Ni-Catalyzed Borylation of Aryl Fluorides via C-F Cleavage

Article abstract of DOI:10.1021/jacs.5b08103

A Ni-catalyzed borylation via C-F activation is described. This protocol is distinguished by a wide scope, including unactivated fluoroarenes, without compromising its efficiency and scalability, thus representing a significant step-forward toward the implementation of C-F activation protocols.

Full text of DOI:10.1021/jacs.5b08103

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Communication
Ni-catalyzed borylation of aryl fluorides via C-F cleavage
Xiang-Wei Liu, Javier Echavarren, Cayetana Zárate, and Ruben Martin
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Ni-catalyzed Borylation of Aryl Fluorides via C–F Cleavage
Xiang-Wei Liu†‡, Javier Echavarren†‡, Cayetana Zarate† and Ruben Martin†§*
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†Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain
§ Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluïs Companys, 23, 08010, Barcelona, Spain
Supporting Information Placeholder
larly activated fluoroarenes8 or heavily fluorinated aro-
ABSTRACT: A Ni-catalyzed borylation via C-F activa-
matic substrates9 (Scheme 2, path b). These features
tion is described. This protocol is distinguished by a
contribute to the perception that direct C–F activation of
unactivated fluoroarenes might constitute a daunting,
yet highly rewarding, scenario in C–heteroatom bond-
formation.10 If successful, this technique will offer un-
conventional new tactics in retrosynthetic analysis while
exploiting new opportunities in the C–F cleavage are-
na.11 Prompted by the versatility and pivotal role of or-
ganoboron reagents as reaction intermediates,12 as well
as our ongoing interest in inert bond-activation,13 we
report herein an unconventional Ni-catalyzed C–F bond-
cleavage/C–B bond-formation of monofluoroarenes
(Scheme 2, path c).14,15 This protocol represents a pow-
erful alternative to other borylation techniques based on
more reactive carbon-halide bonds,16 suggesting that
iterative scenarios might come into play when dealing
with the functionalization of polyhalogenated frame-
works. This method is characterized by its wide sub-
strate cope, and by obviating the need for stoichiometric
organometallic reagents, representing a significant step-
forward for the implementation of C–F bond-cleavage in
cross-coupling reactions.17 Preliminary mechanistic
studies suggest a scenario consisting of a C–F bond-
oxidative addition to Ni(0) complexes.
wide scope, including unactivated fluoroarenes, without
compromising its efficiency and scalability, thus repre-
senting a significant step-forward towards the imple-
mentation of C–F activation protocols.
Metal-catalyzed cross-coupling reactions of organic hal-
ides have become indispensable tools in modern synthet-
ic chemistry.1 The reactivity trend of organic halides is
inversely proportional to their bond-dissociation energy,
with C–F bonds arguably possessing the shortest bond
length in the organic halide series (Scheme 1).2 Indeed,
C–F bonds are the strongest C–heteroatom bonds in na-
ture, conferring a remarkable metabolic activity that
makes them particularly attractive in pharmaceuticals as
bioisosteres of C–H bonds.3 Not surprisingly, these fea-
tures have inspired chemists to design catalytic C–F
cleavage protocols,4 aiming at providing new tools for
lead generation in drug discovery.
Scheme 1. Bond-Strength and Prevalence of C–F Bonds.
