Construction of chiral Ti(IV)-Rh(I)-salenophos complexes and their application in the asymmetric hydroformylation of functionalised olefins

Article abstract of DOI:10.1016/S0957-4166(99)00165-2

The synthesis of new chiral salenophos-type ligands bearing 'hard' and 'soft' coordination sites is described. The polyfunctionalised ligands are used for the construction of monometallic and early-late heterobimetallic complexes. In the reaction with titanium(IV) reagents the salen subunit selectively coordinated to the 'hard' metal. The phosphine groups in turn can be coordinated to rhodium(I). The coordination geometry of the diphosphine- Rh subunit is strongly influenced by the counter ligands COD or CO and Cl, respectively. In the asymmetric hydroformylation of vinyl acetate with one of the Rh-Ti-complexes, the branched aldehyde is predominantly formed with 30% ee.

Full text of DOI:10.1016/S0957-4166(99)00165-2

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1803–1811
Construction of chiral Ti(IV)–Rh(I)–salenophos complexes and
their application in the asymmetric hydroformylation of
functionalised olefins
Michael Quirmbach, Achim Kless, Jens Holz, Vitali Tararov and Armin Börner ∗
Institut für Organische Katalyseforschung an der Universität Rostock e.V., Buchbinderstr. 5/6, D-18055 Rostock, Germany
Received 21 April 1999; accepted 28 April 1999
Abstract
The synthesis of new chiral salenophos-type ligands bearing ‘hard’ and ‘soft’ coordination sites is described.
The polyfunctionalised ligands are used for the construction of monometallic and early–late heterobimetallic
complexes. In the reaction with titanium(IV) reagents the salen subunit selectively coordinated to the ‘hard’ metal.
The phosphine groups in turn can be coordinated to rhodium(I). The coordination geometry of the diphosphine–Rh
subunit is strongly influenced by the counter ligands COD or CO and Cl, respectively. In the asymmetric
hydroformylation of vinyl acetate with one of the Rh–Ti-complexes, the branched aldehyde is predominantly
formed with 30% ee. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
The development of highly efficient catalysts for asymmetric hydroformylation of prochiral olefins
represents a challenging task in academic and industrial research.1 Chiral aldehydes obtained as products
have an enormous potential as building blocks in organic synthesis.2 Hitherto, rhodium complexes
based on chiral phosphorus ligands have been shown to be competent catalysts for highly regio- and
stereoselective hydroformylation of a range of different olefinic substrates.3
As a part of our ongoing research concerned with the construction and application of bifunctional
catalysts4 we focused our attention on the use of chiral bimetallic complexes in asymmetric hydro-
formylation. Particularly attractive are heterobimetallic catalysts containing early as well as late tran-
sition metals.5 The ‘hard’ metal could advantageously activate and adjust an appropriate bifunctional
substrate while the second ‘soft’ metal is executing the hydroformylation. Such complexes showing
cooperative effects in asymmetric catalysis4,6 may have relevance as biomimetic models and synthetic
∗
Corresponding author. E-mail: armin.boerner@ifok.uni-rostock.de
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00165-2

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enzymes (chemzymes) and hold entirely new perspectives in the enantiofacial differentiation of prochiral
substrates.7
In principle, the application of achiral bimetallic complexes with metals of d0–d8-electronic confi-
guration (e.g. Ti(IV)–Rh(I) or Zr(IV)–Rh(I)) for hydroformylation was investigated by Choukroun and
Ziolkowski.8 In some instances they found that the hard metal increased the formation of the terminal
aldehyde when 1-hexene was hydroformylated.8c However, to the best of our knowledge no asymmetric
version of this particular reaction has been elucidated.
