Ruthenium-Catalyzed ortho C?H Borylation of Arylphosphines

Article abstract of DOI:10.1002/anie.201813278

Efficient, phosphine-directed ortho C?H borylation of arylphosphine derivatives was achieved using Ru catalysts for the first time. The reaction is applicable to various tertiary arylphosphine and arylphosphinite derivatives to give (o-borylaryl)phosphoru

Full text of DOI:10.1002/anie.201813278

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Accepted Article
Title: Ruthenium-Catalyzed ortho C–H Borylation of Arylphosphines
Authors: Kazuishi Fukuda, Nobuharu Iwasawa, and Jun Takaya
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10.1002/anie.201813278
Angewandte Chemie International Edition
COMMUNICATION
Ruthenium-Catalyzed ortho C–H Borylation of Arylphosphines
Kazuishi Fukuda[a], Nobuharu Iwasawa[a], and Jun Takaya*[a][b]
Abstract: Efficient, phosphine-directed ortho C–H borylation of
arylphosphine derivatives was achieved using Ru catalysts for the
first time. The reaction is applicable to various tertiary arylphosphine
and arylphosphinite derivatives to give (o-borylaryl)phosphorus
compounds in high yields. This reaction enables easy access to a
variety of functionalized phosphine ligands and ambiphilic
wide substrate generality, enabling easy access to a variety of
functionalized phosphine ligands and ambiphilic phosphine-
boronate compounds.
a) Indirect route
b) Direct route
R
R
X
phosphine-boronate
compounds,
realizing
new
late-stage
PR2
α
δ
α
γ
α
P
PPh2
Ar
PCy2
Ar
β
modification of phosphorus compounds.
β
β
Y
Bpin
γ
γ
Transition
intermolecular C–H bond transformation of tertiary phosphines is
promising approach for design and synthesis of new
metal-catalyzed,
phosphine-directed
(X = O, S)
Ir cat.
Rh cat.
(Shi)
Rh cat.
(Yu, Che)
(Y = alkenyl, aryl)
(Miura, Chang etc.)
(Clark)
a
Functionalization at γ- or δ-position
phosphine compounds, which can be widely utilized in the field
of organic synthesis, organometallic chemistry, and materials
science.[1] However, such reactions have rarely been developed,
and in particular, the phosphine-directed ortho-functionalization
of arylphosphines, such as PPh3, remained unexplored.[2] The
difficulties of the phosphine-directed C–H functionalization stem
from 1) deactivation of the catalysts by strong coordination of
phosphines to transition metal complexes and 2) formation of
unfavorable four-membered metallacycle intermediates in the
case of ortho-functionalization of arylphosphines.[3] To overcome
these problems, phosphine oxides and sulfides-directed
reactions have been developed although an additional reduction
step is required to access tertiary phosphines (Figure 1-a).[4,5] As
a direct route, Clark reported Ir-catalyzed phosphine-directed
sp2C–H borylation of benzylphosphine and biarylphosphine
derivatives with rather limited substrate scope (Figure 1-b).[6]
Recently, Rh-catalyzed direct C–H arylation reactions of
biarylphosphines and 1-naphthylphosphines were developed by
Shi’s and Yu and Che’s groups.[7,8] However, these reactions
occurred at γ- or δ-position of the directing –PR2 group via five-
or six-membered metallacycle intermediates, thus limiting the
substrates to benzyl, biaryl, and 1-naphthyl derivatives.[9]
c) This work
PR2
• Direct ortho-functionalization of arylphosphines
via C–H borylation (β-functionalization)
α
Bpin
β
• Efficient late-stage modification of phosphines
Ru cat.
