Half-Sandwich Ruthenium(II) Complexes as Catalysts for Stereoselective Carbene-Carbene Coupling Reactions

Article abstract of DOI:10.1021/om990582x

Cyclopentadienyl complexes of general formula [RuX(η⁵-ligand)(PR₃)₂] have been found to catalyze the stereoselective decomposition of ethyl diazoacetate (EDA) to form diethyl maleate (DEM) in 95-99% purity with less than 1 mol % of catalyst, at temperatures depending on the bulkiness of the phosphine and the η⁵-ligand as well as the nature of the anionic ligand X. A detailed study of the reaction between [RuCl(η⁵-C₅H₅)(PPh₃) ₂] and EDA suggests that dissociation of one PPh₃ is a key step of the catalytic process, which proceeds via the intermediate [RuCl(η⁵-C₅H₅)(=CHCO ₂Et)(PPh₃)]. Although this electrophilic carbene was not detected in the reaction mixture, it was independently prepared in solution at low temperature starting from [Ru(η²-0₂CMe)(η⁵-C₅H ₅)(PPh₃). The acetate reacts with EDA at -40°C to form the cyclic ester [Ru(CH(CO₂Et)OC(Me)O)(η⁵-C₅H ₅)(PPh₃), which on treatment with Me₂SiCl₂ gives the metal carbene postulated in the catalytic cycle. The stoichiometric reaction of the latter compound with EDA selectively affords the olefin derivative [RuCl-(η⁵-C₅H₅)(η ²-DEM)(PPh₃)], which was also detected in the catalytic cycle. The new complexes [RuCl(η⁵-C₅H₅)(PR₃)₂] (PR₃ = PPh₂(2-MeC₆H₄), PPh₂Cy, P(3-MeC₆H₄)₃), bearing phosphines bulkier than PPh₃, have been prepared in high yield starting from ruthenium trichloride hydrate, cyclopentadiene, and phosphine in refluxing ethanol.

Full text of DOI:10.1021/om990582x

Organometallics 1999, 18, 5091-5096
5091
Ha lf-Sa n d w ich Ru th en iu m (II) Com p lexes a s Ca ta lysts for
Ster eoselective Ca r ben e-Ca r ben e Cou p lin g Rea ction s
Walter Baratta, Alessandro Del Zotto, and Pierluigi Rigo*
Dipartimento di Scienze e Tecnologie Chimiche, Universita` di Udine, Via Cotonificio 108,
33100 Udine, Italy
Received J uly 26, 1999
Cyclopentadienyl complexes of general formula [RuX(η5-ligand)(PR3)2] have been found
to catalyze the stereoselective decomposition of ethyl diazoacetate (EDA) to form diethyl
maleate (DEM) in 95-99% purity with less than 1 mol % of catalyst, at temperatures
depending on the bulkiness of the phosphine and the η5-ligand as well as the nature of the
anionic ligand X. A detailed study of the reaction between [RuCl(η5-C5H5)(PPh3)2] and EDA
suggests that dissociation of one PPh3 is a key step of the catalytic process, which proceeds
via the intermediate [RuCl(η5-C5H5)(dCHCO2Et)(PPh3)]. Although this electrophilic carbene
was not detected in the reaction mixture, it was independently prepared in solution at low
temperature starting from [Ru(η2-O2CMe)(η5-C5H5)(PPh3). The acetate reacts with EDA at
-40 °C to form the cyclic ester [Ru{CH(CO2Et)OC(Me)O}(η5-C5H5)(PPh3), which on treatment
with Me2SiCl2 gives the metal carbene postulated in the catalytic cycle. The stoichiometric
reaction of the latter compound with EDA selectively affords the olefin derivative [RuCl-
(η5-C5H5)(η2-DEM)(PPh3)], which was also detected in the catalytic cycle. The new complexes
[RuCl(η5-C5H5)(PR3)2] (PR3 ) PPh2(2-MeC6H4), PPh2Cy, P(3-MeC6H4)3), bearing phosphines
bulkier than PPh3, have been prepared in high yield starting from ruthenium trichloride
hydrate, cyclopentadiene, and phosphine in refluxing ethanol.
In tr od u ction
fumarate (DEF, the thermodynamic product trans), the
DEM/DEF ratio depending on the catalyst employed.
Highly stereoselective formation of DEM (93-96%) has
been obtained with ruthenium6,7 and osmium8 porphy-
rin derivatives with overall yields above 90%. In com-
parison, Cu(II) salts,9 [Rh2(OAc)4],10 and [MCl2(PPh3)3]11
(M ) Ru or Os) catalyze EDA decomposition with lower
DEM/DEF ratios.
R-Diazo carbonyl compounds have received increasing
attention in recent years because of their manifold
applications in organic synthesis via transition metal
catalyzed reactions.1 Coordinatively unsaturated com-
plexes react with diazo compounds to generate transient
electrophilic metal-carbenes,2 which are key intermedi-
ates in carbon-carbon bond-forming processes of stra-
tegic importance, such as alkene cyclopropanation,2a,3
olefin metathesis,4 and carbon-hydrogen bond inser-
tion.2c,5 Furthermore, metal complexes decompose diazo
compounds with formation of a mixture of cis and trans
alkenes via carbene dimerization. Thus, ethyl diazoac-
etate (EDA) catalytically decomposes affording diethyl
maleate (DEM, the kinetic product cis) and diethyl
Recently, we have found that the readily available
complex [RuCl(η5-C5H5)(PPh3)2] (1a ) converts EDA into
DEM quantitatively and with a purity higher than 99%,
DEF being the only byproduct.12 The DEM/DEF ratio
obtained with 1a is the highest reported to date.
