Half-sandwich ruthenium(II) catalysts for C-C coupling reactions between alkenes and diazo compounds

Article abstract of DOI:10.1021/om0003537

The complex [(η⁵-C₅H₅)Ru(PPh₃) ₂Cl] (1) and other readily available ruthenium(II) derivatives of general formula [(η⁵-ligand)Ru(PR₃)₂X] efficiently catalyze the cyclopropanation of styrene and other electron-rich alkenes in the presence of ethyl diazoacetate with a high cis stereoselectivity. When diphenyldiazomethane is employed as carbene source, the reaction with styrene, catalyzed by 1, affords mainly 1,1,3-triphenylpropene, as result of a formal :CPh₂-:CHCH₂Ph coupling. Furthermore, appreciable amounts of the metathesis and, cyclopropanation products 1,1-diphenylethene and 1,2-diphenylcyclopropanes in a 1,1 molar ratio are observed. The carbene complex [(η⁵-C₅H₅)Ru(=CPh₂)(PPh ₃)Cl] (13), which was detected during the catalytic process, can be readily obtained in 85% isolated yield from 1 and diphenyldiazomethane in a one-pot reaction. With styrene, complex 13 undergoes a stoichiometric carbene transfer reaction, yielding the same organic products observed in the catalytic process with 1.

Full text of DOI:10.1021/om0003537

3664
Organometallics 2000, 19, 3664-3669
Ha lf-Sa n d w ich Ru th en iu m (II) Ca ta lysts for C-C
Cou p lin g Rea ction s betw een Alk en es a n d Dia zo
Com p ou n d s
Walter Baratta,*,† Wolfgang A. Herrmann,*,‡ Roland M. Kratzer,‡ and
Pierluigi Rigo†
Dipartimento di Scienze e Tecnologie Chimiche, Universita` di Udine, Via Cotonificio 108,
I-33100 Udine, Italy, and Anorganisch-chemisches Institut, Technische Universita¨t Mu¨nchen,
Lichtenbergstrasse 4, D-85747 Garching, Germany
Received April 24, 2000
The complex [(η5-C5H5)Ru(PPh3)2Cl] (1) and other readily available ruthenium(II) deriva-
tives of general formula [(η5-ligand)Ru(PR3)2X] efficiently catalyze the cyclopropanation of
styrene and other electron-rich alkenes in the presence of ethyl diazoacetate with a high cis
stereoselectivity. When diphenyldiazomethane is employed as carbene source, the reaction
with styrene, catalyzed by 1, affords mainly 1,1,3-triphenylpropene, as result of a formal
:CPh2-:CHCH2Ph coupling. Furthermore, appreciable amounts of the metathesis and
cyclopropanation products 1,1-diphenylethene and 1,2-diphenylcyclopropanes in a 1:1 molar
ratio are observed. The carbene complex [(η5-C5H5)Ru(dCPh2)(PPh3)Cl] (13), which was
detected during the catalytic process, can be readily obtained in 85% isolated yield from 1
and diphenyldiazomethane in a one-pot reaction. With styrene, complex 13 undergoes a
stoichiometric carbene transfer reaction, yielding the same organic products observed in
the catalytic process with 1.
In tr od u ction
tion pathway. As a rule, reactions of electron-rich
terminal alkenes with R-diazo carbonyl compounds,
catalyzed by transition metal complexes, afford mixtures
containing the trans isomers as the predominant prod-
ucts. When ethyl diazoacetate (EDA) is used, a catalytic
cyclopropane carboxylate formation favoring the cis
isomer was reported for only a few systems: “chiral
wall” porphyrin rhodium(III) complexes,5 pyrazolyborate
and polypyrazole copper(I) derivatives,6 the cationic
complex [(η5-C5H5)Fe(CO)2(thf)][BF4], and most recently
chiral Ru-salen compounds.7 However, most of these
complexes suffer from strong limitations due to the
difficult and low-yielding catalyst synthesis5a or the low
catalyst efficiency.7a,d By way of contrast, high cis
stereocontrolled cyclopropanation of styrene was re-
ported using [W(dCHPh)(CO)5]8 and [(η5-C5H5)Fe-
(dCHPh)(CO)2]+ 9 as a carbene source in stoichiometric
reactions.
Although catalytic carbene transfer reactions, starting
from diazo compounds, have been known for many
decades, a growing number of transition metal com-
plexes address the question of selectivity.1 In this
regard, the asymmetric cyclopropanation of alkenes
with R-diazo carbonyl compounds using chiral transition
metal catalysts was intensively investigated in the past
decade, because of the importance of naturally occurring
cyclopropyl derivatives.2 A remarkable improvement of
enantioselectivity in alkene cyclopropanation was-
achieved with chiral copper, rhodium,3 and, more re-
cently, ruthenium complexes.4 Despite the relatively
sophisticated level of knowledge concerning the enan-
tioselective reaction tuning, few efforts have been
directed to inverting the common stereoselective reac-
* To whom correspondence should be addressed. E-mail: inorg@
dstc.uniud.it (W.B.).
