Synthesis of planar chiral (1,2-disubstituted arene)chromium tricarbonyl compounds and their application in asymmetric hydroboration

Article abstract of DOI:10.1021/om980228j

Optically active (1,2-disubstituted arene)chromium tricarbonyl complexes with a diamine and a phosphorus group in the two ortho benzylic positions were stereoselectively synthesized from a commercially available (+)-(4,6-O-benzylidene)methyl-α-D-glucopyranoside. These chromium complexes have been used as chiral ligands in the preparation of rhodium catalysts for the hydroboration of styrene derivatives. Moderate enantioselectivities were observed in the hydroboration of vinylarenes.

Full text of DOI:10.1021/om980228j

3236
Organometallics 1998, 17, 3236-3239
Syn th esis of P la n a r Ch ir a l (1,2-Disu bstitu ted
a r en e)ch r om iu m Tr ica r bon yl Com p ou n d s a n d Th eir
Ap p lica tion in Asym m etr ic Hyd r obor a tion
Seung Uk Son, Hye-Young J ang, In Su Lee, and Young Keun Chung*
Department of Chemistry and Center for Molecular Catalysis, College of Natural Sciences,
Seoul National University, Seoul 151-742, Korea
Received March 26, 1998
Optically active (1,2-disubstituted arene)chromium tricarbonyl complexes with a diamine
and a phosphorus group in the two ortho benzylic positions were stereoselectively synthesized
from a commercially available (+)-(4,6-O-benzylidene)methyl-R-D-glucopyranoside. These
chromium complexes have been used as chiral ligands in the preparation of rhodium catalysts
for the hydroboration of styrene derivatives. Moderate enantioselectivities were observed
in the hydroboration of vinylarenes.
In tr od u ction
bonylchromium complexes.3 Recently, we reported a
new and easy method for the asymmetric synthesis of
chiral benzaldehyde complexes modified by a sugar
moiety.4 As part of this process, we designed a new type
of chiral ligand, an amine-phosphine hybrid ligand,
from glucopyranoside. The P,N ligand represents a
class of chiral auxiliaries that are amenable to easy
steric and electronic tuning,5 an aspect that is antici-
pated to play an important role in the future develop-
ment of asymmetric catalysis. We investigated the
possibility of preparation of planar chiral (disubstituted
arene)Cr(CO)3 complexes containing phosphine and
amine groups. Herein we report the synthesis of new
chiral diamine ligands having chromium carbonyls and
phosphorus and their rhodium complexes, which exhibit
moderate enantioselectivity in the reduction of styrenes.
(π-Arene)tricarbonylchromium complexes play a very
important role in stereoselective reactions,1 and some
asymmetric syntheses are based on the use of optically
pure chiral complexes as chiral ligands.2 Most of the
reactions studied are restricted to asymmetric stoichio-
metric reactions. A number of groups have established
particular utilities of optically active (arene)chromium
complexes for a variety of synthetic applications. How-
ever, the utilization of optically pure chiral complexes
as chiral ligands has been hampered by the limited
availability of enantiomerically enriched (arene)tricar-
(1) (a) Solladie´-Cavallo, A. In Advances in Metal-Organic Chemistry;
Liebeskind, L. Ed.; J AI: Greenwich, CT, 1989; Vol. 1, pp 99-133. (b)
Uemura, M. In Advances in Metal-Organic Chemistry; Liebeskind, L.
Ed.; J AI: Greenwich, CT, 1991; Vol. 2, pp 199-245. (c) Ku¨ndig, E. P.;
Bernardinelli, G.; Liu, R.; Ripa, A. J . Am. Chem. Soc. 1991, 113, 9676.
(d) Mukai, C.; Miyakawa, M.; Mihira, A.; Hanaoka, M. J . Org. Chem.
1992, 57, 2034. (e) Uemura, M.; Miyake, R.; Nakayama, K.; Shiro, M.
Hayashi, Y. J . Org. Chem. 1993, 58, 1238. (f) Ku¨ndig, E. P.; Xu, L. H.;
Romanens, P.; Bernardinelli, G. Tetrahedron Lett. 1993, 34, 7049. (g)
Davies, S. G.; Hume, W. E. J . Chem. Soc., Chem. Commun. 1995, 251.
(h) Davies, S. G.; Correia, M. A. R. B. J . Chem. Soc., Chem. Commun.
