Enantioselective π-complexation. Synthesis of enantiomerically pure planar chiral ruthenocenes

Article abstract of DOI:10.1021/om950736i

Reaction of half-sandwich complexes Cp*RuL₂^ch, featuring a chiral auxiliary ligand L₂^ch, with prochiral cyclopentadienides Cp^pc- gave enantiomerically pure (>95% ee) or enriched ruthenocenes Cp*RuCp^pc 4. For the chiral auxiliary L₂^ch were tested a pyridyloxazoline (ref 15), the diolefin nopadiene (ref 16), and the amino acids methionine and proline, which form relatively labile complexes with the Ru(II) fragment Cp*Ru⁺. For the prochiral cyclopentadienide Cp^pcLi, Li derivatives of 1-tert-butyl-3-methylcyclopentadienide, 1-tert-butylindenyl, and 1- tert-butyl-3-benzylindenyl were used. Determination of enantiomeric excess was achieved through a derivatization procedure generating fulvene cation complexes [Cp^pcRuC₅-Me₄=CH₂]⁺ (5), which, after the addition of (S)-α-phenylethylamine, yield mixtures of diastereoisomers Cp^pcRuC₅H4CH₂NHCH(Me)Ph (6). The absolute configuration of 4c (Cp^pc = 1-tert-butyl-3-benzylindenyl) was determined by X-ray structure analysis of the respective fulvene salt. Asymmetric induction was greatest with the amino acids and was reverted with the epimeric amino acid; thus, (S)-proline or -methionine gave (S)-ruthenocene 4c and vice versa.

Full text of DOI:10.1021/om950736i

1376
Organometallics 1996, 15, 1376-1383
En a n tioselective π-Com p lexa tion . Syn th esis of
En a n tiom er ica lly P u r e P la n a r Ch ir a l Ru th en ocen es
Ulrich Koelle,* Karin Bu¨cken, and Ulli Englert
Institute for Inorganic Chemistry, Technical University at Aachen, Prof.-Pirlet-Strasse 1,
D-52074 Aachen, Germany
Received September 15, 1995X
ch
Reaction of half-sandwich complexes Cp*RuL2ch, featuring a chiral auxiliary ligand L2
,
with prochiral cyclopentadienides Cppc- gave enantiomerically pure (>95% ee) or enriched
ruthenocenes Cp*RuCppc 4. For the chiral auxiliary L2 were tested a pyridyloxazoline (ref
ch
15), the diolefin nopadiene (ref 16), and the amino acids methionine and proline, which form
relatively labile complexes with the Ru(II) fragment Cp*Ru+. For the prochiral cyclopen-
tadienide CppcLi, Li derivatives of 1- tert-butyl-3-methylcyclopentadienide, 1-tert-butylindenyl,
and 1- tert-butyl-3-benzylindenyl were used. Determination of enantiomeric excess was
achieved through a derivatization procedure generating fulvene cation complexes [CppcRuC5-
Me4dCH2]+ (5), which, after the addition of (S)-R-phenylethylamine, yield mixtures of
diastereoisomers CppcRuC5H4CH2NHCH(Me)Ph (6). The absolute configuration of 4c (Cppc
) 1-tert-butyl-3-benzylindenyl) was determined by X-ray structure analysis of the respective
fulvene salt. Asymmetric induction was greatest with the amino acids and was reverted
with the epimeric amino acid; thus, (S)-proline or -methionine gave (S)-ruthenocene 4c and
vice versa.
Planar chiral arene and dienyl complexes have re-
Strategies to effect face differentiation in the course
of the complexation of prochiral (enantiotopic) π-ligands
were mostly based on an additional chiral substituent,
e.g., a menthyl ester, on this ligand, leading to diaste-
reoisomers with planar chirality as well as central
chirality in the side chain. Although the method was
successful in the diastereospecific complexation of the
Cr(CO)3 unit to an R-hydroxytetraline, with diastereo-
metric induction of >98%,7 in other cases induction was
low and the resulting diastereomeric mixtures had to
be separated by recrystallization or chromatography.8
In view of the synthetic versatility provided by an
element of planar chirality in transition metal com-
plexes on the one hand and the restrictions inherent in
the separation of enantiomers or diastereoisomers on
the other, enantioselective π-complexation would mark
important progress in the direction toward synthesis of
cently proven to be versatile intermediates in synthetic
organic chemistry.1 Due to the complete regiospecificity
in, for example, the addition of a nucleophile to an
electrophilic π-ligand of a transition metal complex,2 the
newly generated asymmetric carbon atom is formed
enantiospecifically depending on the optical purity of
the π-complex. Such methodology has been widely used
to elaborate a host of functionalities in stereocontrolled
+ 3
reactions via, for example, the (dienyl)Fe(CO)3
(arene)Cr(CO)3 moieties.
or
1
Although sandwich and half-sandwich complexes with
planar chirality, particularly ferrocenes, ruthenocenes,
(arene)chromium, and CpMn tricarbonyl derivatives,4
had been synthesized and their chiroptical properties
studied, resolution into enantiomers generally has
rested on classical derivatization procedures via dias-
tereoisomers. These can be generated by the introduc-
tion of a chiral substituent onto either the π-ligand5 or,
in the case of a half-sandwich complex, the metal (or
both).6
enantiomerically pure planar chiral π-complexes.
A
chiral auxiliary ligand, which may or may not be lost
during complexation, could serve as the si/ re controlling
factor during attack of the complexing metal fragment
LnM onto the prochiral π-ligand, e.g., Cppc. The idea
was first pursued by Birch,9 who attempted this kind
of chirality transfer by reacting chiral oxa diene Fe(CO)3
complexes (of 16-dehydropregnenolone acetate or pule-
gone) with suitably substituted cyclohexadienes to
generate optically enriched (cyclohexadiene)Fe(CO)3
X Abstract published in Advance ACS Abstracts, February 1, 1996.
(1) (a) See for example: Schmalz, H.-G. Nachr. Chem. 1994, 42, 1016
and references given therein. (b) Enders, D.; J andeleit, B.; Raabe, G.
Angew. Chem. 1994, 106, 2033.
(2) Ku¨ndig, E. P.; Liu, R.; Ripa, A. Helv. Chim. Acta 1992, 75, 2657
and references given therein.
