Highly selective palladium-catalyzed cross-coupling of secondary alkylzinc reagents with heteroaryl halides

Article abstract of DOI:10.1021/ol502230p

The highly selective palladium-catalyzed Negishi coupling of secondary alkylzinc reagents with heteroaryl halides is described. The development of a series of biarylphosphine ligands has led to the identification of an improved catalyst for the coupling of electron-deficient heterocyclic substrates. Preparation and characterization of oxidative addition complex (L)(Ar)PdBr provided insight into the unique reactivity of catalysts based on CPhos-type ligands in facilitating challenging reductive elimination processes.

Full text of DOI:10.1021/ol502230p

Letter
pubs.acs.org/OrgLett
Open Access on 08/25/2015
Highly Selective Palladium-Catalyzed Cross-Coupling of Secondary
Alkylzinc Reagents with Heteroaryl Halides
Yang Yang, Katrin Niedermann, Chong Han, and Stephen L. Buchwald*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: The highly selective palladium-catalyzed Ne-
gishi coupling of secondary alkylzinc reagents with heteroaryl
halides is described. The development of a series of
biarylphosphine ligands has led to the identification of an
improved catalyst for the coupling of electron-deficient
heterocyclic substrates. Preparation and characterization of
oxidative addition complex (L)(Ar)PdBr provided insight into
the unique reactivity of catalysts based on CPhos-type ligands in facilitating challenging reductive elimination processes.
romatic compounds bearing one or multiple alkyl
components represent ubiquitous structural motifs
with aryl bromides and activated aryl chlorides to deliver a
range of coupling products with good selectivity.4 Organ also
developed a well-engineered NHC-based PEPPSI precatalyst
(Pd-PEPPSI-IPentCl), enabling the selective preparation of
functionalized arenes bearing secondary alkyl substituents.5
Despite these advances, significant challenges still remain.
While the coupling of relatively simple aromatic substrates and
secondary alkyl nucleophiles can be accomplished with good
regioisomeric retention, efforts to combine heteroaryl electro-
philes with secondary alkyl organometallic reagents have been
met with considerably less success. Because of the altered
electronic properties of heterocylic compounds, poor selectivity
for the desired coupling products (4) is usually obtained. In
addition, the presence of heteroatoms capable of coordinating
to the Pd center can lead to catalyst inhibition and deactivation,
thereby rendering the coupling of these heterocycles
particularly challenging.9 Given the importance of heterocyclic
A
among pharmaceuticals and natural products.1 Consequently,
extensive efforts have been devoted to the rapid and direct
construction of sp2−sp3 carbon−carbon bonds in both
industrial and academic settings. One of the most frequently
practiced methods to form sp2−sp3 carbon−carbon bonds
relies on transition-metal-catalyzed coupling reactions.2 How-
ever, the cross-coupling involving secondary alkyl nucleophiles
remains challenging, owing to the competitive β-hydride
elimination and migratory reinsertion that results in the
formation of undesired isomerized products (5) (Scheme 1).
Scheme 1. Major Side Reactions Associated with Pd-
Catalyzed Cross-Coupling Involving Secondary Alkyl
Nucleophiles
compounds in medicinal chemistry and materials science,10
a
general, practical, and selective protocol for the coupling of
heteroaryl halides with secondary alkyl nucleophiles is highly
desirable. Herein we report our efforts in catalyst development
for such coupling reactions. With these monodentate
biarylphosphine-based catalysts, a diverse array of heteroaryl
halides, including those that were unsuccessful substrates with
our previously reported catalyst system, can be combined with
secondary alkylzinc reagents with high levels of regiochemical
fidelity.
Our initial studies focused on the coupling of various types of
heteroaryl halides with isopropylzinc bromide prepared using
Knochel’s procedure (Scheme 2).11 It was found that through
the use of our easily activated palladacycle precatalyst12 (11)
ligated by CPhos13 (L1), a wide range of heteroaryl halides,
including 3-chlorobenzosisothiazole (10a), 4-chloroquinazoline
(10b), 4- and 5-halopyrimidine (10c and 10d), and 3-
To overcome this challenge, the development of catalyst
systems to facilitate reductive elimination while suppressing
competitive β-hydride elimination is of central importance.
