Hydrogen-Bonding-Assisted Br?nsted Acid and Gold Catalysis: Access to Both (E)- and (Z)-1,2-Haloalkenes via Hydrochlorination of Haloalkynes

Article abstract of DOI:10.1021/acscatal.7b03563

We have developed an efficient synthesis of both (Z)- and (E)-chlorohaloalkenes via hydrochlorination of haloalkynes, based on two distinct hydrogen-bond-network-assisted catalytic systems: Br?nsted acid catalysis and gold catalysis. Both systems offer high stereoselectivity, good chemical yields, and diverse functional group tolerance.

Full text of DOI:10.1021/acscatal.7b03563

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Article
Hydrogen Bonding Assisted Brønsted Acid and Gold Catalysis: Access
to both E- and Z- 1,2-Haloalkenes via Hydrochlorination of Haloalkynes
Xiaojun Zeng, Shiwen Liu, Gerald B. Hammond, and Bo Xu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03563 • Publication Date (Web): 15 Dec 2017
Downloaded from http://pubs.acs.org on December 16, 2017
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Hydrogen Bonding Assisted Brønsted Acid and Gold Cataly-
sis: Access to both E- and Z- 1,2-Haloalkenes via Hydrochlo-
rination of Haloalkynes
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Xiaojun Zeng,[a] Shiwen Liu, [a] Gerald B. Hammond[b], * and Bo Xua,*
[a]College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shang-
hai, China. [b]Department of Chemistry, University of Louisville, Louisville, KY 40292, USA
ABSTRACT: We have developed an efficient synthesis of both Z- and E- chlorohaloalkenes via hydrochlorination of haloal-
kynes, based on two distinct hydrogen bond network assisted catalytic systems: Brønsted acid catalysis and gold catalysis.
Both systems offer high stereoselectivity, good chemical yields, and diverse functional group tolerance.
KEYWORDS: Hydrogen bonding, gold catalysis, Brønsted Acid catalysis, hydrochlorination, haloalkynes
but poor regio- and stereoselectivies restrict their useful-
Alkenyl chlorides are important synthetic building blocks
ness.11 Given that the hydrochlorination of alkynes is a
that have been used extensively in cross-coupling reac-
well-established method to prepare chloroalkenes,12 the
tions1 such as Stille couplings2, Suzuki couplings3, So-
hydrochlorination of haloalkynes should be a straightfor-
nogashira couplings4 and Buchwald-Hartwig aminations.5
ward method to prepare 1,2-chlorohaloalkenes.13 Indeed,
In addition, alkenyl chlorides are commonly found in bio-
Zhu and co-workers14 described a clever route to (Z)-1,2-
logically active natural products.6,7 More specifically, 1,2-
chlorohaloalkenes via Pd catalyzed anti-hydrochlorination
chlorohaloalkenes are especially important synthons due
of haloalkynes (Scheme 1b). Ochiai and co-workers re-
to presence of two synthetic handles.8 The possibility of
ported a metal-free Michael type addition of halides to
consecutive transition-metal-catalyzed cross-coupling re-
alkynyl(phenyl)iodonium tetrafluoroborates (Scheme
actions permits stereocontrolled synthesis of highly sub-
1c).11b However, these methods suffer from moderate
stituted alkenes.8 For examples, 1,2-chlorohaloalkenes
chemical yields, and only provide access to anti-addition
have been exploited as key building blocks en route to nat-
products. Herein we present two distinct hydrogen bond
ural products Pan-Bcl-2-Inhibitor9 and chartellines10 (Fig-
network assisted catalytic systems that offer selective ac-
ure 1).
cess to both (E)-and (Z)-chlorohaloalkenes, based on
Brønsted acid and gold catalysis15 (Scheme 1d).
Figure 1. Alkenyl chloride-containing natural products
that can be prepared from 1,2-chlorohaloalkenes.
