Selective Synthesis of Z-Silyl Enol Ethers via Ni-Catalyzed Remote Functionalization of Ketones

Article abstract of DOI:10.1021/jacs.1c01797

We report a remote functionalization strategy, which allows the Z-selective synthesis of silyl enol ethers of (hetero)aromatic and aliphatic ketones via Ni-catalyzed chain walking from a distant olefin site. The positional selectivity is controlled by the directionality of the chain walk and is independent of thermodynamic preferences of the resulting silyl enol ether. Our mechanistic data indicate that a Ni(I) dimer is formed under these conditions, which serves as a catalyst resting state and, upon reaction with an alkyl bromide, is converted to [Ni(II)-H] as an active chain-walking/functionalization catalyst, ultimately generating a stabilized η3-bound Ni(II) enolate as the key selectivity-controlling intermediate.

Full text of DOI:10.1021/jacs.1c01797

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Selective Synthesis of Z‑Silyl Enol Ethers via Ni-Catalyzed Remote
Functionalization of Ketones
Sinem Guven, Gourab Kundu, Andrea Weßels, Jas S. Ward, Kari Rissanen, and Franziska Schoenebeck*
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ABSTRACT: We report a remote functionalization strategy,
which allows the Z-selective synthesis of silyl enol ethers of
(hetero)aromatic and aliphatic ketones via Ni-catalyzed chain
walking from a distant olefin site. The positional selectivity is
controlled by the directionality of the chain walk and is
independent of thermodynamic preferences of the resulting silyl enol ether. Our mechanistic data indicate that a Ni(I) dimer is
formed under these conditions, which serves as a catalyst resting state and, upon reaction with an alkyl bromide, is converted to
[Ni(II)-H] as an active chain-walking/functionalization catalyst, ultimately generating a stabilized η3-bound Ni(II) enolate as the key
selectivity-controlling intermediate.
multicomponent couplings, have therefore also been ex-
plored.34−38
INTRODUCTION
■
An emerging strategy in the synthetic chemist’s toolbox is
chain walking coupled with remote functionalization, in
which a functionality is installed at a certain distance from
the original reaction center.1−5 Such approaches have gained
increasing popularity and significance, as they allow installing
functionalities in a fundamentally different manner, and as
such, they can potentially unlock compatibility with different
functional groups and enable the use of alternative starting
materials or even mixtures of olefin precursors.6−8 While
numerous elegant transformations have been developed,
especially C−C,9 C−Si,10 and C−B11 bond formations or
redox events (e.g., OH to carbonyl)12,13 as terminating
functionalization in this context, to date, there is no
precedence of remote functionalization of ketones.14−16
If realizable, such a process could enable the synthesis of
silyl enol ethers, which have gained prominence as a powerful
synthon for carbon−carbon bond formation and stereo-
selective synthesis.17−20 However, access to silyl enol ethers
under positional and stereochemical control is not free of
challenges. The commonly pursued direct deprotonation of
ketones, followed by silylation of the corresponding enolate,
is incompatible with ketones that contain base-sensitive
functional groups or that contain two alkyl substituents of
similar steric and electronic properties, which will compro-
mise the overall positional selectivity of the double bond (see
Figure 1A).21−23 The alternative approach from α,β-
unsaturated ketones, followed by reductive conjugate
addition, is amenable for cyclic systems,24−27 whereas
hydrosilylation using Rh, Pd, Pt, or Co catalysis has been
predominantly applied to aromatic ketones (Figure 1B).28−31
Alternative α-C−H bond activations of ketones followed by
silylation may give rise to competing over-reductions.32,33
Indirect synthetic strategies, e.g., via rearrangements or
In the context of remote functionalization (Figure 1D), so
far only prefunctionalized silyl ethers with distant double
bonds were shown to undergo precious-metal-catalyzed (Ir,
Ru, Pd) double-bond migrations to yield silyl enol ethers
(Figure 1C).39,40 The majority of these reports made
aldehyde-derived silyl enol ethers resulting from single-bond
olefin migration;41−44 those examples with greater substitu-
tion (i.e., ketone-derived) were generated with non-uniform
E/Z selectivities, which were highly dependent on the α-
substituent.5,39,41,42
RESULTS AND DISCUSSION
■
As part of our ongoing research in metal-catalyzed olefin
migrations,45,46 we set out to explore the possibility of
developing a nonprecious Ni-catalyzed chain walking for the
regio- and stereoselective functionalization of remote ketones
to the corresponding silyl enol ethers (Figure 1E). Such a
transformation of distant ketone olefins requires not only the
migration of a double bond and silylation but formally also a
net addition of a “H”. We hypothesized that this might be
possible through the generation of a [Ni−H] species whose
“H” ultimately remains in the product. This in turn would
require a stoichiometric amount of H-source that ensures a
continuous regeneration of [Ni−H]. Alkyl bromides have
Received: February 17, 2021
Published: May 25, 2021
© 2021 The Authors. Published by
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the dimer [Ni(μ-Br)(IPr)]2 as (pre)catalyst instead of Ni(II)
/
IPr under otherwise identical conditions, product 2 was
obtained with the same yield (89%).
