Asymmetric thermal transformation, a new way to enantiopure biphenyl- bridged titanocene and zirconocene complexes: Efficient catalysts for asymmetric imine hydrogenation

Article abstract of DOI:10.1021/ja983524w

Enantiopure biphenyl-bridged titanocene and zirconocene complexes were obtained, by an asymmetric thermal transformation of the binaphthol complexes formed from the metallocene racemates and subsequent transformation to the corresponding dichlorides, in practically quantitative yields. Increased rates of this transformation in the presence of O₂ gas or TEMPO indicate a radical reaction mechanism. The biphenyl-bridged titanocene enantiomers give rise to an efficient asymmetric catalysis for the hydrogenation of cyclic and noncyclic imines.

Full text of DOI:10.1021/ja983524w

1524
J. Am. Chem. Soc. 1999, 121, 1524-1527
Asymmetric Thermal Transformation, a New Way to Enantiopure
Biphenyl-Bridged Titanocene and Zirconocene Complexes: Efficient
Catalysts for Asymmetric Imine Hydrogenation1
Markus Ringwald,†,‡ Rainer Stu1rmer,§ and Hans H. Brintzinger*,†
Contribution from the Fakulta¨t fu¨r Chemie, UniVersita¨t Konstanz, D-78457 Konstanz, and
Hauptlaboratorium, BASF AG, D-67056 Ludwigshafen, Germany
ReceiVed October 5, 1998
Abstract: Enantiopure biphenyl-bridged titanocene and zirconocene complexes were obtained, by an asymmetric
thermal transformation of the binaphthol complexes formed from the metallocene racemates and subsequent
transformation to the corresponding dichlorides, in practically quantitative yields. Increased rates of this
transformation in the presence of O2 gas or TEMPO indicate a radical reaction mechanism. The biphenyl-
bridged titanocene enantiomers give rise to an efficient asymmetric catalysis for the hydrogenation of cyclic
and noncyclic imines.
Introduction
been considered to be configurationally too stable for thermal
asymmetric transformations to be feasible; to the best of our
knowledge, an equilibrating, dynamic resolution8 of this class
of complexes has not been achieved so far.
Asymmetric catalysis with chiral ansa-metallocene com-
pounds has become a useful tool in chemical research.2 This
application is contingent, however, on efficient access to pure
complex enantiomers. Presently, known methods are quite
limited yet in this regard: While optical resolution of ansa-
metallocene racemates3-5 cannot yield more than 50% of a
particular enantiomer, enantioselective ansa-metallocene syn-
thesis6 has proved chirally inefficient insofar as large parts of
an enantiopure intermediate are wasted by subsequent low-yield
reaction steps. An attractive alternative would be the complete
asymmetric transformation of an ansa-metallocene racemate to
one enantiomer. Whereas such asymmetric transformation can
be achieved photochemically in a limited number of cases,7
ansa-titanocene and -zirconocene complexes have generally
In an attempt to resolve the readily accessible racemate of a
biphenyl-bridged zirconocene complex,9 2,2′-biphenyldiylbis-
(3,4-dimethylcyclopentadienyl)zirconium dichloride (rac-1-Cl2)
by reaction of its dimethyl derivative, rac-1-Me2, with (R)-1,1′-
binaphthol (R-binol), we have recently found, however, that
asymmetric transformations do occur in these reaction systems.10
Here, we report on a new high-yield synthesis for enantiopure
ansa-titanocene and -zirconocene complexes, which we have
developed from these binaphthol-induced asymmetric transfor-
mations.
† Universita¨t Konstanz.
‡ Present address: MCAT, Hermann-von-Vicari-Str. 23, D-78464 Kon-
stanz, Germany, e-mail markus.ringwald@uni-konstanz.de.
§ BASF AG.
(1) ansa-Metallocene Derivatives XLIV, XLIII. Hu¨ttenhofer, M.; Schaper,
F.; Brintzinger, H. H. Angew. Chem., Int. Ed. Engl. 1998, 37, 2268.
(2) For reviews, see: Hoveyda, A. H.; Morken, J. P. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1262. Hoveyda, A. H.; Morken, J. P. In
Metallocenes; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Weinheim,
1998; Vol. 2, p 625, and references therein.
Results and Discussion
(3) Schnutenhaus, H.; Brintzinger, H. H. Angew. Chem., Int. Ed. Engl.
