Sterically controlled alkylation of arenes through iridium-catalyzed C-H borylation

Article abstract of DOI:10.1002/anie.201208203

Complementary chemistry: A one-pot method for the site-selective alkylation of arenes controlled by steric effects is reported. The process occurs through Ir-catalyzed C-H borylation, followed by Pd- or Ni-catalyzed coupling with alkyl electrophiles. This selectivity complements that of the typical Friedel-Crafts alkylation; meta-selective alkylation of a broad range of arenes with various electronic properties and functional groups occurs in good yield with high site selectivity. Copyright

Full text of DOI:10.1002/anie.201208203

Angewandte
Chemie
DOI: 10.1002/anie.201208203
Arene Alkylation
Sterically Controlled Alkylation of Arenes through Iridium-Catalyzed
À
C H Borylation**
Daniel W. Robbins and John F. Hartwig*
Alkylarenes are conventionally prepared by electrophilic
aromatic substitution with carbon electrophiles (Friedel–
Crafts reactions), but these methods have many limitations.
First, arenes containing electron-withdrawing groups are
much less reactive than those containing electron-donating
groups. Second, the electrophiles often undergo rearrange-
ments, limiting the utility for the synthesis of linear alkylar-
enes. Third, the electronic effects prevent formation of
products containing alkyl groups located meta to electron-
donating groups [Eq. (1)].[1]
Herein, we report the discovery of a one-pot method for
the meta-selective allylation, benzylation, and alkylation of
À
arenes through Ir-catalyzed C H borylation, followed by Pd-
and Ni-catalyzed coupling of the resulting arylboronate ester
with allyl, benzyl, and unactivated alkyl electrophiles, includ-
ing secondary alkyl electrophiles. This transformation is
a surrogate for meta-selective Friedel–Crafts arene alkylation
with site-selectivity controlled by steric effects.
Our efforts to develop the alkylation of arenes by the
À
combination of C H borylation and cross-coupling began
with a survey of existing systems reported for the coupling of
alkyl halides with organoboron species. Palladium catalysts
are less effective for the coupling of alkyl halides than for the
coupling of aryl halides because of the slow oxidative addition
of alkyl halides and b-hydride elimination from the alkylpal-
ladium intermediate formed from oxidative addition. To
address these limitations, iron,[6] nickel,[7] and copper[8]
catalysts have been developed for the Suzuki coupling of
alkyl halides or pseudohalides.
À
The site-selectivity for the formation of C C bonds at
aromatic systems can be altered by transition-metal-catalyzed
À
C H bond functionalization. However, transition-metal-cat-
However, the current iron-catalyzed Suzuki coupling of
alkyl halides with aryl boronate esters requires complexation
of the aryl boronate with an alkyl lithium reagent, limiting the
operational simplicity and functional-group tolerance of this
method. The current copper-catalyzed alkylation of pinacol-
substituted arylboronates is limited to a single example, and
this reaction of an alkyl tosylate occurred in modest yield. The
existing nickel-catalyzed Suzuki couplings of alkyl halides are
limited to reactions of boronic acids or 9-BBN-derivatives.
Because transmetallation has been proposed to be the rate-
limiting step in nickel-catalyzed cross-coupling with alkyl
halides, it was unclear whether the less-reactive pinacol
boronate esters would participate in this transformation.[9]
To identify a system that couples alkyl halides with aryl
pinacolboronates, we initially examined various copper and
nickel catalysts for the reaction of phenyl pinacol boronate
with 1-iodooctane or 1-octyl p-toluenesulfonate (Table 1).
The highest yield of 1-phenyloctane was obtained from
reactions conducted in the presence of NiBr2(dme) (dme =
dimethoxyethane) as the precatalyst with trans-N,N’-dime-
thylcyclohexane-1,2-diamine (L4) as the ancillary ligand.[10]
This reaction, catalyzed by NiBr2(dme), L4, and KOtBu base
in 2-butanol and dioxane, formed 1-octylbenzene in 68%
yield after 18 h at 608C (Table 1).
