Nickel-Catalyzed Reductive Cross-Coupling of Aryl Bromides with Alkyl Bromides: Et3N as the Terminal Reductant

Article abstract of DOI:10.1021/acs.orglett.6b01837

Reductive cross-coupling has emerged as a direct method for the construction of carbon-carbon bonds. Most cobalt-, nickel-, and palladium-catalyzed reductive cross-coupling reactions to date are limited to stoichiometric Mn(0) or Zn(0) as the reductant. One nickel-catalyzed cross-coupling paradigm using Et₃N as the terminal reductant is reported. By using this photoredox catalysis and nickel catalysis approach, a direct Csp²-Csp³ reductive cross-coupling of aryl bromides with alkyl bromides is achieved under mild conditions without stoichiometric metal reductants.

Full text of DOI:10.1021/acs.orglett.6b01837

Letter
pubs.acs.org/OrgLett
Nickel-Catalyzed Reductive Cross-Coupling of Aryl Bromides with
Alkyl Bromides: Et3N as the Terminal Reductant
Zhengli Duan, Wu Li,* and Aiwen Lei*
College of Chemistry and Molecular Sciences, the Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, P. R.
China
S
* Supporting Information
ABSTRACT: Reductive cross-coupling has emerged as a direct method
for the construction of carbon−carbon bonds. Most cobalt-, nickel-, and
palladium-catalyzed reductive cross-coupling reactions to date are limited
to stoichiometric Mn(0) or Zn(0) as the reductant. One nickel-catalyzed cross-coupling paradigm using Et3N as the terminal
reductant is reported. By using this photoredox catalysis and nickel catalysis approach, a direct Csp2−Csp3 reductive cross-
coupling of aryl bromides with alkyl bromides is achieved under mild conditions without stoichiometric metal reductants.
eductive cross-coupling, the coupling of two different
electrophiles, recently has emerged as direct methods for
catalysis.7a,11 Crucially, the reduction of Ni(I) to Ni(0) using
an iridium photocatalyst is thermodynamically favorable.
We envision that combining nickel catalysis and photoredox
catalysis could achieve the cross-coupling of two different
electrophiles. We propose a mechanistic cycle for a photoredox-
nickel cocatalyzed reductive cross-coupling approach in Scheme
1. We speculate that an alkyl radical is generated from alkyl
halides reacting with the nickel catalyst or via a single electron
transfer process initiated by visible light irradiation. In fact, the
allylic radical could be generated through single-electron
transfer using Ru(bpy)3(PF6)2 as the catalyst.12 The generated
alkyl radical reacts further with the arylnickel(II) halide (A) as
R
the construction of C−C bonds, which has experienced a surge
of development.1 Compared with the conventional cross-
coupling, such a strategy streamlines the preparation of
electrophiles with air- and moisture-sensitive organometallics
from the corresponding organic halides and avoids the
formation of stoichiometric organometallic species. Recently,
nickel-catalyzed reductive cross-coupling has attained signifi-
cant attention, as one relatively electropositive late transition
metal, the oxidative addition of electrophiles with which tends
to occur quite readily; thus, the reaction conditions could be
mild in accordance with good functional group tolerance. The
Weix,2 Gong,3 and Molander4 groups have achieved consid-
erable progress in this field, and a variety of cross-electrophile
coupling reactions have been developed. In fact, most cobalt-,
nickel-, and palladium-catalyzed reductive cross-coupling
reactions extensively require stoichiometric metal reductants
and have been largely limited to Mn(0) or Zn(0) to complete
the catalytic cycle (eq 1).5 Few efforts have been devoted to
Scheme 1. Proposed Mechanism for Photoredox-Nickel
Cocatalyzed Reductive Cross-Coupling Approach
developing alternative reductants for sustaining the catalytic
cycle. Searching for new reductants would be highly desirable,
and great challenges still remain.
