Switchable Selectivity in the Pd-Catalyzed Alkylative Cross-Coupling of Esters

Article abstract of DOI:10.1021/acs.orglett.8b01646

The Pd-catalyzed cross-coupling of phenyl esters and alkyl boranes is disclosed. Two reaction modes are rendered accessible in a selective fashion by interchange of the catalyst. With a Pd-NHC system, alkyl ketones can be prepared in good yields via a Suzuki-Miyaura reaction proceeding by activation of the C(acyl)-O bond. Use of a Pd-dcype catalyst enables alkylated arenes to be synthesized by a modified pathway with extrusion of CO. Applications of this divergent coupling strategy and the origin of the switchable selectivity are discussed.

Full text of DOI:10.1021/acs.orglett.8b01646

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Switchable Selectivity in the Pd-Catalyzed Alkylative Cross-Coupling
of Esters
Jeanne Masson-Makdissi, Jaya Kishore Vandavasi, and Stephen G. Newman*
Centre for Catalysis Research and Innovation, Department of Chemistry and Biomolecular Sciences, University of Ottawa, 10
Marie-Curie, Ottawa, Ontario K1N 6N5, Canada
S
* Supporting Information
ABSTRACT: The Pd-catalyzed cross-coupling of phenyl esters
and alkyl boranes is disclosed. Two reaction modes are rendered
accessible in a selective fashion by interchange of the catalyst.
With a Pd−NHC system, alkyl ketones can be prepared in good
yields via a Suzuki−Miyaura reaction proceeding by activation of
the C(acyl)−O bond. Use of a Pd−dcype catalyst enables
alkylated arenes to be synthesized by a modified pathway with
extrusion of CO. Applications of this divergent coupling strategy and the origin of the switchable selectivity are discussed.
d-catalyzed cross-coupling reactions have greatly simpli-
Scheme 1. Different Reaction Modes in the Cross-Couplings
of Esters
P
fied the way we construct C(sp2)−C(sp2) and C(sp2)−
C(sp3) bonds. Initially, developments in the field were mostly
focused on expanding the scope of the nucleophilic agent, with
organoboron reagents now being the most utilized coupling
partner due to their accessibility and effectiveness for the
construction of complex molecules.1 While aryl and vinyl
halides are traditionally utilized as the electrophilic reaction
component, the development of cross-coupling reactions that
feature alternative electrophiles can further extend the impact
and utility of the Suzuki−Miyaura reaction by enabling even
more diverse products to be reliably formed from simple
starting materials. Owing to their ubiquity, carboxylic acid
derivatives represent a useful family of alternative coupling
partners. While cross-coupling of acid chlorides has been
known since the pioneering work of Stille2 and recent research
has focused on the use of more robust substrates such as acid
anhydrides,3 thioesters,4 select amides,5 and esters.6 In
particular, the use of phenyl ester coupling partners has
recently been exploited in an array of diverse reactions.7 In
contrast to aryl halides, phenyl esters can be activated by
different reaction modes depending on the choice of catalyst
(Scheme 1A). For instance, Garg and Shi independently
demonstrated activation of the C(aryl)−O bond in couplings
with arylboron reagents.8 Love and Itami later demonstrated
formal cleavage of the C(aryl)−C bond to form biaryls,9 while
our group, in collaboration with the Houk laboratory, recently
disclosed that aryl ketones could be selectively prepared.10
Although a wealth of mechanistic information has
accumulated over the last couple of years on these and related
reactions,11 rationally predicting and controlling selectivity in
cross-couplings of esters remains a considerable challenge.
Moreover, couplings with alkyl nucleophiles constitutes an
underdeveloped area in the field12 due to several challenges
with the use of alkyl boron reagents such as their reluctance to
transmetalate relative to their aryl counterparts and the
intermediacy of alkylmetal species that are prone to β-hydride
elimination.13 Nonetheless, the formation of C(sp2)−C(sp3)
bonds is an important goal due to the prevalence of these types
of linkages in pharmaceuticals and other valuable materials.
In this work, we disclose the first Pd-catalyzed coupling of
phenyl esters with alkyl boranes (Scheme 1B).14 A bulky NHC
ligand enables the reaction to proceed with carbonyl retention
to afford ketones. In contrast, the use of a hemilabile bidentate
phosphine ligand provides the decarbonylated adduct
selectively. Remarkably, β-hydride elimination can be mini-
mized in the latter reaction mode, even at the high temperature
required for decarbonylation.
