Enantiospecific, nickel-catalyzed cross-couplings of allylic pivalates and arylboroxines

Article abstract of DOI:10.1021/ol5016724

We have developed an enantiospecific, nickel-catalyzed cross-coupling of unsymmetric 1,3-disubstituted allylic pivalates with arylboroxines. The success of this reaction relies on the use of BnPPh₂ as a supporting ligand for the nickel(0) catalyst and NaOMe as a base. This method shows excellent functional group tolerance and broad scope in both the allylic pivalate and arylboroxine, enabling the preparation of 1,3-diaryl allylic products in high yields with excellent levels of regioselectivity and stereochemical fidelity.

Full text of DOI:10.1021/ol5016724

Letter
pubs.acs.org/OrgLett
Open Access on 06/13/2015
Enantiospecific, Nickel-Catalyzed Cross-Couplings of Allylic Pivalates
and Arylboroxines
Harathi D. Srinivas,‡ Qi Zhou,‡ and Mary P. Watson*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: We have developed an enantiospecific, nickel-catalyzed cross-
coupling of unsymmetric 1,3-disubstituted allylic pivalates with arylboroxines. The
success of this reaction relies on the use of BnPPh2 as a supporting ligand for the
nickel(0) catalyst and NaOMe as a base. This method shows excellent functional
group tolerance and broad scope in both the allylic pivalate and arylboroxine,
enabling the preparation of 1,3-diaryl allylic products in high yields with excellent
levels of regioselectivity and stereochemical fidelity.
ransition-metal-catalyzed allylic substitution provides an
and carbonates, respectively.24,25 With respect to the stereo-
chemical outcome of these reactions, they have demonstrated
that the arylation of cyclic allylic electrophiles occurs with
inversion of configuration, and Kobayashi has exploited this in
the arylation of cyclopentene diol derivatives to deliver
enantioenriched products (Scheme 1A).24a−e,25 In addition,
T
important method of C−C bond formation.1 In particular,
enantioselective or -specific arylation of readily accessible 1,3-
disubstituted secondary allylic electrophiles enables facile
construction of enantioenriched products equipped with vinyl-
substituted benzylic carbon stereocenters. These products are
useful both as synthetic intermediates and for their potential
biological activity.2 An ideal cross-coupling method to deliver
these valuable enantioenriched products would enable high
levels of enantiospecificity (or enantioselectivity) and regiose-
lectivity, wide functional group tolerance, and the use of an air-
stable coupling partner (such as an arylboronic reagent).3,4
Toward these goals, Rh- and Pd-catalyzed cross-couplings of
allylic electrophiles and arylboronic acids have been developed to
deliver highly enantioenriched products.5−10 However, the
additional goal of developing nonprecious metal catalysts is
also important in lowering the cost, ensuring adequate supplies of
catalysts for large-scale applications, and reducing the environ-
mental impact of this chemistry.11 In contrast to the methods
developed with precious metal catalysts, the use of nonprecious
metal catalysts for enantiospecific or -selective allylic arylations is
less developed and often requires use of air-sensitive aryl
nucleophiles that may have incompatibilities with some func-
tional groups. For example, formation of enantioenriched
products via Cu-catalyzed allylic arylations largely relies on the
use of Grignard, organozinc, or organoaluminum reagents;12−16
only a limited number of methods allow the use of
arylboronates.17−19
Based on our studies of enantiospecific, Ni-catalyzed couplings
of benzylic pivalates and arylboroxines,20 we envisioned that Ni-
based catalysts may also serve as efficient, nonprecious metal
catalysts for highly enantiospecific and regioselective couplings
of 1,3-disubstituted allylic pivalates with aryl boroxines. Prior art
in this area has indeed demonstrated that nickel catalysts
efficiently catalyze the cross-coupling of allylic electrophiles with
aryl Grignard reagents, with both diastereo- and enantioselective
variants reported.21−23 Trost and Kobayashi have also shown
that more functional group tolerant borate and boronic acid
nucleophiles can be used in cross-couplings with allylic amines
Scheme 1. Enantioselective and Enantiospecific Nickel-
Catalyzed Allylic Arylation with Arylboron Reagents
Uemura has reported a moderately enantioselective, Ni-catalyzed
coupling of symmetric and terminal allylic acetates with
arylboronic acids (Scheme 1B).26 However, to our knowledge,
there are no reports of enantiospecific, Ni-catalyzed cross-
couplings of acyclic allylic electrophiles and arylboron reagents to
deliver highly enantioenriched products.
