Cyclopropanation of styrene with ethyl diazoacetate catalyzed by chiral and achiral ruthenium 2,6-bis(imino)pyridyl complexes

Article abstract of DOI:10.1021/om990951p

The optically pure 2,6-bis(imino)pyridyl ligands (R)-2,6-bis{1-[α-methylbenzylimine]ethyl}-pyridine [(R)-2,6-py(NCHMePh)₂] and (S)-2,6-bis{1-(1-naphthyl)ethylimine]ethyl}pyridine [(S)-2,6-py(NCHMeNaph)₂] have been prepared by condensation of 2,6-diacetylpyridine with (R)-(+)-α-methylbenzylamine and (S)-(-)-1-(1-naphthyl)ethylamine, respectively. These two ligands and others bearing different substituents on the imine nitrogen atoms (i.e., c-C₆H₁₁, C₆H₅, 2,6-Me₂Ph) form, in combination with ruthenium(II) fragments, efficient catalysts for the cyclopropanation of styrene with ethyl diazoacetate (EDA). The catalyst precursors have the general formula RuCl₂(PPh₃){2,6-py(NR)₂} (R = Cy, 1; Ph, 6; CHMeNaph, 7; CHMePh, 8). It is generally found that the catalytic activity increases with the size of the substituents on the imine nitrogen atoms. Consistently, the solvento complex RuCl₂{2,6-py(N(2,6-Me₂-Ph₂) ₂}·CH₂Cl₂ is the most efficient in the series. Best results in terms of activity, diastereoselectivity, and enantioselectivity have been obtained with the chiral precursor 7. In the presence of AgPF₆ as cocatalyst, a remarkable improvement in both productivity and chemoselectivity of the cyclopropanation reactions was observed with the catalysts 1 and 6, whereas a substantial decrease in enantioselectivity occurred with the chiral precursors 7 and 8. The carbene complex trans-RuCl₂(=CHCO₂Et){2,6-py(NCy)₂} (trans-3) has been isolated upon reaction of 1 with EDA. A kinetic product, cis-RuCl₂(=CHCO₂Et){2,6-py(NCy)₂}, was intercepted at low temperature. Compound trans-3 was also obtained by reaction of the ethylene complex RuCl₂(η²-H₂C=CH ₂)[2,6-py(NCy)₂] with EDA. Treatment of trans-3 with AgPF₆ led to the formation of the unsaturated complex [RuCl(=CHCO₂Et){2,6-py(NCy)₂}]PF₆. The overall reactivity of the Ru(II) 2,6-bis(imino)pyridyl six-coordinate complexes toward either EDA alone or EDA/styrene mixtures suggests that the carbene transfer from EDA to the olefin is apparently mediated by Ru(II) carbene species in an intermolecular fashion. This mechanistic view may not be true for the reactions performed in the presence of a halide scavenger.

Full text of DOI:10.1021/om990951p

Organometallics 2000, 19, 1833-1840
1833
Cyclop r op a n a tion of Styr en e w ith Eth yl Dia zoa ceta te
Ca ta lyzed by Ch ir a l a n d Ach ir a l Ru th en iu m
2,6-Bis(im in o)p yr id yl Com p lexes
Claudio Bianchini* and Hon Man Lee
Istituto per lo Studio della Stereochimioca ed Energetica dei Composti di Coordinazione,
ISSECC-CNR, Via J . Nardi 39, I-50132 Firenze, Italy
Received December 3, 1999
The optically pure 2,6-bis(imino)pyridyl ligands (R)-2,6-bis{1-[R-methylbenzylimine]ethyl}-
pyridine [(R)-2,6-py(NCHMePh)2] and (S)-2,6-bis{1-(1-naphthyl)ethylimine]ethyl}pyridine
[(S)-2,6-py(NCHMeNaph)2] have been prepared by condensation of 2,6-diacetylpyridine with
(R)-(+)-R-methylbenzylamine and (S)-(-)-1-(1-naphthyl)ethylamine, respectively. These two
ligands and others bearing different substituents on the imine nitrogen atoms (i.e., c-C6H11,
C6H5, 2,6-Me2Ph) form, in combination with ruthenium(II) fragments, efficient catalysts for
the cyclopropanation of styrene with ethyl diazoacetate (EDA). The catalyst precursors have
the general formula RuCl2(PPh3){2,6-py(NR)2} (R ) Cy, 1; Ph, 6; CHMeNaph, 7; CHMePh,
8). It is generally found that the catalytic activity increases with the size of the substituents
on the imine nitrogen atoms. Consistently, the solvento complex RuCl2{2,6-py(N(2,6-Me2-
Ph2)2}‚CH2Cl2 is the most efficient in the series. Best results in terms of activity, diastereo-
selectivity, and enantioselectivity have been obtained with the chiral precursor 7. In the
presence of AgPF6 as cocatalyst, a remarkable improvement in both productivity and
chemoselectivity of the cyclopropanation reactions was observed with the catalysts 1 and 6,
whereas a substantial decrease in enantioselectivity occurred with the chiral precursors 7
and 8. The carbene complex trans-RuCl2(dCHCO2Et){2,6-py(NCy)2} (trans-3) has been
isolated upon reaction of 1 with EDA. A kinetic product, cis-RuCl2(dCHCO2Et){2,6-py(NCy)2},
was intercepted at low temperature. Compound trans-3 was also obtained by reaction of
the ethylene complex RuCl2(η2-H2CdCH2)[2,6-py(NCy)2] with EDA. Treatment of trans-3
with AgPF6 led to the formation of the unsaturated complex [RuCl(dCHCO2Et){2,6-
py(NCy)2}]PF6. The overall reactivity of the Ru(II) 2,6-bis(imino)pyridyl six-coordinate
complexes toward either EDA alone or EDA/styrene mixtures suggests that the carbene
transfer from EDA to the olefin is apparently mediated by Ru(II) carbene species in an
intermolecular fashion. This mechanistic view may not be true for the reactions performed
in the presence of a halide scavenger.
In tr od u ction
nyl)pyridine ligands (II) form some of the most active
catalytic systems for the cyclopropanation of olefins with
diazo compounds.4 Moreover, the great availability of
primary amines in the chiral pool could allow for a large
variety of chiral 2,6-bis[(imino)ethyl]pyridine ligands to
be readily synthesized and eventually employed in
asymmetric cyclopropanation reactions to give optically
pure cyclopropanes, which are valuable building blocks
for the elaboration of important organic molecules.4,5,13
Remarkable advances in homogeneous catalyst tech-
nology have recently been achieved with systems com-
prising late transition metals and 2,6-bis(imino)pyridyl
ligands (I). Successful reactions span from the polym-
erization1 and epoxidation of olefins2 to the enantiose-
lective reduction of prochiral ketones, especially R,â-
unsaturated ones.3 To the best of our knowledge, no
report has ever appeared in the relevant literature
showing the active role of 2,6-bis(imino)pyridyls metal
complexes in the cyclopropanation of olefins. This is
quite surprising, as the structurally related bis(oxazoli-
(1) (a) Small, B. L.; Brookhart, M. J . Am. Chem. Soc. 1998, 120,
7143. (b) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120.
(c) Britovsek, G. J . P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J .;
MacTavish, S. J .; Solan, G. A.; White, A. J . P.; Willimas, D. J . J . Am.
Chem. Soc. 1999, 121, 8728. (d) Britovsek, G. J . P.; Bruce, M.; Gibson,
V. C.; Kimberley, B. S.; Maddox, P. J .; Mastroianni, S.; MacTavish, S.
J .; Redshaw, C.; Solan, G. A.; Stro¨mberg, S.; White, A. J . P.; Williams,
D. J . J . Am. Chem. Soc. 1999, 121, 8728.
In this work, we show that 2,6-bis[(imino)ethyl]-
pyridine ligands (R ) CH3) in conjuction with ruthe-
nium(II) ions give rise to effective catalyst systems for
(2) C¸ etinkaya, B.; C¸ etinkaya, E.; Brookhart, M.; White, P. S. J . Mol.