(3) a) Knoelker, H. J .; Baum, G.; Kosub, M. Synlett 1994, 1012. (b)
Knoelker, H. J .; Bauermeister, M. Helv. Chim. Acta 1993, 76, 2500.
(4) (a) Schlo¨gl, K. Top. Stereochem. (Allinger, N. L., Eliel, E. L., Eds.)
1967, 1, 39. (b) Halterman, R. H. Chem. Rev. 1992, 92, 965. (c) J aouen,
G.; Vessie`res, A. Pure Appl. Chem. 1985, 57, 1865. (d) J aouen, G. Pure
Appl. Chem. 1986, 58, 597. (e) Hofer, O.; Schlo¨gl, K. Tetrahedron Lett.
1967, 36, 3485. (f) Sokolov, V. I. Chirality and Optical Activity in
Organometallic Compounds; Gordon & Breach: New York, 1990; pp
70 ff.
(5) (a) Hofer, O.; Schlo¨gl, K. J . Organomet. Chem. 1968, 13, 457.
(b) Alexakis, A.; Mangeney, P.; Marek, I.; Rose-Munch, F.; Rose, E.;
Semra, A.; Robert, F. J . Am. Chem. Soc. 1992, 114, 8288.
(6) Pertici, P.; Pitzalis, E.; Marchetti, F.; Rosini, C.; Salvadori, P.;
Bennett, M. A. J . Organomet. Chem. 1994, 466, 221.
(7) Schmalz, H.-G.; Millies, B.; Bats, J . W.; Du¨rner, G. Angew. Chem.
1992, 104, 640; Angew. Chem., Int. Ed. Engl. 1992, 31, 631.
(8) (a) Uno, M.; Ando, K.; Komatsuzaki, N.; Takahashi, S. J . Chem.
Soc., Chem. Commun. 1992, 964. (b) Uno, M.; Ando, K.; Komatsuzaki,
N.; Tanaka, T.; Sawada, M.; Takahashi, S. J . Chem. Soc., Chem.
Commun. 1993, 1549. (c) Pearson, J .; Chang, K.; McConville, D. B.;
Youngs, W. J . Organometallics 1994, 13, 4. (d) Schmalz, H.-G.; Hessler,
E.; Bats, J . W.; Du¨rner, G. Tetrahedron Lett. 1994, 35, 4543. (e)
Komatsuzaki, N.; Uno, M.; Shirai, K.; Tanaka, T.; Sawada, M.;
Takahashi, S. J . Organomet. Chem. 1995, 498, 53.
(9) (a) Birch, A. J .; Kelly, L. F. J . Organomet. Chem. 1985, 285, 261.
(b) Birch, A. J .; Raverty, D.; Stephenson, G. R. Organometallics 1984,
3, 1075.
0276-7333/96/2315-1376$12.00/0 © 1996 American Chemical Society

Synthesis of Planar Chiral Ruthenocenes
Organometallics, Vol. 15, No. 5, 1996 1377
complexes. However, neither optical yields, estimated
at best around 40% ee, nor chemical yields were in a
range that encouraged practical use of the method.
Since the intermediate LchFe(CO)3 complexes were not
isolated, their diastereomeric purity could not be as-
sessed,10 leaving the reasons for the relatively low chiral
induction unclear. In a similar experiment by Wink and
co-workers,11 who examined arene exchange at (naph-
thalene)(CO)2PR(OR′)2chCr, inductions likewise were
rather low. In this case the chiral phosphine is not
exchanged during reaction, leading to a mixture of
diastereoisomers.
Amino acids could serve as readily available auxilia-
ries from the chiral pool. CODRu(amino acid) com-
plexes are known with the composition CODRu(amino
acid)Cl2.17 Depending on the additional functional
groups, an amino acid could function as a bi- or
tridentate ligand toward a CpRu* unit. We found that
the coordinatively unsaturated precursor 1 reacted
smoothly with methionine and also with proline to give
3c and 3d , respectively. In the methionine complex 3c,
the amino acid functions as a monoanionic tridentate
ligand through carboxylate, amine, and MeS groups.
Complexation of methionine to Cp*Ru generates two
new chiral centers (at ruthenium and at sulfur), giving
rise to four diastereoisomers. Starting from optically
pure (S)-methionine, NMR spectra of 3c show the
presence of but one diastereoisomer, i.e., the configu-
ration of the amino acid determines the sense of the
tripodal arrangement (O, N, S) as well as the configu-
ration at the complexed MeS group. Such dia-
stereoselectivity was previously observed in CODRu-
(amino acid)Cl2, where the use of a racemic amino acid
led to the mixture of only two enantiomers.17
As a prerequisite for successful enantiosteering of a
prochiral π-ligand, the complexation reaction must
proceed under mild conditions, implying that the aux-
iliary ligand must not be too strongly bound in case it
has to be released in the course of the complexation.
The chiral auxiliary itself should be preferably available
from the natural chiral pool or should be easily synthe-
sized from it. Moreover, if it is released during com-
plexation, it could be recovered. Common (acceptor)
ligands in transition organometallic complexes in gen-
eral do not meet these requirements. We recently found
a class of Cp*Ru complexes where coligands such as
amines or sulfoxides are pure σ-donors12 and can be
expected to be displaced in, for example, a cyclopenta-
dienylation reaction under rather mild conditions. We
thus embarked on the direct synthesis of enantiomeri-
cally enriched planar chiral ruthenocenes by cyclopen-
tadienylation of such labile half-sandwich complexes as
an entry into enantioselective π-complexation. For this
reason, we developed a methodology to incorporate
chiral chelating amines and dienes, as well as amino
acids, as ligands into preferably labile Cp*Ru half-
sandwich complexes. These have proved to directly
yield planar chiral ruthenocenes upon complexation
with appropriately substituted cyclopentadienides in
good chemical yield and, in the most favorable cases,
in rather high optical purity.
The situation is slightly more complicated with pro-
line, which can function only as a bidentate ligand. The
analytical and spectroscopic composition of the bright
yellow, extremely air sensitive (pyrophoric!) solid that
separates from THF is for Cp*Ru(proline). The com-
pound is soluble without decomposition only in coordi-
nating solvents such as acetonitrile. NMR spectra in
this solvent show coordinated proline and one single Cp*
singlet. Although a number of Cp*Ru complexes with
16 valence electrons are well documented,18 no mono-
meric complex with just two σ-donor ligands exists
among them. We suggest a solvent-stabilized mono-
meric structure for 3d in acetonitrile solution and a
polymeric one in the solid, possibly with the free CdO
group of the amino acid as a weak intermolecular link.
