAr at the electron-deficient fluorinated arene with
main-group organometallic reagents. This approach formerly
found application in materials science making use of
organolithium reagents.[12] However, this strategy suffers from
the lack of control in the degree of substitution, leading to
mixtures of mono- and polyarylated fluoroarenes.[10] Li and co-
workers addressed this shortcoming by employing Grignard
reagents, yet with a limited functional-group tolerance on each
aromatic ring, possibly due to the high reactivity of these polar
reagents (Scheme 1, middle).[11] Our laboratory has been
We began our investigation by optimizing the fluoride-
promoted coupling of the 4-tolyl-substituted diazene 1a and
hexafluorobenzene (2a) at room temperature (Table 1). Guided
by our previous work,[13] an initial experiment was performed
using a catalytic amount of CsF with 18-crown-6 as an additive
in THF, affording the polyfluorinated biaryl 3aa after 2 h in 77%
yield (entry 1). Owing to the three-fold excess of 2a employed,
only a minor amount of a regioisomeric mixture of terphenyl 4a
formed. As expected, reducing the amount of the fluoroarene 2a
resulted in an increase of bisarylated 4a (see the Supporting
Information for the complete optimization). Moreover, diazene 1a
is prone to degradation into the corresponding arylsilane 5a
[*]
L. Finck, Prof. Dr. M. Oestreich
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
Homepage: https://www.organometallics.tu-berlin.de
upon activation with
a Lewis base (see the Supporting
Information for details). To prevent this competitive reaction, a
catalytic system without 18-crown-6 was tested. The “less
Supporting information for this article is given via a link at the end of
the document.
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naked” fluoride ion considerably improved the selectivity of the
reaction in favor of the desired coupling product, yet at a
prolonged reaction time (entry 2). With these promising results
as a starting point, we examined the influence of the solvent. It is
known that the solubility of alkali metal fluorides is generally
enhanced in DMF compared to other commonly used polar
aprotic solvents.[16] The reaction conducted in DMF effectively
exhibited both a good selectivity and an improved reaction
kinetics without additives, affording the coupling product 3aa in
excellent yield in less than five minutes (entry 3). Additionally,
various fluoride-based catalysts were probed, albeit CsF had
proven to be a competent catalyst initiating the N–Si bond
cleavage.[17] Alkali metal salts provided slow reaction kinetics
(entry 4) or hardly any conversion (entries 5 and 6), likely owing
to their insolubility.[16] In turn, the more soluble
tetraalkylammonium fluorides rapidly mediated the reaction.
While TBAT afforded 3aa in good yield (entry 7), TMAF
furnished an inferior yield resulting from competing reactions
(entry 8). Similarly, the trimethylsilanolate salt KOSiMe3
(generating the poorly soluble KF in situ) successfully promoted
the reaction albeit with slower reaction rate (entry 9). It is
important to mention that the latter initiator can potentially
engage in SNAr with polyfluoroarenes.[18] Ultimately, a control
experiment confirmed that there is no reaction in the absence of
the initiator (entry 10).
10[e]
none
DMF
24 h
0
0
[a] All reactions were performed on a 0.10 mmol scale in 0.7 mL (0.1 M) of
the indicated solvent. [b] Determined by calibrated GLC analysis with
tetracosane as an internal standard. [c] CsF/18-crown-6 (1.0:1.2 molar
ratio). [d] Yield of isolated product on a 0.30 mmol scale after flash
chromatography on silica gel in parentheses. [e] Incomplete conversion of
1a. TBAT
= tetrabutylammonium difluorotriphenylsilicate, TMAF =
tetramethylammonium fluoride.
