Rhodium-Catalyzed PIII-Directed ortho-C?H Borylation of Arylphosphines

Article abstract of DOI:10.1002/anie.201813452

Transition-metal-mediated metalation of an aromatic C?H bond that is adjacent to a tertiary phosphine group in arylphosphines via a four-membered chelate ring was first discovered in 1968. Herein, we overcome a long-standing problem with the ortho-C?H activation of arylphosphines in a catalytic fashion. In particular, we developed a rhodium-catalyzed ortho-selective C?H borylation of various commercially available arylphosphines with B₂pin₂ through P^III-chelation-assisted C?H activation. This discovery is suggestive of a generic platform that could enable the late-stage modification of readily accessible arylphosphines.

Full text of DOI:10.1002/anie.201813452

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Accepted Article
Title: Rh-Catalyzed P(III)-Directed ortho C-H Borylation of
Arylphosphines
Authors: Zhuangzhi Shi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201813452
Angew. Chem. 10.1002/ange.201813452
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10.1002/anie.201813452
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
C-H Borylation
Rh-Catalyzed PIII-Directed ortho C-H Borylation of Arylphosphines
Jian Wen, Dingyi Wang, Jiasheng Qian, Di Wang, Chendan Zhu, Yue Zhao, and Zhuangzhi Shi*
Abstract: Transition-metal-mediated metallation of an aromatic C-
H bond that is adjacent to a tertiary phosphine group in
arylphosphines via
a four-membered chelate ring was first
discovered in 1968. Here, we overcome a long-standing problem
with ortho C-H activation of arylphosphines in a catalytic fashion.
Rhodium-catalyzed ortho-selective C-H borylation of various
commercially available arylphosphines with B2pin2 through PIII-
chelation-assisted C-H activation has been developed. This
discovery is suggestive of a generic platform that could enable the
late-stage modification of readily accessible arylphosphines.
Transition-metal-catalyzed
C-H
functionalization
is
a
straightforward method for the formation of carbon-carbon and
carbon-heteroatom bonds in organic synthesis, dramatically
simplifying the synthesis of complex molecules.[1] The C-H
activation step is limited by two fundamental challenges involving
the inert nature of most C-H bonds and the requirement of
controlling site selectivity in molecules that contain diverse C-H
bonds. While several methods have been employed to address these
issues, the most common strategies involve the use of a directing
group chelation-assistance with transition metals forming the
kinetically favored five- or six-membered-ring cyclometallated
intermediates.[2] Such directors are usually N- and O-containing
functional groups.[3] As early as the late 1960s, three groups
independently demonstrated the ortho C-H metallation of
triphenylphosphine in complexes of cobalt, iridium and rhodium
salts, in which the formation of a four-membered chelate ring
Ph2P(o-C6H4)M was uncovered (Figure 1a).[4] Since then, many
groups have reported the synthesis, reactivity and structure of
different transition metal complexes containing ortho-metallated
arylphosphines (ArPR2).[5] Due to the strong coordination between
transition metals and tertiary phosphine groups, the catalytic
variation remains a critical challenge. Recently, we and other groups
have successfully explored the use of the PIII atom as a director in
some transition metal-catalyzed C-H activation reactions including
C-H functionalization of phenols (ArOPtBu2),[6] ferrocene-based,[7]
biaryl[8] and 1-naphthyl[9] phosphines, and indoles (N-PtBu2).[10]
Despite these successes, the exquisite selectivity is only displayed
through PIII-chelation-assisted 5/6-membered-ring intermediates. As
a natural extension of this chemistry, we became interested in the
development of a catalytic process that could achieve the ortho C-H
functionalization of PPh3 and other arylphosphines based on the
precedent stoichiometric reactions.
Figure 1. Towards a catalytic process for ortho C-H borylation of
arylphosphines.
significant progress has been made towards the C-H borylation
reactions, and different types of directing groups with many
transition metal complexes have been explored to overcome the site-
selectivity and substrate diversity.[13] In this context, Clark and
coworkers reported an iridium-catalyzed C-H borylation of benzyl
and biaryl phosphines, in which the phosphine was used as a
directing group (Figure 1b).[8a] o-Boronated phosphines R2P(o-
C6H4)B(OR')2 are valuable synthons that can readily be further
functionalized via well-developed organoboron chemistry.
