One-Pot cascade hydration-asymmetric transfer hydrogenation as a practical strategy to construct chiral β-adrenergic receptor blockers

Article abstract of DOI:10.1002/cctc.201500409

The facile construction of biologically active β-adrenergic receptor agonists/blockers and analogues is a great fundamental and practical challenge in medical chemistry. Herein, we report a hydration-asymmetric transfer hydrogenation cascade to realize the one-pot enantioselective transformation of aromatic haloalkynes into chiral aromatic halohydrins, which can be converted readily into chiral β-adrenergicreceptor blockers. Such a one-pot cascade process involves the Au-catalyzed hydration of aryl-substituted haloalkynes to aryl-substituted α-halomethyl ketones and the Ru-catalyzed asymmetric transfer hydrogenation of aryl-substituted α-halomethyl ketones to aryl-substituted 2-haloethanols. The significant benefits of this procedure are that it provides chiral aromatic halohydrins in high yields, with excellent enantioselectivities, and a wide variety of functional groups are tolerated under mild conditions. The study described herein offers a useful approach to construct chiral β-adrenergic blockers, which is an attractive practical organic transformation that is performed in a one-pot manner.

Full text of DOI:10.1002/cctc.201500409

DOI: 10.1002/cctc.201500409
Communications
One-Pot Cascade Hydration–Asymmetric Transfer
Hydrogenation as a Practical Strategy to Construct Chiral
b-Adrenergic Receptor Blockers
Qunqun Ye, Tanyu Cheng,* Yuxi Zhao, Junwei Zhao, Ronghua Jin, and Guohua Liu*[a]
The facile construction of biologically active b-adrenergic re-
ceptor agonists/blockers and analogues is a great fundamental
and practical challenge in medical chemistry. Herein, we report
a hydration–asymmetric transfer hydrogenation cascade to re-
alize the one-pot enantioselective transformation of aromatic
haloalkynes into chiral aromatic halohydrins, which can be con-
verted readily into chiral b-adrenergicreceptor blockers. Such
a one-pot cascade process involves the Au-catalyzed hydration
of aryl-substituted haloalkynes to aryl-substituted a-halomethyl
ketones and the Ru-catalyzed asymmetric transfer hydrogena-
tion of aryl-substituted a-halomethyl ketones to aryl-substitut-
Figure 1. Important medicines prepared from aryl-substituted 2-haloetha-
nols.
ed 2-haloethanols. The significant benefits of this procedure
are that it provides chiral aromatic halohydrins in high yields,
with excellent enantioselectivities, and a wide variety of func-
tional groups are tolerated under mild conditions. The study
described herein offers a useful approach to construct chiral b-
adrenergic blockers, which is an attractive practical organic
transformation that is performed in a one-pot manner.
biologically active b-adrenergic receptor agonists or blockers is
highly desirable.
One-pot cascade reactions are atom economical, and they
are well known to construct various valuable compounds with
minimum isolation/purification processes.[3] As two classical re-
actions for the construction of aryl-substituted 2-haloethanols,
the hydration of a-haloalkynes to 2-haloketones[4] and the
asymmetric transfer hydrogenation (ATH) of 2-haloketones to
chiral 2-haloethanols[5] have been extensively studied theoreti-
cally and practically. However, the one-pot direct transforma-
tion of alkynes into alcohols through a cascade process is still
an unmet challenge,[6] and this can be mainly ascribed to com-
plicated conflicts derived from intrinsic bimetallic incompatibil-
ity and extrinsic reaction conditions. So far, only one successful
example of the one-pot enantioselective transformation of al-
kynes into chiral alcohols reported by Xiao’s group has been
realized, and the formic acid mediated alkyne-to-ketone trans-
formation is performed with a subsequent enantio-relay-cata-
lyzed ATH reaction to chiral alcohols (Scheme 1).[6a] However,
this one-pot transformation of alkynes into alcohols is a two-
step procedure. Clear drawbacks are that the first formic acid
mediated alkyne-to-ketone transformation needs a high tem-
perature (1008C) to reach its hydrated completion, whereas
the second ATH reaction must be controlled through adjusta-
ble pH values and an additional chiral catalyst. Notably, conflict
of extrinsic reaction conditions and the incompatible nature of
the chiral catalyst require the employment of a two-step pro-
cedure. Furthermore, the inherent nature of this one-pot trans-
formation is intolerant to functional groups on the substrates,
and haloalkynes cannot be converted into chiral halo alcohols.
