A dinuclear ruthenium catalyst with a confined cavity: Selectivity in the addition of aliphatic carboxylic acids to phenylacetylene

Article abstract of DOI:10.1039/c2cc38454j

A dinuclear ruthenium catalyst with a rigid anthracene spacer shows excellent regio- and stereo-selectivity in the atom-economic addition of aliphatic carboxylic acids to phenylacetylene, producing exclusively anti-Markovnikov enol-esters with high E/Z ratios of the isomers.

Full text of DOI:10.1039/c2cc38454j

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A dinuclear ruthenium catalyst with a confined cavity:
selectivity in the addition of aliphatic carboxylic acids
to phenylacetylene†
Cite this: DOI: 10.1039/c2cc38454j
Received 4th September 2012,
Accepted 3rd December 2012
Kwong-Chak Cheung,z Wing-Leung Wong,z Ming-Him So, Zhong-Yuan Zhou,
Siu-Cheong Yan and Kwok-Yin Wong*
DOI: 10.1039/c2cc38454j
www.rsc.org/chemcomm
A dinuclear ruthenium catalyst with a rigid anthracene spacer
shows excellent regio- and stereo-selectivity in the atom-economic
addition of aliphatic carboxylic acids to phenylacetylene,
producing exclusively anti-Markovnikov enol-esters with high
E/Z ratios of the isomers.
The development of selective catalysts for atom-economic
reactions is important, as these reactions give the desired
products without by-products or wastes.1 In recent years,
ruthenium catalysts have become attractive for applications
in atom-economic reactions,2,3 in particular the activation of
unsaturated hydrocarbon molecules for selective carbon–
heteroatom bond formation.4 Since the first report by Rotem
Scheme 1 Synthetic route to the dinuclear ruthenium complex.
and Shvo,5 a variety of efficient ruthenium catalysts has been esters as the major product in the catalytic anti-Markovnikov
developed for the addition of carboxylic acids to terminal transformation of phenylacetylene.
alkynes.6–9 The development of this organic transformation is
A synthetic route to the new dinuclear ruthenium complex is
dominated by mononuclear ruthenium–phosphine catalysts: outlined in Scheme 1. The ligand 1,8-bis(2,2-dipyridylamino)
good regioselectivity is usually achieved with a combination anthracene (BDPAA) was constructed to provide a rigid anthracene
of a ruthenium precursor and appropriate phosphine ligands, spacer for the two dipyridylamine units12 at its 1,8 posi-
though the use of dinuclear ruthenium complexes have also tions, respectively, each of which then coordinated to one
been reported.5,9 Depending on the ligands, the ruthenium ruthenium metal.
catalysts are known to promote the electrophilic activation
The X-ray structure§ of the dinuclear complex [(Ru(p-cymene)-
of alkynes via either the classical Markovnikov or the anti- Cl)2BDPAA](CF3SO3)2 is shown in Fig. 1(A). Two RuII(p-cymene)Cl
Markovnikov addition reactions. For the anti-Markovnikov products, moieties independently coordinated with the 2,20-dipyridylamine
the Z-enol esters are usually the preferred isomer.10 Selective moiety, which are attached covalently at the 1,8-position of the
preparation of E-enol ester using ruthenium catalysts, however, anthracene spacer. The two p-cymenes coordinated with the two
is still a challenge.11 We report here, to the best of our knowledge, ruthenium centers, respectively, are located at the outer sides of the
the first example of a dinuclear ruthenium catalyst with N-donor complex. The short bond distances of Ru–cymene (approximately
dipyridylamine ligands and its characteristic production of E-enol 2.2 Å) indicate the strong coordination between the ruthenium
metals and cymene ligands. In contrast, the two chloro ligands
(Ru(1)–Cl(1) and Ru(2)–Cl(2) E 2.4 Å) are found to be pointing
towards each other in the center position of the dinuclear complex.
Department of Applied Biology and Chemical Technology and the State Key
Due to the rigidity of the anthracene spacer, a confined cavity is
created between the two ruthenium centers, with an estimated
distance of about 9.2 Å for Ru(1)–Ru(2).
The active catalyst was prepared by replacing the chloride
ligands in [(Ru(p-cymene)Cl)2BDPAA]2+ with the more labile
triflate ligand by treatment with AgCF3SO3 in methanol to give
[(Ru(p-cymene)CF3SO3)2BDPAA]2+ (1), which effectively catalyzed
Laboratory of Chirosciences, The Hong Kong Polytechnic University, Hunghom,
Kowloon, Hong Kong, China. E-mail: kwok-yin.wong@polyu.edu.hk;
Fax: +852 2364-9932; Tel: +852 3400-3977
† Electronic supplementary information (ESI) available: Detailed synthetic
procedures, characterizations, conditions for catalysis, and X-ray data. CCDC
899405 and 899406. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc38454j
‡ These authors contributed equally to this work.