1979, 18, 777. Wild, F. R. W. P.; Zsolnai, L.; Huttner, G.; Brintzinger, H.
H. J. Organomet. Chem. 1982, 232, 233. Collins, S.; Kuntz, B. A.; Hong,
Y. J. Org. Chem. 1989, 54, 4154. Grossman, R. B.; Doyle, R. A.; Buchwald,
S. L. Organometallics 1991, 10, 1501. Grossman, R. B.; Davis, W. M.;
Buchwald, S. L. J. Am. Chem. Soc. 1991, 113, 2321. Chin, B.; Buchwald,
S. L. J. Org. Chem. Soc. 1996, 61, 5650. Chin, B.; Buchwald, S. L. J. Org.
Chem. Soc. 1997, 62, 2267.
(4) Scha¨fer, A.; Karl, E.; Zsolnai, L.; Huttner, G.; Brintzinger, H.-H. J.
Organomet. Chem. 1987, 328, 87.
(5) Habaue, H.; Sakamoto, H.; Baraki, H.; Okamoto, Y. Macromol.
Rapid. Commun. 1997, 18, 707.
(6) For a review, see: Halterman, R. L. Chem. ReV. 1992, 92, 965 and
references therein. Ellis, W. W.; Hollis, T. K.; Odenkirk, W.; Whelan, J.;
Ostrander, R.; Rheingold, A. L.; Bosnich, B. Organometallics 1993, 12,
4391. Halterman, R. L.; Ramsey, T. M. Organometallics 1993, 12, 2879.
Mitchell, J. P.; Hajela, S.; Brookhart, S. K.; Hardcastle, K. I.; Henling, L.
M.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 1045.
Reaction of rac-1-Me2 with 1 equiv of enantiopure (R)-binol,
which is complete after 20 h at room temperature in toluene,
yields the diastereomers (R)-1-(R)-binol and (S)-1-(R)-binol.
Suprisingly, these diastereomers were found to arise in a ratio
of about 2:1, rather than in the 1:1 ratio expected from the
stoichiometry of the reactants. Obviously, a fraction of ca. 30%
of the reactant (S)-1-Me2 was converted to the product (R)-1-
(R)-binol during or after formation of the binaphthol complex.11
When a mixture of the diastereomers (R)-1-(R)-binol and (S)-
(8) Ward, R. S. Tetrahedron: Asymmetry 1995, 6 (7), 147. Caddik, S.;
Jenkins, K. Chem. Soc. ReV. 1996, 447.
(9) Huttenloch, M. E.; Dorer, B.; Rief, U.; Prosenc, M.-H.; Schmidt,
K.; Brintzinger, H. H. J. Organomet. Chem. 1997, 541, 219.
(10) Ringwald, M. Ph.D. Dissertation, Universita¨t Konstanz, 1998.
(11) The assignment of these diastereomers follows that of the corre-
sponding Ti complexes reported in ref 9.
(7) Schmidt, K.; Reinmuth, A.; Rief, U.; Diebold, J.; Brintzinger, H. H.
Organometallics 1997, 16, 1724.
10.1021/ja983524w CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/05/1999

Asymmetric Thermal Transformation
J. Am. Chem. Soc., Vol. 121, No. 7, 1999 1525
Scheme 1
Scheme 2
1-(R)-binol was kept in deuteriobenzene at 60-110 °C,12
changes in the 1H NMR-spectra indicate that the (S)-1-(R)-binol
diastereomer is epimerized to the (R)-1-(R)-binol isomer. This
epimerization goes to completion when the reaction mixture is
kept in toluene at 100 °C for 2 days. This reaction sequence
can be conducted as a one-pot reaction (Scheme 1).
Scheme 3
We have further extended this new asymmetric transformation
strategy to the corresponding titanium complex rac-2-Me2.
Previously, the binaphthol complex (R)-2-(R)-binol had been
isolated in 48% yield by kinetic resolution with 0.5 equiv of
(R)-binaphthol.9 By reacting rac-2-Me2 with 1 equiv of (R)-
binaphthol in toluene and heating the solution to 100 °C for 4
days, we obtained (R)-2-(R)-binol in 99% isolated yield.
