Nickel-catalyzed cross-couplings of benzylic pivalates with arylboroxines: Stereospecific formation of diarylalkanes and triarylmethanes

Article abstract of DOI:10.1021/ja312087x

We have developed a stereospecific nickel-catalyzed cross-coupling of benzylic pivalates with arylboroxines. The success of this reaction relies on the use of Ni(cod)₂ as the catalyst and NaOMe as a uniquely effective base. This reaction has broad scope with respect to the arylboroxine and benzylic pivalate, enabling the synthesis of a variety of diarylalkanes and triarylmethanes in good to excellent yields and ee's.

Full text of DOI:10.1021/ja312087x

Communication
pubs.acs.org/JACS
Nickel-Catalyzed Cross-Couplings of Benzylic Pivalates with
Arylboroxines: Stereospecific Formation of Diarylalkanes and
Triarylmethanes
Qi Zhou, Harathi D. Srinivas, Srimoyee Dasgupta, and Mary P. Watson*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
Scheme 1. Stereospecific Coupling of Benzylic Pivalates with
Arylboroxines
ABSTRACT: We have developed a stereospecific nickel-
catalyzed cross-coupling of benzylic pivalates with
arylboroxines. The success of this reaction relies on the
use of Ni(cod)2 as the catalyst and NaOMe as a uniquely
effective base. This reaction has broad scope with respect
to the arylboroxine and benzylic pivalate, enabling the
synthesis of a variety of diarylalkanes and triarylmethanes
in good to excellent yields and ee’s.
nickel(0) catalyst. This cross-coupling leads to a wide variety of
diarylalkanes and triarylmethanes.
We selected the cross-coupling of pivalate 1a, which is readily
prepared with >99% ee,14 as our model reaction for
optimization. Given the precedent for activation of benzylic
C−O bonds5e,f,6 and our prior experience in Suzuki cross-
couplings of benzylic electrophiles,8a we anticipated that a
Ni(0) catalyst supported by an electron-rich phosphine ligand
would enable this transformation. However, with Ni(cod)2/
PCy3 (cod = 1,5-cyclooctadiene, Cy = cyclohexyl) as the
catalyst system and phenylboronic acid as the coupling partner,
we observed low yields of the desired product 2 using a variety
of bases commonly employed in Suzuki cross-couplings (Table
1, entries 1 and 2). β-Hydride elimination was also observed
under these reaction conditions. However, when NaOMe was
employed as the base, 2 was formed in 78% yield (entry 3).
Further optimization showed that the use of PCy2Ph as the
ligand resulted in 93% yield (entry 4), but 2 was formed with
only 54% ee under these conditions. In contrast, much higher
chirality transfer was obtained without a detrimental effect on
the yield when Ni(cod)2 alone was used as the catalyst (93%
ee; entry 5). Notably, the absolute configuration of the major
enantiomer was the opposite when the phosphine ligand was
not employed. When a greater amount of boronic acid and
lower reaction temperature (70 °C) were used, a greater extent
of chirality transfer was observed without a significant drop in
yield (entry 6). The nature of the counterion of the methoxide
base had a significant impact on the reaction outcome. With
KOMe, the product was formed with lower ee (entry 7), and
no reaction occurred with LiOMe (entry 8). These results
suggest that the solubility of the base is important; with more
base in solution (as occurs with KOMe), a lower product ee
was observed, but LiOMe may not have been soluble enough in
PhMe to promote the reaction. This hypothesis is consistent
with our solvent studies: with more polar solvents, lower
product ee’s were observed.14 By increasing the concentration
of the reaction mixture, we obtained an 87% yield of 2 with
iarylalkanes and triarylmethanes are important molecules
D
in organic synthesis and pharmaceutical development.1 A
highly efficient and direct route to these valuable chiral
compounds is the metal-catalyzed cross-coupling of a benzylic
electrophile with an organometallic reagent.2 Such couplings
have been accomplished in both enantioselective and
enantiospecific fashion.3−5 Of particular note, the Jarvo group
has demonstrated that nickel-catalyzed cross-couplings of
enantioenriched benzylic ethers with Grignard reagents occur
with excellent levels of chirality transfer.6 Jarvo’s work
highlights the convenience of using readily available enantioen-
riched benzylic alcohol derivatives as starting materials.
However, a limitation to the state of the art in this field is
the requirement for highly nucleophilic coupling partners
(Grignard or organozinc reagents), which restricts the range of
functional groups that are compatible with these reactions. To
our knowledge, no enantioselective couplings of benzylic
electrophiles with the more mild organoboranes are yet
known, and stereospecific couplings with boronic reagents to
deliver these products are rare.7−11 The ability to employ
organoboranes as coupling partners would lead to greatly
expanded scope and functional group tolerance within this
important class of reactions.
