Preparation of axially chiral biphenyl diphosphine ligands and their application in asymmetric hydrogenation

Article abstract of DOI:10.1248/cpb.52.1445

Axially chiral biphenyldiphosphine ligands bearing diphenylphosphino group(s) and/or dicyclohexylphosphino group(s) were prepared in enantiomerically pure form starting from 2,6-dimethylnitrobenzene via 8 steps: iodination, reduction, methoxylation throug

Full text of DOI:10.1248/cpb.52.1445

December 2004
Chem. Pharm. Bull. 52(12) 1445—1450 (2004)
1445
Preparation of Axially Chiral Biphenyl Diphosphine Ligands and Their
Application in Asymmetric Hydrogenation
Toshiaki MORIMOTO,* Kiyoshi YOSHIKAWA, Masanao MURATA, Naoko YAMAMOTO, and Kazuo ACHIWA
School of Pharmaceutical Sciences, University of Shizuoka; 52–1 Yada, Shizuoka 422–8526, Japan.
Received July 12, 2004; accepted September 15, 2004
Axially chiral biphenyldiphosphine ligands bearing diphenylphosphino group(s) and/or dicyclohexylphos-
phino group(s) were prepared in enantiomerically pure form starting from 2,6-dimethylnitrobenzene via 8 steps:
iodination, reduction, methoxylation through diazotization, Ullmann coupling, bromination, phosphorylation,
optical resolution, and silane reduction, and the obtained ligands were used in rhodium-catalyzed asymmetric
hydrogenation.
Key words axially chiral ligand; diphosphine ligand; optical resolution; asymmetric hydrogenation; rhodium complex
ally chiral diphosphines,18—21) which were found to be very
efficient ligands for ruthenium-catalyzed asymmetric hydro-
genation of a b-keto ester or rhodium-catalyzed asymmetric
hydrogenation of an a-amino ketone. In this article we
describe in detail the synthesis of optically pure 3,3ꢀ-
dimethoxy-2,2ꢀ,4,4ꢀ-tetramethyl-1,1ꢀ-biphenyls 8a—c bear-
ing one or two 6,(6ꢀ)-dicyclohexylphosphino group(s) and/or
6,(6ꢀ)-diphenylphosphino group(s), and an application to
rhodium-catalyzed asymmetric hydrogenation.
Development of efficient methods for catalytic enantiose-
lective synthesis has been an important subject for the practi-
cal synthesis of optically active organic compounds.1) Nu-
merous methods of enantioselective synthesis employing var-
ious catalysts such as transition metal complexes coordinated
with many types of chiral ligands have been reported for
more than three decades.1—6) For the development of efficient
chiral catalysts, creation of new types of ligands is a most
significant strategy. Among various types of chiral ligands,
diphosphines are the most effective and are widely applicable
in the catalytic asymmetric hydrogenation of prochiral com-
pounds such as olefins, ketones, and imines using transition
metal (rhodium, ruthenium, iridium, etc.) complexes as cata-
lyst. We previously proposed a novel idea, “respective con-
trol concept” for designing new chiral ligands,7—10) and de-
veloped efficient diphosphine ligands such as (2S-cis)-4-(di-
cyclohexylphosphino)-2-[(diphenylphosphino)methyl]-1-
pyrrolidinecarboxylic acid 1,1-dimethylethyl ester (BCPM)
and its analogs,8,11—13) (4R-trans)-[(2,2-dimethyl-1,3-diox-
olane-4,5-diyl)bis(methylene)]bis[bis(4-methoxy-3,5-di-
methylphenyl)phosphine] (MOD-DIOP) and its analogs,14—17)
etc. (Fig. 1) for rhodium-catalyzed asymmetric hydrogena-
tion of some prochiral ketones (ketopantolactone, a- and b-
amino ketones, etc.), and olefins (itaconic acid and its deriva-
tives, N-acyldehydroamino acids, etc.). Furthermore, we re-
vealed that phosphines bearing electron-donating groups
such as cyclohexyl groups, p-(dimethylamino)phenyl groups,