To prepare the dichloride enantiomers (R)-1-Cl2 and (R)-2-
Cl2, the binaphthol complexes (R)-1-(R)-binol and (R)-2-(R)-
binol were first converted to the dimethyl derivatives (R)-1-
Me2 and (R)-2-Me2, which were then transformed to (R)-1-Cl2
and (R)-2-Cl2, respectively, with suitable chlorinating agents
(vide infra).13
To determine the enantiomeric excess of complex 1-Me2, we
used the commercially available enantiopure alcohol (R)-(+)-
1-phenyl-1-propanol ((R)-PP).14 Reaction of rac-1-Me2 with 2
equiv of the alcohol in toluene yields, after 4 days,15 the bis-
alkoxy diastereomers (R)-1-((R)-PP)2 and (S)-1-((R)-PP)2, which
are cleanly distinguished by their 1H NMR spectra (Scheme 2,
cf. Experimential Section).
Scheme 4
(50% ee). The same reaction at -20 °C gave a (R):(S) ratio of
97:3 (94% ee).16 At still lower temperatures, the binaphthol
complex does not react with methyllithium. With Grignard
reagents the reaction was slow and led to the formation of more
than one product species. With trimethylaluminum as methyl
tranfer agent at -20 °C, finally, racemization was not detectable;
in this case (R)-1-Me2 was isolated with 98% ee (Scheme 3).
While reaction of (R)-2-Me2 with dry HCl in ether at room
temperature gives enantiopure (R)-2-Cl2,9 the analogous reaction
with the zirconocene complex (R)-1-Me2 leads to partially
racemized dichloride product 1-Cl2 and to formation of an
unknown byproduct, apparent in the 1H NMR spectra.17,18 When
this reaction is conducted at -78 °C, racemization is suppressed,
but the byproduct is still formed. Dry dichlorodimethylsilane
was finally found to give a clean reaction, even at room
temperature, according to Scheme 4.
When the optical purity of the dimethyl species 1-Me2,
obtained from reaction of (R)-1-(R)-binol with methyllithium
in ether at room temperature, was determined in this manner,
racemization of the zirconocene complex was indicated by the
formation of (R)-1-((R)PP)2 and (S)-1-((R)PP)2 in a ratio of 3:1
(12) 1H NMR experiments were conducted with toluene or hexameth-
ylbenzene as internal NMR standard, to ensure that the disappearance of
one diastereomer is accompanied by an increase of the other one and not
just by destruction of the complex.
(13) The titanium complex (R)-2-Me2 was found to be enantiopure by
reaction with O-acetyl-(R)-mandelic acid, which yielded only a single
diastereomer of [2,2′-biphenyldiylbis(3,4-dimethylcyclopentadienyl)]tita-
nium-bis(O-acetyl-(R)-mandelate).9
(14) O-Acetyl-(R)-mandelic acid proved unsuitable for this ee determi-
nation since it decomposes rac-1-Me2 under liberation of the biphenyl-
bridged ligand. Reaction with Moshers reagent failed similarly.
(15) The monosubstituted complexes (R)-1-((R)-PP)-Me and (S)-1-((R)-
PP)-Me arise from the reaction of rac-1-Me2 with 1 equiv of the alcohol
in C6D6 after a reaction time of about 1 h. These monosubstituded complexes
are equally suitable for the ee determination of 1-Me2 as the disubstituted
complexes (S)-1-((R)-PP)2 and (R)-1-((R)-PP)2 (see Experimental Section).
When the rate of the asymmetric transformation was deter-
mined in C6D6 solution in closed NMR tubes at a temperature
(16) Scha¨fer et al. also observed partial racemization in the reaction of
(R)-ethylenebis-(4,5,6,7-tetrahydro-1-indenyl)zirconium (R)-binaphthol with
methyllithium in ether at temperatures above 0 °C.4
(17) A racemization in the chlorinating step was also observed by Scha¨fer
in the reaction of (R)-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium
dimethyl with HCl at temperatures obove 0 °C.4
(18) For this ee determination, the dichloride product (R)-1-Cl2 is
converted, with methyllithium in ether at -20 °C, to the dimethyl compound
(R)-1-Me2, which is then reacted with (R)-(+)-1-phenyl-1-propanol as
described above.

1526 J. Am. Chem. Soc., Vol. 121, No. 7, 1999
Ringwald et al.
Scheme 5
Scheme 6
obtained with (R)-2-Cl2, although the catalyst:substrate ratio is
smaller here by a factor of 20-100.
