Base-Free Palladium-Catalyzed Borylation of Aryl Chlorides with Diborons

Article abstract of DOI:10.1002/cctc.201600456

The base-free palladium-catalyzed borylation of aryl chlorides with diborons was achieved. The base-free conditions offered acceptable functional group compatibility. Based on experimental and computational studies, it was shown that smooth boryl transfer from the diborons to the arylpalladium chloride was promoted by strong interaction between the Lewis acidic boron and the chlorine atom on palladium.

Full text of DOI:10.1002/cctc.201600456

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DOI: 10.1002/cctc.201600456
Communications
Base-Free Palladium-Catalyzed Borylation of Aryl Chlorides
with Diborons
Yutaro Yamamoto,[a] Hiroshi Matsubara,*[b] Hideki Yorimitsu,*[a] and Atsuhiro Osuka[a]
The base-free palladium-catalyzed borylation of aryl chlorides
with diborons was achieved. The base-free conditions offered
acceptable functional group compatibility. Based on experi-
mental and computational studies, it was shown that smooth
boryl transfer from the diborons to the arylpalladium chloride
was promoted by strong interaction between the Lewis acidic
boron and the chlorine atom on palladium.
Arylboronate esters are versatile reagents for organic synthesis,
and they are employed in the preparation of various CÀO, CÀ
N, and CÀC bonds.[1] Thus, the development of new methods
to synthesize arylboronate esters under mild reaction condi-
tions is important in modern organic chemistry.[2–19] According-
ly, transition-metal-catalyzed borylation reactions of aryl halides
with diborons have emerged as efficient and reliable meth-
ods.[2–4,5b,6b,9f,g,11] In these catalytic borylation reactions, the
presence of a base is essential to activate the diboron for
smooth boryl transfer. It would be desirable to develop base-
free conditions to achieve the synthesis of arylboronates
bearing base-sensitive functional groups.
Scheme 1. Base-free palladium-catalyzed silylation and borylation of aryl
chlorides with Lewis acidic bimetallic species.
and SPhos (2-dicyclohexylphosphino-2’,6’-dimethoxybiphenyl,
9 mol%) in toluene at 1008C (0.33m) resulted in moderate
conversion and afforded the corresponding borylated product
in 43% yield. After extensive optimization of the reaction con-
Very recently, we reported the activator-free palladium-cata-
lyzed silylation of aryl chlorides with silylsilatrane, which is
a new unsymmetrical disilane having a Lewis acidic silatrane
moiety (Scheme 1).[20] This silylation reaction converts aryl
chlorides without the need for a base by taking advantage of
the strong interaction between the Lewis acidic silicon and the
chlorine atom on the palladium center in the silyl-transfer step.
From these results, we envisioned that Lewis acidic diborons
could serve as borylating agents under base-free conditions on
a mechanistic basis similar to that of silylsilatranes. This is
indeed the case, and we describe herein the palladium-cata-
lyzed borylation of aryl chlorides with diborons without the
use of any activators.
ditions, we discovered that
a higher loading of B2pin2
(1.5 equiv.), a longer reaction time (15 h), and a higher concen-
tration (0.40m) led to the highest yield (93%, as determined
by NMR spectroscopy) of the desired product (Table 1,
entry 1).[21]
With the optimized conditions in hand, the scope of the bor-
ylation reaction was then investigated (Table 1). Interestingly,
we applied these base-free conditions to base-sensitive siloxy-
and NMeFmoc-substituted aryl chlorides (Fmoc=9-fluorenyl-
methoxycarbonyl) without observable deprotection (Table 1,
entries 2–4).[22] As a protecting group of NH2, phthalimido pro-
tection was also tolerated (Table 1, entry 5). Heteroaromatic
chlorothiophenes and p-extended chloronaphthalenes also
participated in the reaction (Table 1, entries 6–9). Not only
meta- and para-substituted aryl chlorides but also a sterically
congested ortho-substituted aryl chloride reacted to afford the
corresponding products in excellent yields (Table 1, entries 10–
12). Electron-deficient substrates such as fluoro-, trifluorometh-
yl-, ethoxycarbonyl-, and cyano-substituted aryl chlorides react-
ed smoothly (Table 1, entries 13–16). Unfortunately, our at-
tempts to borylate aryl chlorides bearing acid-sensitive func-
tional groups such as 4-chloroacetophenone, N-tert-butoxycar-
bonyl (Boc)-protected 4-chloroaniline, and 4-chlorobenzalde-
hyde dimethyl acetal failed, probably because of the high
Lewis acidity of B-chloropinacolborane. The borylation was effi-
cient enough to borylate 1,4-di- and 1,3,5-trichlorobenzene
with a larger amount of the diboron reagent (Scheme 2).