This limitation greatly hinders the practical application of this
procedure for the construction of biologically active b-adrener-
gic receptor agonists or blockers. Thus, the development of
Optically pure halohydrins, especially aryl-substituted 2-haloe-
thanols as important synthetic motifs, have attracted great in-
terest in medical chemistry.[1] Great achievements through the
utilization of aryl-substituted 2-haloethanols as building blocks
are well documented, as these building blocks can be readily
converted into various b-adrenergic receptor agonists and
blockers.[2] As shown in Figure 1, some prominent examples,
such as nifenalol[2a,b] and pronethalol,[2b] are important medi-
cines as b-adrenergic receptor blockers, whereas metaprotere-
nol[2c] and fenoterol[2d] are b-adrenergic receptor agonists. Gen-
erally, the construction of aryl-substituted 2-haloethanols can
be performed through two single-step reactions, in which hy-
dration of haloalkynes gives a-halomethyl ketones and asym-
metric transformation of the a-halomethyl ketones provides 2-
haloethanols. Owing to the demands of atom economy and
minimal workup procedures, the facile construction of aryl-sub-
stituted 2-haloethanols followed by their transformation into
[a] Q. Ye, Dr. T. Cheng, Y. Zhao, J. Zhao, Dr. R. Jin, Prof. Dr. G. Liu
Key Laboratory of Resource Chemistry of Ministry of Education
Key Laboratory of Rare Earth Functional Materials
Department of Chemistry, Shanghai Normal University
No. 100 Guilin Rd. Shanghai 200234 (P.R. China)
E-mail: tycheng@shnu.edu.cn
ghliu@shnu.edu.cn
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500409.
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In the case of the second ATH reaction step, we utilized the
above optimal reaction conditions and screened the extensive-
ly studied Cp*M[(S,S)-TsDPEN] series A–C [h5-Cp*=pentame-
thylcyclopentadiene; M=Ru, Rh, Ir; TsDPEN=N-(p-toluenesul-
fonyl)-1,2-diphenylethylenediamine] and the areneRuTsDPEN
series D and E (h6-arene=p-cymene, 1,3,5-trimethylbenze-
ne).[10,5d,j,k] In this optimization of the catalyst, the ATH reaction
of 2-bromophenylethanone to (R)-2-bromophenylethanol was
performed in the presence of the IPrAuNTf2 (1.5 mol%) com-
plex. As shown in Table 1, catalyst E produced the target (R)-2-
Table 1. Optimization of catalysts for the ATH reaction of 2-bromoaceto-
phenone to (R) À2-bromo-1-phenylethanol.[a]
Scheme 1. One-pot organic transformation of alkynes into alcohols.
a one-pot, one-step cascade hydration–ATH reaction to over-
come the intolerance of the substrate is of great fundamental
and practical interest for the construction of chiral b-adrener-
gic receptor agonists or blockers from achiral haloalkynes.
To obtain compatible reaction conditions for the one-pot,
one-step cascade hydration–ATH reaction of aromatic haloal-
kynes (Scheme 1) on the basis of our previous exploration in
asymmetric catalysis,[7] two single-step catalytic reactions, the
hydration of (bromoethynyl)benzene to 2-bromophenyletha-
none and the ATH reaction of 2-bromophenylethanone to (R)-
2-bromophenylethanol, were investigated separately. In the
first hydration reaction, owing to its air stability we used the
highly efficient [2,5-bis(2,6-diisopropylphenyl)cyclopent-3-en-1-
yl]{1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methylsulfonami-
do}gold (IPrAuNTf2[8]) catalyst reported for the hydration of hal-
oalkynes.[8a,b] Because of compatible considerations for the
second ATH step, we systemically optimized the reaction con-
ditions for the first hydration step[8a] through the addition of
four common hydrogen sources (HCOOH/NEt3, HCOONa/H2O,
iPrOH, and HCOOH) that are often employed in ATH reaction
systems. In this process, the first hydration reaction step was
performed through the use of 1,2-dichloroethane (DCE) as the
solvent and with the hydrogen sources as additives in separate
experiments; the mixtures were stirred at 208C for a certain
amount of time in a parallel manner. With the use of the
HCOOH/NEt3 (5:2) azeotropic mixture and HCOOH as additives,
enhanced yields (from 85%[8a] to more than 99%) were ob-
served, and they were attributed possibly to acid-promoted ac-
celeration of the reaction.[9] However, the use of HCOOH as an
additive did not result in a chiral product in the second ATH re-
action. Thus, the HCOOH/NEt3 (5:2) azeotropic mixture was de-
termined to be the optimal compatible solvent and the hydro-
gen source for the second ATH reaction. To our delight, in this
case, the amount of IPrAuNTf2 in the first hydration step could
be halved (from 3.0 to 1.5 mol%) and quantitative hydration
was still obtained.