c
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Table 1 The investigation of stereoselectivity in the Ru-catalyzed addition
of carboxylic acids to phenylacetylene: catalyst
catalyst 2 = mononuclear Ru-complexa
1
=
dinuclear Ru-complex,
Stereoselectivityc
Entry Catalyst R1
R2
Yieldb/%
E
Z
1
2
3
4
5
6
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH3
95
93
93
92
6.5
9.3
11
9.6
10
1
1
1
1
1
1
1.2
1.4
1.3
1.5
1.6
1.2
1.3
1.2
1.3
1.7
1.5
1.1
n-C2H5
n-C3H7
n-C4H9
n-C5H11 91
n-C6H13 81
3
1
1
1
1
1
1
1
1
1
1
1
1
7d
8d
9d
10
11
12
13
14
15
16d
17d
18d
Ph
85
60
75
95
95
95
94
n-C6H13 Ph
n-C6H13 n-C3H7
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH3
n-C2H5
n-C3H7
n-C4H9
n-C5H11 94
n-C6H13 92
Fig. 1 The X-ray crystal structures: (A) a dinuclear [(Ru(p-cymene)Cl)2BDPAA]2+
complex. (30% probability level for the ellipsoids; two triflate anions (CF3SO3ꢀ),
one CH3CN solvent molecule, and hydrogen atoms are not shown for clarity); (B)
a mononuclear [(Ru(p-cymene)Cl)DPPA]+ complex. (30% probability level for the
ellipsoids; one chloride counterion (Clꢀ), one H2O solvent molecule, and
hydrogen atoms are not shown for clarity).
Ph
90
96
90
n-C6H13 Ph
n-C6H13 n-C3H7
a
Reaction conditions: phenylacetylene (1 mmol), catalyst 4 mol%,
carboxylic acid (1.2 equivalents) in dry toluene (2.5 ml) under
b
nitrogen at 85 1C for 24 h. Yield of enol-esters was determined by
GC-MS based on
estimated by GC-MS. Reaction was performed at 120 1C.
c
1
mmol phenylacetylene. Stereoselectivity was
d
the formation of enol esters in good yields. The result of
the addition reactions and stereoselectivity are summarized
in Table 1.13
yields (60–75%) but no stereoselectivity towards the E-isomer
was observed (entries 8 and 9).
The experimental results demonstrate that the dinuclear
catalyst 1 produces anti-Markovnikov enol esters exclusively
in 100% regioselectivity. In all cases examined in the catalysis,
no gem-enol ester was observed. Moreover, as clearly indicated
by the high isomer ratio of E/Z = 6.5 with acetic acid (Table 1,
entry 1), the stereoselectivity of catalyst 1 was found to be much
more favorable for E-enol ester formation than the Z-isomer.
To understand more on the stereoselectivity of the dinuclear
catalyst for E-enol ester production, aliphatic carboxylic acids of
different chain lengths were examined. The stereoselectivity
was obviously improved as the chain length of the carboxylic
acid was increased (Table 1, entries 1–3). In the case of
propanoic acid, the E/Z ratio is significantly increased to 9.3,
which is a very high stereoselectivity found in ruthenium-
catalyzed addition of carboxylic acids to phenylacetylene.5–11
The E/Z ratio was further enhanced with butanoic acid (entry 3:
E/Z = 11) whereas the yield remained as good as acetic acid
(95%) and propanoic acid (93%). However, when pentanoic
acid or hexanoic acid was used in the reactions, the E/Z ratio
dropped to about 10 (entries 4 and 5). In the case of heptanoic
acid, the stereoselectivity dropped significantly (E/Z = 3) and the
yield decreased to 81%, which is much lower than those shorter
chained carboxylic acids (entry 6). When benzoic acid was
tested, the catalytic results indicated that both the yields and
stereoselectivity of the adduct were poor (entry 7). The reaction
of 1-octyne with butanoic acid and benzoic acid was also
investigated with 1 as the catalyst. The reaction gave moderate
In order to understand more about the stereochemistry of the
dinuclear ruthenium complex in the catalysis, its mononuclear
analogue, [(Ru(p-cymene)Cl)DPPA]+ (DPPA = N,N-di(2-pyridyl)-
phenylamine), was also synthesized and investigated for
comparison. The X-ray structure of [(Ru(p-cymene)Cl)DPPA]+ is
shown in Fig. 1(B). In general, the mononuclear [(Ru(p-cymene)-
(CF3SO3)DPPA]+ complex (2) exhibits similar yields (92–95%) but
much poorer stereoselectivity compared to the dinuclear
complex in the catalysis. It is worthy to note that the mono-
nuclear catalyst 2 gives both Z- and E-isomers, with the Z-isomer
slightly predominant with regard to the E-isomer in the catalysis
(Table 1, Z/E ratio = 1.1–1.7).