We first investigated the possibility of performing alkylative
carbonyl-retentive couplings with phenyl esters and alkyl
boranes. Given the efficiency of Pd−NHC complexes in
related cross-couplings,10,15 we elected Pd(IPr)(cinnamyl)Cl
as a potential catalyst. No conversion was observed in the
absence of base (Table 1, entry 1). However, upon
incorporation of cesium carbonate, 3a was obtained in high
Received: May 25, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01646
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Organic Letters
Letter
a
Table 1. Optimization of the Pd-Catalyzed Coupling of Phenyl Esters and Alkyl Boranes
a
General reaction conditions: ester 1a (0.10 mmol), alkyl borane 2a (0.15 mmol), Pd or Ni catalyst, ligand, base (0.15 mmol), additive (0.15
b
mmol), toluene (0.11 M), 16 h, under nitrogen atmosphere. Yield determined by 1H NMR with 1,3,5-trimethoxybenzene as the internal standard.
c
d
Yield determined by GC/FID with 1,3,5-trimethoxybenzene as the internal standard. dcypp·HBF4 was used along with 20 mol % of K2CO3.
yield (entry 2). Further addition of water reached near
quantitative yield of 3a (entry 3). Increasing the temperature
was found to lead to reduced yields of the ketone product
without any detectable formation of decarbonylated products
(entries 4 and 5). Next, we turned our attention to
decarbonylative alkylative couplings. With the net majority of
decarbonylative couplings of esters occurring at elevated
temperatures >140 °C, it was suspected that preventing β-
hydride elimination under such conditions would pose a
significant challenge. We envisioned that an optimal catalyst
would bear a ligand that favors reductive elimination while
being labile enough to allow decarbonylation. Rationally
designed screenings were conducted on the basis of previous
reports and the mechanistic studies of related couplings.6−10
An initial hit was found with Pd(OAc)2 (5 mol %) and dcype
(10 mol %). As is shown in Table 1, a moderate yield of 4a can
be obtained in the absence of any base or additive (entry 6).
Interestingly, KF was found to both increase the yield and lead
to better selectivity for the formation of the desired adduct 4a
over the reduced product 5a (entry 7). Better conversion was
obtained at higher temperatures, reaching a satisfactory yield of
82% of 4a at 160 °C (entry 8). Among the diverse array of
ligands screened, dcype was found to be uniquely effective for
this transformation (Table S3). For instance, the use of dppe
as the ligand led to full recovery of the starting material 1a
(entry 9), while dcypp was unselective for the formation of
arenes 4a and 5a (entry 10). Interestingly, the replacement of
Pd(OAc)2 by Ni(cod)2 reversed the selectivity, favoring beta-
hydride elimination over reductive elimination to give 60% of
naphthalene 5a (entry 11). To further corroborate that the
selectivity switch between the formation of 3a vs 4a was not
solely temperature-dependent, both catalysts were tried at
different temperatures. At 160 °C, no trace of decarbonylated
adduct 4a was observed with the Pd-NHC catalyst (entry 12).
Conversely, the Pd-dcype catalyst did not provide any ketone
3a at 60 °C (entry 13). Thus, we attribute the requirement for
elevated temperatures in the decarbonylative pathway to the
challenging liberation of CO from the metal center.
The scope of the carbonyl-retentive couplings is illustrated
in Scheme 2. Different esters were tested with alkyl borane 2a.
Simple phenyl naphthoate and benzoate derivatives provided
good to excellent yields (3a−d). Heteroaryl esters such as
a
Scheme 2. Scope of Carbonyl-Retentive Coupling
a
General reaction conditions: ester (0.20 mmol), alkyl borane (0.30
mmol), Pd(IPr)(cinnamyl)Cl (5 mol %), Cs2CO3 (0.30 mmol), water
(0.30 mmol), toluene (0.22 M), 16 h, at 60 °C, under nitrogen
atmosphere. Ar′ = 2-naphthyl.
B
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Organic Letters
Letter
a
furans, benzofurans, and quinolines were also compatible
under the reaction conditions (3e−h). Different B-alkyl-9-
BBN reagents were then tested. A B-alkyl-9-BBN bearing a
perfluorophenyl ring provided 3i in a moderate yield. Styrenyl-
derived and aliphatic boranes also provided good yields (3j−l).
We next explored the scope of the decarbonylative couplings
(Scheme 3). Both electron-neutral (4a and 4b) and electron-
Scheme 4. Derivatization of Bioactive Molecules
a
Scheme 3. Scope of Decarbonylative Coupling
a
General conditions as described in Schemes 1 and 2. Ar = Ph for 3m
b
and 3n, Ar = 3,4-methylenedioxyphenyl for 4m and 4n. 10 mol % of
Pd(IPr)(cinnamyl)Cl was used. 3.0 mmol scale with 3 mol % of
c
[Pd].