Catal. A 1999, 142, 101.
(3) De Matrin, S.; Zassinovich, G.; Mestroni, G. Inorg. Chim. Acta
1990, 174, 9.
(4) Nishiyama, H.; Itoh, Y.; Matsumoto, H.; Park, S. B.; Itoh, K. J .
Am. Chem. Soc. 1994, 116, 2223.
10.1021/om990951p CCC: $19.00 © 2000 American Chemical Society
Publication on Web 04/06/2000

1834 Organometallics, Vol. 19, No. 10, 2000
Bianchini and Lee
spectra were recorded using pulse sequences suitable for
phase-sensitive representations using TPPI at 294 K on
degassed, nonspinning CD2Cl2 solutions. The 1H NOESY
spectrum was acquired with 1024 increments of size 2K (with
16 scans each) covering the full range in both dimensions (ca.
12 000 Hz), with a relaxation delay of 2.0 s and a mixing time
of 700 ms, respectively.11 GC analyses were performed on a
Shimadzu GC-14 A gas chromatograph equipped with a flame
ionization detector and a 30 m (0.25 mm i.d., 0.25 mm FT)
SPB-1 Supelco fused silica capillary column. Calibration curves
were constructed using the GC internal standard biphenyl and
known amounts of diethyl fumarate, diethyl maleate, ethyl
trans-2-phenylcyclopropanecarboxylate, and ethyl cis-2-phe-
nylcyclopropanecarboxylate. Yields of catalytic runs were
determined from the calibration curves using the same GC
standard. GC/MS analyses were performed on a Shimadzu QP
2000 apparatus equipped with a column identical to that used
for GC analysis. Chiral capillary GC analyses were performed
on a Shimadzu GC-17A gas chromatograph equipped with a
flame ionization detector and a chiraldex G-TA capillary
column (40 m × 0.25 mm i.d.).
the metal-mediated carbene transfer from ethyl diazo-
acetate (EDA) to styrene.6 Also, we show that 2,6-bis-
[(imino)ethyl]pyridine ligands may be successfully em-
ployed in asymmetric cyclopropanation reactions when
chiral substituents are introduced onto the imine ni-
trogen atoms.
With cyclohexyl substituents on the imine nitrogen
atoms, the resulting 2,6-{bis(cyclohexylimine)ethyl}-
pyridine ligand has been found to form rather stable
fragments with ruthenium(II) in combination with
various ligating groups, especially with the participative
carbene moiety CH(CO2Et). This has allowed us to gain
insight into the mechanism of the cyclopropanation
reaction of styrene with EDA and ultimately show that
the reactions proceed via the carbenoid path when
coordinatively saturated precursors are employed.
Exp er im en ta l Section
Gen er a l In for m a tion . All manipulations were carried out
under a dinitrogen atmosphere using standard Schlenk tech-
niques. Solvents were distilled under dinitrogen over sodium
benzophenone (hexane, diethyl ether, THF) or calcium hydride
(dichloromethane). The starting materials RuCl2(PPh3)3,7
[(p-cymene)RuCl2]2,8 2,6-bis{1-(cyclohexylimine)ethyl}pyridine
[2,6-py(NCy)2],9 2,6-bis{1-(phenylimine)ethyl}pyridine [2,6-
(S)-(+)-2,6-Bis{1-[1-n a p h t h yl)et h ylim in e]et h yl}p yr i-
d in e [(S)-2,6-p y(NCHMeNa p h th )2]. A mixture of 2,6-di-
acetylpyridine (1.0 g, 6.1 mmol) and (S)-(-)-1-(1-naphthyl)-
ethylamine (4.1 g, 24.0 mmol) without solvent was heated in
a closed flask at 95 °C for 4 days. The residual oil was
recrystallized with CH2Cl2/MeOH to give a yellow crystalline
solid. Yield: 1.7 g, 60%. Mp ) 114-117 °C. Anal. Calcd for
py(NPh)2],10 and Ru{2,6-py(N(Me2C6H3))2}Cl2 [2,6-py(N-
2
C
33H31N3: C, 84.40; H, 6.65; N, 8.95. Found: C, 84.30; H, 6.61;
(Me2C6H3))2) ) 2,6-bis{1-[(2,6-dimethyphenyl)imine]ethyl}-
pyridine] were prepared according to literature methods. All
the other reagents were used as purchased from Aldrich or
Strem Chemical Co. Microanalyses were performed by
ISSECC-CNR. 1H, 31P{1H}, and 13C{1H} NMR spectra were
collected on either a Bruker ACP-200 (200.13, 81.01, and 50.32
MHz, respectively) or a Bruker Advance DRX-500 spectrom-
eter equipped with a variable-temperature control unit ac-
curate to (0.1 °C (500.13, 202.47, and 125.76 MHz, respec-
1
N, 9.00. IR (Nujol, cm-1): ν(CdN) 1632 (s). H NMR (CDCl3,
3
200.13 MHz): δ 1.75 (d, J (HH) ) 6.5 Hz, 6 H, CHCH3), 2.48
(s, 3 H, NdCCH3), 5.66 (q, 3J (HH) ) 6.5 Hz, 2 H, CHCH3);
3
7.46-7.62 (m, 6 H), 7.76-7.93 (m, 7 H), 8.34-8.38 (d, J (HH)
) 7.8 Hz, 4 H); Ar and Py protons. 13C{1H} NMR (CDCl3, 50.33
MHz): δ 14.8 (CH3), 25.1 (CH3), 57.7 (CH), 122.2 (CH), 124.3
(CH), 124.7 (CH), 126.0 (CH), 126.4 (CH), 127.8 (CH), 129.7
(quaternary C), 131.4 (quaternary C), 137.3 (CH), 142.7
(quaternary C), 157.0 (quaternary C), 166.3 (quaternary C).
1
tively). H and 13C NMR chemical shifts are relative to TMS;
[R]20 ) +206.62° (c ) 1.286, CHCl3).
D
31P NMR chemical shifts are relative to 85% H3PO4. 2D NMR
(R )-(-)-2,6-Bis{1-[r-m e t h ylb e n zylim in e ]e t h yl}p yr i-
d in e [(R)-2,6-p y(NCHMeP h )2]. A mixture of 2,6-diacetylpy-
ridine (0.3 g, 2.0 mmol) and (R)-(+)-R-methylbenzylamine (1.2
g, 10.0 mmol) without solvent was heated in a closed flask at
95 °C for 3 days. Then the mixture was subjected to high
vacuum for 4 h at 95 °C to eliminate the excess amine. The
residual oil could be used as a ligand without further purifica-
(5) (a) Doyle, M. P.; Peterson, C. S.; Zhou Q. L.; Nishiyama, H. J .
Chem. Soc., Chem. Commun. 1997, 211. (b) Park, S. B.; Sakata, N.;
Nishiyama, H. Chem. Eur. J . 1996, 2, 303. (c) Nishiyama, H.; Aoki,
K.; Itoh, H.; Iwamura, T.; Sakata, N.; Kurihara, O.; Motoyama, Y.
Chem. Lett. 1996, 1071. (d) Park, S. B.; Murata, K.; Matsumoto, H.;
Nishiyama, H. Tetrahedron: Asymmetry 1995, 6, 2487. (e) Nishiyama,
H.; Itoh, Y.; Sugawara, Y.; Matsumoto, H.; Aoki, K.; Itoh, K. Bull.
