Oxidative Addition Complexes as Precatalysts for Cross-Coupling Reactions Requiring Extremely Bulky Biarylphosphine Ligands

Article abstract of DOI:10.1021/acs.orglett.7b01082

In this report, we describe the application of palladium-based oxidative addition complexes (OACs) as effective precatalysts for C-N, C-O, and C-F cross-coupling reactions with a variety of (hetero)arenes. These complexes offer a convenient alternative to previously developed classes of precatalysts, particularly in the case of the bulkiest biarylphosphine ligands, for which palladacycle-based precatalysts do not readily form. The precatalysts described herein are easily prepared and stable to long-term storage under air.

Full text of DOI:10.1021/acs.orglett.7b01082

Letter
pubs.acs.org/OrgLett
Oxidative Addition Complexes as Precatalysts for Cross-Coupling
Reactions Requiring Extremely Bulky Biarylphosphine Ligands
Bryan T. Ingoglia and Stephen L. Buchwald*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: In this report, we describe the application of palladium-based oxidative addition complexes (OACs) as effective
precatalysts for C−N, C−O, and C−F cross-coupling reactions with a variety of (hetero)arenes. These complexes offer a
convenient alternative to previously developed classes of precatalysts, particularly in the case of the bulkiest biarylphosphine
ligands, for which palladacycle-based precatalysts do not readily form. The precatalysts described herein are easily prepared and
stable to long-term storage under air.
he use of biarylphosphines as supporting ligands in
palladium catalysis has played an important role in the
precatalysts based on these particularly bulky ligands are less
successful than with other ligands. While G3 precatalysts
containing these ligands are readily isolated, they form
carbazole upon catalyst activation, which occasionally can
cause inhibition of the catalyst based on kinetic and mechanistic
studies.5c,6a,8 Precatalysts of type G4 and G5 were prepared to
address this, among other issues. However, G4 and G5-based
precatalysts based on L1−L4 are only formed with difficulty.5d
As an alternative, our group has developed [L−Pd]2(cod)
(cod = 1,5-cyclooctadiene) precatalysts (Figure 1) which are
suitable for L1−L4.9 However, not all biarylphosphine ligands
form precatalysts of this type. Furthermore, these complexes
are generally air sensitive and exhibit poor solubility in organic
solvents, making them less convenient to handle.4a,9,10 Thus, we
set out to find alternative Pd(II) precatalysts that would
overcome these issues.
T
development of synthetic methods that can be used to form C−
C,1 C−N,2 C−O,3 and C−F4 bonds in a wide variety of
settings. To facilitate the use of these ligands, we have
developed several generations of palladacyclic precatalysts
(G1−G5, Figure 1), which are able to accommodate a range
We hypothesized that a potential solution would be to use
oxidative addition complexes (OACs) of general formula L−
Pd(Ar)X as precatalysts. These are readily obtained by
oxidative addition of aryl (pseudo)halides to an in situ
generated L−Pd(0) species. Moreover, since they serve as
presumptive intermediates on the catalytic cycle for cross-
coupling processes, these OACs should be competent
palladium sources for a variety of C−X bond-forming
reactions.11,12
We were particularly interested in the use of OACs for the
development of a simplified and improved protocol for the
palladium-catalyzed fluorination of aryl (pseudo)halides.
Previously, L1 (AlPhos) was found to be an effective
supporting ligand for the palladium-catalyzed fluorination of a
broad range of aryl bromides and triflates.4 However,
preliminary investigation into the use of palladacycle
precatalysts yielded poor outcomes, likely due to the formation
Figure 1. Evolution of precatalysts developed by the Buchwald group
and selected bulky supporting ligands.
of phosphine-based ligands.5,6 These precatalysts can be used to
promote cross-coupling reactions with high efficiency. This is
due, in part, to the preassociation of the ligand with the Pd
center and to the rapid formation of a L−Pd(0) species under
the reaction conditions. In addition, these Pd(II) precatalysts
are easily handled and stable to long-term storage without the
need to carefully exclude air or water.