† Universita` di Udine.
(4) (a) Park, S. B.; Nishiyama, H.; Itoh, Y.; Itoh, K. J . Chem. Soc.,
Chem. Commun. 1994, 1315. (b) Nishiyama, H.; Itoh, Y.; Sugawara,
Y.; Matsumoto, H.; Aoki, K.; Itoh, K. Bull. Chem. Soc. J pn. 1995, 68,
1247. (c) Nishiyama, H.; Aoki, K.; Itoh, H.; Iwamura, T.; Sakata, N.;
Kurihara, O.; Motoyama, Y. Chem. Lett. 1996, 1071. (d) Nishiyama,
H.; Soeda, N.; Naito, T.; Motoyama, Y. Tetrahedron: Asymmetry 1998,
9, 2865.
(5) (a) Maxwell, J . L.; O’Malley, S.; Brown, K. C.; Kodadek, T.
Organometallics 1992, 11, 645. (b) Callot, H. J .; Metz, F.; Piechocki,
C. Tetrahedron 1982, 38, 2365.
‡ Technische Universita¨t Mu¨nchen.
(1) (a) Doyle, M. P. In Comprehensive Organometallic Chemistry II;
Abel, E. W., Stone, F. G., Wilkinson, G., Eds.; Pergamon Press: New
York, 1995; Vol. 12, pp 387, 421. (b) Doyle, M. P. Chem. Rev. 1986, 86,
919. (c) Padwa, A.; Austin, D. J . Angew. Chem., Int. Ed. Engl. 1994,
33, 1797. (d) Noels, A. F.; Demonceau, A. In Applied Homogeneous
Catalysis by Organometallic Complexes; Cornil B., Herrmann, W. A.,
Eds.; Verlag Chemie: Weinheim, 1996. (e) Ye, T.; McKervey, M. A.
Chem. Rev. 1994, 94, 1091. (f) Brookhart, M.; Studabacker, W. B.
Chem. Rev. 1987, 87, 411. (g) Singh, V. K.; Datta Gupta, A.; Sekar, G.
Synthesis 1997, 137.
(6) (a) Pe´rez, P. J .; Brookhart, M.; Templeton, J . L. Organometallics
1993, 12, 261. (b) Tokar, C. J .; Kettler, P. B.; Tolman, W. B.
Organometallics 1992, 11, 2737.
(2) Doyle, M. P.; Forbes, D. C. Chem. Rev. 1998, 911.
(3) (a) Pfaltz, A. Acc. Chem. Res. 1993, 26, 339. (b) Schumacher, R.;
Dammast, F.; Reissig, H.-U. Chem. Eur. J . 1997, 3, 614. (c) Doyle, M.
P.; Westrum, L. J .; Wolthuis, W. N. E.; See, M. M.; Boone, W. P.;
Bagheri, V.; Pearson, M. M. J . Am. Chem. Soc. 1993, 115, 958. (d)
Doyle, M. P. Recl. Trav. Chim. Pays-Bas 1991, 110, 305. (e) Mu¨ller,
P.; Baud, C.; Ene´, D.; Motabelli, S.; Doyle, M. P.; Brandes, B. D.;
Dyatkin, A. B.; See, M. M. Helv. Chim. Acta 1995, 78, 459.
(7) (a) Seitz, W. J .; Saha, A. K.; Hossain, M. M. Organometallics
1993, 12, 2604. (b) Seitz, W. J .; Hossain, M. M. Tetrahedron Lett. 1994,
35, 7561. (c) Theys, R. D.; Vargas, R. M.; Wang, Q.; Hossain, M. M.
Organometallics 1998, 17, 1333. (d) Seitz, W. J .; Saha, A. K.; Casper,
D.; Hossain, M. M. Tetrahedron Lett. 1992, 33, 7755. (e) Ku¨hn, J .; Ru¨ck-
Braun, K. J . Prakt. Chem. 1997, 339, 675. (f) Uchida, T.; Irie, R.;
Katsuki, T. Synlett 1999, 1163.
10.1021/om0003537 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 08/05/2000

Half-Sandwich Ru(II) Catalysts
Organometallics, Vol. 19, No. 18, 2000 3665
Ta ble 1. Cyclop r op a n a tion of Styr en e w ith EDA
Ca ta lyzed by Ha lf-Sa n d w ich Ru th en iu m (II)
Com p lexesa
Ch a r t 1
T
t
cis isomer trans isomer dimersb
catalyst
(°C) (h)
(%)
(%)
(%)
1
2
3
4c
5
6
7
8
45
60
50
25
50
40
80
80
50
4
3
2
4
4
4
3
3
4
65
40
55
46
52
48
36
38
10
31
33
35
33
32
40
60
58
30
4
27
10
21
16
12
4
4
17
RuCl3‚xH2Od
It is noteworthy that the nature of the ancillary
ligands also affects the chemoselectivity in metal-
catalyzed carbene transfer reactions. Thus, diphenyl-
diazomethane reacts with ethene in the presence of
catalysts of general formula [RhL2Cl]2 (L ) phosphine,
stibine), and an insertion of :CPh2 into an olefinic C-H
bond or a remarkable :CPh2-ethene coupling yielding
Ph2CdCHCH3 was observed, according to the type of
the neutral ligand L employed.10
a
0.5% catalyst. Reactions were performed in neat styrene with
total conversion of EDA into the cis and trans isomers and dimers.