Herein we report the first enantiospecific, Ni-catalyzed cross-
coupling of acyclic, 1,3-disubstituted allylic pivalates and
arylboroxines (Scheme 1C). This method enables efficient
transformation of readily available, highly enantioenriched allylic
Received: June 10, 2014
Published: June 13, 2014
© 2014 American Chemical Society
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Letter
a
pivalates to valuable 1,3-diaryl allylic products in good yields with
excellent regioselectivities and levels of stereochemical fidelity.
Our initial investigations focused on the coupling of pivalate
1a, easily prepared in 96% ee via a CBS reduction/pivalation
sequence, with phenylboroxine (Table 1). Under optimal
Scheme 2. Scope of Arylboroxine
a
Table 1. Optimization of Reaction Conditions
b
c
entry mol % [Ni]
ligand (mol %)
temp (°C) yield (%) ee (%)
d
1
10
10
10
10
10
10
10
10
5
none
none
50
50
50
50
50
50
50
rt
95
96
90
30
80
83
85
90
90
92
72
86
18
57
57
27
84
85
87
87
92
94
93
94
2
e
3
dppp (11)
e
4
XantPhos (11)
PCy3 (22)
5
6
CyPPh2 (22)
BnPPh2 (22)
BnPPh2 (22)
BnPPh2 (11)
BnPPh2 (5)
BnPPh2 (5)
BnPPh2 (5)
7
8
9
rt
10
11
12
2
rt
f
2
rt
g
2
rt
a
Conditions: pivalate 1a (96% ee, 0.1 mmol, 1.0 equiv), (PhBO)3 (1.0
equiv), Ni(cod)2, ligand, NaOMe (2.0 equiv), CH3CN (0.25 mL, 0.4
a
b
Conditions: pivalate 1a (96% ee, 0.3 mmol, 1.0 equiv), (PhBO)3 (1.0
equiv), Ni(cod)2 (2 mol %), BnPPh2 (5 mol %), NaOMe (2.0 equiv),
CH3CN (0.75 mL, 0.4 M), rt, unless otherwise noted. Average yields
( 3%) and ee’s ( 1%) of isolated products of duplicate reactions. Ee
determined by HPLC analysis using a chiral stationary phase. es = (ee
1
M), 4 h, unless otherwise noted. Determined by H NMR analysis
c
using 1,3,5-trimethoxybenzene as internal standard. Determined by
d
HPLC analysis using a chiral stationary phase. PhMe instead of
e
CH3CN. dppp = 1,3-bis(diphenylphosphino)propane. XantPhos =
f
4,5-bis(diphenylphosphino)-9,9-dimethylxanthene. PhB(OH)2 (3.0
b
g
of product)/(ee of starting material). 1a (95% ee).
equiv). H2O (1.5 equiv) added.
either electron-donating or -withdrawing groups (2−13). A wide
range of functional groups were tolerated including ether (3),
thioether (4), vinyl (5), halide (6−8), trifluoromethyl (9), ester
(10), ketone (11), nitrile (12), and acetal (13) groups. In
particular, the vinyl, bromide, chloride, and thioether groups
highlight the mildness of these reaction conditions; undesired
Ni-catalyzed reactions of these groups do not occur under our
conditions. Notably, the Ni(0) catalyst generated in situ from
NiBr2, BnPPh2, and Zn was also effective in this arylation,
providing product 2 in the same ee as with Ni(cod)2, albeit in
somewhat reduced yield (eq 1). These conditions enable a
benchtop setup of the arylation reaction.
conditions for the coupling of benzylic pivalates [Ni(cod)2, no
additional phosphine ligand, PhMe], desired coupling product 2
was obtained in excellent yield. However, only 18% ee of product
was observed (entry 1). Changing the solvent to MeCN resulted
in higher enantiomeric excess (ee) of the product (57% ee) in
similar yield (entry 2). In an effort to improve the stereochemical
fidelity further, we investigated phosphine ligands to support the
Ni catalyst. Although bidentate phosphine ligands led to poor
ee’s of product (entries 3, 4), the use of monodentate phosphine
ligands resulted in significant improvement (entries 5−7).
Among the ligands examined, benzyl diphenyl phosphine
(BnPPh2) proved to be the best (entry 7). Reducing the reaction
temperature gave an increased yield (entry 8). Further increases
in yield and ee were obtained when the catalyst loading was
lowered to 2 mol % (entries 9, 10). The observation of higher
levels of stereochemical fidelity at lower catalyst loadings may
suggest a Ni-mediated epimerization pathway.27 Under these
optimized conditions, the use of phenylboronic acid in place of
phenylboroxine led to a lower yield but with the same ee (entry
11). The reason for this difference is currently unclear, but does
not seem to be due to the reaction’s sensitivity to water. Addition
of water to the reaction resulted in only a slightly reduced yield
(entry 12).