Since the pioneering work of Kumada and Hayashi3 in the area
of Ni- and Pd-catalyzed selective cross-coupling of secondary
alkyl nucleophiles, several key advances have been achieved in
the past decade.4−8 In 2009, our group described a catalyst
system based on a diakylbiarylphosphine ligand (CPhos, L1),
which allowed for the coupling of secondary alkylzinc reagents
Received: July 28, 2014
© XXXX American Chemical Society
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Letter
Scheme 2. Cross-Coupling of Heteroaryl Halides with
Isopropylzinc Bromide
Scheme 3. Ligand Effect on the Coupling of 2-
a,b
a
Bromopyrimidine and i-PrZnBr·LiCl
a
Reaction conditions: HetAr-X (1.0 mmol), i-PrZnBr·LiCl (1.3
a
b
mmol), 0 °C to rt, 1−12 h. Yields are isolated yields on average of
Catalyst derived from L12 afforded 15% conversion after 2 h. Only
30% conversion was achieved in the absence of phosphine ligands; no
reaction in the absence of Pd(OAc)2. 13a/13b ratio was determined
by GC analysis of the crude reaction mixture.
b
two runs. 1 equiv of LiCl was used; THF was used as the sole solvent;
c
2 mol % of 15 and 2 mol % of L1 was used. 2.3 equiv of iPrZnBr·LiCl
was used. n = normal product, r = rearranged product. n/r ratio was
determined by GC and/or 1H NMR spectroscopy analysis of the crude
reaction mixture.
drawing substituents to the P-bound Ar- groups did not
provide improved results (L6), and the L5-based catalyst
bearing P-bound 3,5-dimethyl-4-methoxyphenyl groups ex-
hibited the best selectivity for the nonrearranged product. We
next replaced the P-bound cyclohexyl group in L1 by other
alkyl groups. Eventually, L10 (EtCPhos) possessing two less
electron-donating P-bound ethyl substituents16 was identified
to as the most effective ligand for this transformation.
With the new set of reaction conditions in hand, we set out
to explore the substrate scope of electron-deficient six-
membered nitrogen heterocycles (Scheme 4). It was found
that the catalyst based on EtCPhos (L10) accommodated a
wide variety of nitrogen heterocycles including 2-chloropyr-
idine (15a), 2-chloroquinoline (15b), 2-chloropyrazine (15c),
2-chloroquinoxaline (15d), 3-chloropyridazine (15e), deliver-
ing the corresponding coupling products in excellent yields.
Moreover, in most cases (15a−e), improved selectivity for the
nonrearranged product was achieved as compared with the L1-
based catalyst that we previously developed.
To further demonstrate the utility of catalysts based on
CPhos-type ligands, a series of secondary alkylzinc halides were
prepared and coupled with a wide range of heteroaryl halides
(Scheme 5). Coupling of acyclic secondary alkylzinc halides
proceeded with excellent selectivity for the desired product
(18a−c). Notably, rearrangement of the alkyl content was not
observed during the coupling event when benzylzinc reagents
(18d) and cyclic alkylzinc reagents (18e and 18f) were used.
Other cyclic secondary alkylzinc reagents ranging from
cyclopropyl to cyclohexylzinc halides (18g−i) could also be
effectively coupled. We note, however, that haloimidazoles
afforded low yields under the current reaction conditions due to
the competitive reduction of these heteroaromatic substrates.
bromobenzothiazole (10e) could be effectively transformed in
good yields with high level of selectivity. The coupling of 5-
chlorobenzothiazole (10f) proved to be more difficult with this
catalyst system, and we found that the addition of 1 equiv of
LiCl allowed these processes to occur with excellent yields.14
Finally, nitrogen heterocycles with unprotected NH groups
such as 5-bromoindole (10g), 5-bromo-7-azaindole (10h), and
6-bromoindazole (10i) also represented compatible substrates
under our conditions.
The catalyst derived from CPhos (L1) was not effective with
electron-deficient six-membered nitrogen heterocycles. For
example, using the CPhos-based catalyst, Negishi coupling of
2-bromopyrimidine (12) and isopropylzinc bromide (9)
furnished a 75:25 mixture of 2-isopropylpyrimidine (13a) and
2-propylpyrimidine (13b) as determined by GC analysis
(Scheme 3). To overcome this limitation, we set out to further
facilitate the reductive elimination process by preparing and
examining a new series of biarylphosphine ligands (L2−L13).