In the absence of Mn, but otherwise unaltered reaction
conditions with 12 mol % Ni(I) dimer, a combined yield of
12% of product 2 and its desilylated form 2a is generated
(see Figure 2B, details in SI), which indicates that one cycle
per nickel dimer was undergone and that the role of Mn is
likely primarily to regenerate the Ni(I) dimer. To examine this
further, we analyzed the resulting Ni species that is formed in
the reaction under Ni(I) dimer catalysis in the absence of Mn
(Figure 2B) by crystallization of the resulting reaction
mixture. X-ray crystallographic analysis indicated that two
Ni(II) species had formed: the dimeric [Ni(μ-Br)(Br)(IPr)]2
(purple crystals; ∼50%) and monomeric [NiBr3(IPr)][IPrH]
(turquoise crystals; ∼50%) (Figure 2C). However, the latter
monomeric Ni(II) salt potentially forms from fragmentation of
the Ni(II) dimer in solution. By analogy, [FeCl3(SIPr)]-
[SIPrH] salts have been shown to form from [Fe(μ-
Cl)(Cl)(SIPr)]2.54 We further discovered that the subsequent
subjection of Mn (3 equiv) to this mixture of Ni(II) species in
THF gives rise to the formation of the Ni(I) dimer [Ni(μ-
Br)(IPr)]2, as judged by 1H NMR analysis (Figure 2C).
These data indicate that the Ni(I) dimer is a key species in
this transformation. The role of Mn is to regenerate the Ni(I)
dimer after each cycle.
Research in the field of dinuclear metal complexes of
oxidation state I with palladium has shown that the precise
catalytic role and mechanistic involvement of such dinuclear
metal complexes is highly dependent on the type of
transformation, the reaction conditions, and especially the
additives that are present.55 In this context, we previously
showed that a Ni(I) dimer can give rise to Ni(I) metalloradical
reactivity with olefins.45 However, our previous work also
indicated that ketones wouldif not blocked by a Lewis
acidinhibit such radical reactivity.45 As such, the radical
species may also be converted to an alternative species, if
suitable reagents are present in the mixture. In this context,
the alkyl bromide likely funnels the “inhibited” Ni(I)
metalloradical to a [Ni(II)-H],9,47−49 as illustrated in Figure
2D. In line with this proposal, the employment of fully
deuterated iPrBr gave rise to deuterium incorporation at the
terminal site, which is consistent with the initial addition of a
[Ni(II)-D] species (see SI for details). Ultimately, upon chain
walking, a stabilized η3-bound enolate is formed.
Figure 1. (A) Base-mediated silyl enol ether formations; (B) metal-
mediated silyl enol ether formation; (C, D) silyl enol ethers via
isomerization and remote functionalization; (E) this work: remote
functionalization of ketones.
previously been shown to be suited for this endeavor,9,47−49
and we set out to explore suitable conditions.
