A Bronsted acid-catalyzed generation of palladium complexes: Efficient head-to-tail dimerization of alkynes

Article abstract of DOI:10.1039/c3cc43131b

A Bronsted acid Ph₂P(O)OH can efficiently catalyze the reaction of a Pd(0) complex with an alkyne to produce a novel alkenyl(alkynyl)palladium complex via selective hydropalladation and ligand exchange processes. On the basis of this finding, a

Full text of DOI:10.1039/c3cc43131b

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A Brønsted acid-catalyzed generation of palladium
complexes: efficient head-to-tail dimerization
of alkynes†
Cite this: Chem. Commun., 2013,
49, 7498
Received 27th April 2013,
Accepted 18th June 2013
Tieqiao Chen,ab Cancheng Guo,*a Midori Gotob and Li-Biao Han*ab
DOI: 10.1039/c3cc43131b
www.rsc.org/chemcomm
A Brønsted acid Ph2P(O)OH can efficiently catalyze the reaction of a Pd(0)
complex with an alkyne to produce a novel alkenyl(alkynyl)palladium
complex via selective hydropalladation and ligand exchange processes.
On the basis of this finding, an efficient Pd(0)/Ph2P(O)OH mediated
head-to-tail dimerization of alkynes was disclosed.
Palladium complexes can efficiently catalyze a variety of chemical
transformations and are the most popular catalysts used in
modern organic synthesis.1 Herein we disclose an unprecedented
Brønsted acid catalyzed synthesis of alkenyl(alkynyl)palladium
Scheme 1 Metal-catalyzed head-to-tail dimerizations of alkynes.
complexes 1, important intermediates for the formation of mechanistic aspects related to the catalysis are rather ambiguous and
sp2C–spC bonds (eqn (1)).1–4 There are thousands of palladium controversial (Scheme 1).2a,d,3a,b,6e For example, it was suggested that
complex-catalyzed reactions. However, to our knowledge, such a a hydridometal species 4 formed via the activation of the spC–H bond
catalytic preparation of palladium complexes seems unknown. of acetylene was the intermediate, which can subsequently add to
This finding also leads to the disclosure of a highly selective head- acetylene to give either 1 by hydrometallation or 5 by carbometalla-
to-tail dimerization of alkynes affording 3 catalyzed by a combi- tion.2a,d,3a,b,9 However, no clear evidence supporting the presence of
nation of Pd(0)/Ph2P(O)OH (Scheme 1).
such intermediates 1, 4 and 5 in the reactions was obtained.2d,3b
Careful mechanistic studies of the current Pd/Brønsted acid
mediated selective head-to-tail dimerization unambiguously
revealed the reaction paths which are different from those
proposed before i.e. firstly, a selective hydropalladation of
alkynes takes place to form an alkenylpalladium 2 which then
affords 1 via a ligand exchange process. Subsequent reductive
elimination of 1 gives the head-to-tail conjugated dimer 3.
An equimolar mixture of phenylacetylene, Ph2P(O)OH and
Pd(PEt3)4 was dissolved in C6D6 at room temperature (0.05 mmol
in 0.5 mL C6D6). The colour of the solution gradually changed
from brown to light blue. Two characteristic vinyl protons were
observed at 5.09 (s, 1H) and 6.02 (t, 1H, JP–H = 3.6 Hz) by 1H NMR
spectroscopy while at the same time two new signals were also
observed by 31P NMR spectroscopy at 19.22 and 10.72, indicative
of the formation of a vinylpalladium complex 2a via selective
hydropalladation of phenylacetylene.7 Removal of the volatiles
in vacuo afforded complex 2a as a white solid. Good crystals
suitable for X-ray analysis were obtained by recrystallization from
a toluene–hexane mixed solvent at À30 1C and the structure of 2a
was unambiguously confirmed (Fig. 1). It is worth noting that an
internal alkyne with an electron withdrawing phosphoryl group
was also hydropalladated regio and stereoselectively generating
(1)
Transition metal catalyzed dimerization of alkynes, as exemplified
by Scheme 1, is a straightforward and atom-economic approach to
conjugated enynes 3 which are not only useful building blocks in
organic synthesis but also themselves found as the key units in
various natural products such as histrionicotoxin and elatenyne.5,6
This reaction is currently extensively exploited with a variety of metal
catalysts in order to find more efficient and selective ways for the
dimerization of alkynes.2,3 However, despite its high importance,
a College of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, China. E-mail: ccguo@hnu.edu.cn; Fax: +86-731-8821488;
Tel: +86-731-8821314
b National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba,
Ibaraki 305-8565, Japan. E-mail: libiao-han@aist.go.jp; Fax: +81-298-61-4511;
Tel: +81-298-61-4855
† Electronic supplementary information (ESI) available: Experimental proce-
dures, CIF files and CifCheck files of 1f and 2a, and characterization data. CCDC
918882 and 918883. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3cc43131b
c
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Scheme 2 Ph2P(O)OH-catalyzed generation of complexes 1.
