Carbon - Carbon bond formation between furyl and triphenylphosphine ligands in ruthenium complexes

Article abstract of DOI:10.1021/om900882c

The η³-allylic complex 8 was obtained from thermolysis of the neutral ruthenium furyl complex 7 with an unsaturated carbon chain on the furyl ligand. Protonation of complex 8c with HBF₄ generates complex 9c with an oxygen atom andan

Full text of DOI:10.1021/om900882c

38 Organometallics 2010, 29, 38–41
DOI: 10.1021/om900882c
Carbon-Carbon Bond Formation between Furyl and Triphenylphosphine
Ligands in Ruthenium Complexes
Yu-Hao Huang, Wen-Wu Huang, Ying-Chih Lin,* and Shou-Ling Huang
Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, Republic of China
Received October 10, 2009
Summary: The η3-allylic complex 8 was obtained from ther-
obtained by the reaction of an iridium cyclooctadienyl
complex with furan.5 This iridium furyl complex reacted
with tert-butylacetylene by insertion of the alkyne into the
Ir-C bond to form an iridium vinyl hydride complex.6 A few
tungsten furyl complexes were obtained from the reaction of
tungsten propargyl complexes with aldehydes. This furyl
ligand is easily dissociated from the metal fragment and
further reacts with Grignard reagent.7 Previously, we repor-
ted the formation of oxygen addition products in almost
quantitative yield when a simple ruthenium furyl complex
was exposed to air.8 The reaction is proposed to proceed via
the formation of an endoperoxide intermediate.8 In a con-
tinuation of our previous effort on exploring new reactions
of ruthenium furyl complexes, herein we report our unex-
pected observation on the thermal reactions of three ruthe-
nium furyl complexes, each with a pendant unsaturated
hydrocarbon chain on the furyl ring. Two 5,5-disubstituted
propargylic-1,6-enynes, HCCCH(OH)CH2CR2CHdCH2
(2a, R = Ph; 2b, R = Me), prepared in high yields according
to the reported procedures,9 were used for the synthesis of
vinyl-acetylide complexes. Then, three furyl complexes
were synthesized by electrophilic alkylations of the acetylide
complexes followed by a deprotonation process. Thermo-
lysis of the three furyl complexes all led to unusual coupling
products.
molysis of the neutral ruthenium furyl complex 7 with an
unsaturated carbon chain on the furyl ligand. Protonation
of complex 8c with HBF4 generates complex 9c with an
oxygen atom and an olefin group coordinated to the ruthenium
metal.
Metal-mediated processes make possible certain reactions,
which are not feasible without the involvement of the metal
species. In particular, organometallic ruthenium complexes
play important roles in many catalytic reactions, such as
asymmetric hydrogenation,1 olefin metathesis,2 and poly-
merization.3 A better understanding of the mechanism of
these reactions revealed the role of the metal and led to wide
applications of ruthenium in organic synthesis. To further
expand the scope of these applications, it is crucial to explore
new reactivity of various complexes of ruthenium. We pre-
viously reported the synthesis of a ruthenium cyclopropenyl
complex through the deprotonation reaction of a vinylidene
complex containing a -CH2R group at Cβ of the vinylidene
ligand.4 The same approach could also be used for the
synthesis of a neutral ruthenium furyl complex from the
deprotonation of a vinylidene complex containing a -CH2-
CO2R group. Synthesis and reactions of a few furyl com-
plexes of other transition metals have been reported. An
iridium σ-furyl complex containing a hydride ligand can be
As shown in Scheme 1, treatment of [Ru]-Cl ([Ru] =
Cp(PPh3)2Ru) with 2a in CH2Cl2 at room temperature
afforded the vinyl-vinylidene complex 3a exclusively. How-
ever, the reaction of [Ru]-Cl with the 1,6-enyne 2b in the
presence of KPF6 in CH2Cl2 yielded a mixture of the
vinylidene complex 3b and the alkenyl-phosphonium com-
plex 4b in a ratio of ca. 3:2. Separation of 3b and 4b was
achieved by precipitation of 3b from a CH2Cl2/diethyl ether
solution. These reactions proceed via formation of a vinyli-
dene intermediate10 followed by dehydration. Interestingly,
*To whom correspondence should be addressed. E-mail: yclin@ntu.
edu.tw.
