Palladium–Borane Cooperation: Evidence for an Anionic Pathway and Its Application to Catalytic Hydro-/Deutero-dechlorination

Article abstract of DOI:10.1002/anie.201909675

Metal–Lewis acid cooperation provides new opportunities in catalysis. In this work, we report a new type of palladium–borane cooperation involving anionic Pd⁰ species. The air-stable DPB palladium complex 1 (DPB=diphosphine-borane) was prepared and reacted with KH to give the Pd⁰ borohydride 2, the first monomeric anionic Pd⁰ species to be structurally characterized. The boron moiety acts as an acceptor towards Pd in 1 via Pd→B interaction, but as a donor in 2 thanks to B-H-Pd bridging. This enables the activation of C?Cl bonds and the system is amenable to catalysis, as demonstrated by the hydro-/deutero-dehalogenation of a variety of (hetero)aryl chlorides (20 examples, average yield 85 %).

Full text of DOI:10.1002/anie.201909675

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Accepted Article
Title: Palladium-Borane Cooperation: Evidence for an Anionic Pathway
and its Application to Catalytic Hydro- / Deutero-dechlorination
Authors: Hajime Kameo, Jun Yamamoto, Ayaka Asada, Hiroshi
Nakazawa, Hiroyuki Matsuzaka, and Didier Bourissou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201909675
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Palladium-Borane Cooperation: Evidence for an Anionic Pathway
and its Application to Catalytic Hydro- / Deutero-dechlorination
Hajime Kameo,*[a] Jun Yamamoto,[a] Ayaka Asada,[a] Hiroshi Nakazawa,[b] Hiroyuki Matsuzaka,[a] and
Didier Bourissou*[c]
Dedicated to Prof. Pablo Espinet on the occasion of his 70th birthday
Abstract: Metal-Lewis acid cooperation provides new opportunities in
catalysis. In this work, we report a new type of palladium-borane
cooperation involving anionic Pd(0) species. The air-stable DPB
palladium complex 1 (DPB = diphosphine-borane) was prepared and
reacted with KH to give the Pd(0) borohydride 2, the first monomeric
anionic Pd(0) species to be structurally characterized. The boron
moiety acts as an acceptor towards Pd in 1 via PdB interaction, but
as a donor in 2 thanks to B–H–Pd bridging. This enables the activation
of C–Cl bonds and the system is amenable to catalysis, as
demonstrated by the hydro- / deutero-dehalogenation of a variety of
(hetero)aryl chlorides (20 examples, average yield 85%).
Over the past 15 years, polyfunctional ligands featuring Lewis
acid moieties, so-called ambiphilic ligands, have garnered huge
interest.[1] Most noticeable is the ability of Lewis acids to behave
as -acceptor ligands towards transition metals and engage into
rare TM→LA interactions.[1] The presence of a Lewis acid at or
nearby the metal strongly influences its reactivity[2] and opens new
avenues in catalysis.[3] Accordingly, different types of transition
metal-Lewis acid cooperation have been progressively
authenticated and applications in catalysis have started to
Figure 1. Schematic representation of the different modes of transition metal-
Lewis acid cooperation, with selected examples of complexes applied in
catalysis.
emerge (Fig. 1). Typically, Lewis acids have been used to
generate electrophilic metals (Fig. 1A), either by abstracting
intramolecularly an X-type ligand (i),[4] or directly by withdrawing
electron density via TM→LA interactions (ii).[5] In addition,
In general Pd-catalyzed coupling reactions proceeds via
oxidative addition, transmetalation and reductive elimination (Fig.
2, top). With palladium-borane cooperation, the reaction
sequence may be reversed, as follows (Fig. 2, bottom): addition
of the nucleophilic partner (formal insertion of the Y group into the
PdB bond), oxidative addition and reductive elimination.
