Mixed donor ligands based on sp2-hybridized nitrogen donors: Asymmetric hydrosilylation catalyzed by rhodium complexes that contain the 2-(2- oxazolin-2-ylmethyl)pyridine system

Article abstract of DOI:10.1016/S0957-4166(98)00340-1

The synthesis and coordination chemistry of a mixed pyridine-oxazoline bidentate ligand, 2-(2-oxazolin-2-ylmethyl)pyridine, are described. Crystal structure data along with variable temperature NMR studies establish the structures of achiral and chiral versions of this ligand system. In addition, the catalytic asymmetric hydrosilylation of acetophenone has been examined.

Full text of DOI:10.1016/S0957-4166(98)00340-1

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 3191–3202
Mixed donor ligands based on sp2-hybridized nitrogen donors:
asymmetric hydrosilylation catalyzed by rhodium complexes that
contain the 2-(2-oxazolin-2-ylmethyl)pyridine system
Michael D. Fryzuk,∗ Laleh Jafarpour and Steven J. Rettig †
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., V6T 1Z1, Canada
Received 10 June 1998; accepted 17 July 1998
Abstract
The synthesis and coordination chemistry of a mixed pyridine–oxazoline bidentate ligand, 2-(2-oxazolin-2-
ylmethyl)pyridine, are described. Crystal structure data along with variable temperature NMR studies establish the
structures of achiral and chiral versions of this ligand system. In addition, the catalytic asymmetric hydrosilylation
of acetophenone has been examined. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
Chiral chelating ligands that incorporate the oxazoline heterocycle have been of great interest in the
past decade.1–3 This is because their synthesis involves the use of readily available chiral aminoalcohols
and, more importantly, spectacular results in asymmetric synthesis have been obtained using these
oxazoline-containing systems.4 For example, the C2-symmetric tridentate ligands of the type 2,6-
bis(2-oxazoline-2-yl)pyridine (Pybox), 1, have shown remarkable activity and enantioselectivity in
the Rh-catalyzed hydrosilylation of ketones and the Ru-catalyzed cyclopropanation of olefins with
diazoacetates.5–7 Also important are the reports5,8,9 of bidentate systems similar to 1 in which only one
oxazolinyl unit is utilized such as shown in 2.
∗
†
Corresponding author. E-mail: fryzuk@chem.ubc.ca
Professional Officer: UBC Crystal Structure Laboratory.
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00340-1

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In both 1 and 2, the different donors are separated in identical ways such that five-membered chelate
rings are generated upon coordination. Our efforts in this area were spurred by the fact that chelate
ring size is an important parameter in the optimization of chiral ligands for asymmetric catalysis.10
In an effort to probe this particular point with these kinds of systems, we investigated the synthesis,
coordination chemistry and enantioselectivity of chiral bidentate ligands that contain the oxazolinyl unit
and the pyridine separated by a methylene unit, shown here as 3; such a system should show different
behavior to 2 simply because, upon coordination, a six-membered chelate ring would be formed. In
this paper we report the synthesis of achiral and chiral versions of this new bidentate mixed donor
system, their coordination chemistry to Rh(I) and some preliminary results with regard to asymmetric
hydrosilylation of acetophenone.
2. Results and discussion
2.1. Synthetic considerations
The synthesis of substituted 2-(2-oxazolin-2-ylmethyl)pyridines relies on the condensation of the
appropriate aminoalcohol with 2-cyanomethylpyridine according to standard protocol (Eq. 1).11 Thus,
refluxing a mixture of 2-cyanomethylpyridine and the aminoalcohol in chlorobenzene in the presence
of a catalytic amount of zinc chloride produces the desired ligand. Chirality is introduced at the carbon
center next to the nitrogen donor on the oxazoline ring by using chiral aminoalcohols such as valinol
and leucinol. Phenylglycinol did not react with 2-cyanomethylpyridine even after refluxing the reaction