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pubs.acs.org/Organometallics
Published on Web 12/11/2009
2009 American Chemical Society

Communication
Organometallics, Vol. 29, No. 1, 2010 39
Scheme 1
corresponding η3-allylic complexes 8a-c in high yield.12 The
new coupling reaction between the furyl ligand and one
phenyl group of the phosphine ligand, giving 8, is accom-
panied by dissociation of a PPh3 ligand. Characterization of
complexes 8a-c relies on their 1H, 13C, 31P, and 2-D NMR
spectra. For example, the 31P NMR spectrum of 8c displays a
singlet peak at δ 84.27. Resonances of the OCH2 group split
into two multiplets at δ 4.28 and 4.05 in the 1H NMR
spectrum because of the presence of the stereogenic metal
center generated in the formation of 8c. In addition, two
doublet resonances at δ 8.09 and 6.51 with 3JHH = 15.8 Hz
are assigned to the trans protons of the internal olefinic
group. In addition, the 1H NMR spectrum of 8c also displays
one doublet resonance at δ 1.27 with 3JPH = 14.0 Hz and one
singlet resonance at δ 5.86 assigned to two allylic protons.
The 13C NMR spectrum of 8c shows a singlet resonance at δ
35.84 assigned to the center carbon of the allylic ligand and
two resonances at δ 81.82 and 55.86 assigned to other two
allylic carbon atoms.13
The structure of 8c was further confirmed by a solid-state
single-crystal X-ray diffraction analysis. Yellow single crys-
tals suitable for X-ray diffraction were grown via slow
evaporation of a methanol solution of 8c. An ORTEP
drawing is shown in Figure 1. It is apparent that C-C bond
formation between a phenyl group of the coordinated phos-
phine ligand and the furyl ligand has occurred. In addition,
opening of the furyl ligand causes CR and two other ring
carbon atoms to be bound to the ruthenium center in a η3-
allylic bonding mode. The bond lengths C(19)-C(20) and
C(20)-C(28) of the allylic ligand are 1.420(13) and 1.455(13)
no allenylidene complex was observed in these reactions.
Namely, the dehydration reaction took place at C3-C4,
instead of C2-C3, possibly due to the more stable conju-
gated form of the vinyl-vinylidene ligand. The formation of
the minor product 4b, however, might be attributed to the
addition of phosphine to Cγ of the allenylidene ligand.11
Further deprotonation of complexes 3a,b were carried out
with NaOMe to obtain the corresponding acetylide com-
plexes 5a (R = Ph) and 5b (R = Me), respectively. Treat-
ment of complexes 5a,b with XCH2CO2R0 (X = Br, I;
R0 = Me, Et) afforded the cationic vinylidene complexes
[Ru]dCdC(CH2CO2R0)CHdCHCR2CHdCH2[PF6], (6a,
R = Ph, R0 = Et; 6b, R = R0 = Me; 6c, R = Me, R0 =
Et), all in high yield. For example, complex 5b reacted with
ethyl iodoacetate in CH2Cl2 to give 6c, characterized by 1H,
31P, and 13C NMR spectra and analytical data. The 31P
NMR spectrum of 6c displays a singlet resonance at δ 41.48.
In the 13C NMR spectrum of 6c, the resonance assigned to
the RudCR carbon atom appears at δ 362.52 as a triplet. In
addition, one singlet resonance at δ 27.20 is assigned to the
methylene carbon near the ester group of 6c.
˚
A, respectively. Both stay within the range between double-
and single-bond lengths, suggesting π electron delocalization
within the allylic unit. The Ru(1)-C(19), Ru(1)-C(20), and
Ru(1)-C(28) bond distances of 2.164(10), 2.098(9), and
3
˚
2.192 (8) A, respectively, are normal for a η -allylic bonding
mode. The bond lengths are comparable to those of the
ruthenium allylic complex reported by Green et al.14 The
bond angle C(19)-C(20)-C(28) of 114.8(8)° is slightly
smaller than that of the reported allylic complex.15 This
can be explained by the steric effect between the ethyl acetate
group in a syn position and the long unsaturated carbon
chain bound to the central carbon of the allylic ligand.
