Cross-coupling reactivity of 1,1-dichloroalkenes under palladium catalysis: Domino synthesis of diarylalkynes

Article abstract of DOI:10.1039/c7nj05107g

An efficient synthesis of diarylalkynes was achieved from the domino cross-coupling reaction of 1,1-dichloroalkenes with triarylbismuth reagents under palladium-catalyzed conditions. Under the established palladium protocol, 1,1-dichloroalkenes demonstrated hitherto unknown remarkable cross-coupling reactivity with organometallic triarylbismuth reagents to furnish functionalized diarylalkynes.

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Cross-coupling reactivity of 1,1-dichloroalkenes
under palladium catalysis: domino synthesis of
diarylalkynes†
Cite this:DOI: 10.1039/c7nj05107g
L. N. Rao Maddali * and Suresh Meka
Received 24th December 2017,
Accepted 5th February 2018
An efficient synthesis of diarylalkynes was achieved from the domino cross-coupling reaction of
1,1-dichloroalkenes with triarylbismuth reagents under palladium-catalyzed conditions. Under the
established palladium protocol, 1,1-dichloroalkenes demonstrated hitherto unknown remarkable cross-
coupling reactivity with organometallic triarylbismuth reagents to furnish functionalized diarylalkynes.
DOI: 10.1039/c7nj05107g
rsc.li/njc
Introduction
Substituted acetylenes serve as indispensable scaffolds due to
their broader applicability in the synthesis of natural products
and pharmaceuticals in addition to material applications.1 Some
of the known synthetic methods2–8 for the preparation of internal
alkynes, given in Scheme 1, utilize metal-catalyzed (i) coupling of
a terminal alkyne with an aryl halide, popularly known as the
Sonogashira reaction;2 (ii) decarboxylative coupling of alkynoic
acid with an aryl halide;3 (iii) coupling of an alkynyliodonium salt
with arylboronic acid;4 (iv) alkynyl organometallic (B, Al, Sn, Mg
or Li) reagent coupling with an aryl halide;5 (v) 1-haloalkyne
coupling with arylboronic acid;6 (vi) alkynylsulfone coupling with
a Grignard reagent7 and (vii) coupling of 1,1-dibromoalkene with
an arylorganometallic (B, Sn, Bi etc.) reagent.8 The majority of
these methods invoke alkynyl-derived both electrophilic and
nucleophilic coupling partners in conjunction with comple-
mentary aryl coupling reagents to generate internal alkynes.
This essentially involves the conversion of a terminal alkyne to
an internal alkyne through derivatization to either an electro-
philic or nucleophilic reagent followed by its coupling with a
complementary aryl derived partner. However, alleviating the
direct usage of a terminal alkyne or its derivative with viable
alternatives for the preparation of an internal alkyne would be
synthetically more useful. However, methods known for the
Scheme 1 Various synthetic methods for internal alkynes.
alternative synthesis of internal alkynes without the direct (Scheme 1, vii).8,9 Hence, the development of a new method-
involvement of alkynyl derivatives are not many, except for ology employing 1,1-dihaloalkenes is of prime importance. In this
1,1-dihaloalkene coupling with aryl organometallic reagents context, the reactivity of 1,1-dibromoalkene in domino couplings
with different organometallic reagents involving organo boron,
tin and bismuth reagents has been well studied.8 However, the
corresponding studies with 1,1-dichloroalkenes are relatively less
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208 016,
India. E-mail: maddali@iitk.ac.in; Fax: +91 (0)512 259 7532;
known, including (i) Pd-catalyzed stereoselective monoalkylation
Tel: +91 (0)512 259 7532
and arylation with Grignard or organozinc reagents;10a,b (ii) tandem
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7nj05107g
arylations followed by alkylations,10c (iii) the synthesis of Z-chloro
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Table 1 Screening conditionsa,b,c
alkenes using 9-alkyl-9-BBN10d and (iv) iron-catalyzed bis-
couplings with Grignard reagents.10e
Further, Pd-catalyzed C–H bond alkynylations of heteroarenes
and the construction of heterocycles by cascade reactions are
known.11 However, the domino synthesis of internal alkynes with