and p-methoxy-m,mꢀ-dimethylphenyl groups enhance not
only the catalytic activity but also the enantioselectivity in
the rhodium-catalyzed hydrogenations. The p-methoxy-
m,mꢀ-dimethylphenyl (abbreviated to MOD) group was
found to be a useful component for developing efficient lig-
ands on the basis of its steric effect (m-Me) as well as its
electronic effect (p-MeO, m-Me).15) We also reported in pre-
liminary communications the design and preparation of axi-
Results and Discussion
The synthetic route of axially dissymmetric biphenyl-
diphosphine ligands 8a—c bearing one or two dicyclo-
hexylphosphino group(s) and/or one or two diphenylphos-
phino group(s) is described in Chart 1 (8a—c were abbrevi-
ated to BIMOP, Cy-BIMOP, and MOC-BIMOP, respec-
tively). Iodination of 2,6-dimethylnitrobenzene (1) was car-
ried out selectively at the 3-position by heating with iodine at
90 °C for 4 d in the presence of periodic acid in acetic acid
containing dil. sulfuric acid, yielding mono-iodide 2 in high
isolated yield (95%). The nitro group of 2 was reduced by
heating with iron powder in water for 48 h in the presence of
a catalytic amount of sulfuric acid, and the corresponding
amine was isolated as a hydrochloride 3 in high yield (92%).
The amino group was converted to a methoxy group by dia-
zotization with isoamyl nitrite followed by elimination of N2
in methanol, and an anisole derivative 4 was obtained in
good yield. The methoxy group was expected to be useful for
introducing a bromo group at the para-position and for in-
creasing the electron density of a phosphino group intro-
duced later at the same position. Ullmann coupling of 4 was
carried out by heating with copper powder in sulfolane at
220 °C for 15 h, affording the corresponding biphenyl 5.
Bromination of 5 was easily performed by the reaction with
2 eq of bromine in acetic acid, yielding selectively a 6,6ꢀ-di-
Fig. 1. Diphosphine Ligands
∗ To whom correspondence should be addressed. e-mail: morimtt@u-shizuoka-ken.ac.jp
© 2004 Pharmaceutical Society of Japan

1446
Vol. 52, No. 12
Chart 1. Synthesis of (R)-6,6ꢀ-Bis(dialkylphosphino)-3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-tetramethyl-1,1ꢀ-biphenyl ((R)-8a—c)
brominated product 6 in good yield. Dilithiation of 6 with (R)-8a—c in good yields. The absolute configurations of
tert-butyllithium at ꢁ70 °C followed by treatment with 2 eq (R)-8a—c were determined by comparison of their CD spec-
of diphenylphosphinic chloride resulted in the formation tra on the shape and the signs of the major Cotton effects
of a diphosphine dioxide, (RS)-6,6ꢀ-bis(diphenylphosphoryl)- with that of (R)-BIPHEMP whose absolute configuration had
3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-tetramethyl-1,1ꢀ-biphenyl (RS)-7a been determined by X-ray crystallography.22) The CD spectra
in 51% yield. An (RS)-6,6ꢀ-bis(dicyclohexylphosphoryl) de- of (R)-8a—c showed a nearly identical Cotton curve (ex-
rivative (RS)-7b was similarly prepared in 63% yield by the hibiting strong negative maxima at 230—240 nm and weaker
reaction of the dilithiate with chlorodicyclohexylphosphine positive maxima at 266—278 nm) with those of (R)-
followed by oxidation with hydrogen peroxide. An (RS)-6- BIPHEMP and its analogs.22) The absolute configurations of
dicyclohexylphosphoryl-6ꢀ-diphenylphosphoryl
derivative (R)- and (S)-BICHEP bearing a similar structure to 8b had
(RS)-7c was also synthesized in 14% yield by stepwise been also determined by comparison of their CD spectra with