The biphenyl-bridged complex (R)-2-Cl2 in the presence of
n-butyllithium thus yields a remarkably efficient imine hydro-
genation catalyst, based on a metal which is inexpensive and
environmentally safe. We are presently exploring possible
extensions of the concept of asymmetric thermal transformation
with the aim of providing efficient access to a wide range of
enantiomerically pure metallocene catalysts.
of 90 °C, we obtained a linear decrease of the logarithm of the
ratio (S)-1-(R)-binol/((R)-1-(R)-binol + (S)-1-(R)-binol), indi-
cating a first-order rate law. Suprisingly however, the apparent
first-order rate constant measured at three different concentra-
tions of diastereomeric mixtures of (S)-1-(R)-binol and (R)-1-
(R)-binol decreased from k ) 33.3 h-1 at [Zr]tot ) 4.3 mmol/L
to k ) 23.6 h-1 at [Zr]tot ) 8.6 mmol/L and to k ) 17.5 h-1 at
[Zr]tot ) 17.2 mmol/L. This apparent contradiction to a first-
order rate law can be explained by the intervention of some
impurities present in the reaction mixture, which is expected to
be more pronounced at low zirconocene concentrations.
Experimental Section
All reactions were carried out under an Ar or N2 atmosphere using
standard Schlenk and glovebox techniques. Solvents were dried and
distilled from sodium benzophenone or CaH2. NMR spectra were
recorded on Bruker AC 250 MHz, Bruker DRX 600 MHz and JEOL
FX 90Q spectrometers with residual C6HD5 (7.15 ppm), CHCl3 (7.24
ppm), or C6H5CH3 (2.09 ppm) as internal standards.
The reaction is indeed substantially accelerated by introducing
dioxygen gas into the inert gas atmosphere above the reaction
mixture (k) 127 h-1 at [Zr]tot ) 8.6 mmol/L). A similiar, but
somewhat smaller, rate increase is caused by addition of 1 mg
of tetramethylpiperidine N-oxide (TEMPO) (k ) 45.6 h-1 at
[Zr]tot ) 8.6 mmol/L). In Scheme 5 we present a possible
reaction sequence for the catalytic action of TEMPO, where
the radical TEMPO facilitates the release of a ligand radical
similar to that discussed in photochemical ligand exchange
reactions.19
(R)-[2,2′-Biphenyldiylbis-(3,4-dimethylcyclopentadienyl)]zirco-
nium (R)-1,1′-Bi-2-naphtholate ((R)-1-(R)-binol). A solution of 308
mg (1.08 mmol) R(+)-1,1′-bi-2-naphthol in 50 mL CH2Cl2 was added
slowly to 490 mg (1.08 mmol) of biphenyl(3,4-Me2-C5H3)2ZrMe2 (1-
Zr-Me2) in 150 mL of CH2Cl2. When the reaction mixture was stirred,
both diastereomers (R)-1-(R)-binol and (S)-1-(R)-binol were obtained
in ratios depending on the reaction time. Exchange of the solvent against
50 mL of toluene and heating the resulting solution for 2 days to 100
°C gave (R)-1-(R)-binol which was pure by 1H NMR. (R)-1-(R)-
binol: 1H NMR(C6D6, 250 MHz, δ in ppm) 0.87 (s, 6H, CH3), 1.53
(s, 6H, CH3), 5.46 (d, 2H, J ) 3 Hz, C5H), 6.08 (d, 2H, J ) 3 Hz,
C5H), 6.78-7.80 (m, 20H, C6H); 13C NMR (CDCl3, 600 MHz, δ in
ppm) 11.02, 12.96, 109.56, 114.59, 115.82, 120.85, 122.04, 122.60,
125.27, 125.58, 126.54, 126.91, 127.60, 127.70, 128.40, 128.75, 128.82,
132.51, 132.74, 134.19, 135.37, 138.37, 160.48. (S)-1-(R)-binol: 1H
NMR (C6D6, 250 MHz, δ in ppm) 1.18 (s, 6H, CH3), 1.66 (s, 6H, CH3),
5.30 (d, 2H, J ) 2 Hz, C5H), 6.03 (d, 2H, J ) 2 Hz, C5H), 6.76-7.80
(m, 20H, C6H); 13C NMR (CDCl3, 600 MHz, δ in ppm) 11.22, 13.76,
108.99, 110.46, 114.22, 116.13, 117.76, 121.96, 123.20, 125.07, 125.80,
126.13, 126.86, 127.05, 127.24, 127.99, 128.22, 129.03, 130.09, 132.44,
135.61, 138.28, 159.45; [R]589 ) -163°, [R]436 ) -618° (1.20 mg/10
mL of toluene, d ) 10) (lit.9 [R]589 ) -90°, [R]436 ) -400° (1 mg/10
mL of CH2Cl2, d ) 10).