Likewise, complex 1a catalyzes the decomposition of
i
other N2CHCOR compounds [R ) Me, nPr, Pr and
(CH2)10CH3], affording quantitatively RCOCHdCHCOR
carbene dimers, the cis isomer being formed in 95-97%
yield.12 Although reaction intermediates could not be
isolated, the experimental data suggested a mechanism
that implies formation of the carbene intermediate
[RuCl(η5-C5H5)(dCHCO2Et)(PPh3)], generated by the
reaction between the R-diazo carbonyl compound and
the 16-electron complex [RuCl(η5-C5H5)(PPh3)] (Scheme
* To whom correspondence should be addressed. E-mail:
pierluigi.rigo@dstc.uniud.it.
(1) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods
for Organic Synthesis with Diazo Compounds; Wiley & Sons: New
York, 1998.
(2) (a) Doyle, M. P. Chem. Rev. 1986, 86, 919. (b) Ye, T.; McKervey,
M. A. Chem. Rev. 1994, 94, 1091. (c) Zollinger, H. Diazo Chemistry II;
Wiley-VCH: Weinheim, 1995. (d) Do¨rwald, F. Z. Metal Carbene in
Organic Synthesis; Wiley-VCH: Weinheim, 1999.
(3) Doyle, M. P. In Comprehensive Organometallic Chemistry II;
Abel, E. W., Stone, F. G., Wilkinson, G., Eds.; Pergamon Press: New
York, 1995; Vol. 12, p 387.
(4) (a) Ivin, K. J .; Mol, J . C. Olefin Metathesis and Metathesis
Polymerization; Academic Press: San Diego, 1997. (b) Grubbs, R. H.;
Miller, S. J .; Fu, G. C. Acc. Chem. Res. 1995, 28, 446. (c) Moore, J . S.
In Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G., Wilkinson, G., Eds.; Pergamon Press: New York, 1995; Vol. 12, p
1209. (d) Schuster, M.; Blechert, S. Angew. Chem., Int. Ed. Engl. 1997,
36, 2036.
(6) Collman, J . P.; Rose, E.; Venburg, G. D. J . Chem. Soc., Chem.
Commun. 1993, 934.
(7) Galardon, E.; Le Maux, P.; Toupet, L.; Simonneaux, G. Orga-
nometallics 1998, 17, 566.
(8) Woo, L. K.; Smith, D. A. Organometallics 1992, 11, 2344
(9) Oshina, T.; Hagai, T. Tetrahedron Lett. 1980, 21, 1251.
(10) Shankar, B. K. R.; Shechter, H. Tetrahedron Lett. 1982, 23 ,
2277.
(5) Doyle, M. P. In Comprehensive Organometallic Chemistry II;
Abel, E. W., Stone, F. G., Wilkinson, G., Eds.; Pergamon Press: New
York, 1995; Vol. 12, p 421.
(11) Demonceau, A.; Lemoine, C. A.; Noels, A. F.; Chizhevsky, I. T.;
Sorokin, P. V. Tetrahedron Lett. 1995, 36, 8419.
(12) Baratta, W.; Del Zotto, A.; Rigo, P. Chem. Commun. 1997, 2163.
10.1021/om990582x CCC: $18.00 © 1999 American Chemical Society
Publication on Web 11/06/1999

5092 Organometallics, Vol. 18, No. 24, 1999
Baratta et al.
Sch em e 1
Ch a r t 1
1). The hypothesis of formation of a metal-carbene
intermediate suggests that 1a , and related half-sand-
wich ruthenium complexes, should display catalytic
activity in a broad spectrum of processes involving
carbene transfer reactions. The carbeneruthenium(II)
species was not spectroscopically detected in the reac-
tion mixtures, probably owing to its extremely low
concentration in solution. However, preliminary results
show that 1a induces cyclopropanation of styrene from
EDA as well as R-diazo carbonyl compound insertion
into N-H and S-H bonds. All these reactions are
believed to involve metal-carbene species.1,2b
Half-sandwich cyclopentadienyl ruthenium complexes
exhibit a rich and interesting chemistry, which is an
area of current active research.13 Although such com-
plexes have been used in several stoichiometric and
catalytic reactions,14 hitherto their application in cata-
lytic processes involving diazo derivatives has not been
described.
As part of a program directed toward the investigation
of the catalytic potential of this class of compounds, we
have now tested several complexes related to 1a in the
decomposition of EDA, with the aim to understand the
nature of the key steps occurring at the metal center.
To obtain a better insight into the influence of both
steric and electronic factors in the formation of the
carbene intermediate, the complexes reported in Chart
1 have been employed. We also succeeded in the
characterization in solution of [RuCl(η5-C5H5)(dCHCO2-
Et)(PPh3)] and [RuCl(η5-C5H5)(η2-DEM)(PPh3)], previ-
ously proposed as intermediates in the catalytic decom-
position of EDA (Scheme 1).12 Moreover, the novel
complexes [RuCl(η5-C5H5)(PR3)2] (PR3 ) PPh2(2-MeC6H4),
2a ; PPh2Cy, 2b; P(3-MeC6H4)3, 3a ) have been synthe-
sized and characterized.