As a main requisite for each asymmetric heterobimetallic catalysis an appropriate ligand is required
which is able to accommodate two different metals in a defined manner.9 In particular, chiral hydroxy
phosphines are useful hosts owing ‘hard’ (O) and ‘soft’ ligating atoms (P).10 Recently, we described
the synthesis of the polyfunctionalised chiral phosphine (R,R)-diph-salenophos11 (1, Scheme 1) and
its stepwise complexation to Ti(IV) and Pd(II).12 Herein, we wish to report the synthesis of other
salenophos-type ligands and the preparation of the corresponding mono- and heterobimetallic complexes.
Subsequently, the first results of the application of the new complexes in asymmetric hydroformylation
are discussed.
Scheme 1. Salenophos-type ligands. (R,R)-Diph-salenophos 1: R1=(R,R)-1,2-diphenyleth-1,2-diyl; (R,R)-cyc-salenophos 2:
R1=(R,R)-cyclohex-1,2-diyl; (R,R)-bin-salenophos 3: R1=(R)-binaphth-1,10-diyl
2. Results and discussion
2.1. Synthesis of the catalytic system
In general, chiral salenophos-type ligands are easily constructed by a modular approach reacting two
equivalents of 3-diphenylphosphino-2-hydroxybenzaldehyde and the appropriate chiral diamine in boil-
ing ethanol (Scheme 1). Besides the earlier described (R,R)-diph-salenophos 1, we have now succeeded
in the synthesis of the related hydroxy phosphines containing a cyclohexyl- 2 and a binaphthyldiimine
backbone 3, respectively.
Salenophos-type ligands have a unique structure since they contain ‘hard’ as well as ‘soft’ ligating
atoms. They are therefore suitable for the construction of early–late heterobimetallic complexes. For the
synthesis of mononuclear titanium(IV) complexes, salenophos ligands were individually treated with
Ti(OiPr)4 or Ti(OiPr)2Cl2 (Scheme 2). After the reaction with ligands 1 and 2, respectively, we were
able to isolate uniform titanium complexes 4–6 bearing two isopropoxy or chlorine ligands in apical
positions at the metal centre. Unfortunately, the synthesis of cyc-salenophos–Ti(OiPr)2 complex failed.
In this reaction a number of products were yielded which could not be separated by crystallisation.
A characteristic feature of all titanium complexes was the essentially unchanged shift of resonances in
the 31P{1H}-NMR spectrum in comparison to the signals of the hydroxy phosphines 1–3. This clearly
indicated that the phosphine groups do not participate in the coordination of titanium.

M. Quirmbach et al. / Tetrahedron: Asymmetry 10 (1999) 1803–1811
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Scheme 2. Salenophos–Ti(IV) complexes. (R,R)-Diph-salenophos–Ti(OiPr)2 4: R1=(R,R)-1,2-diphenyleth-1,2-diyl,
X=OiPr; (R,R)-diph-salenophos–TiCl2 5: R1=(R,R)-1,2-diphenyleth-1,2-diyl, X=Cl; (R,R)-cyc-salenophos–TiCl2 6:
R1=(R,R)-cyclohex-1,2-diyl, X=Cl; (R)-bin-salenophos–Ti(OiPr)2 7: R1=(R)-binaphth-1,10-diyl, X=OiPr
We next investigated the complexation of diph-salenophos–TiCl2 4 with different rhodium reagents
(Scheme 3). Thus, in the reaction with [Rh(COD)2]BF4 in THF the asymmetric Ti–Rh complex 8
was observed lacking the C2-symmetry of the precursor phosphine. The 31P–31P-coupling constant of
30.5 Hz (JRhP=138.7 Hz and 144.3 Hz) derived from the 31P{1H}-NMR spectrum in CD2Cl2 provided
unambiguous evidence for the cis-coordination of the phosphines to rhodium. Interestingly, the reaction
of 4 with [RhCl(CO)2]2 in CD2Cl2 produced a mixture consisting of two Ti–Rh complexes in different
ratios. The major product 9a was characterised by a 31P–31P-coupling constant of 377.0 Hz (JRhP=130.4
Hz, 133.2 Hz) indicating the trans-relationship of the phosphines. The minor product 9b (C2-symmetric
species with JRhP=127.6 Hz) was probably a dimeric µ-chloro-bridged complex with cis-coordination of
the P-ligating atoms. Unfortunately, all attempts to separate the two complexes were unsuccessful.