Scheme 2. Catalytic C–F Bond-Cleavage Scenarios
In contrast with the wealth of literature data using C–X
bonds (X = I, Br, Cl), catalytic C–F bond-activation is
still relatively rare, an observation that is in line with the
exceptional strength of the C–F bond.4 Prompted by the
seminal work of Kumada,5 the vast majority of these
processes are still based on C–C bond-formation using
stoichiometric and highly reactive organometallic rea-
gents possessing polarized carbon-metal bonds (Scheme
2, path a).6 Alternatively, catalytic dehydrofluorination
events have recently gained considerable momentum.7
Interestingly, a close look into the literature data re-
vealed that C–heteroatom bond-formation has hardly
been considered, remaining essentially confined to nu-
cleophilic aromatic substitution techniques with particu-
We started our investigations with 1a as our model sub-
strate (Table 1). After judicious optimization of all reac-
tion parameters,18 we identified that a protocol consist-
ing of 2a, Ni(COD)2, PCy3 and NaOPh in THF was par-
ticularly suited for our purposes, affording 3a in a 81%
isolated yield (entry 1).19 As expected, the nature of the
ligand played a critical role on the reaction outcome;
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Table 2. Scope of the Borylation Event.a,b
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while otherwise related phosphine ligands did not pro-
1
vide even traces of 3a in the crude mixtures (entry 4), a
markedly inferior results were observed when employ-
ing N-heterocyclic carbenes (entries 2 and 3). Intriguing-
ly, the C–B bond-formation was found to be strongly
dependent on the metal:ligand ratio, with the commonly
employed 1:2 (Ni:L) ratio providing clearly inferior re-
sults.20 Interestingly, a significant erosion in yield was
observed when operating under Ni(II) regimes (entries 5
and 6) or by using [Ni(PCy3)2]2N2 (entry 7). While ten-
tative, these results suggest that COD might be acting as
a non-innocent ancillary ligand to stabilize the active
propagating Ni(0) species.21 We found negligible
amounts of 2a in the crude mixtures when employing
additives known to efficiently activate diboron reagents
such as CsF or NaOtBu (entries 9-11),22 showing that
NaOPh uniquely assisted the borylation event.23 At pre-
sent, we do not have an explanation for such distinctive
reactivity profile. Although a Suzuki-Miyaura coupling
scenario of 1a with in situ formed 3a might come into
play,6c we found that this was not the case.24 As antici-
pated, careful control experiments revealed that all reac-
tion parameters were critical for success (entries 12-14),
suggesting that a nucleophilic aromatic substitution
pathway might not come into play.8 Notably, no boryla-
tion was detected with B2pin2 (2b; entry 15), an observa-
tion that illustrates that a subtle balance of steric effects
on the boron reagent is required for the C–B bond-
forming reaction.
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Table 1. Optimization of the Reaction Conditions.a
a Reaction conditions: as for Table 1 (entry 1). b Isolated
c
yields, average of at least two independent runs. Using
Ni(COD)2 (10 mol%). d Reaction performed with 1k (1.0 g,
6.90 mmol scale).
Prompted by these results, we next focused our attention
on the generality of our Ni-catalyzed borylation protocol
(Table 2). From the results summarized in Table 2, it is
evident that our C–F bond-cleavage/C–B bond-
formation turned out to be rather general. Notably, the
reaction could be easily scaled-up on a gram scale with-
out further optimization, delivering 3k in essentially
quantitative yield (97%). It is worth noting that an oth-
erwise identical reactivity was found for regular mono-
fluoroarenes regardless of the electronic nature of the
aryl fluoride utilized (3m-3q), thus providing additional
compelling evidence that a nucleophilic aromatic substi-
tution scenario is highly unlikely.8 Remarkably, the reac-
tivity of regular monofluoroarenes was comparable to π-
extended systems, an observation that is in sharp con-
trast with other Ni-catalyzed inert bond-activation proto-
cols.25,26 As shown for 3f, the presence of an ortho-
substituent did not significantly hampered the reactivity.
The chemoselectivity of the borylation event was nicely
illustrated by the fact that amines (3o), silyl ethers (3r),
acetals (3d), esters (3t) or amides (3q and 3w), among
others, were perfectly accommodated.27 Similarly, we
a
Reaction conditions: 1a (0.50 mmol), 2a (1.50 mmol),
Ni(COD)2 (5 mol%), PCy3 (20 mol%), NaOPh (1.50
mmol), THF (2 mL; 0.25M) at 110 ºC for 12 h. b GC yields
c
using decane as internal standard. Isolated yield, average
d
of at least two independent runs. Using PCy3 (10 mol%).
B2pin2
=
bis(4,4,5,5,-tetramethyl-1,3,2-dioxaborolane.
B2nep2 = 5,5-dimethyl-1,3,2-dioxaborolane.
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found that nitrogen-containing heterocycles posed no trans coordination geometry. We corroborated its struc-
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problems (3u and 3v). Although Ni complexes are suited
for borylation events via C(sp2)–, C(sp3)–OMe13a,28 or
C(sp2)–NCOR cleavage,29 we found that the presence of
aryl(benzyl) methyl ethers or amides did not compete
with productive C–F cleavage/C–B bond-formation (3e,
3i, 3j, 3l, 3n and 3w). Interestingly, pinacolborane resi-
dues were well tolerated, allowing for preparing bifunc-
tional borylated arenes in a straightforward fashion
(3s).30 In light of these results, we anticipated that our
borylation event could be applied for late-stage diversi-
fication of advanced fluorinated intermediates such as
1x. As shown, this was indeed the case, and 3x could be
prepared under otherwise identical reaction conditions in
moderate yields. Taken together, the results compiled in
Table 2 clearly demonstrates the prospective impact of
our C–B bond-formation, even with unactivated sub-
strates, thus representing a significant step-forward for
increasing the overall synthetic utility of fluoroarenes as
new platforms for molecular diversity.
