Enantioselective and Regioselective Hydroetherification of Alkynes by Gold-Catalyzed Desymmetrization of Prochiral Phenols with P-Stereogenic Centers

Article abstract of DOI:10.1021/acs.orglett.8b02982

The gold(I)-catalyzed enantioselective hydroetherification of alkynes was achieved via desymmetrization of prochiral bisphenols bearing P-stereogenic centers. (S)-DTBM-Segphos(AuCl)₂/AgNTf₂ proved to be a highly efficient catalyst system for this transformation, affording P-chiral cyclic phosphine oxides in good yields with high enantioselectivities (with up to 99% ee). The same catalyst system allowed for the enantioselective desymmetrization of dialkynes. Synthetic transformations of the cyclization products afforded other P-chiral molecules with high enantiospecificity.

Full text of DOI:10.1021/acs.orglett.8b02982

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enantioselective and Regioselective Hydroetherification of Alkynes
by Gold-Catalyzed Desymmetrization of Prochiral Phenols with
P‑Stereogenic Centers
Yin Zheng, Linna Guo, and Weiwei Zi*
State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071,
China
S
* Supporting Information
ABSTRACT: The gold(I)-catalyzed enantioselective hydroether-
ification of alkynes was achieved via desymmetrization of
prochiral bisphenols bearing P-stereogenic centers. (S)-DTBM-
Segphos(AuCl)2/AgNTf2 proved to be a highly efficient catalyst
system for this transformation, affording P-chiral cyclic phosphine
oxides in good yields with high enantioselectivities (with up to
99% ee). The same catalyst system allowed for the enantiose-
lective desymmetrization of dialkynes. Synthetic transformations
of the cyclization products afforded other P-chiral molecules with
high enantiospecificity.
ince the first successful application of P-chiral phosphine
ligand DiPAMP in Rh-catalyzed asymmetric hydrogenation
contrast, gold-catalyzed enantioselective hydrofunctionalization
reactions of alkynes remain rare.11 This paucity partially derives
from the fact that alkynes are not prochiral; therefore,
stereocenters cannot be directly formed during the formation
of Csp2−O or Csp2−N bonds in hydroetherification or
hydroamination reactions of alkynes. However, prochiral
carbon-based nucleophiles have been employed in enantiose-
lective C−C bond formation through gold-catalyzed hydro-
functionalization of alkynes; however, only poor enantioselec-
tivities have been obtained thus far using gold catalysis.12
Czekelius and co-workers first developed a desymmetrization
strategy to establish a quaternary carbon stereocenter in the
gold-catalyzed enantioselective hydroamination of alkynes.11e
More recently, Toste and co-workers have reported a gold(I)-
catalyzed desymmetrization of 1,3-diols for the preparation of
quaternary carbon centers.13a Considering the important
application of cyclic phosphines with P-chirality in transition-
metal catalysis,2,14 we envisioned developing a desymmetriza-
tion strategy to construct the phosphorus-based stereocenters
(Scheme 1). Herein, we disclose the first gold(I)-catalyzed
enantioselective hydroetherification reaction of alkynes, featur-
ing intramolecular desymmetrization of bisphenols or dialkynes,
which provides a highly efficient method to prepare chiral cyclic
phosphine oxides with P-chirality.15
S
reactions by Knowles in 1970s,1 chiral phosphorus compounds
with P-stereogenic centers have been widely utilized in
transition-metal catalysis2 as well as organo-catalysis.3 Tradi-
tionally, access to enantioenriched P-chiral phosphorus
compounds is achieved through the use of chiral reagents- or
auxiliary-assisted transformations.4 Recently, considerable pro-
gress has been made for the introduction of P-chirality via
catalytic asymmetric processes.2c Most prominently, enantiose-
lective synthesis of P-stereogenic phosphines have been
accomplished through dynamic kinetic resolution of racemic
secondary phosphines and their derivatives catalyzed by
transition metals,5 such as palladium,5a−d platinum,5e ruth-
enium,5f,g and copper.5h The preparation of P-stereogenic
centers via the catalytic desymmetrization of prochiral
phosphorus compounds presents an attractive alternative.6