With optimized reaction conditions in hand (Table 1, entry 3),
we explored the substrate scope of this transformation
(Schemes 2 and 3). We started with investigating the coupling
reaction of various aryl-substituted silyldiazenes 1a–o and
hexafluorobenzene (2a) at room temperature (Scheme 2). It is
noteworthy that the addition of the 18-crown-6 was required for
specific substrates that otherwise exhibited a poor reactivity. A
broad range of electron-donating and -withdrawing functional
groups were compatible with our method, affording the coupling
products within a few minutes in high yields. Diazenes bearing
sensitive substituents such as cyano (as in 1d),
methoxycarbonyl (as in 1e), nitro (as in 1f), and trifluoromethyl
(as in 1k) were effectively transferred. A larger excess of
fluoroarene 2a was necessary to provide 3ka in 80% yield due
to the competing degradation of the diazene 3k into the cognate
arylsilane. Coupling products 3ga–ka with halogen substituents
could be successfully accessed. Competitive halogen–metal
exchange was observed for both iodo- and bromo-substituted
aryl nucleophiles 1g and 1h, respectively; the corresponding
Table 1: Selected examples of the optimization of the fluoride-promoted
preparation of polyfluorinated biaryls.[a]
1,4-dihalogenated
benzene
derivatives
and
1,4-
bis(pentafluorophenyl)benzene were found in small quantities
(see the Supporting Information for details). Additionally, the
ortho-fluoro-substituted diazene 1l reacted equally well by
delivering 3la in 70% yield; the possible β-elimination giving the
corresponding aryne intermediate was not seen. Increasing the
steric bulk in the ortho-positions as in 1m and 1n was not
detrimental. Even the bisdiazene 1p, equivalent to an aryl
bisnucleophile, smoothly underwent the SNAr to furnish the
valuable building block 3pa.[19]
Entry
Catalyst
Solvent
Time
Yield [%][b]
5a
3aa
77
1
CsF/18-C-6[c]
CsF
THF
THF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
2 h
15
5
2
48 h
91
3
CsF
< 5 min
24 h
92 (79)[d]
57
7
4[e]
5[e]
6[e]
7
KF
5
NaF
24 h
trace
trace
89
trace
trace
7
LiF
24 h
TBAT
TMAF
KOSiMe3
< 5 min
< 5 min
24 h
8
62
16
3
9[e]
30
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Scheme 2. Scope I: Coupling of various functionalized silylated aryldiazenes
with hexafluorobenzene (2a). All reactions were performed on a 0.30 mmol
scale at room temperature. Yields are isolated yields after flash
chromatography on silica gel. [a] CsF/18-crown-6 (1.0:1.2 molar ratio). [b] 10
equiv of C6F6 (2a) used. [c] The reaction time was 3 h. [d] The reaction time
was 1 h. [e] 20 equiv of C6F6 (2a) used. R = Me, Et.
To further illustrate the chemoselectivity of our method we
achieved the coupling reaction of various aryl-substituted
silyldiazenes with functionalized polyfluoroarenes 2b–j (Scheme
3). The construction of such biaryls possessing functional-group
diversity on both coupling partners is not trivial, and existing
strategies for their preparation are scarce. Considering that the
transformation furnishes
a
mixture of regioisomers,[20] our
primary intention was to investigate the influence of the solvent
on the regioisomers distribution. To this end, we screened
various solvents for the coupling of pentafluoropyridine (2b) and
diazene 1k (see the Supporting Information for details).
Accordingly, we established that the coupling performed in DMF
proceeded with a good level of regiocontrol while it did not in
ethereal solvents. All the reactions were therefore conducted in
DMF to produce the para- or β-substituted regioisomer,
respectively with good to moderate selectivities; the products
were isolated in a regioisomerically pure form after flash
chromatography on silica gel. The pyridine derivative 2b was
hence successfully coupled with diazene 1k to provide 3kb in
67% yield. The trifluoromethyl group was also compatible on the
electron-deficient
ring,
furnishing
3bc
and
3ic.
Pentafluorobenzonitrile
(2d) exhibited
a
moderate
regioselectivity, likely due to the increased electrophilic
character of the ortho-position arising from the neighbouring
cyano group.[20] Competing arylation of the carbonyl group in
benzophenone derivative 2e did not occur, and the
functionalized coupling product 3ie was obtained in excellent
yield and with good regioselectivity. In line with these findings,
polyfluorinated naphtalene 2f and biphenyl 2g displayed a high
reactivity and satisfying selectivity. The steric congestion around
the ortho-position of biphenyl 2g exerted a non negligible effect
on the regioselectivity of the transformation as the SNAr was
partially steerd towards the usually less favorable meta-position.