1,2-Chlorohaloalkenes can be prepared via directly halo-
genation of alkynes (Scheme 1a).11 These reactions usually
give anti addition products via a halonium intermediate,
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a Conditions: 1a (0.2 mmol), HCl source (0.4 mmol), DCM/HFIP (3:1, 0.4
mL), 8 h. b 5 equiv.
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9
We then focused our attention on obtaining the anti-
addition isomer using transition metal catalysis. Firstly, we
evaluated several literature-known alkyne hydrochlorina-
tion catalytic systems, and found that neither TiO2/Au12e
nor CpRuCl(cod)/PPh312d gave good yields and stereoselec-
tivity (Table 1, entries 8-9). Recently, we reported an effi-
cient homogeneous gold-catalyzed hydrochlorination pro-
cedure,19 but when the same conditions were used in this
case, 3a was obtained in low yields, although good stere-
oselectivity was observed (Table 1, entries 10-11). We
reasoned that we could enhance the reactivity by adding
hydrogen bond donor HFIP as a part of the solvent mix-
ture, but this resulted in an E/Z mixture (Table 1, entry
12), possibly due to competition of the Brønsted-acid-
catalyzed process (see Table 1, entry 4). This prompted us
to turn to alternative chloride sources. To our satisfaction,
a combination of LiCl/AcOH with JohnPhosAuCl dramati-
cally increased both chemical yield and anti-selectivity
(Table 1, entry 13). We also investigated chlorination sys-
tems using in situ generated HCl, but they were less effec-
tive (Table 1, entries 14-15).
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Scheme 1. Existing methods for the synthesis of 1,2-
chlorohaloalkenes.
We first tested our newly developed hydrochlorination
reagent DMPU/HCl on model substrate bromoalkyne 1a
(Table 1).16 Although HCl is a strong acid, almost no reac-
tion took place when 1a was treated with DMPU/HCl alone
(Table 1, entries 1-3). Strong hydrogen-bond donating
solvents such as HFIP (hexafluoro-2-propanol) can form
hydrogen bond networks or clusters,17 which have been
shown to provide significant increases in the rate of many
reactions.18 To our delight, by using a two-component sol-
vent mixture of DCM/HFIP, an excellent yield of syn-
addition product 2a was obtained (Table 1, entry 4). Nota-
bly, lower reactivity and stereoselectivity were observed
when commercial solutions of HCl were used (Table 1,
entries 5-7).
With the optimized conditions in hand, we first evaluated
the hydrogen bonding assisted syn-hydrochlorinations
(Table 2). We found that iodo-, bromo- and chloroalkynes
were all suitable substrates (Table 2, 2a-c).20 Our condi-
tions exhibited high compatibility with halogens (F, Cl or
Br) (Table 2, 2d-f) and the acid sensitive acetyl group
(Table 2, 2g). Electron withdrawing groups such as esters
or ketones reduced the reactivity, and a higher tempera-
o
ture (70 C) was needed (Table 2, 2i-j). Furthermore, the
trifluoromethoxy group and the tert-butyl group were also
well tolerated, achieving yields of 80% and 75%, respec-
tively (Table 2, 2k-l). However, aliphatic haloalkynes
showed no reactivity under the same conditions.
Table 1. Optimization of the hydrochlorination of
bromoalkyne 1a.a
Table 2. Substrate scope of (E)-2-halo-1-chloroalkenes.