We decided to preserve the biaryl framework of CPhos as a key
design element for creating more effective ligands, as
mechanistic studies suggested that the dimethylamino
(Me2N-) groups present in the CPhos biaryl backbone are
critical to accelerate reductive elimination and discourage β-
hydride elimination (vide infra). Since electron-deficient ligands
have been demonstrated to accelerate the reductive elimination
step,15 we prepared ligands L2−L7 wherein the cyclohexyl
groups on the phosphine were replaced with less electron-
donating aryl groups. Indeed, a catalyst composed of L3
(PhCPhos) furnished improved selectivity than the original L1-
based catalyst, as demonstrated by a 10-fold increase in the
13a/13b ratio. However, adding additional electron-with-
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(Scheme 6). Reaction of methyl 4-chlorobenzoate and
isopropylzinc bromide employing catalytic amount (1 mol %)
Scheme 4. Cross-Coupling of Electron-Deficient Six-
Membered Heteroaryl Halides with Isopropylzinc Bromide
a
Scheme 6. Synthesis of Oxidative Addition Complex 20
of 20 afforded the same mixture of rearranged and non-
rearranged products (45:1) as when palladacycle precatalyst 11
was used, demonstrating the catalytic competence of 20 for the
coupling of secondary alkylzinc halides. Single-crystal X-ray
diffraction analysis (Figure 1) of 20 revealed a nearly square-
a
Reaction conditions: HetAr-X (1.0 mmol), i-PrZnBr·LiCl (1.3
mmol), Pd(OAc)2 (1 mol %), L10 (2 mol %), 0 °C to rt, 12 h. n =
normal product, r = rearranged product. n/r ratio was determined by
1
GC or H NMR spectroscopy analysis of the crude reaction mixture.
Yields are isolated yields on average of two runs. In most cases, the
rearranged product and the nonrearranged product are chromato-
graphically inseparable.
Scheme 5. Substrate Scope of Secondary Alkylzinc
a
Reagents
Figure 1. ORTEP representation drawing of oxidative addition
complex 20. Hydrogen atoms omitted for clarity; thermal ellipsoids
set at 50% probability.
planar Pd(II) center featuring κ2 bound CPhos ligand through
P atom and ipso-C moiety of the bottom aromatic ring (ipso-
C−Pd bond length = 2.478(3) Å). The solid-state structure of
20 indicates that neither of the dimethylamino substituents of
L1 coordinates to the Pd(II) center, indicating the mono-
dentate nature of L1. Further examination of 20 suggests that
neither of the dimethylamino groups lies in the plane of the
bottom ring of L1, with the dihedral angles C38−N1−C32−
C31 and C40−N2−C36−C31 being 39.12° and 82.24°,
respectively. This observation is suggestive that the lone pair
of the Me2N- group is not in conjugation with the lower ring of
L1 and the Me2N groups presented in the ligand biaryl
backbone could likely serve as electron-withdrawing substitu-
ents, thereby rendering the bottom ring of the phosphine less
electron-donating. In light of this effect, we believe that the use
of CPhos-type ligands may facilitate reductive elimination and
carefully balancing the electron-donating ability of P-bound
a
Reaction conditions: HetAr-X (1.0 mmol), sec-alkylZnBr·LiCl (1.3
mmol), 15 (1 mol %), L1 (1 mol %), 0 °C to rt, 2 h. a. 60 °C. n =
normal product, r = rearranged product. n/r ratio was determined by
GC and/or H NMR analysis of the crude reaction mixture.
1
To gain further insight into the unique reactivity of Pd-based
catalyst system featuring CPhos-type ligands, we prepared
oxidative addition complex [L1·ArPdBr] (Ar = 4-cyanophenyl)
(20) as an air-stable bright yellow solid by treating (COD)-
Pd(CH2TMS)2 with 4-bromobenzonitrile and L1 in THF
C
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(6) Other studies involving secondary alkylzinc reagents: (a) Thaler,
T.; Haag, B.; Gavryushin, A.; Schober, K.; Hartmann, E.; Gschwind, R.
M.; Zipse, H.; Knochel, P. Nat. Chem. 2010, 2, 125. (b) Krasovskiy, A.;
Duplais, C.; Lipshutz, B. H. J. Am. Chem. Soc. 2009, 131, 15592.