At any rate, the configurational lability of the group IV
metallocene complexes considered here appears to be intimately
connected to the presence of a phenoxide ligand. The resolved
enantiomers of the dichlorides (R)-1-Cl2 and (R)-2-Cl2 do not
undergo any change in their specific optical rotation when kept
in toluene solution at 50 °C for several days under exclusion of
air and moisture.20 These derivatives are thus sufficiently stable
for use in enantioselective catalysis reactions.21
Imine Hydrogenation
Catalyst (R)-2-Cl2 in the presence of 2 equiv of n-butyllithium
was used for the asymmetric hydrogenation of imines by
dihydrogen at a pressure of 150 bar.22 The results are represented
in Scheme 6. While the selectivities of both catalytic hydroge-
nations are similar to those obtained by Willoughby and
Buchwald with ethylenebis(4,5,6,7-tetrahydro-1-indenyl)titanium
dichloride under comparable conditions,22 higher yields are
(R)-[2,2′-Biphenyldiylbis(3,4-dimethylcyclopentadienyl)]zirco-
nium Dimethyl ((R)-1-Me2). The (R)-1-(R)-binol (450 mg, 0.63 mmol)
was dissolved in 80 mL toluene, cooled to -20 °C and reacted with 5
mL of a 2 M solution of trimethylaluminum in toluene. The solution
was allowed to warm to room temperature. After 5 h, the reaction
mixture was evaporated to dryness and the resulting residue was taken
up in pentane; the salts were removed by filtration over Celite. The
solvent was evaporated to give 230 mg (0.50 mmol, 80% yield) of
(19) Harrigan, R. W.; Hammond, G. S.; Gray, H. B. J. Organomet. Chem.
1974, 8, 79. Tsai, Z.-T.; Brubaker, C. H., Jr. J. of Organomet. Chem. 1979,
166, 199. Atkinson, J. M.; Brindley, P. B.; Davies, A. G.; Hawari, J. A.-A.
J. Organomet. Chem. 1984, 264, 253 and references therein.
(20) While indefinitely stable as crystals in a nitrogen or argon
atmosphere, the zirconocene complexes gradually decompose in the presence
of air.
(21) The dimethyl derivative (R)-1-Me2 was likewise found to be
configurationally stable in toluene solution at 50 °C even in the presence
of excess methylalumoxane.
(22) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1992, 114,
7562; Willoughby, C. A.; Buchwald, S. L. J. Org. Chem. 1993, 58, 7627.
Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 8952.
Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 11703.
Recently, increased yields and enantioselectivities have been reported for
imine hydrosilylations at catalyst:substrate ratios similar to those used
here: Verdaguer, X.; Lange, U. E. W.; Reding, M. T.; Buchwald, S. L. J.
Am. Chem. Soc. 1996, 118, 6784. Verdaguer, X.; Lange, U. E. W.;
Buchwald, S. L. Angew. Chem., Int. Ed. Engl. 1998, 37, 1103.

Asymmetric Thermal Transformation
J. Am. Chem. Soc., Vol. 121, No. 7, 1999 1527
(R)-1-Me2 as a white solid: 1H NMR (CDCl3, 250 MHz, δ in ppm)
-0.66 (s, 6H, Zr-CH3), 1.88 (s, 6H, CH3), 2.20 (s, 6H, CH3), 4.39 (d,
2H, J ) 2.8 Hz, C5H), 6.21 (d, 2H, J ) 2.8 Hz, C5H), 7.11 (m, 2H,
C6H), 7.26 (m, 4H, C6H), 7.41 (m, 2H, C6H).
