A mild Ni/Cu-catalyzed silylation via C -O cleavage

Article abstract of DOI:10.1021/ja412107b

A Ni/Cu-catalyzed silylation of unactivated C-O electrophiles derived from phenols or benzyl alcohols is described. This transformation is characterized by its wide scope and mild conditions, providing a direct access to synthetically versatile silylated compounds. The protocol allows for the coupling of C(sp ²)-O and even C(sp³)-O bonds with similar efficiency.

Full text of DOI:10.1021/ja412107b

Communication
pubs.acs.org/JACS
A Mild Ni/Cu-Catalyzed Silylation via C−O Cleavage
Cayetana Zarate† and Ruben Martin*,†,§
†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
S
* Supporting Information
Scheme 1. Synthesis of Aryl and Aliphatic Silanes
ABSTRACT: A Ni/Cu-catalyzed silylation of unactivated
C−O electrophiles derived from phenols or benzyl
alcohols is described. This transformation is characterized
by its wide scope and mild conditions, providing a direct
access to synthetically versatile silylated compounds. The
protocol allows for the coupling of C(sp2)−O and even
C(sp3)−O bonds with similar efficiency.
ultimetallic catalysis has recently received a considerable
interest for designing unconventional synthetic strategies
M
that are unattainable by other means.1 Among these, the use of
Pd and Cu catalysts has shown to be particularly effective in
methodologies that have changed the landscape of organic
synthesis, such as the Sonogashira−Hagihara coupling2 or the
Wacker−Tsuji oxidation,3 among others. While Ni catalysts play
a dominant role in the cross-coupling arena,4 it comes as a
surprise that the combination of Ni and Cu catalysts has been
virtually unexplored in homogeneous catalysis.5 Therefore, the
discovery of new protocols within this field might lead to novel
synthetic routes toward advanced intermediates, counterintuitive
at first sight, thus increasing our organic chemist’s repertoire.
In recent years, the utilization of C−O electrophiles has
received considerable attention due to their lack of toxicity and
the readily availability of phenol as compared to commonly
employed organic halides.6 Unlike the use of activated aryl
sulfonates, a limited knowledge has been acquired when
employing simpler aryl esters via catalytic C−O cleavage. This
is probably due to the high activation barrier for C−O cleavage,
the proclivity of esters toward hydrolysis under strong basic
conditions, and the site-selectivity issues in the presence of
multiple C−O reaction sites.6 Despite the advances realized, the
vast majority of C−O bond-cleavage reactions are restricted to
the formation of C−C bonds. Indeed, the development of
catalytic C-heteroatom bond-forming reactions remains an
elusive task in the cross-coupling arena when utilizing
unactivated C−O bonds.7 We envisioned that organic silanes,
valuable synthetic intermediates of considerable interest in
medicinal and material science,8 could be within reach by a C−Si
bond-forming event from unactivated C−O electrophiles under
certain reaction conditions. Such a method would constitute an
alternative to classical metal/halogen exchange (Scheme 1, path
a),9 the coupling of aryl halides with R3SiH10a−c or (R3Si)2
counterparts10d−f (path b) and C−H11a−d or C−CN function-
alization techniques11e,f that are conducted at high
temperatures11 and using ortho-directing groups11a−d (path c).
As part of our ongoing studies in this field,12 we report herein the
discovery of a Ni/Cu catalytic couple that allows for a C−Si
bond-forming reaction via cleavage of unactivated C−O bonds in
ester derivatives (path d).13 The method is distinguished by its
mild conditions, robustness, and wide substrate scope, including
the formation of particularly elusive C(sp3)−Si bonds,14 without
compromising its application profile.
