Pd/NHC-catalyzed enantiospecific and regioselective suzuki-miyaura arylation of 2-arylaziridines: Synthesis of enantioenriched 2-arylphenethylamine derivatives

Article abstract of DOI:10.1021/ja5039616

A palladium-catalyzed stereospecific and regioselective cross-coupling of enantiopure 2-arylaziridines with arylboronic acids under mild conditions to construct a tertiary stereogenic center has been developed. N-heterocyclic carbene (NHC) ligands efficiently promote the coupling, suppressing β-hydride elimination. The enantiospecific cross-coupling allowed us for preparation of a series of biologically important 2-arylphenethylamine derivatives in an enantiopure form.

Full text of DOI:10.1021/ja5039616

Communication
pubs.acs.org/JACS
Pd/NHC-Catalyzed Enantiospecific and Regioselective Suzuki−
Miyaura Arylation of 2‑Arylaziridines: Synthesis of Enantioenriched
2‑Arylphenethylamine Derivatives
Youhei Takeda,*,†,‡ Yuki Ikeda,‡ Akinobu Kuroda,‡ Shino Tanaka,‡ and Satoshi Minakata*,‡
‡
†Frontier Research Base for Global Young Researchers and Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan
S
* Supporting Information
The 2-arylphenethylamine scaffold constitutes a pharmaco-
ABSTRACT: A palladium-catalyzed stereospecific and
regioselective cross-coupling of enantiopure 2-arylaziri-
dines with arylboronic acids under mild conditions to
construct a tertiary stereogenic center has been developed.
N-heterocyclic carbene (NHC) ligands efficiently promote
the coupling, suppressing β-hydride elimination. The
enantiospecific cross-coupling allowed us for preparation
of a series of biologically important 2-arylphenethylamine
derivatives in an enantiopure form.
logically important motif in dopamine receptor agonists, thereby
serving as potential candidates for the treatment of disorders of
the central nervous system such as schizophrenia and Parkinson
disease.12 Not surprisingly, they often exhibit biological
enantiospecificity.13 Therefore, the asymmetric construction of
such a skeleton is important in terms of the exploration of
structurally related bioactive agents and pharmaceuticals.
However, the asymmetric construction of this privileged
structure has been mainly limited to Lewis acid-catalyzed
stereospecific Friedel−Crafts-type ring-opening of enantiopure
2-arylaziridines14 and to a sequence of enantioselective Michael-
addition of arylmetals to β-nitrostyrenes and following
reduction.15 The former intrinsically requires electron-rich
arenes as an aryl reactant, and the latter suffers from difficulty
in controlling enantioselectivity of products. Recognizing that
enantioenriched 2-arylaziridines are accessible by various
methods,16 we assumed that enantiospecific and regioselective
cross-coupling of 2-arylaziridines with an arylmetals would allow
for an efficient, functional-tolerant, and diverse route to
enantioenriched 2-arylphenethylamine derivatives (eq 1).
he construction of stereogenic centers through metal-
T
catalyzed cross-coupling in a highly stereocontrolled
manner has remained a big challenge in organic synthesis.1 An
excellent approach toward achieving this involves enantiocon-
vergent cross-coupling of racemic secondary alkyl electrophiles2
or alkylmetals3 with the aid of a chiral catalyst through dynamic
kinetic resolution. Another comprises stereospecific cross-
coupling employing either enantioenriched α-chiral secondary
alkylmetals4 or alkyl halides5 with an achiral catalyst, through
which the stereochemical information on substrates is efficiently
translated into products (retention or inversion). Meanwhile, the
past decade has seen a significant growth in stereospecific cross-
coupling of α-chiral nonclassical secondary alkyl electrophiles
Hillhouse17 and Wolfe18 independently reported that 2-
alkylaziridines smoothly undergo oxidative addition to Ni(0) and
Pd(0) complexes at the less hindered C−N bond in a SN2 fashion
to give isolable azametallacyclobutanes, respectively. After
several years of their pioneering works, the Doyle group opened
up the way to the utilization of aziridines as a nonclassical alkyl
electrophile in metal-catalyzed cross-coupling by demonstrating
Ni-catalyzed Negishi coupling of 2-aryl and 2-alkylaziridines.19
Until then, metal-catalyzed transformations of nonvinylic
aziridines were limited to the insertion of unsaturated
components like CO20a−c and heterocumullenes,20d,e which
outcompete β-hydride elimination. Despite their elegant works,
the general prediction of stereoconfiguration of products in the
Ni-catalyzed systems is difficult, probably due to the involvement
of single-electron-transfer (SET) mechanism. More recently,
Michael et al. reported a Pd/phosphine-catalyzed regioselective
cross-coupling of 2-alkylaziridines with arylboronic acids which
exclusively gives linear coupled products.21 The stereo-inversion
at the less hindered aziridine carbon showed good agreement
with SN2 oxidative addition mechanism. However, Pd-catalyzed
branch-selective cross-coupling of aziridines to construct a
3
3
3
bearing a *Csp −O or *Csp −N bond (*Csp : stereogenic carbon;
6
7
8
9
+
O = OSO2R, OR, and OC(O)R; N = NMe3(−OTf) and
NTs210): α-chiral alcohols and amines are widely accessible by
established methods,11 and their derivatives are highly stable, less
toxic and exhibit orthogonal reactivities compared with classical
alkyl halides. In this connection, stereospecific cross-coupling
using α-chiral nonclassical electrophiles would provide great
opportunity for diverse access to chiral complex molecules that
are otherwise difficult to synthesize by conventional method-
ologies. Herein we disclose a Pd/NHC-catalyzed stereospecific
and regioselective cross-coupling of enantiopure 2-arylaziridines
with arylboronic acids to produce a series of completely
stereoinverted 2-arylphenethylamine derivatives (eq 1).