1,1-dichloroalkenes has not had broader attention except for two
minor observations, as given in Scheme 2. In one case, the cross-
coupling reaction of 1,1-dichloroalkene with an organozinc reagent
was reported under Pd-catalyzed conditions ((i), Scheme 2).10c
Similar couplings with p-tolylboronic acid under Cu-catalyzed
conditions have been reported with two examples ((ii), Scheme 2).12
Catalyst
Ligand
Base
Additive
Yield
Entry (0.09 equiv.) (0.36 equiv.) (6 equiv.) (equiv.) Solvent (%)
1
2
3
4
5
6
7
8
Pd(PPh3)4
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
Pd(PPh3)4
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
None
—
K3PO4
K3PO4
—
—
—
—
—
DMF
DMF
DMF
DMF
DMF
41
42
PPh3
PPh3
PPh3
dppf
PPh3
PPh3
PPh3
PPh3
PPh3
—
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
—
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K3PO4
59
62d
59e
63
TBAB (3) DMF
TBAB (3) DMSO 70
TBAB (3) DMSO 67d
TBAB (1) DMSO 75
TBAB (1) DMSO 65 f
TBAB (1) DMSO 65
TBAB (1) DMSO 53g
TBAB (1) DMSO 27h
TBAB (1) DMSO 42
TBAB (1) DMSO 25
9
10
11
12
13
14
15
16
17
18
19
K2CO3
NaHCO3 TBAB (1) DMSO 20
Cs2CO3
Cs2CO3
None
TBAB (1) DMSO 44i
TBAB (1) DMSO
TBAB (1) DMSO
—
—
PdCl2
PPh3
a
Reaction conditions: 1a (0.75 mmol, 3 equiv.), BiAr3 (0.25 mmol,
1 equiv.), Pd catalyst (0.09 equiv.), ligand (0.36 equiv.), base (6 equiv.),
b
additive (3 or 1 equiv.), solvent (3 mL), 90 1C, 2 h. Isolated yields.
c
d
e
Biaryl side product formed in minor amount. At 110 1C. With dppf
f
g
h
i
(0.18 equiv.). 3 h. With base (4 equiv.). At 60 1C. For 1 h.
Scheme 2 Synthesis of internal alkynes with 1,1-dichloroalkenes.
Pd(PPh3)4 (0.09 equiv.) in DMF at 90 1C for 2 hours. This
protocol furnished the desired diarylalkyne (2.2) in 41% yield
(entry 1, Table 1) involving the domino coupling process of
1,1-dichloroalkene with the triarylbismuth reagent. Further use
of the PdCl2/PPh3 catalytic system did not improve the yield
significantly (entry 2, Table 1). Marginal improvement in the
yield was observed with Cs2CO3 at different temperatures
(entries 3 and 4, Table 1). An additional check with dppf ligand
failed to improve the yield (entry 5, Table 1). The cross-coupling
efficiency was reasonably increased in the presence of a TBAB
additive (entries 6 and 7, Table 1). Further changes in reaction
conditions to 110 1C proved to be ineffective (entry 8, Table 1).
However, the domino cross-coupling was proved to be more
effective in the presence of 1 equiv. of additive with 75% yield
(entry 9, Table 1). Further screening with 3 h reaction time
gave a lower yield (entry 10, Table 1). Coupling with catalytic
Pd(PPh3)4 proved to be detrimental (entry 11, Table 1).
Lowering either the amount of base (entry 12, Table 1) or reaction
temperature (entry 13, Table 1) was found to be disadvantageous.
Additional screening with K3PO4, K2CO3 and NaHCO3 to check
their efficacy provided inferior results (entries 14–16, Table 1).
The domino coupling with 1 h as reaction time gave a lower
The relatively less explored reactivity of 1,1-dichloroalkenes led
us to investigate their viability in domino couplings for further
functionalizations. Our research group has been involved in
the development of several new protocols and reactions with
1,1-dihaloalkenes for various synthetic functionalizations.13 Hence,
it was decided to investigate the potential domino reactivity of
1,1-dichloroalkenes in couplings with triarylbismuth reagents
under Pd-catalyzed conditions for the direct synthesis of internal
alkynes. Synthetically 1,1-dichloroalkenes can be easily obtained
from aldehydes using CCl4/PPh3 conditions.14 Applications of
triarylbismuths as privileged reagents are well known in the
literature.15 In particular, triarylbismuths serve as threefold
coupling reagents with atom-economic advantage in cross-
coupling reactions.16 With this background, we investigated the
domino coupling reactivity of 1,1-dichloroalkenes to develop
hitherto unknown coupling reactions of 1,1-dichloroalkenes
in the synthesis of internal alkynes under palladium coupling
conditions. These investigations are elaborated hereafter.