Recognizing the wide accessibility of enantioenriched
benzylic alcohol derivatives and the advantage of using mild
and functional-group-tolerant organoborane coupling partners,
we sought to develop a stereospecific cross-coupling of these
reagents. Within our research program focused around the
development of cross-coupling reactions of nontraditional
electrophiles, we have been drawn to the use of pivalate
substrates for aryl C−O bond activation.12,13 We envisioned
that the use of benzylic pivalates would enable the desired
nickel-catalyzed coupling of enantioenriched benzylic alcohol
derivatives with organoborane coupling partners (Scheme 1).
Herein we report the stereospecific, high-yield cross-coupling of
benzylic pivalates with arylboroxines in the presence of a simple
Received: December 11, 2012
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
a
a
Table 1. Optimization of the Cross-Coupling Reaction
Scheme 2. Scope of Boroxines
mol %
Ni
T
yield
b
ee
c
entry
PhBX2 (equiv)
base
CsF
(°C)
(%)
(%)
d
e
1
10
10
10
10
10
10
10
10
5
PhB(OH)2
(3.0)
100
100
100
100
100
70
14
14
78
93
99
83
74
0
n.d.
d
e
e
2
PhB(OH)2
(3.0)
K3PO4
n.d.
n.d.
d
3
PhB(OH)2
(3.0)
NaOMe
NaOMe
NaOMe
NaOMe
KOMe
f
4
PhB(OH)2
(2.0)
54 (R)
93 (S)
99 (S)
69 (S)
5
6
7
8
PhB(OH)2
(2.0)
PhB(OH)2
(3.0)
PhB(OH)2
(3.0)
70
e
PhB(OH)2
(3.0)
LiOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
70
n.d.
g
a
9
PhB(OH)2
(2.5)
70
87
98
59
0
98 (S)
97 (S)
94 (S)
Conditions: pivalate 1a (0.20 mmol, 1.0 equiv), boroxine (0.83
equiv), Ni(cod)2 (5 mol %), and NaOMe (2.0 equiv) in PhMe (0.4
M) at 70 °C for 3 h, unless otherwise noted. Average isolated yields of
duplicate experiments ( 0−2%) and average ee’s of duplicate
experiments ( 0−1%) as determined by chiral HPLC are reported.
g
10
5
(PhBO)3
(0.83)
70
gh
,
11
12
13
5
(PhBO)3
(0.83)
70
b
c
g
g
e
10 mol % Ni(cod)2, 90 °C, 12 h. 40 °C, 24 h.
0
PhB(OH)2
(2.5)
70
n.d.
e
0
(PhBO)3
(0.83)
70
0
n.d.
mildness of these reaction conditions. Increased steric
hindrance on the aryl fragment was well-tolerated, as
demonstrated by the formation of product 6 in 94% yield
with 96% ee. By comparison of product 5 to an authentic
sample recently prepared in our laboratory,8a we confirmed the
absolute configuration of this product, demonstrating that this
reaction occurs with overall inversion of configuration at the
benzylic stereocenter. The absolute configurations of the other
products were assigned by analogy.
a
Conditions: pivalate 1a (0.10 mmol, 1.0 equiv), PhBX2, Ni(cod)2,
and base (2.0 equiv) in PhMe (0.2 M), unless otherwise noted.
b
1
Determined by H NMR analysis using 1,3,5-trimethoxybenzene as
c
an internal standard. Determined by chiral HPLC. The absolute
configurations of the major enantiomers are shown in parentheses.
d
e
Performed with racemic 1a. PCy3 (24 mol %) was added. n.d. = not
f
g
h
determined. PCy2Ph (22 mol %) was added. 0.4 M. 1.0 equiv of
H2O was added.