from Aldrich Chemical Co.; PPh2Cy was purchased from Strem
Chemical Co. The compounds [RuCl(η5-C5H5)(PPh3)2] (1a ),15
[RuI(η5-C5H5)(PPh3)2] (1b),16 [Ru(CN)(η5-C5H5)(PPh3)2] (1c),17
[Ru(N3)(η5-C5H5)(PPh3)2] (1d ),18 [Ru(SnCl3)(η5-C5H5)(PPh3)2]
(1e),19 [Ru(CCPh)(η5-C5H5)(PPh3)2] (1f),20 [RuH(η5-C5H5)(PPh3)2]
(1g),16 [RuCl(η5-C5H5)(PPh2Me)2] (2c),21 [RuCl(η5-C5H5){P-
(OPh)3}2] (3b),22 [RuCl(η5-C5H5)(PPh2CH2CH2PPh2)] (4),23 [RuCl-
(η5-C5H5)(CO)(PPh3)] (5a),24 [RuCl(η5-C5H5)(CNtBu)(PPh3)] (5b),25
[RuCl(η5-C5H4Me)(PPh3)2] (6),26 [RuCl(η5-C5Me5)(PPh3)2] (7),21
[RuCl(η5-C9H7)(PPh3)2] (8),27 and [Ru(η2-O2CMe)(η5-C5H5)-
(PPh3)] (9)28 were prepared according to the literature. NMR
measurements were carried out using a Bruker AC 200
spectrometer. Chemical shifts, in ppm, are relative to TMS
for 1H and 13C, and to 85% H3PO4 for 31P. IR spectra were
recorded with a Nicolet Magna 550 FT-IR spectrometer.
Elemental analysis (C, H, N) were performed by the Microana-
lytical Laboratory of our department.
Syn th esis of [Ru Cl(η5-C5H5){P P h 2(2-MeC6H4)}2] (2a ).
Ruthenium trichloride hydrate (400 mg, 1.53 mmol as RuCl3‚
xH2O) was dissolved in ethanol (10 mL) by bringing the
mixture to a boil. After cooling, freshly distilled cyclopenta-
diene (1 mL, 12.1 mmol) was added. This solution was added
dropwise to diphenyl-2-tolylphosphine (1.50 g, 5.43 mmol)
dissolved in ethanol (40 mL) at the reflux point, and the
(15) Bruce, M. I.; Windsor, N. J . Aust. J . Chem. 1977, 30, 1601.
(16) Wilczewski, T.; Boche´nska, M.; Biernat, J . F. J . Organomet.
Chem. 1981, 215, 87.
(17) Baird, G. J .; Davies, S. G. J . Organomet. Chem. 1984, 262, 215.
(18) Khan, M. M. T.; Bhadbhade, M. M.; Siddiqui, M. R. H.;
Venkatasubramanian, K.; Tikhonova, J . A. Acta Crystallogr., Sect. C
1994, 50, 502.
(19) Blackmore, T.; Bruce, M. I.; Stone, F. G. A. J . Chem. Soc. 1971,
2376.
(20) Bruce, M. I.; Hameister, C.; Swincer, A. G.; Wallis, R. C. Inorg.
Synth. 1982, 21, 82.
(21) Treichel, P. M.; Komar, D. A.; Vincenti, P. J . Synth. React. Inorg.
Met.-Org. Chem. 1984, 14, 383.
(22) Bruce, M. I.; Gardner, R. C. F.; Stone, F. G. A. J . Chem. Soc.,
Dalton Trans. 1976, 81.
Exp er im en ta l Section
Gen er a l Meth od s. All reactions were carried out under an
argon atmosphere using standard Schlenck techniques. Sol-
vents were carefully dried by conventional methods and
distilled under argon before use. RuCl3‚xH2O, P(3-MeC6H4)3,
N2CHCO2Et, Me2SiCl2, and diethyl maleate were purchased
(23) Ashby, G. S.; Bruce, M. I.; Tomkins, J . B.; Wallis, R. C. Aust.
J . Chem. 1979, 32, 1003.
(24) Davies, S. G.; Simpson, S. J . J . Chem. Soc., Dalton Trans. 1984,
993.
(25) Bruce, M. I.; Wallis, R. C. Aust. J . Chem. 1981, 34, 209.
(26) Reventos, L. B.; Alonso, A. G. J . Organomet. Chem. 1986, 309,
179.
(27) Oro, L. A.; J . Ciriano, M. A.; Campo, M.; Foces-Foces, C.; Cano,
F. H. J . Organomet. Chem. 1985, 289, 117.
(28) Daniel, T.; Mahr, N.; Braun, T.; Werner, H. Organometallics
1993, 12, 1475.
(13) (a) Alberts, M. O.; Robinson, D. J .; Singleton, E. Coord. Chem.
Rev. 1987, 79, 1. (b) Daviews, S. G.; McNally, J . P.; Smallridge, A. J .
Adv. Organomet. Chem. 1990, 30, 1. (c) Bennet, M. A.; Khan, K.;
Wenger, E. In Comprehensive Organometallic Chemistry II; Abel, E.
W., Stone, F. G., Wilkinson, G., Eds.; Pergamon Press: New York,
1995; Vol. 7, p 473.
(14) Naota, T.; Takaya, H.; Murahashi, S. Chem. Rev. 1998, 98,
2599.

Half-Sandwich Ru(II) Complexes as Catalysts
Organometallics, Vol. 18, No. 24, 1999 5093
suspension was stirred for 1 h. The yellow-orange product was
filtered off, stirred in Et2O for 4 h, and crystallized from
dichloromethane/heptane. Yield: 890 mg (77%). Anal. Calcd
for C43H39ClP2Ru: C, 68.47; H, 5.21. Found: C, 68.31; H, 5.22.
1H NMR (CDCl3, 25 °C): δ ) 7.96 (q, 2H, 3J (HH) ) 4 Hz;
aromatic protons), 7.32-6.96 (m, 26H; aromatic protons), 3.98
(s, 5H; C5H5), 1.70 (s, 6H; CH3). 13C{1H} NMR (CDCl3, 25 °C):
δ ) 142.2 (t, J (CP) ) 2 Hz; CMe), 138.7 (pseudo t, J (CP) )
18.8 Hz; ipso-C6H4), 133.7 (t, J (CP) ) 9.1 Hz; m-C6H4), 136.3
(pseudo t, J (CP) ) 20.8 Hz; ipso-C6H5), 135.6 (pseudo t,
J (CP) ) 15.9 Hz; ipso-C6H5), 133.2 (t, J (CP) ) 5.0 Hz; o-C6H5),
132.4 (t, J (CP) ) 5.2 Hz; o-C6H5), 131.8 (t, J (CP) ) 4.8 Hz;
o-C6H4), 129.6 (s; p-C6H4), 128.5 (s; p-C6H5), 128.2 (s; p-C6H5),
127.6 (t, J (CP) ) 4.5 Hz; m-C6H5), 127.3 (t, J (CP) ) 4.2 Hz;
m-C6H5), 124.6 (t, J (CP) ) 5.8 Hz m-C6H4), 80.5 (t, J (CP) )
2.0 Hz; C5H5), 23.2 (t, J (CP) ) 2.0 Hz; CH3). 31P{1H} NMR
(CDCl3, 25 °C): δ ) 37.9.
