Selective homogeneous palladium(0)-catalyzed hydrogenation of alkynes to (Z)-alkenes

Article abstract of DOI:10.1002/(SICI)1521-3773(19991216)38:24<3715::AID-ANIE3715>3.0.CO;2-O

Zero-valent palladium precatalysts containing rigid bidentate bis(arylimino)acenaphthene ligands (shown schematically) facilitate the highly stereoselective homogeneous catalytic hydrogenation of alkynes to (Z)- alkenes. Internal, terminal, aryl-substituted, and cyclic alkynes are suitable substrates, as are some enynes, which are chemoselectively hydrogenated to dienes. E=CO₂Me; R¹, R² = 4-OCH₃, 4-CH₃, 2,6-(CH₃)₂.

Full text of DOI:10.1002/(SICI)1521-3773(19991216)38:24<3715::AID-ANIE3715>3.0.CO;2-O

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deficient alkene as the ancillary ligand;[11] however, analogues
of I with electron-rich alkenes are not stable. We reasoned
that since simple alkenes are expected to be readily substi-
tuted by alkynes, if alkynes were hydrogenated in the
presence of molecular hydrogen, subsequent fast substitution
by excess alkyne could give rise to a viable catalytic cycle
(Scheme 1).
Selective Homogeneous Palladium(0)-
Catalyzed Hydrogenation of Alkynes to (Z)-
Alkenes**
Martijn W. van Laren and Cornelis J. Elsevier*
The selective semihydrogenation of alkynes to (Z)-alkenes
seems to be a simple reaction, yet this type of conversion
remains a desirable synthetic tool. A variety of catalysts is
available for the conversion of alkynes into (Z)-alkenes; the
best known and most efficient ones are heterogeneous
catalysts such as the Lindlar catalyst,[1] nickel boride,[2] the
ªP2Niº catalyst,[3] and palladium immobilized on a clay.[4]
Especially the Lindlar catalyst suffers from a number of
major problems in the selective cis-hydrogenation of alkynes,
notably, partial isomerization of the (Z)-alkene product to the
(E)-alkene, shift of the double bond, overreduction to the
alkane, and problems with reproducibility. Only a few
examples are known of homogeneous catalyst systems that
show good selectivity for a wider range of substrates, for
example, rhodium-[5] and Cr(CO)3-catalyzed hydrogenation.[6]
For palladium, however, few homogeneous catalyst systems
are known, and these involve palladium(ii) complexes.[7, 8] The
only homogeneous palladium(0) catalyst is the complex
[Pd2(dppm)3], which shows low activity in the hydrogenation
of propyne and 2-butyne.[9]
Scheme 1. Proposed catalytic cycle for hydrogenation of alkynes.
Here we report the first example of a homogeneous
palladium(0) catalyst bearing a
We conducted hydrogenation reactions of alkynes at 208C
and 1 bar hydrogen pressure with initial sample concentra-
tions of about 0.5m in THF and 1 mol% of Ia as precatalyst.
With 3-hexyne (1) and 4-octyne (3), high stereoselectivity
(>99.5%) for the formation of (Z)-3-hexene and (Z)-4-
octyne was observed, and hydrogenation proceeded with
excellent yield. Various other alkynes were then subjected to
these hydrogenation conditions (Table 1).
bidentate nitrogen ligand that is
able to homogeneously hydro-
genate a wide variety of alkynes
to the corresponding (Z)-al-
kenes with very high selectivity.
Additionally, enynes are selec-
tively converted into dienes.
As part of our continuing
The stereoselectivity for the formation of the (Z)-alkene is
generally excellent and in many cases superior to that
obtained with Lindlar[1] or P2Ni catalysts[3] and comparable
to or in several cases better than those of known homoge-
neous systems.[5±7] Generally, quantitative conversion of the
alkyne to the desired (Z)-alkene takes place, without con-
comitant alkane formation. The selectivity is highest (Z/E >
99/1) for simple alkynes such as 1 and 3. The Lindlar catalyst
gives 95/5 and 94/6 mixtures of Z/E alkenes, respectively, and
only partial conversion in these cases.[1c, d] Excellent results
were also obtained for the homopropargylic alcohol 4, alkynes
with aryl- and/or electron-withdrawing substituents (5 ± 7, 10,
12), and cyclooctyne (13). For 7 the stereoselectivity was
determined by reaction with D2. Semihydrogenation of 7
usually results in the formation of ethylbenzene; an 89/11
mixture of styrene/ethylbenzene was obtained at full con-
version of 7.[1e]
Phenylacetylenes show somewhat lower but still very good
selectivities, comparable to conversions with Lindlar or P2Ni
catalysts, and for 8 and 9 only a small amount of alkane is
formed. In several cases (5, 9, 10, 12, 13) some Z ± E
isomerization occurs, but only to a minor extent. Remarkably,
no isomerization to (E)-stilbene was observed with 11,
whereas the amount of 1,2-diphenylethane is somewhat
interest in carbon ± element
(e.g., C C, C H, C N, C X)
bond-forming reactions,[10] we
employed zerovalent palladium
precatalysts I, which contain the
rigid bidentate nitrogen ligand
bis(arylimino)acenaphthene
(bian). These have previously
been isolated with an electron-
[*] Prof. Dr. C. J. Elsevier, M.Sc. M. W. van Laren
Institute of Molecular Chemistry, Inorganic Chemistry
Universiteit van Amsterdam, Nieuwe Achtergracht 166
NL-1018 WV Amsterdam (The Netherlands)
Fax: (‡31)20-525-6456
E-mail: else4@anorg.chem.uva.nl
[**] Rigid Bidentate Nitrogen Ligands in Organometallic Chemistry and
Homogeneous catalysis, Part 15. We thank Dr. A. Fürstner (MPI für
Kohlenforschung, Mülheim; 5), Dr. U. Siemeling (Universität Biele-
feld; 12) and Prof. Dr. L. Brandsma (Universiteit Utrecht; 14, 15) for
their kind donation of the indicated samples. We also thank R. P.