1996, 1803. (i) Kamikawa, K.; Watanabe, T.; Uemura, M. J . Org. Chem.
1996, 61, 1375. (j) Taniguchi, N.; Kaneta, N.; Uemura, M. J . Org. Chem.
1996, 61, 6088. (k) Schmalz, H.; Siegel, S.; Schwarz, A. Tetrahedron
Lett. 1996, 37, 2947. (l) Semmelhack, M. F.; Schmalz, H. Tetrahedron
Lett. 1996, 37, 3089. (m) Pearson, A. J .; Gontchartov, A. V.; Woodgate,
P. D. Tetrahedron Lett. 1996, 37, 3087. (n) Quattropani, A.; Anderson,
G.; Bernardinelli, G.; Ku¨ndig, E. P. J . Am. Chem. Soc. 1997, 119, 4773.
(2) (a) Baldoli, C.; Del Buttero, P. J . Chem. Soc., Chem. Commun.
1991, 982. (b) Heaton, S. B.; J ones, G. B. Tetrahedron Lett. 1992, 33,
1693. (c) Uemura, M.; Hayashi, Y.; Hayashi, Y. Tetrahedron: Asym-
metry 1993, 4, 2291. (d) J ones, G. B.; Heaton, S. B. Tetrahedron:
Asymmetry 1993, 4, 261. (e) J ones, G. B.; Chapman, B. J .; Huber, R.
S.; Beaty, R. Tetrahedron: Asymmetry 1994, 5, 1199. (f) J ones, G. B.;
Chapman, B. J . Synthesis 1995, 475. (g) J ones, G. B.; Huber, R. S.;
Chapman, B. J . Tetrahedron: Asymmetry 1997, 8, 1797. (h) J ones, G.
B.; Heaton, S. B.; Chapman, B. J .; Guzel, M. Tetrahedron: Asymmetry
1997, 8, 3625.
Resu lts a n d Discu ssion
Syn th esis of P la n a r Ch ir a l (Ar en e)Cr (CO)3 Com -
p ou n d s. The planar chiral N,N-acetal compounds 2A
and 2B were synthesized from 1 (eq 1). Compound 1
(1)
(3) (a) Kondo, Y.; Green, J . R.; Ho, J . J . Org. Chem. 1991, 56, 7199.
(b) Alexakis, A.; Mangeney, P.; Marek, I.; Rose-Munch, F.; Rose, E.;
Semra, A.; Robert, F. J . Am. Chem. Soc. 1992, 114, 8288. (c) Aube´, J .;
Heppert, J . A.; Miligan, M. L.; Smith, M. J .; Zenk, P. J . Org. Chem.
1992, 57, 3563. (d) Price, D. A.; Simpkins, N. S.; MacLeod, A. M.; Watt,
A. P. J . Org. Chem. 1994, 59, 1961. (e) Schmalz, H.; Schellhaas, K.
Tetrahedron Lett. 1995, 36, 5515. (f) Amurrio, D.; Khan, K.; Ku¨ndig,
E. P. J . Org. Chem. 1996, 61, 2258. (g) Siwek, M. J .; Green, J . R. J .
Chem. Soc., Chem. Commun. 1996, 2359. (h) Ewin, R. A.; MacLeod,
A. M.; Price, D. A.; Simpkins, N. S.; Watt, A. P. J . Chem. Soc., Perkin
Trans 1 1997, 401.
(4) Han, J . W.; Son, S. U.; Chung, Y. K. J . Org. Chem. 1997, 62,
8264.
(5) (a) Brown, J . M.; Hulmes, D. I.; Laysell, T. P. J . Chem. Soc.,
Chem. Commun. 1993, 1673. (b) Schnyder, A.; Hintermann, L.; Togni,
A. Angew. Chem., Int. Ed. Engl. 1995, 34, 931. (c) Schnyder, A.; Togni,
A.; Wiesli, U. Organometallics 1997, 16, 255.
