Nickel-Catalyzed Enantioselective Cross-Coupling of N-Hydroxyphthalimide Esters with Vinyl Bromides

Article abstract of DOI:10.1021/acs.orglett.7b00793

An enantioselective Ni-catalyzed cross-coupling of N-hydroxyphthalimide esters with vinyl bromides is reported. The reaction proceeds under mild conditions and uses tetrakis(N,N-dimethylamino)ethylene as a terminal organic reductant. Good functional group tolerance is demonstrated, with over 20 examples of reactions that proceed with >90% ee.

Full text of DOI:10.1021/acs.orglett.7b00793

Letter
pubs.acs.org/OrgLett
Nickel-Catalyzed Enantioselective Cross-Coupling of
N‑Hydroxyphthalimide Esters with Vinyl Bromides
Naoyuki Suzuki,‡ Julie L. Hofstra,‡ Kelsey E. Poremba, and Sarah E. Reisman*
The Warren and Katharine Schlinger Laboratory for Chemistry and Chemical Engineering, Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: An enantioselective Ni-catalyzed cross-coupling
of N-hydroxyphthalimide esters with vinyl bromides is reported.
The reaction proceeds under mild conditions and uses
tetrakis(N,N-dimethylamino)ethylene as a terminal organic
reductant. Good functional group tolerance is demonstrated,
with over 20 examples of reactions that proceed with >90% ee.
As part of our efforts to develop asymmetric cross-coupling
reactionsthatemployavarietyofC(sp3)electrophiles, webecame
interested in the coupling of redox-active N-hydroxyphthalimide
(NHP) esters.10 These electrophiles are prepared from the
corresponding carboxylic acids and have been recently demon-
strated as C(sp3) substrates for Ni-catalyzed Negishi,11a,b
Suzuki,11c and reductive12 cross-coupling reactions to generate
racemic products (Scheme 1).13,14 However, NHP esters have
not been demonstrated as competent coupling partners in Ni-
catalyzed enantioselective cross-coupling reactions. We recognized
thattheuseofNHPestersmightbeadvantageousforsubstratesin
which the corresponding alkyl chlorides are unstable or
challenging to prepare. In this letter, we report the first Ni-
ickel catalyzed cross-coupling reactions have emerged as
3
2
3
Npowerful methods to form C(sp )−C(sp ) and C(sp )−
C(sp3) bonds.1 Whereas pioneering investigations focused on the
canonical cross-coupling of C(sp3) electrophiles with organo-
metallic reagentsvariants of the venerable Negishi,2 Kumada,3
and Suzuki4 reactions, among othersadditional modes of alkyl
cross-coupling using nickel catalysis have recently been disclosed.
Theseincludecross-electrophile “reductive”couplingsthatusean
exogenous, stoichiometric reductant to shuttle electrons to the
nickel catalyst,5 as well as cross-coupling reactions that rely on
synergistic reactivity between nickel and photoredox cocatalysts.6
Taken together, these reactions enable the cross-coupling of a
broad range of C(sp3) substrates, providing access to a variety of
products.
Scheme 1. Ni-Catalyzed Reactions of NHP Esters
Ni-catalyzed reductive cross-coupling reactions have proven
particularly useful for the cross-coupling of secondary alkyl
electrophiles, often providing chiral products as racemic
mixtures.5 Recognizing that the ability to render these trans-
formations enantioselective would enhance their utility,7 our
laboratory has recently developed enantioselective Ni-catalyzed
reductivecross-coupling reactionsof bothbenzylicchlorides8 and
α-chloronitriles.9 An important objective for further improving
the synthetic usefulness of asymmetric reductive cross-coupling
reactionsistodevelopreactionsofnewelectrophileclasses. Justas
in conventional cross-coupling reactions, where different organo-
metallic reagents (e.g., organozinc, organomagnesium, organo-
boron reagents, etc.) bring unique advantages to a specific
synthetic scenario, the ability to cross-couple new electrophile
classes broadens the tool box for strategic C−C bond formation.
However, it can be challenging to apply conditions from
previously developed reductive cross-coupling reactions to new
electrophile classes, especially if there are changes to the
mechanism by which the coupling partner undergoes oxidative
addition; in particular, it can require tuning of either the ligand
structure or the stoichiometric reductant (or both) in order to
develop reactions that proceed in both good yield and
enantioselectivity.
Received: March 16, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00793
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Organic Letters
Letter
catalyzed asymmetric cross-coupling reactions of NHP esters.
These reactions proceed under mild conditions using tetrakis-
(N,N-dimethylamino)ethylene (TDAE) as a homogeneous,
stoichiometric reductant and exhibit high functional group
tolerance.