Yields were determined by GC measurements. bDEM as main
product and DEF. c Under a nitrogen atmosphere. 57% EDA
conversion.
d
the reaction unexpectedly leads to the trisubstituted
olefin 1,1,3-triphenylpropene as the main product.
Resu lts a n d Discu ssion
Among the large number of transition metal com-
plexes that have been used in carbenoid-mediated
reactions,1 ruthenium derivatives have recently emerged
as a new class of versatile catalysts because of their high
selectivity and their relatively low sensitivity for sub-
strate-bearing functional groups.4a,11 Recently we have
found that [(η5-C5H5)Ru(PPh3)2Cl] (1) can decompose
EDA and R-diazo ketones affording diethyl maleate
(DEM) and cis-enediones of 95-99% purity, the highest
values for a stereoselective carbene dimerization process
reported to date.12 These results prompted us to inves-
tigate 1, and the related readily available complexes of
general formula [(η5-ligand)Ru(PR3)2X] (R ) aryl; X )
halogen), for selective carbene transfer reactions from
diazo compounds to terminal alkenes.
Cyclop r op a n a tion of Styr en e Der iva tives w ith
EDA Usin g Ha lf-Sa n d w ich Ru th en iu m (II) Ca ta -
lysts. The half-sandwich ruthenium complexes of gen-
eral formula [(η5-ligand)Ru(PR3)2X] have been proven
to be effective catalysts for EDA dimerization. Thus,
aiming at the catalytic cyclopropanation of styrene
derivatives with EDA, we performed the reaction in
pure alkene without any further solvent to prevent
diethyl maleate formation. Furthermore, a premixed
EDA/alkene solution was added to the catalyst (dis-
solved in alkene) over the course of several hours, to
keep the EDA concentration low. Because cyclopropa-
nation of styrene derivatives with EDA may also take
place in the absence of catalyst and even at room
temperature, the EDA/alkene mixture has to be freshly
prepared prior to use. It should be noted that the
thermal decomposition of EDA in the presence of
styrene affords the corresponding trans and cis cyclo-
propyl derivatives in about 2:1 ratio. With these precau-
tions, cyclopropane products are easily obtained in high
yields and with predominantly cis stereoselectivity. In
the case of precatalyst 1, optimum conditions for the
cyclopropanation of styrene were set at 45 °C and 4 h,
giving rise to 95% yield of ethyl phenylcyclopropane
carboxylate with a stereoselectivity of 67% cis vs 33%
trans (eq 1). To examine the influence of the ligand
We now report the results of a study of reactions
between styrene and EDA or diphenyldiazomethane in
the presence of the complexes shown in Chart 1. With
EDA cyclopropanation was observed with a high cis
stereoselectivity, whereas with diphenyldiazomethane
(8) (a) Casey, C. P.; Polichnowski, S. W.; Shusterman, A. J .; J ones,
C. R. J . Am. Chem. Soc. 1979, 101, 7282. (b) Doyle, M. P.; Griffin, J .
H.; Bagheri, V.; Dorow, R. L. Organometallics 1984, 3, 53. (c) Casey,
C. P.; Polichnowski, S. W.; Tuinstra, H. E.; Albin L. D.; Calabrese, J .
C. Inorg. Chem. 1978, 17, 3045.
(9) (a) Brookhart, M.; Humphrey, M. B.; Kratzer, H. J . Nelson, G.
O. J . Am. Chem. Soc. 1980, 102, 7802. (b) Brookhart, M.; Studabaker,
W. B.; Husk, G. R. Organometallics, 1985, 4, 943.
(10) (a) Werner, H. J . Organomet. Chem. 1995, 500, 331. (b) Wolf,
J .; Brandt, L.; Fries, A.; Werner, H. Angew. Chem., Int. Ed. Engl. 1990,
29, 510.
