Rh(III)-catalyzed C-H alkylation of arenes using alkylboron reagents

Article abstract of DOI:10.1021/acs.orglett.5b01232

Rhodium(III)-catalyzed direct alkylation of arenes using commercially available alkyltrifluoroborates is disclosed. Oximes, heteroarenes, azomethines, N-nitrosoamines, and amides are viable directing groups to entail this transformation. The alkyl group in the boron reagent can be extended to primary alkyls, benzyl, and cycloalkyls, and the reaction proceeded with controllable mono- and dialkylation selectivity when both ortho C-H sites are accessible.

Full text of DOI:10.1021/acs.orglett.5b01232

Letter
pubs.acs.org/OrgLett
Rh(III)-Catalyzed C−H Alkylation of Arenes Using Alkylboron
Reagents
He Wang, Songjie Yu, Zisong Qi, and Xingwei Li*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
S
* Supporting Information
ABSTRACT: Rhodium(III)-catalyzed direct alkylation of arenes using commercially available alkyltrifluoroborates is disclosed.
Oximes, heteroarenes, azomethines, N-nitrosoamines, and amides are viable directing groups to entail this transformation. The
alkyl group in the boron reagent can be extended to primary alkyls, benzyl, and cycloalkyls, and the reaction proceeded with
controllable mono- and dialkylation selectivity when both ortho C−H sites are accessible.
irect C−H bond functionalization has emerged as an
increasingly valuable strategy for the construction of C−
isolated in 34% yield. In contrast, the reaction gave poor or no
conversion when AgSbF6 or [RhCp*Cl2]2 was omitted (entries
3, 13). Screening of the oxidant indicated that the amount of
AgF is crucial (entry 2), and coupling using other oxidants
afforded inferior results (entries 4−8). Lowering the temper-
ature to 80 °C resulted in diminished yield (entry 9). The use
of 3 equiv of the boron reagent seems necessary because a
slightly lower yield was isolated when the amount was reduced
to 2 equiv (entry 10). The reaction proceeded with lower
efficiency when the alkylating reagent was replaced by 2-butyl-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane or butylboronic acid
(entries 11, 12). Optimization of the solvent indicated that
ClCH2CH2Cl seems to be the optimal one (entries 14, 15).
We next explored the scope and limitation of this reaction
(Schemes 1 and 2). Oximes bearing different electron-donating
(3fa) and -withdrawing (3da, 3ga−3ka) groups at the ortho
and meta positions of the phenyl ring all coupled smoothly with
MeBF3K in high monoselectivity. In contrast, a mixture of
mono- and dimethylated products was obtained for non- or
para-substituted oximes. To our delight, lowering the amount
of MeBF3K and AgF can greatly improve the monoselectivity
such that only a trace of the dimethylated product could be
detected (3aa, 3ea−3ka). The oxime substrate is not restricted
to those derived from acetophenones, and oximes with other
ketone platforms also reacted smoothly in comparably high
yield (3la−3pa). Moreover, product 3ha was isolated in 70%
yield in a gram-scale (5 mmol) synthesis using a reduced
loading of the catalyst ([RhCp*Cl2]2 (2.0 mol %) and AgSbF6
(8 mol %)).
D
C bonds in organic synthesis.1 In this regard, direct alkylation
of the C−H bond of arenes remains a great challenge.2
Palladium catalysis has been particularly well-studied for C−H
alkylation.3 Various other transition-metal catalysts have also
been utilized to increase the catalytic efficiency, substrate scope,
and functional group compatibility.4 While alkylation of arenes
was often performed using alkyl halides as the alkylating
reagents,3c,e,l,m,5 organo main group reagents such as organo-
borons6 have been extensively employed in C−H alkylation7
ever since the first report of palladium-catalyzed alkylation of
arenes by the Yu group with these reagents in 2006.3a
Recently, Rh(III)-catalyzed C−H activation of arenes leading
to efficient cross-couplings has attracted increasing attention.8
Despite this attractive catalytic process, the coupling partners
are mostly limited to unsaturated molecules such as alkenes,9
alkynes,10 ketones and aldehydes,11 imines,12 azides,13 iso-
cyanates,14 azides,15 and strained rings.16 Although Rh(III)-
catalyzed direct C−H arylation of arenes has been implemented
using organoboron reagents17 and aryl halides,18 related
alkylation reactions remain unexplored. This is likely associated
with the low reactivity of rhodium(III) alkyl species toward
C(sp2)−C(sp3) reductive elimination. However, we reasoned
that the competitive β-H elimination of Cp*Rh(DG-Ar)(alkyl)
is suppressed due to the coordinative saturation enforced by the
Cp* and the cyclometalated group. We now report Rh(III)-
catalyzed oxidative C−H alkylation of diverse arenes bearing an
ortho-directing group using potassium alkyltrifluoroborates.