Variation of the pivalate partner led to a variety of
diarylalkane and triarylmethane products in good to excellent
yields and enantiospecificities (Table 2). In the formation of
diarylalkanes, increased steric bulk of the alkyl substituent (R)
was well-tolerated, giving excellent levels of chirality transfer in
the cross-coupling reaction (entries 1−4). Notably, even
pivalate 1c with a bulky isobutyl substituent underwent the
reaction (entry 4). The coupling of diaryl-substituted pivalate
1d also proceeded in excellent yields to deliver triarylmethanes
17 and 18 with good levels of chirality transfer (entries 5 and
6). A variety of functional groups on the naphthyl fragment
were well-tolerated. The coupling of 6-methoxynaphthyl-
substituted pivalate 1e gave diarylethane 19 in 87% yield with
93% ee (entry 7). In particular, the successful couplings of
ester- and nitrile-substituted pivalates 1f and 1g highlight the
increased functional group tolerance enabled by the use of an
arylboroxine coupling partner (entries 8 and 9). The couplings
of benzylic pivalates with aryl groups other than 2-naphthyl
were also successful. The reaction of 1-naphthyl-substituted
pivalate 1h gave diarylethane 22 in 73% yield with 85% es
(entry 10). We hypothesize that the slightly lower level of
chirality transfer in this case may be due to the increased steric
hindrance arising from ortho substitution on the aromatic
group. Both electron-rich and electron-poor heteroaromatics
underwent the desired coupling (entries 11 and 12). Finally, we
excellent chirality transfer using PhB(OH)2 and only 5 mol %
Ni(cod)2 (entry 9). Nearly quantitative yield was observed
when phenylboroxine was employed as the coupling partner
under these conditions (entry 10). Although the difference in
yield was not particularly significant for this model reaction, as
we examined other arylboronic reagents, we generally observed
better yields and higher levels of chirality transfer with
boroxines, indicating that water likely has a detrimental effect
on this reaction. Indeed, the addition of H2O (1.0 equiv) to the
reaction mixture resulted in diminished yield and chirality
transfer (entry 11).14 Finally, we performed control reactions in
the absence of Ni(cod)2 and found that no cross-coupling
occurred with either phenylboronic acid or phenylboroxine
(entries 12 and 13). In these reactions, pivalate 1a was
recovered quantitatively with >99% ee.
Under our optimized conditions (Table 1, entry 10), we
observed broad scope with respect to the arylboroxine (Scheme
2).15 High yields and excellent chirality transfer were observed
with both electron-rich and electron-poor aryl groups.
Furthermore, a wide range of functional groups were tolerated,
including amino (3), ether (4), fluoro (7, 12), chloro (8),
trifluoromethyl (9), and acetal (10) functionalities. The
formation of chloride 8 is particularly impressive and highlights
the advantage of using a boronic coupling partner and the
B
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Journal of the American Chemical Society
Communication
a
were pleased to observe that the cross-coupling of biphenyl-
substituted pivalate proceeded with 90% es (entry 13). The
diminished yield in this case clearly demonstrates the
importance of the aryl group in the oxidative addition of
nickel to the pivalate.
Table 2. Scope of Pivalates
We hypothesize that this reaction proceeds via oxidative
addition of the nickel catalyst to the benzylic C−O bond, with
subsequent transmetalation and reductive elimination deliver-
ing the C−C bond of the arylated product. In an effort to
understand the stereochemical outcome of the oxidative
addition, we attempted a series of β-hydride elimination
experiments using deuterated pivalate 1l (Scheme 3).16 If the
Scheme 3. β-Hydride Elimination Experiment
oxidative addition occurs with retention of configuration at the
benzylic carbon, then products 26 and/or 27 will result.
However, if inversion of configuration occurs upon oxidative
addition, then products 28 and/or 29 will be observed. We first
attempted to induce β-hydride elimination using stoichiometric
Ni(cod)2. However, we observed no reaction, even at reaction
temperatures of up to 130 °C. In contrast, when a phosphine
ligand was added, facile β-hydride elimination was observed.
With PCy3, an 80% yield of styrene 26 and a 5% yield of
styrene 28 were observed after only 15 min at 70 °C (Scheme
3). These results are consistent with the conclusion that the
predominant pathway for oxidative addition proceeds with
retention of configuration at the benzylic carbon when a
phosphine-supported nickel catalyst is used. Although it seems
somewhat surprising that the C−D bond was selectively
cleaved to form 26, this result is likely due to the significant
steric hindrance between the naphthyl and methyl groups in the
transition state leading to product 27. As shown in entry 4 of
Table 1 and in the Supporting Information, the cross-coupling
occurs with overall retention of configuration with phosphine-
supported nickel catalysts, suggesting that the transmetalation
and reductive elimination also occur with retention of
configuration. Such retention of configuration during trans-
metalation and reductive elimination is well-precedented with
similar organometallic intermediates.16,17 However, retention of
configuration upon oxidative addition of a benzylic electrophile
is rare and suggests that the oxidative addition may be more
complicated with this catalyst system. In contrast, when only
Ni(cod)2 was used without any phosphine ligand, we observed
overall inversion of configuration at the benzylic carbon,
suggesting that the oxidative addition step likely occurs with
inversion of configuration via an SN2 pathway under these
conditions.
