Enantioselective synthesis of atropisomeric vinyl arene compounds by palladium catalysis: A carbene strategy

Article abstract of DOI:10.1002/anie.201509571

An efficient palladium-catalyzed asymmetric synthesis of axially chiral vinyl arenes from aryl bromides and hydrazones is reported. The products were easily oxidized to axially chiral biaryl compounds, and the phosphine oxides were readily reduced to phosphine ligands.

Full text of DOI:10.1002/anie.201509571

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International Edition: DOI: 10.1002/anie.201509571
German Edition: DOI: 10.1002/ange.201509571
Biaryls
Enantioselective Synthesis of Atropisomeric Vinyl Arene Compounds
by Palladium Catalysis: A Carbene Strategy
Jia Feng+, Bin Li+, Yun He, and Zhenhua Gu*
Abstract: An efficient palladium-catalyzed asymmetric syn-
thesis of axially chiral vinyl arenes from aryl bromides and
hydrazones is reported. The products were easily oxidized to
axially chiral biaryl compounds, and the phosphine oxides
were readily reduced to phosphine ligands.
Lassaletta, and Stoltz prepared a class of useful chiral 1-
arylisoquinoline derivatives by palladium-catalyzed kinetic
dynamic resolution (KDR; Scheme 1b).[11] The groups of
Miller, Akiyama, You, and Colobert developed direct asym-
À
metric C H halogenation or olefination reactions for the
construction of axially chiral compounds with good to
excellent enantioselectivity.[12] A bond-cleavage KDR strat-
egy for the synthesis of axial compounds from small-ring-
bridged biaryls was used by the groups of Bringmann,
Clayden, and Hayashi.[13–15] Transition metal catalyzed
[2+2+2] cycloadditions have been proven as useful ways for
the synthesis of atropisomeric molecules (Scheme 1c).[16]
Additionally atropisomeric molecules could also be accessed
by chirality transfer aromatization reactions from chiral
compounds with stereogenic atoms (Scheme 1d).[17,18]
A
tropisomeric molecules exhibit an axis of chirality because
of the inhibited bond rotations, and are significantly different
from stereogenic atoms with four different substituents.
Represented by 1,1’-bi-2-naphthol (BINOL), biaryl axially
chiral compounds became one of the most successful chiral
chiral ligands in organic synthesis. Natural enantiopure
atropisomers such as vancomycin and (À)-steganone have
significant biological activities, thus increasing the scientific
interest in axially chiral compounds.
A straight forward way to synthesize axially chiral biaryl
compounds[1] is the coupling of two arenes by either oxidative
dimerization or cross-coupling (Scheme 1a). Representative
Although these achievements are important, catalytic
asymmetric synthesis of atropisomeric compounds is still in its
infancy. The tedious preparation of multisubstituted and
hindered aryl (pseudo)halides or organometallic reagents
hampered the progress in the area of biaryl cross-coupling.
Thus, the development of new versatile methods using easily
available materials to access synthetically valuable atropiso-
meric compounds is urgent and challenging.
contributions come from the groups of Brussee,[2] Smrcina,
ˇ
Kocovsky,[3] Gong,[4] Hayashi,[5] Espinet,[6] Buchwald,[7] Lin,[8]
and Tang,[9] as well as others.[10] The groups of Fernµndez and
ˇ
´
During a survey on the synthesis of axially chiral
molecules, it was found that axially chiral vinyl arenes were
=
rarely studied (Scheme 2a). The axially chiral C C bond
might act as a new class of ligands, such as (P, Olefin) ligands.
Scheme 1. Catalytic asymmetric synthesis of axially chiral compounds.
[*] J. Feng,[+] B. Li,[+] Y. He, Prof. Dr. Z. Gu
Department of Chemistry
University of Science and Technology of China
96 Jinzhai Road, Hefei, Anhui 230026 (China)
E-mail: zhgu@ustc.edu.cn
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201509571.
Scheme 2. The synthesis of axially chiral vinyl arenes.
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Furthermore, it would provide a new protocol for the
activity. Furthermore, the enantioselectivity declined to
synthesis of biaryl atropisomers after aromatization. We
reasoned that b-hydride elimination of a quaternary carbon
around 30% when the bulky ligands L9 and L10 were used.
The introduction of an electron-withdrawing trifluoromethyl
group onto the ligand (L11) substantially improved the
catalytic activity, and the yield increased to 91%. Pleasingly,
the p-fluorophenyl-derived ligand L12 proved superior to
others, and the enantioselectivity increased to 77%. The
increased catalytic activity of Pd/L12 allowed the reaction to
be performed at 508C, and as a result the ee value increased to
90%. The evaluation of other ligands, such as L13 and L14,
showed that the 1,3-dioxolane skeleton is critical for both
highly catalytic activity and selectivity.
À
atom bearing a C Pd bond would form vinyl arene com-
pounds. The challenge is that both the R- and S-configured
intermediates have two conformers, wherein Int1A and Int2A
give (S)-3, and Int1B and Int2B give (R)-3 (Scheme 2b). The
palladium/carbene-involved insertion, migration, and b-hy-
dride elimination process,[19] which was extensively studied for
the synthesis of multisubstituted alkenes by the groups of van
Vranken,[20] Barluenga and ValdØs,[21] and Wang,[22] as well as
others,[23] provides an ideal model to realize our hypothesis.
However, to our knowledge there is no catalytic asymmetric
Firstly we tested the generality regarding different
substituted diaryl phosphine oxides (Table 2). The reaction
with either electron-donating or electron-withdrawing groups
=
study on this C C formation reaction. Herein we report an
asymmetric palladium-catalyzed synthesis of axially chiral
vinyl arenes from aryl bromides and hydrazones, which are