We initiated our studies with the screening of the conditions
for the coupling of methyl 4-(1-(methoxyimino)ethyl)benzoate
with potassium n-butyltrifluoroborate (Table 1). Using
[RhCp*Cl2]2 (4.0 mol %) as a catalyst, a desired coupling
occurred in the presence of AgSbF6 (16 mol %) and AgF (3.0
equiv) in ClCH2CH2Cl (entry 1), and the product 3hb was
To define the scope of the directing group further, the
alkylation of other arenes, especially those bearing a
Received: April 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01232
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a b
,
Table 1. Optimization Studies
Scheme 2. Scope of Chelation-Assisted C−H Methylation
b
entry
oxidant (equiv)
AgF (3.0)
solvent
yield
1
2
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
1,4-dioxane
34%
89%
28%
31%
nd
AgF (4.0)
c
3
AgF (4.0)
4
5
6
7
8
Ag2O (4.0)
AgOAc (4.0)
Ag2CO3 (4.0)
CuF2 (4.0)
AgF/Ag2O (2.0/2.0)
AgF (4.0)
44%
trace
47%
79%
80%
64%
25%
ND
d
9
e
10
AgF (4.0)
f
11
AgF (4.0)
g
12
AgF (4.0)
h
13
AgF (4.0)
14
15
AgF (4.0)
25%
18%
AgF (4.0)
toluene
a
Reactions were carried out using methyl 4-(1-(methoxyimino)ethyl)-
n
benzoate (0.2 mmol), BuBF3K (0.6 mmol), [RhCp*Cl2]2 (4.0 mol
%), and AgSbF6 (16 mol %) in a solvent (3 mL) under nitrogen at 100
°C for 24 h. Isolated yield. No AgSbF6 was used. Reaction was
b
c
d
e
n
performed at 80 °C. Reaction was performed using BuBF3K (2.0
f
equiv). Reaction was performed using butylboronic acid (0.6 mmol).
g
Reaction was performed using 2-butyl-4,4,5,5-tetramethyl-1,3,2-
h
dioxaborolane (0.6 mmol). No catalyst was used.
ab
,
Scheme 1. Methylation of Oximes
a
Reaction conditions: arene (0.20 mmol), MeBF3K (0.60 mmol), AgF
(0.56 mmol), [RhCp*Cl2]2 (0.008 mmol), AgSbF6 (0.032 mmol),
ClCH2CH2Cl (3.0 mL), 100 °C, 24 h, sealed tube under nitrogen.
b
c
Isolated yield after column chromatography. Reaction was
d
performed using MeBF3K (0.6 mmol), AgF (0.6 mmol). Reaction
was performed using MeBF3K (0.4 mmol), AgF (0.6 mmol). Reaction
was performed using MeBF3K (0.7 mmol), AgF (0.6 mmol). Reaction
e
f
was performed using MeBF3K (0.8 mmol), AgF (0.6 mmol).
N-(2-pyridine)indoles and N-(2-pyrimidyl)indoles (4aa−4ha,
42−94% yields). The arene substrate can be further extended
to N-nitrosoanilines,20 and products 4ia and 4ja were isolated
in good yield, where the introduction of a meta-methyl or
-chloro blocking group ensured monoselectivity. Using
azomethine imine as a directing group, the meta methyl
substituted substrate coupled in moderate yield with mono-
selectivity (4ka). However, the para methyl substituted
azomethine substrate reacted in dimethylation selectivity
(4la) in the presence of an excess of MeBF3K and AgF.