Chem. Soc. J pn. 1995, 68, 1247. (f) Park, S. B.; Nishiyama, H.; Itoh,
Y.; Itoh, K. J . Chem. Soc., Chem. Commun. 1994, 1315. (g) Galardon,
E.; Le Maux, P.; Toupet, L.; Simonneaux, G. Organometallics 1998,
17, 565. (h) Galardon, E.; Le Maux, P.; Simonneaux, G. J . Chem. Soc.,
Chem. Commun. 1997, 927. (i) Lo, W. C.; Che, C. M.; Cheng, K. F.;
Mak, T. C. W. J . Chem. Soc., Chem. Commun. 1997, 1205. (j)
Frauenkron, M.; Berkessel, A. Tetrahedon Lett. 1997, 38, 7175. (k)
Fraile, J . M.; Carc´ıa, J . I.; Mayoral, J . A.; Tarnai, T. J . Mol. Catal. A
1999, 144, 85. (l) Christenson, D. L.; Tokar, C. J .; Tolman, W. B.
Organometallics 1995, 14, 2148. (m) Fritschi, H.; Leutenegger, U.;
Pfaltz, A. Helv. Chim. Acta 1988, 71, 1553.
1
tion. Yield: 0.7 g, 97%. IR (Nujol, cm-1): ν(CdN) 1634 (s). H
3
NMR (200.13 MHz, CDCl3): δ 1.58 (d, J (HH) ) 6.5 Hz, 6 H,
3
CHCH3), 2.48 (s, 3 H, NdCCH3), 4.95 (q, J (HH) ) 6.5 Hz, 2
H, CHCH3), 7.23-7.55 (m, 10 H, Ph), 7.77 (t, 3J (HH) ) 7.7
Hz, 1 H, py H-4), 8.27 (d, 3J (HH) ) 7.7 Hz, 2 H, py H-3,4).
13C{1H} NMR (CDCl3, 50.33 MHz): δ 14.5 (CH3), 25.5 (CH3),
61.0 (CH), 122.2 (CH), 127.4 (CH), 129.1 (CH), 146.7 (quater-
nary C), 157.1 (quaternary C), 165.6 (quaternary C). [R]20
-36.26° (c ) 2.530, CHCl3).
)
D
(6) (a) Singh, V. K.; DattaGupta, A.; Sekar, G. Synthesis 1997, 137.
(b) Aratani, T. Pure Appl. Chem. 1985, 57, 1839. (c) Doyle, M. P. Chem.
Rev. 1986, 86, 919. (d) Brookhart, M.; Studabaker, W. B. Chem. Rev.
1987, 87, 411. (e) Doyle, M. P. In Catalytic Asymmetric Synthesis;
Ojima, I., Ed.; VCH: New York, 1993; Chapter 3, pp 63-99. (f) Davies,
H. L. M. Aldrichim. Acta 1997, 30, 107. (g) Zhou, S. M.; Deng, M. Z.;
Xia, L. J .; Tang, M. H. Angew. Chem., Int. Ed. 1998, 37, 2845.
(7) Hallman, P. S.; Stephenson, T. A.; Wilkinson, G. Inorg. Synth.
1970, 12, 237.
(8) Bennett, M. B.; Huang, T.-N.; Matheson, T. W.; Smith, A. K.
Inorg. Synth. 1982, 21, 75.
(9) Sacconi, L.; Morassi, R.; Midollini, S. J . Chem. Soc. (A) 1968,
1510.
(10) Midollini, S.; Bacci, M. J . Chem. Soc. (A) 1970, 2964.
(11) (a) Sklener, V.; Miyashiro, H.; Zon, G.; Miles, H. T.; Bax, A.
FEBS Lett. 1986, 208, 94. (b) J eener, J .; Meier, B. H.; Bachmann, P.;
Ernst, R. R. J . Chem. Phys. 1979, 71, 4546.
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Granda, S. Inorg. Chem. 1999, 38, 2874.
cis-Ru Cl2(P P h 3){2,6-p y(NCy)2} (1). An acetone solution
(20 mL) containing 2,6-py(NCy)2 (150 mg, 0.462 mmol) and
RuCl2(PPh3)3 (398 mg, 0.415 mmol) was refluxed under
nitrogen for 4 h. During the reaction, the color of the solution
changed to deep red and a purple microcrystalline solid was
formed. The volume of the solution was reduced to ca. 10 mL
under a strong stream of nitrogen, and 20 mL of diethyl ether
was slowly added to ensure the complete formation of the
microcrystalline solid, which was collected on a frit, washed
with diethyl ether, and dried under a slow stream of nitrogen.
Yield: 230 mg (73%). Anal. Calcd for C39H46Cl2N3PRu: C,
1
61.65; H, 6.10; N, 5.53. Found: C, 61.60; H, 6.18; N, 5.49. H
NMR (200.13 MHz, CD2Cl2): δ 0.65-1.77 (m, 18 H, Cy), 2.48
(s, 6 H, NdCCH3), 2.55 (br, 2 H, Cy), 4.98 (br, 2 H, dNCH);
7.10-7.31(m, 12 H), 7.60-7.70 (m, 6 H), aromatic protons. 31P-
{1H} NMR (81.02 MHz, CD2Cl2): δ 31.4 (s).
(13) Nishiyama, H.; Park S. B.; Itoh, K. Chem. Lett. 1995, 599.

Cyclopropanation of Styrene
Organometallics, Vol. 19, No. 10, 2000 1835
tr a n s-Ru Cl2(η2-H2CdCH2){2,6-p y(NCy)2} (2). A solution
of 2,6-py(NCy)2 (150 mg, 0.462 mmol) and [(p-cymene)RuCl2]2
(122 mg, 0.208 mmol) in 20 mL of dichloromethane was
refluxed under nitrogen for 15 min and then under ethylene
overnight. The volume of the solution was reduced under a
strong stream of ethylene until a purple solid began to form.
The precipitation was completed by addition of petroleum
ether. The solid was collected on a frit, washed with petroleum
ether, and dried under a slow stream of nitrogen. Yield: 192
mg (88%). Anal. Calcd for C23H35Cl2N3Ru: C, 52.57; H, 6.71;
N, 8.00. Found: C, 52.90; H, 6.53; N, 7.98. 1H NMR (200.13
MHz, CD2Cl2): δ 0.51-2.67 (m, 22 H, Cy), 2.76 (s, 6 H,
NdCCH3), 2.90 (s, 4 H, ethylene), 7.57-7.71(m, 3 H, aromatic
protons).
cis-Ru Cl2(P P h 3){2,6-p y(NP h )2} (6). A solution of THF (30
mL) containing 2,6-py(NPh)2 (230 mg, 0.734 mmol) and RuCl2-
(PPh3)3 (633 mg, 0.660 mmol) was refluxed under nitrogen for
2 days. During the reaction, the color of the solution changed
to deep red. The volume of the solution was reduced to ca. 10
mL under a strong stream of nitrogen, and 20 mL of diethyl
ether was slowly added to give a dark brown solid, which was
collected on a frit, washed with diethyl ether, and dried under
a slow stream of nitrogen. Yield: 400 mg, (81%). Anal. Calcd
for C39H34Cl2N3PRu: C, 62.65; H, 4.58; N, 5.62. Found: C,
62.70; H, 4.56; N. 5.65. 1H NMR (200.13 MHz, CD2Cl2): δ 2.35
(s, 6 H, CH3), 6.89-7.45 (m, aromatic protons). 31P{1H} NMR
(81.02 MHz, CD2Cl2): δ 33.6 (s).