Scheme 3. Reaction of (R,R)-diph-salenophos–TiCl2 4 with different rhodium precursors. (i) 1.0 equiv. [Rh(COD)2]BF4, THF,
30 min, rt, 45%; (ii) 0.5 equiv. [RhCl(CO)2]2, CD2Cl2, 10 min, rt

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2.2. Application in asymmetric hydroformylation
The mono- and bimetallic precatalysts for hydroformylation of functionalised olefins (Scheme 4)
were prepared in situ by mixing salenophos ligands 1–3 or the salenophos–Ti(IV) ‘ligands’ 4–6 with
Rh(acac)(CO)2 in a ligand:Rh ratio of 5:1. This procedure turned out to be more advantageous than
the direct use of the rhodium complexes. An excess of the salenophos ligand prevents the formation of
highly reactive but achiral complexes such as HRh(CO)3. The hydroformylation was performed with a
1:1 mixture of H2:CO at 60 bar and 80°C in THF as solvent.
Scheme 4. Hydroformylation of allyl acetate and vinyl acetate
We first investigated the salenophos ligands and salenophos–Ti complexes, respectively, in the hydro-
formylation of allyl acetate. Representative results are listed in Table 1. For runs 1, 4 and 6 monometallic
Rh complexes were employed. Diph-salenophos– and cyc-salenophos–rhodium complexes were active
catalysts whereas the bin-salenophos–rhodium performed rather sluggishly. In all trials, predominant
formation of the linear aldehyde was observed. This result may be attributed to the large bite angle of
salenophos ligands. According to mechanistic studies of Casey13 and van Leeuwen14 a large bite angle of
the diphosphine ligand favoured the n-selectivity of the hydroformylation. Unfortunately, the branched
aldehyde that we obtained as a minor product was racemic.
By application of heterobimetallic Ti–Rh complexes (runs 2, 5 and 7) we noted that only those
Ti complexes bearing isopropoxy ligands were active. TiCl2 complexes displayed no activity in the
hydroformylation; presumably, due to the rapid formation of the catalytically inactive µ-chloro-bridged
Ti–Rh species. The linear carbaldehyde was again predominantly formed. However, in comparison to the
monometallic catalysts and in contrast to the reports of Choukroun and Ziolkowski8 a significant increase
in the formation of the branched product was caused by the titanium. Unfortunately, in these trials also
only the formation of racemic product was observed.
Table 1
Hydroformylation of allyl acetatea

M. Quirmbach et al. / Tetrahedron: Asymmetry 10 (1999) 1803–1811
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Table 2
Hydroformylation of vinyl acetatea
We then studied the hydroformylation of vinyl acetate. With this substrate the formation of the
branched aldehyde is generally more favoured. This phenomenon is caused by the heteroatom adjacent
to the olefin which stabilises the intermediate isoalkylmetal species better than the isomeric n-alkylmetal
complex. Insertion of CO occurs predominantly at the 1-position of the olefin. Chiral 2-acetoxypropanal
formed is a valuable precursor of threonine and a source of 2-hydroxypropanal being a useful building
block for the synthesis of steroids, antibiotics and peptides.
The hydroformylation was carried out under the conditions described for allyl acetate. In Table 2
relevant results are recorded. As anticipated, all active catalysts preferentially produced the branched
aldehyde. Similar to the reaction with allyl acetate in the series of monometallic catalysts the Rh–(R,R)-
diph-salenophos complex provided the highest conversion (run 1). Poor reactivity was obtained with the
catalyst based on (R)-bin-salenophos (run 4). Although the conversion with cyc-salenophos–rhodium
was low, asymmetric induction (6% ee) was observed. Similar to the reaction with allyl acetate, only
Ti–Rh complexes with isopropoxy as co-ligand catalysed the hydroformylation of vinyl acetate (runs
2 and 5). The use of diph-salenophos–Ti(OiPr)2–rhodium diminished the iso/n-selectivity, however, 2-
acetoxypropanal was obtained in 77% chemical yield and 30% ee. This represents the first example of the
successful application of a chiral early–late heterobimetallic catalyst in asymmetric hydroformylation.