Visible-light-photoredox catalysis has been established as a
powerful technique to facilitate activation of organic molecules,
enabling achievement of a wide variety of new chemical
reactions.6 Extensive efforts have been made since the seminal
studies from the groups of MacMillan,7 Yoon,8 and
Stephenson.9 Visible-light-photoredox catalysis has emerged
as one of the most active research topics in organic synthesis
and made tremendous advances in the past decade.10 The
application of photoredox/nickel dual catalysis via single
electron transfer provides new opportunities for nickel
Received: June 22, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01837
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the result of the Ni(0) catalyst oxidative addition to aryl halides,
forming the intermediate Ni(III) (B). The organometallic (B)
species reductive elimination would forge the new C−C bond
simultaneously delivering the reductive cross-coupling product.
Finally, the expelled Ni(I) species was reduced by the Ir(II)
reductant, completing the nickel catalytic cycles. By using
photoredox/nickel dual catalysis, the research groups of
Molander and MacMillan and Doyle have described carbon−
carbon formation with an organoboron and amino acid as
coupling reagents (Scheme 1).7d,11b Herein, we report one
nickel/photoredox catalyzed reductive cross-coupling of aryl
bromides with alkyl bromides, using Et3N as the terminal
reductant (Scheme 1, D = Et3N).13
desired product was detected without the ligand (entry 9). No
desired product was obtained without the catalyst Ni(acac)2
and Ir(ppy)2(dtbbpy)PF6 (entries 12 and 13). Without light
irradiation, no product could be produced (entry 14).
With the optimized conditions, we focused our attention on
the scope of the substrates and a range of aryl bromides were
tested first (Scheme 2). Overall, all of the substrates studied
a
Scheme 2. Scope of Aryl Bromides
Our initial optimization for the synthesis cyclopentylbenzene
(3a) from the reductive cross-coupling of bromobenzene (1a)
and bromocyclopentane (2a) is summarized in Table 1. The
a
Table 1. Effects of Reaction Parameters
yield
b
entry
deviation from “standard conditions”
(%)
c
1
2
3
4
5
6
7
8
9
none
72 (72)
60
NiBr2 instead of Ni(acac)2
Ni(cod)2 instead of Ni(acac)2
DMA instead of DMF
69
52
MeCN instead of DMF
toluene instead of DMF
THF instead of DMF
54
34
trace
0
Ru(bpy)3(PF6)2 instead of [Ir]
Ir[CF3(df)ppy](dtbbpy)PF6 instead of Ir(ppy)2(dtbbpy)
PF6
68
10
11
12
13
14
no MgCl2
no dtbbpy
no Ni(acac)2
no [Ir]
13
trace
0
0
no light
0
a
Standard reaction conditions: 1a (0.2 mmol), 2a (1.0 mmol),
Ni(acac)2 (0.02 mmol), Ir(ppy)2(dtbbpy)PF6 (0.002 mmol), MgCl2
(0.2 mmol), Et3N (0.6 mmol), dtbbpy (0.02 mmol), DMF (2 mL), rt
b
for 24 h. Yields were determined by GC using diphenyl as an internal
c
standard. Yield of isolated product.
combination of Ni(acac)2 (acac = acetylacetonate), Ir-
(ppy)2(dtbbpy)PF6, (ppy = 2-phenylpyridine, dtbbpy = 4,4′-
di-tert-butyl-2,2′-bipyridine), MgCl2, and Et3N in DMF (N,N-
dimethylformamide) at room temperature, and then irradiated
with the blue LEDs for 24 h, achieved the best result for the
desired coupling in 72% yield. When Ni(acac)2 was replaced by
NiBr2 or Ni(cod)2 (cod = 1,5-cyclooctadiene), the yield of the
desired product decreased (entries 2 and 3). As to solvent,
DMA (dimethylacetamide), MeCN, and toluene led to a
decreased yield respectively (entries 4, 5, and 6), while THF
(tetrahydrofuran) proved to be ineffective (entry 7). Under
equivalent conditions, Ru(bpy)3(PF6)2 showed no efficiency in
terms of the desired product yield (entry 8). A slightly
decreased yield was obtained when another common iridium
photocatalyst Ir[dF(CF3)ppy]2(dtbbpy)PF6 (dF(CF3)ppy = 2-
(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine) was used
(entry 9). Notably, the additive was crucial for the reaction;
the yield decreased dramatically without MgCl2 (entry 10). The
dtbbpy was also indispensable, as only a trace amount of
a
Reaction conditions: 1a (0.2 mmol), 2a (1.0 mmol), Ni(acac)2 (0.02
mmol), [Ir] (0.002 mmol), MgCl2 (0.2 mmol), Et3N (0.6 mmol),
dtbbpy (0.02 mmol), solvent (2 mL), rt for 24 h.