We began our investigations by examining the reactivity of 1a
using nickel catalysts (Table 1).15,16 While silylboranes have
extensively been employed for promoting silylborylation of
unsaturated C−C bonds,17 to the best of our knowledge their
utilization en route to aryl silanes has not been explored. As
shown in entry 3, we found promising results when employing
readily available 2a,18 Ni(COD)2 as the catalyst, and PCy3 as the
supporting ligand at 50 °C. In line with our studies in the field, we
found that the absence of COD had a deleterious impact on
reactivity (entries 1 and 2), suggesting that non-innocent
ancillary ligands might stabilize the active species within the
catalytic cycle.12 As anticipated,17 additives played a crucial role
by activating the Si−B bond (entries 4−15). As shown in entries
6−8, the inclusion of AgF or CuF2 was rather promising for our
purposes, suggesting the intermediacy of MSiEt3 species (M =
Ag(I), Cu(II)).19 While the addition of CsF (1 equiv) in
combination with CuF2 (30 mol %) had a cooperative effect on
reactivity, obtaining a 90% isolated yield of 3a (entry 10), the use
of structurally related KF and more soluble fluoride sources such
as TBAF or K3PO4, among others, had a deleterious effect (entry
9). We believe these results indicate an intimate and unique
interplay of CuF2 and CsF and suggest that CuF2 does not simply
act as a soluble fluoride source.20 Such perception was further
corroborated when comparing the results shown in entry 5 and
entries 7−9 with entry 10. A simple filtration of the crude mixture
through a plug of Celite was necessary, thus greatly simplifying
the workup. Interestingly, the counterion and oxidation state of
the additives utilized were found to be critical factors for success
(entries 12−15). Control experiments in which the Ni catalyst
Received: November 27, 2013
Published: January 30, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
a
ab
,
Table 1. Optimization of the Reaction Conditions
Table 2. Ni-Catalyzed Silylation of Naphthyl Pivalates
b
entry
Ni catalyst
CsF (x) additive (mol %)
3a (%)
1
2
Ni(PCy3)2(C2H4)
NiCl2(PCy3)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
−
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
−
−
−
1
c
O
3
11
10
10
20
10
4
CsF (30)
CsF (100)
AgF (30)
CuF2 (30)
CuF2 (50)
CuF2 (30)
CuF2 (30)
CuF2 (30)
AgF (30)
CuBr2 (30)
CuS04 (30)
Cul (30)
CuF2 (30)
5
6
7
d
8
18
e
f
g
9
0 , 1 , 24
h
10
11
12
13
14
15
16
94(90)
hi
,
88 (85)
21
0
20
0
0
a
Reaction conditions: 1 (0.50 mmol), 2a (0.60 mmol), Ni(COD)2 (10
a
Reaction conditions: 1a (0.25 mmol), 2a (0.30 mmol), Ni(COD)2
(0.025 mmol), PCy3 (0.05 mmol) in toluene (2.0 mL), 50 °C, 2 h.
mol %), PCy3 (20 mol %), CuF2 (30 mol %), CsF (1 equiv) in toluene
b
(3.0 mL) at 50 °C. Isolated yields, average of at least two
b
c
c
d
GC yields using decane as an internal standard. Zn (0.25 mmol) was
independent runs. PhMe2SiBpin (2aa) was utilized. 80 °C.
d
e
used. Similar result was obtained with CuF2 (100 mol %). TBAF (1
equiv) as additive. K3PO4 (1 equiv) as additive. KF (1 equiv) as
additive. Isolated yield. 110 °C.
f
g
a b
,
Table 3. Ni-Catalyzed Silylation of Phenyl Pivalates
h
i
was omitted resulted in no product formation (entry 16).
Similarly, little conversion was observed in the absence of CuF2
(entries 3 and 5), suggesting that both Ni and Cu assist the C−O
cleavage/C−Si bond-forming event. Although silylboranes have
been used as a platform for preparing aryl boronates,18,21 no
traces of C−B bond-formation were detected by NMR
spectroscopy of the crude mixture, hence showing the distinctive
features of our protocol.