Although in principle two diastereomeric complexes 3d
can be formed from either (S)- or (R)-amino acids,
respectively, only one has been observed in complexes
such as Cp*Rh(proline)Cl or (arene)Ru(proline)Cl,19
which means that the configuration of the newly formed
chiral center at nitrogen is determined by the configu-
ration of the R-carbon. Therefore, also in the case of
3d , from the two diastereomeric forms possible in
solution, whether rapidly interconverting or not, one is
expected to be present in large excess.
Cp *Ru Lch Com p lexes w ith Bid en ta te Ch ir a l
2
Au xilia r y Liga n d s Lch 2. Starting from either [Cp*Ru-
(OMe)]2 (1) or [Cp*RuCl]4 (2), complexes of the type
Cp*RuL2X were readily prepared with a wide variety
of ligands L, including pure σ-donors.12-14 In analogy
to Cp*Ru(bipy)Cl,13 the corresponding complex with a
chiral 2-(2-pyridinyl)-2-oxazoline15 compound 3a , known
as a ligand in RhI complexes that catalyze asymmetric
hydrosilylation, was prepared. The NMR spectrum of
3a indicated a 1:10 ((S,R):(R,R)) mixture of diastereoi-
somers that did not change upon recrystallization and
is believed to be the thermodynamic mixture. Complex
3b, which was obtained from 2 and the chiral diene (+)-
nopadiene16 (Scheme 1), showed but one diastereoisomer
in the NMR spectrum.
Cyclop en ta d ien yla tion w ith Un sym m etr ica lly
Su bstitu ted Cyclop en ta d ien id es. Prochiral cyclo-
pentadienes CppcH, 1-tert-butyl-3-methylcyclopentadi-
ene, 1-tert-butylindene, and 1-tert-butyl-3-benzylindene,
were synthesized according to or adopted from literature
methods.20 The corresponding lithium cyclopentadien-
ides were prepared by the action of a stoichiometric
amount n-BuLi on cyclopentadienes. These were added
to complexes 3a -3d at low temperature (-78 °C), and
(10) Complexation of Cp*Ru on pullegone to give the the Cp*Ru-
(oxadiene) complex analogous to the (CO)3Fe complex of ref 9a (Koelle,
U.; Pasch, R. To be published) showed almost no diastereoselectivity,
which may be the reason for the low enantiomeric excess in transfer
reactions described in ref 9.
(16) Salzer, A.; Schmalle, H.; Stauber, R.; Streiff, S. J . Organomet.
Chem. 1991, 408, 403.
(11) Nirchio, P. C.; Wink, D. J . Organometallics 1991, 10, 336.
(12) Koelle, U.; Wang, H.; Englert, U. J . Organomet. Chem. 1993,
453, 127.
(13) (a) Koelle U.; Kossakowski, J . J . Chem. Soc., Chem. Commun.
1988, 549. (b) Koelle, U.; Kossakowski, J . J . Organomet. Chem. 1989,
362, 383.
(17) Sheldrick, W. S.; Exner, R. J . Organomet. Chem. 1990, 386,
375.
(18) Bickford, C. C.; J ohnson, T. J .; Davidson, E. R.; Caulton, K. G.
Inorg. Chem. 1994, 33, 1080 and references given therein.
(19) Kra¨mer, R.; Polborn, K.; Wanjek, H.; Zahn, I.; Beck, W. Chem.
Ber. 1990, 123, 767.
(14) Fagan, P. J .; Mahoney, W. S.; Calabrese, J . C. Organometallics
1990, 9, 1843.
(15) Brunner, H.; Obermann, U. Chem. Ber. 1989, 122, 499.
(20) (a) Besanc¸on, J .; Top, S. J . Organomet. Chem. 1977, 127, 139.
(b) Okuda, J .; Chem. Ber. 1989, 122, 1075. (c) Solov’yanov, A. A.;
Beletskaya, I. P.; Reutov, O. A. Zh. Org. Khim. 1982, 18, 1373.

1378 Organometallics, Vol. 15, No. 5, 1996
Sch em e 1. Syn th esis of Ch ir a l Cp *Ru Ha lf-Sa n d w ich Com p lexes
Koelle et al.
the mixture was slowly warmed until the reaction
started, which occurred around -30 to -10 °C. The
mixture was then stirred at this temperature for some
hours before workup. Ruthenocenes were purified by
chromatography over silylated silica. They are pale
yellow oils that were spectroscopically pure, but elemen-
tal analysis was difficult in cases where the compound
did not solidify. Chemical and optical yields of ru-
thenocenes 4a -4c are collected in Scheme 2. For the
sake of comparison, the same ruthenocenes were syn-
thesized as racemates by cyclopentadienylation of 1 with
the respective prochiral lithium cyclopentadienide (Cppc)
in THF, a reaction that gives ruthenocenes in yields of
60-65%.
Der iva tiza tion , En a n tiom er ic P u r ity, a n d Abso-
lu te Con figu r a tion . In the present case, determina-
tion of the enantiomeric composition poses a particular
problem. Ruthenocenes lacking functional groups are
not split in the NMR by chiral shift reagents. They are
not volatile enough for GC, nor did HPLC on chiral
material (DIONEX) give any separation of enantiomers.
To determine the enantiomeric composition, we resorted
to a derivatization procedure, the principles of which
we developed many years ago.21 Ruthenocenes featur-
ing a Cp* ligand are susceptible to hydride abstraction
from a methyl group, giving cationic fulvene complexes
in high yield. These are readily recrystallized, which,
when followed by hydride addition, can serve as a
convenient purification procedure for the ruthenocene.
Fortunately hydride abstraction in the present cases
occurred exclusively at the Cp* ligand, even if the other
Cp ring carried methyl groups as well.
Since these cationic fulvenes are prone to add various
nucleophiles at the exocyclic dCH2 group,21 the specific
addition of (S)-R-phenylethylamine (Scheme 3) was a
straightforward way to obtain diastereoisomers that
could be distinguished in the proton (in some cases also
the carbon) NMR. Optical yields in Chart 1 are based
1
on these spectra. Figure 1 shows a representative H
NMR spectrum of 6c to illustrate the method. The
derivatization as applied to racemates gave diastereoi-
somers in an exactly 1:1 ratio, proving that no diaste-
reoselectivity is inherent in any of its reaction steps.