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3193
mixture for one week; starting materials were recovered. The new ligands 3a–c were characterized by
NMR spectroscopy and elemental analysis.
(1)
Ligands 3a–c react with [Rh(COD)(THF)2]PF6 to form the complexes [Rh(COD)(3a)]PF6, [Rh-
(COD)(3b)]PF6, and [Rh(COD)(3c)]PF6, 4a–c, respectively, as yellow microcrystalline solids in good
yields (Eq. 2).
(2)
Yellow needle shaped crystals of 4a and 4b, suitable for X-ray analysis, were grown from CH2Cl2
or THF solutions layered with hexanes. The X-ray structure determinations for these two complexes
have been undertaken and their molecular structures are shown in Figs 1 and 2; Table 1 contains
the crystallographic data and Table 2 has a selection of bond distances and bond angles of interest.
Comparison of the two structures reveals that they both have a slightly distorted square planar geometry
at Rh.
Of particular interest are the conformations of the pyridyl–methyloxazoline ligand system as found in
the solid state. Upon coordination, a tricyclic ring system is generated that consists of the six-membered

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Fig. 1. Ball and stick representation of the cationic portion of 4a
Fig. 2. Ball and stick representation of the cationic portion of 4b
chelate ring flanked by the two heterocyclic rings containing the sp2-nitrogen donor atoms. The chelate
ring itself adopts a boat conformation with both of the oxazoline and pyridine rings folded towards the
‘bottom’ of the ‘boat’. In the solid state, the oxazoline ring of 4a is twisted slightly as evidenced by
the torsion angle of N1–C1–O1–C2 of 6.4(6)°; in chiral 4b, this ring is flatter with the same torsion
angle now being −2.4(6)°. A key feature in these molecular structures is the orientation of the oxazoline
substituents; in achiral 4a, the geminal methyl groups are disposed such that one is perpendicular to the
square planar rhodium center and the other is almost parallel to this plane. For chiral 4b, the isopropyl
group is oriented perpendicular to the square planar rhodium center. As a result, the Rh–N1 distance in
4a (2.110(3) Å) is longer than in 4b (2.033(4) Å), possibly a result of the larger steric effect of the two
geminal methyl groups as compared to the single isopropyl substituent. Other bond lengths and angles
compare favorably with similar rhodium systems that incorporate related nitrogen ligands.12–14
1
In solution, 4a and 4b show very different NMR spectral characteristics. The H NMR spectrum of
4a contains very broad resonances that suggest that the molecule is undergoing some fluxional process,
while the proton NMR spectrum of 4b consists of sharp resonances at all temperatures. Cooling a solution
of 4a and monitoring the resulting NMR spectra reveals that a process can be slowed sufficiently on the
NMR time scale to generate sharp resonances. Thus, at room temperature, the two methyl groups of
the oxazoline unit appear as a singlet at 1.3 ppm, while the protons of the backbone methylene (C4),

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Table 1
Crystallographic data
the oxazoline methylene (C2) and the olefinic protons of COD appear as two broad singlets at 4.2 and
4.7 ppm, overlapping an extremely broad third resonance. Upon lowering the temperature to 0°C, the
very broad resonance decoalesces into two broad peaks that eventually split into two doublets at 4.3 and
5.2 ppm (2JHH=27 Hz) at −30°C; these are caused by the backbone methylene protons. The methylene

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Table 2
Selected bond lengths (Å) and angles (deg) for 4a and 4b
protons of the oxazoline ring (C2) decoalesce at −75°C, and at −90°C appear as two broad singlets, at
4.8 and 5.0 ppm.
It is proposed that the fluxional process being exhibited by achiral 4a is caused by the conformational
flipping or inversion of the six-membered chelate ring as shown in Fig. 3. This process of conformational
flipping exchanges the backbone methylene protons in the fast exchange limit as well as the methylene
protons of the oxazoline ring and the methyl substituents (R=R0=Me). Using line shape analysis of the
spectral data from bridging methylenic protons, one can obtain activation parameters from the Arrhenius
and Eyring equations of Ea=3.0 kcal mol−1, ∆G‡ (298 K)=5.4 kcal mol−1, ∆H‡=2.5 kcal mol−1 and
∆S‡=−10.5 cal K mol−1
.
In contrast, no fluxional process was observed for chiral 4b at any temperature. All the resonances
1
can be assigned in the room temperature H NMR spectrum of 4b by NMR spectroscopy (COSY
and NOE experiments). For achiral 4a, the conformational isomers shown in Fig. 3 with R=R0=Me
are enantiomeric; however, for chiral 4b, these conformers A and B are diastereomeric, and should in
principle have different energies. Therefore, the fact that no fluxional process occurs for this system
suggests that the solid state structure observed, in which the isopropyl substituent is pointed perpendicular
to the square plane of the complex, is maintained in solution; in other words, only species A in Fig. 3 is
found since conformer B has the R=Pri pointed parallel to the square plane.
2.2. Hydrosilylation of acetophenone with diphenylsilane
The in situ catalysts were prepared from the rhodium(I) precursors [Rh(COD)Cl]2 or [Rh(COE)2Cl]2
with the chiral ligands 3b or 3c, in neat acetophenone. The standard hydrosilylation was initiated by
addition of diphenylsilane to this mixture of substrate and catalyst at 0°C. The reaction mixture was
allowed to slowly warm up to room temperature and was stirred for the periods indicated in Table 3.
The enantiomeric excesses were determined by GC.15 Addition of CCl4 to the reaction mixture showed