Examples include Rh-catalyzed [2 + 2 + 2] cycloaddition,6a
Mo-catalyzed asymmetric ring-closing (ARC) reaction,6b and
Pd or Rh-catalyzed asymmetric C−H activation reactions.6c−f
Organo-catalysts, such as Brønsted acids7a and N-heterocyclic
carbenes,7b have also been employed. Despite those achieve-
ments, the developments of efficient methods to prepare P-
stereogenic phosphorus compounds containing novel architec-
tures remains highly desirable.8
We began our investigation employing bisphenol 1a as the
model substrate. On the basis of previous works,13a a chiral
counteranion strategy was investigated as a means to achieve the
During the past decade, asymmetric gold catalysis has
attracted intensive studies, leading to successful applications of
gold catalysis in the enantioselective formation of C−O, C−N,
and C−C bonds.9 Enantioselective reactions based on hydro-
functionalization of allenes and alkenes are among the most
successful achievements in this area;10 however, in sharp
Received: September 18, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02982
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Gold(I)-Catalyzed Asymmetric Desymmetrization
To Construct Quaternary Centers
Table 1. Optimization of the Reaction Conditions
b
c
entry
L
AgX
solvent
time (h) yield (%) ee (%)
1
2
3
4
5
6
7
8
Ph3P
L1
L2
L3
L4
L5
L3
L3
L3
L3
L3
L3
L3
(S)-AgTrip
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgOTf
AgOTs
AgBF4
AgSbF6
AgNTf2
AgNTf2
AgNTf2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
o-xylene
toluene
24
1
1
1
1
1
1
20
3
1
86
90
86
85
84
84
93
>95
80
81
92
94
>95
50
75
80
85
80
66
73
21
76
71
91
89
96
9
10
11
12
1
1
24
e
13
a
Reaction conditions: 5 mol % gold catalyst, 10 mol % AgX, 0.10
b
mmol substrate, 1 mL of solvent, room temperature. Yields were
determined by H NMR spectroscopic analysis using CH2Br2 as the
internal standard. Enantiomeric excess values were determined by
chiral HPLC analysis. 5 mol % Ph3PAuCl, 5 mol % (S)-TripAg. 0.2
mmol scale; reaction was conducted at −20 °C.
1
c
desired asymmetric desymmetrization reaction using homoge-
neous gold catalysis. Catalyzed by a combination of Ph3PAuCl
and (S)-AgTrip, the reaction proceeded smoothly in dichloro-
methane to deliver the desired cyclization product 2a in 86%
yield after 24 h. Although the enantioselectivity was moderate
(50% ee, Table 1, entry 1), high regioselectivity was obtained
with only the 6-endo-dig cyclization product being detected.16
Moreover, despite the noted tendency for N-oxides and
sulfoxides to undergo redox reactions under gold catalysis,
such a reaction of the phosphine oxide was not observed.17
Further optimization of the enantioselectivity by structural
tuning of the chiral phosphates was met with failure, which
prompted us to evaluate other catalyst systems. In Toste’s recent
work on gold(I)-catalyzed asymmetric tandem alkoxylation/
Claisen rearrangement reaction, sterically demanding bi-
sphosphine ligands provided excellent results in the asymmetric
desymmetrization of 1,4-dialkenes.13b In contrast, Czekelius and
co-workers reported that bisphosphine-type ligands gave no
enantioselectivity in the desymmetrization of 1,4-dialkynes via a
5-exo-dig hydroamination reaction.11e In our system, we
anticipated that the 6-endo-dig pathway would allow the
prochiral phosphorus center to be much closer to the chiral
environment of the chiral ligand, resulting in increased
enantioinduction in the cyclization event.9e
d
e
much bulkier groups at 3,5 positions on Ar (L4 vs L3) or with
smaller dihedral angles18 (L1 vs L3 vs L5) gave better
enantioselectivities. Efforts to improve the ee by changing
AgNTf2 to other silver salts (AgOTs, AgOTf, AgBF4, or AgSbF6)
exclusively failed.19 A brief survey of solvents revealed that a
nonpolar solvent, such as toluene, resulted in significantly
improved enantioinduction (91% ee) with a modest improve-
ment in yield (entry 11). Finally, optimal results were obtained
by lowering the reaction temperature to −20 °C, which afforded
the desired cyclic chiral phosphine oxide 2a in 95% yield with
96% ee, albeit prolonging the reaction time to 24 h (entry 13).