easily obtained on large scale with versatile functionalities.
The primary screening with 1a and 2a as substrates
indicated that 1,4-dioxane and tBuOLi were the optimum
solvent and base (Table 1). The reaction with (R)-DIOP (L1)
as the ligand afforded 3a in only 40% ee. The use of the
ligands L2, L3a, and L3b did not result in satisfactory
selectivity. TADDOL-based phosphoramidites proved to be
a class of promising ligands, and by examination of the
substituents on N, the pyrrolidine moiety (L5) was superior to
others, such as morpholine (L4), indoline (L6), cis-octahydro-
1H-isoindole (L7), and other acyclic amines. Inspired by these
findings, further screening focused on the modification of
pyrrolidine-based phosphoramidites. Replacing the phenyl
ring with electron-rich aryls (L8–L10) decreased the catalytic
Table 2: Substrate scope with respect to the phosphine oxides.[a]
Table 1: Optimization of reaction conditions.[a]
[a] For details see the Supporting Information. Yield is that of the isolated
product. The ee values were determined by HPLC analysis using a chiral
stationary phase.
at the para-position of the phenyl rings proceeded unevent-
fully with excellent yields and enantioselectivity (3b–f).
Notably, a longer reaction time was required when the
phenyl ring bears a bulky substituent (3c and 3d). Both the
meta-methoxyphenyl and meta-methylphenyl phosphine
oxides (3g and 3h), and the disubstituted phenyl phosphine
oxides (3i and 3j) worked smoothly to give the corresponding
products in 87–91% ee. Both b- and a-naphthyl phosphine
oxides readily underwent the coupling reaction to deliver 3k
and 3l, respectively, in excellent enantioselectivity, albeit the
one with the bulkier a-naphthyl phosphine oxide resulted in
a lower relative yield.
The reactions of both 7- and 6-subsituted tetralone-
derived hydrazones smoothly furnished the products in
decent yields and excellent ee values (Table 3; 3m–q). 5-
Methoxyl-1-tetralone-derived hydrazone was a good sub-
strate, and it readily reacted with 2-bromonaphthylenes to
deliver the products in up to 97% ee (3r–t). We were pleased
to see that 4,4-disubstituted hydrazones were compatible
[a] The reaction was conducted with 1a (0.20 mmol, 1.0 equiv) and 2a
(3.0 equiv). Yield is that of the isolated product. The ee values were
determined by HPLC analysis using a chiral stationary phase. [b] The
reaction was conducted with 2a (1.5 equiv), Pd(OAc)2 (10 mol %), L12
(20 mol %), LiOtBu (2.5 equiv) at 508C for 24 h. Ts =4-toluenesulfonyl.
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Table 3: Substrate scope.[a]
Scheme 3. Transformations and application. DCE=1,2-dichloroethane,
DCM=dichloromethane, DDQ=2,3-dichloro-5,6-dicyanobenzo-
quinone, THF=tetrahydrofuran.
[a] For details see Supporting Information. Yield is that of the isolated
product. The ee values were determined by HPLC analysis using a chiral
stationary phase.
substrates, where the quaternary carbon center did not affect
the enantioselectivity (3u–x). Cheerfully, 6-isopropoxycar-
bonyl-(1-bromonaphthalen-2-yl)-diphenylphosphine oxide
displayed good reactivity, and the corresponding reactions
gave excellent results in both yield and selectivity (3y).
The scalability of this reaction was evaluated by perform-
ing the reaction with 2.0 mmol of 1a. The reaction gave 88%
yield and 89% ee of 3a, and the ee value was further improved
by recrystallization (Scheme 3a).[25] Gratifying, the phosphine
oxide could be readily reduced under mild reaction conditions
with excellent yield and ee value (Scheme 3b). In the
presence of DDQ, 3a was easily aromatized to the biaryl
atropisomer (R)-5a with a high ee value, thus the absolute
configuration was determined by comparison of the optical
rotation of (R)-5a with the reported data (Scheme 3c). The
utility of (P, Olefin) ligands[24] was primarily demonstrated by
a benchmark asymmetric allylation reaction between (E)-1,3-
diphenylallyl acetate and the indole 6. In the presence of Pd/
4a (1:1), the reaction afforded the N-allylation product 7 in
88% yield and 83% ee (Scheme 3d).[26]
Scheme 4. Plausible catalytic cycle.
enantioselectivity was partially affected by the p–p interac-
tion of the phenyl ring of the phosphine oxide with the
tetrahydronaphthalene moiety in either B or C, since a control
reaction with diethyl phosphonate as a substrate gave 8a in
7% ee (Scheme 4b).
In conclusion, we have reported a palladium-catalyzed
asymmetric synthesis of axially chiral 1-vinylnaphthalen-2-yl
phosphine oxides from aryl bromides and hydrazones. The
use of carbene precursors as coupling partners resulted in
mild reaction conditions and thus a broad functional-group
tolerance.
A catalytic cycle is proposed (Scheme 4a). The oxidative
addition of Pd0 with 1a would give A, which reacts with the
diazo compound (generated in situ from hydrazones in the
presence of tBuOLi) to afford the carbene-coordinated
complex B. Migration/insertion of B affords a quaternary
À
carbon center bearing a C Pd bond, and delivers the final
product by b-hydride elimination. It is possible that the
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This work was supported by the “973” project
(2015CB856600), NSFC (21272221, 21472179), the Recruit-
ment Program of Global Experts, and the Fundamental
Research Funds for the Central Universities (WK
2060190028, 2060190026). We thank Prof. Q. Zhou and Prof.
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Keywords: biaryls · chirality · enantioselectivity · hydrazones ·
palladium
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Received: October 13, 2015
Revised: November 14, 2015
Published online: December 28, 2015
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