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3197
Fig. 3. Proposed conformational change of the six-membered chelate ring; for R=R0=Me, the process is fast on the NMR time
scale; for R=H and R0=Pri, no exchange is observed and only structure A is observed both in solution and in the solid state
mixed results with respect to enantioselectivity. While it increased the enantioselectivity by a factor
of 30 when [Rh(COD)Cl]2 was used, the enantioselectivity plummeted to zero with [Rh(COE)2Cl]2.
The so-called ‘CCl4 effect’ was observed in the asymmetric hydrosilylation of ketones when ligand 2
was used; although the effect is not understood, the use of CCl4 has been found to lead to increases in
enantioselectivity.3–5
(3)
The best result was obtained when the catalyst system was [Rh(COD)Cl]2, 3b, and CCl4 (entry 4,
Table 3). The enantioselectivities for the catalyst precursor [Rh(COE)2Cl]2 were very low (entries 1–3, 7,
Table 3). Ligand 3b results in a higher enantioselectivity than 3c (entries 5, 8, Table 3) probably because
the chiral isobutyl substituent in 3c is further away from the reaction center as a result of the presence of
the extra methylene group. Our best result, 42% ee, is considerably lower than that observed for similar
catalyst systems using the related ligands 1 and 2. Two explanations can be put forward to rationalize

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Table 3
Catalytic asymmetric hydrosilylation of acetophenone with diphenylsilane
our modest results: in the chiral ligands 3b,c, the presence of only one chiral center on a bidentate ligand
could result in lower enantioselectivities given that the catalytic cycle involves octahedral Rh(III) type
species;10 however, a more serious effect is the conformation of the six-membered ring which orients the
chiral substituent away from the rhodium center towards the bottom of the boat. This is shown here for a
generic octahedral Rh(III) complex.