°C): δ ) 274.3 (d, 2J (CP) ) 15.3 Hz; RudCH), 181.7 (CO),
137-128 (C arom), 94.9 (d, 2J (CP) ) 2.2 Hz; C5H5), 59.9 (CH2),
14.4 (CH3). 31P{1H} NMR (toluene-d8, -40 °C): δ ) 55.9.
Sp ectr oscop ic Evid en ce of [Ru Cl(η5-C5H5)(η2-DEM)-
(P P h 3)] (12). A red solution of [Ru(η2-O2CMe)(η5-C5H5)(PPh3)]
(9) (19 mg, 0.0390 mmol) in 0.4 mL of benzene-d6 was treated
with diethyl maleate (DEM) (5.7 µL, d ) 1.06 g/mL, 0.035
mmol). Dichlorodimethylsilane (4.5 µL, 0.037 mmol) was added
to the yellow solution, and the NMR spectra were registered
1
at room temperature. H NMR (C6D6, 20 °C): δ ) 7.70-6.80
3
(m, 15H; C6H5), 5.02 (s, 5H; C5H5), 4.20 (q, 2H, J (HP) ) 7.0
Hz, CH2), 4.16 (d, H, 3J (H,H) ) 9.4 Hz; CHdCH), 3.98 (q, 2H,
3J (HP) ) 7.2 Hz, CH2), 3.70 (dd, H, 3J (H,H) ) 9.4 Hz,
3J (HP) ) 14.0 Hz; CHdCH), 1.08 (t, 3H, 3J (HP) ) 7.0 Hz, CH2),
0.96 (t, 3H, 3J (HP) ) 7.2 Hz, CH2). 13C{1H} NMR (C6D6, 20
2
°C): δ ) 170.2 (CO), 136-128 (aromatic C), 91.0 (d, J (CP) )
2.6 Hz; C5H5), 60.8 (2CH2), 57.6 (br, CHdCH), 50.7 (br, CHd
CH), 14.6, (CH3), 14.4 (CH3). 31P{1H} NMR (C6D6, 20 °C):
δ ) 49.6.
Syn th esis of [Ru Cl(η5-C5H5)(P P h 2Cy)2] (2b). The syn-
thesis of 2b was carried out as described for the corresponding
complex 2a by using PPh2Cy in place of PPh2(2-MeC6H4).
Yield: 667 mg (59%). Anal. Calcd for C41H47ClP2Ru: C, 66.70;
H, 6.42. Found: C, 66.62; H, 6.35. 1H NMR (CDCl3, 25 °C):
δ ) 7.38-7.15 (m, 20H; C6H5), 3.93 (s, 5H; C5H5), 2.62 (d, 3J (H,
Gen er a l P r oced u r e for th e Ca ta lytic EDA Decom p osi-
tion . The samples were typically prepared as follows: EDA
(0.2 mmol) was added to 0.5 mL of benzene-d6 containing the
catalyst (2 µmol) under argon, and the solution was gently
heated until nitrogen evolution started. After a few minutes
the reaction was completed and the products were analyzed
by NMR. No cyclopropanation products or other byproducts
were detected in the reaction mixtures by NMR and GC-mass
spectrometry.
3
H) ) 11.6 Hz, 2H; Cy), 2.31 (pseudo t, J (H,H) ) 9.9 Hz, 2H;
Cy) 1.75-0.85 (m, 18H; Cy). 13C{1H} NMR (CDCl3, 25 °C):
138.5 (pseudo t, J (CP) ) 16.6 Hz; ipso-C6H5), 136.7 (pseudo t,
J (CP) ) 18.5 Hz; ipso-C6H5), 133.8 (t, J (CP) ) 4.9 Hz; o-C6H5),
132.5 (t, J (CP) ) 4.2 Hz; o-C6H5), 128.7 (s; p-C6H5), 128.2 (s;
p-C6H5), 127.7 (t, J (CP) ) 3.9 Hz; m-C6H5), 127.1 (t, J (CP) )
4.5 Hz; m-C6H5), 80.3 (t, J (CP) ) 2.3 Hz; C5H5), 39.8 (t,
J (CP) ) 9.1 Hz; CH), 28.7 (s; CH2), 28.5 (s; CH2), 27.6-27.1
(m; CH2), 26.4 (s; CH2). 31P{1H} NMR (CDCl3, 25 °C): δ ) 38.7.