PP)2: 0.88 (t, 6H, J ) 8 Hz, CH2-CH3), 1.52-1.87 (m, 4H, CH2) 1.82
(s, 6H, C5CH3), 1.95 (s, 6H, C5CH3), 5.01 (t, 2H, J ) 7 Hz, CH), 5.30
(d, 2H, J ) 3 Hz, C5H), 6.02 (d, 2H, J ) 3 Hz, C5H) 6.98-7.51 (m,
18H, C6H); MS (EI): m/z 696 (M+, 16%)
(R)-[2,2′-biphenyldiylbis(3,4-dimethylcyclopentadienyl)]zirco-
nium Dichloride ((R)-1-Cl2). The (R)-1-Me2 (230 mg, 0.50 mmol)
was dissolved in 10 mL of toluene at room temperature and reacted
with 10 mL of Me2SiCl2 for a period of 4 days. Evaporation gave 300
mg (0.48 mmol, 96% yield) of a yellow solid of enantiopure (R)-1-
Cl2) (ee g 98%). The solid was stirred with ether for 15 min, filtered
over a glass frit, and washed with ether. The overall yield was 76%
based on rac-1-Me2. A pure sample of the product was obtained by
column chromatography over a small column of silanized silica gel
with toluene as eluent: [R]589 ) +201 (7.70 mg/10 mL toluene, d )
10). Anal. Calcd for C26H24ZrCl2: C, 62.63; H, 4.85. Found: C, 62.73;
H, 5.03.
(R)-[2,2′-Biphenyldiylbis(3,4-dimethylcyclopentadienyl)]-
-titanium (R)-1,1′-Bi-2-naphtholate ((R)-2-(R)-binol). To a solution
of 260 mg (0.63 mmol) biphenyl(2,3-Me2-C5H3)2TiMe2 (rac-2-Me2)
in 50 mL of toluene was added 180 mg (0.63 mmol) of (R)-(+)-1,1′-
bi-2-naphthol. The solution was stirred for 4 days at 100 °C. After
evaporation, 415 mg (0.62 mmol, 99% yield) of a deep red solid was
isolated. A pure sample of the complex (R)-2-(R)-binol was isolated
by filtration of a pentane solution over Celite. Anal. Calcd for C46H36O2-
Ti: C, 82.63; H, 5.43. Found, C, 82.73; H, 5.76.
(R)-[2,2′-Biphenyldiylbis(3,4-dimethylcyclopentadienyl)]-
titanium Dimethyl ((R)-2-Me2). Following a literature procedure,9 415
mg (0.62 mmol) (R),(R)-2-binol was dissolved in 60 mL of ether and
reacted with 2.5 mL of 1.6 M methyllithium in ether. The color of the
solution immediately changed to dark yellow. Extraction with pentane
yielded 210 mg (0.51 mmol, 82% yield) of pure (R)-2-Me2 as a yellow
powder: [R]589 ) +333, [R]546 ) +383 (1.20 mg/10 mL of toluene, d
) 1). Anal. Calcd for C28H30Ti: C, 81.15; H, 7.30. Found: C, 80.98;
H, 7.46.
Determination of the Enantiomeric Purity of (R)-1-Me2. A 10-
mg sample of a mixture of (R)-1-Me2 and (S)-1-Me2 was reacted in an
NMR tube with a slight excess of (R)-(+)-1-phenyl-1-propanol ((R)-
PP). After 2 h only the monosubstituted complexes (R)-[2,2′-biphen-
yldiylbis(3,4-dimethylcyclopentadienyl)]zirconium methyl((R)-1-phen-
yl-1-propanolate) (R)-1-(Me)-((R)-PP) and (S)-[2,2′-biphenyldiylbis-
(3,4-dimethylcyclopentadienyl)]zirconium-methyl((R)-1-phenyl-1-pro-
panolate) (S)-1-(Me)-((R)-PP) were detected. In the cyclopentadienyl
region, eight doublets were observed. 1H NMR (C6D6, 250 MHz, δ in
ppm): (R)-1-(Me)-((R)-PP) 4.78, 5.19, 5.95, 6.17 and (S)-1-(Me)-
((R)-PP) 4.81, 5.22, 5.92, 5.93.
Asymmetric Hydrogenation Reactions. In a dry Schlenk flask, 9.1
mg (20 µmol) of (R)-biphenyl(2,3-Me2-C5H3)2TiCl2 ((R)-2-Cl2) was
dissolved in 20 mL of toluene under an argon atmosphere at room
temperature. A solution of 40 µmol n-butyllithium (0.16 M in hexane)
was added dropwise. The color changed rapidly to a dirty brown-green.