Fig. 1 ORTEP drawings of complexes 2a and 1f. Thermal ellipsoids are drawn at
50% probability. H atoms are omitted for clarity.
a trans manner. It was noted that alkenyl(alkynyl)palladium com-
plexes 1 represents the first palladium complexes bearing sp2C–Pd
and spC–Pd bonds generated via direct hydropalladation of alkynes.
Although this kind of complexes have long been proposed as
intermediates in the dimerization of alkynes2,3 and other reactions,4
they have never been obtained directly (Scheme 2).9
the corresponding 2b with palladium attached to the more
electronegative carbon (eqn (2)).
(2)
These stoichiometric reactions indicate the possibility of a highly
selective head-to-tail dimerization of alkynes catalyzed by the combi-
nation of Pd(0)/Ph2P(O)OH. This indeed was the case. As shown in
Table 1, in the presence of 0.25 mol% Pd2(dba)3, 0.5 mol% dppe and
1 mol% Ph2P(O)OH, 1-octyne could be selectively converted to the
corresponding head-to-tail dimerization product 3a in 90% yield in
toluene at 80 1C (entry 1).10 Under the present catalytic conditions, a
variety of aliphatic alkynes could be employed as the substrates to give
the corresponding conjugated enynes in good yields (entries 1–7).
Valuable functional groups such as carboxylic ester (entry 2), chloro
(entry 6) and nitrile (entry 7) were all compatible with the present
system. In the presence of the amino group, the dimerization of
dibutylpropargylamine progresses slowly and more catalysts were used
(entry 5). The expected product 3c was also obtained in high yield
from the bulky t-butylacetylene (entry 3). Compared to alkyl alkynes,
selective dimerization of aromatic alkynes was complicated due to side
reactions such as trimerization and the polymerization of the product
enynes.2a,11 By carrying out the catalytic reaction at room tempera-
ture,11 aromatic terminal alkynes could also be readily dimerized to
give the corresponding enynes in good yields. Thus, the combination
of 0.5 mol% Pd2(dba)3, 1 mol% dppe and 2 mol% Ph2P(O)OH could
efficiently catalyze the head-to-tail dimerization of phenylacetylene in
Interestingly, it was found that ligand exchange between 2a and
phenylacetylene could take place easily at room temperature to
generate a novel alkenyl(alkynyl)palladium complex 1a (eqn (3)).8
Thus, an equimolar mixture of 2a and phenylacetylene (0.05 mmol)
was dissolved in 0.5 mL of toluene-d8 at room temperature. After a
few minutes, two new vinyl protons at 5.33 and 6.59 were observed by
1H NMR spectroscopy while a new singlet at 13.95 was also observed
by 31P NMR spectroscopy. As described later, these new signals are
due to the formation of a new palladium complex 1a. As estimated
1
from H NMR spectroscopy, 1a was generated in ca. 45% yield
after 2 h, showing that the ligand exchange between 2a and phenyl-
acetylene took place smoothly. Similarly, complex 2b could react with
phenylacetylene to produce 1b in ca. 75% yield after 1 h (eqn (3)).
