Ruthenium-promoted Z-selective head-to-head dimerization of terminal alkynes in organic and aqueous media

Article abstract of DOI:10.1021/om050355x

[RuH(CH₃CN)(NP₃)]OTf effectively catalyzes the dimerization of both aliphatic and aromatic alkynes to give (Z)-enynes in high regio- and stereoselectivity. The catalytic system can tolerate a number of functional groups, which allows for its use in organic and aqueous medium.

Full text of DOI:10.1021/om050355x

4330
Organometallics 2005, 24, 4330-4332
Communications
Ruthenium-Promoted Z-Selective Head-to-Head
Dimerization of Terminal Alkynes in Organic and
Aqueous Media
Xingguo Chen,† Peng Xue,† Herman H. Y. Sung,† Ian D. Williams,†
Maurizio Peruzzini,*,‡ Claudio Bianchini,*,‡ and Guochen Jia*,†
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water
Bay, Kowloon, Hong Kong, and Istituto di Chimica dei Composti Organometallici
(ICCOM-CNR), Via Madonna del Piano, snc, 50019 Sesto Fiorentino (Firenze), Italy
Received May 6, 2005
Summary: [RuH(CH3CN)(NP3)]OTf effectively catalyzes
the dimerization of both aliphatic and aromatic alkynes
to give (Z)-enynes in high regio- and stereoselectivity. The
catalytic system can tolerate a number of functional
groups, which allows for its use in organic and aqueous
medium.
for Z-selective head-to-head dimerization of both aro-
matic and aliphatic alkynes are very rare.
We have previously shown that tripodal phosphine
complexes such as [RuH(X2)(PP3)]BPh4, [OsH(N2)(PP3)]-
BPh4, and RuH2(PP3) (X ) N2, H2; PP3 ) P(CH2CH2-
PPh2)3)4 are excellent catalytic precursors for the selec-
tive dimerization of PhCtCH and Me3SiCtCH to give
(Z)-enynes. In a related study, Dahlenburg et al. have
shown that the ruthenium complex RuH(Ph)(NP3) (NP3
) N(CH2CH2PPh2)3)5 catalyzes the dimerization of
n-BuCtCH to give (Z)-(n-Bu)CHdCHCtC(n-Bu) with
a selectivity >95%. Encouraged by these findings, we
have synthesized the new complex [RuH(CH3CN)(NP3)]-
OTf (3; OTf ) trifluoromethanesulfonate) and studied
its catalytic property for alkyne dimerization, to see
whether the Z selectivity can be achieved for a wider
range of substrates. Indeed, the new complex was found
to promote the selective dimerization of both aliphatic
and aromatic alkynes to give (Z)-enynes. In addition,
the system can tolerate a number of functional groups,
including NH2 and CO2H, and the catalytic reactions
can be carried out in organic and aqueous media.
The catalytic precursor [RuH(CH3CN)(NP3)]OTf (3)
was synthesized by the route shown in Scheme 1.
Treatment of RuCl2(NP3) (1)5 with NaBH4 in THF/
ethanol produced the monohydride complex RuHCl(NP3)
Transition-metal-catalyzed dimerization reactions of
terminal alkynes are attractive, atom-economic C-C
bond forming reactions that can give products contain-
ing key structural units in natural products and materi-
als.1 Alkyne dimerization reactions are also potentially
useful for making conjugated polymers or oligomers
from diynes.2c Dimerization reactions of RCtCH can
produce several isomeric products, and the most com-
mon ones include (Z)-RCHdCHCtCR, (E)-RCHdCHCt
CR, CH2dC(R)CtCR, cis-RHCdCdCdCHR, and trans-
RHCdCdCdCHR. For practical applications in organic
synthesis, ideally, one needs a catalytic system that
gives only one desired isomer for a wide range of
substrates. However, many of the reported catalytic
systems actually give a mixture of some of the isomeric
products, and the selectivities of the isomeric products
often vary with the substituents of alkynes. Recently,
there have been considerable efforts devoted to the
understanding of the origins of the selectivity and the
development of selective dimerization of terminal
alkynes.1-6 Where Z-selective head-to-head dimerization
of terminal alkynes is concerned, several catalysts that
can mediate selective dimerization of aromatic alkynes2
or aliphatic alkynes3 (with Z selectivity >90% or 100%)
have been disclosed; however, systems that are effective
(3) Z-Selective head-to-head dimerization of aliphatic alkynes: (a)
Qu, J. P.; Masui, D.; Ishii, Y.; Hidai, M. Chem. Lett. 1998, 1003. (b)
Jun, C.-H.; Lu, Z.; Crabtree, R. H. Tetrahedron Lett. 1992, 33, 7119.