de Boer and D. Schildknegt for conducting several experiments and
Dr. S. B. Duckett (University of York), Prof. J. Bargon, and Dr. A.
Harthun (Universität Bonn) for their interest and involvement.
Part 14: ref. [10b].
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can be hydrogenated with I as a precatalyst,[13] but styrene and
stilbene reacted sluggishly. In this context is is noteworthy that
conjugated enynes such as 14 and 15 are cleanly (though in the
case of 14 sluggishly) converted into the corresponding dienes
by hydrogenation with Ia, with no trace of hydrogenation of
the double bond(s); hydrogen uptake stops completely after
hydrogenation of the triple bond, without any decomposition
of the catalyst. Employing a Lindlar catalyst with 14 led to
significant overreduction (86/8/6 mixture of diene/ethylcyclo-
hexene/ethynylcyclohexene) after uptake of one equivalent of
H2.[14]
Table 1. Product distribution in the hydrogenation of alkynes with Ia as
precatalyst.[a]
Substrate
Product distribution [%][b]
(Z)-alkene
(E)-alkene
alkane
1[c]
2[c]
> 99
±
±
1-hexene exclusively
3
4
> 99
> 99
±
±
±
±
The effect of the substituents on the N-Ar moieties of bian
on the selectivity was briefly adressed for the hydrogenation
of 1-phenyl-1-propyne (8; Table 2). The selectivity for (Z)-1-
phenyl-1-propene decreases markedly with decreasing elec-
tron-donor character of the substituents on the N-aryl groups.
For entries 1 ± 3, the Z/E ratio decreases from 46 to 11, with a
5
95
5
±
6
7
> 98
< 1
< 1
> 99[d]
±
±
8
9
92
91
2
6
6
3
Table 2. Product distribution in the hydrogenation of 1-phenyl-1-propyne
to 1-phenyl-1-propene and 1-phenylpropane.
Entry Precatalyst
Product distribution [%]
(Z)-alkene
(E)-alkene
alkane
10
95
87
5
±
±
1
2
3
4
Ia
Ib
Ic
Id
92
85
80
62
2
5
7
3
6
10
13
35
11
12
13
97
96
3
4
±
±
concomitant increase of the amount of alkane formed.
Especially the sterically congested complex Id (entry 4)
exhibits a rather low selectivity and leads to formation of
larger amounts of alkane. The latter result may be due to
steric crowding, since Id may form a more stable complex
with the (Z)-alkene than with the alkyne.
Use of zero-valent palladium catalyst precursors with other
ligandsÐsuch as [Pd(dba)2], [Pd(bpy)(alkene)], and [Pd(dab)-
(alkene)] (Table 3)Ðleads to unstable complexes and/or
13
14
ethenylcyclohexene exclusively
ethenylcyclooctene exclusively
15
[a] Reaction conditions: 80 mm substrate and 0.8 mm precatalyst Ia in THF
at 208C and 1 bar H2 pressure. [b] The reaction was monitored by GC
analysis. The product distribution was determined by GC and 1H NMR
analysis at >99.5% conversion of alkyne. [c] Hydrogenated with Ic as
precatalyst. [d] Determined by reaction with D2.
Table 3. Product distribution in the hydrogenation of 1-phenyl-1-propyne.