À
alyzed alkylation of aromatic C H bonds has typically been
limited to reactions that are either intramolecular or occur at
positions ortho to a directing group.[2]
Iridium-catalyzed borylation could provide a method for
the alkylation of arenes with alternative selectivity. The site-
À
selectivity of iridium-catalyzed C H borylation is predom-
inantly controlled by steric factors [Eq. (1)].[3] Therefore,
a sterically controlled arene alkylation could be envisioned to
À
occur by the combination of C H borylation, followed by
alkylation of the resulting arylboronate ester. However,
methods to form sp3 carbon–aryl linkages with the products
of the C H borylation are not known.[4] The pinacolboronate
À
À
esters formed by C H borylation are significantly less
reactive than other aryl boron species such as aryl boronic
acids, for which coupling reactions with alkyl halides are
known.[4a,5]
[*] D. W. Robbins, Prof. J. F. Hartwig[+]
Department of Chemistry, University of Illinois
Urbana-Champaign, Urbana, IL 61801 (USA)
E-mail: jhartwig@berkeley.edu
[+] Current address: University of California, Department of Chemistry,
Having identified conditions for the coupling of aryl
pinacolboronate esters with primary alkyl halides, we devel-
oped a one-pot procedure for the alkylation of arenes.
Reactions of 1,3-disubstituted arenes with bis(pinacolato)di-
boron (B2pin2) in the presence of a catalyst formed in situ
from the combination of [{Ir(cod)OMe}2] (cod = 1,5-cyclo-
octadiene) and 4,4’-di-tert-butylbipyridine (dtbpy) in THF at
718 Latimer Hall, Berkeley, CA 94720 (USA)
[**] We thank the NSF (CHE-0910641) for support, Johnson-Matthey for
a gift of iridium, and Allychem for a gift of B2pin2. We also thank the
American Chemical Society Division of Organic Chemistry (spon-
sored by Boehringer Ingelheim) for fellowship support (D.W.R.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208203.
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Table 1: Effect of conditions on the coupling of PhBpin with alkyl
(pseudo)halides.
Entry
Catalyst[a]
Ligand
X
Yield [%][b]
1
2
CuI
CuI
CuI
none
PPh3
1,10-phen
L1
L2
L3
L4
L5
L4
L4
OTs
OTs
OTs
I
I
I
I
I
44
57
32
44
26
44
58
27
68
55
69
3[c]
4
NiCl2(dme)
NiCl2(dme)
NiCl2(dme)
NiCl2(dme)
NiCl2(dme)
NiBr2(dme)
NiBr2(dme)
NiBr2(dme)
5
6
7
8
9
I
Br
Br
10
11[d]
L4
[a] Cu-catalyzed reactions conducted with LiOtBu (2 equiv); Ni-catalyzed
reactions conducted with KOtBu (1.2 equiv) and 2-butanol (2 equiv).
[b] Yield determined by GC with an internal standard. [c] 10 mol% ligand
used. [d] NaI (1 equiv) added. 1,10-phen=1,10-phenanthroline, Ts=
p-toluenesulfonyl.
Scheme 1. Scope of the alkylation with unactivated alkyl electrophiles.
[a] Alkylation conducted with alkyl bromide and NaI. [b] Borylation
conducted with [{Ir(cod)OMe}2] (0.5 mol%) and dtbpy (1.0 mol%).
[c] Borylation conducted with [{Ir(cod)OMe}2] (1.0 mol%) and dtbpy
(2.0 mol%). [d] Alkylation conducted with L1. Boc=tert-butoxycar-
bonyl.
808C,[11] followed by addition of the nickel catalyst, KOtBu,
2-butanol, and dioxane to the crude arylboronate ester and
heating at 608C, gave the meta-alkylation product in good
yield.[12]
The scope of the meta-selective alkylation with unacti-
vated alkyl halides in the presence of the iridium and nickel
catalysts is shown in Scheme 1. Arenes containing a variety of
substituents reacted, including those containing ethers, esters,
nitriles, amides, and trifluoromethyl groups. The coupling of
an alkyl halide occurred in preference to the coupling of an
aryl halide. The reaction with benzofuran occurred at the
2-position in good yield, thus providing a method for the
alkylation of five-membered ring heteroarenes at the less
electron-rich position. The pinacolboronates generated by the
iridium-catalyzed borylation ortho to fluorine also coupled
with alkyl iodides in good yield, despite the propensity of the
corresponding boronic acids to undergo protodeborylation in
other Suzuki coupling processes.[13]
The examples in Scheme 1 also reveal the range of alkyl
halides that undergo this arene alkylation process. Alkyl
halides containing a TBS-protected (TBS = tert-butyldime-
thylsilyl) alcohol and a nitrile coupled in acceptable yield.
Alkyl iodides containing a branch point at the position b to
the halogen also coupled in good yield. Primary alkyl
bromides also coupled with pinacol boronates in good yield
when the reactions were conducted with added NaI to initiate
halogen exchange, followed by coupling of the alkyl iodide
generated in situ with the aryl pinacol boronate ester. In each
case, no rearrangement, which would be observed for a true
Friedel–Crafts reaction, occurred.
nickel catalyst bearing diamine L4 as the ancillary ligand, only
trace amounts of the product from the coupling of phenyl
pinacolboronate ester with iodocyclohexane was observed.