(c) Slugovc, C.; Mereiter, K.; Zobetz, E.; Schmid, R.; Kirchner, K.
Organometallics 1996, 15, 5275. (d) Pavlik, S.; Gemel, C.; Slugovc, C.;
Mereiter, K.; Schmid, R.; Kichner, K. J. Organomet. Chem. 2001, 617,
301.
(4) (a) Bianchini, C.; Peruzzini, M.; Zanobini, F.; Frediani, P.;
Albinati, A. J. Am. Chem. Soc. 1991, 113, 5453. (b) Bianchini, C.;
Frediani, P.; Masi, D.; Peruzzini, M.; Zanobini, F. Organometallics
1994, 13, 4616. (c) Barbaro, C.; Bianchini, C.; Peruzzini, M.; Polo, A.;
Zanobini, F.; Frediani, P. Inorg. Chim. Acta 1994, 220, 5. (d) Bianchini,
C.; Bohanna, C.; Esteruelas, M. A.; Frediani, P.; Meli, A.; Oro, L. A.;
Peruzzini, M. Organometallics 1992, 11, 3837.
† The Hong Kong University of Science and Technology.
‡ Istituto di Chimica dei Composti Organometallici (ICCOM-CNR).
(1) Reviews: (a) Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999,
32, 311. (b) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102,
1731. (c) Katayama, H.; Ozawa, F. Coord. Chem. Rev. 2004, 248, 1703.
(d) Yi, C. S.; Liu, N. Synlett 1999, 281.
(2) Z-Selective head-to-head dimerization of aromatic alkynes: (a)
Nishiura, M.; Hou, Z.; Wakatsuki, Y.; Yamaki, T.; Miyamoto, T. J. Am.
Chem. Soc. 2003, 125, 1184. (b) Tazelaar, C. G. J.; Bambirra, S.; van
Leusen, D.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics
2004, 23, 936. (c) Katayama, H.; Nakayama, M.; Nakano, T.; Wada,
C.; Akamastsu, K.; Ozawa, F. Macromolecules 2004, 37, 13. (d) Yi, C.
S.; Liu, N. Organometallics 1996, 15, 3968. (e) Echevarren, A. M.;
Lopez, J.; Santos, A.; Montoya, J. J. Organomet. Chem. 1991, 414, 393.
(5) Dahlenburg, L.; Frosin, K.-M.; Kerstan, S.; Werner, D. J.
Organomet. Chem. 1991, 407, 115.
(6) Additional examples of recent work: (a) Schafer, M.; Wolf, J.;
Werner, H. Organometallics 2004, 23, 5173. (b) Horacek, M.; Stepnicka,
P.; Kubista, J.; Gyepes, R.; Mach, K. Organometallics 2004, 23, 3388.
(c) Yang, C.; Nolan, S. P. J. Org. Chem. 2002, 67, 591. (d) Rubina, M.;
Gevorgyan, V. J. Am. Chem. Soc. 2001, 123, 11107. (e) Lee, C. C.; Lin,
Y. C.; Liu, Y. H.; Wang, Y. Organometallics 2005, 24, 136.
10.1021/om050355x CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/23/2005

Communications
Organometallics, Vol. 24, No. 18, 2005 4331
Table 1. Dimerization of Terminal Alkynes
Catalyzed by 3
Figure 1. Molecular structure of the complex cation [RuH-
(CH3CN)(NP3)]+. Selected bond distances (Å) and angles
(deg): Ru(1A)-N(1A), 2.024(4); Ru(1A)-N(2A), 2.169(4);
Ru(1A)-P(1A), 2.2835(14); Ru(1A)-P(3A), 2.2983(15); Ru-
(1A)-P(2A), 2.3386(14); N(1A)-Ru(1A)-N(2A), 178.16(16);
N(1A)-Ru(1A)-P(1A), 96.36(12); N(2A)-Ru(1A)-P(1A),
84.42(11); N(1A)-Ru(1A)-P(3A), 94.66(12); N(2A)-Ru-
(1A)-P(3A), 83.86(11); P(1A)-Ru(1A)-P(3A), 149.34(5);
N(1A)-Ru(1A)-P(2A), 97.36(12); N(2A)-Ru(1A)-P(2A),
84.04(11); P(1A)-Ru(1A)-P(2A), 102.70(5); P(3A)-Ru-
(1A)-P(2A), 104.13(5).
Scheme 1
(2). Treatment of 2 in acetonitrile with TlOTf produced
the cationic complex [RuH(CH3CN)(NP3)]OTf (3). The
new complexes 2 and 3 have been characterized by NMR
spectroscopy and elemental analysis.7 The structure of
3 has also been confirmed by an X-ray diffraction study.