Scheme 3. Scope II: Coupling of various silylated aryldiazenes with
functionalized polyfluorarenes. All reactions were performed on a 0.30 mmol
scale at room temperature. Regioisomeric ratios (para:ortho:meta or β:α) were
determined by quantitative 19F NMR analysis with hexafluorobenzene as an
internal standard. Yields are isolated yields of para (β for 3hf) regioisomers
after purification. [a] 10 equiv of the polyfluorarene used. [b] The reaction time
was 12 h. [c] Yield of isolated product on a 1.30 mmol scale after flash
chromatography on silica gel in parentheses. [d] Yield determined by
quantitative 19F NMR analysis with hexafluorobenzene as an internal standard.
n.d. = not determined.
Although our method displayed good chemoselectivity and
regiocontrol, a few substrates were not suitable (Scheme 3, gray
box). The less reactive pentafluoroanisole (2h) reacted poorly,
and the diazene 1h instantaneously decomposed into the
corresponding
arylsilane.
Chloropentafluorobenzene
(2i)
underwent mainly halogen–metal exchange, leading to
oligomeric mixtures of chloroperfluoropolyphenyls.[21] As a result
of the high basicity of the in-situ released aryl nucleophile,
pentafluorobenzene (2j) was deprotonated and further
decomposed (see the Supporting Information for details).
Similarly, 1,2-difluorobenzene underwent ortho-formylation
through deprotonation followed by addition to the solvent (not
shown, see the Supporting Information for details). Nucleophilic
addition of the aryl nucleophile to DMF had never been
observed with our system before.[14a]
To demonstrate the synthetic utility of the coupling products,
difficult-to-make partially fluorinated biaryls 3ck and 3dk
containing a styrene unit were prepared and further derivatized
to trans-stilbene derivatives (Scheme 4). Accordingly,
pentafluorostyrene (2k) reacted chemoselectively with the aryl
pronucleophiles 1c and 1d in good yields under the standard
reaction conditions. The electronic properties of the substituents
attached to the diazene (OMe for 1c and CN for 1d) had no
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effect on the regioselectivity (Scheme 4, top). Compounds 3ck
and 3dk were subsequently converted into the partially
fluorinated stilbene derivatives 7cka and 7dkb by a Heck
reaction in high yields (Scheme 4, gray box).
[1]
[2]
[3]
T. Hiyama, Organofluorine Compounds: Chemistry and Applications,
Springer, Heidelberg, 2000.
P. Kirsch, Modern Fluoroorganic Chemistry: Synthesis, Reactivity,
Applications, Wiley-VCH, Weinheim, 2004.
For recent reviews, see: a) H. Amii, K. Uneyama, Chem. Rev. 2009,
109, 2119-2183; b) M. He, J.-F. Soulé, H. Doucet, ChemCatChem 2014,
6, 1824-1859; c) T. Ahrens, J. Kohlmann, M. Ahrens, T. Braun, Chem.
Rev. 2015, 115, 931-972; d) T. D. Moseev, M. V. Varaksin, D. A. Gorlov,
V. N. Charushin, O. N. Chupakhin, Org. Biomol. Chem. DOI:
10.1039/d1ob00472g.
[4]
[5]
[6]
For recent examples of the preparation of polyfluorinated biaryls under
transition-metal-catalyzed cross-coupling, see: a) R. Shang, Y. Fu, Y.
Wang, Q. Xu, H.-Z. Yu, L. Liu, Angew. Chem. Int. Ed. 2009, 48, 9350-
9354; Angew. Chem. 2009, 121, 9514-9518; b) M. Ohashi, R. Doi, S.
Ogoshi, Chem. Eur. J. 2014, 20, 2040-2048; c) V. V. Bardin, A. Y.