Although the enantioselectivity is only moderate, this example provides evidence that transfer of
chirality from the backbone to the catalytically active metal mediated by a second metal is possible.15 Our
preliminary results make salenophos-bimetallic complexes attractive catalysts for further investigations
with the potential for additional synthetic power and the impetus for mechanistic information arising
from the role of the second metal. Currently, work is in progress to test other functionalised olefins in
this reaction.
3. Experimental
3.1. General procedures
All solvents were dried and distilled under argon. Reactions involving phosphines and organometallic
compounds were performed under an Ar atmosphere by using standard Schlenk techniques. Tetrahydro-

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furan (THF), Et2O and toluene were distilled from sodium/benzophenone ketyl, CH2Cl2 was distilled
from CaH2. Commercial reagents were used without further purification. Thin-layer chromatography
was performed on precoated TLC plates (silica gel 60 F254, Merck). Flash chromatography was carried
out with silica gel 60 (particle size 0.040–0.063 mm, Merck). Melting points are corrected. The optical
rotation was measured on a ‘gyromat-HP’ instrument (Fa. Dr. Kernchen). NMR spectra were recorded
on a Bruker ARX 400 instrument at 303 K. Spectra were obtained at the following frequencies: 400.13
1
(1H), 100.63 (13C), 161.98 (31P). Chemical shifts of H and 13C NMR spectra are given relative to the
residual solvent peak (δ). Chemical shifts of 31P NMR spectra are reported in ppm referred to H3PO4 as
external standard. The mass spectra were recorded on an AMD 402 instrument (Fa. Intectra).
3.2. General procedure for the synthesis of salenophos ligands
A Schlenk tube was charged with 17.3 mmol (5.29 g) of 3-diphenylphosphino-2-hydroxy-
benzaldehyde11,12 and ethanol (120 ml) and heated to 70°C. After the aldehyde was completely
dissolved, a solution of the appropriate diamine (8.64 mmol) in ethanol (20 ml) was added dropwise.
The reaction mixture was kept for a further 1 h at this temperature and then cooled to room temperature.
After addition of water (30 ml) a precipitate was formed. Stirring was continued for 20 min. The product
was allowed to settle, filtered and dried in vacuo. If necessary, the crude product was purified by flash
chromatography.
3.3. (R,R)-N,N-Bis(3-diphenylphosphino-2-hydroxybenzylidene)-cyclohexan-1,2-diamine
3.3.1. Cyc-salenophos 2
Following the general procedure for the synthesis of salenophos ligands, 9.56 mmol (2.93 g)
3-diphenylphosphino-2-hydroxybenzaldehyde were treated with 4.78 mmol (0.55 g) (R,R)-1,2-
diaminocyclohexane. The obtained crude product was already analytically pure. Yield 2.9 g (90%); mp
118–121°C; [α]25=−527.3 (c 1.01, CHCl3); 31P NMR (CDCl3) δ=−16.0 (s, P, PPh); 1H NMR (CDCl3)
D
δ=12.50 (s, 2H, OH), 8.13 (s, 2H, CH_N), 7.30–6.35 (m, 26H, Ph), 2.95 (m, 2H, CHN), 1.92–1.35 (m,
8H, CH2); 13C NMR (CDCl3) δ=164.7 (CH_N), 163.4 (d, 2JPC=18.2 Hz, CO), 136.4–117.5 (Ph), 72.3
˜
(CHN), 32.9 (CH2), 24.1 (CH2); IR (Nujol) ν=1624 (s), 1597 (m), 1432 (s, P–C), 1294, 1266, 1148,
1094, 1070, 1027, 954 (w), 744, 696 (s, C–Harom); MS (EI, 70 eV) [m/z] (rel. int. %): 690 [M+] (100),
385 (48), 304 (50), 183 (28); calcd C44H40N2O2P2 (690.76): C, 76.51; H, 5.84; N, 4.06; P, 8.97; found:
C, 75.70; H, 5.92; N, 4.02; P, 8.63.