Results and discussion
The cross-coupling reaction of 1,1-dichloroalkene (1a) was inves- yield (entry 17, Table 1). The desired cross-coupling product was
tigated in conjunction with tri-p-tolylbismuth as a nucleophilic not formed without catalyst or in basic conditions (entries 18
coupling partner under different palladium coupling reaction and 19, Table 1). In summary, the above investigation provided an
conditions (Table 1).
optimized protocol with PdCl2 (0.09 equiv.) /4 PPh3 (0.36 equiv.)
A preliminary study was carried out using 1a (3 equiv.) with involving Cs2CO3 (6 equiv.) and TBAB (1 equiv.) in DMSO at 90 1C
tri-p-tolylbismuth (1 equiv.) in the presence of K3PO4 base and and for 2 h (entry 9, Table 1). During this screening, the formation
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of a homo-coupled biaryl from triarylbismuth as a side product functionalized with 4-methyl, 4-methoxy and 4-isopropoxy groups
was obtained in minor amounts and this is inevitable under gave unsymmetrical diarylalkynes 2.2, 2.9 and 2.7 in 63–75%
metal-catalyzed conditions.15 Significantly, the above screening yields. A similar efficient reactivity of 1a was witnessed with
helped to realize hitherto unknown cross-coupling reaction of trinaphthalen-2-ylbismuth to give a mixed diarylalkyne (2.6) in
1,1-dichloroalkene with triarylbismuth reagent with an efficient 73% yield. The domino reactivity of 1a with tri-m-tolylbismuth
delivery of diarylalkyne under viable palladium coupling conditions. and tris(3-methoxyphenyl)bismuth also afforded the diaryl-
To realize general applicability, the above protocol was alkynes 2.8 and 2.10 in 66% and 68% yields, respectively. This study
investigated with 1,1-dichloroalkene (1a) against electronically revealed the facile cross-coupling reactivity of 1,1-dichloroalkene (1a)
different triarylbismuth reagents and the results are summar- with various triarylbismuth reagents under the optimized protocol
ized in Table 2. Under the established protocol conditions, conditions.
1,1-dichloroalkene 1a reacted efficiently with triphenylbismuth
Further elaboration of the scope was examined using electro-
to afford unsymmetrically substituted diarylalkyne (2.1) in 71% nically variant 1,1-dichloroalkenes and triarylbismuth reagents
yield. The domino cross-coupling study of 1a with electronically (Table 3). Each one of the 1,1-dichloroalkenes was tested against
deficient triphenylbismuths functionalized with 4-F, 4-Cl and different triarylbismuths under the established conditions. This
3-Cl groups afforded the corresponding unsymmetrically substi- work furnished an efficient cross-coupling reactivity of various
tuted diarylalkynes (2.3–2.5) in 45–73% yields. The corresponding 1,1-dichloroalkenes. Elaborately, the general coupling ability of
reactivity of 1a with electronically rich triphenylbismuths (2,2-dichlorovinyl)benzene (1b) was proved to be viable with
different triarylbismuth reagents to afford diarylalkynes (3.1–3.4)
in 50–78% yields. Furthermore, 1,1-dichloroalkenes 1c and 1d
functionalized with electron withdrawing 4-CN and 4-NO2 reacted
efficiently to afford diarylalkynes (3.5–3.11) in moderate to good
yields. In addition, electronically rich 1,1-dichloroalkenes
(1e and 1g) also fared well to give diarylalkynes (3.12–3.16
and 3.21–3.23) in 41–62% and 38–59% yields, respectively.
The facile domino cross-coupling reactivity of 2-(2,2-dichloro-
vinyl)naphthalene (1f) afforded the corresponding mixed diaryl-
alkynes (3.17–3.20) in 57–69% yields. Interestingly, the coupling
reactivity of p-bromo and p-chloro functionalized 1,1-dichloroalkenes
(1h and 1i) demonstrated chemo-selective reactivity to give the
corresponding bromo or chloro functionalized diarylalkynes 3.24
and 3.25 in 69% and 67% yields. In these cases, the expected cross-
coupling reaction with the bromo or chloro aryl terminus of 1h or 1i
was not observed.16d
Our established domino cross-coupling process was not
restricted to aryl systems alone as the corresponding high
reactivity was observed with heteroaryl furanyl and thienyl
derived 1,1-dichloroalkenes (1j and 1k). In these cases, the
corresponding heteroaryl/aryl substituted alkynes 3.26–3.30 were
obtained in 42–62% yields. Overall, this study employing a
variety of aryl and heteroaryl 1,1-dichloroalkenes with triaryl-
bismuth reagents allowed the synthesis of a library of variously
functionalized internal alkynes under palladium-catalyzed
conditions.