Figure 1. Phosphine-directed intermolecular C–H bond transformation of
tertiary phosphines
It was found that ortho-borylation of triphenylphosphine with
1.0 equivalent of pinacolborane (HBpin) proceeded in the
presence of various ruthenium complexes in n-octane at 150 ˚C
(Table 1). The commercially available [RuCl2(p-cymene)]2 turned
out to be the best catalyst among tested to afford the o-
borylphenyl(diphenyl)phosphine 1a in 63% yield (Entry 7). The
yield was further improved by carrying out the reaction in
benzene at 100 ˚C with 1.1 equivalent of HBpin, giving 1a in
80% yield after purification by silica gel column chromatography
even with 0.5 mol% [RuCl2(p-cymene)]2 (Entry 8).[11] The
borylation occurred selectively at the ortho-position of one of the
phenyl rings under the reaction conditions. Ru-catalyzed sp2C–H
borylation reactions have been rather rare and required pyridine,
imine, or amide as directing groups.[12,13] It should be noted that
the Clark's Ir-catalyst did not promote C–H borylation of PPh3 at
all,[6] thus demonstrating unique reactivity of the Ru catalyst.
Herein we report the first example of phosphine-directed
ortho C–H borylation of arylphosphines catalyzed by ruthenium
complexes. The C–H borylation is the most suitable reaction for
the late-stage modification of phosphines since the resulting C–
B bonds can be transformed to various substituents and
functional groups.[10] This reaction exhibited high efficiency and
This reaction is applicable to various arylphosphine
derivatives (Table 2). Triarylphosphines bearing an electron-
donating or -withdrawing substituent at the 4-position were
smoothly borylated under the optimized conditions to afford the
(o-borylphenyl)phosphine derivatives 1b-d in good yields. It
should be noted that various halogens and ester substituents
were tolerated under the reaction conditions, giving
functionalized triarylphosphine derivatives in 75-93% yields
[a]
[b]
Kazuishi Fukuda, Prof. Dr. Nobuharu Iwasawa, Dr. Jun Takaya
Department of Chemistry, School of Science
Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8551, Japan
E-mail:takayajun@chem.titech.ac.jp
Dr. Jun Takaya
(Entries
4-7).
The
borylation
of
meta-substituted
JST, PRESTO
Honcho, Kawaguchi, Saitama, 332-0012, Japan
triarylphosphines proceeded at the less hindered position, para
to the substituent, preferentially, affording 2a-c in moderate to
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201813278
Angewandte Chemie International Edition
COMMUNICATION
Table 2. Borylation of triarylphosphines.
Ar1
Ar1
HBpin (1.1 equiv)
[RuCl2(p-cymene)]2 (0.5 mol%)
Table 1. Screening of reaction conditions.
P
P
Ru catalyst
(2.0 mol% Ru atom)
Ar2
Ar2
Bpin
R
R
PPh2
PPh2
Bpin
benzene, 100 ºC, 6 h
+
HBpin
6 h
1a
Entry
1[a]
2
Product
R
Yield
Entry
Ru catalyst
None
Conditions[a]
Yield[b]
0%
Me 1b
CF3 1c
MeO 1d
F 1e
83%
1
2
3
4
5
6
7
8
A
A
A
A
A
A
A
B
quant.
RuCl3•3H2O
trace
79%[b]
PAr2
Bpin
3
[c]
Ru3(CO)12
35%
4
84%
R
1
(Ar = 4-RC6H4)
(Cp*RuCl2)n
25%
5
Cl 1f
93%
RuCl2(dmso)4
50%
6
Br 1g
75%
[RuCl2(cod)]n
59%
7
COOMe 1h
Me 2a
87%
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
63%
8[a]
9
91% (0%)[c]
57% (6%)[c]
54% (8%)[c]
Bpin
R
PAr2
R
PAr2
91% (80%)[e]
[d]
Cl 2b
+
Bpin
10
MeO 2c
[a] Conditions A: HBpin (1.0 equiv) in n-octane at 150 ˚C. Conditions B:
HBpin (1.1 equiv) in benzene at 100 ˚C. [b] NMR yield. [c] Ru3(CO)12 (2
mol%). [d] [RuCl2(p-cymene)]2 (0.5 mol%). [e] Isolated yield.