Next, we turned our attention to study the preparative scope of
our reaction. As shown in Table 2, the outcome was largely
insensitive to changes in the electronic nature of the substrates
utilized, obtaining the desired compounds in excellent yields
using 2a as coupling counterpart. Interestingly, we found that the
coupling of 2aa resulted in moderate yields of 3aa, hence
showing the superior activity of 2a.22 The chemoselectivity
profile of the method is nicely illustrated by the fact that ethers
(3b, 3d), esters (3e, 3h), amides (3g), or silyl ethers (3i) were
perfectly tolerated under our optimized protocol. As shown for
3h, the reaction was not seriously hampered by the presence of
ortho-substituents, although slightly higher temperatures were
required in this case. While C(sp2)−F bonds are prone to
oxidative addition in the presence of Ni catalysts with PCy3 as
supporting ligand,23 we found that such motifs remained intact
under our reaction conditions (3f). Similarly, the presence of
nitrogen-containing heterocycles did not interfere with the
productive C−Si bond-forming reaction (3j).
A closer look into the literature data indicates that the
inclusion of π-extended systems greatly accelerates the rate of
C(sp2)−O bond-cleavage reactions.24 Encouraged by the
findings in Table 2, we speculated that our mild Ni/Cu-catalyzed
silylation event could be even extended to the use of simpler, yet
challenging, phenyl pivalates. As shown in Table 3, this was
indeed the case, and a wide variety of phenyl pivalates, regardless
of the electronic effects on the aryl ring, could be coupled in high
a
b
As for Table 2. Isolated yields, average of at least two independent
c
runs. GC yields using decane as an internal standard due to volatility
issues. Reaction conducted at 80 °C. 2a (1.0 mmol) and CsF (1.70
equiv).
d
e
yields (5a−5k). It is worth noting that the coupling of phenyl and
naphthyl pivalates (Tables 2 and 3) operates under otherwise
identical reaction conditions, an observation that demonstrates
the robustness and generality of our Ni/Cu-catalyzed event. In
analogy with the results in Table 2, the presence of ortho-
substituents did not hinder the C−Si bond-forming reaction (5g,
5h). Likewise, a number of functional groups such as boronic
esters (5c), ketones (5d), amines (5f), acetals (5i, 5k), and
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Journal of the American Chemical Society
Communication
heteroaromatics (5j) could also be equally accommodated in
good to excellent yields.
mechanistic studies and other related transformations are
currently underway in our laboratories.
To the best of our knowledge, a catalytic C-heteroatom bond-
forming reaction via the cleavage of unactivated C(sp3)−O
bonds has no precedents in the literature.25 Gratifyingly, we
observed that primary and even secondary benzylic pivalates
could be cross-coupled with 2a in good yields (Table 4).
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and spectral data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
a b
,
Table 4. C(sp3)−O Cleavage/C−Si Bond Formation
AUTHOR INFORMATION
Corresponding Author
rmartinromo@iciq.es
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank ICIQ Foundation, the European Research Council
(ERC-277883), and MICINN (CTQ2012-34054) for financial
support. Johnson Matthey, Umicore, and Nippon Chemical
Industrial are acknowledged for a gift of metal and ligand sources.
Dr. N. Cabello at ICIQ and X. Ribas from UdG are
acknowledged for HRMS as well as insightful mechanistic
discussions.
a
b
As for Table 2. Isolated yields, average of at least two independent
c
runs. Reaction conducted at 80 °C.
Importantly, no reoptimization of the reaction conditions was
required, a rather valuable finding that demonstrates the
outcome of our method. Overall, we believe the results shown
in Tables 2−4 nicely illustrate the excellent reactivity and the
wide application profile of our Ni/Cu-catalyzed C(sp2)− and
C(sp3)−O bond-cleavage protocol, suggesting that other
conceivable scenarios might be discovered in the near future.26
Although a detailed mechanistic picture requires further
studies, we tentatively propose a scenario consisting of two
catalytic cycles (Scheme 2). Thus, we favor an initial oxidative
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