(S)-cis-Ru Cl2(P P h 3){2,6-p y(NCHMeNa p h )2} (7). A solu-
tion of THF (30 mL) containing (S)-2,6-py(NCHMeNaph)2 (200
mg, 0.428 mmol) and RuCl2(PPh3)3 (369 mg, 0.385 mmol) was
refluxed under nitrogen for 6 h. During the reaction, the color
of the solution changed to deep red. The volume of the solution
was reduced to ca. 10 mL under a strong stream of nitrogen,
and 20 mL of diethyl ether was slowly added to give a red
solid, which was collected on a frit, washed with diethyl ether,
and dried under a slow stream of nitrogen. Yield: 240 mg,
(91%). Anal. Calcd for C51H46Cl2N3PRu: C, 67.77; H, 5.13; N,
4.65. Found: C, 67.90; H, 4.95; N, 4.69. 1H NMR (200.13 MHz,
CD2Cl2): δ 1.02 (d, 3J (HH) ) 7.2 Hz, 3 H, CHCH3), 2.00 (d,
3J (HH) ) 7.2 Hz, 3 H, CHCH3′), 2.41 (s, 3 H, NdCCH3), 2.64
(s, 3 H, NdCCH3′); 6.84-7.89 (m, 32 H), 7.99 (d, 3J (HH) ) 8.4
Hz, 1 H), 8.21 (d, 3J (HH) ) 8.4 Hz, 1 H), aromatic protons
and dNCH. 31P{1H} NMR (81.02 MHz, CD2Cl2): δ 29.8 (s).
(R)-cis-Ru Cl2(P P h 3){2,6-p y(NCHMeP h )2} (8). The com-
plex was prepared similarly to 7. (R)-2,6-py(NCHMePh)2 (352
mg, 0.953 mmol) and RuCl2(PPh3)3 (777 mg, 0.810 mmol) were
used. A red solid was obtained. Yield: 500 mg, (77%). Anal.
Calcd for C43H42Cl2N3PRu: C, 64.26; H, 5.27; N, 5.23. Found:
Rea ction of tr a n s-Ru Cl2(η2-H2CdCH2){2,6-p y(NCy)2}
a n d P P h 3. To a CD2Cl2 solution (0.5 mL) of 2 (12.0 mg, 0.023
mmol) in a 5 mm NMR tube was added PPh3 (6.0 mg, 0.023
1
mmol). H and 31P NMR spectra were recorded immediately.
In the 31P NMR spectrum, the only signal observed was that
due to 1 at 31.4 ppm. In the 1H NMR spectrum, besides the
signals of the phosphine complex, a singlet due to the free
ethylene gas was observed at 5.40 ppm.
tr a n s-Ru Cl2(dCHCO2Et){2,6-py(NCy)2} (tr a n s-3). Meth -
od A. To a dichloromethane solution (10 mL) containing 2 (150
mg, 0.285 mmol) was added EDA (54 µL, 0.513 mmol). The
solution was allowed to stir for 2 h at room temperature. The
volume was then reduced to ca. 1 mL under high vacuum.
Petroleum ether was added to give a purple solid, which was
collected on a frit, washed with petroleum ether, and dried
under a slow stream of nitrogen. Yield: 143 mg (86%). Meth od
B. To a dichloromethane solution (30 mL) containing 1 (543
mg, 0.712 mmol) was addded EDA (300 µL, 2.853 mmol). The
solution was allowed to stir for 6 h at room temperature. The
volume was reduced under a stream of nitrogen, and 30 mL
of diethyl ether was slowly added until a purple microcrys-
talline solid was formed. The solid was collected on a frit,
washed with diethyl ether, and dried under a slow stream of
nitrogen. Yield: 370 mg (89%). Anal. Calcd for C25H37Cl2N3-
O2Ru: C, 51.48; H, 6.39; N, 7.20. Found: C, 51.75; H, 6.51; N.
1
C, 64.44; H, 5.30; N, 5.31. H NMR (200.13 MHz, CD2Cl2): δ
1.06 (d, 3J (HH) ) 7.2 Hz, 3 H, CHCH3), 2.01 (s, 3 H, NdCCH3),
2.06 (d, 3J (HH) ) 6.8 Hz, 3 H, CHCH3′), 2.18 (s, 3 H,
NdCCH3′), 6.36 (m, 2 H, dNCH); 6.58 (m, 2 H), 6.90-7.36
(m, 19 H), 7.79-7.96 (m, 7 H), aromatic protons. 31P{1H} NMR
(81.02 MHz, CD2Cl2): δ 29.3 (s).
1
7.22. H NMR (200.13 MHz, CD2Cl2): δ 1.23-1.84 (m, 20 H,
Va r ia ble-Tem p er a tu r e NMR Stu d y of th e Rea ction
betw een Ru Cl2(P P h 3){2,6-p y(NCy)2} a n d EDA. To a CD2-
Cl2 solution (0.5 mL) of 1 (15.0 mg, 0.020 mmol) was added
EDA (6.2 µL, 0.059 mmol) at -78 °C. 1H and 31P NMR spectra
at -80 °C were obtained immediately. In the 31P NMR spec-
trum, the signals due to the starting complex and to the ylid
PPh3dCHCO2Et, roughly in a ratio of 9:1, were observed at
33.3 and 18.4 ppm, respectively. In the low-field region of the
1H NMR spectrum were observed two singlets at 20.14 and
20.77 ppm that we assign to the carbene C-H protons of
trans-3 and cis-RuCl2(dCHCO2Et){2,6-py(NCy)2} (cis-3), re-
spectively. On warming to -70 °C, the intensities of the two
signals increased. On further warming to -60 °C, the signals
of the cis isomer at ca. 20.8 ppm decreased in intensity, while
that of the trans isomer at ca. 20.2 ppm increased. At -50 °C,
the signal of the cis isomer disappeared. On further warming,
the signal due to the trans isomer gained in intensity.
Eventually, the signal was observed at 20.41 ppm at room
temperature.
Ca ta lytic Cyclop r op a n a tion Rea ction s. A dichlorometh-
ane solution (1 mL) of EDA (1.5 mmol) was added to a
dichloromethane solution (1 mL) containing the catalyst
precursor (0.02 mmol) and styrene (1.72 mL, 15 mmol) at room
temperature over ca. 8 h using a syringe pump. Then the
mixture was allowed to stir for another 16 h. In some cases,
similar reactions were carried out in the presence of an excess
of benzyltriethylammonium chloride (BzEt3NCl).
3
Cy), 1.42 (t, J (HH) ) 7.2 Hz, 3 H, CO2CH2CH3), 2.91 (s, 6 H,
3
NdCCH3), 3.81 (br s, 2 H, dNCH), 4.49 (q, J (HH) ) 7.2 Hz,
2 H, CO2CH2CH3), 8.02-8.17 (m, 3 H, aromatic protons), 20.41
(s, 1 H, dCH). 13C{1H} NMR (50.33 MHz, CD2Cl2): δ 15.3
(CH2, Cy), 19.3 (CH2, Cy), 25.1(CH3), 26.2 (CH3), 30.0 (CH2,
Cy), 33.8 (CH2, Cy), 60.3 (dNCH), 68.5 (CO2CH2CH3), 123.5
(CH, py), 137.8 (CH, py), 151.3 (quaternary C, py), 170.0
(NdC), 186.7 (CO2CH2CH3), 299.9 (dCH).
[Ru Cl(dCHCO2Et){2,6-p y(NCy)2}]P F 6 (4). Into a 5 mm
NMR tube containing a CD2Cl2 solution (1 mL) of trans-3 (10.0
mg, 0.017 mmol) was introduced AgPF6 (5.0 mg, 0.020 mmol).
The color of the solution changed immediately from deep red
to light red. A proton NMR spectrum was acquired imme-
1
diately at room temperature. H NMR (200.13 MHz, CD2Cl2):
δ 0.97-1.79 (m, 20 H, Cy), 1.60 (t, 3J (HH) ) 7.1 Hz, 3 H, CO2-
CH2CH3), 2.99 (s, 6 H, NdCCH3), 3.74 (m, 2 H, dNCH), 4.55
(q, 3J (HH) ) 7.1 Hz, 2 H, CO2CH2CH3), 8.23-8.45 (m, 3 H,
aromatic protons), 21.34 (s, 1 H, dCH). All our attempts to
isolate this product in the solid state by scaling up the reaction
were unsuccessful due to its great instability.
