Nickel-catalyzed cross couplings of benzylic ammonium salts and boronic acids: Stereospecific formation of diarylethanes via C-N bond activation

Article abstract of DOI:10.1021/ja3089422

We have developed a nickel-catalyzed cross coupling of benzylic ammonium triflates with aryl boronic acids to afford diarylmethanes and diarylethanes. This reaction proceeds under mild reaction conditions and with exceptional functional group tolerance. Further, it transforms branched benzylic ammonium salts to diarylethanes with excellent chirality transfer, offering a new strategy for the synthesis of highly enantioenriched diarylethanes from readily available chiral benzylic amines.

Full text of DOI:10.1021/ja3089422

Article
pubs.acs.org/JACS
Nickel-Catalyzed Cross Couplings of Benzylic Ammonium Salts and
Boronic Acids: Stereospecific Formation of Diarylethanes via C−N
Bond Activation
Prantik Maity,† Danielle M. Shacklady-McAtee,† Glenn P. A. Yap, Eric R. Sirianni, and Mary P. Watson*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: We have developed a nickel-catalyzed cross coupling of
benzylic ammonium triflates with aryl boronic acids to afford
diarylmethanes and diarylethanes. This reaction proceeds under mild
reaction conditions and with exceptional functional group tolerance.
Further, it transforms branched benzylic ammonium salts to diaryl-
ethanes with excellent chirality transfer, offering a new strategy for the
synthesis of highly enantioenriched diarylethanes from readily available
chiral benzylic amines.
Despite these impressive advances in stereospecific cross
couplings of benzylic reagents, several challenges remain,
including identification of a class of benzylic reagents generally
available in exceptional enantiopurity and the ability to employ
commercially available and functional group tolerant coupling
partners. In considering these challenges, we have been drawn
to the use of benzylic ammonium salts as substrates.10 Benzylic
ammonium salts are readily prepared from amine precursors.11
Further, these substrates offer a functional group handle
orthogonal to both halides and ethers, and both amines and
ammonium salts are stable to long-term storage. Importantly,
highly enantioenriched benzylic amines are readily available via
classical resolution or a variety of catalytic asymmetric
iarylmethanes and diarylethanes are important molecules
1
Din organic synthesis and pharmaceutical development.
These valuable targets can be prepared via cross couplings of
classic electrophiles, such as benzylic halides, with Grignard,
organozinc, and organoboron reagents.2 In fact, Carretero has
shown that enantioenriched diarylethanes can be formed via
stereospecific cross couplings of benzylic bromides with aryl
Grignard reagents (Scheme 1A-1).2c However, these classic
electrophiles are highly reactive, can decompose upon
prolonged storage, typically mandate that the cross coupling
occur at the beginning of a synthetic sequence, and are difficult
to prepare in high enantiopurity. Cross couplings of benzylic
carbonates, acetates, and phosphates have also been developed
to enable use of more stable starting materials.3 Recent reports
have also demonstrated that the use of benzylic ethers and
alcohols as coupling partners overcomes some of these
challenges, offering greater substrate stability and orthogon-
ality.4 Further, the Jarvo group has shown that nickel-catalyzed
cross couplings of benzylic ethers with Grignard reagents
occurs in a stereospecific fashion (Scheme 1A-2).5,6 In contrast,
Tian has reported that copper-catalyzed couplings of benzylic
sulfonimides proceed with only low levels of chirality transfer
(Scheme 1A-3).7 Notably, almost all cross couplings of benzyl
electrophiles to deliver enantioenriched diarylethanes require
Grignard or organozinc coupling partners.2b−d,5 No examples
of enantioselective couplings of benzylic electrophiles with
boronic acids are yet known, and only a single stereospecific
coupling of a benzylic α-cyanohydrin mesylate with a boronic
acid partner has been reported to our knowledge.8
methods.12 Csak
́
y et al. have elegantly demonstrated the
̈
rhodium-catalyzed cross couplings of boronic acids with
ammonium iodides derived from gramines (3-aminomethylin-
doles) to form achiral 3-benzyl- and 3-allylindoles.10d However,
to our knowledge, a general method for the cross coupling of
benzylic ammonium salts to yield diarylmethanes and
enantioenriched diarylethanes has not yet been realized.