Biarylphosphine ligands bearing bis(tert-butyl) and bis(1-
adamantyl) phosphino groups (e.g., L1−L4) are useful
supporting ligands for catalysts that promote C−O and C−F
cross-coupling reactions.7 Among other things, these ligands are
believed to facilitate the challenging reductive elimination steps
of these processes.3c,4 Methods to prepare palladacycle
Received: April 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01082
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Letter
of inhibiting byproducts (e.g., HF or carbazole) during catalyst
activation, as well as the inefficiency of catalyst activation itself
in the absence of a suitable exogenous base.4c We expected
OACs to address both of these issues as no base is needed for
activation, while the only byproduct is simply another aryl
fluoride, which is presumably innocuous.
Table 1. Additive Effects on the Fluorination of 1-Bromo-4-
n-butylbenzene with P1 as Precatalyst
a
To test this hypothesis, precatalysts P1−P3 were prepared by
combining the biarylphosphine ligand, aryl bromide, or triflate,
and (cod)Pd(CH2TMS)2 in pentane, from which the desired
complex precipitated. Filtering and washing the precipitate
afforded the pure complexes in moderate to good isolated yield,
while the remaining free ligand may be recovered from the
filtrate (Scheme 1, see Supporting Information (SI) for details).
entry
[Pd]
additive
2% cod
yield A(B)
4
1
2
3
[L1−Pd]2(cod)
87% (5%)
22% (1%)
91% (6%)
P1
P1
a
Yields reported as 19F NMR yields using 1-fluoronaphthalene as an
internal standard: cy = cyclohexane; cod = 1,5-cyclooctadiene.
a
Scheme 1. Synthesis of Oxidative Addition Complexes
products. To address this issue, we prepared triflate analogue
P2. For aryl triflate substrates, this precatalyst exhibits the same
catalytic activity as P1, while avoiding the formation of aryl
bromide side products. However, P2 does not efficiently
transform aryl bromides into aryl fluorides, perhaps due to
inefficient activation of the precatalyst by the fluoride sources
present under the reaction conditions. Thus, we employed P1
and P2 for the fluorination of aryl bromides and triflates,
respectively. To gain a sense of the generality of these new
protocols, a small collection of substrates were examined under
these conditions. Substrates containing potentially sensitive
functional groups, including an internal alkyne (2a), an
aldehyde (2c), and an azo (2f) were fluorinated in good to
excellent yield, with a regioisomeric side product observed only
in the case of 2e (Scheme 2).14
After investigating OAC precatalysts in the context of C−F
bond-forming reactions, we explored the applicability of this
method to other bond-forming processes. To facilitate
purification of the cross-coupling product, we considered
precatalysts which would produce easily removed byproducts
upon activation. In particular, we designed an OAC bearing a
a
Structures of OACs used as precatalysts. Crystallographically
determined X-ray structure of P2 shown (thermal ellipsoid plot at
50% probability, triflate anion and hydrogen atoms are omitted).
4-Trifluoromethylphenyl was selected as the aryl group for P1
and P2, since catalyst activation via the formation of 4-
fluorobenzotrifluoride under the reaction conditions was
expected to be facile. Moreover, this byproduct was expected
to be inert under the reaction conditions and readily removed
due to its volatility. The structures of P1 and P2 were
confirmed by NMR spectroscopy as well as by X-ray
crystallography in the case of P2, providing the first crystal
structure of a complex derived from an aryl triflate with L1 as
the supporting ligand (Figure 1). The complexes are stable
under air in a benchtop desiccator in the solid state for at least
10 months, as judged by 31P NMR spectroscopy.13
Scheme 2. Fluorination of Aryl Bromides and Triflates with
a b
,
P1 and P2
Our reported conditions for the palladium-catalyzed
fluorination of aryl bromides and triflates call for [L1−
Pd]2(cod) as the source of ligand and palladium.4a We initially
employed P1 alone in place of the cyclooctadiene complex for
the fluorination of 1-bromo-4-n-butylbenzene and found that it
performed poorly compared to the previously developed
conditions (Table 1, entries 1 vs 2). However, when exogenous
cyclooctadiene was added, the yield increased substantially
(Table 1, entry 3). We postulate this additive may improve the
yield by stabilizing any off-cycle Pd(0) species and averting
irreversible catalyst deactivation. Encouraged by this result, we
examined the generality of these conditions toward the
fluorination of aryl triflates. Indeed, P1 also served as a
competent precatalyst for the fluorination of aryl triflates,
although small amounts of the corresponding aryl bromides
were also formed, complicating isolation of the desired
a
Isolated yields are reported as an average of two runs. General
conditions: ArBr (1.0 mmol), AgF (2.0 mmol), KF (0.5 mmol),
b
cyclohexane (10 mL). ArOTf (1.0 mmol), CsF (3.0 mmol),
cyclohexane (10 mL). Toluene was the reaction solvent.