Entry Precatalyst[a]
Product distribution (%)
(Z)-alkene
(E)-alkene
alkane
1
2
3
4
5
[Pd(dba)2]
70
73
±
80
92
8
6
±
2
2
22
21
±
18
6
[Pd(bpy)(dmfu)]
[Pd(dab)(dmfu)][b]
[Pd(dppe)(dmfu)]
[Pd(bian)(dmfu)]
higher than is obtained with Lindlar procedures; a 93/2/5
mixture of (Z)-alkene/(E)-alkene/alkane was reported.[1f]
In the case of 1 ± 6, 10, and 12 very little or no alkane is
formed. The high selectivity in the case of dimethyl butyne-
dioate (6) is noteworthy, as it indicates that hydrogenation of
the Pd alkyne complex is faster than formation of the
palladacycle with a second molecule of butynedioate, which
is known to be a fast reaction.[10b, 12] A Pd ± alkyne complex
similar to II was observed as an intermediate in the formation
of such a palladacycle.[10a, b]
Evidently, the hydrogenation of alkynes takes place with
high chemoselectivity towards the triple bond. No hydro-
genation of functional groups such as ester, carboxylic acid or
nitro substituents has been observed, but I is capable of
hydrogenating the carbon ± carbon double bond in several
cases. It was previously known that electron-deficient alkenes
[a] bpy ˆ bipyridyl, dab ˆ p-anisyldiazabutadiene, dba ˆ dibenzylidenea-
cetone, dmfu ˆ dimethyl fumarate, dppe ˆ bis(diphenylphosphanyl)-
ethane. [b] The precatalyst decomposed almost immediately.
serious decomposition under the conditions employed. As a
result, in these cases heterogeneous hydrogenation of 8 with
low selectivity and a high degree of overreduction to n-
propylbenzene was observed. The use of a complex containing
a chelating biphosphane ligand such as dppe gives very stable
alkyne and alkene complexes, and no decomposition is
observed, but hydrogenation is very slow, and larger amounts
of n-propylbenzene are formed. Thus, the selectivity of the
Pd(bian) complexes I for hydrogenation of alkynes is much
higher than those of the other precatalysts in Table 3.
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Elsevier, Organometallics 1998, 17, 1812; c) R. van Asselt, E. E. C. G.
Gielens, R. E. Rülke, K. Vrieze, C. J. Elsevier, J. Am. Chem. Soc. 1994,
116, 977; d) C. J. Elsevier, Coord. Chem. Rev. 1999, 185 ± 186, 809.
[11] a) R. van Asselt, C. J. Elsevier, A. L. Spek, R. Benedix, Recl. Trav.
Chim. Pays-Bas 1994, 113, 88; b) R. van Asselt, C. J. Elsevier, W. J. J.
Smeets, A. L. Spek, Inorg. Chem. 1994, 33, 1521.
[12] K. Moseley, P. M. Maitlis, J. Chem. Soc. Chem. Commun. 1971, 1604.
[13] R. van Asselt, C. J. Elsevier, J. Mol. Catal. 1991, 65, L13.
[14] E. N. Marvell, J. Tashiro, J. Org. Chem. 1965, 30, 3991.
[15] Only after all of the alkyne had been quantitatively reduced to the
alkene and/or alkane did the catalyst decompose to form heteroge-
neous palladium species in some cases. Only then was cyclohexene
hydrogenated. The ªmercury testº cannot be employed with Pd/bian
systems.[13]
The homogeneity of the hydrogenation was monitored by
the addition of a tenfold excess of cyclohexene to the reaction
mixture in several cases. Cyclohexene is not or only very
slowly hydrogenated by homogeneous Pd species, but is
readily hydrogenated by colloidal or heterogeneous Pd. As we
did not observe the formation of any cyclohexane during the
hydrogenation of the alkynes, it can be concluded that the
Pd(bian)-catalyzed hydrogenation reaction is homogene-
ous.[15]
The mechanism of this selective hydrogenation of alkynes is
as yet unclear. The complexes I should be considered
precatalysts, and the actual catalyst is probably a [Pd(bian)(al-
kyne)] complex II (Scheme 1). The stability of II relative to
the corresponding alkene complex I is probably important for
the observed chemoselectivity of hydrogenation. For most or
all alkynes a high ratio II/I would be expected. Hydrogenation
could readily take place by addition of hydrogen to II and
subsequent insertion/elimination or by a pairwise transfer of
hydrogen atoms. The latter was substantiated in similar
hydrogenations by means of the para-hydrogen-induced
[16] A. Harthun, R. Giernoth, C. J. Elsevier, J. Bargon, Chem. Commun.
1996, 2483.
Self- and Directed Assembly of Hexaruthenium
Macrocycles**
1
2
polarization in H and H NMR spectroscopy during hydro-
genation.[16] Currently, we are trying to extend the scope and
to elucidate the mechanism of the reaction.
George R. Newkome,* Tae Joon Cho, Charles N.