We anticipated that nickel catalysts would enable the cross
coupling of a wide variety of benzylic ammonium salts with
mild and highly functional group compatible coupling partners,
such as commercially available boronic acids. Because
ammonium salts can be readily prepared with a wide variety
of counterions, we envisioned facile tuning of the reactivity of
these electrophiles. In particular, we predicted that weakly
coordinating counterions, such as triflate, would be highly
effective. In contrast to halide or alkoxide counterions formed
in cross couplings with benzylic halides and ethers, the
byproducts of oxidative addition into an ammonium triflate
are neutral trimethylamine and the triflate counterion. We
envisioned that the resulting oxidative addition intermediate
In an alternative bond-disconnection approach to the
synthesis of enantioenriched diarylethanes, Crudden has
shown that enantioenriched benzylic boronic esters undergo
stereospecific cross couplings (Scheme 1A-4).9 Stereospecific
protodeborylation of tertiary boronic esters also delivers
enantioenriched diarylethanes.6e
Received: September 8, 2012
© XXXX American Chemical Society
A
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Article
a
Scheme 1. Stereospecific Cross Couplings To Form
Table 1. Optimization of Reaction Conditions
Diarylethanes
b
entry
X
ligand (mol %)
base
temp (°C) yield (%)
1
I
IMes·HCl (11)
PCy3 (22)
CsF
60
60
60
60
60
40
40
40
r.t.
40
r.t.
40
40
40
21
30
59
54
100
96
16
82
68
99
96
0
2
I
CsF
3
I
PPhCy2 (22)
PPh2Cy (22)
PPh2Cy (22)
PPh2Cy (22)
PPhCy2 (22)
PPh3 (22)
CsF
4
I
CsF
5
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
CsF
6
CsF
7
CsF
8
CsF
9
PPh2Cy (22)
PPh2Cy (22)
PPh2Cy (22)
none
CsF
10
11
K3PO4
K3PO4
CsF
c
12
c
13
none
K3PO4
CsF
0
14
IMes·HCl (11)
28
a
Conditions: ammonium triflate 1 (0.10 mmol, 1.0 equiv), boronic
acid (1.2 equiv), Ni(cod)2 (10 mol %), ligand, CsF or K3PO4 (1.3
b
equiv), dioxane (0.33 M), 24 h, unless noted otherwise. Determined
by 1H NMR analysis using 1,3,5-trimethoxybenzene as internal
c
standard. No Ni(cod)2 used.
examination of the N-heterocyclic carbene IMes·HCl under the
optimized conditions again showed phosphines to be superior
ligands.
Using the optimized conditions (Table 1, entries 6 and 10),
we observed broad substrate scope for both the boronic acid
and ammonium triflate in the cross couplings of unbranched
benzylic ammonium triflates. We generally observed similar
results with CsF and K3PO4. However, for products with base-
sensitive functional groups, such as 6 (Scheme 2), CsF proved
advantageous. For the boronic acid, substitution was well
tolerated at the ortho, meta, and para positions of the aromatic
ring. High yields were observed for arylboronic acids with both
electron-donating (4) and electron-withdrawing (5−9) sub-
stituents. Further, these mild reaction conditions tolerated a
wide variety of functional groups, including ethers (4, 10, 14,
17, 18, 21), fluorides (5, 12, 13, 15), esters (6), secondary
amides (7), sulfones (8), trifluoromethyls (9), free alcohols
(11), and acetals (19). Naphthyl, pyridyl, and quinolinyl
boronic acids were also effective partners (20−23).17
For the ammonium triflate partner, a variety of functional
groups were tolerated at the ortho, meta, and para positions of
the phenyl group (Scheme 3). Products containing ether (24,
26−28), trifluoromethyl (25), ketone (29), and fluoride (30)
substituents were formed in high yields. Notably, these reaction
conditions selectively activate the benzylic C−N bond in the
presence of either aryl or benzylic C−O bonds (24, 26−28),
highlighting the complementarity of the benzylic ammonium
salts to ethers. This method also provides orthogonal reactivity
to aryl halides. Although chloride-substituted benzyl ammo-
nium triflates undergo cross coupling at both the C−Cl and C−
NMe3 bonds, aryl bromides can be effectively cross coupled in
the presence of benzylic dimethylamino groups.18 Finally,
naphthyl and indolyl10d substrates are also effective in this
reaction (31, 32).