2
3
(Ar = 3-RC6H4)
[a] HBpin (1.5 equiv). [b] Isolated after BH3-protection. [c] Yield of isomer 3
was shown in the parenthesis.
good yields (Entries 8-10). Furthermore, this reaction can be
applied not only to triarylphosphines but also to more electron
rich (alkyl)n(aryl)3-nphosphines. The reaction of various
alkyl(diphenyl)phosphines (alkyl = iPr, Cy, tBu, Et) afforded
tertiary phosphines 4a-d having three different substituents on
phosphorus in moderate to good yields (Table 3, Entries 1-4).
Borylation of diethyl(phenyl)phosphine also proceeded with 1.5
Table 3. Borylation of other phosphines and phosphinites.
R1
HBpin (1.1 equiv)
[RuCl2(p-cymene)]2 (0.5 mol%)
R1
mol%
borylphenyl)phosphine 5a in 37% yield (Entry 5). Notably, the
reaction of phosphinite derivative, (–)-menthyl
of
[RuCl2(p-cymene)]2,
giving
diethyl(o-
H3B
P
P
R2
R2
benzene, 100 ºC, 6 h
then BH3•thf
Bpin
diphenylphosphinite, proceeded without problem to afford an (o-
borylphenyl)phosphinite derivative 6 in good yield (dr = 50:50,
Entry 6).
Entry
1[a]
2
R1
Ph
Ph
Ph
Ph
Et
R2
Yield
iPr
Cy
tBu
Et
4a
4b
4c
4d
5a
6
62%
50%
32%
32%
37%
68%[d]
Next, the site-selectivity of this Ru-catalyzed C–H borylation
was investigated with substrates having two different aryl
moieties to be borylated. Diphenyl(1-naphthyl)phosphine reacted
at one of the two phenyl rings, not at the 8-position of the
naphthyl group, to give the ortho-borylation product 7 selectively
in 73% yield (Scheme 1-a). The borylation of 2-
biphenyl(diphenyl)phosphine also proceeded at the phenyl
group to give 8 although the catalyst efficiency was not
satisfactory due to large steric repulsion (Scheme 1-b). These
results demonstrated different regioselectivity from that reported
in the Rh-catalyzed C–H arylation reactions.[7,8] On the other
hand, the reaction of benzyl(diphenyl)phosphine proceeded
3[b]
4[a]
5[b]
6[c]
Et
iPr
Ph
O
smoothly at the benzyl group
to
give
benzyl(2-
borylbenzyl)phenylphosphine 9 in 74% yield (Scheme 1-c).
Notably, the Ru-catalyst enabled double borylation at 2,6-
position of the benzyl group to give 10 in 82% yield by using 3
equivalent of HBpin.[14] Therefore, the site-selectivity for the
[a] HBpin (1.5 equiv). [b] [RuCl2(p-cymene)]2 (1.5 mol%), HBpin (1.5 equiv).
[c] In n-octane at 150 ˚C. [d] dr = 50:50.
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10.1002/anie.201813278
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COMMUNICATION
Table 4. Competition Experiments.
PAr1 PAr2
Ru-catalyzed borylation is demonstrated as benzyl > phenyl >>
2-biphenyl, 1-naphthyl. Further investigation on the origin of the
site-selectivity is underway and will be reported in due course.