(3)
Remarkably, a trace amount of Ph2P(O)OH could efficiently toluene at 25 1C to generate the corresponding conjugated enyne 3h in
catalyze the formation of complex 1a via the combination of 81% yield (entry 8). Other aromatic alkynes bearing electron-donating
hydropalladation and ligand exchange reactions as described above. groups could also be dimerized to the corresponding enynes selec-
Thus, in the presence of 1 mol% Ph2P(O)OH (based on Pd), 1 mmol tively in good yields (entries 9–13). p-Fluorophenylacetylene was also a
phenylacetylene and 0.5 mmol Pd(PEt3)4 smoothly reacted at 25 1C good substrate for this dimerization (entry 14). However, an aromatic
and selectively generated the corresponding vinyl(alkynyl)palladium alkyne bearing an electron-withdrawing group only produced the
1a quantitatively within 20 h. 1a is a pale yellow oil which enyne in a low yield because of severe side reactions (entry 15).
precipitates out when cooling down the solution at À78 1C (83%
On the other hand, a cross dimerization between a terminal
isolated yield). There is no need to mention that such a complex as alkyne and an internal alkyne could also be achieved by using this
1a could not be formed at all from phenylacetylene and Pd(PEt3)4 in Pd(0)/Ph2P(O)OH catalyst. For example, phenylacetylene reacted
the absence of Ph2P(O)OH. In order to clearly determine the smoothly with 4-phenylbut-3-yn-2-one and diethyl 2-phenylethynyl-
structure by X-ray analysis, the preparation of a solid 1 was phosphonate at 25 1C to generate the corresponding dimerization
thoroughly investigated. Unfortunately, although other alkynes such products 3p and 3q in high yields with high selectivity (Scheme 3).
as p-tolylacetylene, p-t-butylphenylacetylene, and p-phenylphenyl-
On the basis of the results of stoichiometric reactions (eqn (1)–(3)),
acetylene all gave the corresponding products in high yields, all this Pd(0)/Ph2P(O)OH mediated head-to-tail dimerization of terminal
the complexes were oils. Finally, when p-anisylacetylene was used as alkynes could be clearly rationalized to take place via a catalytic cycle
the substrate, the corresponding complex 1f was obtained as a involving the hydropalladation of alkynes with Ph2P(O)OH to generate
yellow solid in 70% isolated yield. Recrystallization of the solid from an alkenylpalladium complex 2, followed by ligand exchange of 2 with
hexane–toluene at À30 1C gave good crystals suitable for X-ray another molecule of alkyne to produce the vinyl(alkynyl)palladium
analysis, which, as depicted in Fig. 1, unambiguously confirmed the intermediate 1, which underwent reductive elimination to give the
structure showing an alkenyl and an alkynyl group attached to Pd in corresponding products enyne 3 (Scheme 4).12
c
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Table 1 Palladium(0)/Brønsted acid Ph2P(O)OH mediated head-to-tail dimer-
ization of terminal alkynesa
This work was partially supported by Fundamental Research
Funds for the Central Universities (Hunan university) and the
Canon Foundation.
Notes and references
Run
1
Alkynes
Product
Isolated yield (%)
90
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Scheme 4 Proposed mechanism for the Pd(0)/Ph2P(O)OH mediated head-to-
tail dimerization of alkynes.
In summary, by successful isolation and characterization of relevant 10 For more information, see ESI†-E.
11 The dimerization products from aromatic terminal alkynes easily
intermediates, we reported, for the first time, a novel Brønsted acid
catalyzed formation of alkenyl(alkynyl)palladiums from the reaction of
polymerize at high temperatures. For example, at 80 1C, all the
enynes polymerized and no 3h could be obtained.
a Pd(0) complex with alkynes, via efficient hydropalladation and sub- 12 Trost has reported that Pd(OAc)2/phosphine was an effective catalytic
system for the head-to-tail dimerization of alkynes.3a,b However, mecha-
sequent ligand exchange reactions. On the basis of this finding, selec-
nistic aspects were not clear. We guess the current mechanism is
applicable to Trost’s system since Pd(0)/acetic acid was also effective
tive head-to-tail dimerization of alkynes efficiently affording enynes
catalyzed by the combination of Pd(0)/Brønsted acid was developed.
for the dimerization of terminal alkynes (see ESI†-E).
c
7500 Chem. Commun., 2013, 49, 7498--7500
This journal is The Royal Society of Chemistry 2013