may thus more readily undergo transmetalation with a boronic
acid. Such an effect has been observed in cross couplings of aryl
ammonium triflates.13
Herein we report the first example of a general method for
the cross coupling of benzylic ammonium salts with boronic
acids (Scheme 1B). To our knowledge, this is the first cross
coupling of an aryl boronic acid with a benzylic amine
derivative other than gramine and the first stereospecific
coupling of an acyclic benzylic amine derivative with excellent
chirality transfer.7,14,15 This method enables the preparation of
diarylmethanes with broad substrate scope and diarylethanes
with excellent enantioenrichment.
We began by investigating the cross coupling of p-tolyl
boronic acid and benzyl ammonium salts 1, which are easily
prepared in quantitative yield via methylation of dimethyl
benzyl amine.11 Using conditions similar to those reported for
Suzuki reactions of aryl ammonium salts,13a low yields of
diarylmethane 2 were observed when iodide 1a was employed
(Table 1, entry 1). In contrast, use of monodentate phosphine
ligands, particularly mixed aryl/alkyl phosphines, provided
increased yields (entries 2−4). As we had anticipated, the use of
the less coordinating triflate counterion resulted in greater
reactivity. Quantitative yield was observed when triflate 1b was
employed as the substrate (entry 5). Lowering the reaction
temperature to 40 °C, PPh2Cy proved to be the best ligand
(entries 6−8). K3PO4 was as effective as CsF, resulting in nearly
quantitative yield (entry 10). Notably, no product is observed
in the absence of Ni catalyst (entries 12 and 13).16 Re-
Encouraged by this broad substrate scope, we turned to the
cross coupling of branched benzylic ammonium triflate 33 (Np
B
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a
= 2-naphthyl), which was readily prepared from (1S)-1-(2-
naphthyl)ethanamine, purchased in 99.6% ee. Although
application of the previously optimized conditions gave no
desired product at 40 °C, high yields of diarylethane 34 were
realized at a higher reaction temperature (Table 2, entry 1).
Scheme 2. Scope in Boronic Acid
a
Table 2. Optimization of Branched Ammonium Salt
b
c
entry
ligand (mol %)
temp (°C)
yield (%)
ee (%)
1
2
3
4
5
6
7
PPh2Cy (22)
PPhCy2 (22)
PPh3 (22)
100
100
100
100
100
100
70
84
97
83
(91)
15
71
94
95
0
81
79
81
98
40
98
98
98
t-Bu-XantPhos (12)
XantPhos (12)
P(o-Tol)3 (22)
P(o-Tol)3 (22)
P(o-Tol)3 (22)
none
d
8
70
e
f
9
70
n.d.
e
f
10
none
100
0
n.d.
a
Conditions: ammonium triflate 33 (0.10 mmol, 1.0 equiv), boronic
acid (1.2 equiv), Ni(cod)2 (10 mol %), ligand, K3PO4 (1.3 equiv),
b
1
dioxane (0.4 M), 4 h, unless noted otherwise. Determined by H
NMR analysis using 1,3,5-trimethoxybenzene as internal standard.
c
d
e
Determined by chiral HPLC. PhMe replaced dioxane as solvent. No
f
Ni(cod)2 used. n.d. = not determined.
However, only moderate chirality transfer was observed. A
screen of ligands revealed that ligand affects the chirality
transfer with 9,9-dimethyl-4,5-bis(di-tert-butylphosphino)-
xanthene (t-Bu-XantPhos) and tri(o-tolyl)phosphine proving
best (entries 4 and 6). Due to ligand cost, we selected P(o-
Tol)3 for further optimization and found that the reaction
proceeds in high yield and enantiospecificity at 70 °C in either
dioxane (entry 7) or PhMe (entry 8). Under these conditions,
the reaction proceeds with excellent chirality transfer and yield,
providing a powerful route to enantioenriched diarylethanes.