(11) (a) Grubbs, R. H.; Miller, S. J .; Fu, G. C. Acc. Chem. Res. 1995,
28, 446. (b) Ivin, K. J .; Mol, J . C. In Olefin Metathesis and Metathesis
Polymerization; Academic Press: San Diego, 1997. (c) Doyle, M. P.;
Peterson, C. S.; Zhou, Q. L.; Nishiyama, H. Chem. Commun. 1997,
211. (d) Schwab, P.; France, M. B.; Ziller, J . W.; Grubbs, R. H. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2039. (e) Schwab, P.; Grubbs, R. H.;
Ziller, J . W. J . Am. Chem. Soc. 1996, 118, 100. (f) Stumpf, A. W.; Saive,
E.; Demonceau, A.; Noels, A. F. J . Chem. Soc., Chem. Commun. 1995,
1127. (g) Herrmann, W. A.; Schattenmann, W. C.; Nuyken, O.; Glander,
S. C. Angew. Chem., Int. Ed. Engl. 1996, 35, 1087. (h) Weskamp, T.;
Schattenmann, W. C.; Spiegler, M.; Herrmann, W. A. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 2490. (i) Nishiyama, H.; Itoh, Y.; Matsumoto,
H.; Park, S.-B.; Itoh, K. J . Am. Chem. Soc. 1994, 116, 2223. (j)
Demonceau, A.; Simal, F.; Noels, A. F.; Vin˜as, C.; Nun˜ez, R.; Teixidor,
F. Tetrahedron Lett. 1997, 38, 4079. (k) Galardon, E.; Le Maux, P.;
Simonneaux, G. Chem. Commun. 1997, 927. (l) Galardon, E.; Le Maux,
P.; Toupet, L.; Simonneaux, G. Organometallics 1998, 17, 565. (m) Lee,
H. M.; Bianchini, C.; J ia, G.; Barbaro, P. Organometallics 1999, 18,
1961.
sphere toward the product selectivity and to elucidate
aspects of the reaction mechanism, the half-sandwich
ruthenium compounds 2-8 were employed (Chart 1).
The results of the catalytic cyclopropanation of styrene
are shown in Table 1. Compared to 1, no improvement
of selectivity for the cis cyclopropyl isomer was observed
by using the iodide derivative 2 or the methylcyclopen-
tadienyl compound 3. The use of ruthenium complexes
4 and 6, bearing bulkier phosphines such as PPh2(2-
(12) Baratta, W.; Del Zotto, A.; Rigo, P. Chem. Commun. 1997, 2163.

3666 Organometallics, Vol. 19, No. 18, 2000
Baratta et al.
Ta ble 2. Cyclop r op a n a tion of F u n ction a lized
Alk en es w ith EDA Ca ta lyzed by 1a
Sch em e 1
T
cis isomer trans isomer dimersb
(°C)
alkene
(%)
(%)
(%)
45
45
45
p-methyl styrene
R-methyl styrene
n-butyl vinyl ether
45
56
45
24
32
30
31
12
25
a
0.5% catalyst, 4 h. Reactions were performed in neat alkene
with total conversion of EDA into the cis and trans isomers and
dimers. Yields were determined by GC measurements. bDEM as
main product and DEF.
the reaction of EDA with different terminal electron-
rich olefins, and the results of the cycloaddition reac-
tions are displayed in Table 2. In line with the previous
data obtained with styrene as starting material, the
reactions of EDA with p-methyl styrene, R-methyl
styrene, and butyl vinyl ether afford the corresponding
cyclopropanes in good yields and with predominately cis
stereoselectivity. By contrast, alkyl-substituted alkenes
(e.g., 2,5-dimethyl-1,5-hexadiene) give mainly DEM and
only minor cyclopropanation products. Apparently, the
terminal electron-rich olefins show a higher reactivity
toward the carbene intermediate, and thus dimerization
of the diazo compounds can be suppressed. In summary
the easily available complex 1 is an efficient catalyst
for cyclopropanation of styrene and other terminal
olefins with EDA, the cis selectivity being similar to that
reported for the most selective systems. Work is needed
to address the mechanistic issues of this ruthenium-
catalyzed stereoselective process.
F or m a tion of 1,1,3-Tr ip h en ylp r op en e fr om Sty-
r en e a n d Dip h en yld ia zom eth a n e Ca ta lyzed by 1
a n d 4. Regarding the chemistry of diphenyldiaz-
omethane, it is well-known that treatment of styrene
with N2CPh2 gives 1,1,2-triphenylcyclopropane 9 under
thermal or photochemical elimination of dinitrogen.14
Unexpectedly, when a catalytic amount of complex 1 (1
mol % referred to N2CPh2) is added to a benzene-d6
solution of styrene and diphenyldiazomethane (1.2:1
molar ratio), 1,1,3-triphenylpropene 10 (58%) together
with a 1:1 mixture of 1,1-diphenylethene 11 (17%) and
1,2-diphenylcyclopropanes15 12a ,b (16%) form, as es-
tablished by 1H,1H-2D COSY, 13C NMR DEPT, and GC
measurements (eq 2). Only 9% of the expected cyclo-
CH3C6H4) and P(3-CH3C6H4)3, respectively, allows cy-
clopropanation of styrene at temperatures below 45 °C,
but without increase of the cis/ trans cyclopropyl ratio.
Employment of ruthenium complexes with one carbonyl
ligand 7 or with a chelate phosphine 8 yields at 80 °C a
mixture of cis and trans isomers similar to that observed
in the reaction performed without catalyst, suggesting
that these complexes are not catalytically active. Fur-
thermore, commercial ruthenium trichloride RuCl3‚
xH2O has been found to catalyze the cyclopropanation
of styrene with the trans isomer as the main product.