Moorefield, Gregory R. Baker, Randy Cush, and
Paul S. Russo
Experimental Section
Elegant work in the area of self-assembly by Stang,[1] Lehn
et al.,[2] and many others[3±7] has prompted our investigation of
the potential to spontaneously construct Ru-based (macro)-
molecules. More specifically, our goal involved the design and
preparation of polyterpyridyl ligands that would form the
basis of a ªmodular building block setº[8] capable of being
used to access ªhigher orderº (fractal) architectures. Herein
we report the construction of a bis(terpyridyl) monomer that
facilitates the preparation of hexaruthenium macrocycles.
Linear bis(terpyridyl) monomers have been employed for
the formation of layered polyelectrolyte films,[9] RuII-based
dendrimers,[10] helicating ligands,[11] grids,[12] racks,[13] and
photoactive molecular-scale wires,[14] to mention but a few.
Whereas progress in directed synthesis of cyclic rigid struc-
tures[15] can be found in ªshape-persistentº phenylacety-
lenes,[16±18] diethynylbenzene macrocycles,[19] and a hexagon
containing 24 phenylene units,[20] advances in self-assembly
have yielded, for example, chiral[21] and achiral circular
Typical procedure for the catalytic hydrogenation of alkynes: Ia[11b] (25 mg,
0.04 mmol) and the alkyne (4.0 mmol) were added to 50 mL of dry THF in
a Schlenk tube under nitrogen atmosphere. The solution was then subjected
to a hydrogen atmosphere of 1 bar by first flushing with hydrogen and then
slowly blowing hydrogen over the surface while the solution was vigorously
stirred at 208C. The reaction was monitored by GC (Varian 3300; DB-5
column) and stopped when all alkyne had been consumed (conversion
99.5 ± 100%). The composition of the reaction mixture was determined by
1
GC and H NMR spectroscopic analysis.
Received: June 24, 1999
Revised version: August 24, 1999 [Z13620IE]
German version: Angew. Chem. 1999, 111, 3926 ± 3929
Keywords: alkenes ´ alkynes ´ hydrogenations ´ N ligands ´
palladium
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Dubuis, Org. Synth. 1966, 46, 89; c) O. R. Brown, P. H. Middleton,
T. L. Threfall, J. Chem. Soc. Perkin Trans. 2 1984, 955; d) D. J.
Hastings, A. C. Weedon, Can. J. Chem. 1991, 69, 1171; e) J. Rajaram,
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f) J.-J. Brunet, P. Caubere, J. Org. Chem. 1984, 49, 4058.
[*] Prof. Dr. G. R. Newkome, Dr. T. J. Cho, Prof. Dr. C. N. Moorefield,
Prof. Dr. G. R. Baker
Center for Molecular Design and Recognition
University of South Florida
Tampa, FL 33620 (USA)
[2] J. Choi, N. M. Yoon, Tetrahedron Lett. 1996, 37, 1037.
[3] a) C. A. Brown, V. K. Ahuja, J. Chem. Soc. Chem. Commun. 1973,
553; b) C. A. Brown, V. K. Ahuja, J. Org. Chem. 1973, 38, 2226.
[4] B. M. Choudary, G. Vasantha, M. Sharma, P. Bharathi, Angew. Chem.
1989, 101, 506; Angew. Chem. Int. Ed. Engl. 1989, 28, 465.
[5] R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2143.
[6] M. Sodeoka, M. Shibasaki, J. Org. Chem. 1985, 50, 1147.
[7] P. Pelagatti, A. Venturini, A. Leporati, M. Carcelli, M. Costa, A.
Bacchi, G. Pelizzi, C. Pelizzi, J. Chem. Soc. Dalton Trans. 1998, 2715.
[8] A. Bacchi, M. Carcelli, M. Costa, P. Pelagatti, C. Pelizzi, G. Pelizzi,
Gazz. Chim. Ital. 1994, 124, 429.
Fax: (‡1)813-974-6564
E-mail: gnewkome@research.usf.edu
R. Cush, Prof. Dr. P. S. Russo
Department of Chemistry, Louisiana State University
Baton Rouge, LA 70803 (USA)
[**] We gratefully thank the National Science Foundation (DMR-9901393
to G.R.N and BIR-9420253 to P.S.R) and the Office of Naval
Research (N00014-99-1-0082 to G.R.N). Also acknowledged are the
Louisiana Board of Regents Fellowship (R.C.) and a post doctoral
fellowship provided by the Korea Research Foundation (T.J.C.). We
also wish to thank Prof. Goeffrey A. Ozin and Dr. Srebri Petrov at the
University of Toronto for insightful and helpful comments.
[9] E. W. Stern, P. K. Maples, J. Catal. 1972, 27, 120.
[10] a) R. van Belzen, H. Hoffmann, C. J. Elsevier, Angew. Chem. 1997,
109, 1833; Angew. Chem. Int. Ed. Engl. 1997, 36, 1743; b) R.
van Belzen, R. A. Klein, H. Kooijman, N. Veldman, A. L. Spek, C. J.
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