diphenylphosphorylation and dicyclohexylphosphination fol- those of (R)- and (S)-BIPHEMP.23)
lowed by oxidation. Effective optical resolution of (RS)-7a, b
Catalytic asymmetric hydrogenation of functionalized
was attained by using (2R,3R)-(ꢁ)-2,3-O,Oꢀ-dibenzoyltar- olefins such as itaconic acid (9), a-piperonylidenesuccinic
taric acid (DBTA) as a resolving agent. When a hot solution acid monomethyl ester (11), and (Z)-a-acetamidocinnamic
of (RS)-7a and (ꢁ)-DBTA in chloroform was diluted with acid (13) was carried out by using cationic rhodium com-
hot ethyl acetate, cooled, and seeded with a small amount of plexes of (R)-8a—c. The results are summarized in Tables 1
pure (R)-7a, a less soluble diastereomer precipitated as crys- and 2. Using 0.1 mol% of cationic rhodium(I) complex cata-
tals. Repeated recrystallization from a mixed solvent of chlo- lysts which were prepared by mixing bis(norbornadiene)-
roform and ethyl acetate gave a diasteromerically pure com- rhodium(I) perchlorate [[Rh(nbd)2]ꢂClOꢁ4 ] and (R)-8a—c in
plex of (R)-7a and the dibenzoyltartaric acid in a theoretical dichloromethane and concentrated in vacuo, the hydrogena-
47% isolated yield. Free (R)-7a was isolated quantitatively tion of itaconic acid (9) was carried out in methanol under
by treatment with alkaline. Optically pure (R)-7b was also 5 atm of hydrogen, yielding the hydrogenation product 10
isolated in 38% overall yield from (RS)-7b by resolution quantitatively in 51% ee (S) with (R)-8a, 80% ee (R) with
with (2R,3R)-(ꢁ)-2,3-O,Oꢀ-dibenzoyltartaric acid in (R)-8b, and 71% ee (R) with (R)-8c, respectively (Table 1).
methanol. Optical resolution of (RS)-7c was found to be dif- In the presence of triethylamine the enantioselectivity with
ficult with the dibenzoyltartaric acid; therefore, (R)-7c was (R)-8a was reduced to 19% ee (S). Analogous a-piperonyli-
isolated by preparative HPLC with a chiral column, Chiral- denesuccinic acid monomethyl ester (11) was also hydro-
pack OT(ꢂ) using hexane/2-propanol (4/1) as an eluent. Re- genated under the same conditions as described above, and
duction of the phosphine oxides (R)-7a—c was carried out similar results were obtained (56% ee (S) with (R)-8a, 73%
without any racemization in a sealed tube by heating with an ee (R) with (R)-8b, 59% ee (R) with (R)-8c). To our surprise,
excess of trichlorosilane in the presence of triethylamine in the chirality of the products 10 and 12 ((S)-configuration) in-
chlorobenzene at 140 °C (bath temp.) for 5 h. Purification by duced by (R)-8a was reversed in comparison with that in-
silica gel column chromatography (eluent: chloroform for 8a duced by (R)-8b, c, although all the ligands (R)-8a—c have
or toluene for 8b, c) afforded the corresponding phosphines the same axial chirality backbone. (R)-BINAP did not show

December 2004
1447
Table 1. Asymmetric Hydrogenationa) of Itaconic Acid and a-Piperonyl-
idenesuccinic Acid Monomethyl Ester Catalyzed by Rhodium(I) Complexes
of Ligands (R)-8a—c
Substrate
Ligand
Yield (%)b)
E.e. (%)c) Confign.d)
9
9
9
11
11
11
(R)-8a
(R)-8b
(R)-8c
(R)-8a
(R)-8b
(R)-8c
100
100
100
72
100
61
51
80
71
56
73
59
S
R
R
S
R
R
a) The reaction was carried out using 0.5 M solution of the substrate in MeOH under
the conditions: [Subst.]/[Rh]ꢃ1000, 5 atm (H2), 30 °C, 20 h. b) Determined by 1H-
NMR analysis. c) Calculated on the basis of the specifical rotation value of the pure
(R)-enantiomer 10, [a]D20 ꢂ16.88° (cꢃ2.16, EtOH) [Berner E., Leonardsen R., Ann.