Several advantages may be envisioned for such an anionic
pathway: (i) stabilized active species (the electron-rich metal
center is engaged into Pd→B interaction), (ii) oxidative addition to
an activated anionic Pd(0) complex, and (iii) B to Pd transfer of
the nucleophilic group assisting the R–Y bond formation.[8]
Herein, we disclose a diphosphine-borane Pd complex
displaying this unprecedented mode of reactivity and substantiate
its catalytic relevance in hydro- and deutero-dechlorination
reactions.
cooperative activation of -bonds by addition across TM→LA
interactions has been shown feasible under stoichiometric and
even catalytic conditions (Fig. 1B).[6] This represents
a
conceptually new approach in metal-ligand cooperation which
usually involves electron-rich or redox-active ligands. As a further
extension, we reasoned that hydride insertion into TM→LA
interactions may give access to anionic complexes in which the
TM is electronically enriched by LA–H–M bridging (Fig. 1C).[7,8]
[a]
Dr. H. Kameo, J. Yamamoto, A. Asada, Prof. H. Matsuzaka
Department of Chemistry, Graduate School of Science, Osaka
Prefecture University
Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531 (Japan)
E-mail: h.kameo@c.s.osakafu-u.ac.jp
[b]
[c]
Prof. H. Nakazawa
Department of Chemistry, Graduate School of Science, Osaka City
University
Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
Dr. D. Bourissou
Laboratoire Hétérochimie Fondamentale et Appliquée
Université Paul Sabatier/CNRS UMR 5069
118 Route de Narbonne, 31062 Toulouse Cedex 09 (France)
E-mail: dbouriss@chimie.ups-tlse.fr
Supporting information and the ORCID idendification number(s) for
the author(s) of this article can be found under:
https://doi.org.10.1002/anie.XXX
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Scheme 1. Synthesis of the anionic palladium(0) complex 2 and its reaction with
chlorobenzene.
Figure 2. Classical pathway for Pd-catalyzed cross-couplings (top) and
envisioned anionic pathway involving Pd-borane cooperation (bottom, this work).
Figure 3. Molecular structures of complexes 1 (left) and 2 (right) (thermal
ellipsoids at 40% probability). For clarity, the [K([2.2.2]-cryptand)] cation and
hydrogen atoms (except the hydride bonded to B and Pd) are omitted, and only
the Cipso carbon atoms of the Ph groups at P are shown.
The neutral Pd(0) complex 1 (Scheme 1) was readily prepared
by phosphine exchange between Pd(PPh3)4 and the diphosphine-
borane (DPB) ligand.[9] It was isolated in 92% yield as an air-
stable yellow powder (no sign of decomposition detected after 3
days standing in air).[10] The molecular structure of 1 was
unambiguously established by multi-nuclear NMR spectroscopy
and X-ray diffraction analysis (Fig. 3, left). It resembles those of
the (DPB)Pd(L) complexes (L = Py, ) recently reported by
Tauchert[11,12] and is also reminiscent of DPB Ni(0)[6a] and Cu(I)[13]
complexes. The Pd−B (2.294(2) Å) distance is only slightly longer
than the sum of the covalent radii (2.23 Å).[14] A relatively short
Pd∙∙∙Cipso contact is also observed (2.638(2) Å), in line with 2-BC
coordination of the BPh moiety.[15,16] Complex 1 shows no sign of
reaction with substrates such as chlorobenzene, bromobenzene,
phenylsilane and dihydrogen even upon heating at 60°C, in line
with the depletion of electron density at Pd by the Lewis acid
moiety.
Unlike the neutral (PdB) complex 1, 2 reacts with
chlorobenzene (1 equiv.) at 60°C to give benzene (73% yield) with
quantitative regeneration of 1 (Scheme 1).[22] This transformation
prompted us to test the catalytic dehalogenation of chlorobenzene
(Table 1). Using 5 mol% of complex 1, 1.5 equiv. of KH and 10
mol% of [2.2.2]-cryptand or 18-crown-6, the transformation
indeed proceeded, affording benzene in 68-76% yield (entries 1
and 2). Control experiments with Pd(PPh3)4 alone or combined
with BPh3 as external Lewis acid led to much lower yields (< 30%),
substantiating the role of the (DPB)Pd complex.[10] Of note, NMR
analysis of a catalytic run performed with [2.2.2]-cryptand
revealed the presence of both complexes 1 and 2 in about 3/1
ratio during catalysis. The anionic pathway disclosed in Fig. 4 is
proposed to account for the catalytic hydrodechlorination of PhCl
via complexes 1 and 2.