[Ru Cl(P P h 3)(dCHCO2Et){2,6-p y(NCy)2}]P F 6 (5). To a
NMR solution of 4 prepared as described above was added
PPh3 (10.2 mg, 0.039 mmol). A color change from red to pale
yellow occurred immediately. 1H and 31P NMR spectra were
recorded. 1H NMR (200.13 MHz, CD2Cl2): δ 0.80-1.79 (m, 20
3
H, Cy), 1.61 (t, J (HH) ) 7.1 Hz, 3 H, CO2CH2CH3), 2.93 (s, 3
H, NdCCH3), 3.02 (s, 3 H, NdCCH3′), 3.74 (m, 2 H, dNCH),
Alternatively, the reactions were carried out as above but
in the presence of either silver hexafluorophosphate or silver
triflate (0.03 mmol). Independent reactions with either silver
salt did not show any cyclopropanation activity.
3
4.60 (q, J (HH) ) 7.1 Hz, 2 H, CO2CH2CH3), 8.19-8.41 (m, 3
3
H, py), 7.39-7.57 (m, 15 H, PPh3), 20.98 (d, J (HP) ) 6.1 Hz,
1 H, dCH). 31P{1H} NMR (81.02 MHz, CD2Cl2): δ 18.2 (s).

1836 Organometallics, Vol. 19, No. 10, 2000
Bianchini and Lee
Ta ble 1. Styr en e Cyclop r op a n a tion w ith EDA by Ru (II) 2,6-Bis(im in o)p yr id yl Com p lexesa
entry
catalyst
co-reagentb
A+B (%)c (A:B)
C+D (%) (C:D)
total yield (%)
ee of B (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
8
8
7
7
1
1
2
2
2
2
6
6
68.1 (86:14)
62.3 (38:62)
64.8 (86:14)
60.2 (53:47)
21.3 (82:18)
58.6 (65:35)
17.6 (80:20)
43.3 (63:19)
52.2 (60:40)
18.3 (79:21)
47.5 (73:27)
70.1 (52:48)
10.0 (80:20)
52.9 (58:42)
43.0 (61:39)
71.2 (84:16)
3.3 (21:79)
71.4
62.3
68.9
62.2
39.6
58.6
40.3
48.1
52.2
29.4
58.1
70.1
14.8
52.9
45.7
75.4
41.3 (1S, 2R)
4.1 (1S, 2R)
75.6 (1R, 2S)
4.0 (1R, 2S)
1.5 AgPF6
1.5 AgPF6
AgPF6
4.1 (29:71)
2.0 (14:86)
18.3 (10:90)
22.7 (6:94)
4.8 (13:87)
AgOTf
AgPF6
[Et4N]Cl
11.1 (13:87)
10.6 (19:81)
1.5 AgPF6
trans-3
trans-3
trans-3
11
4.8 (33:67)
AgPF6
AgOTf
2.7 (9:91)
4.2 (27:73)
a
Catalytic conditions: catalyst (0.02 mmol), EDA (1.5 mmol), styrene (15 mmol), dichloromethane (2 mL), 8 h addition followed by 16
h stirring. % ee of trans: not determined. All data represent average of at least three runs. Equivalents of silver salt. c Based on EDA.
b
Sch em e 1
equiv of the corresponding chiral amine without solvent
at ca. 95 °C for 3-4 days led to complete formation of
the desired products. In the case of (R)-2,6-py(NCH-
MePh)2, the excess amine could be distilled away under
reduced pressure. The residual oil could then be used
as ligand without further purification. In the case of (S)-
2,6-py(NCHMeNaph)2, recrystallization of the residue
with dichloromethane/methanol gave a yellow crystal-
line solid.
Syn th esis of Ru th en iu m Com p lexes w ith 2,6-Bis-
(im in o)pyr idyl Ligan ds. The ruthenium triphenylphos-
phine complex RuCl2(PPh3){2,6-py(NCy)2} (1) was syn-
thesized by refluxing an acetone solution of 2,6-py(NCy)2
and RuCl2(PPh3)3 for 4 h. Subsequent reduction of
the solvent and precipitation with diethyl ether gave 1
as a purple microcrystalline solid. The reactions be-
tween RuCl2(PPh3)3 and the chiral ligands (R)-2,6-
py(NCHMePh)2 and (S)-2,6-py(NCHMeNaph)2 in the
same solvent were found to be slow and incomplete. By
changing to THF instead, the ruthenium complexes (S)-
cis-RuCl2(PPh3){2,6-py(NCHMeNaph)2} (7) and (R)-cis-
RuCl2(PPh3){2,6-py(NCHMePh)2} (8) were obtained
readily in 6 h. The reaction of RuCl2(PPh3)3 with the
less basic ligand 2,6-py(NPh)2 was sluggish, and the
complex cis-RuCl2(PPh3)[2,6-py(NPh)2] (6) was only
obtained after a prolonged reflux in THF for 2 days. A
related complex RuCl2(PPh3){(2,6-py(CH3CdN(4-MeO-
C6H4)2} has been prepared recently in a similar fashion
by refluxing the tridentate ligand with RuCl2(PPh3)3 in
toluene.2
The product composition was determined by GC; biphenyl
was used as internal standard. The percent ee were deter-
mined by chiral capillary GC. The results of all reactions are
collected in Table 1.
Isola tion of th e Meta l P r od u cts a fter th e Cyclop r o-
p a n a tion Rea ction s w ith 2,6-p y(NCy)2 P r ecu r sor s. The
solvent was removed completely under high vacuum. Petro-
leum ether (5 mL × 4) was added to dissolve the biphenyl,
cyclopropanes, and residual styrene. The solution was carefully
removed via a cannula. The residual solid was pumped dried
1
under vacuum. H and 31P NMR spectra were then obtained.
When 1 and 2 were the catalyst precursors, the carbene
complex trans-3 was isolated as confirmed by NMR spectros-
copy. When the carbene complexes was used as catalyst
precursor, it was recovered intact after the catalytic run.
Resu lts a n d Discu ssion
The 31P{1H} NMR spectra of the ruthenium com-
plexes 1 and 6 show singlets at δ 31.4 and 33.6,
respectively, indicating that only one PPh3 molecule is
Syn t h esis of Ch ir a l 2,6-Bis(im in o)p yr id yl Li-
ga n d s. The new tridentate ligands (R)-2,6-bis{1-[R-
methylbenzylimine]ethyl}pyridine [(R)-2,6-py(NCHMe-
Ph)2] and (S)-2,6-bis{1-(1-naphthyl)ethylimine]ethyl}-
pyridine [(S)-2,6-py(NCHMeNaph)2] were prepared by
condensation of 2,6-diacetylpyridine with the com-
mercially available optically pure primary amines (R)-
(+)-R-methylbenzylamine and (S)-(-)-1-(1-naphthyl)-
ethylamine, respectively, as shown in Scheme 1. The
synthesis of these ligands by employing the common
procedure of refluxing 2,6-diacetylpyridine with 2 equiv
of the corresponding primary amine in the presence of
a catalytic amount of acid in an alcohol for a period of
even 1 week still led to incomplete reactions. Instead,
we found that by heating 2,6-diacetylpyridine with 4
1
coordinated to the ruthenium center. In the H NMR
spectra, the presence of only one methyl resonance is
in accord with the existence of a symmetry plane
bisecting the two halves of the coordinated ligand and
hence the ligand coordinated in a meridional fashion.