Received: April 21, 2014
Published: May 29, 2014
© 2014 American Chemical Society
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Communication
a
Table 1. Effect of Pd Catalysts in the Cross-Coupling
Scheme 1. Enantiospecificity of the Cross-Coupling
Table 2. Pd-Catalyzed Enantiospecific Cross-Coupling of (R)-
a
1a with ArB(OH)2
a
Reaction conditions: 1a (0.2 mmol), 2a (0.22 mmol), Pd catalyst (8
μmol), ligand, and Na2CO3 (0.22 mmol) in toluene/H2O (1.2 mL, v/v
b
5:1) at room temperature under N2 atmosphere for 1 h. Determined
c
with GC. Isolated yield.
tertiary stereogenic center in a highly stereocontrolled manner
has not yet been reported (eq 1).
We began by identifying the catalytic system capable of
efficient C−C bond formation in the cross-coupling of racemic 2-
phenylaziridine [( )-1a] with phenylboronic acid (2a) as a
model reaction (Table 1). Since stereochemical scrambling was
of special concern,19 we chose Pd catalysts as an alternative
option. In terms of synthetic diversity, readily available and easily
handled arylboronic acids were selected as a nucleophilic partner.
Screening of privileged Pd catalysts22 revealed that Pd/PR3
systems typically suffered from production of 4 and 5, which
were probably derived through β-hydride elimination (entries
1−3, Table 1).23,24 In sharp contrast, the use of NHCs as an
adventitious ligand facilitated the regioselective formation of
coupled product 3aa without forming regioisomeric product, 4,
and 5, albeit in low yields (entries 4−6). After extensive
screening, NHC-ligated Pd precatalysts were found highly
effective for the coupling (entries 7−9), and among tested, a
bench-stable catalyst [SIPr-Pd(cinnamyl)Cl]25 gave 3aa in an
excellent yield even at room temperature (entry 9).
Having optimized reaction conditions, we then probed
stereochemical outcomes of the reaction by applying both
enantiomers of aziridine 1a separately to the coupling with
boronic acid 2b (Scheme 1). Analysis of the coupled product
obtained from (R)-1a and 2b with HPLC equipped with a chiral
column revealed that the product was enantiomerically pure
(99% ee).26 The absolute configuration of the product was
unambiguously determined by X-ray crystallography of its single
crystal to be R,26 confirming the complete stereo-inversion at the
benzylic carbon of aziridine. As predicted, the other enantiomeric
product (S)-3ab was obtained by the coupling of (S)-1a with 2b.
To investigate the scope of this enantiospecific coupling,
enantiopure (R)-1a was coupled with a variety of arylboronic
acids (Table 2). The coupling with ArB(OH)2 bearing an
electron-donating and a neutral group at the p-position
a
Reaction Conditions: (R)-1a (1.0 mmol), 2 (1.1 mmol), and Na2CO3
(1.1 mmol) in toluene/H2O (6 mL, v/v 5:1) at room temperature;
enantiomeric excess (ee) was determined by chiral HPLC analysis.
b
[SIPr-Pd(allyl)Cl] was used as an alternative catalyst.
proceeded smoothly to give stereo-inverted products (R)-3ac,
3ad, 3ae, and 3af in good to high yields as the single enantiomer.
In contrast, the couplings with electron-deficient boronic acids
(p-Cl, CF3, and CO2Me) were rather sluggish and gave low yields
(20−50%) of the corresponding products under the optimized
conditions. As the results of modifying conditions,22 precoordi-
nated allylic ligands on the Pd center were found to significantly
affect the reaction efficiencies,22 and [SIPr-Pd(allyl)Cl] gave the
best result. It is noteworthy that electron-deficient arylboronic
acids were coupled with aziridines under such mild conditions
because such products are difficult to access by conventional
Friedel−Crafts methods.14 The generality of this stereospecific
coupling was also confirmed by the reactions with m-tolyl-,
naphthyl-, and heteroaromatic boronic acids, although the
reactions with N-heterocyclic aromatic boronic acids like 3-
pyridyl- and N-Boc-2-pyrrole boronic acids did not work under
the optimal conditions.