First, we utilized the reaction conditions of our previous sily-
lation[20a] for the base-free borylation. Treatment of 4-chloro-
anisole with B2pin2 [bis(pinacolato)diboron, 1.2 equiv.] in the
presence of Pd2(dba)3 (3 mol%; dba=dibenzylideneacetone)
[a] Y. Yamamoto, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
[b] Prof. Dr. H. Matsubara
Department of Chemistry, Graduate School of Science
Osaka Prefecture University
Naka-ku, Sakai 599-8531 (Japan)
E-mail: matsu@c.s.osakafu-u.ac.jp
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600456.
ChemCatChem 2016, 8, 1 – 5
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The scope of the diborons was surveyed (Scheme 3). The re-
action proceeded with bis(hexyleneglycolato)diboron to give
3a in good yield. Bis(neopentyl glycolato)diboron was less re-
active yet participated in the borylation with the aid of higher
loadings of Pd2(dba)3 and SPhos. Unfortunately, bis-
(catecholato)diboron reacted sluggishly.
Table 1. Borylation of aryl chlorides with B2pin2.
Entry
Ar
1
Yield[a] [%]
Attempted borylation of aryl bromides and triflates (OTf) re-
sulted in sluggish conversions under similar reaction condi-
tions (Table 2, entries 1–4). Similar phenomena were observed
1
2
3
4
4-MeOC6H4
4-tBuMe2SiOC6H4
4-TMSOC6H4
1a
1b
1c
1d
93 (82)
92 (86)
87
4-FmocMeNC6H4
90 (84)
Table 2. Borylation of aryl bromides and triflates.
5
1e
96 (91)
6
7
8
9
10[b]
11
12
13
14
15
16
2-thienyl
3-thienyl
1 f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
90 (79)
86 (76)
86 (68)
95 (92)
83 (82)
91 (89)
95 (90)
91 (84)
88 (77)
93 (78)
94 (92)
1-naphthyl
2-naphthyl
2-MeC6H4
3-MeC6H4
4-MeC6H4
4-FC6H4
4-F3CC6H4
4-EtO2CC6H4
4-NCC6H4
Entry
R
X
Solvent
LiCl [equiv.]
Yield[a] [%]
1
2
3
4
5
6
7
8[b]
OMe
OMe
CO2Et
CO2Et
OMe
OMe
CO2Et
CO2Et
Br
OTf
Br
OTf
Br
OTf
Br
toluene
toluene
toluene
toluene
dioxane
toluene
dioxane
toluene
0
0
0
0
1.5
1.5
1.5
1.5
4
12
1
2
70
79
79
70
OTf
[a] Yield was determined by NMR spectroscopy by using 1,1,2,2-tetra-
chloroethane as an internal standard. Yield of isolated product is given in
parentheses and is slightly lower than the corresponding yield deter-
mined by NMR spectroscopy owing to the instability of arylboronates on
silica gel (see the Supporting Information). [b] Pd2(dba)3 (5 mol%), SPhos
(15 mol%).
[a] Yield was determined by NMR spectroscopy. [b] Pd2(dba)3 (5 mol%),
SPhos (15 mol%).
in our previous silylation, and we proved that the chloride of
the arylpalladium chloride plays a key role in successful silyl
transfer to palladium.[20] To replace the bromide or triflate on
the palladium center with chloride after oxidative addition, we
added lithium chloride to promote boryl transfer to palladium.