It is worth highlighting that the viable formation of diaryl-
alkyne from 1,1-dichloroalkene and triarylbismuth reagents
under domino cross-coupling conditions could be similar to
what is known for 1,1-dibromoalkenes under metal-catalyzed
conditions.8 Accordingly the probable mechanism is given in
Scheme 3.
To confirm the above proposal, a few control reactions were
performed. Firstly, the formation of 1-chloroalkyne from
1,1-dichloroalkene under base-mediated conditions was reported
in the literature.11a,14a It was investigated under our domino
coupling conditions but in the absence of triarylbismuth
reagent. In this case, 1-chloroalkyne (4b) was formed from
1,1-dichloroalkene (4a) as a base-mediated elimination product
Table 2 Domino couplings of 1a with BiAr3 reagentsa,b,c
BiAr3
Product
Entry
1
Yield (%)
71
2
3
4
75
73
51
5
45
6
7
8
9
73
63
66
72
68
10
a
Reaction conditions: 1a (0.75 mmol, 3 equiv.), BiAr3 (0.25 mmol,
1 equiv.), PdCl2 (0.0225 mmol, 0.09 equiv.), PPh3 (0.09 mmol, 0.36 equiv.),
Cs2CO3 (1.5 mmol, 6 equiv.), TBAB (0.25 mmol, 1 equiv.) and DMSO
b
(3 mL), 90 1C, 2 h. Isolated yields based on three aryl couplings from
c
BiAr3 reagents. Biaryl side product in minor amount.
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Table 3 Domino couplings of different 1,1-dichloroalkenes with BiAr3 reagentsa,b,c
BiAr3
Product
Entry
1
1,1-Dichloroalkene
Yield (%)
74
2
3
4
5
6
7
8
78
50
71
50
75
70
51
9
51
55
10
11
12
13
14
45
61
61
53
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Table 3 (continued)
BiAr3
Product
Entry
15
1,1-Dichloroalkene
Yield (%)
41
16
17
62
63
18
19
20
69
66
57
21
22
23
59
55
38
24
25
26
69
67
61
27
62
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Table 3 (continued)
BiAr3
Product
Entry
28
1,1-Dichloroalkene
Yield (%)
62
29
42
44
30
a
Reaction conditions: 1,1-dichloroalkenes (0.75 mmol, 3 equiv.), BiAr3 (0.25 mmol, 1 equiv.), PdCl2b(0.0225 mmol, 0.09 equiv.), PPh3 (0.09 mmol,
0.36 equiv.), Cs2CO3 (1.5 mmol, 6 equiv.), TBAB (0.25 mmol, 1 equiv.) and DMSO (3 mL), 90 1C, 2 h. Isolated yields based on three aryl couplings
from BiAr3 reagents. Biaryl side product in minor amount.
c
in high yields. Additionally, chemo-selective domino couplings with
aryl bromo and chloro substituted 1,1-dichloroalkenes furnished
halogen functionalized diarylalkynes in good yields.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Scheme 3 Proposed mechanistic cycle.
Suresh Meka acknowledges the Council of Scientific and
Industrial Research (CSIR), New Delhi for the research fellow-
ship. The authors also acknowledge financial support from the
Council of Scientific and Industrial Research (CSIR), New Delhi
[02(0091)/12/EMR-II].
in 55% yield. In this reaction, a homo-coupled 1,3-diyne from
4b was obtained in 22% yield. During the screening studies
(entry 19, Table 1), a control reaction without base did not afford
the 1-chloroalkyne (4b) and cross-coupling product (4e). A con-
trol reaction without catalyst also furnished the 1-chloroalkyne
(4b). Based on this, the proposed catalytic cycle was expected
Notes and references
to involve the initial formation of 1-chloroalkyne (4b) followed
by its involvement in the Pd-catalyzed cross-coupling process
through oxidative addition (4c), transmetallation (4d) and reduc-
tive elimination to give diarylalkyne (4e). The repeated involve-
ment of the arylbismuth species in the transmetallation step
accounts for the observed threefold coupling from the triaryl-
bismuth reagent.15
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We have developed for the first time a palladium-catalyzed
domino synthesis of diarylalkynes with the combination of
1,1-dichloroalkenes and triarylbismuth reagents. Under the
established palladium-catalyzed conditions, this study describes
the remarkable reactivity of 1,1-dichloroalkenes to give diarylalkynes
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