Photochemical Nickel-Catalyzed Reductive Migratory Cross-Coupling of Alkyl Bromides with Aryl Bromides

Article abstract of DOI:10.1021/acs.orglett.8b00413

A novel method to access 1,1-diarylalkanes from readily available, nonactivated alkyl bromides and aryl bromides via visible-light-driven nickel and iridium dual catalysis, wherein diisopropylamine (ⁱPr₂NH) is used as the terminal stoichiometric reductant, is reported. Both primary and secondary alkyl bromides can be successfully transformed into the migratory benzylic arylation products with good selectivity. Additionally, this method showcases tolerance toward a wide array of functional groups and the presence of bases.

Full text of DOI:10.1021/acs.orglett.8b00413

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Photochemical Nickel-Catalyzed Reductive Migratory Cross-Coupling
of Alkyl Bromides with Aryl Bromides
Long Peng, Zheqi Li, and Guoyin Yin*
The Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072, P. R. China
S
* Supporting Information
ABSTRACT: A novel method to access 1,1-diarylalkanes from readily
available, nonactivated alkyl bromides and aryl bromides via visible-light-
driven nickel and iridium dual catalysis, wherein diisopropylamine
(iPr2NH) is used as the terminal stoichiometric reductant, is reported.
Both primary and secondary alkyl bromides can be successfully
transformed into the migratory benzylic arylation products with good
selectivity. Additionally, this method showcases tolerance toward a wide array of functional groups and the presence of bases.
1,1-Diarylalkanes are a significant class of pharmacophores
present in drugs and many other relevant biologically active
molecules.1 Therefore, the synthesis of this framework has
aroused enormous interest and attention. Transition-metal-
catalyzed cross-coupling reactions of benzylic electrophiles with
arylmetallic reagents or aryl halides are the most common
strategy to construct this class of compounds.2 The direct
catalytic arylation of benzylic C−H bonds is a more attractive
protocol to access this skeleton, and progress has been made by
Walsh3 with palladium and by Stahl4 and Liu5 with copper.
Very recently, further progress toward avoiding the use of
activated but potentially unstable benzylic halides as well as
arylmetallic reagents was made independently by us6 and Zhu
and co-workers.7 We demonstrated that 1,1-diarylalkanes can
be efficiently prepared from nonactivated alkyl electrophiles
and aryl electrophiles under nickel catalysis in the presence of a
stoichiometric reducing agent (Figure 1a, top). However, this
approach still relied on the use of a stoichiometric amount of a
metal reductant such as manganese or zinc metal dust to
complete the catalytic cycle.8 Efforts toward exploring
alternative reducing agents and new catalytic systems would
therefore be highly beneficial to broaden the synthetic utility of
this strategy further.
Visible-light-driven photoredox catalysis has been well-
established as a powerful tool in the construction of carbon−
carbon and carbon−heteroatom bonds in organic synthesis
over the past decade.9 The merging of photoredox catalysis
with transition metal catalysis has shed new light on transition
metal catalysis, by not only providing new coupling partners
but also altering the catalytic cycles.10 Numerous studies on the
redox neutral and oxidative systems11 have previously been
described. However, in comparison, reports on the trans-
formation involving photochemical transition-metal catalysis
under reductive conditions have to date remained scarce.12,13
Herein, we report a photochemical, nickel-catalyzed migratory
cross-coupling of nonactivated, aryl substituted alkyl bromides
with aryl bromides using a cheap organic amine as the terminal
electron donor,14 allowing access to 1,1-diaraylalkanes with
high efficiency and selectivity under mild conditions (Figure 1a,
bottom). It is noteworthy that, even in the presence of a large
amount of organic base, the reaction selectively affords the
migratory cross-coupling products, rather than olefins or the
Heck type products,15 probably due to the reductive
elimination from the Ni−H species being reversible16 (Figure
1b).