These data show that ruthenium cyclopentadienyl
complexes of general formula [(η5-C5H5)Ru(PR3)2X] can
induce carbene transfer reactions to styrene with pre-
dominant cis stereoselectivity. The cis/trans ratio values
obtained in these reactions are probably the minimum
ones because the uncatalyzed cyclopropanation occurred
under the present reaction conditions. The catalytic
activity of this class of ruthenium complexes depends
on the relative facility of dissociation of one phosphine.
It is conceivable that this reaction proceeds via the
ruthenium carbene intermediate [(η5-C5H5)Ru(dCHCO2-
Et)(PR3)Cl], as proposed in our recent investigation for
the dimerization of EDA (Scheme 1).13 Apparently, the
methyl-substituted aryl phosphines are more weakly
bound to ruthenium compared with PPh3 and, therefore,
more prone to dissociation in the initial step. This may
explain why compounds 4 and 6 catalyze the cyclopro-
panation reaction even at lower temperatures than 1.
Conversely, the monocarbonyl derivative 7 and the
complex 8 containing a chelating phosphine ligand show
only poor catalytic performance due to the hampered
ligand dissociation. In the second step of the catalytic
cycle the 16-electron complex promptly reacts with EDA,
through nitrogen extrusion, affording a cyclopentadienyl
ruthenium carbene. The subsequent nucleophilic attack,
performed by styrene, affords the cyclopropane carboxy-
late derivative, whereas the concomitant reaction of the
carbene with EDA gives DEM. Although no spectro-
scopic evidence for the formation of [(η5-C5H5)Ru(dCH-
CO2Et)(PR3)Cl] was obtained in the reaction mixture,
the high cis stereocontrol in carbene transfer to styrene
suggests that the ruthenium carbene is a key interme-
diate in the cyclopropanation reaction. Steric effects,
induced by the cyclopentadienyl and phosphine ligands,
apparently limit the conformations of the forming
cyclopropane and therefore favor the formation of the
less sterically demanding isomer.12,13 Having estab-
lished that 1 shows the best catalytic performance for
the cis cyclopropanation of styrene, we have investigated
propyl derivative 9 is present in the final solution. If
the catalyst 4 is used instead of 1, the propene deriva-
tive 10 is obtained at lower temperature and in higher
(14) (a) Tomioka, H.; Ohno, K.; Izawa, Y.; Moss, R. A.; Munjal, R.
C. Tetrahedron Lett. 1984, 25, 5415. (b) Becker, R. S.; Edwards, L.;
Bost, R.; Elam, M.; Griffin, G. J . Am. Chem. Soc. 1972, 94, 6584. (c)
Hadel, L. M.; Platz, M. S.; Scaiano, J . C. J . Am. Chem. Soc. 1984, 106,
283. (d) Closs, G. L.; Rabinow, B. E. J . Am. Chem. Soc. 1976, 98, 8190.
(15) (a) Bloodworth, A. J .; Lampman, G. M. J . Org. Chem. 1988,
53, 3, 2668. (b) Overberger, C. G.; Anselme, J .-P. J . Am. Chem. Soc.
1964, 86, 658. (c) Schmidbauer, H.; Bublak, W.; Schier, A.; Reber, G.;
Mu¨ller, G. Chem. Ber. 1988, 121, 1373.
(13) Baratta, W.; Del Zotto, A.; Rigo, P. Organometallics 1999, 18,
5091.

Half-Sandwich Ru(II) Catalysts
Organometallics, Vol. 19, No. 18, 2000 3667
Ta ble 3. Rea ction of Dip h en yld ia zom eth a n e w ith
Styr en e Ca ta lyzed by 1 a n d 4a
catalyst T (°C) t (h) 9 (%) 10 (%) 11 (%) 12a ,bb (%)
during catalysis. The metal carbene compound 13 was
previously obtained by Werner and co-workers in a
multistep synthesis starting from 1 via formation of the
acetato complex [(η5-C5H5)Ru(η2-O2CMe)(PPh3)].16 We
have now found that the green product 13 is easily
available in 85% isolated yield by direct reaction of 1
with diphenyldiazomethane (eq 3). Heating of 1 at 65
1
4
c
65
40
80
2.5
2.0
3.0
9
<2
55
58
67
17
15
16
16
a
1% catalyst. Reactions were performed in C6D6, under an
1
argon atmosphere. Yields were determined by H NMR measure-
ments. Tetraphenylethene was detected in trace amounts by GC
b
measurements. Cis and trans isomers of 1,2-diphenylcyclopro-
pane. c Without catalyst 9 was the only product of the carbene
transfer to styrene.
yield (67%), whereas 9 forms in trace amounts (Table
3). When 1 is employed, the H, 13C{1H}, and 31P{1H}
1
°C in toluene is necessary to cause phosphine dissocia-
tion,13 and an excess of N2CPh2 has to be added during
the reaction to obtain complete conversion of 1 into 13,
because of the slow N2CPh2 decomposition.