Moreover, they also act as a class of important frustrated Lewis
pairs (FLPs),[14] which have been studied in a number of intriguing
stoichiometric and catalytic systems for small-molecule activation
and reaction.[15] Traditionally, the preparation of such compounds is
achieved by borylation of the o-bromophenyl phosphines with
BuLi.[16] Here, we report a general method for ortho C-H borylation
of arylphosphines with diboron compounds[17] through rhodium-
catalyzed,[18] PIII-directed C-H activation to rapidly construct a
library of o-boronated phosphines with different steric and
electronic properties (Figure 1c). This in situ and late-stage
modification strategy will greatly improve the atom and step
economy compared to the traditional routes.
To establish a platform for this chemistry, we initially selected
the cross-coupling of PPh3 (1a, 1.0 equiv) with B2pin2 (2a, 1.0
equiv) (Table 1). The reaction was first carried out using Clark’s
reaction conditions in the presence of 1.5 mol% [Ir(OMe)cod]2 at
120° C under an Ar atmosphere in toluene. However, we did not
observe any C-H borylation product (entry 1). To our satisfaction,
the addition of 1.0 equiv KOAc to the above system furnished the
desired ortho C-H borylation product 3aa in 16% yield with a trace
amount of diborylation product 4aa (entry 2). To facilitate isolation
and purification, further treatment of the mixture with BH3·THF
provided two borane complexes 3aa' and 4aa' by column
chromatography, which were confirmed by NMR and X-ray
analysis, respectively (Figure 2). Critical to the promotion of this
reaction was the use of rhodium catalysis (entries 3-4), in which
70% yield of 3aa was obtained with a catalytic amount of
[Rh(coe)2Cl]2 (entry 4). An examination of the additives revealed
that a slightly higher yield (74%) was obtained with a catalytic
amount of HBpin instead of KOAc (entry 5). The bipyridine-type
ligand, commonly used in Ir-catalyzed C-H borylation, resulted in
diminished reactivity in our system (entry 6). Recently, we have
Arylboronates are versatile synthetic intermediates in modern
organic synthesis.[11] They are most often prepared via the
transmetalation of highly reactive aryl-lithium or -magnesium
reagents with electrophilic boron species.[12] During the past decade,
[] Dr. J. Wen, D. Wang, J. Qian, D. Wang, Dr. C. Zhu, Dr. Y.
Zhao, and Prof. Dr. Z. Shi
State Key Laboratory of Coordination Chemistry, School of
Chemistry and Chemical Engineering, Nanjing University,
Nanjing, 210093, China
Prof. Dr. Z. Shi
State Key Laboratory of Elemento-Organic Chemistry, Nankai
University, Tianjin, 300071, China
E-mail: shiz@nju.edu.cn
Homepage: http://hysz.nju.edu.cn/zzshi/
Supporting information and the ORCID identification
number(s) for the author(s) for this article can be found under
http://www.angewandte.org.