Scheme 5. Proposed Mechanism
a
General reaction conditions: ester (0.20 mmol), alkyl borane (0.30
mmol), Pd(OAc)2 (5 mol %), dcype (10 mol %), KF (0.30 mmol),
toluene (0.22 M), 40 h, at 160 °C, under nitrogen atmosphere. Ar′ =
b
2-naphthyl. Pd(COD)(CH2TMS)2 (5 mol %) was used instead of
Pd(OAc)2.
rich (4c and 4d) arenes provided good to excellent yields. An
ester bearing a methyl ester functionality selectively reacted
with the phenyl ester moiety (4e). Pleasingly, heteroaryl esters
with pyridine and pyrazine motifs were suitable coupling
partners (4f−h). Aliphatic boranes provided moderate yields
under the reaction conditions (4i and 4k), while a safrole-
derived B-alkyl-9-BBN nucleophile provided high yield of
product 4j. Notably, an alkyl borane having a benzylic β-
hydrogen was also an efficient coupling partner (4l).
The ability to transform a single starting material into
different classes of products is most powerful when the
preparation of diverse, potentially bioactive molecules is sought
(Scheme 4). Toward this goal, functional group-rich phenyl
ester 1b, derived from the antidiabetic drug repaglinidine,
could be utilized to form both the ketone 3m and
decarbonylated product 4m. Ester 1c, derived from the gout
and hyperuricemia drug probenecid, could be similarly
diversified to 3n and 4n on 3 mmol scale with reduced
catalyst loading, providing good yields of both products.
A general mechanism outlining the different reaction modes
is proposed in Scheme 5. Three products were observed in the
coupling of phenyl esters and B-alkyl-9-BBN reagents: ketone
adduct 3, alkylated arene 4, or reduced product 5. All
mechanisms can begin with oxidative addition of the catalyst
into the weak C(acyl)−O bond, as has been proposed in
related couplings10,16 and demonstrated experimentally in
stoichiometric studies with Ni/ICy.6d
The palladium acyl intermediate A can then undergo
transmetalation with the organoboron species to form B,
with the respective additives facilitating transmetalation (X =
OPh, F, OH, or carbonate).17 Depending on the nature of the
C
DOI: 10.1021/acs.orglett.8b01646
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Letter
ligand, direct reductive elimination leads to ketone 3, or
decarbonylation and CO extrusion can occur to form
intermediate C. Most reported cross-couplings of phenyl esters
proceed by the decarbonylative pathway, many of which utilize
dcype as a ligand.18 Previous DFT calculations on similar
transformations with carbon-based nucleophiles suggest trans-
metalation occurs prior to decarbonylation.9a,11a,d The
dissociation of one of the phosphine arms of dcype is
subsequently required to open up a coordination site for the
challenging decarbonylation/extrusion.11a,b In contrast, the
electron-rich Pd-IPr catalyst blocks decarbonylation due to the
steric bulk of the NHC ligand, favoring reductive elimination.10
Lastly, after decarbonylation, there are again two possibilities.
Reductive elimination yields the desired product 4, while β-
hydride elimination would lead to the reduced product 5. The
minimization of β-hydride elimination at elevated temperatures
with Pd−dcype is particularly notable.
In summary, the use of esters as electrophilic coupling
partners in the Pd-catalyzed Suzuki−Miyaura with alkyl
nucleophiles was developed. While many related cross-
couplings of carboxylic acid derivatives have been reported,
controlling which product class forms is challenging. In this
work, two reaction modes can be accessed selectively by
interchange of the ligands. An air-stable Pd−NHC catalyst
enabled carbonyl-retentive coupling to form a broad range of
alkyl ketones. In parallel, a Pd−dcype catalyst was shown to
selectively form alkylated arenes via a decarbonylative pathway.
We expect that these disclosed transformations will provide
rapid access of diverse product-types from a single starting
material and will encourage academic chemists to exploit the
broad knowledge accumulated in the field to rationally design
selective cross-coupling reactions.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01646.
Experimental procedures, characterization of organic
molecules, and optimization tables (PDF)
AUTHOR INFORMATION
(7) Takise, R.; Muto, K.; Yamaguchi, J. Cross-Coupling of Aromatic
Esters and Amides. Chem. Soc. Rev. 2017, 46, 5864−5888.
■
Corresponding Author
*E-mail: stephen.newman@uottawa.ca.
ORCID
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Stephen G. Newman: 0000-0003-1949-5069
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this work was provided by the University
of Ottawa, the National Science and Engineering Research
Council of Canada (NSERC), and the Canada Research Chair
program. The Canadian Foundation for Innovation (CFI) and
the Ontario Ministry of Economic Development and
Innovation are thanked for essential infrastructure. J.M.M.
thanks OGS and NSERC for graduate scholarships. Roxanne
́
Clement is thanked for assistance with high-throughput
experimentation.
D
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