Although thermally stable, 13 can transfer quantita-
tively its carbene ligand :CPh2 to styrene in a stoichio-
metric reaction. So, treatment of 13 with styrene in 1:3
molar ratio in C6D6 at 45 °C affords, within 1.5 h,
compounds 10, 11, and 12a ,b (eq 4), with a distribution
NMR spectra of the reaction mixture reveal the forma-
tion of the carbene complex [(η5-C5H5)Ru(dCPh2)(PPh3)-
Cl], 13,16 which is the only ruthenium intermediate
detected during the reaction. Moreover, tetraphenyl-
ethene, the product of :CPh2 dimerization, was detected
in very low concentration in the reaction mixture, thus
indicating that the carbene 13 ought to undergo a faster
attack by the alkene compared to the diazo compound,
in contrast to the aforementioned species [(η5-C5H5)-
Ru(dCHCO2Et)(PR3)Cl]. It is generally accepted that
many metal-catalyzed C-C bond forming reactions
starting from diazo compounds imply generation of
transient electrophilic metal carbenes. However, it is
important to note that there is a striking paucity of
catalytically active ruthenium carbene complexes that
have been isolated11a,m,17 or directly observed during
catalysis.11e,18 As regard to the mechanism involved in
the formation of the trisubstituted alkene 10, it is
unlikely that this product results from a cyclopropane
rearrangement of 9. In fact, although a number of
functionalized cyclopropyl derivatives have been specif-
ically transformed to their corresponding ring-opened
alkenes via transition-metal-assisted reactions,19 no
conversion of the cyclopropane 9 to the alkene 10 occurs
using 1 as catalysts in benzene-d6 at 70 °C, even after
4 h. Probably, the cyclopropyl product 9 forms in our
mixture as a result of a thermal side reaction from
styrene and diphenyldiazomethane, as suggested by the
observation that using catalyst 4, which works at lower
temperature, a minor amount of 9 is obtained. Note-
worthy, the related complexes [(η5-C5Me5)Ru(PPh3)2Cl]
or [(η5-C9H7)Ru(PPh3)2Cl] show a lower catalytic activity
in the formation of 10, whereas [RuCl2(PPh3)3] and [(p-
cymene)RuCl2]2 have been found not active under our
experimental conditions.
fairly similar to that observed in the reaction of styrene
with N2CPh2 catalyzed by 1 and closer to that of the
reaction catalyzed by 4 at lower temperature. It should
be emphasized that under these experimental conditions
no cyclopropanation of styrene was observed, thus
confirming that compound 9, which was observed in the
catalytic reaction, forms as result of a thermal side
reaction from styrene and diphenyldiazomethane. Al-
though the 1H and 31P{1H} NMR spectra of 13 with
styrene in excess suggest formation of [(η5-C5H5)Ru-
(styrene)(PPh3)Cl] (δH ) 4.09 ppm for Cp; δP ) 53.6 ppm
in toluene-d8) in line with our previous work with
diethylmaleate,13 we were not able isolate the olefinic
complex.
Concerning the mechanism of the catalytic process,
it is important to note that the composition and distri-
bution of the products formed in the stoichiometric
reaction between 13 and styrene is similar to that
observed in the catalytic cycle. Employment of solvents
different from benzene-d6, such as CDCl3 or acetone-
d6, or addition of NEt4Cl (CDCl3) to prevent chloride
ligand dissociation from 1 or 4 during catalysis, results
in no change of chemoselectivity. Therefore it is reason-
able to assume that the initial stage of the catalytic
process implies the generation of the carbene intermedi-
ate [(η5-C5H5)Ru(dCPh2)(PR3)Cl], B, by reaction of the
diazo compound with the 16-electron complex [(η5-C5H5)-
Ru(PR3)Cl], A, formed by displacement of one phosphine
from 1 or 4.13 Formally 10 results from a C-C coupling
With the aim to elucidate the mechanism of formation
of the trisubstituted olefin 10 from styrene and the diazo
derivative and to understand the nature of the key steps
occurring at the metal center, we have studied the
reaction of styrene with the carbene complex 13, which
is the intermediate observed in the reaction mixture
(16) (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.
(17) Park, S.-B.; Sakata, N.; Nishiyama, H. Chem. Eur. J . 1996, 2,
303
(18) Schattenmann, W. C. Ph.D. Thesis, University of Mu¨nchen,
1997.
(19) (a) Doyle, M. P.; van Leusen, D. J . Am. Chem. Soc. 1981, 103,
5917. (b) Doyle, M. P.; van Leusen, D. J . Org. Chem. 1982, 47, 5326.
(c) Alonso, M. E.; J ano S., P.; Herna´ndez, M. I. J . Org. Chem. 1980,
45, 5299.

3668 Organometallics, Vol. 19, No. 18, 2000
Baratta et al.