To our delight, we identified that a catalytic amount of
NiBr2(dme) along with IPr (1,3-bis(2,6-diisopropylphenyl)-
1,3-dihydro-2H-imidazol-2-ylidene) ligand, iPrBr, and Mn
powder allowed for the selective functionalization of ketone
1 with Et3SiCl to yield 2 (Figures 2 and 3).
The omission of either Mn, Et3SiCl, or Ni(II)/IPr from the
reaction mixture left the starting ketone 1 untouched. The
i
employment of PhMe2SiH instead of Et3SiCl and PrBr gave
rise to a net reductive silylation of ketone without olefin
migration.
Our computational studies57,58 suggest that the η3-
coordination is likely the origin of positional selectivity and
stereoselectivity, as it fixes the enol geometry and impedes
further chain walking (see Figure 2D). After silylation the
formed Ni(II) is reduced with Mn to (re)form the Ni(I) dimer.
With the mechanism identified, we subsequently explored
the scope of the transformation (Figure 3) using conditions
that allow using commercially available catalyst components,
i.e., those that start from Ni(II), and that efficiently generate
the Ni(I) dimer in situ after prestirring.
We reacted our model substrate 1 with different
chlorosilane sources, which gave the corresponding silyl
enol ethers (−OSiMe3 (3), −OSi(iPr)Me2 (4)) in good
yields and excellent Z-selectivities, indicating that different
silyl chloride sources are equally efficient in the trans-
formation. With this in mind, we subsequently evaluated the
functional group tolerance using Et3SiCl. We studied various
Given nickel’s propensity to form dinuclear Ni(I) complexes
with IPr ligands,50−52 we investigated the likely speciation of
the Ni(II) precatalyst under these reductive conditions.
Indeed, we observed that the subjection of Mn and IPr to
a solution of NiBr2(dme) in THF resulted in the formation
of Ni(I) dimer [Ni(μ-Br)(IPr)]2 as the major product within
15 min at 35 °C. Our crystallization of the mixture resulted
in ∼91% of [Ni(μ-Br)(IPr)]2 (green crystals) along with
minor amounts of NiBr2(IPr)2 (red crystals; ∼9%) (see
Figure 2A).53 In line with these observations and suggesting
that the initial reduction to Ni(I) is mechanistically critical, we
found that an initial premixing of NiBr2(dme), IPr, and Mn
in THF for 15 min prior to the addition of the substrate and
remaining reagents led to optimal conversion to the products.
For example, 2 was formed in 89% yield and high Z-
selectivity (Z:E 96:4, shown in Figure 3), while it formed in
only 60% yield without premixing. Moreover, when we used
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Figure 2. (A) Formation of Ni(I) dimer, (B) reactivity in the absence of Mn, (C) re-formation of the Ni(I) dimer, (D) proposed mechanism.
Free energies calculated at SMD (THF) M06L/def2-TZVP//ωB97XD/6-31G(d)(SDD for Ni) in kcal/mol.21 IPr refers to 1,3-bis(2,6-
diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene.
aromatic as well as aliphatic ketones with different chain
lengths (up to four-carbon).
silyl enol ether was observed, which is especially notable for
product 24, where a rather acidic benzylic proton is present
(Figure 3).
We found that electron-rich aromatic (p-OMe (5), p-
N(Me)2 (6), m-CH3 (9)) as well as electron-deficient (p-Cl
(7), m-CF3 (10)) aromatic ketones reacted equally efficiently.
Similarly, biphenyl (11) and polyaromatic ketones (naph-
thalene (12) or anthracene (13)) as well as heterocyclic
examples (1,3-benzodioxole (14), furan (15), or indole (16))
were converted to the corresponding silyl enol ethers in high
yields.59 Nonaromatic acyclic ketones were equally effective
(17), tolerating even a bulky adamantyl group (18). Notable
in this context is the selective formation of silyl enol ether 19,
for which no further isomerization to the thermodynamically
favored, alternative conjugated silyl enol ether was observed.