(Figure 1). The reaction exhibits good tolerance of Lewis basic
functional groups: for example, anilines (3c), nitriles (3e), and
Our efforts began with ester 2, which is prepared in one step
from commercially available 2-phenylpropanoic acid. Subjection
of 2 to our optimal conditions developed for the reductive cross-
coupling of vinyl bromides and benzyl chlorides provided only
trace quantities of product (Table 1, entry 1),8b highlighting the
a
Table 1. Reaction Optimization
b
c
entry
X
reductant additive temp (°C) yield (%) ee (%)
1
2
3
4
5
6
Cl (4a)
Cl
Mn
none
0
trace
27
−
Mn
TMSCl
TMSCl
TMSCl
TMSBr
TMSBr
TMSBr
none
0
90
64
95
92
93
96
88
Cl
Zn
0
38
Cl
TDAE
TDAE
TDAE
TDAE
TDAE
0
61
Br (4b)
Br
0
75
−7
−7
−7
79
d
e
7
Br
80
d
8
Br
19
b
a
Reactions conducted on 0.2 mmol scale under N2. Determined by
Figure 1. Scope of the vinyl bromide coupling partner. Reactions are
conducted on 0.2 mmol scale under N2. Isolated yields are provided; ee is
determined by SFC using a chiral stationary phase. For 3g, 1.5 equiv of
NHPester wereused. For3m, 2.0 equivofTMSBrwereused;thealcohol
is silylated under the reaction conditions. NMR yield of 3m versus an
internal standard is provided in parentheses.
c
1H NMR versus an internal standard. Determined by SFC using a
d
e
chiral stationary phase. 1.5 equiv of TDAE. Isolated yield.
esters (3f, 3j) can be incorporated into the substrate without
detriment totheyield orenantioselectivity. Apyridine-containing
vinyl bromide also performs well, providing 3i in 67% yield and
95% ee. In addition, alkyl-substituted vinyl bromides react
smoothly, providing the corresponding products in good yield
and ee(3j−3o). Avinylbromide possessingafree alcoholcouples
efficiently, although silylation occurs under the reaction
conditions to give silyl ether 3m. The silyl ether can easily be
cleaved with a mild acid workup; in this case it was preserved in
order to facilitate purification. It is notable that alkenyl MIDA-
boronate 3n and alkenyl silane 3o can be prepared in 97% ee from
commercially available vinyl bromides. To demonstrate that this
method can be used preparatively, the coupling between 1a and 2
was conducted on 5.0 mmol scale, which delivered 918 mg (77%
yield) of 3a in 91% ee.
The reaction also exhibits a broad scope for the NHP ester
coupling partner, delivering good yields and high enantioselec-
tivities for a range of substrates bearing substitution on the arene
or at the benzylic position (Figure 2). In certain cases (e.g., 7a−
7c), the NHP esters cross-couple with improved yield relative to
the corresponding benzyl chlorides (under the previously
reported conditions).8b Moreover, aniline 7e can be prepared in
66% yield and 94% ee; this compound could not be accessed via
our previously reported benzylic chloride coupling8b due to
challenges in preparing and handling 4-(chloro(phenyl)methyl)-
N,N-dimethylaniline under standard conditions.21 Higher
substitution at the benzylic position is also tolerated (7g−l),
challenges presented by this new C(sp3)-electrophile. Slightly
improved results could be obtained under the same conditions by
adding 1.0 equiv of TMSCl (entry 2). An investigation of
alternative reductants revealed that TDAE substantially increased
the reactivity, delivering 3a in 61% yield and 95% ee (entries 2−
4).15
Monitoring the reaction progress at room temperature
determined that (E)-1-(2-chlorovinyl)-4-methoxybenzene (5)
was forming and accumulating under the reaction conditions,
presumably through a Ni-catalyzed halide exchange process.16
Since vinyl chloride 5 does not readily engage in the cross-
coupling reaction, we hypothesized that the yield of 3a could be
improved by removing chloride from the reaction and thus
preventing formation of this unproductive side product. Indeed,
the use of TMSBr instead of TMSCl, and the use of L·NiBr2 (4b)
as the catalyst, furnished product 3a in 75% yield and 92% ee
(entry 5). By decreasing the reaction temperature to −7 °C, and
using only 1.5 equiv of TDAE, 3a could be obtained in 80% yield
and96%ee(entry7).17,18 Runningthereactionundertheoptimal
conditions, but omitting TMSBr, confirms that this additive is
critical for obtaining high yields of 3a (entry 8).19,20 We note that
these optimal reaction conditions employ a 1:1 stoichiometry of
the two electrophiles and only a 10 mol % catalyst loading.
To demonstrate the scope of the reaction, a series of (E)-vinyl
bromides were cross-coupled with NHP ester 2, providing the
corresponding products (3) in uniformly good yield and high ee
B
DOI: 10.1021/acs.orglett.7b00793
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Letter
ester 8 proceeds smoothly, furnishing allylic ether 9 in good yield
and ee. This highlights an advantage of the NHP ester for certain
C(sp3) electrophiles, as the corresponding α-chloroether
substrate is unstable and difficult to work with.