Both cis and trans dispositions of the chlorides would
be possible, however. Efforts to get crystals suitable for
X-ray analyses were unsuccessful. Recently, similar
complexes with the formula trans- and cis-[RuCl2(PPh3)-
{κ3-N,N,N-(SS)-iPr-pybox)}] (trans-9 and cis-9) contain-
ing the pybox ligand 2,6-bis(dihydrooxazolin-2′-yl]-
pyridine have been synthesized via the substitution

Cyclopropanation of Styrene
Organometallics, Vol. 19, No. 10, 2000 1837
reaction of trans-[RuCl2(η2-H2CdCH2){κ3-N,N,N-(SS)-
iPr-pybox)}] with PPh3.12 The thermodynamically stable
cis isomer was indeed obtained from the trans isomer
by refluxing the latter in acetone. Under our experi-
mental conditions, the thermodynamically stable cis
products would similarly be formed. Moreover, trans-9
shows a slightly downfield phosphorus resonance as
compared to cis-9 (δ 38.11 vs 35.34), while the 31P{1H}
NMR signal in the achiral complex trans-[RuCl2(PPh3)-
{κ3-N,N,N-pybox)}] (10) was observed at δ 46.48.12 On
the basis of these spectroscopic data, 1 and 6 may be
assigned a cis geometry.
at δ 2.90. Examples of ruthenium(II) complexes with
fluxional ethylene ligands have previously been re-
ported.14 Consistent with the presence of a labile eth-
ylene ligand, the reaction of 2 with PPh3 in CH2Cl2 gave
1 as the only product (Scheme 2). No intermediate trans
isomer was observed by NMR spectroscopy.
R ea ct ion s of R u Cl2(P P h 3){2,6-p y(NR 2)} a n d
Ru Cl2(η2-H2CdCH2){2,6-p y(NCy)2} w ith Eth yl Di-
a zoa ceta te. To prove the formation of a carbene
complex in the cyclopropanation of styrene with EDA
catalyzed by the present Ru(II) bis(imino)pyridyl com-
plexes, we have investigated their stoichiometric reac-
tions with EDA (Scheme 2). A carbene complex was
successfully isolated only from the reactions with either
1 or 2, i.e., when R ) Cy (Scheme 2). In all the other
cases, no well-defined Ru compound was isolated while
the dimerization of EDA to a mixture of diethyl fuma-
rate and diethyl maleate occurred.
In the 31P{1H} NMR spectra of the chiral complexes
7 and 8, singlets are observed for the coordinated PPh3
group at δ 29.8 and 29.3, respectively. These chemical
shifts are quite comparable to those of 1 and 6; hence a
cis disposition of the chlorides is assigned to the chiral
complexes too. The cis structural formulation for 7 and
1
8 was confirmed by the H NMR spectra that contain
In the reaction of 2 with EDA, the carbene complex
trans-RuCl2(dCHCO2Et){2,6-py(NCy)2} (trans-3) was
cleanly isolated. The reaction presumably proceeds via
the thermodynamically favorable ethylene substitution
by EDA with formation of nitrogen and ethylene gases
as byproducts. It is also possible that the initial EDA
attacks the coordinated ethylene to give ethyl cyclopro-
panecarboxylate, which opens up a coordination site.
However, no cyclopropanation product was detected by
GC/MS of the reaction mixture. By reacting 1 with EDA,
the same carbene complex trans-3 was obtained. In this
case, the reaction involves the nucleophilic attack by
EDA onto PPh3 with formation of the ylid PPh3dCHCO2-
Et as byproduct (31P{1H} NMR signal δ 18.0).15,16 The
formation of ylid-type compounds has also been ob-
served in cyclopropanation reactions catalyzed by copper
phosphine complexes.16
two different sets of resonances for the methyl groups
on both the stereocenters and the CdN groups. Indeed,
this will only be possible with a cis geometry (III and
IV) in which there is no C2 symmetry to render the two
halves of the complexes equivalent as in the case of the
trans isomer.
Compound trans-3 is stable in both the solid state and
solution. Similar ruthenium carbene complexes RuCl2-
(ttp)(dCHCO2Et) [ttp ) Ph(CH2CH2CH2PPh2)2] and
RuCl2(ttp*)(dCHCO2Et) [ttp* ) (S,S)-Ph(CH2CHMeCH2-
PPh2)2] have recently been isolated by treating the
corresponding five-coordinate ruthenium dichloride com-
plexes with EDA.17
The presence of a carbene ligand in trans-3 is unam-
1
biguously shown by the 13C{1H} and H NMR spectra
which contain typical resonances at 299.9 and 20.44
ppm for the carbene carbon atom and its hydrogen atom,
respectively.5b,13,17 NMR spectroscopy shows also that
the two halves of the tridentate ligand are magnetically
equivalent, as indicated by the presence of only one
signal for the methyl groups on the CdN nitrogen
atoms. The equivalence of these methyl groups can take
place when the carbene ligand lies either trans to the
pyridine nitrogen atom and the molecule possesses a
C2 axis or in a mirror plane perpendicular to the Ru-
N3 plane and encompassing the Ru and pyridine nitro-
gen atoms. Since the 1H NOESY spectrum of trans-3
shows strong NOE cross-peaks relating the protons
belonging to the ethyl group of the carbene ligand with
The ethylene complex RuCl2(η2-H2CdCH2)[2,6-py-
(NCy)2] (2) was prepared following the procedure re-
ported by Nishiyama et al. for trans-[RuCl2(η2-H2-
CdCH2){κ3-N,N,N,-(SS)-iPr-pybox)}],4,13 by refluxing a
mixture of [RuCl2(p-cymene)2]2 with 2,6-py(NCy)2 in
dichloromethane under an ethylene atmosphere. Sub-
sequent solvent reduction and slow precipitation by
petroleum ether gave 2 as a purple solid. The compound
is stable in the solid state. However, loss of ethylene
occurs in solution, as evidenced by the presence of the
1
free ethylene signal at δ 5.40 in the H NMR spectrum.
(14) Bianchini, C.; Masi, D.; Peruzzini, M.; Casarin, M.; Maccato,
C.; Rizzi, G. A. Inorg. Chem. 1997, 36, 1061.
(15) Speziale, A. J .; Ratts, K. W. J . Am. Chem. Soc. 1963, 85, 2790.
(16) D´ıaz-Requejo, M. M.; Nicasio, M. C.; Pe´rez, P. J . Organome-
tallics 1998, 17, 3051.
(17) Lee, H. M.; Bianchini, C.; J ia, G.; Barbaro, P. Organometallics
1999, 18, 1961.
In contrast to the rigidity of the ethylene ligand in trans-
[RuCl2(η2-H2CdCH2){κ3-N,N,N,-(SS)-iPr-pybox)}], which
1
shows well-distinguishable H NMR multiplets for the
two sets of CH2 groups,4,13 only one singlet for the four
hydrogens of the coordinated ethylene is observed for 2

1838 Organometallics, Vol. 19, No. 10, 2000
Bianchini and Lee
Sch em e 2
cyclohexyl protons, the RudC bond most likely lies in
the plane of the tridentate ligand, and thus a trans
coordination of the chlorides can be hypothesized for this
product as represented in Scheme 2.
complex was readily formed, as evidenced by an im-
mediate color change from deep red to a lighter color
and by the appearance in the H NMR spectrum of a
1
signal due to a carbene hydrogen at δ 21.34, slightly
downfield with respect to the analogous resonance in
trans-3 (δ 20.41). The other proton signals are similar
to those of trans-3. On the basis of these spectroscopic
data, the product obtained by scavenging a chloride
ligand from trans-3 is assigned the formula [RuCl-
(dCHCO2Et){2,6-py(NCy)2}]PF6 (4). Several attempts
have been made to isolate the complex, which, however,
is too unstable in solution to allow for its separation.