Resu lts a n d Discu ssion
Syn th esis of Half-San dwich Ru th en iu m (II) Com -
p lexes. The readily available complex 1a has been
employed as suitable starting product, because of its
facility to give chloride or PPh3 exchange reactions
under relatively mild experimental conditions. However,
attempts to obtain complexes from 1a with phosphines
bulkier than PPh3, such as PPh2Cy or P(3-MeC6H4)3,
resulted in the formation of only a very small amount
of the corresponding mixed complexes [RuCl(η5-C5H5)-
(PPh3)(PR3)]. Therefore, substitution of PPh3 in 1a
seems to be strongly controlled by the steric properties
of the incoming phosphine. Similarly the syntheses of
1a, the half-sandwich complexes [RuCl(η5-C5H5)(PPh2R)2]
(R ) 2-MeC6H4, 2a ; Cy, 2b), and [RuCl(η5-C5H5){P(3-
MeC6H4)3}2] (3a ) were obtained by addition of freshly
distilled cyclopentadiene to a mixture of ruthenium
trichloride hydrate and the corresponding phosphine in
boiling ethanol. These compounds, bearing phosphines
with cone angles larger than that of PPh3 (145°), were
isolated in high yield and characterized by NMR and
elemental analysis. Their 1H NMR spectra show the
expected singlet for the η5-coordinated C5H5 in the range
δ 4.08-4.31, together with the appropriate resonances
of phosphine protons, whereas the 31P{1H} NMR spectra
exhibit a signal in the range δ 38-40. Characteristic
13C{1H} NMR signals are observed for the cyclopenta-
dienyl carbon atoms at ca. δ 81. It should be noted that
with phosphines having steric requirements higher than
Syn th esis of [Ru Cl(η5-C5H5){P (3-MeC6H4)3}2] (3a ). The
synthesis of 3a was carried out as described for the corre-
sponding complex 2a by using P(3-MeC6H4)3 in place of PPh2-
(2-MeC6H4). Yield: 682 mg (55%). Anal. Calcd for C47H47ClP2-
Ru: C, 69.66; H, 5.85. Found: C, 69.39; H, 5.89. 1H NMR
(CDCl3, 25 °C): δ ) 7.52-7.38 (m, 12H; C6H4), 7.30-7.23 (m,
12H; C6H4), 4.34 (s, 5H; C5H5), 2.39 (s, 18H; CH3). 13C{1H}
NMR (CDCl3, 25 °C): δ ) 138.5 (pseudo t, J (CP) ) 20.1 Hz;
ipso-C6H4), 136.6 (t, J (CP) ) 4.9 Hz; m-C6H4), 134.3 (t,
J (CP) ) 5.2 Hz; o-C6H4), 131.0 (t, J (CP) ) 4.9 Hz; o-C6H4),
129.4 (s; p-C6H4), 127.2 (t, J (CP) ) 4.9 Hz; m-C6H4), 81.4 (t,
J (CP) ) 2.3 Hz; C5H5), 21.6 (CH3). 31P{1H} NMR (CDCl3, 25
°C): δ ) 39.1.
Sp ectr oscop ic Evid en ce of [Ru {CH(CO2Et)OC(Me)O}-
(η5-C5H5)(P P h 3)] (10). The complex Ru(η2-O2CMe)(η5-C5H5)-
(PPh3) (9) (21 mg, 0.043 mmol) was dissolved in 0.4 mL of
toluene-d8. The orange solution was cooled at -50 °C, and N2-
CHCO2Et (4.5 µL, 0.043 mmol) was added, affording a yellow
1
solution after rapid nitrogen evolution. H NMR (toluene-d8,
-40 °C): δ ) 7.65-7.55 (m, 6H), 7.05-6.85 (m, 9H), 6.29 (d,
1H, 3J (HP) ) 15.6 Hz, PRuCH), 4.27 (s, 5H; C5H5), 4.24 (q,
2H, 3J (H,H) ) 7.3 Hz, CH2CH3), 1.20 (s, 3H; CH3), 1.17 (t, 3H,
3J (H,H) ) 7.1 Hz, CH2CH3). 13C{1H} NMR (toluene-d8, -40
3
°C): δ ) 181.6 (C(O)OEt), 181.4 (d, J (CP) ) 2.8 Hz; OC(O)-
CH3), 136.6 (d, 1J (CP) ) 36.2 Hz; ipso-C6H5), 134.2 (d,
2J (CP) ) 11.4 Hz; o-C6H5), 129.1 (m-C6H5), 128.0 (p-C6H5), 88.5
2
2
(d, J (CP) ) 11.4 Hz; RuCH), 76.4 (d, J (CP) ) 2.9 Hz; C5H5),
58.5 (CH2), 17.8 (CH3CO), 15.1 (CH3CH2). 31P{1H} NMR
(toluene-d8, -40 °C): δ ) 62.5.
PPh2(2-MeC6H4), such as PPh2(2,6-Me2C6H3), PPh3-n
-
Sp ectr oscop ic Evid en ce of [Ru Cl(η5-C5H5){dCH(CO2-
Et)}(P P h 3)] (11). Dichlorodimethylsilane (6.2 µL, 0.051 mmol)
was added to the solution of complex 10 at -40 °C, prepared
as described above. A change of color from yellow to green
occurred slowly, and the spectra were recorded after 30 min.
1H NMR (toluene-d8, -40 °C): δ ) 15.26 (d, 3J (HP) ) 16.9;
RudCH), 7.7-6.8 (m, 15H), 5.11 (s, 5H; C5H5), 4.00 (q, 2H,
3J (H,H) ) 7.4 Hz, CH2CH3), 1.70 (s, 3H; CH3), 0.97 (t, 3H,
3J (H,H) ) 7.2 Hz, CH2CH3). 13C{1H} NMR (toluene-d8, -40
(2-MeC6H4)n (n ) 2, 3), and PCy3, we were unable to
isolate complexes of general formula [RuCl(η5-C5H5)-
(PR3)2] by this route.
Liga n d s Effects in th e Ca ta lytic Decom p osition
of EDA. To explore the catalytic potential of complexes
1a -9 in the stereocontrolled carbene dimerization, their
reactions with excess EDA in benzene-d6 have been
investigated.