Sch em e 2
The catalytic cycle reported in Scheme 2 should be
considered as an attempt to rationalize the formation
of 10, 11, and 12a ,b. Further investigations are needed
to give definitive evidence of the proposed reaction paths
and the features of the intermediates.
Con clu d in g Rem a r k s
Cyclopentadienyl ruthenium complexes of general
formula [(η5-C5H5)Ru(PR3)2X] have been found to cata-
lyze cis cyclopropanation and an unusual C-C coupling
reaction from diazo compounds and terminal alkenes.
Although there is strong evidence that carbene inter-
mediates of the type [(η5-C5H5)Ru(dCR′R′′)(PR3)X] (R′
) H, R′′ ) Ph, CO2Et; R′, R′′ ) Ph) easily form during
the catalytic reactions, their stability and reactivity is
strongly affected by the R′ and R′′ substituents. The
presence of two aryl groups (R′, R′′ ) Ph) apparently
stabilizes the metal carbene, in comparison with the
highly reactive analogue [(η5-C5H5)Ru(dCHCO2Et)-
(PPh3)Cl], as suggested by the detection of 13 during
catalysis and its isolation. The different behavior of the
cyclopentadienyl ruthenium carbene intermediates in
:CR′C′′ transfer reactions to olefins, according to the R′
and R′′ substituents, holds promise for new catalytic
conversions at this metal center. Current investigations
in our laboratories focus on expanding the preparative
scope of the readily accessible half-sandwich ruthenium
catalysts,22 as well as gaining further mechanistic
insight to understand the factors that govern the chemo-
and stereoselectivity.
reaction between the :CPh2 fragment and the styrene
isomer :CHCH2Ph. A similar catalytic reaction was
reported by Werner and co-workers, who found that the
rhodium complex [Rh(PiPr3)2Cl]2 catalyzes the synthesis
of 1,1-diphenylpropene starting from ethene and diphe-
nyldiazomethane. According to the mechanism proposed
by Werner, we tentatively assume that also in our case
in the second stage of the reaction both substrates, the
olefin and the carbene, coordinate to the metal, affording
a metallacyclobutane C derivative (Scheme 2).10,20 This
complex can give, via an η3-allylhydrido intermediate
D and subsequent hydride migration to the terminal CH
carbon of the allyl group, the alkene 10, affording the
coordinatively unsaturated species A. However, in the
absence of a deeper investigation, the mechanism and
the nature of the intermediate in the formation of the
trisubstituted olefin still remain to be confirmed.
The metallacycle C, postulated in the principal cata-
lytic cycle, also has the structural features of the key
species for olefin metathesis reactions and can account
for the formation, in a side reaction, of small amounts
of 11 and 12a ,b (Scheme 2). Thus, the proposed met-
allcycle C may give a second competitive reaction, which
leads via metathesis to one molecule of alkene 11 and
the complex E containing the carbene moiety :CHPh.
Subsequent transfer of the electrophilic phenylcarbene
from ruthenium to styrene terminates the cycle, afford-
ing the cis and trans cyclopropane derivatives 12a ,b in
almost equimolar amount with respect to 11. In this
regard we have to point out that we have also investi-
gated the reaction between styrene and N2CHPh21
catalyzed by 1. In this process, which leads to the
formation of the expected cyclopropane derivatives
12a ,b, in a cis/trans ratio of ca. 1, the ruthenium car-
bene intermediate E is likely to form in accordance with
the aforementioned results with styrene and EDA.
Exp er im en ta l Section
Gen er a l P r oced u r es. Toluene and heptane were distilled
over sodium. Styrene (Aldrich) was freshly distilled before use,
whereas EDA (Aldrich) was used without further purification.
The starting materials [(η5-C5H5)Ru(PPh3)2Cl] (1),23 [(η5-C5H5)-
Ru(PPh3)2I] (2),24 [(η5-C5H4Me)Ru(PPh3)2Cl] (3),25 [(η5-C5H5)-
Ru(PR3)2Cl] (PR3 ) PPh2(2-MeC6H4), 4; PPh2Cy, 5; P(3-Me-
C6H4)3, 6),13 [(η5-C5H5)Ru(CO)(PPh3)Cl] (7),26 [(η5-C5H5)Ru(PPh2-
CH2CH2PPh2)Cl] (8),27 and Ph2CN2 were synthesized accord-
28
ing to literature methods. 1H, 13C{1H}, and 1H,1H-2D COSY
NMR experiments were recorded on a Brucker AC200 spec-
trometer. GC-MS measurements were performed with Fisons
TRIO 2000 and Hewlett-Packard 5970 B instruments.