Formation of complex 8 can be accounted for by the
possible processes depicted in Scheme 2. Dissociation of a
Deprotonation of the vinylidene complex 6a by n-Bu4-
NOH in acetone is accompanied by a cyclization reaction,
affording the neutral furyl complex 7a. Under similar con-
ditions, complexes 6b,c also reacted with n-Bu4NOH to give
7b,c, respectively. No three-membered cyclopropenyl com-
plex,4 supposedly from deprotonation at Cγ followed by a
direct intramolecular cyclization, was found. The 31P NMR
spectrum of 7c displays a singlet resonance at δ 50.95. In the
13C NMR spectrum of 7c, three singlet resonances at δ
164.07, 149.04, and 110.04 are assigned to the carbonyl
carbon and two terminal olefinic carbon atoms on the basis
of the 1H-13C HMQC and 1H-13C HMBC spectra. The 1H
NMR spectrum of 7c shows a set of coupled doublet
resonances at δ 6.86 and 5.90 with 3JHH = 15.9 Hz assigned
to the trans protons of the internal olefinic group.
(12) Thermolysis of 7c (0.10 g, 0.11 mmol) was carried out in toluene
under nitrogen by heating the solution to reflux for 2 days. The solvent
was then removed under vacuum, and the crude product was purified by
neutral Al2O3 column chromatography with 1/10 Et2O/hexanes as
eluent to yield 8c (0.05 g, yield 75%). Slow evaporation of a methanol
solution of 8c generated yellow single crystals suitable for X-ray
diffraction. Selected spectroscopic data for 8c: 1H NMR (C6D6) δ 8.05
(d, 1H, 3JHH = 15.8 Hz, CHd), 6.51 (d, 1H, 3JHH = 15.8 Hz, CHd),
6.15 (dd, 1H, 3JHH = 17.3 Hz, 2JHH = 10.5 Hz, 1H, CHd), 5.86 (s, 1H,
allylic CH), 5.26 (dd, 1H, 3JHH = 17.3 Hz, 2JHH = 1.1 Hz, CH2d), 5.14
(dd, 1H, 3JHH = 10.5 Hz, 2JHH = 1.1 Hz, CH2d), 4.58 (s, 5H, Cp), 1.27
(d, 1H, 3JPH = 14.0 Hz, allylic CH); 31P NMR (C6D6) δ 84.27; 13C NMR
(C6D6) δ 176.23 (CdO), 148.28 (CHd), 134.22 (CHd), 131.02 (CHd),
110.62 (CH2d), 83.27 (Cp), 81.82 (allylic CH), 55.86 (allylic CH), 35.84
(central C). Anal. Calcd for C36H37O2PRu: C, 68.23; H, 5.89. Found: C,
68.47; H, 6.05. Complete spectroscopic data are given in the Supporting
Information.
(13) Brady, L. A.; Dyke, A. F.; Garner, S. E.; Knox, S. A. R.; Irving, A.;
Nicholls, S. M.; Orpen, A. G. J. Chem. Soc., Dalton Trans. 1993, 487–488.
(14) Crocker, M.; Green, M.; Nagle, K. R.; Orpen, A. G.; Neumann,
H. P.; Morton, C. E.; Schaverien, C. J. Organometallics 1990, 9, 1422–
1434.
Thermolysis of three complexes 7a-c by heating their
toluene solutions to reflux for up to 2 days afforded the
(15) Xue, P.; Bi, S.; Sung, H. H. Y.; Williams, I. D.; Lin, Z.; Jia, G.
(11) Cadierno, V.; Gimeno, J. Chem. Rev. 2009, 109, 3512–3560.
Organometallics 2004, 23, 4735–4743.

40 Organometallics, Vol. 29, No. 1, 2010
Huang et al.
Scheme 3
intermediate with a four-membered ring.17 Afterward, a
hydride shift to CR of the furyl ligand is followed by opening
of the furyl ring to form a zwitterionic intermediate. Then, as
shown in the lower part of Scheme 2, a subsequent reductive
elimination of the furyl and the phenyl groups with con-
comitant coordination of the double bond to the ruthenium
center forms 8. Alternatively, the direct cleavage of the PPh3
ortho C-H bond by the Ru-C1-O bond of the furyl group
with formation of C1-Cortho and O-H bonds leading to
compound 9 is also depicted in the upper part of Scheme 2.