3.4. (R)-N,N-Bis(3-diphenylphosphino-2-hydroxybenzylidene)-[1,10 ]binaphthalenyl-2,20 -diamine
3.4.1. Bin-salenophos 3
Following the general procedure for the synthesis of salenophos ligands, 3.80 mmol (1.16 g) 3-
diphenylphosphino-2-hydroxybenzaldehyde were treated with 1.90 mmol (0.54 g) (R)-2,20-diamino-
1,10-binaphthalene. The crude product was purified by flash chromatography (n-hexane:EtOAc, 4:1).
Yield 1.4 g (86%); mp 102–110°C; [α]25=−317.7 (c 0.9, CHCl3); 31P NMR (CDCl3) δ=−16.3 (s, P,
D
PPh); 1H NMR (CDCl3) δ=12.61 (s, 2H, OH), 8.25 (s, 2H, CH_N), 7.77–6.50 (m, 38H, Ph); 13C NMR
(CDCl3) δ=162.3 (d, 2JPC=16.2 Hz, CO), 156.0 (CH_N), 144.1–117.2 (Ph); IR (Nujol) ν=1600, 1583,
˜
1558 (m), 1433 (m, P–C), 1292, 1262, 1204, 1144 (w), 741, 694 (m, C–Harom); MS (EI, 70 eV) [m/z] (rel.
int. %): 860 [M+] (83), 676 (10), 572 (92), 556 (100), 539 (13), 478 (32), 430 (28); calcd C58H42N2O2P2
(860.93): C, 80.92; H, 4.92; N, 3.25; P, 7.20; found: C, 80.11; H, 5.12; N, 3.66; P, 6.81.

M. Quirmbach et al. / Tetrahedron: Asymmetry 10 (1999) 1803–1811
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3.4.2. (R,R)-Diph-salenophos–TiCl2 4
To a stirred solution of 1.74 mmol (1.37 g) (R,R)-diph-salenophos 1 in CH2Cl2 (10 ml) was added
dropwise via syringe a 0.5 M solution of Ti(OiPr)2Cl2 (3.5 ml) in CH2Cl2. During the addition the colour
of the reaction mixture changed from yellow to deep red. After stirring overnight at room temperature
Et2O (20 ml) was added. The precipitated complex was filtered off, washed with Et2O (2×10 ml) and
dried in vacuo to yield the deep red complex 4. Yield: 1.4 g (89%); mp >300°C; 31P NMR (CD2Cl2)
1
δ=−16.7 (s, P, PPh); H NMR (CD2Cl2) δ=7.70 (s, 2H, CH_N), 7.41–6.80 (m, 36H, Ph), 5.45 (s, 2H,
2
CHN); 13C NMR (CD2Cl2) δ=163.4 (d, JPC=9.1 Hz, CO), 162.9 (CH_N), 139.7–122.5 (Ph), 76.7
˜
(CHN); IR (Nujol) ν=1615, 1578 (m), 1549 (w), 1419 (w, P–C), 1379, 1263, 877 (m), 749 (w), 716 (m),
695 (w), 676 (m), 581 (m); MS (EI, 70 eV) [m/z] (rel. int. %): 905 [M+] (71), 868 [M+−H−HCl] (71),
44 (100); calcd C52H40Cl2N2O2P2Ti (905.64): C, 68.97; H, 4.45; N, 3.09; found: C, 68.50; H, 4.20; N,
3.23.