Ca ta lytic Cyclop r op a n a tion of Alk en es w ith EDA. All
experimental details are given in Tables 1 and 2. In a typical
procedure, 0.05 mmol of the ruthenium complex was dissolved
in the corresponding alkene (100 mmol). A mixture of EDA/
alkene (10 mmol/100 mmol) was added to the stirred solution
containing the catalyst over the course of several hours at the
appropriate temperature. Then, the reaction mixture was
stirred for 30 min, and the product distribution was deter-
mined by integration of the GC signals. With catalysts 1, 5,
and styrene as well as catalyst 1 and n-butyl vinyl ether the
1
organic product distribution was also checked by H NMR. In
(22) Del Zotto, A.; Baratta, W.; Rigo, P. J . Chem. Soc., Perkin Trans.
1 1999, 3079.
(23) Bruce, M. I.; Windsor, N. J . Aust. J . Chem. 1977, 30, 1601.
(24) Wilczewski, T.; Boche´nska, M.; Biernat, J . F. J . Organomet.
Chem. 1981, 215, 87.
(20) (a) Werner, H.; Mo¨hring, U. J . Organomet. Chem. 1994, 475,
277. (b) Werner, H.; Bosch, M.; Schneider, M. E.; Hahn, C.; Kukla, F.;
Manger, M.; Windmu¨ller, B.; Weberndo¨rfer, B.; Laubender, M. J .
Chem. Soc., Dalton Trans. 1998, 3549.
(21) a) C¸ etinkaya, B.; O¨ zdemir, I.; Dixneuf, P. H. J . Organomet.
Chem. 1997, 534, 153. (b) Davies, H. M. L.; Bruzinski, P. R.; Fall, M.
J . Tetrahedron Lett. 1996, 37, 4133.
(25) Reventos, L. B.; Alonso, A. G. J . Organomet. Chem. 1986, 309,
179.
(26) Davies, S. G.; Simpson, S. J . J . Chem. Soc., Dalton Trans. 1984,
993.
(27) Ashby, G. S.; Bruce, M. I.; Tomkins, J . B.; Wallis, R. C. Aust.
J . Chem. 1979, 32, 1003.
(28) Miller, J . B. J . Org. Chem. 1959, 24, 560.

Half-Sandwich Ru(II) Catalysts
Organometallics, Vol. 19, No. 18, 2000 3669
this case the excess alkene was removed under reduced
pressure and the product distribution of the residual oil was
determined by 1H NMR spectroscopy. To purify the crude
product, the oil was chromatographed on a column of silica
gel and eluted with a 1% ethyl acetate/pentane mixture to yield
a mixture of cyclopropane isomers and a mixture of the
byproducts DEM and DEF, which are the only compounds
detected in the reaction mixtures by NMR and GC-MS.
Ca ta lytic F or m a tion of 1,1,3-Tr ip h en ylp r op en e (10).
Diphenyldiazomethane (55 mg, 0.283 mmol) and 1 (2 mg, 2.8
µmol) were added to a solution of styrene (40 µL, d ) 0.91
g/mL, 0.350 mmol) in 0.5 mL of C6D6. The red solution was
heated at 65 °C for 2.5 h under argon, allowing dinitrogen
evolution. The distribution of the organic products was deter-
heptane (25 mL) was added, and a green product separated.
After crystallization from toluene/heptane the resulting com-
plex was dried under reduced pressure and identified as 13
by comparison of the 1H, 13C{1H}, and 31P{1H} NMR data with
those reported in the literature.16 Yield: 450 mg, 85%.
Rea ction betw een 13 a n d Styr en e. Complex 13 (20 mg,
0.032 mmol) was dissolved in 0.5 mL of C6D6, and styrene (9
µL, d ) 0.91 g/mL, 0.079 mmol) was added under argon. The
green solution was gently heated at 45 °C for 1.5 h, resulting
in complete conversion of 13 into the organic products 10, 68%,
11, 16%, and 12a ,b, 16% yield, as established by 1H NMR
measurements.
Ack n ow led gm en t. This work was supported by the
Conferenza dei Rettori delle Universita` Italiane (CRUI),
the Ministero della Ricerca Scientifica e Tecnologica
(MURST), the Bayerische Forschungsstiftung (FOR-
KAT), and the Deutscher Akademischer Austauschdi-
enst (DAAD). The authors thank Prof. G. Verardo and
Dr. P. Martinuzzi of the Dipartimento di Scienze e
Tecnologie Chimiche, Universita` di Udine, for perform-
ing the mass and NMR measurements.
1
mined by H and 13C{1H} NMR and GC-MS.
Similarly, compound 4 was used as catalyst at 45 °C,
causing the reaction to be complete after 2 h.
Syn th esis of [(η5-C5H5)Ru (dCP h 2)(P P h 3)Cl] (13). Com-
pound 1 (609 mg, 0.839 mmol) and N2CPh2 (510 mg, 2.63
mmol) were dissolved in 30 mL of toluene, and the solution
was stirred at 65 °C under argon (1 atm) for 30 h. During this
period, new N2CPh2 (130 mg, 0.669 mmol) was added after 10
h, and a second addition of N2CPh2 (90 mg, 0.463 mmol) was
made after 20 h. The solution was concentrated to about 5 mL,
OM0003537