Catalytic enantioselective cyclization/cross-coupling with alkyl electrophiles

Article abstract of DOI:10.1021/ja500706v

As part of our ongoing effort to expand the scope of cross-coupling reactions of alkyl electrophiles, we have pursued a strategy wherein the nucleophilic coupling partner includes a pendant olefin; after transmetalation by such a substrate, if β-migratory insertion proceeds faster than direct cross-coupling, an additional carbon-carbon bond and stereocenter can be formed. With the aid of a nickel/diamine catalyst (both components are commercially available), we have established the viability of this approach for the catalytic asymmetric synthesis of 2,3-dihydrobenzofurans and indanes. Furthermore, we have applied this new method to the construction of the dihydrobenzofuran core of fasiglifam, as well as to a cross-coupling with a racemic alkyl electrophile; in the latter process, the chiral catalyst controls two stereocenters, one that is newly generated in a β-migratory insertion and one that begins as a mixture of enantiomers.

Full text of DOI:10.1021/ja500706v

Communication
pubs.acs.org/JACS
Catalytic Enantioselective Cyclization/Cross-Coupling with Alkyl
Electrophiles
Huan Cong†,‡ and Gregory C. Fu*,†,‡
†Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
As part of our ongoing effort to expand the scope of
ABSTRACT: As part of our ongoing effort to expand the
scope of cross-coupling reactions of alkyl electrophiles, we
have pursued a strategy wherein the nucleophilic coupling
partner includes a pendant olefin; after transmetalation by
such a substrate, if β-migratory insertion proceeds faster
than direct cross-coupling, an additional carbon−carbon
bond and stereocenter can be formed. With the aid of a
nickel/diamine catalyst (both components are commer-
cially available), we have established the viability of this
approach for the catalytic asymmetric synthesis of 2,3-
dihydrobenzofurans and indanes. Furthermore, we have
applied this new method to the construction of the
dihydrobenzofuran core of fasiglifam, as well as to a cross-
coupling with a racemic alkyl electrophile; in the latter
process, the chiral catalyst controls two stereocenters, one
that is newly generated in a β-migratory insertion and one
that begins as a mixture of enantiomers.
enantioselective cross-couplings of alkyl electrophiles, we are
pursuing an approach wherein an organometallic reagent that
bears a pendant olefin is employed as the nucleophilic coupling
partner (bottom of Figure 1).4−6 In the presence of a chiral
catalyst, transmetalation and then β-migratory insertion (left
side of Figure 2), followed by alkyl−alkyl coupling, could lead
Figure 2. Complementary approaches to generating a precursor (A)
for catalytic enantioselective cyclizations.
n recent years, significant progress has been reported on the
Idevelopment of methods for the transition-metal-catalyzed
cross-coupling of alkyl electrophiles to generate carbon−carbon
bonds, including enantioselective processes.1 To date, most
investigations of asymmetric catalysis have focused on stereo-
convergent reactions of racemic secondary electrophiles,2
although an advance has also been described with a racemic
secondary nucleophile (top of Figure 1).3
to the formation of two carbon−carbon bonds and a new
stereocenter (bottom of Figure 1). This strategy complements
asymmetric coupling processes wherein an intermediate of type
A is generated through oxidative addition of an electrophile
(right side of Figure 2).7
In this report, we establish that a transmetalation−insertion
sequence can indeed be used to generate two, rather than one,
carbon−carbon bonds in a cross-coupling with an alkyl
electrophile and that this process can be achieved with good
enantioselectivity. Specifically, we describe couplings of
arylboron reagents that bear a pendant olefin with unactivated
alkyl halides, thereby furnishing 2,3-dihydrobenzofurans8,9 and
indanes10,11 in high ee (eq 1).
In order to enhance the likelihood of cyclization (β-
migratory insertion) prior to coupling with the electrophile,
we chose to focus on an organometallic coupling partner that
could form a five-membered ring upon insertion, since such
cyclizations are often facile. At the outset, it was unclear what
catalyst would enable the desired sequence of bond-forming
processes, much less achieve high enantioselectivity.
Received: January 22, 2014