A view of the complex cation is shown in Figure 1. In
the solid state, complex 3 is reasonably air stable and
can be exposed to air for at least 2 days without
appreciable decomposition.
a Not isolated; percent conversion determined in situ by NMR.
b Contain 4% head-to-tail dimer. c Contains 3% trans dimer.
d Contains 5% head-to-tail dimer.
lene were all selectively converted to the corresponding
(Z)-enynes when heated in toluene in the presence of 1
mol % of 3. Complex 3 is also catalytically active for
the dimerization of silylalkynes and aliphatic alkynes,
although the reactions were slower than those with aryl
alkynes. In contrast, the selectivity was maintained:
Me3SiCtCH and Me3CCtCH were transformed to
(Z)-Me3SiCHdCHCtCSiMe3 and (Z)-Me3CCHdCHCt
CCMe3, respectively, upon heating in benzene. When
straight-chain aliphatic alkynes RCtCH, for example
1-hexyne and 1-octyne, were used, the selectivity in the
Z isomers decreased slightly and the reactions produced
a mixture of (Z)-RCHdCHCtCR and CH2dC(R)CtCR
in ratios of ca. 100:4 and 100:5, respectively. The lower
Z selectivity in the reactions of 1-hexyne and 1-octyne
is not unexpected, as a similar trend in Z selectivity has
been also reported for the dimerization of n-BuCtCH
and t-BuCtCH mediated by TpRuCl(PPh3)2 (Tp )
hydrotris(pyrazol-1-yl)borate)3c and [Ir(biph)(PMe3)3Cl]/
2AgBF4 (biph ) biphenyl-2,2′-diyl).3b For example, in
the reactions assisted by TpRuCl(PPh3)2, the Z selectiv-
ity is 100% for t-BuCtCH and only 25% for n-BuCt
CH.
We initially tested the catalytic behavior of complexes
1-3 for the dimerization of PhCtCH in toluene. When
a toluene solution of PhCtCH containing 0.5 mol % of
3 was heated for 11 h, the phenylacetylene was com-
pletely consumed. After purification by chromatography,
the isolated product was identified to be (Z)-PhCHd
CHCtCPh. The stereochemistry was assigned on the
3
basis of the observation of a J(HH) value of 12.0 Hz
for the olefinic protons. The other regio- and stereoiso-
meric dimers were apparently not produced or only
produced in trace amounts, as they were not detected
by NMR. Under similar conditions, complexes 1 and 2
were inactive for the catalytic dimerization of PhCtCH,
however.
We then explored the catalytic dimerization reactions
of other terminal alkynes with 3. The results are
summarized in Table 1. Aromatic alkynes such as
4-MeC6H4CtCH, 4-FC6H4CtCH, and 9-anthrylacety-
(7) See the Supporting Information for experimental and spectro-
scopic details of the new compounds.

4332 Organometallics, Vol. 24, No. 18, 2005
Communications
The present catalytic system can tolerate a number
of functional groups. Under conditions similar to
those used for the dimerization of phenylacetylene,
4-BrC6H4CtCH, 4-NH2C6H4CtCH, and Me2C(OH)Ct
CH were all selectively converted to the corresponding
(Z)-enynes; the straight-chain aliphatic alkynol HO-
(CH2)4CtCH was converted to (Z)-HO(CH2)4CHdCHCt
C(CH2)4OH and CH2dC((CH2)4OH)CtC(CH2)4OH in a
ratio of ca. 100:4. The selectivity of the latter reaction
is similar to that with 1-hexyne. It is interesting to note
that the propargyl alcohol Me2C(OH)CtCH is dimerized
to a dienone rather than to enynes under the influence
of [CpRu(CH3CN)3]BF4.8
Our catalytic system can also tolerate the aldehydic
group. Thus, the aldehyde 4-HCOC6H4CtCH was con-
verted predominantly to (Z)-HOCC6H4CHdCHCtCC6H4-
CHO, although the reaction produced also a small
amount (ca. 3%) of (E)-enyne. Even p-HO2CCH2C6H4Ct
CH, which has a carboxylic acid group, can be trans-
formed to the corresponding (Z)-enyne when heated in
2-propanol in the presence of 3. In contrast, ruthenium
complexes such as Ru(PR3)2(allyl)2 and Cp*Ru(COD)
promote the addition of carboxylic acids to HCtCR to
give exclusively alkenyl esters9a or dimerized dienyl
esters.9b It is noteworthy that the regioselective forma-
tion of alkenyl esters is catalyzed by the Rh(I) NP3
derivative [RhH(NP3)].10
For liquid substrates, the dimerization reactions can
be carried out neat without additional organic solvents.
