Ni-Catalyzed Stannylation of Aryl Esters via C?O Bond Cleavage

Article abstract of DOI:10.1002/anie.201611720

A Ni-catalyzed stannylation of aryl esters with air- and moisture-insensitive silylstannyl reagents via C_sp² ?O cleavage is described. This protocol is characterized by its wide scope, including challenging combinations, thus enabling access to versatile building blocks and orthogonal C?heteroatom bond formations.

Full text of DOI:10.1002/anie.201611720

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201611720
German Edition: DOI: 10.1002/ange.201611720
Carbon–Heteroatom Coupling
ꢀ
Ni-Catalyzed Stannylation of Aryl Esters via C O Bond Cleavage
Yiting Gu and Rfflben Martín*
ꢀ
Abstract: A Ni-catalyzed stannylation of aryl esters with air-
At present, the existing precedents for C heteroatom
ꢀ
ꢀ
and moisture-insensitive silylstannyl reagents via Csp2
cleavage is described. This protocol is characterized by its
wide scope, including challenging combinations, thus enabling
O
bond formation of aryl esters via C O cleavage remain
[4a,d]
ꢀ
ꢀ
essentially confined to C P and C N bond formations.
Aimed at providing better flexibility in synthesis design
through further derivatization techniques, aryl esters have
recently been converted into aryl trialkylsilanes or aryl
ꢀ
access to versatile building blocks and orthogonal C hetero-
atom bond formations.
boronates via C O cleavage;[4b,c] unfortunately, only a limited
ꢀ
ꢀ
O
wing to the low cost, benign character, and availability of
number of transformations via cleavage of the C Si bond of
ꢀ
phenol, C O electrophiles have emerged as powerful alter-
aryl trialkylsilanes are within reach, whereas high temper-
natives to aryl halides in the cross-coupling arena.[1] Although
atures and noble catalysts are needed to forge C B bonds
from aryl esters, thus reinforcing a change in strategy.
Prompted by our interest in C O functionalization,
questioned whether an umpolung strategy could be designed
to convert electrophilic aryl esters into nucleophilic organotin
ꢀ
ꢀ
predisposed to site-selectivity issues with multiple C O
[4c,5]
ꢀ
reaction sites, aryl esters have become attractive counterparts
due to their accessibility, thermal/moisture stability, and
exquisite orthogonality with aryl halides, representing an
added value when compared to highly reactive organic
we
ꢀ
reagents, superb reaction intermediates for subsequent C Sn
sulfonates.[1] In contrast to commonly employed C C bond
cleavage (Scheme 1b).[6] Indeed, the Migita–Kosugi–Stille
(MKS) reaction of organotin reagents remains one of the
most robust, versatile, mild, and widely applicable cross-
couplings.[7] Not surprisingly, the MKS reaction is frequently
used in the total synthesis of natural products[8] or for
preparing densely functionalized polyheterocyclic cores,[9]
privileged motifs in a wide variety of pharmaceuticals,
which are not particularly trivial to assemble by classical
cross-coupling reactions.[10] Herein, we describe a Ni-cata-
ꢀ
formations using organometallic species (Scheme 1a,
path a),[2,3] the paucity of C heteroatom bond formations of
ꢀ
aryl esters is certainly striking (Scheme 1a, path b),[4] a testa-
ment to the attenuated reactivity of heteroatom-based
nucleophiles. Undoubtedly, such void terrain constitutes
a unique opportunity for discovering new fundamental
reactivity while expanding our synthetic repertoire for
accessing essential molecular architectures.
ꢀ
lyzed stannylation of aryl esters via Csp2 O cleavage. The
transformation is distinguished by its wide scope, including
the coupling of non-p-extended arenes or even heterocyclic
cores, setting the basis for designing iterative cross-coupling
ꢀ
procedures as well as further derivatization techniques via C
Sn cleavage. Initial mechanistic studies suggest that a catalytic
cycle initiated by oxidative addition comes into play.
We started our investigations by reacting 1a with 2a,
a bench-stable stannyl reagent that can be prepared quanti-
tatively in one step and in bulk quantities.[11–13] After system-
atic experimentation (Scheme 2),[14] a combination of [Ni-
(cod)2], L1, and CsF in toluene at 908C provided the best
results, affording 3a in 91% yield (entry 1). In line with our
expectations, the nature of the ligand proved to be critical.