PAr1
PAr2
2
2
2
2
Bpin
+
Bpin
a
+
a)
Ph
P
Ph
P
[RuCl2(p-cymene)]2 (2.5 mol%)
HBpin (1.5 equiv)
HBpin
R2
R2
R1
R3
R1
R3
n-octane, 150 ˚C, 6 h
(Ar1 = 4-R1C6H4)
(Ar2 = 3-R2-4-R3C6H3)
Bpin
7 73%
b)
c)
Ph
Ph
P
[RuCl2(p-cymene)]2 (5.0 mol%)
HBpin (2.0 equiv)
Ph
Entry
R1
H
R2
R3
Yield[b]
P
1
2
3
4
H
H
H
CF3
MeO
MeO
H
1a 18%
1a 16%
1c 64%
1a 52%
1c 76%
1d 53%
1d 16%
2c 14%
mesitylene, 180 ˚C, 6 h
Bpin
8 23%
H
BH3
BH3
Ph
CF3
H
P
PPh2 Bpin
PPh2
[RuCl2(p-cymene)]2
HBpin (x equiv)
MeO
Bpin
or
Bpin
benzene, 100 ˚C, 6 h
then BH3•thf
9 74%
(x = 1.0)[a]
10 82%
(x = 3.0)[b]
[a] PAr13:PAr23:HBpin=5:5:1, [RuCl2(p-cymene)]2 (5.0 mol% relative to the
amount of HBpin), benzene, 100 ˚C, 6 h. [b] NMR yield in the crude products.
[a] [RuCl2(p-cymene)]2 (1.0 mol%), HBpin (1.0 equiv), benzene, 100 ˚C, 6 h.
[b] [RuCl2(p-cymene)]2 (3.0 mol%), HBpin (3.0 equiv), benzene, 100 ˚C, 6 h.
Scheme 1. Site-selectivity for the C–H borylation.
PPh2
P(C6D5)2
Bpin
a)
Bpin
or
To shed light on the mechanism, the effect of electronic
nature of triarylphosphines on the reactivity was investigated. A
set of competition experiments among PPh3, P(4-CF3C6H4)3,
P(4-MeOC6H4)3, and P(3-MeOC6H4)3 revealed that the reaction
was faster with P(4-CF3C6H4)3 > P(4-MeOC6H4)3 > PPh3 > P(3-
MeOC6H4)3 (Table 4). These results suggested that the
electronic nature of the phosphorus atom did not affect the
reactivity, and the reaction rate increased as the ortho-carbon to
be borylated became electron deficient. On the other hand,
deuterium labeling experiments with PPh3/P(C6D5)3 exhibited a
KIE of 2.0 from two parallel reactions and 2.2 from an
intermolecular competition (Scheme 2-a and 2-b). An
intramolecular KIE was also observed in the reaction of
PPh2(C6D5) (kH/kD = 1.3, Scheme 2-c). Furthermore, H/D
exchange at the ortho-position of P(C6D5)3 was observed with
HBpin, thus suggesting that the C–H activation step is reversible
(Scheme 2-d). These results support that the C–H activation
could be involved as a pre-equilibrium before a rate-determining
step.[15] The substituent effect demonstrated in Table 4 can be
partly explained as the pre-equilibrium C–H activation is
accelerated by the electron deficient ortho-carbon although the
rate-determining step is not clear yet.[16] Further investigation on
the reaction mechanism through detailed kinetic studies and
characterization of intermediates is in progress.
[a]
[a]
PPh3 or P(C6D5)3
+
+
HBpin
1a
kH/kD = 2.0
11
(10 equiv)
D4
b)
PPh3
+
P(C6D5)3
(5 : 5 : 1)
HBpin
+
1a
11
kH/kD = 2.2
PPh(C6D5) PPh2
c)
Bpin
+
Bpin
[a]
PPh2(C6D5)
+
HBpin
12
13
D4
(1.1 equiv)
kH/kD = 1.3
d)
P
D
(ca. 20%H)
H/D
3
H/D
D
[c]
P(C6D5)3
+
HBpin
(10 equiv)
+
11
ca. 50%
2 h
D
ca. 50%
[a] [RuCl2(p-cymene)]2 (0.5 mol%), benzene, 100 ˚C, 6 h. [b] [RuCl2(p-
cymene)]2 (1.5 mol%), HBpin (10 equiv), [D6]benzene, 100 ˚C, 2 h.