S0276-7333(98)00228-3 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 06/26/1998

(1,2-disubstituted arene)Cr(CO)3 Compounds
Organometallics, Vol. 17, No. 15, 1998 3237
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 2A
Cr-C1 1.811(8) Cr-C2 2.214(5) Cr-C5 2.216(5) P-C2 1.853(5)
C1-C2 1.439(7) P-C24 1.832(5) C1-C7 1.538(7) C7-N1 1.448(7)
C1-C2-P
118.4(4) C2-C1-C7 120.7(5) C2-P-C24 100.7(2)
C1-C7-N1 112.0(4) N1-C7-N2 108.3(5) C7-N1-C8 118.5(5)
the plane of the arene, and the nitrogen atom (N1) is
located 1.199 Å above the plane of the arene. The
distance between P and N1 is 3.351 Å. When we use
2A as a chiral chelate ligand, the nitrogen atom N2 is
not in the direction of a metal and is far away from the
metal. Thus, 2A acts as a chiral P,N ligand, not a P,N,N
ligand.
Asym m etr ic Hyd r obor a tion . Although borane and
its derivatives react with olefinic substrates in the
absence of catalysts, the reaction is markedly acceler-
ated by addition of transition-metal catalysts.6 The
transition-metal catalyst also has a significant effect on
the regio- and stereochemistry of the reaction.7
Rhodium complexes with 2A or 2B have been exam-
ined with regard to their catalytic activity and enanti-
oselectivity in the reaction of vinylarenes with cate-
cholborane. The results are summarized in Table 3. To
prevent the reaction catalyzed by unmodified rhodium
species, 1.2-2.0 equiv of free ligands was added to [Rh-
(COD)2]BF4 for catalytic reactions.
F igu r e 1. Molecular structure of 2A giving the labeling
scheme.
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
Deta ils for 2A
empirical formula
fw
C32H23CrN2O3P
566.49
cryst syst
orthorhombic
P212121
space group
unit cell dimens
a, Å
13.095(4)
b, Å
13.693(5)
Initially we started to study the hydroboration of
4-methoxystyrene (entries 3-6 in Table 3), which reacts
otherwise with catecholborane only at elevated temper-
atures.8 Hydroboration of 4-methoxystyrene with cat-
echolborane in the presence of 2 mol % of [Rh(COD)2]-
BF4 and 1.2 equiv of 2A in THF at -15 °C for 18 h,
followed by oxidation with alkaline hydrogen peroxide,
gives the corresponding sec-alcohol in 87% yield with
62% ee (entry 1 in Table 3). Many studies indicate that
phosphine-to-rhodium ratios are critical in hydrobora-
tions of styrene derivatives: higher ratios favor forma-
tion of the secondary alcohol even with neutral cata-
lysts.9 High regioselectivity was observed for other
cationic rhodium complexes having tertiary phosphine
ligands.10 When we tried the same reaction using 2.0
equiv of 2A, the corresponding sec-alcohol was obtained
in 86% yield with 65% ee (entry 2). However, the effect
of the concentration of the chiral ligand was not great.
The in situ cationic rhodium complex generated by the
c, Å
14.972(2)
2684.6(13)
4
1.402
0.710 73
293(2)
1168
2.02-24.97
0 e h e 15, 0 e k e 16, 0 e l e 17
2668
2668 (R(int) ) 0.0000)
full-matrix least squares on F2
2668/0/360
V, Å3
Z
d(calcd), g/cm3
λ, Å
temp, K
F(000)
θ range for data collecn, deg
index ranges
no. of rflns collected
no. of indep rflns
refinement method
no. of data/restraints/params
GOF
1.074
final R indices (I > 2σ(I))
R indices (all data)
abs structure param
R1 ) 0.0410, wR2 ) 0.0898
R1 ) 0.0838, wR2 ) 0.1045
-0.06(4)
largest diff peak and hole, e Å-3 0.261 and -0.427
was previously prepared by us through the asymmetric
lithiation and addition of electrophile.4 Treatment of 1
with 1,8-naphthalenediamine in the presence of molec-
ular sieves (4 Å) gave 2A in 85% yield. Compound 2B
was prepared in 95% yield by the reaction of 2A with
KOtBu followed by addition of MeI. At first we expected
an imino compound as a product and wanted to prepare
a C2-symmetric diimino compound. However, the 1H
(6) Rh complexes: (a) Ma¨nnig, D.; No¨th, H. Angew. Chem., Int. Ed.
Engl. 1985, 24, 878. Zr complexes: (b) Pereira, S.; Srebnik, M.
Organometallics 1995, 14, 3127. (c) Pereira, S.; Srebnik, M. J . Am.
Chem. Soc. 1996, 118, 909. Ir complexes: (d) Evans, D. A.; Fu, G. C.;
Hoveyda, A. H. J . Am. Chem. Soc. 1992, 114, 6671. Ni complexes: (e)
Gridnev, I. D.; Miyaura, N.; Suzuki, A. Organometallics 1993, 12, 589.