The effect of lithium chloride was dramatic in promoting the
borylation (Table 2, entries 5–8).[23,24]
To clarify the positive effect of chloride for boryl transfer,
DFT calculations were performed with Gaussian 09.[25,26] We
chose boryl transfer from B2gly2 (2,2’-bi-1,3,2-dioxaborolane) to
SM_X (PhPdX(2-dimethylphosphino-2’-methoxybiphenyl) as
a model reaction for computational simplicity (Figure 1). The
oxygen atom on the phosphine ligand in SM_Cl weakly coordi-
nates to palladium. Boryl transfer from B2gly2 to SM_Cl was cal-
culated to proceed concertedly via four-membered transition
state TS_Cl with an activation barrier of 51.6 kJmolÀ1. The
lengths of the developing PdÀB bond and the dissociating BÀ
B bond in TS_Cl were calculated to be 2.121 and 2.426 ꢁ, re-
spectively. Boryl transfer results in the formation of Prod_Cl, in
which the chloride atom weakly coordinates to the palladium
center and the oxygen atom on the phosphine ligand is disso-
ciated from palladium. The overall reaction is endothermic by
38.8 kJmolÀ1. Moreover, the activation energy for boryl transfer
from B2gly2 to SM_Br was calculated to be 80.5 kJmolÀ1, and
TS_Br is more difficult to reach than TS_Cl by 28.9 kJmolÀ1.
The length of the PdÀB bond in TS_Br is slightly shorter than
that in TS_Cl by 0.019 ꢁ, whereas the length of the BÀB bond
is longer by 0.207 ꢁ. These differences in the bond lengths
imply that TS_Br is a later transition state than TS_Cl. The for-
Scheme 2. Double and triple borylation. Yields of the isolated products are
given in parentheses.
Scheme 3. Scope of the diboron reagents. Yields of the isolated products
are given in parentheses.
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In conclusion, we developed the base-free palladium-cata-
lyzed borylation of aryl chlorides with diborons. These boryla-
tion reactions allowed the synthesis of arylboronates bearing
base-sensitive siloxy and N-(9-fluorenylmethoxycarbonyl)-N-
methyl groups without observable deprotection owing to the
absence of a basic activator. Experimental and computational
investigations disclosed that the success of the base-free bory-
lation reaction depends on smooth boryl transfer from the di-
boron reagent to the arylpalladium chloride with the help of
the affinity between the chloride atom on the palladium
center and the Lewis acidic boron. This concept was applied to
the palladium-catalyzed hydrodechlorination of aryl chlorides
with pinacolborane. Further studies to realize new base-free
reactions are underway in our laboratory.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Numbers
JP24685007, JP26620081, JP16H01019, and JP16H04109. Y.Y. ac-
knowledges JSPS for a Predoctoral Fellowship. H.Y. acknowledges
the Japan Association for Chemical Innovation for financial
support.
Figure 1. Energy profile of boryl transfer obtained by DFT calculations at the
M06-2X/6-31G*+ECP(Pd,P,Cl,Br) level. Energies are in kJmolÀ1
.
mation of product Prod_Br is significantly endothermic
(72.5 kJmolÀ1), which also correlates with TS_Br as a later tran-
sition state of higher energy. These results show that the effi-
cient borylation of aryl chlorides is based on an intrinsically
strong interaction between the boron and chlorine atoms in
the boryl transfer.
Keywords: boron · borylation · noncovalent interactions ·
palladium · transmetalation
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[21] For details of optimization of conditions, see Table S1 in Supporting
Information.
[22] Admirably, borylations of siloxy- and NMeFmoc-substituted aryl
chlorides under Buchwald’s protocol using KOAc as a base in Ref. 4c
afforded the corresponding borylated products in good yields without
observable deprotection.
[23] This ligand exchange process was confirmed by 31P NMR analysis in our
previously reported silylation reaction in ref. 20a.
[24] The borylation of 4-chloroanisole with lithium chloride (1.5 eq) resulted
in a lower conversion. This result implies that the chloride ion would
not activate the diboron for smooth boryl transfer through the forma-
tion of chloride-coordinated borate.
[25] Gaussian 09, rev. C.01, M. J. Frisch, et al., Gaussian, Inc., Wallingford, CT,
2010. For full citation, see Supporting Information.
[26] Computational perspectives on the mechanisms of Pd-catalyzed cross-
coupling reactions were summarized nicely: a) M. Garcꢄa-Melchor,
A. A. C. Braga, A. Lledꢆs, G. Ujaque, F. Maseras, Acc. Chem. Res. 2013, 46,
2626; b) L. Xue, Z. Lin, Chem. Soc. Rev. 2010, 39, 1692; c) M. Sumimoto,
N. Iwane, T. Takahama, S. Sakaki, J. Am. Chem. Soc. 2004, 126, 10457.
Also see: ref. 4c.
[27] Palladium-catalyzed hydrodeiodination of aryl iodide with pinacol-
borane in the presence of base was reported in ref. 7a.
Received: April 19, 2016
Published online on && &&, 0000
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