Notably, no product is observed in the absence of nickel
catalyst at these elevated temperatures (entries 9 and 10).16
With these conditions in hand, we examined the scope of this
transformation. Upon increasing the reaction scale, we found
that the catalyst loading could be lowered to 3 mol % without
detrimental effect on yield or enantioselectivity of diarylethane
34 (Table 3, entry 1). In some cases, lower catalyst loading
resulted in higher enantiospecificities, suggesting a possible Ni-
mediated epimerization pathway (entries 1 vs 2, 12 vs 13).
Electron-rich and electron-neutral aryl boronic acids perform
well in this reaction (entries 1−11). Further, a wide variety of
functional groups are tolerated, including ethers, olefins,
fluorides, esters, nitriles, and sulfones. In terms of electron-
poor boronic acids, we were surprised that the cross coupling of
4-fluorophenyl boronic acid and ammonium triflate 33 resulted
in low yield when K3PO4 was used as base (entry 6). However,
exceptional yield and chirality transfer were observed when CsF
was employed (entry 7). Notably, the successful formation of
ester 39 (entry 8), nitrile 40 (entry 9), and sulfone 41 (entry
10) particularly highlights the advantage of using a boronic acid
as the coupling partner. For some of these base-sensitive
functional groups, we replaced K3PO4 with CsF (entry 8).
Finally, we were pleased to observe that the reaction of vinyl
a
Conditions: ammonium triflate 1b (0.30 mmol, 1.0 equiv), boronic
acid (1.2 equiv), Ni(cod)2 (10 mol %), PPh2Cy (22 mol %), CsF or
K3PO4 (1.3 equiv), dioxane (0.33 M), 40 °C, 24 h. Average isolated
yield of duplicate experiments reported ( 0−12%), unless otherwise
b
noted. Result of a single experiment.
a
Scheme 3. Scope in Ammonium Salt
a
Conditions: ammonium triflate (0.20 mmol, 1.0 equiv), boronic acid
(1.2 equiv), Ni(cod)2 (10 mol %), PPh2Cy (22 mol %), CsF or K3PO4
(1.3 equiv), dioxane (0.33 M), 40 °C, 24 h. Average isolated yield of
duplicate experiments reported ( 0−9%), unless otherwise noted.
b
Result of a single experiment.
C
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a
precursors to α-arylpropanoic acid analgesics, such as
naproxen.19
Table 3. Formation of Enantioenriched Diarylethanes
With respect to the scope of the ammonium salt, electron-
poor aryl groups are well tolerated (entries 14−17).20 In every
case, the benzylic ammonium triflates are available in greater
than 98% ee, highlighting one of the advantages of using
benzylic amine derivatives in these stereospecific cross
couplings. However, with these substrates, higher catalyst
loadings are required. In some cases, we also observed higher
yields and greater chirality transfer when t-Bu-XantPhos was
employed as ligand (entries 14 and 15). Although we do not
yet fully understand the basis for the effect of ligand on the
chirality transfer, the benefit of higher catalyst loadings for these
substituted benzene derivatives suggests that a nickel-catalyzed
epimerization pathway is not significant for these substrates.
We established the absolute configuration of diarylethane 35,
whose racemate displays anti-cancer activity,1d,e by comparison
of its optical rotation to the reported value.5b The absolute
configuration of (R)-38 was determined by X-ray crystallog-
raphy (Figure 1).11,21 Collectively, these assignments demon-
strate that these reactions proceed with overall inversion of
configuration.
Figure 1. Crystal structure of (R)-38. Molecular diagram of (R)-38
with ellipsoids at 30% probability. Tertiary H-atom depicted with
arbitrary radius. All other H-atoms and a second symmetry-unique
compound molecule are omitted for clarity.