Ru complexes: (f) Burgess, K.; J aspars, M. Organometallics 1993, 12,
4197. Pd complexes: (g) Matsumoto, Y.; Naito, M.; Hayashi, T.
Organometallics 1992, 11, 2732. (h) Matsumoto, Y.; Naito, M.; Uozumi,
Y.; Hayashi, T. J . Chem. Soc., Chem. Commun. 1993, 1468. Ln
complexes: (i) Harrison, K. N.; Marks, T. J . J . Am. Chem. Soc. 1992,
114, 9220.
(7) (a) Evans, D. A.; Fu, C.; Hoveyda, A. H. J . Am. Chem. Soc. 1988,
110, 6917. (b) Evans, D. A.; Fu, C.; Anderson, B. A. J . Am. Chem. Soc.
1992, 114, 6679. (c) Burgess, K.; van der Donk, W. A.; Westcott, S. A.;
Marder, T. B.; Baker, R. T.; Calabrese, J . C. J . Am. Chem. Soc. 1992,
114, 9350.
1
NMR spectrum of 2A did not match with the H NMR
spectrum of the expected diimino compound, as we could
see the peaks of amino protons. Thus, we suspected
that the product was not a diimino compound. We grew
single crystals of 2A and solved the X-ray crystal
structure of 2A. Figure 1 shows the three-dimensional
molecular structure of 2A with the atomic numbering.
Crystal data and structure refinement details of 2A are
given in Table 1, and selected bond distances and angles
are given in Table 2. The stereochemistry of 2A is the
same as that of 1, as expected.4 The naphthalene group
is upright to the plane of arene coordinated to Cr(CO)3
(the angle between the plane of the arene (C1-C2-C3-
C4-C5-C6) and the plane N1-C8-C9-C10-N2 is
93.0°). The phosphorus atom is located 0.024 Å above
(8) (a) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T. J .
Am. Chem. Soc. 1992, 114, 8863. (b) Pelter, A.; Smith, K.; Brown, H.
C. Borane Reagents; Academic Press: New York, 1988.
(9) (a) Burgess, K.; van der Donk, W. A.; Kook, A. M. J . Org. Chem.
1991, 56, 2949. (b) Zhang, J .; Lou, B.; Guo, G.; Dai, L. J . Org. Chem.
1991, 56, 1670.
(10) Hayashi, T.; Matsumoto, Y.; Ito, Y. J . Am. Chem. Soc. 1989,
111, 3426.

3238 Organometallics, Vol. 17, No. 15, 1998
Ta ble 3. Ca ta lystic Asym m etr ic Hyd r obor a tion w ith Ca tech olbor a n ea
Son et al.
OH
NaOH, H2O2
catecholborane, 2 mol% L*Rh
Ar
Ar
THF, –15 °C, 18 h
entry no.
substrate
catalyst (amt, mol%)
ligand (equiv of L/Rh)
yield, %b
ee, %
1
2
3
4
5
6
7
8
4-methoxystyrene
4-methoxystyrene
4-methoxystyrene
4-methoxystyrene
4-bromostyrene
styrene
Rh(COD)2BF4 (2)
Rh(COD)2BF4 (2)
[Rh(COD)Cl]2 + AgBF4 (2)c
Rh(COD)2BF4 (2)
Rh(COD)2BF4 (2)
Rh(COD)2BF4 (2)
Rh(COD)2BF4 (2)
Rh(COD)2BF4 (2)
2A (1.2)
2A (2.0)
2A (1.2)
2B (1.2)
2A (1.2)
2A (1.2)
2A (1.2)
2A (1.2)
87
86
65
80
80
95
65
94
62
65
32
6
19
53
75
81
3,4-dimethoxystyrene
2,4-dimethylstyrene
a
b
All reactions were carried out in THF at -15 °C for 18 h. Isolated yields by column chromatography. c The cationic rhodium catalyst
was prepared in situ.