No. solvent
Catalyst
HCl Source
temp yield
oC
(2a:3a)
%
1
2
3
4
5
6
7
8
DCM
DCE
-
-
-
HCl/DMPU 50
HCl/DMPU 70
HCl/DMPU 80
trace
trace
trace
PhCH3
DCM/HFIP (3/1) -
HCl/DMPU 50 98 (20:1)
DCM/HFIP (3/1)
DCM/HFIP (3/1)
DCM/HFIP (3/1)
dioxane
-
-
-
HCl/Et2O
50
35 (4:1)
98 (8:1)
97 (11:1)
0
HCl/i-PrOH 50
HCl/dioxane 50
TiO2/Au, 3A MS HCl/dioxane 90
CpRuCl(cod)/
HCl/DMPU 50
PPh3
9
DCM
46 (1:0.6)
10 DCM
11 DCE
JohnPhosAuCl HCl/DMPU 50
JohnPhosAuCl HCl/DMPU 80
10
37 (1:17)
98 (1:0.5)
We next investigated the scope of the gold-catalyzed
anti-hydrochlorination of bromoalkynes (Table 3, entries
3a-3w). Due to higher reactivity and milder conditions,
this system offers outstanding functional group tolerance
and excellent chemical yields: aromatic alkynes substitut-
12 DCM/HFIP (3/1) JohnPhosAuCl HCl/DMPU 50
13 HOAc
14 DCE
15 DCE
JohnPhosAuCl LiClb
JohnPhosAuCl TMSClb/H2O 80
JohnPhosAuCl AcClb/H2O
80
80 97 (<1/99)
50 (1/ 49)
71 (1/ 8)
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ed with halogens in both the ortho and para positions
Cl
X
JohnPhosAuCl (5%)
LiCl (5 equiv), AcOH
X
R
1
(Table 3, 3b-e); acid sensitive groups such as esters (3f,
3j), nitriles (3h) and sulfonates (3k); ketones (3g), nitro
groups (3i), and ethers (3m) all worked very well. The
aromatic diyne 3l was also shown to be a suitable sub-
strate. In general, substrates with electron withdrawing
groups on the phenyl ring gave higher yields (3f-i) than
those with electron-donating groups such as 3m. Sub-
strates containing heteroaromatics such as thiophene (3n)
and pyridine (3o) also reacted efficiently, albeit with lower
stereoselectivity. Contrary to the Brønsted acid catalyzed
process, we also found aliphatic haloalkynes to be suitable
substrates with good to excellent yields (3p-w) in the gold-
catalyzed process. A variety of functional groups, including
benzyl groups (3p), phthalimides (3q), alkenes (3t), alco-
hols (3u), and carboxylic acids (3v) were all well tolerated.
Even the cholesterol-tethered substrate 3w gave a high
yield and selectivity. We also explored the substrate scope
of chloroalkynes (3x-3ab). It should be noted that some
functional groups such as -OTs and -OH were chlorinated
or esterified (3s, 3u) under the reaction condition. While
chloroalkynes are less reactive, thus requiring high tem-
R
80 or 100 o
C
1
H
3
2
3
4
5
6
7
8
9
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Br
Br
Br
Ph
3a
H
H
H
H
Cl
H
Br
F
3b
, 94%
, 94%
Cl
3c, 95%
3d
, 89%
Cl
3e
, 93%
Cl
Br
Cl
Br
Br
Br
H
H
O
H
H
H
AcO
O2N
NC
O
OMe
3j
, 94%
Cl
3i, 92%
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3g
, 94%
3h, 91%
3f,
96%
Cl
Cl
Cl
Br
Br
Br
Br
Cl
S
H
H
MeO
H
Br
H
Br
N
OMs
3k
Cl
3l, 98%
H
3o
,
89% (E/Z = 1/1)
3m, 56%
3n
, 85% (E/Z = 8/1)
, 95%
Cl
Br
O
Cl
Cl
Br
H
H
Br
BnO
N
R
R = Cl
Br
Cl
, 97%
H
H
b
3p
, 84%
Cl
O
3s
, 94%
3r
3q
, 90%
Br
Cl
O
Cl
Br
Br
o
H
R
peratures (100 C), the excellent yields and exclusive anti-
HO
R = OAc
3u, 84%c
H
3t, 90%
H
selectivity were maintained. Various functional groups
such as tert-butyl (3y), trifluoromethoxyl (3z), and chloro-
alkyl (3ab) were well tolerated. We also tested reactions of
iodoalkynes, but they did not give clean transformations.
3v, 92%
Cl
Cl
Cl
Cl
Cl
O
Ph
H
Br
H
O
Table 3. Scope of gold-catalyzed hydrochlorination of
haloalkynes.