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3. Conclusions
The incorporation of a methylene unit into the backbone of mixed pyridine–oxazoline systems, such
as 2a–c, generates a six-membered ring upon coordination. Solid state structural studies on two square
planar rhodium complexes show the presence of a folded boat conformation; for the chiral system 4b, the
isopropyl substituent on the oxazoline ring is oriented towards the bottom of the boat. The use of these
ligand systems in the homogeneous asymmetric hydrosilyation of acetophenone results in rather low
enantiomeric excesses (42% ee), presumably because the six-membered ring orients the chiral substituent
away from the incoming substrate.
4. Experimental section
4.1. General procedures and reagent syntheses
All reactions were carried out under an atmosphere of nitrogen unless otherwise stated. Tetrahydro-
furan and hexanes were predried over CaH2 and distilled from sodium benzophenone ketyl under argon
prior to use. Dichloromethane was dried and distilled from CaH2 under argon. 2-pyridylacetonitrile,
diphenylsilane, acetophenone, and carbon tetrachloride were purchased from Aldrich and used without
further purification. [Rh(COD)Cl]2 and [Rh(COE)2Cl]2 were prepared by the published methods. Chiral
amino acids were purchased from Acros. CDCl3 was purchased from CIL and dried and distilled over
CaH2. d8-THF was purchased from CIL and dried and vacuum transferred over potassium. It was then
degassed by three ‘freeze–pump–thaw’ cycles. The chiral amino alcohols were prepared from chiral
amino acids by the literature procedure.16
1H NMR spectra were recorded on Varian XL-300, Bruker WP-200, Bruker WH-400, or Bruker AM-
500 spectrometers with CDCl3 and d8-THF as solvents; the spectra were referenced to CHCl3 at 7.24
ppm and C4D7HO at 3.58 ppm.
Elemental analyses were carried out by P. Borda of this department.
4.2. General procedure for the synthesis of the ligands 2-(2-(4,-R,4-R0)oxazolin-2-ylmethyl)pyridine
(R=R0=Me; R=i-Pr, R0=H; R=sec-Butyl R0=H), 3a–c
In a 250 ml flask, ZnCl2 (0.3 g, 2.12 mmol) was melted under vacuum and cooled under nitrogen, then
2-pyridyl acetonitrile (5 g, 42 mmol), the appropriate amino alcohol (84 mmol), and chlorobenzene (60
ml) were added to the flask. The mixture was refluxed under nitrogen for one day and then was allowed
to cool to room temperature. The solvent was evaporated under reduced pressure and the resulting orange
brown slurry was dissolved in CH2Cl2 (60 ml), and was washed with water (3×30 ml). The aqueous phase
was then extracted with CH2Cl2 (2×25 ml). The organic phases were combined, dried over Na2SO4 and
the volatiles were removed under reduced pressure. The brown residue was distilled under vacuum (at
0.1 mmHg 76°C 3a; 80°C 3b; 90°C 3c).
For 3a: 1H NMR (CDCl3): δ 7.7 (d, 1H, J=10.0 Hz, pyridine), 6.8 (t, 1H, J=8.0 Hz, pyridine), 6.5 (d,
1H, J=10.0 Hz, pyridine), 6.1 (t, 1H, J=8.0 Hz, pyridine), 3.1 (s, 2H, CH2 oxazoline), 2.95 (s, 2H, CH2
aliphatic); 0.4 (s, 6H, CH3). 13C{1H} NMR: δ 162 (C-ipso), 155 (quat, oxazoline), 149 (o-C), 137 (m-C),
124 (p-C), 121 (m-C), 78 (CH2, oxazoline), 66 (CMe2), 37 (CH2, aliphatic), 28 (CH3). Anal. calcd for
C11H14N2O: C, 69.45; H, 7.42; N, 14.72. Found: C, 69.22; H, 6.99; N, 15.01.

