Arylboration of 1-Arylalkenes by Cooperative Nickel/Copper Catalysis

Article abstract of DOI:10.1021/acs.orglett.6b01675

A method for the arylboration of 1-arylalkenes with bis(pinacolato)diboron and aryl chlorides or tosylates by cooperative Ni/Cu catalysis has been developed, which affords 2-boryl-1,1-diarylalkanes in high regio- and stereoselectivity. Under the applied conditions, this method is tolerant toward various functional groups, including silyl ether, alkoxycarbonyl, and aminocarbonyl moieties.

Full text of DOI:10.1021/acs.orglett.6b01675

Letter
pubs.acs.org/OrgLett
Arylboration of 1‑Arylalkenes by Cooperative Nickel/Copper
Catalysis
Kazuhiko Semba,* Yasuhiro Ohtagaki, and Yoshiaki Nakao*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: A method for the arylboration of 1-arylalkenes with bis(pinacolato)diboron and aryl chlorides or tosylates by
cooperative Ni/Cu catalysis has been developed, which affords 2-boryl-1,1-diarylalkanes in high regio- and stereoselectivity.
Under the applied conditions, this method is tolerant toward various functional groups, including silyl ether, alkoxycarbonyl, and
aminocarbonyl moieties.
lkylboranes are useful synthetic intermediates in organic
synthesis, as the C−B bonds can be readily converted into
A
C−C, C−O, or C−N bonds.1 Conventionally, such C−B bonds
are prepared via the hydroboration of alkenes2 or the
nucleophilic substitution of B(OR)3 with organolithium or
organomagnesium reagents.3 Transition-metal-catalyzed carbo-
borations4 of alkenes represent another powerful tool for the
preparation of alkylboranes, mostly on account of their high
functional group tolerance and the readily available variety of
alkenes.5 To date, both the intra- and intermolecular
carboborations of alkenes have been accomplished by catalytic
methods based on a single metal. Recently, our group and that
of Brown independently reported the intermolecular arylbora-
tion of alkenes with bis(pinacolato)diboron (B2(pin)2) and aryl
halides by cooperative Pd/Cu catalysis.6 However, the use of
Pd catalysts is not cost-effective, and phenol-derived electro-
philes, which are generally more readily available compared to
aryl halides, are restricted to aryl triflates. Conversely, aryl
tosylates, which are easier to access and handle than aryl
triflates, cannot be used as substrates in such reactions. Herein,
we report the arylboration of alkenes with aryl chlorides or
tosylates and B2(pin)2 by cooperative Ni/Cu catalysis.7,8
To test the viability of a cooperative Ni/Cu catalysis, we
initially examined the cross-coupling of alkylcopper 1a,9 which
was prepared by a borylcupration of styrene (2a), with p-
chlorotoluene (3a) (eq 1). Even though 4a was gratifyingly
obtained in 41% yield, alkenylboronate 5a and toluene were
also formed in 40% and 41% yield, respectively. In the absence
of Ni(cod)2, the formation of 4a was not observed. These
preliminary results encouraged us to investigate and develop
cross-coupling reactions based further on cooperative Ni/Cu
catalysis.
catalytically and furnished 4a in 53% yield. Subsequently, we
screened the reaction conditions with respect to various nickel
and copper sources, phosphine ligands, and bases. As a result,
we found that the arylboration of 2a (1.0 mmol) with 3a (1.0
mmol) and B2(pin)2 (1.0 mmol) afforded 4a in 72% yield in
the presence of Ni(acac)2 (2.0 mol %), CuCl (1.0 mol %),
PCyp3 (6.0 mol %), and LiOtBu (1.5 mmol), under
concomitant formation of small amounts of 5a, 6a, and 7a
(entry 1, Table 1). The effects of varying the monodentate
phosphine ligand are summarized in entries 2−6 (Table 1).10
When PPh3, PnPr3, and PtBu3 were employed, the formation of
4a was not observed (entries 2, 3, and 6). The use of PiPr3 and
PCy3 resulted in the formation of 4a, albeit in lower yield
compared to using PCyp3 (entries 4 and 5). Bidentate ligands
such as 1,2-bis(diphenylphosphino)ethane (dppe) and 2,2′-
bipyridine (bpy) did not promote the formation of 4a (entries
7 and 8). Moreover, the presence of LiOtBu was found to be
crucial in order to obtain 4a in good yield. Other bases such as
LiOMe, NaOtBu, or KOtBu afforded 4a in very low yield
(entries 9−11). The arylboration of 2a did not proceed in the
absence of Ni(acac)2, and without CuCl, the yield of 4a
For that purpose, we carried out the arylboration of 2a with
3a and B2(pin)2 in the presence of Ni(cod)2 (2.0 mol %),
tricyclopentylphosphine (PCyp3, 4.0 mol %), (IPr)CuCl (1.0
mol %), and LiOtBu (1.5 mmol). The reaction proceeded