The feasibility of the cleavage of ortho agostic C-H bonds
by deprotonation assisted by both metal and ligand has been
demonstrated by DFT calculations.18 Previously we re-
ported dioxygen addition to a furyl ligand with no hydro-
carbon chain to give an endoperoxide intermediate.8
However, complexes 7a-c are stable in air, most likely
because of the presence of the pendant unsaturated hydro-
carbon chain. The ring-opening reaction of a furyl ring
system in organometallic lanthanide complexes has been
reported. A CH2SiMe3 group has been used as a proton
sponge for the furyl ligand, resulting in opening of the five-
membered ring.19 Also, a rare example of ortho metalation
with concomitant C-C bond formation has been reported in
the UV irradiation of an iron thiocarbonyl phosphite com-
plex, affording the thioaldehyde complex by insertion of the
thiocarbonyl C atom into an ortho C-H bond of an axially
bound P(OPh)3 phenyl group.20 In our complexes, the
intramolecular coupling reaction involves the furyl ligand
with a long unsaturated hydrocarbon chain, resulting in
connecting this chain to the phosphine ligand, and the furyl
ligand serves as a hydride acceptor for the opening of the
five-membered ring. To our knowledge, this intramolecular
coupling reaction to bring a long hydrocarbon chain to a
phosphine ligand is the first example of this type.
Figure 1. ORTEP drawing of 8c, with the phenyl groups of the
phosphine ligands (except the ipso carbons and the phenyl
group with C-C bond formation) and hydrogen atoms omitted
˚
for clarity. Selected bond distances (A) and angles (deg): Ru-
(1)-C(19), 2.164(10); Ru(1)-C(20), 2.098(9); Ru(1)-C(28),
2.192(8); C(18)-C(19), 1.501(13); C(19)-C(20), 1.420(13); C-
(20)-C(21), 1.517(13); C(21)-C(22), 1.300(14); C(20)-C(28),
1.455(13); C(28)-C(29), 1.450(14); C(29)-O(1), 1.251(11); C-
(18)-C(19)-C(20), 123.6(8); C(19)-C(20)-C(28), 114.8(8); C-
(29)-C(28)-C(20), 122.6(8); C(28)-C(20)-C(21), 121.7(8);
C(21)-C(20)-C(19), 123.4(9).
Scheme 2
The protonation reaction of 8c was also explored by using
HBF4. Treatment of 8c in acetone with HBF4 afforded the
cationic complex 9c (Scheme 3) by direct addition of a proton
to one of the terminal carbon of the η3-allyl ligand having the
ester group. The structure of complex 9c is supported by
(17) (a) Del Piero, G.; Perego, G.; Zazzetta, A.; Cesari, M. Cryst.
Struct. Commun. 1974, 3, 725–729. (b) Klein, H.-F.; Schneider, S.; He, M.;
Floerke, U.; Haupt, H.-J. Eur. J. Inorg. Chem. 2000, 2295–2301. (c)
Bennett, M. A.; Clark, A. M.; Contel, M.; Rickard, C. E. F.; Roper, W. R.;
Wright, L. J. J. Organomet. Chem. 2000, 601, 299–304. (d) Clark, G. R.; Lu,
G.-L.; Rickard, C. E. F.; Roper, W. R.; Wright, L. J. J. Organomet. Chem.
2005, 690, 3309–3320.
phosphine ligand on 7 occurred when a toluene solution of 7
was heated to reflux. One pathway presumably involves
ortho metalation of a phenyl group of the remaining PPh3
on complex 7, resulting in formation of a Ru(IV)16 hydride
(18) Cooper, A. C.; Clot, E.; Huffman, J. C.; Streib, W. E.; Maseras,
F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999, 121, 97–106.
Davies, D. L.; Donald, S. M. A.; Macgregor, S. A. J. Am. Chem. Soc. 2005,
127, 13754–13755. These references and the alternative mechanism were
suggested by a reviewer.
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775–781.
(20) Cromhout, N. L.; Gallagher, J. F.; Manning, A. R.; Paul, A.
(16) (a) Sanchez-Castro, M. E.; Paz-Sandoval, M. A. Organometal-
lics 2008, 27, 6083–6089. (b) Ng, S. Y.; Tan, G. K.; Koh, L. L.; Leong,
W. K.; Goh, L. Y. Organometallics 2007, 26, 3352–3361. (c) Cadierno, V.;
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Organometallics 1999, 18, 1119–1121.

Communication
Organometallics, Vol. 29, No. 1, 2010 41
diffraction study. An ORTEP drawing of 9c is shown in
Figure 2. For 9c, the shorter lengths of Ru(1)-C(24)
˚
˚
(2.196(8) A) and Ru(1)-C(25) (2.260(8) A) and the longer
˚
length of C(24)-C(25) (1.433(11) A) compared to those in
complex 8c reveal that the degree of π-back-donation in
complex 9c might be stronger than that in 8c. The C(25)-C-
(26) and C(26)-C(27) bond distances of 1.509(11) and
˚
1.494(11) A, respectively, are normal for a C-C single bond.