Chem., 1, 538 (1939).] or by HPLC analysis of the corresponding morpholino deriva-
tive of 12 on a chiral column, Chiralcel OD. d) Determined by the sign of the specific
rotation in comparison with that of (R)-10 or (R)-12, [a]D20 ꢂ30.4° (cꢃ2, MeOH)
[Brown E., Daugan A., Tetrahetron Lett., 26, 3997—3998 (1985).].
Chart 2. Correlation between the Chirality of Bidentate Ligands and the
Absolute Configuration of the Asymmetric Hydrogenation Products
Table 2. Asymmetric Hydrogenationa) of (Z)-2-Acetamidocinnamic Acid
Catalyzed by Rhodium(I) Complexes of Ligands (R)-8a—c
genation or palladium-catalyzed asymmetric allylic alkyla-
tion. We first proposed a P/M chirality concept,24) where the
positioning array of four phenyl rings of diphosphine ligands
closely correlates with the absolute configuration of products
in asymmetric hydrogenation, and later presented a more
general concept, Pr/Mr chirality,25) for showing all chiral
bidentate ligands (e.g. P,P-ligand, P,N-ligand, S,N-ligand,
N,N-ligand, etc.) (Chart 2). The ligands (R)-8a—c were clas-
sified to bear Pr-chirality by comparison with (R)- or (S)-
BINAP, whose metal complexes had been structurally re-
solved by X-ray crystallography.26,27) Based on our previous
elucidation of the correlation between the Pr/Mr chirality of
ligands and the hydrogenation products,24,25) it is reasonable
that Pr-chirality ligands show R- and S-selectivities in the hy-
drogenation of itaconic acids and (Z)-a-acetamidocinnamic
acid, respectively. Therefore, the ligands (R)-8b, c have nor-
mal selectivity in the rhodium-catalyzed hydrogenation. Sim-
ilar R- and S-selectivities were observed with (R)-BICHEP in
the hydrogenation of itaconic acids and (Z)-a-acetamidocin-
namic acid.28) Although the reason why (R)-8a shows re-
versed chirality is not clearly demonstrated, the reversed chi-
rality might be rationalized by a possible explanation that a
dinuclear complex26,29) of Rh(I) and a triaryl-type ligand (R)-
8a (probably formed in situ from a diene coordinate starting
complex) can show reversed chirality. Other reversal of chi-
rality induced by MOD-DIOP in rhodium-catalyzed asym-
metric hydrogenation of enamides was reported previously.30)
It is well known that the rhodium-catalyzed asymmetric hy-
drogenation of (Z)-a-acetamidocinnamic acid proceeds by
Halpern’s mechanism31) where an intermediate of a minor di-
asteromeric complex of the substrate and the catalyst is much
more active for oxidative addition of hydrogen (a rate-deter-
mining step) than a major intermediate, and, therefore, the
minor intermediate affords a predominant enantiomeric prod-
uct. In the case of a possible dinuclear complex of (R)-8a, a
minor diastereomeric complex may not be so active such that
Ligand
Yield (%)b)
E.e. (%)c)
Confign.d)
(R)-8a
(R)-8b
(R)-8c
42
100
100
30
79
61
R
S
S
a) The reaction was carried out using 0.1 M solution of the substrate in MeOH under
the conditions: [Subst.]/[Rh]ꢃ500, 10 atm (H2), 50 °C, 24 h. b) Determined by 1H-
NMR analysis. c) Calculated on the basis of the specifical rotation value of the pure
(S)-enantiomer 14, [a]D20 ꢂ40.1° (cꢃ1.0, MeOH) [Vineyard B. D., Knowles W. S.,
Sabacky M. J., Bachman G. L., Weinkauff D. J., J. Am. Chem. Soc., 99, 5946—5952
(1977).]. d) Determined by the sign of the specific rotation.