To begin to understand the mechanism of this trans-
formation, we studied the stoichiometric reaction of
ammonium triflate 47 with Ni(cod)2 and PPh2Cy. This
reaction produced alkylnickel(II) triflate 48 in 51% isolated
yield (Scheme 4). The structure of this complex was confirmed
1
by X-ray crystallography, as well as H, 13C, 19F, and 31P NMR
analysis.11,21 It is consistent with oxidative addition of nickel
into the benzylic C−N bond. The chelating carbonyl likely
stabilizes this complex in the η1 form. Upon treatment with p-
tolylboronic acid, complex 48 is converted to diarylmethane 29
in 95% yield (1H NMR, eq 1). Further, complex 48 is
a
Conditions: ammonium triflate (0.26 mmol, 1.0 equiv), boronic acid
(1.2 equiv), Ni(cod)2, P(o-Tol)3, K3PO4 (1.3 equiv), dioxane (0.4 M),
70 °C, 6 h unless otherwise noted. Starting materials were ≥99% ee,
b
unless otherwise noted. Average isolated yield of duplicate experi-
1
ments ( 5%). Yields in parentheses determined by H NMR analysis
c
using 1,3,5-trimethoxybenzene as internal standard. Average ee from
duplicate experiments as determined by chiral HPLC analysis ( 1%).
d
e
f
Result of a single experiment. Performed on 0.2 mmol scale. n.d. =
g
h
not determined. CsF used instead of K3PO4. Performed on 0.5
mmol scale. Performed on 0.1 mmol scale. Performed on 1.36 mmol
scale. Starting material was 98% ee.
i
j
k
catalytically competent; product 29 was formed in quantitative
yield (1H NMR) when 48 was used as catalyst (eq 2). These
results suggest that complex 48 is a viable intermediate in the
catalytic reaction. We thus propose that these reactions proceed
boronic acids also proceeds in high yield and enantiospecificity
(entry 12). Such enantioenriched allyl arenes are known
D
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Scheme 4. Synthesis and Crystal Structure of Oxidative
Addition Complex 48
Scheme 5. Proposed Reaction Mechanism
a
coupling of a benzylic electrophile that provides highly
enantioenriched products. Given the ready availability of highly
enantioenriched chiral amines, this method offers a powerful
approach to the synthesis of enantioenriched diarylethanes.
Studies to determine further mechanistic details of this cross
coupling as well as to expand the scope of this transformation
are underway in our laboratories.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data and spectra of
new compounds, and X-ray crystal structures of (R)-38 and 48.
This material is available free of charge via the Internet at
http://pubs.acs.org.
a
Molecular diagram of 48 with ellipsoids at 30% probability. H-atoms
omitted for clarity.
via oxidative addition of the electron-rich Ni(0) complex into
the C−N bond to generate either an η1- or η3-bound benzyl
nickel(II) intermediate.22 Transmetalation with the activated
boronic acid and subsequent reductive elimination then delivers
the diaryl product.
We have attempted to use a similar substrate to determine
whether the oxidative addition of nickel into the C−N bond
occurs with retention or inversion of configuration. However,
the cross coupling of branched ammonium triflate 49 occurs
with poor chirality transfer, delivering diarylethane in only 33%
ee (eq 3). This result suggests that the carbonyl group
AUTHOR INFORMATION
Corresponding Author
mpwatson@udel.edu
■
Author Contributions
†P.M. and D.M.S.-M. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Acknowledgement is made to NIH COBRE (P20 RR017716)
and the Donors of the American Chemical Society Petroleum
Research Fund for generous support of this research. We also
thank AstraZeneca for donated boronic acids. NMR and other
data were acquired at UD on instruments obtained with the
assistance of NSF and NIH funding (NSF MIR 0421224, NSF
CRIF MU CHE0840401, NIH P20 RR017716).
REFERENCES
■
promotes an epimerization pathway. Unfortunately, attempts to
isolate the oxidative addition adducts of substrates without such
chelating groups have been unsuccessful to date. Nonetheless,
retention of configuration during transmetalation and reductive
elimination is well precedented for alkyl metal species.23 Thus,
we propose that the oxidative addition of the nickel catalyst
into the benzylic C−N bond likely occurs via an SN2
mechanism, resulting in inversion of configuration of the
benzylic stereocenter (Scheme 5). This mechanistic proposal is
consistent with the overall inversion of configuration observed
in this cross coupling reaction.
In summary, we have developed mild conditions for a high-
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boronic acids to give diarylmethanes. This nickel-catalyzed
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