However, the same reaction catalyzed by NiBr2(dme) with
bathophenanthroline (L1) as the ancillary ligand occurred in
67% yield. Moreover, these conditions were compatible with
the one-pot protocol involving borylation of the arene and
coupling of the arylboronate with a secondary alkyl halide.
As shown by examples in Scheme 1, cyclic and acyclic
secondary alkyl iodides and bromides coupled in good yield
À
with several arenes generated by the C H borylation
chemistry. The alkylation of arenes possessing methoxy,
chloro, and trifluoromethyl groups with iodocyclopentanes
and cyclohexanes occurred regioselectively. The alkylation of
3-chloroanisole with 3-iodotetrahydrofuran and an iodo-
piperidine derivative occurred without any rearrangement
of the alkyl group during the coupling process. Even the
reaction of the acyclic 2-bromopentane occurred without
rearrangement. The primary side products from the alkyla-
tion reaction were those from protodehalogenation of the
alkyl halide.
À
The success of this combination of C H borylation and
the coupling of alkylhalides led us to extend this strategy to
the allylation and benzylation of arenes with steric control.
Few examples of the catalytic direct allylation of arenes have
been published,[14] and the known direct benzylation of
The sterically controlled, meta-selective alkylations of
arenes also occurred with secondary alkyl halides. With the
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À
aromatic and heteroaromatic C H bonds have required
[15]
À
either acidic C H bonds or a directing group. If methods
for the allylation and benzylation of pinacolate-substituted
arylboronate esters could be identified, then a method for the
allylation and benzylation of arenes with stereocontrol would
result.
Several methods for the coupling of organoboranes with
allylic acetates, alcohols, and halides catalyzed by phosphine-
ligated palladium complexes have been developed.[16] After
surveying previously reported reaction conditions for the
allylation of electron-rich and electron-deficient, pinacol-
substituted arylboronate esters with cinnamyl acetate, we
found that [Pd(dba)2] (dba = dibenzylideneacetone) with KF
as an additive and no added ligand in MeOH solvent at room
temperature provided the meta-allylation products of aryl
pinacolboronate esters in good yield.[17]
Scheme 2. Scope of the allylation of 1,3-disubstituted arenes with
Several methods for the Pd-catalyzed coupling of organ-
ometallic reagents with benzylic electrophiles, including
halides and phosphates, have also been developed.[18] After
examining catalysts formed from the combination of several
common phosphine ligands with [Pd(dba)2], we found that
catalysts containing phosphine ligands gave higher yields for
the coupling of the aryl boronate esters with benzylic halides
than did ligandless catalysts. The simple catalyst formed
in situ from the combination of [Pd(dba)2] and PPh3 with
Na2CO3 in a mixture of THFand H2O at 1008C provided good
yields of the product from the coupling of electron-rich and
electron-deficient pinacol-substituted aryl boronate esters
with 4-methylbenzyl bromide.
cinnamyl acetate. [a] Borylation conducted with [{Ir(cod)OMe}2]
(0.5 mol%) and dtbpy (1.0 mol%). [b] Borylation conducted with
[{Ir(cod)OMe}2] (1.0 mol%) and dtbpy (2.0 mol%). [c] Allylation con-
ducted with cinnamyl chloride.
With these methods for the coupling of aryl pinacolbor-
onate esters with cinnamyl acetate and 4-methylbenzyl
bromide, we developed the one-pot meta-allylation and
meta-benzylation of arenes. Reactions of 1,3-disubstituted
arenes with B2pin2 in the presence of a catalyst formed in situ
from the combination of [{Ir(cod)OMe}2] and dtbpy in THFat
808C,[11] followed by addition of the appropriate Pd catalyst,
the allylic or benzylic electrophile, the additive, and solvent
gave good yield of the meta-allylation or meta-benzylation
product.
Scheme 3. Scope of the allylation with various allylic electrophiles.
[a] Allylation conducted with [Pd(dba)2] (2 mol%). [b] Borylation con-
ducted with [{Ir(cod)OMe}2] (0.5 mol%) and dtbpy (1.0 mol%).
[c] 14:1 mixture of linear/branched isomers.