To probe for the intermediacy of a radical species, NHP ester
10 was prepared and subjected to the standard cross-coupling
conditions (Scheme 2b, 10 mol % 4b). A 42% combined yield of
the coupled products 11a−c was obtained. It has been shown that
for phenyl substituted cyclopropylcarbinyl radicals, the ring
opening is reversible and that the cyclopropane species is favored
at lower temperatures.22 The fact that 11b and 11c predominate,
even though they derive from the minor equilibrium species,
perhaps indicates that the rate of radical recombination with
nickel is sensitive to the steric profile of the radical. When the
catalyst loading of 4b was varied, the ratio of 11a to total ring
opened product (11b + 11c) was found to increase at higher
nickel concentrations. This Ni-dependent behavior suggests that
themechanismproceedsthroughacage-escapedradical, which, at
higher concentrations of 4b, can competitively recombine with
nickel before undergoing ring scission.23 Further studies of the
mechanism are ongoing; it is unclear at this time whether the
absolute stereochemistry is set during the oxidative addition or
reductive elimination steps.24 We do note, however, that the
products are formed in similar ee when using either the NHP
esters under the conditions reported here or the benzylic chloride
using the conditions reported previously,8b which suggests that
the reactions proceed through the same stereochemistry-
determining step.25
In summary, these results demonstrate that Ni-catalyzed
reductive cross-coupling reactions of NHP esters can be rendered
highly enantioselective, thus broadening the scope of C(sp3)
electrophiles available for asymmetric C−C bond formation. In
contrast to the related reductive cross-couplings of benzyl
chlorides,8b optimal results are obtained when TDAE is used as
the terminal reductant. A preliminary result demonstrates that
these conditions canbe used tocross-couple α-alkoxy NHP esters
and other substrates for which the corresponding benzylic
chloridecouldbedifficulttoprepareorunstable. Theabilitytouse
both NHP esters (this study) and benzylic chlorides in
asymmetric reductive alkenylation reactions allows users to select
from either electrophile depending on factors such as commercial
availability of the corresponding carboxylic acid or benzylic
chloride starting material, and improves the overall scope of this
transformation. The further development of asymmetric cross-
electrophile coupling reactions of NHP esters and other C(sp3)
electrophiles is the focus of ongoing work in our laboratory.
Figure 2. Scope of the NHP ester coupling partner. Reactions are
conducted on 0.2 mmol scale under N2. Isolated yields are provided; ee is
determined by SFC using a chiral stationary phase.
although the yield begins to decrease with larger groups (e.g., iPr,
7i). Notable products include 7j, which incorporates a siloxy
group, 7k, containing an alkene, and 7l, which has an alkyl
chloride. Perfect chemoselectivity for cross-coupling of the NHP
ester over the primary alkyl chloride is observed.
Although the primary focus of this study was the cross-coupling
of NHP esters with alkyl substituents at the benzylic position, we
have also have begun to investigate substrates containing
heteroatom substitution (Scheme 2a). Reaction of α-methoxy
Scheme 2. Additional Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00793.
Experimental procedures, characterization, and spectral
data (PDF)
Crystallograhic data for 4b (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: reisman@caltech.edu.
ORCID
Kelsey E. Poremba: 0000-0002-7446-257X
Sarah E. Reisman: 0000-0001-8244-9300
C
DOI: 10.1021/acs.orglett.7b00793
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Author Contributions
‡N.S. and J.L.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
WethankthefollowingCaltechstafffortheirhelp:Dr. ScottVirgil
of the Caltech Center for Catalysis and Chemical Synthesis for
access to experimental and analytical equipment; Dr. Michael K.
Takase and Larry M. Henling for assistance in collecting X-ray
diffraction data; and Dr. Mona Shahgholi and Naseem Torian for
assistance with mass spectrometry measurements. We also thank
Kevin Sokol and Dr. Alan Cherney (both of Caltech) for
preparing NHP esters 6g and 6h and vinyl bromides 1g, 1h, and
1l, respectively. Fellowship support was provided by the National
Science Foundation (graduate research fellowship to J.L.H. and
K.E.P., Grant No. DGE-1144469) and Shionogi & Co., Ltd.
(postdoctoral fellowship to N.S.). S.E.R. is an American Cancer
Society Research Scholar and Heritage Medical Research
Institute Investigator. Financial support from the NIH
(NIGMS RGM097582-01, R35GM118191) is gratefully ac-
knowledged.
(12) Huihui, K. M. M.; Caputo, J. A.; Melchor, Z.; Olivares, A. M.;
Spiewak, A.M.;Johnson, K. A.;DiBenedetto, T. A.;Kim,S.;Ackerman,L.
K. G.; Weix, D. J. J. Am. Chem. Soc. 2016, 138, 5016.
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