Shabalin, N. Y. Adonin, Beilstein J. Org. Chem. 2015, 11, 608-616; d) L.
Chen, D. R. Sanchez, B. Zhang, B. P. Carrow, J. Am. Chem. Soc. 2017,
139, 12418-12421; e) Y. P. Budiman, A. Friedrich, U. Radius, T. B.
Marder, ChemCatChem 2019, 11, 5387-5396; f) Z.-J. Luo, H.-Y. Zhao,
X. Zhang, Org. Lett. 2018, 20, 2543-2546.
For recent examples of the preparation of polyfluorinated biaryls under
transition-metal-catalyzed oxidative direct arylation, see: a) M. Lafrance,
C. N. Rowley, T. K. Woo, K. Fagnou, J. Am. Chem. Soc. 2006, 128,
8754-8756; b) H.-Q. Do, O. Daugulius, J. Am. Chem. Soc. 2008, 130,
1128-1129; c) M. Simonetti, G. J. P. Perry, X. C. Cambeiro, F. Julia-
Hernandez, J. N. Arokianathar, I. Larrosa, J. Am. Chem. Soc. 2016,
138, 3596-3606; d) M. Hofer, A. Genoux, R. Kumar, C. Nevado, Angew.
Chem. Int. Ed. 2017, 56, 1021-1025; Angew. Chem. 2017, 129, 1041-
1045; e) A. Dahiya, C. Fricke, F. Schoenebeck, J. Am. Chem. Soc.
2020, 142, 7754-7759.
Scheme 4. Scope III and IV: Construction of polyfluorinated biaryls having a
styrene unit (top) and further diversification of coupling products by a Heck
reaction (bottom). Regioisomeric ratios (para:ortho:meta) were determined by
quantitative 19F NMR analysis with hexafluorobenzene as an internal standard.
Yields are isolated yields of para regioisomers after purification. Reaction
conditions for the Heck reaction: iodobenzene (6a, 1.5 equiv, for 3ck) or
methyl 4-iodobenzoate (6b, 1.5 equiv, for 3dk), Pd(OAc)2 (2.0 mol%), Et3N
(5.0 equiv), DMF, 100 °C, 20 h.
For recent examples of the preparation of polyfluorinated biaryls under
transition-metal-catalyzed cross-dehydrogenative coupling, see: a) Y.
Wei, W. Su, J. Am. Chem. Soc. 2010, 132, 16377-16379; b) C.-Y. He,
S. Fan, X. Zhang, J. Am. Chem. Soc. 2010, 132, 12850-12852; c) R. G.
Kalkhambkar, K. K. Laali, Tetrahedron Lett. 2011, 52, 5525-5529; d) H.
Li, J. Liu, C.-L. Sun, B.-J. Li, Z.-J. Shi, Org. Lett. 2011, 13, 276-279; e)
X. C. Cambeiro, N. Ahlsten, I. Larrosa, J. Am. Chem. Soc. 2015, 137,
15636-15639.
[7]
[8]
S. Senaweera, J. D. Weaver, J. Am. Chem. Soc. 2016, 138, 2520-2523
(photocatalytic C–F activation).
To summarize, a reliable method for the preparation of
partially fluorinated, functionalized biaryls proceeding through an
SNAr has been developed. This strategy relies on the fluoride-
initiated, in-situ release of aryl nucleophiles decorated with
sensitive substituents from readily available N-aryl-N’-
silyldiazenes.[13] Compared to the prior art, the new transition-
metal-free protocol operates under mild reaction conditions and
requires short reaction times, thereby securing excellent
functional group tolerance.
For early investigations on the metal-free preparation of polyfluorinated
biaryls through radical coupling, see: a) Q.-Y. Chen, Z. T. Li, J. Org.
Chem. 1993, 58, 2599-2604; b) Q.-Y. Chen, Z.-T. Li, J. Chem. Soc.,
Perkin Trans. 1 1993, 14, 1705-1710; c) D. Kosynkin, T. M. Bockman, J.