We initiated this study with the cross-coupling of (3-
bromopropyl)benzene (1a) with bromobenzene (2a). By
extensive screening of the reaction parameters, we found that
the combination of [Ir]-1, NiBr2·DME, bathocuproine (BC),17
i
MgBr2 (as an additive), and Pr2NH as the stoichiometric
reductant at room temperature in DMF under blue LEDs
irradiation for 24 h leads to the migratory cross-coupling
product 1,1-diphenylpropane (3a) in 84% yield with a 32/1
regioisomeric ratio (remote/ipso coupling). The ligand screen-
ing efforts again showed that the migratory product 3a is
preferred when 6,6′-dimethyl-2,2′-bipyridine (6,6′-dmbpy) or
neocuproine was used as a ligand (Table 1, entries 2 and 3),
while using 4,4′-di(tert-butyl)-2,2′-bipyridine (4,4′-dtbbpy)
Figure 1. Nickel-catalyzed reductive migratory cross-coupling
reactions.
Received: February 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00413
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Effects of Reaction Parameters
Scheme 1. Investigation of Substrate Scope
b
entry
variation of standard conditions
yield/% (3a/4a)
c
1
none
92 (32/1)
2
6,6′-dmbpy instead of BC
neocuproine instead of BC
4,4′-dtbbpy instead of BC
[Ir]-2 instead of [Ir]-1
[Ir]-3 instead of [Ir]-1
MgCl2 instead of MgBr2
nBu4NBr instead of MgBr2
no MgBr2
71 (44/1)
79 (16/1)
74 (1/92)
85 (28/1)
83 (36/1)
81 (42/1)
86 (42/1)
83 (33/1)
35 (13/1)
2 (3/1)
0
3
4
5
6
7
8
9
i
10
11
12
13
14
Et3N instead of Pr2NH
i
Et2NH instead of Pr2NH
i
no Pr2NH
no NiBr2·DME/BC
no Ir complex or light
0
0
a
General conditions: [Ir]-1 (1 mol %), NiBr2·DME (5 mol %), BC (5
mol %), MgBr2 (20 mol %), 1a (0.45 mmol), 2a (0.3 mmol), iPr2NH
(1.2 mmol), DMF (3 mL), rt, 24 h. Yields and regioisomeric ratios
b
are determined by GC-FID with naphthalene as the internal standard.
c
84% isolated yield.
a
Standard conditions: [Ir]-1 (1 mol %), NiBr2·DME (5 mol %), BC (5
mol %), MgBr2 (20 mol %), aryl bromide (0.3 mmol), alkyl bromide
(0.45 mmol), iPr2NH (1.2 mmol), DMF (3 mL), rt, 24−36 h. Isolated
yields, by average of two runs. Regioisomeric ratios are determined by
yielded predominantly the ipso-product 4a (Table 1, entry 4).
Replacing the iridium photocatalyst [Ir]-1 with [Ir]-2 or [Ir]-3
resulted in a slight decrease in the reaction yield (Table 1,
entries 5 and 6). The addition of MgBr2 led to a further
increase in the yield (Table 1, compare entries 1, 7, 8, and 9).
The choice of electron donor also turned out to be crucial for
b
GC-FID. 1.0 mmol scale reaction.
(3n), unprotected indoles (3s, 3t), and amides (3u, 3v) were
all compatible under the photochemical reductive coupling
conditions.
i
the photochemical transformation, as replacing Pr2NH with
Et3N or Et2NH led to a significant drop in the yield (Table 1,
compare entries 1, 10, and 11), maybe due to less steric
hindrance.18 Finally, control experiments showed that the
nickel, ligand, organic electron donor, iridium complex, and
light are all required for the coupling reaction to occur (Table
1, entries 12, 13, and 14).
Attention was then turned to the reactivity of the secondary
alkyl bromides under these coupling conditions. As shown in
Scheme 2, the choice of ligand was also able to successfully
dictate the selectivity of the cross-coupling product. Bath-
ocuproine promotes coupling at the remote-site, while using
4,4′-dtbbpy the ipso-site products are preferred, which is
consistent with the report of Lei and co-workers.12b It is worth
pointing out that the 4-bromofluorobenzene exhibited poor
reactivity in the migratory coupling reaction (Scheme 2, 3b).
The construction of a quaternary steric center is still a challenge
for the current reductive system (Scheme 3, eq 1). As a final
note, a mixture of primary and secondary alkyl bromides 1a and
1w provided the migratory benzylic product in 87% yield
(Scheme 3, eq 2).