1
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10.1002/anie.201813452
Angewandte Chemie International Edition
Table 1. Reaction development.[a]
Table 2. The scope of arylphosphines.[a]
catalyst
(mol%)
additive
(equiv)
3aa/
Yield
(%)[c]
entry
L
4aa[b]
1[d] [Ir(OMe)cod]2 (1.5)
2[d] [Ir(OMe)cod]2 (1.5)
-
-
-
-
-
0
16
KOAc (1.0)
KOAc (1.0)
KOAc (1.0)
HBpin (0.05)
95/5
69/31
84/16
76/24
72/28
89/11
92/8
> 99/1
-
3
4
[Rh(cod)2Cl]2 (1.0)
[Rh(coe)2Cl]2 (1.0)
[Rh(coe)2Cl]2 (1.0)
41
70
5
74
6
[Rh(coe)2Cl]2 (1.0) L1 HBpin (0.05)
[Rh(coe)2Cl]2 (1.0) L2 HBpin (0.05)
[Rh(coe)2Cl]2 (1.0) L3 HBpin (0.05)
[Rh(coe)2Cl]2 (1.0) L3 HBpin (0.05)
56
7
87
8
92 (86)[e]
9[f]
75
10
-
L3 HBpin (0.05)
0
[a] Reaction conditions: 1a (0.60 mmol), 2a (0.60 mmol) and 0.05-1.0
equiv additive in toluene at 110° C for 12 h under argon. [b]
Determined by GC analysis. [c] GC yield of 3aa using dodecane as
an internal standard. [d] At 120°C. [e] Isolated yield of 3aa'. [f] At
100°C. L1 = 4,4'-di-tert-butyl-2,2'-bipyridine; L2 = 2,2'-biphenol; L3 =
catechol.
demonstrated that bisphenol ligands can play a crucial role in PIII-
directed selective C-H activation of indoles at the C7 position in the
presence of rhodium catalysis.[10a] The addition of the 2,2'-biphenol
(L2) indeed improved the C-H activation efficiency (entry 7).
Further screening had shown catechol (L3) to be the optimal ligand
for this reaction, affording product 3aa in 92% yield (entry 8).
Notably, lowering the reaction temperature to 100 °C still
maintained a good reactivity (entry 9). Finally, a control experiment
confirmed that the coupling process did not occur in the absence of
the Rh catalyst (entry 10).
[a] Reaction conditions: 1.0 mol% [Rh(coe)2Cl]2, 2.0 mol% L3, 5.0
mol% HBpin, 1 (0.60 mmol), 2a (0.60 mmol) in 2.5 mL toluene at 110
°C under argon, 12 h; then added BH3·THF for additional 2 h;
Isolated yields after chromatography. [b] Using 3.0 equiv 2a. [c]
Determined by GC analysis.
Figure 2. ORTEP representation of 3aa' and 4aa' with thermal
ellipsoid at 30% probability level.
substituents including methyl (1b), methoxy (1c) and F (1d) groups
at the para position underwent facile ortho C-H borylation,
With the optimized reaction conditions in hand, the scope of this
PIII-directed ortho C-H borylation was then examined (Table 2). In
general, a wide range of arylphosphines containing electronically
and sterically varied groups reacted with B2pin2 (2a) smoothly,
yielding the corresponding boronate phosphines, in modest to
excellent yields. Triarylphosphines 1b-2d, with different aryl
moieties bearing electron-neutral, donating and withdrawing
affording
products
3ba'-3da'
in
74-81%
yields.
Diarylalkylphosphines 1e-1h bearing primary (1e, 1f) and secondary
(1g, 1h) aliphatic groups afforded the desired products, 3ea'-3ha', in
45-85% yields. Dialkylarylphosphine 1i with
a cyclohexyl
substituent was also successfully transformed in good yield under
the reaction conditions. The reaction of tri-m-tolylphosphine (1j),
which contains two sites for possible C-H borylation, showed
2
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10.1002/anie.201813452
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extremely high selectivity for activation of a less hindered C-H
bond. When (2-methoxyphenyl)diphenylphosphine (1k) with two
different aryl moieties was investigated, we obtained a product 3ka'
with C-H activation at the phenyl core. Interestingly, we also
observed some byproducts including 3aa', probably from PPh3 (1a),
and trace amount of tris(2-methoxyphenyl)phosphine (1l).
Phosphines 1a and 1l could be generated from C-P bond metathesis
between two molecules 1k by reversible arylation.[19] Further
investigation showed that the sterically hindered phosphine 1l could
not transform into the corresponding product 3la'. Other substrates
including heteroaromatic phosphines such as 1m-1n, and a bidentate
phosphine ligand 1o did not work in our system as well.
Encouraged by the success with this PIII-directed mono C-H
borylation, we went on to explore the feasibility to selectively form
di-borylation products (Scheme 1). By employing 5.0 equiv B2pin2
(2a) with PPh3 (1a) in the slightly different reaction conditions, we
obtained the di-borylation product 4aa' in 87% yield with a very
small amount of mono-substituted byproduct 3aa (4aa/3aa = 96/4).