a
Conditions: pivalate (0.20 mmol, 1.0 equiv), boroxine (0.83−1.3
equiv), Ni(cod)2 (5 mol %), and NaOMe (2.0 equiv) in PhMe (0.4
M) at 70 °C for 3 h, unless otherwise noted. Determined by chiral
b
In summary, we have developed a stereospecific nickel-
catalyzed cross-coupling of enantioenriched benzylic pivalates
with arylboroxines. The successful development of this reaction
depended on the identification of Ni(cod)2 as the optimal
catalyst and NaOMe as a uniquely effective base. This reaction
has broad scope with respect to the arylboroxine and benzylic
c
HPLC. Average isolated yields of duplicate experiments ( 0−2%),
d
unless otherwise noted. Average ee’s of duplicate experiments ( 0−
e
1%), as determined by chiral HPLC, unless otherwise noted. % es =
enantiospecificity = [(ee of product)/(ee of starting material)] ×
100%. 10 mol % Ni(cod)2. 80 °C. 12 h. 90 °C. Results of a single
experiment. 50 °C. 32 °C. 24 h. 100 °C.
f
g
h
i
j
k
l
m
n
C
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Journal of the American Chemical Society
Communication
(4) For an enantiospecific coupling of benzylic bromides with
pivalate, enabling the synthesis of a variety of diarylalkanes and
triarylmethanes with good to excellent levels of chirality
transfer. Our current efforts are directed toward expanding
the versatility and scope of this cross-coupling reaction as well
as deepening our mechanistic understanding to enable rational
improvements in the catalyst system.
Grignard reagents, see: Lop
Lett. 2009, 11, 5514.
́ ́
ez-Perez, A.; Adrio, J.; Carretero, J. C. Org.
(5) For examples of benzylic cross-couplings to give achiral or
racemic products, see: (a) Molander, G. A.; Elia, M. D. J. Org. Chem.
2006, 71, 9198. (b) Kuwano, R. J. Synth. Org. Chem., Jpn. 2011, 69,
1263. (c) Kofink, C. C.; Knochel, P. Org. Lett. 2006, 8, 4121.
(d) McLaughlin, M. Org. Lett. 2005, 7, 4875. (e) Guan, B.-T.; Xiang,
S.-K.; Wang, B.-Q.; Sun, Z.-P.; Wang, Y.; Zhao, K.-Q.; Shi, Z.-J. J. Am.
Chem. Soc. 2008, 130, 3268. (f) Yu, D.-G.; Wang, X.; Zhu, R.-Y.; Luo,
S.; Zhang, X.-B.; Wang, B.-Q.; Wang, L.; Shi, Z.-J. J. Am. Chem. Soc.
2012, 134, 14639.
(6) (a) Greene, M. A.; Yonova, I. M.; Williams, F. J.; Jarvo, E. R. Org.
Lett. 2012, 14, 4293. (b) Taylor, B. L. H.; Harris, M. R.; Jarvo, E. R.
Angew. Chem., Int. Ed. 2012, 51, 7790. (c) Taylor, B. L. H.; Swift, E. C.;
Waetzig, J. D.; Jarvo, E. R. J. Am. Chem. Soc. 2011, 133, 389.
(7) For a stereospecific coupling of an α-cyanohydrin mesylate with a
boronic acid, see: He, A.; Falck, J. R. J. Am. Chem. Soc. 2010, 132,
2524.
(8) (a) For our recently developed stereospecific cross-coupling of
benzylic ammonium triflates with boronic acids, see: Maity, P.;
Shacklady-McAtee, D. M.; Yap, G. P. A.; Sirianni, E. R.; Watson, M. P.
J. Am. Chem. Soc. 2013, 135, 280. (b) Tian has reported the copper-
catalyzed coupling of benzylic sulfonimides with Grignard reagents.
See: Li, M.-B.; Tang, X.-L.; Tian, S.-K. Adv. Synth. Catal. 2011, 353,
1980.
(9) For examples of a distinct approach, stereospecific coupling of a
benzylic boronic reagent with an aryl halide, see: (a) Imao, D.;
Glasspoole, B. W.; Laberge, V. S.; Crudden, C. M. J. Am. Chem. Soc.
2009, 131, 5024. (b) Ohmura, T.; Awano, T.; Suginome, M. J. Am.
Chem. Soc. 2010, 132, 13191. (c) Awano, T.; Ohmura, T.; Suginome,
M. J. Am. Chem. Soc. 2011, 133, 20738.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data, and spectra of
new compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
mpwatson@udel.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge Prof. E. R. Jarvo for informing us of
her work in this area prior to publication. We also thank D. M.
Shacklady-McAtee for insightful discussions. Research reported
in this publication was supported by NSF (CHE 1151364) and
an Institutional Development Award (IDeA) from the National
Institute of General Medical Sciences of the NIH
(P20GM103541). NMR and other data were acquired at UD
on instruments obtained with the assistance of NSF and NIH
funding (NSF MIR 0421224, NIH P20 RR017716).
(10) For examples of stereospecific cross-couplings of alkylboronic
reagents with aryl halides to form benzylic stereocenters, see:
́
(a) Sandrock, D. L.; Jean-Gerard, L.; Chen, C.-y.; Dreher, S. D.;
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