As one can see from Figure 1, signals for diastereoi-
somers are doubled for Cp* methyl protons, for residual
protons in Cppc, and for the AB pattern of the exocyclic
CH2 group. NOE experiments on 6a suggest a confor-
mation where the CH2NHCHMe(Ph) group is located
above H2 of the Cppc ring, exerting a maximum chemical
(21) Koelle, U.; Grub, J . J . Organomet. Chem. 1985, 289, 133.

Synthesis of Planar Chiral Ruthenocenes
Organometallics, Vol. 15, No. 5, 1996 1379
Sch em e 2. Syn th esis of Ra cem ic a n d Op tica lly En r ich ed Ru th en ocen es
Sch em e 3. Der iva tiza tion of Ch ir a l Ru th en ocen es (e.g., 4c) Givin g d ia ster eom er s (e.g., 6c).
t
anisotropy in this proton as well as in the Me and Bu
groups. Shift differences are large enough to locate
signals from individual diastereomers and, thus, iden-
tify (R)- and (S)-ruthenocenes for 4a -c independently.
In this way it was possible to assess the formation of
epimeric ruthenocenes (S)-4c and (R)-4c as the majority
enantiomers with the same enantiomeric excess when
(S)- and (R)-proline and -methionine, respectively, were
the auxiliaries.
to the structure solution, the measurement of anoma-
lous dispersion of Friedel pairs allowed one to determine
the absolute configuration. It is thus shown that (S)-
methionine and also (S)-proline induce a planar (S)-
configuration (pS) of this particular ruthenocene. NMR
assignment gives the other diastereoisomer with either
(R)-amino acid (using (S)-R-phenylethylamine); thus,
(R)-methionine and (R)- proline induce a (pR)-configu-
ration.22
To determine the absolute configuration of one of the
chiral ruthenocenes, a single-crystal X-ray structure
determination was performed. Since none of the ru-
thenocenes showed a tendency to form good crystals,
particularly when it was enantiomerically pure or
enriched, we resorted to crystallizing the corresponding
fulvene cations, which must have the same configura-
tion as the parent ruthenocene. Crystals of (tetram-
ethylfulvene)(1-tert-butyl-3-benzylindenyl)ruthenium tet-
rafluoroborate (5c), derived from ruthenocene 4c, were
grown by diffusion of ether into a concentrated meth-
ylene chloride solution of the fulvene salt. In addition
Discu ssion
As can be seen by inspection of Chart 1, asymmetric
induction in the course of π-complexation increases with
increasing steric bulk of the substituents at Cppc.
Moreover, the highest enantiomeric purities were ob-
tained in cases where the reaction started at low
(22) The configuration is designated by using the adaptation of the
Cahn-Ingold-Prelog Rules to Cp transition metal complexes: Sloan,
T. E. In Topics in Inorganic and Organometallic Stereochemistry;
Geoffrey, G., Ed.; Wiley: New York, 1981; Vol. 12, p 32.

1380 Organometallics, Vol. 15, No. 5, 1996
Koelle et al.
Ch a r t 1. Yield s a n d En a n tiom er ic Excess of Ch ir a l Ru th en ocen es Obta in ed w ith Va r iou s Ch ir a l
Au xilia r ies.
temperature, but the Cp*Ru half-sandwich complex was
not completely dissolved in the reaction mixture.
anion at the metal of a carboxylic acid complex can be
compensated for within the carboxylate group, avoiding
a negatively charged metal fragment. In the case of 3d ,
a dynamic equilibrium between the transition complex
of Figure 3 and its constituents, with high preference
for one diastereoisomer, could still lead to high optical
induction in the product. Obviously the low-energy
conformation for this particular case is that depicted in
A
σ-cyclopentadienyl complex as depicted in Figure 3 is
generally considered as an intermediate in this type of
complexation.23 For intermediates such as the (σ-
cyclopentadienyl)(diene)RuCp*, formed in the case of the
nopadiene complex 3b, this complex is anticipated to
be relatively more stable, i.e., the barrier for σ f π
conversion should be higher than if the coligands are
pure σ-donors. A low barrier for σ f π conversion can
be anticipated for amino acid complexes.
t
Figure 3 (benzyl group directed toward N, Bu directed
toward O), since it leads to the experimentally observed
configuration of the resulting ruthenocene.
Since the orientation of the incoming Cppc with
respect to the Cp*Ru residue will be restricted to that
shown in Figure 3 (H on C2 pointing toward Cp*), site
differentiation is determined by the attack of Cppc from
“left or right”. For configurationally rigid precursor
complexes such as 3a -c, the site of attack is then
determined by the most labile (i.e., the most readily
substituted) atom at Ru. It therefore seems essential
that in these cases precursor half-sandwich complexes
are formed as one single diastereoisomer with respect
to the configuration at the metal. But even an auxiliary
such as bidentate proline, which emerged in this study
as the most efficient chiral auxiliary, may still offer a
reaction path where the chiral information provided by
the amino acid can be retained to a late stage at the
reaction coordinate. Figure 3 displays the σ-cyclopen-
tadienyl intermediate that could be formed from 3d and
1-tert-butyl-3-benzylindenyl. Note that the attack of an
In 1,3-disubstituted cyclopentadienides with R1 > R3,
like the 1-tert-butyl-3-methylcyclopentadienide, a 4-σ-
complex is the most probable intermediate. Differentia-
tion is then effected through the difference in size
between R3 and H, which may lead to a single preferred
conformation in the transition state and ultimately to
a single enantiomer. In 1-indenyls a 3-σ-complex would
be formed, where stereodifferentiation is between H
atoms at the Cp and the arene ring, whereas 1,3-
disubstituted indenyls are restricted to form a 2-σ-
complex in the transition state (as depicted in Figure
3). In this latter case, R1 and R3 are both adjacent to
the Ru-C σ-bond providing the full effect of size
differentiation, which may explain the high induction
found for 1-tert-butyl-3-benzylindene. For a ligand
transfer reaction that does not pass through a σ-transi-
tion state, the factors that determine enantioface dif-
ferentiation are less well defined, but substituents of
different size can be expected to exert a major influence.