5094 Organometallics, Vol. 18, No. 24, 1999
Baratta et al.
phine cone angle.30 As regards the influence of the η5-
ligand, the temperature at which nitrogen evolution
starts decreases in the order η5-C5H5 > η5-C5H4Me >
η5-C9H7 > η5-C5Me5. This is consistent with the kinetic
studies performed by Gamasa and co-workers,31 who
have found that complex 8 dissociates PPh3 an order of
magnitude faster than 1a , and also with the observation
that isosteric phosphines are more strongly bound in
the η5-C5H5 compared to the η5-C5Me5 system.29b It is
likely that η5-ligands in [RuCl(η5-ligand)(PPh3)2], which
can behave as an electron reservoir toward the metal
fragment, favor the ruthenium-phosphorus bond cleav-
age and stabilize the 16-electron intermediate. Accord-
ingly, indenyl and methylcyclopentadienyl have already
been described as stronger donors with respect to
cyclopentadienyl.32
Ta ble 1. Tem p er a tu r es of EDA Decom p osition
In d u ced by Ha lf-Sa n d w ich Ru th en iu m Com p lexesa
complex
T (°C)b
1e, 2c, 3b, 4, 5a , 5b
c
1c, 1d , 1f, 1g
1a , 1b
3a
7
2b
8
6
9
2a
>70
65
60
55
50
45
35
20
<20
a
b
In benzene, 1 mol % catalyst. Temperature at which dini-
trogen evolution starts. c No EDA decomposition was observed
even at 80 °C.
Complexes 1e, 2c, 3b, 4, 5a , and 5b are not catalyti-
cally active for EDA decomposition even at 80 °C. All
other compounds decompose EDA with almost quanti-
tative conversion into DEM and DEF, the former being
present in the reaction mixture in yields higher than
95%. The best selectivity for the cis isomer (>99%) was
however obtained with complex 1a . Although the dimer-
ization reaction takes place within a few seconds, as
inferred by dinitrogen evolution, the temperature at
which the decomposition reaction starts depends on the
ruthenium complex employed (Table 1). Thus, for in-
stance, complex 2a , which contains the bulky ligand
PPh2(2-MeC6H4), promotes evolution of N2 below 20 °C,
whereas even at 80 °C the derivative 2c with the ligand
PPh2Me is catalytically nonactive. It is generally ac-
cepted that, in nonpolar solvents, one phosphine ligand
in [RuCl(η5-ligand)(PR3)2] complexes can be displaced
at high temperature, affording a very reactive coordi-
natively unsaturated species [RuCl(η5-ligand)(PR3)].21,24
The mechanism reported in Scheme 1 is in agreement
with such a statement. Moreover, systematic studies on
the reaction of 1a with EDA have shown a strong
inhibition of the reaction on addition of PPh3, which
implies that phosphine dissociation is most likely the
rate-determining step. The lack of reactivity of com-
plexes 3b, 5a , and 5b, containing the low sterically
demanding ligands P(OPh)3, CO, and CNtBu, and 4,
bearing a chelate diphosphine, is probably due to a high
activation barrier to ligand dissociation. In contrast,
complex [Ru(η2-O2CH3)(η5-C5H5)(PPh3)] (9), in which
unsaturation can be achieved through a η2 to η1 conver-
sion of the acetato ligand, is catalytically active even at
20 °C. The results collected in Table 1 suggest that the
temperature of decomposition of EDA could be related
to the lability of the phosphine ligand, namely, to the
relative facility of formation of the 16-electron interme-
diate. In complexes [RuCl(η5-C5H5)(PR3)2] the temper-
ature at which EDA decomposition occurs decreases in
the order PPh2Me > PPh3> P(3-MeC6H4)3 > PPh2Cy >
PPh2(2-MeC6H4). This trend agrees with the results of
Nolan and co-workers,29 who suggest that the ruthe-
nium-phosphorus bond in complexes of the type [RuCl-
(η5-ligand)(PR3)2] weakens with the increase of phos-
Moreover, as evidenced by the data reported in Table
1, for complexes [RuX(η5-C5H5)(PPh3)2] the temperature
at which nitrogen evolution starts also depends on the
nature of the ancillary ligand X. Apparently, when X
has electron lone pairs, as Cl and I, the dissociation of
one phosphine is favored, probably because the π-donor
ability of the halide can stabilize the coordinatively
unsaturated species.33
Mech a n istic Stu d ies. The decomposition reaction of
EDA catalyzed by 1a has been examined in detail, and
the results agree with the mechanism depicted in
Scheme 1. When a toluene solution of 1a is treated at
65 °C with excess EDA (Ru:EDA ≈ 1:1000) a rapid
evolution of N2 occurs with quantitative formation of
DEM and trace amounts of DEF. At room temperature
no reaction is observed, and heating at 65 °C is neces-
sary to start EDA decomposition. The 31P{1H} NMR
spectrum of the reaction mixture shows that during the
catalytic reaction a new complex (δ 50.3) and the ylide
EtCO2CHdPPh3 are formed in almost 1:1 molar ratio,
along with trace amounts of the phosphazine EtCO2-
CHN2dPPh3. The new ruthenium derivative was sub-
sequently identified as [RuCl(η5-C5H5)(η2-DEM)(PPh3)]
(12) by comparison of the NMR data with those of an
authentic sample prepared from [RuCl(η5-C5H5){dCH-
CO2Et}(PPh3)] (11) and EDA. The 31P{1H} NMR spectra
recorded during the catalytic process show that the
concentration of 1a decreases by addition of subsequent
amounts of EDA, with concomitant formation of 12 and
EtCO2CHdPPh3. Removal of solvent and DEM from the
final solution affords a dark brown mixture of 1a , 12,
and EtCO2CHdPPh3, whose relative amounts depend
on the number of catalytic runs performed. It is note-
worthy that addition of PPh3 to the reaction mixture
converts promptly the labile complex 12 into 1a . The
phosphazine is formed in solution by an equilibrium
reaction between EDA and the phosphine, without
formation of the corresponding ylide even at 90 °C. For
(30) Tolman, C. A. Chem. Rev. 1977, 77, 313.