˚
The C(27)-O(1) bond length is 1.236(10) A, which is com-
parable with that in 8c (1.251(11) A), possibly indicating
˚
weak σ-donation from O(1) to Ru(1).
Notably, complex 9c represents a rare example of an Ru
complex featuring a tridentate ligand donating six electrons
to the ruthenium metal via phosphine, an oxygen atom, and
an olefin group. In the literature, a bimetallic ruthenium
complex of biquinone, also with an olefinic and an oxygen
binding, was synthesized from a ruthenium cyclooctatriene
complex and p-biquinone.21 In another case, a ruthenium
hydrido acyl complex was reacted with diphenylacetylene to
afford a ruthenium complex with the R,β-unsaturated ketone
moiety bound to the metal in an η4 coordination mode,
involving both a classical η2 coordination of the olefinic
bond and a rather uncommon side-on coordination of the
CO bond.22 Here we demonstrated the transformation of an
allylic ligand with an ester substituent to a β,γ-unsaturated
ester group by protonation.
In conclusion, we have synthesized propargylic enyne
compounds 2a,b, from which the three neutral ruthenium
complexes 7a-c were obtained, each containing a furyl
ligand with an unsaturated hydrocarbon chain in high yield.
Unexpectedly, upon heating the toluene solution of 7 to
reflux, the furyl complexes 7a-c transformed into complexes
8a-c, respectively, each with an allylic ligand binding to the
ruthenium center in a η3 coordinating mode. Obviously the
long chain on the furyl ligand plays a key role in changing the
reactivity of these complexes. Further reactivity of these
furyl and allylic complexes is currently under investigation.
Figure 2. ORTEP drawing of 9c with the phenyl groups on the
phosphine ligand (except the ipso carbon and the phenyl group
with formation of a C-C bond) and hydrogen atoms omitted
˚
for clarity. Selected bond distances (A) and angles (deg): Ru(1)-
C(24), 2.196(8); Ru(1)-C(25), 2.260(8); Ru(1)-O(1), 2.144(6);
Ru(1)-P(1), 2.325(2); C(23)-C(24), 1.515(11); C(24)-C(25),
1.433(11); C(25)-C(26), 1.509(11); C(26)-C(27), 1.494(11);
C(27)-O(1), 1.236(10); C(27)-O(2), 1.320(10); C(30)-C(31),
1.323(11); Ru(1)-C(24)-C(25), 73.7(5); C(24)-C(25)-C(26),
120.7(8); C(25)-C(26)-C(27), 112.1(7); C(24)-C(25)-C(30),
118.1(7); C(26)-C(25)-C(30), 117.3(7).
NMR spectra and solid-state structure determination using
X-ray diffraction analysis. The 1H NMR spectrum of com-
plex 9c displays two doublet resonances at δ 3.95 and 2.70
2
with JHH = 19.0 Hz assignable to the methylene protons
near the ester group, showing correlations to the 13C reso-
Acknowledgment. We thank the National Science
Council, Taiwan, Republic of China, for financial sup-
port. We are grateful for a reviewer suggesting the alter-
native mechanism for the formation of 8 from 7.
1
1
nance at δ 39.33 in the H-13C HSQC spectrum. The H
signal at δ 6.13 is assigned to the proton at CR (see Scheme 3
for the R-labeling), showing correlation with the 13C reso-
nance at δ 77.98. Two doublet resonances attributable to the
internal olefinic trans protons appear at δ 6.25 and 5.98 with
3JHH = 16.0 Hz. The 13C NMR spectrum of 9c displays a
distinctive resonance at δ 187.95 assigned to the carbonyl
carbon, which shifts more downfield than that of complex 8c
due to the coordination of the oxygen atom to the ruthenium
center. The ESI mass spectrum of complex 9c exhibits a
signal at m/z 635.16, corresponding to the parent ion.
Supporting Information Available: Text, figures, CIF files,
and a table giving synthetic and spectroscopic data of the
compounds and complete crystallographic data for 8c and 9c.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(21) Ura, Y.; Sato, Y.; Shiotsuki, M.; Suzuki, T.; Wada, K.; Kondo,
T.; Mitsudo, T. Organometallics 2003, 22, 77–82.
ꢁ
(22) Benhamou, L.; Cesar, V.; Lugan, N.; Lavigne, G. Organometal-
Slow diffusion of pentane into a solution of 9c in CH2Cl2
allowed isolation of single crystals suitable for an X-ray
lics 2007, 26, 4673–4676.