K. Kochi, J. Am. Chem. Soc. 1997, 119, 4846-4855; d) D. Kosynkin, T.
M. Bockman, J. K. Kochi, J. Chem. Soc., Perkin Trans. 2 1997, 10,
2003-2012.
[9]
A. U. Meyer, T. Slanina, C.-J. Yao, B. Kꢀnig, ACS Catal. 2016, 6, 369-
375.
[10] For seminal work on the transition-metal-free preparation of
polyfluorinated biaryls through nucleophilic aromatic substitution, see:
a) P. J. N. Brown, R. Stephens, J. C. Tatlow, Tetrahedron, 1967, 23,
4041-4045; b) W. L. Respess, C. Tamborski, J. Organomet. Chem.
1970, 22, 251-263; c) L. S. Chen, G. J. Chen, C. Tamborski, J.
Organomet. Chem. 1981, 215, 281-291.
Acknowledgements
This
research
was
supported
by
the
Deutsche
Forschungsgemeinschaft (Oe 249/23-1). M.O. is indebted to the
Einstein Foundation Berlin for an endowed professorship. We
thank Dr. Clément Chauvier (postdoctoral fellow funded by the
Alexander von Humboldt Foundation, 2018–2019) for his
contributions in the early phase of this study.
[11] Y. Sun, H. Sun, J. Jia, A. Du, X. Li, Organometallics 2014, 33, 1079-
1081.
[12] D. J. Morrison, T. K. Trefz, W. E. Piers, R. McDonald, M. Parvez, J. Org.
Chem. 2005, 70, 5309-5312.
[13] For the preparation of the N-aryl-N’-silyldiazenes, see: C. Chauvier, L.
Finck, S. Hecht, M. Oestreich, Organometallics 2019, 38, 4679-4686.
[14] For a recent application of the N-aryl-N’-silyldiazenes, see: a) C.
Chauvier, L. Finck, E. Irran, M. Oestreich, Angew. Chem. Int. Ed. 2020,
59, 12337-12341; Angew. Chem. 2020, 132, 12436-12440; see also: b)
J. C. Bottaro, J. Chem. Soc. Chem. Commun. 1978, 990.
[15] For a relevant review on SNAr reactions, see: A. J. J. Lennox, Angew.
Chem. Int. Ed. 2018, 57, 14686-14688; Angew. Chem. 2018, 130,
14898-14900.
Conflict of interest
The authors declare no conflict of interest.
Keywords: biaryls • chemoselectivity • Lewis bases •
nucleophilic aromatic substitution • silicon
[16] D. A. Wynn, M. M. Roth, B. D. Pollard, Talanta 1984, 31, 1036-1040.
[17] J. H. Clark, Chem. Rev. 1980, 80, 429-452.
[18] K. Bürglová, J. Hlaváč, Synthesis 2018, 50, 1199-1208.
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[19] M. Schnitte, S. Lipinski, E. Schiebel, S. Mecking, Organometallics 2020,
39, 13-17.
[20] For a study on the regioselectivity of SNAr reactions with substituted
polyfluoroarenes, see: R. D. Chambers, P. A. Martin, G. Sandford, D. L.
H. Williams, J. Fluorine Chem. 2008, 129, 998-1002.
[21] G. A. Artamkina, P. K. Sazonov, V. A. Ivushkin, I. P. Beletskaya, Chem.
Eur. J. 1998, 4,1169-1178.
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Suggestion for the Entry for the Table of Contents
COMMUNICATION
L. Finck, M. Oestreich*
Page No. – Page No.
Transition-Metal-Free Coupling of
Polyfluorinated Arenes and
Functionalized, Masked Aryl
Nucleophiles
A fluoride-promoted, stepwise nucleophilic aromatic substitution of silicon-based
aryl pronucleophiles decorated with sensitive functional groups and polyfluorinated
arenes enables the synthesis of partially fluorinated biaryls. The mild transformation
operates at room temperature, requiring less than one hour for completion. Hence,
the functional-group compatibility is superb, and regioselectivities are improved
compared to known couplings.
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