(23) Cotton, F. A.; Marks, T. J . J . Am. Chem. Soc. 1969, 91, 7523.

Synthesis of Planar Chiral Ruthenocenes
Organometallics, Vol. 15, No. 5, 1996 1381
F igu r e 3. Assumed configuration of the intermediate σ-Cp
complex formed from 3d and 1-tert-butyl-3-methylindenide.
UV spectra were recorded on an X-dap diode array spectro-
photometer, and CD spectra were recorded on a J asco J 41c
instrument. Elemental analyses were from Microanalytical
Laboratories Engelskirchen or the microanalytical laboratory
of this institute. 1-tert-butyl-3-methylcyclopentadiene, 1-tert-
butylindene20 and (R)-(+)-4-ethyl-2-(2-pyridinyl)-2-oxazoline14
were synthesized by adapting literature procedures.
1-ter t-Bu tyl-3-ben zylin d en e. To a solution of 5.95 mmol
of 1-tert-butylindene in 20 mL of THF cooled to -78 °C was
added 2.38 mL of BuLi (2.5 M solution in hexane). The
mixture was slowly warmed to ambient temperature and
stirred for one additional hour. To the solution of 1-tert-
butylindenyllithium cooled again to -78 °C was added 5.95
mmol of benzyl chloride. After the solution was warmed to
room temperature and stirred for 1 h, the solvent was removed
and the oily residue was extracted with pentane. The com-
bined extracts were filtered through Celite, the solvent was
stripped again, and the residual oil was dried at high vacuum
to obtain 4.1 mmol (68% yield) of product (14:1 mixture of
tautomers). 1H NMR (500 MHz, C6D6, 25 °C): 1-tert-butyl-3-
benzylindene δ 7.48-6.68 (m, 9H, Ph, H4-7), 6.00 (d, 3J (H2,H3)
) 2.1 Hz, 1H, H2), 3.49 (ABX, 3J (H3,H2) ) 2.1 Hz, 3J (H3,-
PhCHH) ) 8.2 Hz, 3J (H3,PhCHH) ) 6.4 Hz, 1H, H3), 2.89, 2.55
(ABX, J AB ) 13.4 Hz, 3J (PhCHH,H3) ) 8.2 Hz, 3J (PhCHH,H3)
) 6.4 Hz, 2H, PhCH2), 1.26 (s, 9H, tBu); 1-benzyl-3-tert-
butylindene δ 5.80 (s, 1H, H2), 2.10 (s, 1H, H3), 2.99, 2.94 (ABq,
F igu r e 1. Part of the 1H NMR spectrum of 6c showing
signals that are doubled in the diastereomeric mixture.
Major signals are from (R,S)-6c and minor signals (black)
from (S,S)-6c.
t
J AB ) 13.4 Hz, 2H, PhCH2), 1.03 (s, 9H, Bu). 13C NMR (500
MHz, C6D6, 25 °C): 1-tert-butyl-3-benzylindene δ 153.6, 149.3,
144, 140.6 (C8,9,1, Ph-Cipso), 131 (C2), 129.2, 128.5, 126.6 (Ph),
126.4, 124.4, 123.8, 122.5 (C4-7), 49.6 (C3), 38.6 (PhCH2), 33.0,
29.4 (tBu).
3a . To a solution of 0.25 g (0.23 mmol) of [Cp*RuCl]4 (2) in
30 mL of Et2O was added 0.16 g (0.92 mmol) of (R)-(+)-4-ethyl-
2-(2-pyridinyl)-2-oxazoline; the color directly turned mauve.
The solution was stirred for 2 h, filtered through Celite, and
concentrated. At -40 °C, 0.322 g (0.72 mmol, 78% yield) of
product separated as a dark mauve powder: de 88%; 1H NMR
(500 MHz, C6D6, 25 °C) δ 8.97 (d/d/d, 3J (H10,H9) ) 5.5 Hz,
f
4J (H10,H8) ) 1.5 Hz, J ) 1.0 Hz, 1H, H10), 7.15 (m, 1H, H7),
3
4
6.68 (1H, t/d, J (H8,H7,H9) ) 7.5 Hz, J (H8,H10) ) 1.5 Hz, 1H,
H8), 6.55 (d/d/d, 3J (H9,H10) ) 5.5 Hz, 3J (H9,H8) ) 7.5 Hz,
4J (H9,H7) ) 1.5 Hz, 1H, H9), 4.15 (d/d, 3J (H4,H5) ) 8.5 Hz,
3J (H4,H5′) ) 6.0 Hz, 1H, H4), 3.97 (d/d/d, J (H5,H5′) ) 8.5 Hz,
2
3J (H5,H4) ) 8.5 Hz, J (H5′,H4) ) 6.0 Hz, 2H, H5,H5′), 2.25 (m,
3
1H, CH2), 1.36 (m, 1H, CH2), 0.75 (t, 3J (Me,CH2) ) 7.5 Hz,
3H, Me), 1.74 (s, 15H, Cp*).
F igu r e 2. Ortep representation of the cation of (S)-
(tetramethylfulvene)(1-tert-butyl-3-benzylindenyl)ruthe-
nium tetrafluoroborate (5c): Ru-Cind 2.23(1), 2.24(1),
2.20(1), 2.265(9), 2.25(1) Å, Ru-Cfulv 2.14(1), 2.25(1), 2.252-
3b. To 0.3 g (0.29 mmol) of [Cp*RuCl]4 (2) in 30 mL of Et2O
was added, at -78 °C, 0.17 g (1.16 mmol) of (+)-nopadiene.15
After slow warming to room temperature, the mixture was
stirred for an additional 3 h. The solvent was stripped, the
residue was extracted with hexane, and the solution was
filtered through celite, concentrated, and kept at -78 °C. 3b
separated as an orange powder: yield 80% (0.39 g, 0.92 mmol);
(9), 2.15(1) Å; Ru-CdCH2exocl 2.07(1) Å; Ru-CdCH2
exocl
2.30(1) Å.
Exp er im en ta l Section
3
All manipulations were conducted under an inert atmo-
sphere of nitrogen using conventional Schlenk tube techniques.
Solvents were dried and distilled under nitrogen atmosphere
before use. The nuclear magnetic resonance spectra were
recorded on Varian Unity 500 or Bruker WP-80 spectrometers.