Anionic Pd(0) species are generally formed by addition of
nucleophiles, typically halides, to Pd(0) centers.[17] In the case of
complex 1, the borane moiety present in the coordination sphere
of Pd can serve as an acceptor to give access to anionic Pd(0)
borate species. This chemical behavior was realized by reacting
1 with KH at 60 °C in the presence of [2.2.2]-cryptand. The Pd(0)
borohydride complex 2 was thereby obtained in 43% isolated yield
after workup.[10] Upon hydride addition, the 11B NMR signal shifts
to high field (from  27 ppm for 1 to  −7.2 ppm for 2) and splits
1
into a doublet due to JB-H coupling (70.2 Hz).[18] Single crystals
suitable for X-ray diffraction analysis were grown by slow diffusion
of Et2O into a THF solution of 2 (Fig. 3, right). The complex adopts
a separate ion pair structure. The additional hydrogen atom
(which was located in the difference Fourier map and refined
isotropically) bridges the B and Pd centers (B−H, 1.21(4) Å and
Pd−H, 1.889(X) Å).[19,20] The boron center is in tetrahedral
environment (the sum of the three C−B−C bond angles reduce to
335.3(5)º), while the Pd center adopts a rare trigonal pyramidal
geometry ((P−Pd−P) = 358.3(7)º) with the hydride in the apical
position. To our knowledge, this is the first monomeric anionic
Pd(0) complex to be structurally characterized.[21]
Figure 4. Anionic pathway proposed to account for the catalytic
hydrodechlorination of PhCl via complexes 1 and 2.
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Table 1. Screening of conditions for the catalytic dehalogenation of
chlorobenzene.
Entry
Catalyst
Reductant / Activator
Conv.[a]
Yield[a]
1
2
1
1
KH / [2.2.2]-cryptand
KH / 18-crown-6
>99
>99
68
76
3
4
5
6
7
8
1
1
1
1
1
2
NaH / 15-crown-5
LiH / 12-crown-4
trace
trace
trace
89
trace
trace
trace
89
HCOONa / 15-crown-5
HCOOK / [2.2.2]-cryptand
HCOOK / 18-crown-6
HCOOK / [2.2.2]-cryptand
38
38
88
88
Figure 5. Reaction profile computed for the hydrodechlorination of PhCl
mediated by the anionic Pd(0) complex 2 (G in kcal/mol, [M]=K[2.2.2]-cryptand).
[a] Determined by GC.
Table 2. Catalytic hydro / deutero-dechlorination catalyzed by the PdB
complex 1: scope of (hetero)aryl chlorides.[a]
DFT calculations performed on the real system support this
mechanism and shed light into the interplay between Pd and B.[10]
Fig. 5 shows the most favorable reaction profile. Following the
endergonic displacement of PPh3 by PhCl at Pd, leading to
complex A1, C‒Cl activation takes places (the overall energy
barrier amounts to 26.7 kcal/mol) to afford the Pd(II) complex 3Cl.
After KCl elimination, B to Pd hydride transfer[8] and reductive
elimination are very exergonic and proceed with very low
activation barriers to give complex A3. Displacement of benzene
by PPh3 is further downhill in energy and affords 1.
The catalytic performance of complex 1 was then assessed
further.[10] First, different hydrides were surveyed. While LiH, NaH,
and HCOONa proved inefficient (Table 1, entries 3-5) probably
due to unfavourable trapping of the Li/Na cations, the use of
HCOOK with [2.2.2]-cryptand improved the selectivity of the
reaction and afforded benzene in 89% yield (entry 6).[23] The
borohydride complex 2 exhibited comparable catalytic activity
(entry 8). The scope of chloroarenes was then explored (Table 2).
The transformation displays high functional group tolerance (ether,
CF3, NO2, CN, ester, amide, F, OH, and ketone). It proceeds well
with chloronaphthalenes and chloroheteroarenes including
thiophene, isoquinoline, and quinolone frameworks. However,
significantly lower yields were obtained with chlorobenzenes
featuring methyl and methoxy groups in the para position,
indicating that the reaction is very sensitive to the electron density
of the aromatic ring. This feature enabled the selective mono-
dechlorination of dichloroquinolines. The broad interest of
deuterium-labelled compounds, in particular in pharmacology and
drug discovery,[24] prompted us to investigate catalytic deutero-
dechlorination with the PdB complex 1. Gratifyingly, the use of
[a] Yields were determined by GC. [b] Reactions were performed in close
systems using pressure resistant vessels. [c] Isolated yields of deuterium-
labeled products. [d] Determined by 1H NMR.
In summary, the air-stable PdB complex 1 was found to
react with potassium hydride to give the B–H–Pd bridged complex
2, the first monomeric anionic Pd(0) complex to be structurally
characterized. Insertion of the hydride in the PdB bond
increases the electron density at Pd and promotes the activation
of chlorobenzene. Upon B to Pd hydride transfer and reductive
elimination, benzene is obtained. This anionic pathway is
amenable to catalysis and accordingly, complex 1 was found to
efficiently promote the hydro and deutero-dehalogenation of a
range of (hetero)aryl chlorides. Future studies will aim to develop
further this new type of palladium-borane cooperation, including
towards catalytic carbon-heteroatom coupling reactions.