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1
For 3b: H NMR (CDCl3): δ 8.1 (d, 1H, J=7.0 Hz, pyridine), 7.2 (t, 1H, J=8.0 Hz, pyridine), 6.9
(d, 1H, J=10.0 Hz, pyridine), 6.7 (t, 1H, J=8.0 Hz, pyridine), 3.8 (m, 1H, CH, oxazoline), 3.6 (m, 2H,
CH2, oxazoline), 3.4 (s, 2H, CH2, aliphatic), 1.3 (m, 1H, CHMe2), 0.6 (d, 3H, J=12.0 Hz, CH3), 0.5 (d,
3H, J=12.0 Hz, CH3). 13C{1H} NMR: δ 164 (C-ipso), 155 (quat, oxazoline), 148 (o-C), 137 (m-C), 123
(p-C), 121 (m-C), 76 (CH2, oxazoline), 56 (CHMe2), 37 (CH2, aliphatic), 19 (CHMe2), 18 (CH3), 17
(CH3). Anal. calcd for C12H16N2O: C, 70.56; H, 7.89; N, 13.71. Found: C, 70.58; H, 7.85; N, 13.95.
1
For 3c: H NMR (CDCl3): δ 8.4 (d, 1H, J=10.0 Hz, pyridine), 7.3 (t, 1H, J=8.0 Hz, pyridine), 7.2
(d, 1H, J=10.0 Hz, pyridine), 7.0 (t, 1H, J=8.0 Hz, pyridine), 4.3 (m, 2H, CH2 oxazoline), 4.1 (m, 1H,
CH oxazoline), 3.7 (m, 2H, CH2 aliphatic), 1.4 (m, 2H, CH2CHMe2), 1.2 (m, 1H, CHMe2), 0.7 (d, 3H,
J=10.0 Hz, CH3), 0.6 (d, 3H, J=10.0 Hz, CH3). 13C{1H} NMR: 164 (C-ipso), 155 (quat, oxazoline), 149
(o-C), 137 (m-C), 123 (p-C), 122 (m-C), 77 (CH2, oxazoline), 64 (CHC4H9), 45 (CH2CHMe2), 37 (CH2,
aliphatic), 25 (CHMe2), 23 (CH3). Anal. calcd for C13H18N2O: C, 71.53; H, 8.31; N, 12.83. Found: C,
70.91; H, 7.94; N, 13.47.
4.3. Synthesis of {Rh(COD)[2-(2-(4,-R,4-R0)oxazolin-2-ylmethyl)pyridine]}PF6, 4a–c
[Rh(COD)Cl]2 (200 mg, 0.4 mmol) was dissolved in THF (30 ml) and a solution of AgPF6 (210 mg,
0.8 mmol) in THF (10 ml) was added to it. A yellow precipitate was formed immediately, the flask then
was covered with aluminum foil and the reaction mixture was stirred for 1 h. The precipitate (AgCl)
was filtered through Celite on a Schlenk frit. The solution of the ligand (160 mg, 0.8 mmol) in THF (10
ml) was degassed and added to the filtrate. The reaction mixture was then allowed to stir overnight. The
solvent evaporated under reduced pressure, the yellow powder was washed with hexanes and ether to
give yellow semi-crystalline solid which was recrystallized from 1:1.5:1.5 CH2Cl2:hexanes:ether (1.8 g,
81%).
For 4a, room temperature 1H NMR (CDCl3): δ 8.4 (d, 1H, J=6.0 Hz, pyridine), 7.9 (t, 1H, J=7.5 Hz,
pyridine), 7.6 (d, 1H, J=9.0 Hz, pyridine), 7.4 (t, 1H, J=6.0 Hz, pyridine), 4.7 (br s, 3H, CH2 oxazoline
and aliphatic), 4.2 (broad, 5H, CH COD and CH2 aliphatic), 2.5 (br s, 2H, CH2 COD), 1.9 (br s, 2H,
COD aliphatic), 1.3 (s, 6H, CH3). 1H NMR (d8-THF) at −90°C: δ 5.2 (δ, 1H, J=15 Hz, CH2 aliphatic),
4.9 (br s, 1H, CH2 oxazoline), 4.8 (br s, 1H, CH2 oxazoline), 4.4 (d, 1H, J=9.0 Hz, COD olefinic), 4.3
(d, 1H, J=15.0 Hz, CH2 aliphatic), 4.2 (br s, 1H, COD olefinic), 4.0 (d, 1H, J=9.0 Hz, COD olefinic),
3.9 (br s, 1H, COD olefinic), 1.3 (s, 3H, CH3), 1.2 (s, 3H, CH3). Anal. calcd for C19H26N2OPF6Rh: C,
41.77; H, 4.80; N, 5.13. Found: C, 41.74; H, 4.70; N, 4.98.
For 4b, 1H NMR (CDCl3): δ 8.4 (d, 1H, J=6.0 Hz, pyridine), 7.9 (t, 1H, J=7.5 Hz, pyridine), 7.6 (d,
1H, J=9.0 Hz, pyridine), 7.4 (t, 1H, J=6.0 Hz, pyridine), 4.9 (d, 1H, J=19.0 Hz, CH2 aliphatic), 4.6 (t,
1H, J=9.0 Hz, CH2 oxazoline), 4.3 (m, 2H, COD olefinic), 4.2 (br s, 2H, COD olefinic), 4.0 (d, 1H,
J=19.0 Hz, CH2 aliphatic), 3.8 (m, 1H, CH2 oxazoline), 2.7 (m, 2H, COD aliphatic), 2.4 (m, 3H, COD
aliphatic), 2.2 (br s, 3H, COD aliphatic), 1.9 (m, 1H, CHMe2), 1.8 (m, 1H, CH oxazoline), 0.8 (d, 3H,
J=10.0 Hz, CH3), 0.6 (d, 3H, J=10.0 Hz, CH3). Anal. calcd for C20H28N2OPF6Rh: C, 42.87; H, 5.04; N,
5.00. Found: C, 42.73; H, 5.06; N, 5.13.
1
For 4c, H NMR (CDCl3): δ 8.4 (d, 1H, J=6.0 Hz, pyridine), 7.9 (t, 1H, J=7.5 Hz, pyridine), 7.6
(d, 1H, J=9.0 Hz, pyridine), 7.4 (t, 1H, J=6.0 Hz, pyridine), 4.8 (m, 1H, CH2 aliphatic), 4.4 (m, 2H,
COD olefinic), 4.2 (d, 1H, J=20.0 Hz, CH2 aliphatic), 3.9 (m, 1H, CH2 oxazoline), 2.8 (m, 2H, COD
aliphatic), 2.5 (m, 3H, COD aliphatic), 2.3 (m, 3H, COD aliphatic), 1.8 (m, 2H, CH2CHMe2), 1.4 (m,
2H, CH oxazoline and CHMe2), 1.0 (d, 6H, J=10.0 Hz, CH3).