The principal decomposition path (80%) is, however, an
intermolecular carbene trasfer to give the trans and cis
olefins, which were detected by GC/MS in a 1:9 ratio,
respectively.
Consistent with the coordinatively unsaturated na-
ture of 4, the addition of a slight excess of PPh3 to a
dichloromethane solution of 4 resulted in the formation
of [RuCl(PPh3)(dCHCO2Et){2,6-py(NCy)2}]PF6 (5). An
immediate color change from red to pale yellow was
actually observed upon addition of PPh3, while a doublet
at δ 20.98 (J (HP) ) 6.1 Hz) appeared in the region of
carbene hydrogens. In the new product, the two methyl
groups on the CdN moiety give rise to two different
signals, indicating that the two halves of the meridion-
ally coordinated ligand are not magnetically equivalent.
Like the unsaturated species 4, the phosphine adduct
5 is unstable even in solution, thus preventing a reliable
characterization. Consistent with the incorporation of
PPh3 in the complex structure, the decomposition path
of 5 is different from that of 4, as the ylid PPh3dCHCO2-
Et and not the olefin is formed on standing in solution.
In an attempt to intercept intermediate species
preceding the formation of trans-3, the reaction between
2 and EDA was carried out in a 5 mm NMR tube at low
temperature. Already at -80 °C, the 31P NMR spectrum
contained the signals due to the starting complex and
to the ylid PPh3dCHCO2Et, roughly in a ratio of 9:1,
at 33.3 and 18.4 ppm, respectively. In the low-field
1
region of the H NMR spectrum, two singlets at 20.14
and 20.77 ppm were observed that we assign to the
carbene C-H protons of trans-3 and its cis isomer cis-
RuCl2(dCHCO2Et){2,6-py(NCy)2} (cis-3), respectively
(Scheme 2). On warming to -70 °C, the intensities of
the two signals increased while also the concentration
of the ylid increased. On further warming to -60 °C,
the signals of the cis isomer at ca. 20.8 ppm decreased
in intensity, while that of the trans isomer at ca. 20.2
ppm increased. At -50 °C, the signal of the cis isomer
disappeared. Elimination of the solvent under reduced
pressure gave trans-3 as the only product. The low-
temperature experiments thus suggest the formation of
a kinetic carbene product in which the carbene ligand
is trans to chloride instead of nitrogen (Scheme 2). Given
the paucity of spectroscopic data available for the low-
temperature product, other structural formulations
cannot be disregarded. However, the selective formation
of trans-3 and the 1H NMR signal at 20.77 ppm are
indeed consistent with a carbene structure similar to
that of trans-3.
As coordinatively unsaturated Ru(II) carbene com-
plexes are being extensively used in metathesis reac-
tions,18 it would be of great interest to obtain a five-
coordinate Ru(II) carbene complex with bis(imino)pyridyl
ligands and then study its catalytic performance.
By treating a degassed CD2Cl2 solution of trans-3 with
an equivalent amount of AgPF6 in a NMR tube, a new
(18) (a) Grubbs, R. H.; Dias, E. L. Organometallics 1998, 17, 2758
(b) Weskamp, T.; Kohl, F. J .; Hieringer, W.; Gleich, D.; Herrmann, W.
A. Angew. Chem., Int. Ed. 1999, 38, 2416. (c) Schwab, P.; Grubbs, R.
H.; Ziller, J . W. J . Am. Chem. Soc. 1996, 118, 100. (d) Nguyen, S. T.;
Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc. 1993, 115, 2790. (e)
Nguyen, S. T.; J ohnson, L. K.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem.
Soc. 1992, 114, 3974.

Cyclopropanation of Styrene
Organometallics, Vol. 19, No. 10, 2000 1839
A structural assignment for 5 may tentatively be
made on the basis of the 31P chemical shift of the PPh3
ligand (δ 18.2), which is that at the lowest field in the
present series of bis(imino)pyridyl-PPh3 complexes.
Provided the 31P NMR trend observed by Gimeno12 and
by us is valid, then the PPh3 group should not be trans
to the pyridine nitrogen atom, and hence a structure
like that shown in Scheme 2 may be proposed for 5. In
accord with this structural formulation, the carbene
hydrogen atom shows a rather large coupling to the
phosphine (ca. 60 Hz). The value of this constant, the
chemical shift of PPh3, which is close to that of the ylid,
and the facile elimination of the ylid from 5, taken
altogether, indeed suggest that the phosphine and
carbene ligands are spatially close to each other as
occurs in a cis structure. Since the two methyl groups
on the CdN moieties of 5 give rise to two different
syringe pump.6 We have found that the addition over a
period of 8 h with a catalyst/EDA/styrene ratio of 1:75:
750 gives partially optimized results.
All the six-coordinated ruthenium bis(imino)pyridyl-
PPh3 complexes described in this work are effective
cyclopropanation catalysts with activities decreasing in
the order 8 > 7 > 6 > 1 (entries 1, 3, 11, 5). The new
chiral catalysts 7 and 8 are indeed very selective and
efficient cyclopropanation catalysts with a good trans-
cis diastereoselectivity (86:14). A good enantioselectivity
(75.6% ee) was also obtained with 7, bearing the ster-
ically demanding naphthyl substituent on the stereo-
centers (entry 3). Complex 6, with phenyl substituents
on the imine nitrogen atom, gave an average activity,
while 1, with cyclohexyl substituents, was the least
efficient and gave only 21.3% of cyclopropanation prod-
ucts (entries 11, 5). The poor activity of 1 might indeed
be due to the stability of the carbene derivative trans-
3, which is also the termination metal product of the
catalytic cyclopropanation reactions.5b Consistently, the
ethylene complex 2 (Scheme 2) gave a cyclopropane
production similar to that for the analogous PPh3
complex 1 (17.6 vs 21.3%) (entries 5, 7).
As we discovered that 1 and 2 react with EDA
stoichiometrically to give the carbene complex trans-3
as thermodynamic product, this carbene complex was
employed as catalyst precursor in the same experimen-
tal conditions. Complex trans-3 was indeed found to be
an active precursor for styrene cyclopropanation, but to
our suprise, it was less efficient than 1 and 2 (entry 13).
A reasonable interpretation of these results has been
provided by the variable-temperature in situ study of
the reaction between 1 and EDA. A kinetic carbene
complex, assigned as cis-3, is actually formed at low
temperature, which might be more reactive than the
thermodynamic isomer trans-3 for the transfer of the
carbene moiety to the olefin. This might explain why
the reactions catalyzed by either 1 or 2 produce more
cyclopropanes than those carried out in the presence of
isolated trans-3. Increased cyclopropanation activity
would occur at the early stages of the reaction when a
larger concentration of the kinetic carbene complex is
present in solution. The results obtained by adding
silver hexafluorophosphate as cocatalyst confirm, to a
certain extent, this interpretation (see below).
So far the results obtained shows that ligands with
larger substituents on the nitrogen atoms display higher
activities. To further verify this point, complex 11, with
2,6-Me2Ph substituents on the imine nitrogen atoms,
was also tested in styrene cyclopropanation. As shown
in entry 16, 11 catalyzed very efficiently the reaction,
yielding 71.2% of cyclopropanes. The similarities of
catalytic activities and trans-cis selectivities displayed
by 7, 8, and 11 might imply the same degree of steric
bulkiness around the metal center. As the 2,6-bis-
(imino)pyridyl ligand 2,6-py{N(2,6-iPr2-Ph)}2 is being
extensively used in catalytic polymerization of ethylene,1
it would be of interest to see whether a ruthenium com-
plex of this ligand, which bears even bulkier substitu-
ents on the imine nitrogen atoms than those in complex
11, would give a better result. Several attempts had
been made using both RuCl2(PPh3)3 and [(p-cymene)-
1
signals in the H NMR spectrum, the two halves of the
meridionally coordinated bis(imino)pyridyl ligand are
magnetically inequivalent: this feature is consistent
with the proposed structure in which PPh3 is perpen-
dicular to the coordination plane of the nitrogen atoms
and cis to the carbene ligand. In this eventuality, an
out-of-plane steric interaction between PPh3 and the
cyclohexyl rings may indeed account for the inequiva-
lence of the methyl groups as reported for cis-9.12 In the
latter complex, an X-ray structure revealed an out-of-
plane steric interaction between PPh3 and the isopropyl
substituent, causing one of the oxazoline rings to be
puckered. This type of steric interaction might probably
occur in 5 too due to the presence of even more bulkier
substituents on the imino nitrogen atoms.