3x
, 98%
3y, 95%
3w
, 94%
H
Cl
Cl
Cl
Cl
Cl
Cl
H
Cl
H
F3CO
H
3aa, 96%
3z
3ab, 95%
, 94%
aReaction conditions: haloalkynes (0.2 mmol), LiCl (1.0 mmol), JohnPhosAuCl (5 mol%)
and HOAc (0.5 mL), 80 oC (for bromoalkynes) or 100 oC (for chloroalkynes). b Start from
1s (R = -OTs). c Start from 1u (R = -OH).
1
The proposed mechanism is shown in Scheme 2.21 The
hydrogen bonding cluster generated from HFIP forms a
hydrogen-bonding complex with HCl (A) and grants the
HCl increased acidity, facilitating the rate-determining
proton transfer step (Scheme 2a). This gives rise to inter-
mediate vinyl cation B,22 which should have a linear geom-
etry,23 which we confirmed with DFT calculations (Scheme
2a). Orbitals from the bromine atom participate in stabiliz-
ing the LUMO of B. Attack at the upper face is hindered by
the bulky bromine atom, thus favoring chloride attack syn
to the H atom24
On the other hand, the gold-catalyzed process gave the
expected anti-selectivity, consistent with typical homoge-
neous gold catalysis (Scheme 2b).25 Contrary to traditional
silver-based cationic gold catalysis, we used hydrogen
bonding network to activate cationic gold catalyst precur-
sor (L-Au-Cl).26 We proposed that the H-bond network
generated from acetic acid might weaken the Au-Cl bond,
thereby generating a gold species [Au] with a partial posi-
tive charge that coordinated to the haloalkyne (Scheme
2b). The chloride nucleophile would then attack the gold-
activated haloalkyne to generate a vinyl—gold intermedi-
ate C in an anti-manner. Subsequent protodeauration of C
by (AcOH)n would therefore afford the anti-product.27 This
is consistent with our deuterium-labeling experiment: us-
ing d4-AcOH as the solvent, the deuterated product was
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obtained in 92% yield (eq 1). This result indicates that
AcOH is the proton source, hence, this method could be
used to synthesize other deuterium labeled haloalkenes.
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Scheme 3. Larger-scale reactions and further trans-
formations of Z- and E- 1,2-haloalkens.
In summary, we have developed a widely applicable, high-
ly efficient synthesis of both Z- and E- 1,2-haloalkenes via
hydrochlorination of haloalkynes. Other applications of
hydrogen bonding network-assisted Brønsted acid and
gold catalytic systems are currently being investigated in
our laboratories.
AUTHOR INFORMATION
Corresponding Author
Scheme 2. Proposed mechanism. LUMO of vinyl cation
was calculated at B3LYP/6-311+G(2df,2p) level of the-
ory.
* Email: bo.xu@dhu.edu.cn; gb.hammond@louisville.edu
SUPPORTING INFORMATION
Experimental details and copies of NMR spectra. The Support-
ing Information is available free of charge on the ACS Publica-
tions website.
ACKNOWLEDGMENT
Our protocol is scalable: similarly good yields were ob-
tained at a larger scale (Scheme 3a). Alkenyl chlorides
have become increasingly suitable substrates in cross-
coupling reactions due to the advances in catalyst design.28
Taking advantage of the difference in reactivity between
chloro-and bromo/chloro-alkenes, we were able to con-
duct sequential cross-coupling reactions to synthesize
both E- and Z- tri-substituted alkenes: Sonogashira4b and
Suzuki-Miyaura29 coupling reactions provided an efficient
and selective way to make both Z- and E- enynes (Scheme
3b-c).14 What’s more, our method represents a simple way
of accessing chlorinated 1,3-dienes such as 3t (Table 3),
which can be further transformed into multifunctional
aromatics in good yields via tandem Diels–Alder-
aromatization.14
We are grateful to the National Science Foundation of China
(NSFC-21472018). G.B.H. is grateful to the National Institutes
of Health (R01GM121660). X.Z is grateful to the China Schol-
arship Council, and to Mr. Daniel M. Sedgwick for assisting in
the preliminary editing of the manuscript.
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