The scope of the meta-allylation of 1,2-, and 1,3-substi-
tuted arenes with cinnamyl acetate is summarized in
Scheme 2. These reactions formed the meta-allylation prod-
ucts in good yield from electron-rich and electron-deficient
arenes and tolerated a variety of functional groups, including
aryl fluorides, chlorides, and bromides, as well as esters,
amides, ethers, and trifluoromethyl groups. Ketones on the
arene did not undergo reduction of the carbonyl group.
Tetrasubstituted arenes were also synthesized from 1,2,4-
trisubstituted arenes by borylation ortho to a fluorine sub-
stituent on the arene. Site-selective allylation was performed
on a thiophene derivative in good yield with cinnamyl
chloride as the allylic electrophile in place of cinnamyl
acetate.
This allylation process occurred with a series of allyl
electrophiles (Scheme 3). The conversion of arenes to allylar-
enes occurred with substituted cinnamyl acetates, as well as
allyl chloride itself. Cinnamyl electrophiles that underwent
this process include those containing electron-donating
groups, electron-withdrawing groups, and ortho-substituents.
Cinnamyl acetates containing chloride and bromide substitu-
ents reacted without cleavage of the aromatic carbon–halogen
bond. The reaction with the parent allyl chloride occurred in
good yield. Allylbenzenes, the products of these reactions, are
known to undergo olefin cross metathesis to generate 1,2-
disubstituted olefins.[19] Moreover, one allylarene, elemicin,[20]
is a natural product studied for its antibacterial activity[21] and
was prepared by a one-pot borylation–allylation sequence in
67% yield.
The scope of the meta-benzylation is summarized in
Scheme 4. These examples also include reactions of electron-
rich and electron-deficient arenes containing a variety of
functional groups. Benzylation of benzothiophene occurred at
the C2 position, whereas the uncatalyzed benzylation of five-
membered ring heterocycles is known to occur at C3.[22] Thus,
this approach to arene alkylation leads to site selectivity that
is orthogonal to that of the alkylation of heteroarenes
mediated by Lewis acids. The meta-selective borylation and
benzylation was performed with a variety of benzyl halides.
Similar yields were obtained when benzylic bromides and
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The reaction mixture was then cooled to room temperature, filtered
through silica gel (which was then washed with EtOAc), and
concentrated under vacuum. The residue was then purified by
column chromatography to give the product.
Received: October 11, 2012
Published online: December 11, 2012
Keywords: alkylation · arenes · iridium · nickel · palladium
.
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Scheme 4. Scope of the benzylation of 1,3-disubstituted arenes with
benzylic bromides and chlorides. [a] Borylation conducted with
[{Ir(cod)OMe}2] (0.5 mol%) and dtbpy (1.0 mol%). [b] Borylation con-
ducted with [{Ir(cod)OMe}2] (1.0 mol%) and dtbpy (2.0 mol%).
[c] Benzylation conducted with a benzyl chloride.
benzylic chlorides were used as the alkyl electrophile.
Benzylic halides containing electron-donating groups, elec-
tron-withdrawing groups, and ortho-substituents all reacted in
good yield.
In conclusion, a method for the site-selective alkylation,
allylation, and benzylation of arenes through Ir-catalyzed
borylation and Ni- or Pd-catalyzed coupling has been
developed. The method is effective with various allylic,
benzylic, primary alkyl, and secondary alkyl electrophiles.
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selectivity that is complementary to that of Friedel–Crafts
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À
alkylation, C H functionalization methods controlled by
À
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Experimental Section
General Procedure for Alkylation of Arenes with Primary Alkyl
Halides: Inside a glove box, [{Ir(cod)OMe}2] (2.5 mg, 0.0038 mmol,
0.0038 equiv), dtbpy (2.0 mg, 0.0075 mmol, 0.0075 equiv), B2pin2
(229 mg, 0.900 mmol, 0.900 equiv), arene (1.50 mmol, 1.50 equiv),
and THF (3 mL) were added to a dry vial with a magnetic stir bar. The
vial was sealed and heated at 808C for 18 h. The reaction mixture was
cooled to room temperature, and the volatile materials were removed
under vacuum. NiBr2(dme) (30.9 mg, 0.100 mmol, 0.100 equiv), trans-
N,N’-dimethylcyclohexane-1,2-diamine
(28.5 mg,
0.200 mmol,
0.200 equiv), an alkyl halide (1.00 mmol, 1.00 equiv), KOtBu
(135 mg, 1.20 mmol, 1.20 equiv), 2-butanol (149 mg, 2.00 mmol,
2.00 equiv) and 1,4-dioxane (2 mL) were added consecutively to the
reaction mixture. The reaction mixture was stirred at 608C for 15 h.
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