With respect to the allylic pivalate, pivalates with both
electron-rich and -deficient aryl substituents efficiently under-
went arylation (Scheme 3, 14−20). As had been true of the
arylboroxine, aryl chlorides are also well tolerated on the allylic
pivalate, again highlighting the orthogonality of this reaction to
aryl chlorides (17−19). In addition to the functional groups
already demonstrated on the arylboroxine, a tertiary amine was
well tolerated (18). Allylic pivalates with naphthyl and heteroaryl
substituents were also successfully coupled (21, 22). In terms of
the alkyl substituent (R) on the allylic pivalate, increased steric
hindrance did not diminish reactivity, but lower levels of
Under our optimized conditions (Table 1, entry 10), we
observed a broad scope with respect to the arylboroxine (Scheme
2).28 In all cases in Schemes 2 and 3, excellent regioselectivity and
E/Z selectivity were observed; only α-aryl products with E-
olefins were formed. High yields and excellent levels of
stereochemical fidelity were obtained with arylboroxines bearing
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Letter
a
Consistent with previous Ni-catalyzed allylations, we propose
that this reaction proceeds through a π-allylnickel intermedi-
ate.21−26,31 To test this hypothesis, we subjected pivalate 27
(96% ee), a regioisomer of 1a, to the reaction conditions.
Product (R)-2 was formed in 81% yield and 83% ee with
inversion of the absolute stereochemistry (Scheme 5). The
Scheme 3. Scope of Pivalates
Scheme 5. Regioselectivity Studies
reason for the mild erosion of ee is unclear, but may be due to
epimerization of a faster formed π-allylnickel intermediate. The
observation that both regioisomers of starting material (1a and
27) lead to the same product (2) supports the intermediacy of an
η3-allylnickel complex. Also consistent with an η3-allylnickel
intermediate, the arylation of 1,3-diaryl-substituted 28 resulted in
a 1:1.5 ratio of regioisomers 29 and 30 (Scheme 5). Despite the
modest regioselectivity, it is notable that 30 was obtained with
91% es.
In conclusion, we have developed an enantiospecific, Ni-
catalyzed cross-coupling of enantioenriched secondary allylic
pivalates and arylboroxines to deliver 1,3-diaryl allyl products.
The method features mild conditions that allow broad functional
group tolerance on both the allylic pivalate and arylboroxine. In
all cases, good yields and excellent levels of regioselectivity, E/Z
selectivity, and stereochemical fidelity were observed. In
addition, air-stable, less expensive NiBr2 can be used as an
alternative to Ni(cod)2. Studies exploring other substrate classes
as well as details of the mechanism are currently ongoing and will
be reported in due course.
a
Conditions: pivalates 1b−1i (0.3 mmol, 1.0 equiv), (PhBO)3 (1.0
equiv), Ni(cod)2 (2 mol %), BnPPh2 (5 mol %), NaOMe (2.0 equiv),
CH3CN (0.75 mL, 0.4 M), rt, unless otherwise noted. Average yields
( 3%) and ee’s ( 1%) of isolated products of duplicate reactions. Ee
determined by HPLC analysis using a chiral stationary phase. es = (ee
of product)/(ee of starting material); corresponding starting materials
b
and their ee’s in parentheses. Single experiment.
stereochemical fidelity were observed with cyclohexyl-substi-
tuted 1i (23, 24). The X-ray crystallographic analysis of product
17 shows that its absolute configuration is S,29 which indicates
that the reaction proceeds with inversion of configuration. To
confirm further the stereochemical outcome and demonstrate
the utility of this arylation method, compound (R)-12, prepared
in 95% ee via the arylation of (S)-1a, was converted to (S)-
ketoprofen (26), an anti-inflammatory drug, via a two-step
manipulation (Scheme 4). Comparison of the optical rotation of
26 to previously reported values confirms its absolute
configuration is S.30 The absolute configurations of other
products were assigned by analogy.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental data, details on methods and starting materials,
and copies of spectral data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: mpwatson@udel.edu.
Author Contributions
Scheme 4. Synthesis of (S)-Ketoprofen
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
NSF (CAREER CHE 1151364) is gratefully acknowledged. We
thank AstraZeneca for donated boronic acids. Data were
acquired at UD on instruments obtained with the assistance of
NSF and NIH funding (NSF CHE0421224, CHE 1229234, and
CHE0840401; NIH P20 GM103541 and S10 RR02692).
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Letter
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