While only traces of 3a, if any, were observed with PCy3 and
L3, which have shown to be particularly useful in a myriad of
ꢀ
Scheme 1. Catalytic C O bond cleavage and stannylation of aryl esters.
other C O bond functionalizations (entries 2 and 4),[1] the use
ꢀ
of structurally related L2 resulted in considerably lower yields
of 3a (entry 3). Although 2a could serve as a sacrificial
reducing agent, lower results were found when using NiII
precatalysts, suggesting that cod might be stabilizing the
[*] Y. Gu, Prof. R. Martꢀn
Institute of Chemical Research of Catalonia (ICIQ)
The Barcelona Institute of Science and Technology
Av. Paꢁsos Catalans 16, 43007 Tarragona (Spain)
E-mail: rmartinromo@iciq.es
transient metal species within the catalytic cycle.[15]
A
significant erosion of yield was also observed when using
solvents, bases, and stannyl reagents other than toluene, CsF,
or 2a, respectively (entries 7–10). As expected, rigorous
control experiments demonstrated that all of the reaction
parameters were crucial for the stannylation to occur
(entry 11).[16]
Prof. R. Martꢀn
ICREA
Passeig Lluꢁs Companys 23, 08010 Barcelona (Spain)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201611720.
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could be executed at gram scale (5 mmol) without noticeable
erosion of yield. As shown for 3d, a two-fold stannylation was
easily achieved by carefully adjusting the stoichiometry of the
reaction. Although heterocycles containing nitrogen donors
could potentially hinder the reaction, this was not the case
(3g, 3r). Notably, the reaction could be extended to primary
or secondary benzylic pivalates possessing b-hydrogen atoms,
delivering 3m and 3n in good yields. In addition, the use of 1-
(2-naphthyl)allyl pivalate gave rise to 3o in 89% yield. To put
these results into perspective, 3m–3o could not be obtained in
the absence of Ni catalyst.
ꢀ
Despite the formidable advances realized in C O func-
tionalization, non-p-extended coupling counterparts are not
as commonly used as one might anticipate.[18] As shown in
Scheme 4, we found that our stannylation protocol can also be
applied with regular aryl pivalates. In line with our expect-
ations, the stannylation could be performed independently of
whether electron-rich or electron-poor substituents were
located at either meta, para, or ortho positions. Likewise,
aryl pivalates containing ester (5e, 5h), nitrile (5j), or fluorine
substituents (5d) could be coupled with similar ease. Notably,
extensions to benzyl or allyl pivalates posed no problems (5l
and 5m).[19] Particularly interesting was the tolerance for
heterocycle cores containing basic nitrogen donors, as these
motifs could compete with L1 for metal binding (5k and 5m).
Next, we questioned whether we could implement
orthogonal reactions in the presence of aryl halides with
conventional Cu or Pd catalysts (Scheme 5a). Specifically, we
found that a Cu-catalyzed amidation took place exclusively at
the aryl bromide terminus (!7).[20] A subsequent Pd-
catalyzed Suzuki–Miyaura protocol based on XPhos resulted
in 8,[21] which led to 9 under our optimized stannylation
conditions. These results represent a testament to the robust-
ness of aryl esters, an added value when compared to their
organic sulfonate congeners. The synthetic applicability of our
stannylation protocol is further illustrated in Scheme 5b. As
Scheme 2. Optimization of the reaction conditions.
[a] 1a (0.20 mmol), 2a (0.26 mmol), [Ni(cod)2] (10 mol%), L1
(10 mol%), CsF (0.20 mmol) in PhMe (0.20m) at 908C. [b] Yields were
determined by GC using decane as internal standard. [c] Yield of
isolated product. [d] [Ni(cod)2] (5 mol%). [e] With NaOtBu (20 mol%).