reaction of [Rh(COD)Cl]2 with AgBF4 was not effective
(entry 3). Introduction of a methyl group on the amine
nitrogen (entry 4) did not increase the ee value, pre-
sumably because of a steric constraint: rhodium metal
suffered a great steric congestion to coordinate to the
nitrogen atom. Thus, there would be no enantio-
differentiation, and a low ee value was eventually
obtained. All of the substituted styrenes were trans-
formed into optically active 1-arylethanols. The regi-
oselective formation of (1-arylethyl)boranes was perfect
for all the styrenes examined, irrespective of the electron-
releasing or electron-withdrawing nature of the sub-
stituent group on the phenyl. The enantioselectivity
was dependent on the substrate. The enantioselectivity
for substituted styrenes ranged from 19% to 81% ee. The
order of ee is as follows: 2,4-dimethylstyrene > 3,4-
dimethoxystyrene > 4-vinylanisole > styrene > 4-bro-
mostyrene. The ee values (entries 1, 2, 3, and 7) seem
to be sensitive to the electronic effect of the substituent
on the phenyl. As the electron-donating ability (Br <
H < OMe) of the substituent increases, the vinyl group
binds more strongly and closely to the rhodium metal
and is more easily affected by the chiral ligand. Thus,
as the electron-donating ability of the substituent on
the phenyl increases, the ee value increases. The origin
of the high ee value for entry 8 may be attributable
primarily to the steric repulsion between the Me and
Ph groups.
product. However, the transition state B is less favored
because of the repulsive steric interaction between the
phenyl group on the phosphorus and the phenyl group
on the styrene.
Con clu sion
We have demonstrated that the planar chiral N,N-
acetal compounds 2A and 2B can be synthesized from
the planar chiral chromium benzaldehyde complex 1
and can be used as chiral P,N ligands in the preparation
of rhodium catalysts for the hydroboration of styrene
derivatives. Moderate enantioselectivities were ob-
served, and the ee values depended on the electronic
effect of the substituent on the styrene ring and the
steric effect of the ortho substituent on the styrene ring.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All solvents were purified by
standard methods, and all synthetic procedures were done
under a nitrogen atmosphere. Reagent grade chemicals were
used without further purification.
Elemental analyses were performed at the Chemical Ana-
lytic Center, College of Engineering, Seoul National University
or the Chemical Analytic Center. 1H NMR spectra were
obtained with a Bruker 300 or a Bruker AMX-500 instrument.
Infrared spectra were recorded on a Shimadzu IR-470 spec-
trometer. Compound 1 was prepared by the published method.4
Syn th esis of 2A. To compound 1 (0.20 g, 0.46 mmol) in
20 mL of Et2O was added molecular sieves (4 Å) and 1,8-
naphthalenediamine (0.11 g, 0.70 mmol). The resulting solu-
tion was heated to reflux for 12 h. After the solution was
cooled, any precipitates were filtered off and the filtrate was
concentrated and chromatographed on a silica gel column with
hexane/Et2O (v/v, 10:1) as eluent. After removal of the solvent,
0.23 g of product was isolated (yield 85%). Single crystals were
grown by slow evaporation of a solution of 2A in hexane and
Our catalytic system with 2A and 2B gives the R
absolute configuration for all secondary alcohol prod-
ucts. The stereochemical outcome can be explained by
assuming the two hypothetical transition states A and
B for the formation of a styrene-rhodium complex. The
CH2Cl2. IR: νCO 1974, 1881 cm-1 1H NMR (CDCl3): δ 7.62
.
(t, 5.9 Hz, 1 H), 7.53 (m, 4 H), 7.39 (m, 5 H), 7.27 (t, 7.8 Hz, 1
H), 7.18 (d, 8.3 Hz, 1 H), 7.13 (d, 8.1 Hz, 1 H), 7.06 (t, 7.4 Hz,
1 H), 6.67 (d, 7.2 Hz, 1 H), 6.17 (d, 9.1 Hz, 1 H), 5.77 (d, 4.5
Hz, 1 H), 5.50 (d, 7.3 Hz, 1 H), 5.44 (t, 6.3 Hz, 1 H), 5.24 (t, 6.1
Hz, 1 H), 4.75 (d, 6.4 Hz, 1 H), 4.54 (s, 1 H), 3.61 (s, 1 H) ppm.
Anal. Calcd for C32H23CrN2O3P: C, 67.84; H, 4.09; N, 4.94.
Found: C, 67.62; H, 3.78; N, 5.22. [R]18 ) +158 (c 0.1, CH2-
D
Cl2).
Syn th esis of 2B. To 2A (0.10 g, 0.18 mmol) in 10 mL of
THF was added KOtBu (0.42 mL in 1.0 M in THF). After the
solution was stirred for 15 min, MeI (0.1 mL) was added.