Functionalized 2-arylaziridines and 2,3-disubstituted aziridines
were applicable to the stereospecific coupling to give perfectly
stereo-inverted products in a regioselective manner (Table 3).
Notably, enantiopure aziridines bearing an electron-withdrawing
aryl moiety [(R)-1i, (S)-1g, (S)-1o] were smoothly cross-
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Journal of the American Chemical Society
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a
Table 3. Scope of Aziridine Substrates
Scheme 2. Derivatization of Coupling Products
Scheme 3. A Plausible Catalytic Cycle
A plausible catalytic cycle is illustrated in Scheme 3. Since the
stoichiometric reaction of [SIPr-Pd(cinnamyl)Cl] with PhB-
(OH)2 in the absence of aziridine gave 1,3-diphenylpropene as a
major product (53% yield) along with a small amount of
cinnamyl alcohol (8% yield), we postulate that activation of the
precatalyst would mainly occur through direct nucleophilic
attack of ArB(OH)2 on the cinnamyl position or trans-
metalation/reductive elimination.30 Oxidative addition of
enantiopure aziridine to the resulting singly ligated [(SIPr)-
Pd(0)] complex would proceed in a SN2 manner to form stereo-
inverted alkylpalladium A or B. The regioselectivity could be
facilitated by the precoordination of aryl moiety of aziridines to
Pd(0) center31 or the contribution of π-benzyl palladium
species.19 Transmetalation and simultaneous protonation of
amide anion by boronic acid or water would form T-shaped Pd
complex C, which undergoes reductive elimination to give 3.
Stereoretention through transmetalation and reductive elimi-
nation is well established.32 Since prereduction of Pd(II) to
Pd(0) with a catalytic amount of phenylboronic acid as a
sacrificial reductant did not improve the yield of 3ah (18%), the
low reaction efficiencies of the coupling using electron-deficient
boronic acids could be ascribed to slow transmetalation and/or
reductive elimination. Although the precise roles of SIPr ligand
are unclear at this point, we conjecture that (i) the strong σ-
donating nature of SIPr would allow the Pd center for smooth
oxidative addition; (ii) weak interaction operating between
methyl hydrogen atoms of i-Pr groups of SIPr and the Pd center
could suppress β-hydride elimination through coordinatively
saturating the Pd center with multiple methyl hydrogens;33 (iii)
the steric bulk of SIPr would facilitate reductive elimination.
In conclusion, we have developed a Pd-catalyzed enantiospe-
cific and regioselective cross-coupling of 2-arylaziridines with
a
Reaction Conditions: 1 (1.0 mmol), ArB(OH)2 (1.1 mmol), and
Na2CO3 (1.1 mmol) in toluene/H2O (6 mL, v/v 5:1) at room
temperature for the time indicated.
coupled with PhB(OH)2 (2a) to afford the corresponding
products (S)-3ai, (R)-3ag, and (R)-3ao in good to high yields
(entries 1−3): these products were obtained in poor yields when
electron-neutral aziridine 1a was coupled with electron-deficient
boronic acids under the same conditions (Table 2). Racemic
trans-1p and cis-1q were successfully arylated in a regioselective
and stereospecific manner to give trans-3cp and trans-3aq as the
single diastereomer, respectively (entries 5 and 6). Furthermore,
enantiopure 1r was converted into stereo-inverted product (S,S)-
3ar (entry 7).27
Making use of the excellent enantiospecificity, coupled
products were transformed into cyclic amine derivatives
(Scheme 2). Enantiopure 3ar was efficiently transformed into
fused tetracyclic amine (S,R)-5, keeping the ee intact through
Pictet−Spengler reaction and Ts-deprotection (Scheme 2a).
Such tetracyclic amine scaffold is a ubiquitous motif in dopamine
D1 agonists.12 Furthermore, (R)-3ao was smoothly cyclized
under the modified Pd-catalyzed intramolecular amination
conditions28 to give enantiopure indoline (R)-6 in a quantitative
yield (Scheme 2b). Such three-substituted indoline motif is also
often found in natural products and structurally related drugs.29
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and spectroscopic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
takeda@chem.eng.osaka-u.ac.jp
minakata@chem.eng.osaka-u.ac.jp
Notes
■
(c) Kaiser, H. M.; Zenz, I.; Lo, W. F.; Spannenberg, A.; Schroder, K.;
̈
Jiao, H.; Gordes, D.; Beller, M.; Tse, M. K. J. Org. Chem. 2007, 72, 8847.
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(d) Hajra, S.; Maji, B.; Mal, D. Adv. Synth. Catal. 2009, 351, 859.
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2004, 126, 1954. (b) Wang, Z.-Q.; Feng, C.-G.; Zhang, S.-S.; Xu, M.-H.;
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This research was supported by Grant-in-Aid for Young
Scientists (B) (no. 25810061) from MEXT, Japan (to Y.T.).
■
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