With these optimized conditions in hand, the reaction scope
was examined (Scheme 2). The reaction shows excellent scope
with regard to aryl alkynes. For example, the aryl rings
substituted with an electron-withdrawing (Cl, F) or electron-
donating (MeO, Me, t-Bu) group at the para-position was well-
tolerated (entries 2−6). Substituents on the meta-position gave
satisfactory results as well (entry 7−9); however, substrates with
sterically demanding groups (e.g., MeO) at the ortho-position
gave diminished enantioselectivity (96% yield, 87% ee) (entry
10). A substrate with a thiophene ring was also examined under
the standard conditions, providing the desired compound 2l in
94% yield with 93% ee. In most cases, excellent enantioselectiv-
ities and yields were obtained for substrates with different
substituents on the phenol (entries 13 to 16). Only the para-
bromophenol-derived substrate gave slightly decreased ee
(entry 17). Finally, substrate with two methyl groups on the
phenol ring maintained reactivity (entry 18). To determine the
absolute stereochemistry of the substrate, 2a (96% ee) was
With this hypothesis, we turned our attention to the use of
prevalent chiral bisphosphine ligands for the gold-catalyzed
desymmetrization reaction (Table 1). When (S)-DTBM-
Biphep(AuCl)2/AgNTf2 was employed as catalyst, the reaction
was completed within 1 h, with 2a being isolated in 75% ee.
Commercially available bisphosphines with different backbones
were then surveyed. Changing the ligand from (S)-DTBM-
Biphep (L1) to (S)-DTBM-Segphos (L3) increased the
observed enantioselectivity to 85% ee. Bisphosphines with
B
DOI: 10.1021/acs.orglett.8b02982
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b c
, ,
however, with extremely low conversion under the standard
conditions even after prolonged reaction times (<30%
conversion, −20 °C for 3 days).20 In order to obtain
considerable conversion for 3a, the reaction temperature was
increased to room temperature, which resulted in formation of
4a in 89% yield after 12 h. Notably, no apparent erosion of
enantioselectivity was observed (95% ee) at this higher
temperature. The substrate scope is summarized in Scheme 3.
Scheme 2. Scope of Bisphenols Substrates
a b c
, ,
Scheme 3. Scope of Dialkyne Substrates
a
Reaction conditions: 5 mol % (S)-DTBM-Segphos(AuCl)2, 10 mol
% AgNTf2, 0.10 mmol substrate, 2 mL of toluene at room
b
temperature (20−25 °C). Isolated yields. Enantiomeric excess
values were determined by chiral HPLC analysis of the pure product.
c
Absolute stereochemistry assigned by comparison with 4a.
a
Reaction conditions: 5 mol % (S)-DTBM-Segphos(AuCl)2, 10 mol
b
% AgNTf2, 0.20 mmol substrate, 2 mL of toluene at −20 °C. Isolated
yields. Enantiomeric excess values were determined by chiral HPLC
analysis of the pure product. Absolute stereochemistry assigned by
comparison with 2a. 0.1 mmol scale, 2 mL of toluene.
The gold-catalyzed alkyne desymmetrization tolerated both
electron-poor and electron-rich aromatic substituents, although
the yields were relatively low compared to bisphenols substrates.
Further synthetic transformation of the cyclization products is
illustrated in Scheme 4. Reduction of the phosphine oxide under
HSiCl3/Et3N, followed by reaction with borane gave borane
adduct 2aa with only slightly reduced chirality.21 Selective
bromination of the phenol 2m, in the presence of the phenyl
ether, gave brominated adduct 2ma, which is positioned for
subsequent cross-coupling reactions. Attempts to open the
cyclic vinyl ether ring were conducted under heterogeneous
hydrogenolysis conditions, which afforded the acyclic chiral
phosphine oxide 2ac in 87% yield without loss of enantiopurity.
c
d
recrystallized in hexanes/dichloromethane to give a more
enantiopure sample (91% yield, 98% ee). A single crystal X-
ray diffraction experiment gave the absolute configuration as S
on the phosphorus center.
The success in preparing P-chiral cyclic phosphine oxides via
desymmetrization of bisphenols prompted us to examine
whether this catalyst system could be employed for dialkynes.
In the event, cyclization product 4a was obtained in excellent ee,
C
DOI: 10.1021/acs.orglett.8b02982
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Letter
Scheme 4. Synthetic Transformation of the Products
ACKNOWLEDGMENTS
■
This work was supported by the National “Young Thousand
Talents Plan,” the National Natural Science Foundation of
China (Nos. 21871150), and the Fundamental Research Funds
for Central University. We gratefully acknowledge the State Key
Laboratory of Elemento-organic Chemistry and College of
Chemistry of Nankai University for generous financial support.
We thank Dr. Mark Levin for assistance in the preparation of this
manuscript. We thank Prof. F. Dean Toste for helpful
discussions.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02982.
Experimental details (PDF)
Accession Codes
CCDC 1531462 and 1575903 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail:zi@nankai.edu.cn
ORCID
Weiwei Zi: 0000-0002-7842-0174
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.8b02982
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DOI: 10.1021/acs.orglett.8b02982
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