good enantioselectivity (almost racemic) in the hydrogena-
tion of 9 and 11 under the same reaction conditions. Contrary
to these results, ruthenium complexes of both (R)-8a and
(R)-BINAP showed the same chirality induction (R) in the
hydrogenation of a b-keto ester, methyl 3-oxobutanoate
(98—99% ee (R)) and an a,b-unsaturated carboxylic acid,
tiglic acid (87—91% ee (R)).18) Next, the hydrogenation of
(Z)-a-acetamidocinnamic acid (13) was carried out with
0.2 mol% of the same rhodium complexes of (R)-8a—c
under 10 atm of hydrogen, and the hydrogenation product
was obtained quantitatively (except when using (R)-8a) in
30% ee (R) with (R)-8a, 79% ee (S) with (R)-8b, and 61% ee
(S) with (R)-8c, respectively. In this case, reversal of chilarity
induction was also observed between (R)-8a (R-selective)
and (R)-8b, c (S-selective). It should be noted that the R and
S selectivities (or vice versa) in hydrogenation of itaconic
acids and (Z)-a-acetamidocinnamic acid, respectively, are the
same in a stereochemical sense, though the R and S are su-
perficially different.
We previously reported the correlation between the chiral-
ity of bidentate ligands and the absolute configuration of the
products obtained by rhodium-catalyzed asymmetric hydro-

1448
Vol. 52, No. 12
Table 3. Asymmetric Hydrogenationa) of 2-Aminoacetophenone Hydro-
chloride Catalyzed by Rhodium(I) Complexes of Ligands (R)-8a—c
ple of 2 was obtained by recrystallization from ethanol as pale yellow nee-
dles, mp 69—70 °C. IR (KBr) cmꢁ1: 1520, 1370, 860. 1H-NMR d (CDCl3):
2.24 (3H, s, CH3), 2.38 (3H, s, CH3), 6.86 (1H, d, Jꢃ8 Hz), 7.81 (1H, d,
Jꢃ8 Hz). Anal. Calcd for C8H8INO2: C, 34.68; H, 2.91; N, 5.06. Found: C,
34.80; H, 2.75; N, 4.73.
3-Iodo-2,6-dimethylaniline Hydrochloride (3) A mixture of 2 (277 g,
1.0 mol), water (500 ml), conc. H2SO4 (20 g, 0.2 mol), and iron powder
(168 g, 3.0 mol) was mechanically stirred and heated under reflux for 48 h.
After the reaction mixture was cooled to 70 °C, toluene (400 ml) was added
and the mixture was stirred for 5 min at the same temperature. Precipitated
inorganic materials were filtered off and washed successively with toluene
(300 ml) and water (200 ml). The filtrates were combined and the organic
layer was separated. The aqueous layer was ice-cooled and aq. 1 M NaOH
was added until the pH of the solution became 9—10. The alkaline solution
was extracted with toluene. The toluene extract and the above organic layer
were combined and dried over MgSO4. Evaporation of the solvent in vacuo
left a crude aniline as a red oil. To a solution of the crude aniline in
methanol (500 ml) under ice-cooling was added a methanol solution contain-
ing 25% dry HCl enough for forming its hydrochloride as precipitates.