To gain further insights into the transformation, additional
control experiments were conducted. A stoichiometric amount
of nonactivated olefin 5 was added to the cross-coupling of 4-
carbon alkyl bromide 1l with 2a, and a mixture of products
formed with the olefin-based 3a being the major one (Scheme
3, eq 3). These results indicate that the nickel chain-walking is
not limited to an intramolecular process. Furthermore, catalytic
conditions similar to those for MacMillan’s reductive photo-
With the optimal conditions in hand, we then turned our
attention to the generality of these migratory cross-coupling
conditions. As shown in Scheme 1, a series of aryl bromides
were first examined, and as a result, both aryl and heteroaryl
bromides were converted to the corresponding cross-coupling
products in moderate to high yields under the standard
conditions. Notably, low selectivity was observed with electron-
deficient or π-extended aryl bromides (3g, 3h, and 3i),
probably due to the fact that the reductive elimination offering
the ipso-products is faster than the electron-rich ones. Primary
alkyl bromides with aliphatic chains consisting of two to five
carbons and a terminal aryl substituent were tested next for the
cross-coupling reaction. The results show that the electronic
property of aromatic rings attached to the alkyl bromides had
no obvious influence on either the reactivity or selectivity of the
reductive cross-coupling reactions. Furthermore, functional
groups such as electron-rich pyridine (3e), esters (3f), alcohol
B
DOI: 10.1021/acs.orglett.8b00413
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Investigation of Secondary Alkyl Bromides
Scheme 4. Proposed Mechanism
a
Standard conditions: [Ir]-1 (1 mol %), NiBr2·DME (5 mol %), BC (5
mol %), MgBr2 (20 mol %), aryl bromide (0.3 mmol), alkyl bromide
(0.45 mmol), Pr2NH (1.2 mmol), DMF (3 mL), rt, 36 h. Isolated
i
we cannot rule out the possibility of chain-walking via a Ni(I)7
or Ni(II)21 mediated process.
yields, average of two runs. Regioisomeric ratios are determined by
GC-FID.
In summary, a visible-light-driven reductive redox-relay cross-
coupling reaction by nickel catalysis has been developed. Cheap
organic electron-donor diisopropylamine is used as the terminal
stoichiometric reducing agent. The pharmaceutically important
1,1-diarylalkanes can be synthesized from the nonactivated alkyl
bromides and aryl bromides efficiently. This robust reductive
migratory coupling reaction tolerant of some of the more
common functional groups and also the presence of an organic
base. A radical chain mechanistic profile with nickel chain-
walking as a key step is tentatively proposed to rationalize this
transformation. Further mechanistic investigation is currently
ongoing in our laboratory.
Scheme 3. More Details on the Photochemical Reductive
Migratory Cross-Coupling Reaction
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.8b00413.
Details on the conditions investigation, extended data
about the photoreaction, NMR data and characterization
(PDF)
catalytic system,12a using tris(trimethysilyl)silane (TTMSS) as
the stoichiometric reducing agent, were also effective in the
migratory cross-coupling, giving a 58% yield and a 29/1
regioisomeric ratio (Scheme 3, eq 4).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yinguoyin@whu.edu.cn.
ORCID
Based on the current literature, a radical chain mechanism19
is proposed to rationalize the light-driven nickel-catalyzed
reductive migratory cross-coupling reaction. As illustrated in
Scheme 4, an aryl bromide oxidative addition to nickel(0) I
resulted in the formation of Ni(II) complex II. Meanwhile, a
single-electron transfer process generates an alkyl radical from
an alkyl bromide, which binds to the complex II, leading to the
formation of the Ni(III) intermediate III. Then a nickel chain-
walking process20 occurs until the thermodynamically most
stable benzylic-nickel complex VI is formed. Reductive
elimination from Ni(III) affords the benzylic arylation product
3 along with the corresponding Ni(I) species. The latter is
reduced back to the Ni(0) catalyst by reductive quenching. The
formation of ipso-product 4 is rationalized by the direct
reductive elimination from the intermediate III. At this stage,
Guoyin Yin: 0000-0002-2179-4634
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Great appreciation is shown to Prof. Qianghui Zhou and Prof.
Wen-Bo Liu (Wuhan University) for generously sharing lab
space and the basic instruments. The authors are grateful for
the financial support from Wuhan University and the National
Natural Science Foundation of China (21702151). The
reviewers are acknowledged for helpful suggestions and
comments.
C
DOI: 10.1021/acs.orglett.8b00413
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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D
DOI: 10.1021/acs.orglett.8b00413
Org. Lett. XXXX, XXX, XXX−XXX