3.4.3. (R,R)-Cyc-salenophos–TiCl2 5
To a stirred solution of 0.26 mmol (0.18 g) (R,R)-cyc-salenophos 2 in toluene (5 mL) was added
dropwise via syringe a solution of Ti(OiPr)2Cl2 (0.05 g) in toluene (4 ml) at −50°C. During the addition
the colour of the reaction mixture changed from yellow to brown. After stirring overnight at room
temperature half of the solvent was removed by distillation whilst the complex started precipitating.
The product was allowed to settle, filtered and dried in vacuo to yield the dark brown complex 5. Yield:
1
0.1 g (48%); mp >300°C; 31P NMR (CD2Cl2) δ=−16.7 (s, P, PPh); H NMR (CD2Cl2) δ=8.32 (s, 2H,
CH_N); 7.59–7.01 (m, 26H, Ph), 3.95 (d, 3J=6.4 Hz, 2H, CHN), 2.55 (d, J=11.1 Hz, 2H, CH2), 2.55 (d,
J=7.6 Hz, 2H, CH2), 2.05 (m, 2H, CH2), 1.49 (m, 4H, CH2); 13C NMR (CD2Cl2) δ=163.6 (d, 2JPC=19.1
˜
Hz, CO), 159.3 (CH_N), 139.4–122.4 (Ph), 67.9 (CHN), 28.8 (CH2), 24.1 (CH2); IR (Nujol) ν=1616,
1576 (m), 1547 (w), 1421 (w, P–C), 1386, 1265, 871 (m), 751 (w), 718 (m), 692 (w), 678 (m); MS (FAB
pos., NBA) [m/z] (rel. int. %): 889 [M+−2Cl+NBA] (100); calcd C44H38Cl2N2O2P2Ti (807.53): C, 65.44;
H, 4.74; N, 3.47; found: C, 65.06; H, 4.62; N, 3.44.
3.4.4. (R)-Bin-salenophos–Ti(OiPr)2 6
To a stirred solution of 1.62 mmol (1.39 g) (R)-bin-salenophos in toluene (35 ml) 1.60 mmol (0.46
g) freshly distilled Ti(OiPr)4 was slowly added at room temperature. Stirring was continued overnight
and then the volatiles were removed by distillation. The yellow residue was dried in vacuo to give the
analytically pure titanium complex. Yield: 1.6 g (98%); mp 180°C (dec.); 31P NMR (CDCl3) δ=−16.3 (s,
3
P, PPh); 1H NMR (CDCl3) δ=7.80–6.40 (m, 40H, Ph, CH_N), 4.57 (sept, J=6.1 Hz, 2H, CHO), 0.92
(d, 3J=6.1 Hz, 6H, CH3), 0.86 (d, 3J=6.1 Hz, 6H, CH3); 13C NMR (CDCl3) δ=168.1 (d, 2JPC=19.1 Hz,
˜
CO), 164.9 (CH_N), 149.7–116.4 (Ph), 78.9 (s, CHO), 25.5 (s, CH3), 25.4 (s, CH3); IR (Nujol) ν=1608,
1583, 1536 (m), 1433 (m, P–C), 1410 (m), 1292, 1214, 1143, 1123 (w), 850, 744, 695, 621 (w, C–Harom);
MS (EI, 70 eV) [m/z] (rel. int. %): 906 [M+−2OiPr] (100); calcd C64H54N2O4P2Ti (1024.97): C, 75.00;
H, 5.31; N, 2.73; found: C, 74.51; H, 5.41; N, 2.68.
3.4.5. (R,R)-[Diph-salenophos–TiCl2–Rh(COD)]BF4 8
A Schlenk tube was charged with 0.56 mmol (0.51 g) diph-salenophos–TiCl2 3 and THF (10 ml).