Scheme 2. Deuterium Labeling Experiments.
to give (o-biphenyl)diphenylphosphine 17 in 47% yield.[18] In
addition to these derivatization, the (o-borylphenyl)phosphine
derivatives themselves are also highly useful as ambiphilic
phosphine-boronate compounds, which have been reported to
exhibit unique reactivity for small molecule activation and
catalysis.[19,20] These strategies, the direct borylation of easily
available phosphines and transformation of the resulting C–B
bond, would expand structural diversity of tertiary phosphines,
leading to their new utilization in synthetic chemistry.
Finally, transformation of the borylation product enabled
easy access to various tertiary phosphines (Scheme 3).
Oxidation of the C–B bond of BH3-protected 1a with H2O2
afforded (o-hydroxyphenyl)phosphine derivative 14 quantitatively.
Bench-stable trifluoroborate 15 was obtained by the treatment of
1a with KHF2.[17] Moreover, Pd-catalyzed cross coupling reaction
of boronic acid 16 with PhI proceeded using SPhos as a ligand
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10.1002/anie.201813278
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COMMUNICATION
1a
Lett. 2003, 44, 8665. d) Q.-S. Liu, D.-Y. Wang, J.-F. Yang, Z.-Y. Ma, M.
Ye, Tetrahedron 2017, 73, 3591.
[a]
[c]
[b]
H3B
[6]
[7]
[8]
[9]
K. Crawford, T. Ramseyer, C. Daley, T. Clark, Angew. Chem., Int. Ed.
2014, 53, 7589.; Angew. Chem. 2014, 126, 7719.
PPh2
PPh2
PPh2
PPh2
BF3– K+
B(OH)2
Ph
OH
[d]
X. Qiu, M. Wang, Y. Zhao, Z. Shi, Angew. Chem., Int. Ed. 2017, 56,
7233.; Angew. Chem. 2017, 129, 7339.
X. Luo, J. Yuan, C.-D. Yue, Z.-Y. Zhang, J. Chen, G.-A. Yu, C.-M. Che,
Org. Lett. 2018, 20, 1810.
14 quant.
15 quant.
16 65%
(86% brsm[e]
17 47%
)
a) Tobisu and Chatani reported Pd-catalyzed intramolecular cyclization
of biarylphosphines to give phosphole derivatives via phosphonium
intermediates. K. Baba, M. Tobisu, N. Chatani, Angew. Chem., Int. Ed.
2013, 52, 11892.; Angew. Chem. 2013, 125, 12108. b) Wang reported
Cu-catalyzed intramolecular cyclization reactions of tertiary phosphines
with alkynes to give cyclic phosphonium compounds. Q. Ge, J. Zong, B.
Li, B. Wang, Org. Lett. 2017, 19, 6670.
[a] 1) BH3•thf, 2) H2O2, NaOH aq., rt, 12 h. [b] KHF2, MeOH, rt, 3 h. [c]
(HOCH2CH2)2NH, Et2O, rt, 18 h. [d] Pd(dba)2 (10 mol%), SPhos (12 mol%),
PhI (1.5 equiv), K3PO4 (1.5 equiv), 1,4-dioxane, 100 ˚C, 6 h. [e] Based on
recovered starting material.
Scheme 3. Transformation of 1a.
[10] a) I. Mkhalid, J. Barnard, T. Marder, J. Murphy, J. Hartwig, Rev. 2010,
110, 890. b) L. Xu, G. Wang, S. Zhang, H. Wang, L. Wang, L. Liu, J.
Jiao, P. Li, Tetrahedron 2017, 73, 7123.
In conclusion, we have developed Ru-catalyzed, phosphine-
directed ortho sp2C–H borylation of arylphosphines for the first
time.[21] The high efficiency and wide substrate generality
[11] CAUTION! This reaction should be conducted inside an explosion-proof
wall. This is performed in a sealed glass tube at 100 ˚C which is over
the boiling point of benzene, and H2 is generated during the reaction.