(7) Boron-based secondary alkyl nucleophile: (a) Dreher, S. D.;
Dormer, P. G.; Sandrock, D. L.; Molander, G. A. J. Am. Chem. Soc.
2008, 130, 9257. (b) van den Hoogenband, A.; Lange, J. H. M.;
Terpstra, J. W.; Koch, M.; Visser, G. M.; Visser, M.; Korstanje, T. J.;
Jastrzebski, J. T. B. H. Tetrahedron Lett. 2008, 49, 4122.
(8) Tin-based secondary alkyl nucleophile: Li, L.; Wang, C.-Y.;
Huang, R.; Biscoe, M. R. Nat. Chem. 2013, 5, 607.
(9) (a) Shen, Q.; Shekhar, S.; Stambuli, J. P.; Hartwig, J. F. Angew.
Chem., Int. Ed. 2005, 44, 1371. (b) Su, M.; Buchwald, S. L. Angew.
Chem., Int. Ed. 2012, 51, 4710.
(10) (a) Joule, J. A.; Mills, K. Heterocyclic Chemistry, 5th ed.; Wiley:
Chichester, 2010. (b) Leurs, R.; Bakker, R. A.; Timmerman, H.; de
Esch, I. J. P. Nat. Rev. Drug Discovery 2005, 4, 107.
substituents and the biaryl backbone is critical to the success of
selective coupling of secondary alkyl nucleophiles.
In summary, we have developed general catalyst systems
allowing for the highly selective cross-coupling of secondary
alkylzinc reagents and heteroaryl halides under mild conditions.
Our protocol is effective with a broad spectrum of heteroaryl
halides, delivering an array of complex heterocycles possessing
secondary alkyl substituents that are frequently found in
biologically active compounds. Furthermore, design and
evaluation of a series of biarylphosphine ligands bearing a
2,6-bis(dimethylamino)phenyl group proximal to the phos-
phine have led to a new catalyst that demonstrated superior
selectivity for the coupling of electron-deficient heteroaryl
halides. Application of these newly developed catalysts in cross-
coupling reactions where transmetalation or reductive elimi-
nation remains challenging is topic of onging investigation in
our laboratory.
(11) Krasovskiy, A.; Malakhov, V.; Gavryushin, A.; Knochel, P.
Angew. Chem., Int. Ed. 2006, 45, 6040.
(12) (a) Bruno, N. C.; Tudge, M. T.; Buchwald, S. L. Chem. Sci.
2013, 4, 916. (b) Yang, Y.; Oldenhuis, N. J.; Buchwald, S. L. Angew.
Chem., Int. Ed. 2013, 52, 615. (c) Yang, Y.; Mustard, T. J. L.; Cheong,
P. H.-Y. Angew. Chem., Int. Ed. 2013, 52, 14098.
(13) CPhos (CAS no. 1160556-64-8) is now commercially available
from Aldrich (catalog no. 759171).
(14) For the effect of added LiCl, see: McCann, L. C.; Hunter, H. N.;
Clyburne, J. A. C.; Organ, M. G. Angew. Chem., Int. Ed. 2012, 51, 7024.
(15) For reviews on mechanistic aspects of reductive elimination, see:
(a) Hartwig, J. F. Organotransition Metal Chemistry: from Bonding to
Catalysis; University Science Books: Sausalito, 2009. (b) Hartwig, J. F.
Inorg. Chem. 2007, 46, 1936.
(16) Significant differences between Et- and Cy-substituents are
evident by comparing Tolman’s electronic parameters for PEt3 and
PCy3, showing that Et- is less electron-donating than Cy-: PEt3, 2061.7
cm−1; PCy3, 2056.4 cm−1. For a review, see: Tolman, C. A. Chem. Rev.
1977, 77, 313.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, characterization, and spectral data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sbuchwal@mit.edu.
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 co-workers receive royalty payments.
ACKNOWLEDGMENTS
■
Financial support is provided by National Institutes of Health
(Grant No. GM46059). K.N. thanks the Swiss National Science
Foundation for a postdoctoral fellowship. We thank Dr. Peter
Muller (MIT) for X-ray crystal structure solution and Dr. Tom
̈
Kinzel (MIT) for preliminary mechanistic studies. The content
is solely the responsibility of the authors and does not
necessarily represent the official views of the National Institute
of Health.
REFERENCES
■
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