Mass spectra were recorded on a Varian MAT-95 spectrometer,
1H NMR (500 MHz, C6D6, 25 °C) δ 3.87 (d/d, J (H8,H9′) ) 9.5
Hz, 3J (H8,H9) ) 7.3 Hz, 1H, H8), 3.44 (d br, 3J (H3,H4′) ) 6.1
Hz, 1H, H3), 2.85 (d/d, 3J (H9,H8) ) 7.3 Hz, 2J (H9,H9′) ) 2.14
3
2
Hz, 1H, H9), 1.75 (d/d, J (H9′,H8) ) 9.5 Hz, J (H9′,H9) ) 2.14
Hz, 1H, H9′), 2.22 (d br, 2J (H4,H4′) ) 17.5 Hz, 1H, H4), 2.03
(d/d/d/d, J (H4′,H4) ) 17.5 Hz, J (H4′,H3) ) 6.1 Hz, J (H4′,H5)
2
3
3

1382 Organometallics, Vol. 15, No. 5, 1996
Koelle et al.
f
3
) 2.5 Hz, J ) 1.5 Hz, 1H, H4′), 1.95 (t/d, J (H1,H7) ) 5.5 Hz,
90.4, 80.8 (C8,9,1,3), 82, 10.6 (Cp*), 77.2 (C2), 44 (C10), 32.1, 31.8
(tBu); UV/vis (n-hexane) λmax (ꢀ) 271 (1643), 289 (1591), 394
nm (329); CD ((S)-4c, n-hexane) λmax (∆ꢀ) 259 (-6.3), 307 (1.6),
332 (-7 × 10-1), 409 nm (6.35); MS (70 eV, EI) m/z (relative
fJ ) 1.0 Hz, 1H, H1), 1.85 (m, 1H, H5), 1.54 (m, 1H, H7), 1.10
(d, J ) 8.6 Hz, 1H, H7′), 1.36 (s, 15H, Cp*), 1.23 (s, 3H, Me6),
2
0.86 (s, 3H, Me6′); 13C NMR (500 MHz, C6D6) δ 125.2 (C2), 47-
(C9), 83.2, 74.6 (C8,3), 93.8, 9.6 (Cp*), 46, 42 (C1,5), 38 (C6), 35.4,
30.2 (C4,7), 26, 21.6 (Me6, Me6′).
intensity) 498 (9) [M+], 483 (14) [M+ - Me], 441 (2) [M+
-
tBu], 372 (100) [Cp*2Ru].
(RRu ,SC)- or (SRu ,RC)-3c. To 0.19 g (0.36 mmol) of 1 in 20
mL of MeOH was added 0.11 g (0.71 mmol) of (S)- or
(R)-methionine, respectively, and the mixture was stirred for
12 h at ambient temperature. The solvent was stripped, the
residue was extracted with toluene, and the solution was
filtered through Celite. To the solution, concentrated to about
2 mL, was added ether to precipitate the product as a yellow
powder, which was washed successively with ether and hexane
5a -c. 4a -c were each dissolved in CH2Cl2, an equimolar
amount of Ph3CBF4 was added, and the solution was stirred
for 30 min. After concentration, the solution was diluted with
ether to precipitate 5a -c, respectively, as a yellow powder.
The yield in each case after washing and drying was around
90%. 5a : 1H NMR (80 MHz, CDCl3, 25°C) δ 5.22, 5.19, 5.09
(s, s, s, 1H, 1H, 1H, H5,2,4), 4.87 (m, 2H, CH2), 2.17, 2.11, 1.82,
1.75, 1.68 (s, s, s, s, s, 3H, 3H, 3H, 3H, 3H, Me), 1.07 (s, 9H,
tBu). Anal. Calcd for C20H29BF4Ru (457.33): C, 52.53; H, 6.39.
Found: C, 52.74; H, 6.16. 5b: 1H NMR (80 MHz, CDCl3, 25
1
and dried at high vacuum: yield 0.19 g (0.5 mmol (71%)); H
NMR (500 MHz, C6D6, 25 °C) δ 4.67 (s br, 1H, H2), 2.46 (m,
1H, H3), 2.31 (m, 1H, H3′), 2.15 (m, 2H, H4,4′), 2.20 (s, 3H, SMe),
1.78 (s, 15H, Cp*); 13C NMR (500 MHz, C6D6) δ 181 (CO2), 76,
10.4 (Cp*), 57.4 (C2), 30.6, 27.4 (C3,4), 21.2 (SMe); MS (70 eV,
EI) m/z (relative intensity) 385 (16) [M+], 341 (3) [M+ - CO2],
291 (23), 282 (25) [M+ - MeSC2H4CHNH2], 233 (20) [Cp*Ru
- 4H], 44 (100) [CO2]. Anal. Calcd for C15H25NO2RuS
(384.5): C, 46.86; H, 6.55; N, 3.64. Found: C, 47.1; H, 6.40;
N, 3.57.
3
°C) δ 7.48-7.16 (m, 4H, H4-7), 5.54 (d/d, J (H3,H2) ) 3.0 Hz,
3
4J (H3,H4) ) 1.0 Hz, 1H, H3), 5.32 (d, J (H2,H3) ) 3.0 Hz, 1H;
H2), 5.25, 3.62 (ABq, J AB ) 14.6 Hz, 2H, CH2), 2.05, 1.95, 1.65,
t
1.00 (s, s, s, s, 3H, 3H, 3H, 3H, Me), 1.31 (s, 9H, Bu). 5c: 1H
NMR (80 MHz, CDCl3, 25 °C) δ 7.44-7.20 (m, 9H, Ph, H4-7),
5.54 (s, 1H, H2), 4.31, 3.65 (ABq, J AB ) 14.7 Hz, 2H, PhCH2),
5.29, 3.65 (ABq, J AB ) 14.6 Hz, 2H, CH2), 1.77, 1.74, 1.66, 1.26
t
(s, s, s, s, 3H, 3H, 3H, 3H, Me), 1.34 (s, 9H; Bu); MS (FABS)
m/z (relative intensity) [C30H35Ru]+ 497 (100) [M+], 481 (16)
[M+ - Me - H], 441 (7), 405 (3), 233 (4); MS[BF4]- 87 (100).
Anal. Calcd for C30H35BF4Ru (583.48): C, 61.76; H, 6.05.
Found: C, 59.55; H, 5.79.