Beyond these two-carbon chain walks, three- or four-
carbon chain walks were similarly efficient, although four-
carbon walks needed longer reaction times (48 h). Products
20 and 21, which possess a diphenyl acetyl group, were
obtained exclusively after three-carbon chain walking without
further isomerization to the conjugated enol (21). Notably, a
methyl group could be tolerated along the walk (23), and
once again fully regio- and stereoselective formation of the
We also tested the feasibility of using an internal rather
than a terminal alkene as a starting material (Figure 4). An
internal alkene (Z:E 5:95) separated from the ketone by a
two-carbon chain afforded product 20 efficiently. Also using a
50:50 mixture of terminal and internal alkene isomers in a
three- and two-carbon distance from the ketone, respectively,
led to the same product (20) in high yield. The analogous
reaction outcome was observed for the aliphatic analogue 25
and also in the case of 26, for which the starting olefin was
stabilized by conjugation. Lastly, formation of the corre-
sponding cyclic silyl enol ethers was also possible (27, 28).
Given the sterically and electronically similar substituents of
the ketones, the selective formation of 28 showcases that the
positional selectivity is dictated solely by the directionality of
the chain walk.
CONCLUSION
■
In conclusion, the first remote functionalization of ketones
has been developed, which enabled the regio- and stereo-
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Figure 3. Scope of the reaction.56 Method A: Premix NiBr2dme (10 mol %), IPr (10 mol %), PrBr (2 equiv), and Mn (3 equiv) in THF (1.0
i
i
M) for 15 min. Method B: Without premixing, add [Ni(μ-Br)(IPr)]2 (5 mol %), Mn (3 equiv), PrBr (2 equiv), and THF (1.0 M) in a vial;
then add ketone (1.0 equiv) in DMF (1.0 M) and chlorosilane (1.5 equiv). Substrates 7, 19, 22, and 24 were synthesized using method B; all
other substrates were synthesized using method A. The reaction was carried out in a glovebox, based on a 0.3 mmol scale unless otherwise
stated (see SI). Isolated yields. Z:E ratios are based on GC-MS analysis.
selective synthesis of Z-silyl enol ethers in high yields via Ni-
catalyzed chain walking. The methodology is operationally
simple and compatible with aliphatic as well as (hetero)-
aromatic ketones and is independent of the employed silyl
chloride or the distal olefin (terminal or E/Z internal).
Mechanistic studies indicate the formation of a Ni(I) dimer as
a key intermediate, which ultimately converts to a Ni(II)-H
upon reaction with an alkyl bromide and undergoes the chain
walk.
orcid.org/0000-0003-0047-0929;
Email: franziska.schoenebeck@rwth-aachen.de
Authors
Sinem Guven − Institute of Organic Chemistry, RWTH
Aachen University, 52074 Aachen, Germany; orcid.org/
0000-0002-1503-3484
Gourab Kundu − Institute of Organic Chemistry, RWTH
Aachen University, 52074 Aachen, Germany
Andrea Weßels − Institute of Organic Chemistry, RWTH
Aachen University, 52074 Aachen, Germany
Jas S. Ward − Department of Chemistry, University of
Jyvaskyla, 40114 Jyväskylä, Finland; orcid.org/0000-
0001-9089-9643
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c01797.
■
sı
Kari Rissanen − Department of Chemistry, University of
Jyvaskyla, 40114 Jyväskylä, Finland; orcid.org/0000-
0002-7282-8419
Experimental details, characterization data of com-
pounds, crystallographic data, and computational details
(PDF)
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c01797
Accession Codes
CCDC 2009557−2009560 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_-
request/cif, or by emailing data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
F.S. acknowledges the RWTH Aachen University and the
European Research Council (ERC-864849) for funding. K.R.
acknowledges the Alexander von Humboldt Foundation (for
the AvH Research Award) and J.S.W. the Finnish Cultural
Foundation Central Fund (grant number 00201148) for
financial support. Calculations were performed with comput-
AUTHOR INFORMATION
Corresponding Author
Franziska Schoenebeck − Institute of Organic Chemistry,
RWTH Aachen University, 52074 Aachen, Germany;
■
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