Received: June 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01675
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Optimization of the Reaction Conditions for the
Arylboration of 2a with 3a and B2(pin)2
Table 2. Arylboration of 2 with 3 and B2(pin)2
variation from the
standard conditions
yield 4a yield 5a yield 6a yield 7a
a a a a
entry
(%)
(%)
(%)
(%)
1
2
3
4
5
6
7
8
9
none
72
<1
<1
47
56
<1
<1
<1
5
3
2
2
3
3
5
3
5
9
1
1
PPh3 instead of PCyp3
PnPr3 instead of PCyp3
PiPr3 instead of PCyp3
PCy3 instead of PCyp3
PtBu3 instead of PCyp3
dppe instead of PCyp3
bpy instead of PCyp3
<1
24
4
<1
<1
<1
5
2
2
<1
<1
<1
<1
15
12
2
LiOMe instead of
LiOtBu
10
11
NaOtBu instead of
LiOtBu
<1
<1
5
4
12
35
<1
<1
KOtBu instead of
LiOtBu
12
13
without Ni(acac)2
without CuCl
<1
11
<1
5
14
<1
<1
10
a
The yield was estimated by GC, using n-C13H28 as an internal
standard.
decreased dramatically (entries 12 and 13). These results
demonstrate that the cooperative Ni/Cu catalysis is indis-
pensable for the present arylboration.
With optimized conditions in hand, we examined the
substrate scope of this reaction (Table 2). Electron-donating
and -withdrawing substituents at the para-position of the
benzene ring of the aryl chlorides did not affect the yield,
affording the corresponding products in moderate to good yield
(entries 1−7), while the use of o-tolyl chloride (3h) resulted in
a lower yield (entry 8). Under the identical conditions, p-tolyl
bromide (8a) afforded 4a in high yield (entry 9), whereas p-
tolyl triflate (9a) reacted sluggishly (entry 10). Aryl tosylates
gave the corresponding 1,1-diarylethanes albeit in modest yields
(entries 11−13). Both electron-rich and -poor styrene
derivatives were suitable substrates for the present reaction
(entries 14 and 15). Various functional groups such as silyl
ether, alkoxycarbonyl, aminocarbonyl, and trifluoromethyl
moieties remained unaffected under the applied reaction
conditions. The reaction of 1-octene (2d) proceeded sluggishly,
most likely on account of the low reactivity of 2d toward the
borylcupration (entry 16).11 The arylboration of (E)-1-phenyl-
1-propene (2e) and (Z)-1-phenyl-1-propene (2f) proceeded to
furnish syn-4l as a major diastereomer in both cases (eqs 2 and
3).12
a
b
c
Isolated yield. Scale: 0.50 mmol. Run at 100 °C using Ni(acac)2
d
(5.0 mol %), CuCl (1.0 mol %), and PCyp3 (12 mol %). The yield
was estimated by GC, using n-C13H28 as an internal standard. Run at
100 °C using NiCl2(dme) (5.0 mol %), CuCl (5.0 mol %), and PCyp3
(20 mol %).
e
A plausible reaction mechanism for the present reaction is
outlined in Scheme 1. In situ generated LNi(0) (11) should
oxidatively add across 3 to afford LNi(Ar)Cl (12) (step a in the
Ni cycle). Transmetalation between 12 and alkylcopper 1,
which should be generated from the borylcupration of 2, was
followed by reductive elimination from 13 (steps b and c in the
Ni cycle; eq 1). The catalytic cycle would be completed by the
regeneration of borylcopper 14 from B2(pin)2 and LCuOtBu
16, which would have been obtained from 15 and LiOtBu
(steps e and f in the Cu cycle). The arylboration of (E)-1-
phenyl-1-propene was stereospecific whereas that of the (Z)-
isomer was not (eqs 2 and 3). The reason is not clear, but
configurational isomerization of the alkylcopper intermediate
derived from the (Z)-olefin may be responsible for this
observation.13
B
DOI: 10.1021/acs.orglett.6b01675
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: semba.kazuhiko.5n@kyoto-u.ac.jp.
*E-mail: nakao.yoshiaki.8n@kyoto-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by a Grant-in-Aid for Young
Scientists (B) (26810058) from MEXT and the CREST
program “Establishment of Molecular Technology towards the
Creation of New Functions” Area from JST.
Scheme 1. A Plausible Mechanism for the Arylboration of 1-
Arylalkenes by Cooperative Ni/Cu Catalysis
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In conclusion, we have developed a method for the
arylboration of alkenes with aryl chlorides or tosylates and
B2(pin)2 by cooperative Ni/Cu catalysis. The reaction proceeds
regio- and stereoselectively to afford 2-boryl-1,1-diarylalkanes,
which represent potent intermediates for the generation of 1,1-
diarylalkanes. This novel Ni/Cu catalysis should provide
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b01675.
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C
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DOI: 10.1021/acs.orglett.6b01675
Org. Lett. XXXX, XXX, XXX−XXX