After the complex was completely dissolved 0.56 mmol (0.23 g) [Rh(COD)2]BF4 were added in one
portion. Stirring was continued for 30 min at room temperature, followed by the addition of Et2O (50
ml). The product was allowed to settle, filtered and dried in vacuo yielding the brown complex 8. Yield:
0.30 g (45%); mp >230°C, 31P NMR (CD2Cl2) δ=25.3 (dd, JPP=30.5 Hz, JRhP=144.3 Hz), 13.7 (dd,
JPP=30.5 Hz, JRhP=138.7 Hz); 1H NMR (CD2Cl2) δ=8.0–6.5 (m, 38H, Ph, CH_N), 5.49 (m, 2H, CHN),

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M. Quirmbach et al. / Tetrahedron: Asymmetry 10 (1999) 1803–1811
5.02 (m, 1H, CH), 4.64 (m, 1H, CH), 4.16 (m, 1H, CH), 3.79 (m, 1H, CH), 2.5–1.5 (m, 8H, CH2); calcd
C60H52BCl2F4N2O2P2RhTi (1203.53): C, 59.88; H, 4.35; N, 2.33; found: C, 59.65; H, 4.30; N, 2.35.
4. Hydroformylations
Hydroformylation reactions were performed in a 40 ml stainless steel autoclave (Fa. E. Haage) contain-
ing a glass beaker and a magnetic stirring bar. The beaker was charged with 0.01 mmol Rh(acac)(CO)2.
Subsequently, 0.05 mmol ligand and 1 mmol substrate dissolved in tetrahydrofuran (15 ml) were added
under anaerobic conditions. The autoclave was closed, purged with 60 bar of syn-gas (CO:H2=1:1)
followed by heating the reaction mixture to 80°C. The gas pressure was set at 60 bar and reactions
were conducted with vigorous stirring. After 24 h the autoclave was cooled to room temperature and
depressurised. Analyses of samples were performed by gas chromatography.
4.1. GC conditions
Substrate: allyl acetate; determination of n/iso: HP 101, 25 m: tR/min=15.4 (iso), 16.9 (n); determina-
tion of ee: Lipodex E, 25 m, 85°C; tR/min=10.7, tR/min=11.1.
Substrate: vinyl acetate; determination of n/iso: HP 5, 30 m: tR/min=12.9 (iso), 13.8 (n); determination
of ee: Lipodex E, 25 m, 100°C; tR/min=1.7, tR/min=2.3.
Acknowledgements
We are grateful for the financial support provided by the Deutsche Forschungsgemeinschaft (DFG) for
a grant given to M. Q., the European Commission (Inco-Copernicus, ERBIC 15 CT 960722) for a grant
given to V. T., and the Fonds der Chemischen Industrie. We thank Mrs. C. Pribbenow and Mrs. G. Voß
for skilled technical assistance and Mrs. K. Kortus for GC analysis of hydroformylation products. It is
a pleasure to thank Professor Dr. Y. Belokon (Moscow) and Dr. habil. D. Heller for helpful discussions
and support of this work.
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10. For a review, see: Holz, J.; Quirmbach, M.; Börner, A. Synthesis 1997, 983.
11. Kless, A.; Kadyrov, R.; Börner, A.; Holz, J.; Kagan, H. B. Tetrahedron Lett. 1995, 36, 4601.
12. Kless, A.; Lefeber, C.; Spannenberg, A.; Kempe, R.; Baumann, W.; Holz, J.; Börner A. Tetrahedron 1996, 52, 14599.
13. Casey, C. P.; Whitecker, G. T.; Melville, M. G.; Petrovich, L. M.; Gavney, J. A.; Powell, D. R. J. Am. Chem. Soc. 1992,
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14. van der Veen, L. A.; Boele, M. D. K.; Bregmann, F. R.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje,
J.; Schenk, H.; Bo, C. J. Am. Chem. Soc. 1998, 120, 11616 and references cited therein.
15. It has to be taken into consideration that the chiral subunit is more than 10 Å away from the reaction centre (see Ref. 12).