DCOOK provided straightforward access to
a variety of
deuterium-labelled compounds with high deuterium incorporation
(>95%). Mono and selective deutero-dechlorination of polychloro
heteroarenes, as achieved here for the first time, is particularly
noteworthy.
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Lu, J. Am. Chem. Soc. 2017, 139, 6570–6573; e) H. Kameo, Y.
Hashimoto, H. Nakazawa, Organometallics 2012, 31, 3155–3162.
For reviews dealing with hydrogen transfer between TM and LA, see: a)
G. R. Owen, Chem. Soc. Rev. 2012, 41, 3535–3546; b) G. R. Owen,
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Acknowledgements
[8]
[9]
This work was supported by Grant-in-Aid for Scientific Research
(C) (No. 18K05151 and 18K05152) of the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan. H.K.
acknowledges the financial support from the Tonen general
sekiyu research/development encouragement & scholarship
foundation. The Centre National de la Recherche Scientifique
(CNRS), the Université Paul Sabatier (UPS) and the Agence
Nationale de la Recherche (ANR-15-CE07-0003) are
acknowledged for financial support of this work.
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partners, see ref. 11a.
Keywords: Anionic Complexes • Boranes • Catalysis •
Dehalogenation • Deuteration • Palladium
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[2]
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[17] The role of anionic Pd(0) ate complexes in Heck and cross-coupling
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[18] HBPh3 salts display similar data: 11B –8.2 ppm, 1JB-H = 76-79 Hz, see:
D. Mukherjee, H. Osseili, T. P. Spaniol, J. Okuda, J. Am. Chem. Soc.
2016, 138, 10790–10793.
[3]
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[20] Wiberg bond indexes of 0.757 (B–H) and 0.139 (Pd−H) were computed
by DFT for complex 2.[10]
[21] For an anionic Pd(0) complex adopting a dinuclear ligand-bridged
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[22] The presence of excess PPh3 strongly alters this reaction, less than 10%
conversion being observed when 10 equiv. of PPh3 are added. The
activation of chlorobenzene probably requires PPh3 dissociation to open
a coordination site at Pd.
[23] DFT calculations also suggest an anionic pathway under these
conditions: coordination of formate to boron, C‒Cl activation, B to Pd
formate migration, decarboxylation, reductive elimination (C‒H
coupling).[10] Attempts to generate the formate analog of 2 failed.
Reaction of complex 1 with HCO2K/[2.2.2]-cryptand was unproductive
and
the
formate
adduct
of
the
diphosphine
borane
[(DPB)OCHO][K([2.2.2]-cryptand)] reacted with Pd(PPh3)4 to give the
PdB complex 1.[10]
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[7]
The weak and adaptative character of TMLA interactions make Z-type
ligands very efficient in stabilizing electron-rich TM complexes. This
feature has been exploited in catalytic N2 and CO2 reduction. See for
examples: a) M. –E. Moret, J. C. Peters J. Am. Chem. Soc. 2011, 133,
18118–18121; b) J. S. Anderson, J. Rittle, J. C. Peters, Nature 2013, 501,
84–87; c) P. A. Rudd, S. Liu, L. Gagliardi, V. G. Young Jr., C. C. Lu, J.
Am. Chem. Soc. 2011, 133, 20724–20727; d) M. V. Vollmer, J. Xie, C. C.
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Zhong, W.-Y. Wang, L.-Y. Wu, W. Liu, B. M. Stolz, W.-B. Liu, J. Am.
Chem. Soc. 2018, 140, 10970–10974.
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Entry for the Table of Contents
COMMUNICATION
A palladium-borane dance: an
anionic reaction pathway involving
hydride insertion into PdB bond, C–
Cl bond activation by an anionic Pd(0)
species and B-assisted C–H bond
formation has been discovered and
leveraged into catalysis.
Hajime Kameo,* Jun Yamamoto, Ayaka
Asada, Hiroshi Nakazawa, Hiroyuki
Matsuzaka, Didier, Bourissou*
Page No. – Page No.
Palladium-Borane Cooperation:
Evidence for an Anionic Pathway and
its Application to Catalytic Hydro /
Deutero-dechlorination
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