Entry
Catalyst
t
[h]
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
A
B
C
D
E
6
6
6
4
4
95
95
96
96
99
94
82
70
97
98
[a] Reactions were performed with 2-bromoacetophenone (0.30 mmol),
water (0.90 mmol), HCOOH/NEt3 (5:2, 0.75 mL, 9.0 mmol), IPrAuNTf2
(4.50 mmol, 3.89 mg), and E (4.50 mmol, 2.80 mg) in DCE (3.0 mL) at 208C.
[b] Yield of isolated product. [c] Determined by HPLC with a Daicel Chiral-
cel OD-H column.
bromophenylethanol product with the highest enantiomeric
excess (ee) value in quantitative yield (Table 1, entry 5). Such an
ee value was markedly better than those obtained with the
Cp*MTsDPEN series (Table 1, entries 1–3) and slightly higher
than that obtained with cymeneRuTsDPEN (Table 1, entry 4).
Thereby, areneRuTsDPEN complex E (arene=1,3,5-trimethyl-
benzene) was identified as the optimal catalyst for the second
reaction.
Finally, the combination of both catalysts, IPrAuNTf2 and E,
in a one-pot cascade hydration–ATH process was explored. As
expected, both IPrAuNTf2 and E enabled the highly efficient
hydration–ATH of (bromoethynyl)benzene; (R)-2-bromopheny-
lethanol was afforded in 93% yield and the ee value was re-
tained (Table 2, entry 1). To confirm that areneRuTsDPEN (E)
was the optimal catalyst for the cascade reaction, the other
three hydrogen sources (HCOONa/H2O, iPrOH, and HCOOH) in-
stead of the HCOOH/NEt3 (5:2) azeotropic mixture were investi-
gated under the same conditions. The cascade reaction with
HCOONa/H2O as the hydrogen source did not occur, whereas
that with iPrOH as the hydrogen source only gave (R)-2-bromo-
phenylethanol in 42% yield with a very poor ee value. As pre-
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Table 2. One-pot cascade hydration–ATH reactions of aromatic halo-
alkynes to chiral aromatic halohydrins.[a]
Entry
1
X
Ar
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Ph
93
87
83
89
90
88
81
82
86
89
92
90
92
78
89
94
92
92
98
97
98
99
98
97
95
93
99
95
99
95
99
97
99
97
98
97
4-MeC6H4
4-MeOC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-O2NC6H4
4-F3CC6H4
3-FC6H4
3-ClC6H4
3-BrC6H4
3,4-Cl2C6H3
2-thienyl
2-naphthyl
Ph
9
10
11
12
13
14
15
16
17
18
Figure 2. Time course of the transformation of (bromoethynyl)benzene into
(R)-2-bromophenylethanone. Reactions were performed with (bromoethy-
nyl)benzene (1 equiv.), IPrAuNTf2 (1.5 mol%), E (1.5 mol%), and HCOOH/NEt3
(5:2, 30 equiv.) at 208C.
4-FC6H4
4-ClC6H4
3,4-F2C6H3
the maximum amount of B is produced to provide the target
(R)-2-bromophenylethanone (C) product. Evidence to support
this reaction time course came from a comparison of the two
single-step reaction times and the cascade reaction time, for
which the hydration of (bromoethynyl)benzene (A) to 2-bro-
mophenylethanone (B) required 14 h and the ATH reaction of
2-bromophenylethanone (B) reached completion to (R)-2-bro-
mophenylethanone within 4 h in two single-step reactions,
whereas the total reaction time in the cascade hydration–ATH
reaction was 14 h.
[a] Reactions were performed with haloalkyne (0.30 mmol), water
(0.90 mmol), HCOOH/NEt3 (5:2, 0.75 mL 9.0 mmol), IPrAuNTf2 (4.50 mmol,
3.89 mg), and E (4.50 mmol, 2.80 mg) in DCE (3.0 mL) at 208C for 14 h.
[b] Yield of isolated product. [c] Determined by HPLC with a Daicel Chiral-
cel OD-H, OJ-H, OB-H, or AS-H column.
sented in Xiao’s report,[6a] the reaction with HCOOH as the hy-
drogen source afforded 2-bromophenylethanone in 98% yield
as the intermediate; however, no target product was obtained.