Excess Et2O (30 mL) and aqueous saturated NH4Cl (20 mL)
were added to quench the reaction and extract the chromium
transition state A is free from the repulsive steric
interaction, with styrene fitting better to the chiral
environment. The coordination depicted in A is consis-
tent with the observed absolute stereochemistry of the

(1,2-disubstituted arene)Cr(CO)3 Compounds
Organometallics, Vol. 17, No. 15, 1998 3239
1
compound. The ether extract was concentrated and chromato-
graphed on a silica gel column with hexane/Et2O (v/v, 10:1)
as eluent. After evaporation of solvent, 0.10 g of 2B was
diastereomers and calculated the ee by the inspection of H
NMR spectra of diastereomers.
X-r a y Cr ysta l Str u ctu r e Deter m in a tion of 2A. Crystals
of 2A were grown by slow evaporation of a solution of 2A in
hexane. Diffraction was measured on an Enraf-Nonius CAD4
diffractometer by the ω-2θ scan method. Unit cells were
determined by centering 25 reflections in the approximate 2θ
range. Other relevant experimental details are in the Sup-
porting Information. The structure was solved by direct
methods using SHELXS-86 and refined by full-matrix least
squares with SHELXL-93. All non-hydrogen atoms and H1′
and H2′ were refined with anisotropic temperature factors;
hydrogen atoms except for H1′ and H2′ were refined isotro-
pically using the riding model, with equivalent isotropic
temperature factors of 1.2 times the factors for the atoms to
which they are attached. Crystal data and experimental
details are given in Table 1.
isolated (95%). IR: νCO 1954, 1879 cm-1
.
1H NMR (CDCl3):
δ 7.52 (t, 5.9 Hz, 1 H), 7.44 (m, 4 H), 7.35 (m, 5 H), 7.35 (t, 7.9
Hz, 1 H), 7.27 (d, 8.4 Hz, 1 H), 7.11 (d, 8.4 Hz, 1 H), 6.99 (t,
7.9 Hz, 1 H), 6.49 (d, 7.5 Hz, 1 H), 5.83 (d, 4.3 Hz, 1 H), 5.17
(d, 7.2 Hz, 1 H), 5.11 (t, 5.6 Hz, 1 H), 4.97 (t, 6.1 Hz, 1 H), 4.82
(d, 6.1 Hz, 1 H), 4.78 (dd, 2.7, 6.2 Hz, 1 H), 3.33 (s, 3 H), 2.87
(s, 3 H) ppm. Anal. Calcd for C34H27CrN2O3P: C, 68.68; H,
4.58; N, 4.71. Found: C, 68.47; H, 4.74; N, 4.94. [R]20D ) +52
(c 0.1, CH2Cl2).
Ca ta lytic Asym m etr ic Hyd r obor a tion . A typical pro-
cedure is given for 4-vinylanisole. A mixture of [Rh(COD)2]-
BF4 (7 mg, 0.017 mmol) and 2A (11.7 mg, 0.021 mmol) in 3
mL of THF was stirred under nitrogen at room temperature
for 1.5 h, and 4-vinylanisole (116 mg, 0.86 mmol) was added
at -15 °C. Catecholborane (1.7 mL in 1 M in THF, 1.72 mmol)
was added at -15 °C, and the mixture was stirred at -15 °C
for 18 h and then quenched with 5 mL of ethanol. To the
mixture was added 5 mL of 3 M NaOH and 0.5 mL of 35%
H2O2, and it was stirred at room temperature for 3 h.
Extraction with Et2O followed by chromatography on a silica
gel column with hexane/diethyl ether (v/v, 10:1) as eluent gave
113 mg (86%) of (R)-1-(4-methoxyphenyl)ethanol. The regio-
isomer was not detected by 1H NMR analysis of the crude
reaction mixture. To determine the enantiomeric excess (ee)
values, we used (S)-(+)-mandelic acid as a chiral derivatizing
reagent to convert an enantiomeric mixture to a pair of
Ack n ow led gm en t. This work was supported by the
Ministry of Education (Grant No. BSRI 97-3415) and
the KOSEF through the Center for Molecular Catalysis
and Grant No. 96-0501-03-0-3.
Su ppor tin g In for m ation Available: Tables giving atomic
coordinates, thermal parameters, and bond distances and
angles for 2A (8 pages). Ordering information is given on any
current masthead page.
OM980228J