[12] a) G. Battagliarin, C. Li, V. Enkelmann, K. Müllen, Org. Lett. 2011,13,
3012. b) J. Zhang, S. Singh, D. Hwang, S. Barlow, B. Kippelen, S.
Marder, J. Mat. Chem. C 2013, 1, 5093. c) J. Fernández-Salas, S.
Manzini, L. Piola, A. Slawin, S. Nolan, Chem. Commun. 2014, 50, 6782.
d) S. Okada, T. Namikoshi, S. Watanabe, M. Murata, ChemCatChem
2015, 7, 1531. e) S. Sarkar, P. Kumar, L. Ackermann, Chem. Eur. J.
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Lett. 2018, 59, 2537.
enabled
easy
access
to
various
tertiary
(o-
borylphenyl)phosphines, which are highly useful as precursors
to various functionalized phosphines and ambiphilic phosphine-
boronate compounds. These results demonstrated unique
reactivity of the Ru-catalyst for C–H borylation distinct from Ir.
Further utilization of the borylated tertiary phosphines are
underway in our group.
[13] For Ru-catalyzed electrophilic borylation of indoles, see; T. Stahl, K.
Müther, Y. Ohki, K. Tatsumi, M. Oestreich, J. Am. Chem.
Soc. 2013, 135, 10978.
Acknowledgements
[14] The same site-selectivity to give 9 was observed in the Ir-catalyzed
reaction (ref [6]). However, the selective formation of the double
borylation product was not reported with the Ir-catalyst.
This research was supported by JSPS KAKENHI Grant
Numbers 15H05800, 17H03019 (Gran-in-Aid for Scientific
Research (B)), 18H04646 (Hybrid Catalysis), and JST, PRESTO
Grant Number JY290145, Japan.
[15] E. Simmons, J. Hartwig, Angew. Chem., Int. Ed. 2012, 51, 3066.;
Angew. Chem. 2012, 124, 3120.
[16] One of the possible candidates for the rate-determining step could be
the C–B bond forming reductive elimination. In some Ru-catalyzed
sp2C–H bond activation reactions, acceleration of C–C bond forming
reductive elimination with electron-withdrawing substituents on the
arene was reported. See: a) F. Kakiuchi, H. Ohtaki, M. Sonoda, N.
Chatani, S. Murai, Chem. Lett. 2001, 30, 918. b) M. Schinkel, I. Marek,
L. Ackermann, Angew. Chem., Int. Ed. 2013, 52, 3977.; Angew. Chem.
2013, 125, 4069.
Keywords: Phosphine • C–H activation • Borylation • Ruthenium
• Late-stage modification
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[21] Related papers on borylation of tertiary phosphines have appeared
during the revision process, see; a) J. Wen, D. Wang, J. Qian, D. Wang,
C.
Zhu,
Y.
Zhao,
Z.
Shi,
Angew.
Chem.,
Int.
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Ed. 10.1002/anie.201813452. b) S. Wright, S. Richardson-Solorzano, T.
Stewart, C. Miller, K. Morris, C. Daley, T. Clark. Angew. Chem., Int.
Ed. 10.1002/anie.201812857.
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Kazuishi Fukuda, Nobuharu Iwasawa,
Jun Takaya*
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Ruthenium-catalyzed ortho C–H
Borylation of Arylphosphines
Efficient, phosphine-directed ortho C–H borylation of arylphosphine derivatives was
achieved using Ru catalysts. The reaction is applicable to various tertiary
arylphosphine and arylphosphinite derivatives to give (o-borylaryl)phosphorus
compounds in high yields. This reaction enables easy access to a variety of
functionalized phosphine ligands and ambiphilic phosphine-boronate compounds,
realizing new late-stage modification of phosphorus compounds.
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