As shown in Scheme 3, our stannylation method turned
out to be widely applicable regardless of the electronic and
steric environments on the aryl ring.[17] Interestingly, amides
(3 f and 3l), silyl ethers (3c), aryl silanes (3i), nitriles (3j),
esters (3h and 3k), carbazoles (3p), and benzofurans (3q)
could all be easily obtained. Notably, the stannylation of 1a
Scheme 3. Scope of p-extended aromatic pivalates synthesized. Reac-
tion conditions: as Scheme 2, entry 1; yields of isolated products as
averages of at least two independent runs. [a] 1a (5.0 mmol). [b] 2a
(2.3 equiv). [c] t=6 h. [d] 2a (2.0 equiv). [e] T=1108C. [f] 1-(2-Naph-
thyl)allyl pivalate (1o) as substrate.
Scheme 4. Scope of non-p-extended aryl pivalates synthesized. Reac-
tion conditions: as Scheme 2, entry 1; yields of isolated products as
averages of at least two independent runs. [a] 2a (2.0 equiv).
2
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Scheme 6. Mechanistic considerations.
protocol are the broad scope and excellent chemoselectivity
profile, allowing for the use of non-p-extended arenes or
heteroaryl motifs, even in iterative-type scenarios. Further
ꢀ
extensions to other C O electrophiles are currently underway
in our laboratories.
Acknowledgements
Scheme 5. Orthogonal reactions and their applicability.
We thank ICIQ, the European Research Council (ERC-
277883), MINECO (CTQ2015-65496-R & 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. Y. G. thanks Severo Occhoa for
a scholarship.
shown, 10 or 11 could be easily prepared from 3a upon
treatment with either N-fluorobenzenesulfonimide (NFSI) or
iodine, thus representing a formal ipso halogenation of aryl
esters. Although polyheterocyclic motifs are traditionally
difficult to assemble by classical Pd-catalyzed cross-coupling
reactions,[10] we found that 12 and 13 could be obtained in high
yields under neutral conditions with [Pd(PPh3)4] as catalyst,
thus demonstrating the generality and flexibility of the MKS
coupling. Taken together, the results of Schemes 3–5 show the
prospective impact of our method.
Conflict of interest
Although unraveling all mechanistic intricacies of this
transformation should await further investigations, we
decided to study the reactivity of the putative oxidation
species (Scheme 6a), as Ni-1a could be readily prepared upon
exposure of 1a to [Ni(cod)2] and L1.[22,23] Interestingly, Ni-1a
was found to be competent as reaction intermediate, as 3a
was invariably formed in good yields regardless of whether
Ni-1a was used in a stoichiometric or catalytical manner.
Although we cannot rigorously rule out other conceivable
scenarios,[24] at present we propose a mechanistic rationale
based on the initial formation of Ni-1a followed by trans-
metalation of 2a mediated by the fluoride source (Sche-
The authors declare no conflict of interest.
ꢀ
Keywords: aryl esters · C O cleavage · cross-coupling · nickel
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ꢀ
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ꢀ
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[17] The yields of the reactions in Scheme 3 account for unreacted
starting material and reduced product.
ꢀ
[18] Selected examples for catalytic C O bond cleavages limited to
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[23] A slightly modified procedure to that described in Ref. [21] was
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[24] The available experimental data do not allow us to rule out the
intermediacy of NiI intermediates generated by comproportio-
nation. See for example, Ref. [5e].
[25] This notion gains credence by in situ monitoring the reaction of
1a with 2a and [Ni(cod)2]/L1, observing the clean formation of
Ni-1a in the crude reaction mixture; see Ref. [14].
[26] Me3SiF was formed quantitatively, as judged by 19F NMR and
1
29Si NMR spectroscopy. Likewise, 113Cs and H NMR spectros-
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[27] Stoichiometric experiments with 2a and [Ni(cod)2]/L1 revealed
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ꢀ
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Manuscript received: December 1, 2016
Final Article published: && &&, &&&&
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Carbon–Heteroatom Coupling
Y. Gu, R. Martꢁn*
&&&& — &&&&
Ni-Catalyzed Stannylation of Aryl Esters
ꢀ
via C O Bond Cleavage
An excellent chemoselectivity profile and
a broad substrate scope characterize the
title reaction, which contributes to
functionalization. It can be applied to
challenging substrate combinations and
iteratively, which shows the prospective
impact of this protocol in complex set-
tings.
expanding the rather limited portfolio of
ꢀ
ꢀ
C heteroatom bond formations via C O
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