Other substrates such as tri-p-tolylphosphine (1b) and tris(4-
fluorophenyl)phosphine (1d) were also borylated with 2 B2pin2 (2a),
generating 4ba' and 4da'in 81% and 71% yields, respectively.
benzylic carbon atom, leading to a thermodynamically favored five-
membered chelate ring.[21] When the reaction of dicyclohexyl(o-
tolyl)phosphine (9) was coupled with B2pin2 (2a) under standard
reactions, C-H borylation only proceeded at the benzylic position
(10') in our system (Scheme 3a). It was reported that heating of
Rh(PPh3)3Me in solution gave an orange ortho-metallated four-
membered ring complex Ph2P(o-C6H4)Rh with methane.[4b] Using
the stoichiometric Wilkinson's catalyst (11), a well-defined RhI
complex with the same geometry of Rh(PPh3)3Me, can also directly
react with B2pin2 (2a), affording the borylation products 3aa' and
4aa' (Scheme 3b). These results suggest that the four-membered-
ring cyclometallated species[22] could be a possible intermediate in
this catalytic reaction. Additional investigation is necessary to fully
elucidate the reaction mechanism.
Scheme 3. Further investigations.
In summary, a series of o-boronated phosphines have been
developed by RhI-catalyzed, PIII-directed ortho C-H borylation of
various arylphosphines, in which the mono and di-selectivity can be
controlled. Given the widespread arylphosphine ligands, we
anticipate that this strategy will simplify the synthesis and structural
elaboration of o-boronated phosphine targets for chemists. These
present results represent an important discovery that is expected to
be substantially extended to other new transformation. Additional
applications of the borylated products and more detailed mechanistic
studies are also ongoing in our laboratory.
Scheme 1. PIII-Directed dual C-H borylation of arylphosphines.
The efficient construction of boronate phosphines opens the door
for the study of such compounds (Scheme 2). To demonstrate the
practical utility of our method, a deprotection reaction was first
conducted, and products 3aa were obtained in 87% yield by
treatment with DABCO. The formed C-B bond can be readily
converted into C-C and C-heteroatom bonds. For example, the
oxidation reaction of 3aa with NaBO3·4H2O was highly effective
and provided the hydroxylation product 5 in 89% yield. Treatment
of 3aa with Cu(OAc)2 could undergo methoxylation, affording
product 1k' in 60% yield. The boronate 3aa' can transform to the
potassium trifluoroborate salt 6 with KHF2 in 95% yield, which can
act as an efficient P, F-bidentate ligand in Pd-catalyzed dimerization
of ethylene reactions.[20] Significantly, o-boronated phosphines can
Acknowledgments
We thank the “1000-Youth Talents Plan”, NSF of China (Grant
2167020084) and the “Innovation & Entrepreneurship Talents Plan”
of Jiangsu Province for their financial support.
Keywords:
arylphosphines
·borylation
·C-H
also undergo
a cascade Pd-catalyzed Suzuki reaction with
activation ·rhodium ·ligands
iodoarenes and deprotection for rapid construction of Buchwald-
type phosphine ligands. For example, the cross-coupling of 3aa and
PhI (7) with a catalytic amount of Pd(OAc)2 and Sphos was
successful in providing the CyJohnphos (8) in good yield.
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3
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10.1002/anie.201813452
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
C−H Borylation
Jian Wen, Dingyi Wang, Jiasheng Qian,
Di Wang, Chendan Zhu, and Zhuangzhi
Shi* ___ Page – Page
Rh-Catalyzed PIII-Directed ortho C-H
Borylation of Arylphosphines
More than a ligand: A rhodium catalytic system has been established for direct
and ortho-selective borylation of commercially available arylphosphines via PIII-
directed C-H activation. A series of o-boronated phosphines containing mono and
di-aryl substituted groups with different steric and electronic properties were
obtained in high yields.
5
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