After 5 min, 20 mmol (1000 equiv) of the imine in 10 mL of toluene
were added. The resulting solution was transferred under argon to a
Roth high-pressure autoclave, charged with 150 bar of hydrogen gas
(99.999+%), and allowed to stir for 12 h at 80 °C. After cooling to
room temperature, the vessel was carefully vented and opened. The
amine product was freed from solvent in vacuo and purified by
Kugelrohr distillation. The enantiomeric purity of the amines was
determined by trifluoracetylation and subsequent gaschromatography
on a BP-H-column (Astec)
(R)-(+)-N-Benzyl-1-phenylethylamine (3a). The general procedure
with 0.1 mol % catalyst was used to hydrogenate 4.2 g (20 mmol) of
a anti:syn (9:1) mixture of (3) N-(R-methylbenzylidene)benzylamine;
4.0 g (19 mmol) (R)-(+)-N-benzyl-1-phenylethylamine was isolated
in 95% yield and 76% ee. The spectroscopic properties (1H, 13C) of
the amine match those from an authentic sample.
(R)-(+)-1-Phenylpyrrolidine (4a). The general procedure with 0.1
mol % catalyst was used to reduce 2.9 g (20 mmol) of 1-phenylpyrroline
(4). The resulting clear, yellow solution afforded, after purification, a
colorless oil. 2.81 g (19.2 mmol) of (R)-(+)-1-phenylpyrrolidine was
isolated in 96% yield and 98% ee: 1H NMR (CDCl3, 300 MHz, δ in
ppm) 1.6-1.9 (m, 3H), 2.15 (m, 1H), 2.6 (br s, 1H), 2.9 (m, 1H), 3.2
(m, 1H), 4.1 (t, 1H, J ) 6.9 Hz), 7.1-7.5 (m, 5H).
(R)-[2,2′-Biphenyldiylbis(3,4-dimethylcyclopentadienyl)]-
titanium Dichloride ((R)-2-Cl2). To a solution of 210 mg (0.51 mmol)
(R)-2-Me2 in 50 mL of ether was added a solution of 0.5 mL of 3.6 M
HCl in ether at room temperature. The color of the solution changed
to a dark red. After evaporation, 230 mg (0.51 mmol, 99% yield) of
enantiopure (ee g 98%) (R)-2-Cl2 was isolated. The overall yield was
81% based on the amount of rac-2-Me2 used for the reaction with (R)-
binol: [R]589 ) +786, [R]546 ) -918 (0.49 mg/5 mL of CHCl3, d )
1) (ref 9: [R]589 ) +700 (1.78 mg/10 mL of CHCl3, d ) 1).
Reaction of rac-1-Me2 with (R)(+)-1-Phenyl-1-propanol. rac-1-
Me2) in toluene (31.6 mg, 0.07 mmol) was treated in one portion with
20 mg (0.15 mmol) of (R)-(+)-1-phenyl-1-propanol. After a reaction
time of 4 days, the solvent was removed to afford a yellow crystalline
powder of the two diastereomers (R)-[2,2′-biphenyldiylbis(3,4-dimeth-
ylcyclopentadienyl)]zirconium bis((R)-1-phenyl-1-propanolate), (R)-1-
((R)-PP)2, and (S)-[2,2′-biphenyldiylbis(3,4-dimethylcyclopentadienyl)]-
zirconium bis((R)-1-phenyl-1-propanolate), (S)-1-((R)-PP)2. (R)-1-((R)-
Acknowledgment. We thank Dr. Armin Geyer and Ms.
Monika Cavegn for the determination of 600-MHz NMR spectra
and Merck KGaA for a gift of enantiopure binaphthol. This
work has been supported by Fonds der Chemischen Industrie
and by funds of the University of Konstanz.
Supporting Information Available: NMR spectra of (R)-
+ (S)-1-(R)-binol, R-1-(R)-binol, (R)-1-Cl2, (R)-2-Me2, (R)-2-
Cl2, and (R)- + (S)-1-((R)-PP)2 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
1
PP)2: H NMR (C6D6, 600 MHz, δ in ppm) 0.73 (t, 6H, J ) 8 Hz,
CH2-CH3), 1.52-1.87 (m, 4H, CH2) 1.71 (s, 6H, C5CH3), 1.99 (s,
6H, C5CH3), 4.92 (t, 2H, J ) 7 Hz, CH), 5.28 (d, 2H, J ) 3 Hz, C5H),
6.40 (d, 2H, J ) 3 Hz, C5H), 6.98-7.51 (m, 18H, C6H). (S)-1-((R)-
JA983524W