(31) Gamasa, M. P.; Gimeno, J .; Gonzales-Bernardo, C.; Martin-
Vaca, B. M.; Monti, D.; Bassetti, M. Organometallics 1996, 15, 302.
(32) (a) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987. (b) Ambrosi, L.;
Bassetti, M.; Buttiglieri, P.; Mannina, L.; Monti, D.; Bocelli, G. J .
Organomet. Chem. 1993, 455, 167. (c) Shen, J .; Stevens, E. D.; Nolan,
S. P Organometallics 1998, 17, 3000.
(33) (a) Bickford, C. C.; J ohnson T. J .; Davidson, E. R.; Caulton, K.
G. Inorg. Chem. 1994, 33, 1080. (b) J ohnson T. J .; Folting K. Streib,
W. E.; Martin J . D.; Huffman J . C.; J ackson, S. A.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1995, 34, 488.
(29) (a) Luo, L.; Nolan, S. P. Organometallics 1994, 13, 4781. (b)
Cucullu, M. E.; Luo, L.; Nolan, S. P.; Fagan, P. J .; J ones, N. L.;
Calabrese, J . C. Organometallics 1995, 14, 289. (c) Serron, S.; Luo, L.;
Li, C.; Cucullu, M. E.; Nolan, S. P Organometallics 1995, 14, 5290. (d)
Smith, D. C.; Haar, C. M.; Luo, L.; Li, C.; Cucullu, M. E.; Mahler, C.
H.; Nolan, S. P.; Marshall, W. J .; J ones, N. L.; Fagan, P. J . Organo-
metallics 1999, 18, 2357. (e) Serron, S. A.; Luo, L.; Stevens, E. D.;
Nolan, S. P.; J ones, N. L.; Fagan, P. J . Organometallics 1996, 15, 5209.

Half-Sandwich Ru(II) Complexes as Catalysts
Organometallics, Vol. 18, No. 24, 1999 5095
conceivable that complex [Ru(η1-O2CMe)(η5-C5H5)(dCH-
CO2Et)(PPh3)] is indeed the initial product of the
reaction between 9 and EDA, but subsequently a
nucleophilic attack of the η1-acetate group on the Rud
C bond yields the metallacycle 10. Similarly, Werner
has reported that 9 reacts with HCtCCO2Me to give,
instead of the expected [Ru(η1-O2CMe)(η5-C5H5)(dCd
CRR′)(PPh3)] complex,28 a cyclic vinyl ester formed by
a nucleophilic attack of the acetate carbonyl oxygen on
the R carbon atom of the vinylidene intermediate. A
similar cyclic ester has been obtained by reaction of
sodium acetate with the carbene complex [RuCl(η2-
COPh)(dCH2)(PPh3)2].35
Sch em e 2
Complex 10 is relatively stable in toluene-d8 solution
at temperatures below -30 °C, even in the presence of
EDA, whereas at room temperature it slowly reacts with
EDA to afford DEM. The conversion of 10 into the
carbene complex 11 quickly occurs with chloride trans-
fer agents via displacement of the acetato moiety. Thus,
by addition of dicholorodimethylsilane to a toluene-d8
solution of 10 at -40 °C, the color turns green and the
signal at δ 62.5 in the 31P{1H} NMR spectrum disap-
pears, whereas a new resonance appears at δ 55.9. The
13C{1H} NMR spectrum exhibits a doublet at δ 274.3
the associative process log K values of about 2 and 1 at
20 and 80 °C, respectively, have been obtained by 31P-
{1H} NMR measurements in benzene-d6. Catalytic runs
(EDA/1a ) 100) carried out in the presence of added
PPh3 show an increase of the temperature necessary to
start the reaction. Thus, with equimolar PPh3 and EDA,
no evolution of N2 was observed within 5 min at 65 °C.
In the same conditions, at 80 °C no DEM was detected,
but the solution contains PPh3, phosphazine, and ylide
in a 1:4:1.5 molar ratio. After 45 min the molar ratio
changed to a 2:1:9, and DEM was also present in trace
amounts, thus suggesting that PPh3 is a better nucleo-
phile for the carbene than EDA. All these experimental
evidence are in agreement with the dissociative mech-
anism depicted in Scheme 1, which involves formation
of the carbeneruthenium(II) complex 11, which promptly
reacts with EDA or PPh3 to give DEM or EtCO2CHd
PPh3, respectively. The presence of additional PPh3
hinders the displacement of one phosphine from 1a and
the formation of the 16-electron complex [RuCl(η5-C5H5)-
(PPh3)]. Although detection of the highly reactive elec-
trophilic carbene 11 in the reaction mixture failed, this
compound was independently prepared and character-
ized in solution at low temperature, according to the
procedure adopted by Werner and co-workers for the
synthesis of the analogue [RuCl(η5-C5H5)(dCPh2)-
(PPh3)].34 When an equimolar amount of EDA was
added to a toluene-d8 solution of 9 at -50 °C, the orange
solution turned yellow and N2 evolution took place.
However, the product formed was not the carbene [Ru-
(η1-O2CMe)(η5-C5H5)(dCHCO2Et)(PPh3)], for which a
13C{1H} NMR signal at very low field (δ > 250) attribut-
able to the RudC carbon atom is expected.2d Hence, a
careful and complete analysis of both 1H and 13C{1H}
NMR spectra was carried out to identify the product,
and all resonances have been unambiguously assigned.
The most characteristic feature in the 13C{1H} NMR
spectrum is the presence of a signal at δ 88.5 (2J (PC) )
1
(J (PC) ) 15.3 Hz) and the H NMR spectrum shows a
doublet at δ 15.26 (J (PH) ) 16.9 Hz), which are
consistent with the formation of a RudCHCO2Et moi-
1
ety.36 These and the other features of both H and 13C-
{1H} NMR spectra agree with structure 11.