Evaporation of the solvent left a solid. After three-times dissolution in
methanol (300 ml) followed by evaporation, ether (700 ml) was added to the
residue and the mixture was stirred and heated under reflux for 1 h. After
cooling to room temperature, the precipitates were collected by filtration,
washed with ether (300 ml), and dried, affording 3 as a white solid (261 g,
92%) (pure enough to be used in the next step). An analytical sample of 3
was obtained by recrystallization from methanol as colorless needles, mp
185—186 °C. IR (KBr) cmꢁ1: 2770, 1970, 1590, 1515. 1H-NMR d (CDCl3):
2.40 (3H, s, CH3), 2.56 (3H, s, CH3), 6.97 (1H, d, Jꢃ8 Hz), 7.83 (1H, d,
Jꢃ8 Hz). Anal. Calcd for C8H11ClIN: C, 33.89; H, 3.91; N, 4.94. Found: C,
33.89; H, 3.54; N, 4.66.
Ligand Rhb) [15]/[Rh] atm/°C/d Yield (%)c) E.e. (%)d) Confign.d)
(R)-8a
(R)-8b
(R)-8c
RhN
RhN
Rhꢂ
500
500
1000
50/50/4
50/50/4
90/50/7
61
100
100
8
55
93
S
R
R
a) The reaction was carried out using 0.5 M solution of the substrate in MeOH in the
ꢁ
presence of triethylamine ([Et3N]/[Rh]ꢃ5). b) Rhꢂ: [Rh(nbd)2]ꢂClO4
,
RhN:
[Rh(nbd)Cl]2. c) Determined by 1H-NMR analysis. d) Determined by HPLC
analysis of the corresponding N-benzoyl derivative on a chiral column, Chiralcel OD, in
comparison with an authentic specimen.
the chirality of the product might be controlled by a major
diastereomer.
Furthermore, asymmetric hydrogenation of a functional-
ized ketone, 2-aminoacetophenone hydrochloride (15) was
carried out in the presence of 0.1—0.2 mol% of a cationic or
a neutral rhodium(I) complex of (R)-8a—c under the condi-
tions described in Table 3. In this case a similar trend was
observed for the chirality of the product 16 (8% ee (S)
with (R)-8a, 55% ee (R) with (R)-8b, 93% (R) with (R)-8c).
It is noteworthy that the ligand (R)-8c bearing both a
diphenylphosphino group and a dicyclohexylphosphino
group (electronically dissymmetrized phosphino groups)
showed a very high enantioselectivity. It has been demon-
strated that other backbone-type ligands, BCPMs bearing
both dissymmetrized phosphino groups, also show high
enantioselectivity in similar rhodium-catalyzed asymmetric
hydrogenation of various amino ketones.12,13)
3-Iodo-2,6-dimethylanisole (4) To a stirred and cooled (ꢁ5—0 °C) so-
lution of 3 (142 g, 0.5 mol) in acetic acid (250 ml) and 1,4-dioxane (250 ml)
was added dropwise isoamyl nitrite (64.5 g, 0.55 mol) during 1 h, and the
stirring was continued at 0 °C for another 1 h. Ice-cooled 1,4-dioxane
(250 ml) was added to the reaction mixture containing precipitates of the
corresponding diazonium salt formed. The precipitates were collected by fil-
tration and washed with cooled 1,4-dioxane (250 ml). [Caution: Since the di-
azonium salt is thermally quite unstable (explosive), it should be isolated
with much care and used immediately in the next step.] The obtained diazo-
nium salt was carefully added into cooled methanol (1000 ml), and the mix-
In summary, we have described the synthesis of enantiop- ture was stirred at 0 °C for 1 h and at room temperature for 1 h, and then
heated under reflux for 15 h. After cooling to room temperature, the reaction
mixture was concentrated in vacuo. Purification of the residue by silica gel
ure biphenyl ligands bearing one or two diphenylphosphino
group(s) and/or one or two dicyclohexylphosphino group(s),
and their application to rhodium(I)-catalyzed asymmetric hy-
drogenation of itaconic acid and its analogs, (Z)-a-acetami-
docinnamic acid, and 2-aminoacetophenone hydrochloride.
column chromatography (eluent: hexane/tolueneꢃ2/1) afforded 4 (97 g,
1
74%) as an oil. H-NMR d (CDCl3): 2.23 (3H, s, CH3), 2.39 (3H, s, CH3),
3.68 (3H, s, OCH3), 6.73 (1H, d, Jꢃ8 Hz), 7.47 (1H, d, Jꢃ8 Hz). Anal.