Notably, the cascade reaction in this case could be driven to
completion to give the target (R)-2-bromophenylethanol prod-
uct in 92% yield with 98%ee through adjustment of the pH
value (pH 8.0) of the mixture by adding aqueous sodium hy-
droxide and by prolonging the reduction time; this suggests
the benefit of this one-pot, one-step cascade hydration–ATH
reaction. As a comprehensive result, the one-pot enantioselec-
tive cascade reaction with IPrAuNTf2 (1.5 mol%) and areneR-
uTsDPEN (E, 1.5 mol%) as catalysts, HCOOH/NEt3 (5:2) as the
hydrogen source, and DCE as the solvent performed at 208C
was determined as a compatible reaction system. More inter-
estingly, in sharp contrast to Xiao’s one-pot, two-step process,
this one-pot, one-step cascade hydration–ATH reaction is also
suitable for the substrates examined by Xiao, for which a repre-
sentative substrate, ethynylbenzene, was converted smoothly
into (R)-1-phenylethanol in 95% yield with 97%ee, which con-
firmed the superiority of this cascade hydration–ATH reaction.
Having established a clean cascade hydration–ATH reaction
of (bromoethynyl)benzene to (R)-2-bromophenylethanol, the
reaction time course was explored. As shown in Figure 2, the
hydration of (bromoethynyl)benzene (A) proceeds fast with
concomitant formation of 2-bromophenylethanone (B) in
a maximum yield of 33%. Subsequently, the ATH reaction of 2-
bromophenylethanone occurs quickly for the point at which
On the basis of this clean cascade hydration–ATH reaction,
one-pot transformations of aryl-substituted haloalkynes into
chiral aryl-substituted 2-haloethanols were investigated to test
the general applicability of this procedure with a series of sub-
strates. As shown in Table 2, in general, high yields and excel-
lent enantioselectivities were obtained for all of the tested sub-
strates. Also, the structures and electronic properties of sub-
stituents at the 3- and 4-positions of the Ar group did not
affect significantly the enantioselectivities, and various elec-
tron-withdrawing and electron-donating substituents on the
Ar group were equally efficient (Table 2, entries 2–12). More-
over, substrates bearing thienyl and naphthyl groups, 2-(bro-
moethynyl)thiophene (1m) and 2-(bromoethynyl)naphthalene
(1n), were also converted smoothly into the corresponding
bromohydrins in high yields with excellent enantioselectivities
(Table 2, entries 13 and 14).
In addition to aromatic bromoalkynes, this cascade hydra-
tion–ATH reaction was also expanded to the one-pot transfor-
mations of aryl-substituted chloroalkynes; four representative
aryl-substituted chloroalkynes were obtained successfully in
high yields with excellent enantioselectivities (Table 2, en-
tries 15–18). All these results indicate that this cascade hydra-
tion–ATH reaction could serve as a general approach to con-
struct various optically pure halohydrins.
An important aim of this cascade hydration–ATH reaction
was to realize the efficient preparation of biologically active
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Acknowledgements
Table 3. One-pot synthesis of b-adrenergic receptor blockers.[a]
We are grateful to the National Natural Science Foundation of
China (NSF) (21402120), the Shanghai Sciences and Technologies
Development Fund (13ZR1458700), the Shanghai Municipal Edu-
cation Commission (14YZ074, 13CG48, Young Teacher Training
Project), Specialized Research Fund for the Doctoral Program of
Higher Education (20133127120006), and Shanghai Key Labora-
tory of Chemical Biology for financial support.
Entry
Product
Yield
[%][b]
ee
[%][c]
1
2
3
86
71
78
99
99
99
Keywords: asymmetric catalysis
·
domino reactions
·
halohydrins · hydration · transfer hydrogenation
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kynes to a-halomethyl ketones and the Ru-catalyzed ATH con-
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thanols allowed the efficient construction of various aromatic
halohydrins with up to 99% enantioselectivity under mild reac-
tion conditions. Furthermore, taking use of the benefit of this
cascade hydration–ATH reaction, we prepared three biological-
ly active b-adrenergic receptor agonists through a one-pot se-
quential hydration–ATH and amination reaction to afford desir-
able chiral medicines in acceptable yields with excellent enan-
tioselectivities. The study described herein offers an interesting
approach for the practical construction of various biologically
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Received: April 14, 2015
Published online on May 28, 2015
ChemCatChem 2015, 7, 1801 – 1805
www.chemcatchem.org
1805
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