were conveniently converted into the corresponding reductive
cross-coupled 2a−2m, and most of the products were isolated
in good yields, indicating the high versatility of this catalyst
system. Substitution at the para position of the aryl bromide
was well tolerated with both electron-donating and -with-
drawing (3b−3d, 3l, and 3m) substituents. Importantly, the
free proton containing substrates, such as phenol, underwent
the reaction smoothly and provided 3e in moderate yields.
Substitution at the meta-position of aryl bromide had been
explored, and efficient coupling occurred affording products in
moderate yield. Several bifunctional substrates were tested and
gave moderate to good yields (3i−3k). Similarly, 2-
B
DOI: 10.1021/acs.orglett.6b01837
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Letter
bromonaphthalene and substituted 2-naphthyl bromide could
be coupled efficiently to afford the desired products in
moderate to good yields (3n and 3o).
In addition to demonstrating the scope of this reaction, the
substrate scope of an alkyl bromide was also tested (Scheme 3).
process, Et3N acts as the terminal reductant. A deep
understanding of the exact role of MgCl2 and the final products
of bromine in the current process would be the subject for
future work. Finally, although the reaction mechanism is still
under investigation, these results demonstrate the potential of
combining nickel and visible-light-photoredox catalysis for
reductive cross-coupling. We envision this dual catalytic system
could be applied in other reactions using cobalt, nickel, or
palladium as the catalyst and Mn(0) or Zn(0) as the reductant.
a
Scheme 3. Scope of Alkyl Bromides
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b01837.
Experimental procedures and spectral data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: liwu@whu.edu.cn.
*E-mail: aiwenlei@whu.edu.cn.
a
Reaction conditions: 1a (0.2 mmol), 2a (1.0 mmol), Ni(acac)2 (0.02
mmol), [Ir] (0.002 mmol), MgCl2 (0.2 mmol), Et3N (0.6 mmol),
dtbbpy (0.02 mmol), solvent (2 mL), rt for 24 h.
Notes
The authors declare no competing financial interest.
Generally, moderate to high yields of the desired coupling
products were obtained. Alkyl bromides with branching couple
well with bromobenzene and gave high yields of the coupling
products (3q and 3r). In addition, ethyl 4-bromobutanoate
successfully coupled with bromobenzene and gave a moderate
yield (3s). Remarkably, bromoethane and 1-bromooctane also
coupled efficiently to afford products in 61% and 66% isolated
yields (3t and 3u), independently.
ACKNOWLEDGMENTS
■
This work was supported by the 973 Program
(2012CB725302), the National Natural Science Foundation
of China (21390400, 21520102003, 21272180, and 21302148),
the Hubei Province Natural Science Foundation of China
(2013CFA081), The Research Fund for the Doctoral Program
of Higher Education of China (2015M580665, 2016T90720),
and The Ministry of Science and Technology of China
(2012YQ120060). The Program of Introducing Talents of
Discipline to Universities of China (111 Program) is also
appreciated.
Radical trapping experiments were subsequently performed
in order to understand the mechanism of this nickel/
photoredox catalyzed reductive cross-coupling (Scheme 4).
Scheme 4. Mechanistic Studies
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