(RRu ,SC)- or (SRu ,RC)-3d . To 0.27 g (0.50 mmol) of [Cp*Ru-
(µ2-OMe)]2 (1) in 20 mL of THF was added 0.12 g (1.0 mmol)
of (S)- or (R)-proline, and the mixture was stirred for 3 h at
ambient temperature; a yellow powder precipitated. This was
collected, washed with ether and hexane, and dried in vacuo
to give 0.3 g (0.86 mmol (87%)) of 3d : 1H NMR (500 MHz,
CD3CN, 25 °C) δ 4.21 (br d, 1H, H2), 3.27 (m, 2H, H5,5′), 2.62,
2.35, 1.79, 1.70 (m, s, m, m, 1H, 1H, 1H, 1H, H3,3′,4,4′), 1.57 (s,
15H, Cp*); 13C NMR (500 MHz, CD3CN) δ 181.5 (CO2), 74.6,
10.3 (Cp*), 63.6 (C1), 53.8, 30.4, 27.4 (C2-C4). Anal. Calcd
for C15H23NO2Ru (350.42): C, 51.41; H, 6.62; N, 4.00. Found:
C, 51.25; H, 6.71; N, 4.14.
6a -c. 5a -c were each dissolved in CH2Cl2, an equimolar
amount of (S)-phenylethylamine and Et3N was added, and the
solution was stirred for 30 min. The solvent was evaporated
and the residue was extracted with hexane. After filtration,
the hexane solution was stripped in vacuo to give 6a -c,
respectively, as yellow viscous oils in quantitative yield. 6a :
1
1H NMR (500 MHz, C6D6, diastereoisomer /2) δ 7.43-7.08 (m,
5H, Ph), 4.05 (d/d, 3J (H5,H4) ) 2.5 Hz, 4J (H5,H2) ) 1.5 Hz, 1H,
4a -c fr om 3a -d . The half-sandwich complex 3, as speci-
fied in Chart 1, was dissolved or suspended in 20 mL of solvent
(Chart 1), and an equimolar amount of the appropriate lithium
cyclopentadienide, generated in situ from the cyclopentadiene
and n-BuLi, was added at -78 °C. The temperature was
slowly increased until a color or solubility change was evident.
The mixture was stirred at this temperature for 1-2 h and
then warmed to room temperature. The solvent was stripped,
the residue was extracted with hexane, and the solution
filtered over Celite and chromatographed over silylated silica
(Merck 60 treated with Me3SiCl) with hexane (to remove
volatiles). The solvent was then evaporated and the residual
yellow oil was dried at high vacuum. Yields and ee values are
4
4
H5), 3.99/4.00 (t, J (H2,H4) ) 1.5 Hz, J (H2,H5) ) 1.5 Hz, 1H,
H2), 3.82 /3.81 (d/d, 3J (H4,H5) ) 2.5 Hz, 4J (H4,H2) ) 1.5 Hz,
1H, H4), 3.79 (q, 3J (H,Me) ) 6.5 Hz, 1H, CH), 3.33, 3.30 (ABq,
J AB ) 12.2 Hz, 2H, CH2), 1.92, 1.89 (s, s, 3H, 3H, Me7,10), 1.86/
1.87, 1.85/1.83 (s, s, 3H, 3H, Me8,9), 1.65 (s, 3H, Me3), 1.29 (d,
t
3J (Me,H) ) 6.5 Hz, 3H, Me), 1.14 (s, 9H, Bu); 13C NMR (500
MHz, C6D6) δ 145.5, 127.2, 125.6 (Ph), 104.4, 87.2 (C1,3), 83.8-
83.1 (C6-10), 72.5 (C4), 69.9 (C2), 68.2 (C5), 57.8 (CH), 43.2 (C11),
30, 29.2 (tBu), 24 (Me), 12.2 (Me3), 10.8-10.2 (Me7-10); MS (70
eV, EI): m/z (relative intensity) 491 (33.6) [M+], 434 (32.8) [M+
t
- Bu], 386 (6.4) [M+- CHMePh], 371 (100) [M+ - CHMe-
t
PhNH], 329 (12) [M+ - Bu - CHMePH]. 6b: 1H NMR (500
MHz, C6D6, 25 °C, diastereoisomer 1/2) δ 7.40-7.01 (m, 5H,
given in Chart 1. 4a : [R]20 ) 15.8° cm2 (from [Cp*Ru((R)-
436
3
Ph), 6.79-6.68 (m, 4H, H4-7), 4.45/4.44 (d/d, J (H3,H4) ) 2.4
pro)], c ) 2.9 × 10-2 g cm-3 in n-hexane); 1H NMR (500 MHz,
4
3
Hz, J (H3,H4) ) 1.0 Hz, 1H, H3), 4.33/4.34 (d, J (H2,H3) ) 2.4
3
5
C6D6, 25 °C) δ 4.08 (d/d, J (H5,H4) ) 2.4 Hz, J (H5,H2) ) 1.5
Hz, 1H, H5), 3.99 (d/d, 5J (H2,H5) ) 1.5 Hz, 5J (H2,H4) ) 1.5 Hz,
1H, H2), 3.83 (d/d, 3J (H4,H5) ) 2.4 Hz, 5J (H4,H2) ) 1.5 Hz, 1H,
Hz, 1H, H2), 3.73 (q, J (H,Me) ) 6.5 Hz, 1H, CH), 3.18, 3.15/
3
3.19, 3.14 (ABq, J AB ) 12.2 Hz, 2H, CH2), 1.69/1.73, 1.66/1.62,
1.60/1.61, 1.48/1.46 (s, s, s, s, 3H, 3H, 3H, 3H, Me11-14), 1.36/
H4), 1.86 (s, 15H, Cp*), 1.71 (s, 3H, Me3), 1.20 (s, 9H, tBu); 13
C
t
1.363 (s, 9H, Bu), 1.26 (d, 3J (Me,H) ) 6.5 Hz, 3H, Me); 13C
NMR (500 MHz, C6D6) δ 105.6 (C1,3), 74.0, 71.1, 69.7 (C2,4,5),
84.2, 12.1 (Cp*), 31.4, 30.8 (tBu); UV/vis (n-hexane) λmax (ꢀ)
263 (286), 302 nm (149); CD (n-hexane) λmax (∆ꢀ) 258 (-2.9 ×
10-1), 283 (8.4 × 10-2), 317 nm (-1.2 × 10-1); MS (70 eV, EI)
m/z (relative intensity) 372 (100) [M+], 357 (73) [M+ - Me],
315 (32) [M+ - tBu], 231 (4) [Cp*Ru - 6H]. 4b: 1H NMR (500
MHz, C6D6, 25 °C) δ 7.44, 6.91, 6.83-6.74 (m, m, m, 1H, 1H,