M. D. Fryzuk et al. / Tetrahedron: Asymmetry 9 (1998) 3191–3202
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4.4. Hydrosilylation standard procedure
The in situ catalysts were prepared by dissolving the precursor [Rh(COD)Cl]2 or [Rh(COE)2Cl]2
(0.04 mmol, 0.05 mol%) and the chiral ligand (0.34 mmol) in neat acetophenone (0.73 g, 6 mmol). The
hydrosilylation was started by adding diphenylsilane (1.1 ml, 6.6 mmol) at 0°C. When [Rh(COD)Cl]2
was used as the precursor, CCl4 (5 ml) was also added to the reaction mixture. The deep red mixture
was slowly warmed to room temperature and stirred for the periods indicated in Table 3. For workup, the
reaction mixture was cooled in an ice bath and hydrolyzed by adding methanol (10 ml) and HCl (1N, 14
ml). After stirring at 0°C for 2 h followed by 30 min at room temperature, the mixture was extracted with
ether (2×30 ml). The organic phase was dried over Na2SO4, and the solvent was removed using a rotary
evaporator. Kugelrohr distillation yielded the product and the ratio of the secondary alcohol/ketone was
determined by 1H NMR spectroscopy.
4.5. Determination of optical purity
To measure the enantiomeric excess, the product (20 mg) was mixed with isopropylisocyanate (0.3
ml) and one drop of triethylamine in a Reacti-vial. It was heated at 60°C for 12 h, cooled to room
temperature, and the excess isocyanate was evaporated in a stream of nitrogen. The urethane thus formed
was dissolved in ether (2 ml). 0.5 ml of this solution was injected onto a chiral column, Chirasil-Val
III. Gas chromatographic analysis was performed isothermally at 130°C with the injector and detector
temperature of 220°C. The enantiomeric excess was taken as the average of 3–4 injections.
4.6. X-Ray crystallographic analyses of 4a and 4b
Crystallographic data appear in Table 1. The final unit-cell parameters were obtained by least-
squares on the setting angles for 25 reflections with 2θ=19.1–31.2° for 4a and 30 921 reflections with
2θ=4.0–60.1° for 4b. The data were processed17 and corrected for Lorentz and polarization effects, decay
(4a), and absorption (empirical: based on azimuthal scans for three reflections for 4a; based on a three-
dimensional analysis of symmetry-equivalent data for 4b).
The structures were solved by direct methods. The PF6 anion of 4a was modeled as (64:36) two-fold
disordered with respect to rotation about the F(1)–P(1)–F(2) axis. There are two independent molecules
in the asymmetric unit of 4b. One isopropyl methyl carbon atom of 4b (C(25)) was modeled as (54:46)
two-fold disordered. All nonhydrogen atoms were refined with anisotropic thermal parameters. Hydrogen
atoms were fixed in calculated positions with C–H=0.98 Å and BH=1.2 Bbonded atom. A correction for
secondary extinction (Zachariasen type) was applied for 4a, the final value of the extinction coefficient
being 9.0(2)×10−7. Neutral atom scattering factors and anomalous dispersion corrections were taken
from the International Tables for X-Ray Crystallography.18 The absolute configuration of 4b (for the
particular crystal used for data collection) was determined by parallel refinement of the mirror-image.
The R and Rw factor ratios were 1.026 and 1.043, respectively.
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