Based on the X-ray structure of [(Ru(p-cymene)Cl)2BDPAA]2+,
the space between the two ruthenium centers was estimated to
be about 9.2 Å. This confined space is not large enough to
accommodate two ruthenium phenylethenylidene species14
because the active sites are facing each other in the cavity. In
contrast, the cavity is a good fit to hold two molecules of
carboxylic acids for the reaction. Therefore, it is more favourable
for the ruthenium phenylethenylidene species to orient in such a
manner that the phenylethenylidenes are pointing out of the
cavity (Fig. 2) to give the E-enol ester exclusively in the catalysis.
This can explain why stereoselectivity towards the E-isomer was
only observed for phenylacetylene but not 1-octyne. Nevertheless,
the pathway with one phenylethenylidene pointing towards the
cavity (pathway B in Fig. S1, ESI†), though less favorable, can still
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Fig. 2 A possible mechanism for the dinuclear ruthenium catalyst to produce the E-enol ester in the catalysis (DPA = 2,20-dipyridylamine unit; spacer = anthracene
connected to the two DPA units at the 1,8-position).
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the mononuclear ruthenium catalyst, the phenyl ring of the
ruthenium phenylethenylidene species encounters least steric
hindrance when it is furthest away from the cymene group
(pathway D in Fig. S1, ESI†). This orientation of the ruthenium
phenylethenylidene species leads to the more favourable production
of Z-enol esters in the catalysis. The ruthenium phenylethenylidene
intermediates can be detected by ESI-MS and 13C NMR. A broad low-
field resonance signal of the ruthenium carbene species appears at
d B 199 ppm (Fig. S6 and S7 in ESI†) which is similar to those
reported in the literature (d B 200 ppm).15 The ruthenium phenyl-
ethenylidene species of 1 and 2 were also observed in the mass
spectra (Fig. S8 and S9, ESI†) after reaction with phenylacetylene in
solution. The mass spectrum of the dinuclear complex confirmed
the formation of two ruthenium phenylethenylidene moieties in the
same molecule (Fig. S9, ESI†).
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In conclusion, we have demonstrated that a dinuclear
ruthenium complex with a confined cavity can be a selective
catalyst in the atom-economic addition of aliphatic carboxylic
acid to phenylacetylene to give exclusively the anti-Markovnikov
enol esters with favourable E/Z stereoisomer ratios.
We acknowledge the support of the Hong Kong Polytechnic
University, the Innovation and Technology Commission, the
Research Grants Council (PolyU 5015/07P) and the Special
Equipment Grant (SEG_PolyU01) of the University Grants
Committee.
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Notes and references
§ Selected crystal data for [(Ru(p-cymene)Cl)2BDPAA](CF3SO3)2: formula =
Ru2Cl2(C54H52N6)ꢁCH3CNꢁ(CF3SO3)2; M = 1397.25; monoclinic; P2(1)/c;
a = 16.2205(2), b = 14.5792(2), c = 25.0080(3) Å; b = 95.4810(10)1;
V = 5886.90(13) Å3; T = 296(2) K; Z = 4; m = 0.750 mmꢀ1; reflections
collected = 50 320; independent reflections = 13 291 (Rint = 0.0488); final
R values [I > 2s(I)]: R1 = 0.0550, wR2 = 0.1520; final R values (all data):
R1 = 0.0884, wR2 = 0.1699.
˜
A. Bacchi, M. Balordi, S. Bolano, F. Calbiani, L. Elviri, L. Gonsalvi,
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12 W.-L. Wong, K.-C. Cheung, P.-H. Chan, Z.-Y. Zhou, K.-H. Lee and
K.-Y. Wong, Chem. Commun., 2007, 2175–2177.
Selected crystal data for [(Ru(p-cymene)Cl)DPPA](Cl): formula =
[RuCl(C26H27N3)]Clꢁ1.5(H2O);
V = 5141.53(17) Å3; T = 296(2) K; Z = 8; m = 0.844 mmꢀ1; reflections
collected = 40 721; independent reflections = 5888 (Rint = 0.0572); final
M
c
= 580.50; monoclinic; C2/c; a =
= 14.4679(3) Å; b = 93.8880(10)1;
17.8467(3),
b
=
19.9586(4),
13 A small amount of acetophenone (o5%) was also produced as a
side-product in all cases of the catalysis investigated.
R values [I > 2s(I)] R1 = 0.0389, wR2 = 0.1150; R values (all data) R1
=
14 (a) J. M. Lynam, Chem.–Eur. J., 2010, 16, 8238–8247; (b) S. Rigaut,
D. Touchard and P. H. Dixneuf, Coord. Chem. Rev., 2004, 248,
1585–1601; (c) H. Katayama and F. Ozawa, Coord. Chem. Rev.,
2004, 248, 1703–1715.
0.0488, wR2 = 0.1300.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.