NMR (500 MHz, C6D6) δ 146.8, 128.6-127 (Ph), 126.4, 125.6,
121.4, 121 (C4-7), 99.0, 93.4, 90.2 (C8,9,1), 85.6, 83-82.4 (C10-14),
75.6, 67.4 (C3,2), 59 (CH), 44.2 (C15), 32.2, 32 (tBu), 25.2 (Me),
11.2-10.2 (Me11-14). 6c: 1H NMR (500 MHz, C6D6, 25 °C,
(S,S)-6c/(R,S)-6c) δ 7.48-6.68/7.42-6.70 (m, 14H, Ph, C4-7),
4.36/4.35 (s, 1H, H2), 3.83, 3.53/3.82, 3.50 (ABq, J AB ) 15.6/
3
16.0 Hz, 2H, PhCH2), 3.72/3.72 (q, J (H17,Me17) ) 6.5 Hz, 1H,
3
5
2H, H4-7), 4.47 (d/d, J (H3,H2) ) 2.4 Hz, J (H3,H4) ) 1.0 Hz,
1H, H3), 4.38 (d, 3J (H2,H3) ) 2.4 Hz, 1H, H2), 1.59 (s, 15H,
Cp*), 1.41 (s, 9H, tBu); MS (70 eV, EI) m/z (relative intensity)
H17), 3.12 (s, 2H, CH2), 3.15, 3.09 (ABq, J AB ) 12.5 Hz, 2H,
CH2), 1.67/1.68, 1.64/1.63, 1.52/1.53, 1.47/1.46 (s, s, s, s, 3H,
3H, 3H, 3H, Me12-15), 1.264/1.260 (d, 3J (Me17,H17) ) 6.5 Hz,
408 (17) [M+], 393 (41) [M+ - Me], 372 (5) [Cp*2Ru], 351 (3)
t
3H, Me17), 1.31/1.31 (s, 9H, Bu); 13C NMR (500 MHz, C6D6) δ
[M+ - Bu], 231 (4) [Cp*Ru - 6H]. 4c: [R]20 ) 157.6° cm2
t
D
((S)-4c, c ) 5.4 × 10-3 g cm-3 in n-hexane); 1H NMR (500 MHz,
C6D6, 25 °C) δ 7.48-6.73 (m, 9H, Ph, H4-7), 4.37 (s, 1H, H2),
3.87, 3.55 (ABq, J AB ) 15.7 Hz, 2H, PhCH2), 1.55 (s, 15H, Cp*),
144 (Ph-Cipso),142 (Ph-Cipso), 128.8-126.8 (Ph), 126.1, 123.8,
121.5, 121.1 (C4-7), 98.8, 92.8, 90.5, 85.2 (C8,9,1,3), 82.9-82.4,
81.0 (C11-15), 77.0 (C2), 59.0 (C17), 44.2 (C16), 33.8 (C10), 32.1,
31.8 (tBu), 25.0 (Me17), 1.0-10.0 (Me11-14); MS (70 eV, EI) m/z
(relative intensity) 617 (16) [M+], 560 (10) [M+ - tBu], 512 (11)
1.37 (s, 9H, Bu); 13C NMR (500 MHz, C6D6) δ 142 (Ph-Cipso),
t
128.8-126.8 (Ph), 126.1, 123.8, 121.5, 121.1 (C4-7), 98.6, 92.6,

Synthesis of Planar Chiral Ruthenocenes
Organometallics, Vol. 15, No. 5, 1996 1383
[M+ - PhCHMe], 498 (28) [M+ - CHMePhN], 106 (100) [CH2-
MePh].
(6)). For the alternative polarity, Rw ) 0.061 was obtained. A
final difference Fourier synthesis showed maximal residual
electron density of 1.4 e/Å3 close to one of the Ru atoms.
Cr ysta l Str u ctu r e Deter m in a tion of 5c. Reflections of
an orange plate of approximate dimensions 0.60 × 0.45 × 0.18
mm were collected at 258 K on an ENRAF-Nonius CAD4
diffractometer using graphite-monochromated Mo KR radia-
tion (λ ) 0.7107 Å). The compound crystallizes in the
monoclinic space group P21 (No. 4) with a ) 10.045(2) Å, b )
14.488(4) Å, c ) 18.514(3) Å, â ) 90.21(2)°, V ) 2694(1) Å3, Z
) 4, dcalc ) 1.438 g cm-3, µ(Mo KR) ) 6.13 cm-1, F(000) ) 1200;
6896 reflections were recorded within 3 < θ < 24° in the
+h(k(l hemisphere. Intensity data were collected in the ω
scan mode. An empirical absorption correction (min transmis-
sion 0.827, max transmission 0.998) on the basis of azimuthal
scans24 was applied before averaging over symmetry equiva-
lent reflections. The structure was solved by conventional
Patterson and difference Fourier methods; least-squares full-
matrix refinement on F25 converged for 648 variables and 5149
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft and by Fonds der
Chemischen Industrie, Frankfurt/M. A loan of aqueous
RuCl3 from J ohnson-Matthey, Reading, England, is
gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
parameters, displacement parameters, and bond distances and
angles for 5cBF4 (21 pages). Ordering information is given
in any current masthead page. Additional data on the crystal
structure of 5cBF4 (atomic coordinates, anisotropic displac-
ment parameters, and structure factors) are also available
from the Fachinformationszentrum Karlsruhe, D-76433 Egg-
enstein-Leopoldshafen, Germany, by quoting the depository
number CSD 404544.
observations with I > 1.0 σ(I) at R ) 0.059, Rw ) 0.059 (w-1
)
1/σ2(Fo)) for the correct enantiomorph (Flack′s enantiopol
refinement as implemented in SHELXL9326 converged to 0.01-
OM950736I
(24) North, A .C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
(25) Frenz, B. A. Enraf-Nonius, SDP, Version 5.0; Enraf-Nonius:
Delft, 1988.
(26) Sheldrick, G. M. SHELXL-93, Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1993.