ture by an authentic sample of 4 that was prepared from
5 by anion metathesis.31 Importantly, upon exposure of 4
to B2nep2 and NaOPh in THF we found that 3k was ob-
tained in 64 % yield, an otherwise similar result to that
shown in Table 2 under catalytic conditions. While we
cannot certainly rule out other conceivable reaction
pathways,32 at present we support a scenario consisting
of an initial oxidative addition into the C(sp2)–F bond
(II) followed by boryl transfer assisted by NaOPh (III)
and a final C–B bond-reductive elimination, thus deliv-
ering the targeted borylated arene while regenerating the
active propagating Ni(PCy3)2 species (I).
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In summary, we have described the first Ni-catalyzed
borylation of monofluoroarenes. This work constitutes a
rare example of C–heteroatom bond-formation via cata-
lytic C–F cleavage of unactivated fluoroarenes. The pro-
tocol is distinguished by its wide scope without com-
promising its practicability, efficiency and scalability.
Further investigations aimed at promoting related C–
heteroatom bond-formations are currently ongoing in
our laboratories.
Scheme 3. Mechanistic Rationale
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and
spectral data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* rmartinromo@iciq.es
Author contributions
‡ These authors contributed equally to this work
Funding Sources
No competing financial interests have been declared.
ACKNOWLEDGMENT
We thank ICIQ, the European Research Council (ERC-
277883), MINECO (CTQ2012-34054 & Severo Ochoa
Excellence Accreditation 2014-2018, SEV-2013-0319) and
Cellex Foundation for support. Johnson Matthey, Umicore
and Nippon Chemical Industrial are acknowledged for a
gift of metal & ligand sources. C. Z. thank MINECO for a
FPU scholarship. We sincerely thank Dr. Fedor Miloserdov
for providing an authentic sample of 4 and Prof. Vladimir
V. Grushin for insightful mechanistic discussions.
Taking into consideration the high activation energy of
C–F bonds,2 it comes as no surprise that oxidative addi-
tion complexes via C(sp2)–F activation have essentially
been isolated and characterized with particularly activat-
ed polyfluoroarenes or substrates containing proximal
directing groups.4 In sharp contrast, a direct oxidative
addition of an unactivated monofluoroarene to Ni(0) via
C(sp2)–F cleavage has not yet been observed. To such
end, we decided to in situ monitor the course of the reac-
tion of 1k with Ni(COD)2/PCy3 in THF by NMR spec-
troscopy (Scheme 3). Notably, we cleanly observed a
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2014, 43, 8081. (b) Tobisu, M.; Chatani, N. Acc. Chem. Res.
2015, 48, 1717.
(29) Tobisu, M.; Nakamura, K.; Chatani, N. J. Am. Chem. Soc. 2014,
136, 5587.
(13) Recent selected references: (a) Zarate, C.; Manzano, R.; Martin,
R. J. Am. Chem. Soc. 2015, 137, 6754. (b) Cornella, J.; Jackson,
E. P.; Martin, R. Angew. Chem. Int. Ed. 2015, 54, 4075. (c)
Correa, A.; Martin, R. J. Am. Chem. Soc. 2014, 136, 7253. (d)
Zarate, C.; Martin, R. J. Am. Chem. Soc. 2014, 136, 2236. (e)
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(30) Little amounts of aryl bis(neopentyl)boronates were observed in
the crude reaction mixtures.
1
(31) We sincerely thank F. Miloserdov and V. V. Grushin for
providing an authentic sample of 4, that was prepared from 5
using [Me4N]F: Jover, J.; Miloserdov, F. M.; Benet-Buchholz,
J.; Grushin, V. V.; Maseras, F. Organometallics 2014, 33, 6531
(32) For the intermediacy of Ni(0)-ate complexes, see: (a) Yu, D.–G.;
Li, B.–J.; Zheng, S.–F.; Guan, B.–T.; Wang, B.–Q.; Shi, Z.–J.
Angew. Chem. Int. Ed. 2010, 49, 4566. (b) Hatakeyama, T.;
Hashimoto, S.; Ishizuka, K.; Nakamura, M. J. Am. Chem. Soc.
2009, 131, 11949. (c) Cámpora, J.; Reyes, M. L.; Hackl, T.;
Monge, A.; Ruiz, C. Organometallics 2000, 19, 2950.
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