Ca ta lytic Cyclop r op a n a tion w ith th e Ru th e-
n iu m Com p lexes. The catalytic performance of the
newly synthesized Ru(II) bis(imino)pyridyl complexes
for styrene cyclopropanation with EDA has been inves-
tigated. At the time of this work, a series of solvento
complexes with the general formula RuCl2{2,6-py-
(CH3CdN(Rn-Ar)2}‚S (n ) 1 or 2) had already been
described in the literature.2 The catalytic activity of one
of these complexes, namely, RuCl2{2,6-py(N(2,6-Me2-
Ph2)2}‚CH2Cl2 (11), has also been studied by us. Selected
results are listed in Table 1.
The slow addition of EDA to a dichloromethane
solution containing excess styrene and one of the
catalyst precursors listed in Table 1 led generally to the
formation of both cyclopropanation (A, B) and dimer-
ization products (C, D) (eq 1). No evidence for the
formation of metathesis products, i.e., PhCHdCH(CO2-
Et), was obtained by either GC/MS or 1H NMR spec-
troscopy.17
In metal-catalyzed decompositions of diazo com-
pounds in the presence of olefins, the formation of C
and D may kinetically compete with that of cyclopro-
panes.6,17,19 However, the formation of the olefins can
be reduced by slow addition of EDA to the catalytic
solution containing an excess amount of olefin via a
(19) D´ıaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Prieto,
F.; Pe´rez, P. J . Organometallics 1999, 18, 2601.

1840 Organometallics, Vol. 19, No. 10, 2000
Bianchini and Lee
Sch em e 3
RuCl2]2 as starting ruthenium materials. Unfortunately,
due to steric reasons, no well-defined complex was
isolated in either case.
In the presence of AgPF6, a remarkable improvement
in both productivity and chemoselectivity of the cyclo-
propanation reactions was observed with the catalyst
precursors 1 and 6 (entries 6, 12). Indeed, the yield of
cyclopropanes obtained with 1 increased by more than
250%, while 6 gave 70.1% of cyclopropanes with no
formation of dimerization products. The chiral com-
plexes 7 and 8 were slightly less active in the presence
of AgPF6, however (entries 2, 4).
As shown in Table 1, the addition of AgPF6 to the
reaction mixture reduced the trans-cis diastereoselec-
tivity, in general. Indeed, the trans-cis selectivity was
practically reversed with 8 in the presence of the silver
salt and the cis-cyclopropane became the predominant
product (entry 2). Also, the enantioselectivity was
greatly reduced, as an almost racemic mixture of cis-
cyclopropane was obtained (entries 2, 4).
Notably, the co-presence of AgPF6 in the reaction
mixtures makes 1, 2, and trans-3 equally active cata-
lysts (entries 6, 9, 14). The formation of the same
catalytically active species, most likely the unsaturated
carbene 4, would account for the comparable catalytic
activity exhibited by the precursors 1, 2, and trans-3 in
the presence of AgPF6.
To further verify whether coordinatively and elec-
tronically unsaturated carbenes of the formula [RuCl-
(dCHCO2Et){2,6-py(NR)2}]+ are more efficient and
chemoselective catalysts as compared to the saturated
analogues with PPh3, some reactions were carried out
in the presence of silver triflate (AgOTf), whose anion
possesses nucleophilic character.20 In all the cases
investigated, the substitution of AgOTf for AgPF6 led
to a remarkable decrease in the cyclopropanation activ-
ity as well as chemoselectivity (entries 8, 15). On the
contrary, the addition of an excess of chloride ions (from
tetraalkylammonium salts) to the catalytic mixtures did
not affect the activity in general (entry 10).
haps the interception of a kinetic Ru(II) carbene complex
differing from the thermodynamic isomer by the mutual
positions of the carbene and chloride ligands and, most
importantly, by a higher catalytic activity. This finding
opens up new perspectives in the development of more
efficient cyclopropanation catalysts through the design
of ligand systems capable of controlling the coordination
site of the carbene group.
Within the family of six-coordinate 2,6-bis(imino)-
pyridyl catalysts, the activity increases with the size of
the substituents on the imine nitrogen atoms. This
experimental observable may be interpreted qualita-
tively in terms of both steric and electronic effects. The
presence of bulky groups around the metal center would
promote the elimination of the cyclopropane product,
while a weaker bonding interaction between the metal
center and the carbene ligand would enhance the
electrophilic character of the latter and ultimately favor
the carbene transfer. On the other hand, the good
activity and enantioselectivity exhibited by the chiral
complex 7 witness the importance of the substituents
on the imine nitrogen atoms in controlling both the rate
and the stereoselectivity of the carbene transfer.
Removing one chloride ligand from the six-coordinate
complexes using AgPF6 as halide scavenger has been
found to generally improve the catalytic activity and
chemoselectivity but not the stereoselectivity. A remark-
able decrease in the enantioselectivity is observed, in
fact. Most likely, a change in the cyclopropanation
mechanism occurs due to the participation in the
catalytic cycle of five-coordinate carbene species [RuCl-
(dCHCO2Et){2,6-py(NR)2}]+ that can simultaneously
coordinate carbene and olefin moieties. Experiments
with a silver salt containing a nucleophilic anion support
this hypothesis. No clear-cut conclusion about the
possible occurrence of an intramolecular mechanism
(Scheme 3b), involving the concomitant coordination of
the olefin and carbene groups,6,17 can be drawn at this
stage, however. Indeed, the higher activity of the
catalyst systems comprising AgPF6 as cocatalyst may
simply be due to the increased electrophilic character
of the carbene moiety in electronically unsaturated Ru-
(II) species.
Con clu d in g Rem a r k s
The overall reactivity of the Ru(II) 2,6-bis(imino)-
pyridyl six-coordinate complexes toward either EDA
alone or EDA/styrene mixtures is quite comparable to
that of the pybox analogues.4,5b,13 The latter, however,
are generally more enantioselective with ee’s up to
100%. Like the pybox catalysts, the carbene transfer
from EDA to the olefin is apparently mediated by Ru-
(II) carbene species in an intermolecular fashion (Scheme
3a). The only Ru(II) 2,6-bis(imino)pyridyl carbene com-
plex that has been isolated is just the one exhibiting
the lowest catalytic activity, which again is consistent
with the chemistry of the pybox-based catalysts.5b
The most intriguing result obtained with the catalysts
supported by the 2,6-bis(imino)pyridyl ligands is per-
Ack n ow led gm en t. Thanks are due to Fabrizio
Zanobini for technical assistance in the synthesis of the
2,6-bis(imino)pyridyl ligands. The European Commis-
sion is gratefully acknowledged for financial support
(COST Action D17; RTN contract 1 1999 00164).
(20) (a) Rhodes, L. F.; Sorato, C.; Venanzi, L. M.; Bachechi, F. Inorg.
Chem. 1988, 27, 604. (b) Humphrey, M. B.; Lamanna, W. M.;
Brookhart, M.; Husk, G. R. Inorg. Chem. 1983, 22, 3355.
OM990951P