c
B
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trimethylsilyl ethyl (TMSE) ester, which can be selectively
cleaved with fluoride to liberate the corresponding carboxylate,
which is easier to separate from the desired product. On the
basis of previously reported C−O and C−N cross-coupling
processes,3,15 L2 (t-BuBrettPhos) was selected as the ligand for
the precatalyst. Complex P3 was therefore prepared as a
precatalyst for these types of coupling reactions. The new
precatalyst was first employed for the N-arylation of amino acid
esters under mild and stereoretentive conditions. By employing
P3, the desired arylation products were formed in moderate to
good yields with high levels of enantioretention (Scheme 3).
zine (4c) readily underwent coupling to afford the bis-
(methoxylation) product in excellent yield.
In summary, we have demonstrated that oxidative addition
complexes based on palladium, supported by extremely bulky
dialkylbiaryl phosphine ligands, can serve as active precatalysts
for a wide variety of challenging, synthetically useful carbon−
heteroatom bond-forming reactions. The complexes are well-
characterized, easily handled outside of a glovebox, and provide
a means of introducing L−Pd(0) into a reaction without the
formation of potentially inhibitory byproducts.
ASSOCIATED CONTENT
* Supporting Information
■
a
Scheme 3. Amino Acid Ester Arylation with P3
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01082.
Experimental procedures and characterization data for
new compounds (PDF)
Crystal data of P2 (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sbuchwal@mit.edu.
ORCID
a
Isolated yields are reported as an average of two runs. General
conditions: ArOTf (1.0 mmol), amino acid ester (1.1 mmol), Cs2CO3
(3.0 mmol), P3 (5 mol %), 2-methyltetrahydrofuran (2 mL).
Enantiomeric excess (ee, average of two runs) was determined by
HPLC analysis using chiral stationary phases.
Bryan T. Ingoglia: 0000-0002-9049-6227
Notes
The authors declare the following competing financial
interest(s): MIT has patents on some of the ligands and
precatalysts described in this work, from which S.L.B. as well as
former or current coworkers receive royalty payments.
Treatment with TBAF, if necessary, was mild enough to cleave
the aminated TMSE ester present in the byproduct selectively,
simplifying purification of the desired product (3b, 3d) without
eroding the enantiomeric excess and with merely one percent
ACKNOWLEDGMENTS
■
1
We thank Aldrich for a gift of t-BuBrettPhos (L2). We thank
Dr. Mycah Uehling (MIT), Dr. Nicholas White (MIT), and Dr.
Yi-ming Wang (MIT) for assistance in the preparation of this
manuscript. We thank Dr. Aaron Sather (MIT), Dr. Sandra
King (MIT), and Anni Zhang (MIT) for providing aryl triflate
starting materials for the project. We thank Dr. Kurt Armbrust
(MIT) and Dr. Pedro Arrechea (MIT) for helpful discussions.
We thank Jonathan Becker (MIT) for solving the X-ray
structure of P2. The X-ray diffractometer was purchased with
the help of funding from the National Science Foundation
(Grant No. CHE 0946721). The Varian 500 MHz instrument
used for portions of this work was supported by the National
Science Foundation (Grant No. CHE 9808061). Research
reported in this publication was supported by the National
Institutes of Health under Award Nos. GM46059 and
GM58160. The content is solely the responsibility of the
authors and does not necessarily represent the official views of
the National Institutes of Health.
or less of the byproduct as judged by the H NMR spectrum
after chromatographic purification (see SI for details). Other
compounds (3a, 3c) could be purified without the addition of
fluoride. Precatalyst P3 was also found to be suitable for the
cross-coupling of oxygen nucleophiles (Scheme 4). Hetero-
cyclic aryl halides, including a quinoxaline (4a), an indazole
(4b), and an isoquinoline (4d), were hydroxylated or
methoxylated in good yield. An N-benzyl dibromophenothia-
a b
,
Scheme 4. Alcohol and Hydroxide Coupling with P3
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■
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