When an equimolar amount of EDA is added to a
solution of 11 at -40 °C, nitrogen evolution occurs and
the color turns yellow. The 31P{1H} NMR spectrum of
the solution shows a single resonance at δ 50.2 at -40
°C in toluene-d8, which can be attributed to the DEM
derivative 12. Thus, ruthenium carbene 11, whereas
stable for a few hours at low temperature, promptly
undergoes a nucleophilic attack by EDA affording DEM.
Compound 12 can also be obtained in solution from 9
by stepwise addition of DEM and Me2SiCl2 (Scheme 2).
Hence, addition of an equimolar amount of DEM to 9
in benzene-d6 at room temperature results in the
formation of a yellow solution whose NMR features
suggest the formation of the adduct [Ru(η1-O2CMe)(η5-
C5H5)(η2-DEM)(PPh3)] 31P{1H} NMR resonance at δ
49.7). Subsequent treatment with Me2SiCl2 affords a
product whose 31P{1H} NMR spectrum exhibits the
expected resonance at δ 50.2 attributed to 12. The
resonances of the olefinic protons are observed in the
1H NMR spectrum as the AB part of an ABX spin
system, where X is the 31P nucleus, (δ HA ) 4.16, δ
HB ) 3.70; J (HAHB) ) 9.4, J (PHB) ) 14.0 Hz), as
inferred by 1H, 1H-2D COSY, and 31P-decoupled 1H
NMR measurements. In the 13C{1H} NMR spectrum the
resonances of the olefinic carbon atoms appear at δ 57.6
and 50.7. These data can be compared to those of the
related olefin-osmium complex [OsCl(η5-C5H5)(η2-DEM)-
(PiPr3)], isolated by Esteruelas and co-workers from
[OsCl(η5-C5H5)(PiPr3)2] and EDA and characterized by
NMR and X-ray analysis.37 It should be noted that the
1
11.4 and J (CH) ) 154.4 Hz), which can be attributed
to the carbon atom bound to ruthenium of the metal-
lacyclo derivative [Ru{(CH(CO2Et)OC(Me)O}(η5-C5H5)-
(PPh3)] (10) (Scheme 2). Furthermore, the 1H NMR
spectrum shows a doublet at δ 6.29 (J (PH) ) 15.6 Hz),
integrating for one proton, which is consistent with a
C-H directly bound to ruthenium. It seems therefore
(35) Bohle, D. S.; Clark, G. R.; Rickard, C. E. F.; Roper, W. R.;
Wright, L. J . J . Organomet. Chem. 1988, 358, 411.
(36) (a) Park, S.-B.; Sakata, N.; Nishiyama, H. Chem. Eur. J . 1996,
2, 303. (b) Schwab, P.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc.
1996, 118, 100. (c) Stumpf, A. W.; Saive, E.; Demonceau, A.; Noels, A.
F. J . Chem. Soc., Chem. Commun. 1995, 1127. (d) Djukic, J .-P.; Smith,
D. A.; Young, V. G.; Woo, L. K. Organometallics 1994, 13, 3020.
(34) (a) Braun, T.; Gevert, O.; Werner, H.; J . Am. Chem. Soc. 1995,
117, 7291. (b) Werner, H.; Braun, T.; Daniel, T.; Gevert, O.; Schulz M.
J . Organomet. Chem. 1997, 541, 127.

5096 Organometallics, Vol. 18, No. 24, 1999
Baratta et al.
direct interaction of 1a with DEM in benzene-d6 does
not lead to displacement of PPh3, even if an excess of
olefin is used. By contrast, in the case of complex 2a an
equilibrium reaction takes place at room temperature
with formation of the PPh2(2-MeC6H4) analogue of 12.
lower temperature. The most characteristic feature of
the decomposition reaction is the high selectivity for the
cis-olefin, and this study has revealed that in half-
sandwich ruthenium complexes the nature of the ancil-
lary ligands has a minor influence on the stereoselec-
tivity of the process. The origin of this stereoselectivity
may be due to the steric influences of the ligands that
force EDA to attack the electrophilic metal-carbene
intermediate to give the less crowded cis π-olefin metal
complex. Studies are currently in progress to scrutinize
the catalytic potential of half-sandwich cyclopentadi-
enyl- and pentamethylcyclopentadienylruthenium com-
plexes in other reactions with diazo compounds, ranging
from cyclopropanation and dipolar addition to insertion
reactions.
Con clu d in g Rem a r k s
In summary, we have shown that a number of half-
sandwich ruthenium(II) complexes of general formula
[RuX(η5-ligand)(PR3)2] are effective catalysts for the
stereoselective decomposition of R-diazo carbonyl com-
pounds to cis-olefins. The catalytic activity of the half-
sandwich complexes depends on the formation of the
coordinatively unsaturated [RuX(η5-ligand)(PR3)] spe-
cies, which react with diazo compounds, affording metal-
stabilized carbene intermediates. Apparently, the EDA
decomposition starts at the temperature at which phos-
phine dissociation takes place, to form the catalytically
active 16-electron complex. Bulky phosphines, electron-
releasing η5-ligands, and π-donor X favor reactions at
Ack n ow led gm en t. This work was supported by the
Italian Ministero dell’Universita` e della Ricerca Scien-
tifica e Tecnologica (Rome). The authors thank Dr. P.
Martinuzzi of the Dipartimento di Scienze e Tecnologie
Chimiche, Universita` di Udine, for assistance with the
NMR measurements.
(37) Esteruelas, M. A.; Lopez, A. M.; Ruiz, N.; Tolosa, J . I. Orga-
nometallics 1997, 16, 4657.
OM990582X