Interestingly, N-phenylbenzo[d][1,3]dioxole-5-carboxamide is
also a viable arene source, and the reaction proceeded at the
more sterically hindered ortho position (4ma) in moderate
yield.
a
Reaction conditions: oximes (0.2 mmol), MeBF3K (0.6 mmol), AgF
(0.8 mmol), [RhCp*Cl2]2 (0.008 mmol), AgSbF6 (0.032 mmol),
ClCH2CH2Cl (3.0 mL), 100 °C, 24 h, sealed tube under nitrogen.
b
c
Isolated yield after column chromatography. Reaction was
performed using MeBF3K (0.5 mmol) and AgF (0.6 mmol).
functionalizable19 directing group, was performed under the
optimized reaction conditions (Scheme 2). The coupling of 2-
phenylpyridines bearing different EDG, methyl, and halogen
groups at the ortho, meta, and para positions all proceeded well
(3qa−3za). This methylation reaction was fully applicable to
B
DOI: 10.1021/acs.orglett.5b01232
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We next examined the scope of the boron reagent under the
optimized reaction conditions (Scheme 3). Our results revealed
To probe the relevancy of C−H activation, rhodacyclic
complex 5 was applied as a catalyst (8 mol %) for coupling of 2-
PhPy with MeBF3K, and product 3qa was isolated in 64% yield,
suggesting the relevancy of C−H activation (Scheme 4b). The
kinetic isotope effect (KIE) was thus measured to gain
mechanistic details. An intermolecular competition using 2-
PhPy and 2-PhPy-d5 with MeBF3K has been performed at low
conversion, and a small value of KIE = 1.4 was obtained on the
a b
,
Scheme 3. Scope of Other Alkylating Reagents
1
basis of H NMR analysis (Scheme 4c), indicating that C−H
bond cleavage is likely not involved in the rate-limiting step. In
addition, the coupling of 2-PhPy with MeBF3K was performed
in the presence of typical radical inhibitors such as TEMPO and
BHT (Scheme 4d). The fact that the yield of product 3qa was
only slightly affected by both reagents (64% and 60%) suggests
the irrelevancy of organic radical species. Thus, the coupling
most likely involves transmetalation between the boron reagent
and a rhodacyclic intermediate, followed by C−C reductive
elimination (Scheme 5). The active Rh(III) catalyst is
regenerated when the Rh(I) is reoxidized by Ag(I).
Scheme 5. Proposed Catalytic Cycle
a
Reaction conditions: arene (0.2 mmol), RBF3K (0.6 mmol), AgF (0.8
mmol), [RhCp*Cl2]2 (0.008 mmol), AgSbF6 (0.032 mmol),
ClCH2CH2Cl (3.0 mL), 100 °C, 24 h, sealed tube under nitrogen.
b
Isolated yield after column chromatography.
that the coupling yield (67−82%) is not adversely affected in
the butylation of oximes, 2-phenylpyridines, N-(2-pyridine)-
indole, and N-(2-pyrimidyl)indole (3hb−4cb). Other function-
alization such as pentylation (4ac), cycloalkylation (4ad, 4ae),
and benzylation (4af) has also been readily realized in
moderate to good yields.
Several experiments have been performed to explore the
reaction mechanism (Scheme 4). To probe the electronic
Scheme 4. Mechanistic Studies
In summary, we have developed the first rhodium-catalyzed
direct alkylation of arenes using alkyltrifluoroborates. Both
mono- and dialkylation have been achieved when both ortho
C−H sites are accessible, and this selectivity is controllable by
control of the reaction stoichiometry. Introduction of ortho and
meta blocking groups ensured exclusive monoselectivity. This
Rh(III)-catalyzed alkylation reaction expands the arsenal of
coupling partners. With functionalizable directing groups
installed, further useful chemical transformations of the coupled
products can be expected. Future studies on Rh(III)-catalyzed
C−H activation of other types of C−H bonds are underway in
our laboratories.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures, analytical data, and copies of
NMR spectra of the products. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.orglett.5b01232.
preference of the oxime substrate, an intermolecular competi-
tion experiment has been carried out using an equimolar
mixture of oximes that differ in electronic effects at the 4-
position (Scheme 4a). 1H NMR analysis of the product mixture
indicated that the 4-CO2Me substrate showed only slightly
higher reactivity.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: xwli@dicp.ac.cn.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.5b01232
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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̈
ACKNOWLEDGMENTS
■
Financial support from the NSFC (Nos. 21472186 and
21272231) and the dedicated fund for new technology of
methanol conversion from Dalian Institute of Chemical
Physics, Chinese Academy of Sciences is gratefully acknowl-
edged.
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