Published: February 27, 2014
Figure 1. Asymmetric cross-couplings of alkyl electrophiles.
© 2014 American Chemical Society
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bromide was replaced with the corresponding alkyl chloride,
essentially no 2,3-dihydrobenzofuran was observed (entry 9).16
We next examined the scope of this method for asymmetric
cyclization/cross-coupling with alkyl bromides (Table 2).17
A
Table 2. Catalytic Enantioselective Cyclization/Cross-
a
Coupling with Alkyl Electrophiles
Interestingly, we have determined that a nickel/1,2-diamine-
based catalyst, which we have found to be useful for
enantioconvergent alkyl−alkyl couplings,3,12 is also effective
for the desired cyclization/cross-coupling sequence (Table 1,
Table 1. Catalytic Enantioselective Cyclization/Cross-
Coupling with an Alkyl Electrophile: Influence of Reaction
a
Parameters
a
b
All data are the average of two experiments. Yield of purified
c
product. 15% NiBr2·glyme and 17% ligand 1 were used.
a
b
All data are the average of two experiments. The yield was
determined by GC analysis with the aid of a calibrated internal
standard.
range of functionalized electrophiles serve as suitable reaction
partners, furnishing the desired 2,3-dihydrobenzofuran in very
good enantiomeric excess. A silane, an acetal, and an imide are
compatible with the reaction conditions. The method is not
limited to unhindered primary alkyl bromides; a β-branched
primary and a secondary bromide also undergo cyclization/
cross-coupling (entries 6 and 7).
Under similar conditions, indane derivatives can also be
produced in high ee, although modest yield (eq 2).17c An
attempt to generate a quaternary stereocenter furnished a
promising initial result (eq 3).
A number of optically active 2,3-dihydrobenzofurans exhibit
interesting biological activity,8,9 including fasiglifam (Takeda
Pharmaceuticals: TAK-875), which progressed to phase 3
clinical trials for type 2 diabetes until being withdrawn due to
concerns about liver safety.18 We have applied our method to a
catalytic asymmetric synthesis of the dihydrobenzofuran core of
fasiglifam (Scheme 1).
entry 1). Thus, in the presence of NiBr2·glyme and ligand 1,
both of which are commercially available, the target 2,3-
dihydrobenzofuran is generated in good ee and yield. Under
these conditions, essentially none of the product of direct cross-
coupling (without cyclization of the nucleophile) or of endo
cyclization is observed (<5%).
In the absence of NiBr2·glyme, ligand 1, or i-BuOH, the
desired cyclization/cross-coupling product did not form in
appreciable yield (Table 1, entries 2−4).13 Furthermore, the
use of a smaller excess of the arylboron reagent led to a
somewhat lower ee and yield (entry 5).14 Other ligands that we
have found to be useful for enantioconvergent couplings of
alkyl electrophiles were not effective for this new asymmetric
cross-coupling with an alkyl halide (entries 6−8).15 If the alkyl
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available), we have established that this strategy enables the
synthesis of highly enantioenriched 2,3-dihydrobenzofurans
and indanes through couplings with a range of alkyl halides. We
have applied this new method to the generation of the
dihydrobenzofuran core of fasiglifam, as well as to a
transformation wherein the chiral catalyst controls the
stereochemistry of two rather different processes: a β-migratory
insertion and an enantioconvergent coupling of a racemic alkyl
halide. Ongoing studies are directed at further enlarging the
scope of cross-coupling reactions of alkyl electrophiles, as well
as elucidating the mechanisms of these transformations.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and compound characterization data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Scheme 1. Catalytic Asymmetric Synthesis of the 2,3-
Dihydrobenzofuran Core of Fasiglifam
Corresponding Author
gcfu@caltech.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support has been provided by the National Institutes of Health
(National Institute of General Medical Sciences: R01-
GM62871). We thank Yufan Liang, Dr. Allen G. Oliver
(University of Notre Dame), Dr. Nathan D. Schley, and Dr.
Scott C. Virgil (Caltech Center for Catalysis and Chemical
Synthesis, supported by the Gordon and Betty Moore
Foundation) for assistance.
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