Thus, PhCtCH and 1-octyne can be converted to the
corresponding (Z)-enynes by heating in the presence of
3. The catalytic efficiency of 3 for the dimerization of
PhCtCH is remarkable. For example, almost complete
conversion of phenylacetylene to the (Z)-enyne can be
achieved in 2 h with a catalyst/substrate ratio of 5/1000
or in 28 h with a catalyst/substrate ratio of 3/10 000.
Most previously reported dimerization reactions of
alkynes were carried out with more than 1% catalyst.
Chemical reactions in aqueous medium are of growing
importance because of many potential advantages, such
as alleviation of environmental problems associated
with the use of organic solvents.11 Since the catalytic
system can tolerate polar functional groups and complex
3 has some solubility in polar solvents, the dimerization
reactions were attempted in water. Surprisingly, PhCt
CH, 4-NH2C6H4CtCH, 1-octyne, and HO(CH2)4CtCH
could all be dimerized in water with a selectivity for the
Z isomers essentially identical with that observed in
organic medium. To our knowledge, these are the first
examples of selective catalytic dimerization of alkynes
to give (Z)-enynes carried out in water.12 It is known
that ruthenium complexes (e.g. CpRuCl(PPh3)2 and
(PNP)RuCl2(PPh3)) can promote either the hydration13
or the hydrolysis of alkynes.14 In our case, no product
resulting from either reaction was detected.
A detailed mechanism for the dimerization reactions
mediated by the [Ru(NP3)]+ system is not yet clear, as
the intermediates of the dimerization reactions have not
been identified. On the other hand, it has been estab-
lished that dimerization of RCtCH mediated by the
very similar precursor [RuH(X2)(PP3)]BPh4 (X2 ) H2, N2)
proceeds through the coupling of vinylidene and acetyl-
ide ligands.4 Thus, it is very likely that the present
formation of (Z)-enynes may involve a similar mecha-
nism. The small amounts of head-to-tail dimers in the
reactions of 1-octyne and 1-hexyne are presumably
formed by direct insertion of alkynes into metal-
acetylide bonds.
In summary, we have found that the ruthenium
complex [RuH(CH3CN)(NP3)]OTf can efficiently mediate
the dimerization of both aliphatic and aromatic alkynes
in a highly regio- and stereoselective manner to give (Z)-
enynes. The catalyst is reasonably air stable and can
tolerate a number of functional groups, which allows
for its use in organic and aqueous media.
Acknowledgment. This work was supported by the
Hong Kong Research Grant Council (Project No. 601804),
National Natural Science Foundation of China, through
the Outstanding Young Investigator Award Fund (Project
No. 20429201) and the EC through the programs COST
(D17 and D29) and AQUACHEM.
Supporting Information Available: Text and tables
giving experimental procedures and characterization data and
a CIF file giving X-ray crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM050355X
(8) Trost, B. M.; Rudd, M. T. J. Am. Chem. Soc. 2001, 123, 8862.
(9) (a) Doucet, H.; Martin-Vaca, B.; Bruneau, C.; Dixneuf, P. H. J.
Org. Chem. 1995, 60, 7247. (b) Le Paih, J.; Monnier, F.; Derien, S.;
Dixneuf, P. H.; Clot, E.; Eisenstein, O. J. Am. Chem. Soc. 2003, 125,
11964.
(10) Bianchini, C.; Meli, A.; Peruzzini, M.; Zanobini, F.; Bruneau,
C.; Dixneuf, P. H. Organometallics 1990, 9, 1155.
(11) Joo, F. Aqueous Organometallic Catalysis; Kluwer: Dordrecht,
The Netherlands, 2001.
(12) Rh-catalyzed dimerization reactions of arylalkynes to give
mixtures of enynes in toluene/pyridine/water are known: Baidossi, W.;
Goren, N.; Blum, J.; Schumann, H.; Hemling, H. J. Mol. Catal. 1993,
85, 153.
(13) Tokunaga, M.; Suzuki, T.; Koga, N.; Fukushima, T.; Horiuchi,
A.; Wakatsuki, Y. J. Am. Chem. Soc. 2001, 123, 11917. (b) Grotjahn,
D. B.; Lev, D. A. J. Am. Chem. Soc. 2004, 126, 12232.
(14) Bianchini, C.; Casares, J. A.; Peruzzini, M.; Romerosa, A.;
Zanobini, F. J. Am. Chem. Soc. 1996, 118, 4585.