Calcd for C9H11IO: C, 41.25; H, 4.23. Found: C, 40.96; H, 4.20.
3,3ꢀ-Dimethoxy-2,2ꢀ,4,4ꢀ-tetramethyl-1,1ꢀ-biphenyl (5) A mixture of
4 (52.4 g, 0.5 mol), sulfolane (50 ml), and copper powder (20.4 g, 0.32 mol)
was mechanically stirred and heated at 220 °C for 15 h. After cooling to
70 °C, the reaction mixture was diluted with toluene (80 ml). Precipitates
were filtered by suction and washed with toluene (2ꢄ60 ml). The combined
filtrates were washed successively with water (2ꢄ100 ml) and saturat-
ed brine (60 ml), dried over MgSO4, and concentrated in vacuo. Purifica-
tion of the residue by silica gel column chromatography (eluent:
hexane/tolueneꢃ2/1) gave the corresponding biphenyl (5) (16.2 g, 60%) as a
white solid: mp 81—82 °C. 1H-NMR d (CDCl3): 1.98 (6H, s, 2ꢄCH3), 2.34
(6H, s, 2ꢄCH3), 3.75(6H, s, 2ꢄOCH3), 6.79 (2H, d, Jꢃ7.5 Hz), 7.03 (2H, d,
Jꢃ7.5 Hz). Anal. Calcd for C18H22O2: C, 79.96; H, 8.20. Found: C, 79.69; H,
8.16.
Experimental
Melting points were determined on a micro hot-stage apparatus and are
1
uncorrected. H-NMR spectra were recorded in a CDCl3 solution using a
JEOL JNM-EX 270 spectrometer. Chemical shift values are expressed in
ppm based on tetramethylsilane for 1H. IR spectra were measured on a
JASCO IR-810 spectrometer. Optical rotations were measured on a JASCO
DIP-140 digital polarimeter. CD spectra were recorded on a JASCO J-20
spectropolarimeter, and l values are expressed in nm. Column chromato-
graphic isolation was conducted using silica gel (Kieselgel 60, 70—230
mesh, Merck). Kieselgel 60 F254 aluminum plates (Merck) were employed
for TLC. In general, all organic reagents were used as purchased. THF was
distilled over sodium metal/benzophenone ketyl and used as peroxide-free.
Sulfolane and chlorobenzene were dried over Molecular Sieves 4A. For the
catalytic reactions, dehydrated methanol and dichloromethane were pur-
chased and used after being degassed and filled with argon.
2,4-Dimethyl-3-nitroiodobenzene (2) To a mixture of 2,6-dimethylni-
trobenzene (1) (151 g, 1.0 mol), acetic acid (1200 ml), and conc. H2SO4
(60 ml) were added iodine (102 g, 0.4 mol) and periodic acid dihydrate
(205 g, 0.90 mol), and the mixture was stirred and heated at 90 °C for 4 d.
After cooling to room temperature, the reaction mixture was diluted with
water (2000 ml) and extracted with dichloromethane (1000 ml). The extract
was ice-cooled and washed successively with aqueous 2 M NaOH
(3ꢄ500 ml), water (3ꢄ500 ml), and brine (300 ml), and dried over MgSO4.
Evaporation of the solvent in vacuo gave 2 as a yellowish solid (263 g, 95%).
The product was pure enough to be used in the next step. An analytical sam-
(RS)-6,6ꢀ-Dibromo-3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-tetramethyl-1,1ꢀ-biphenyl
((RS)-6) To a stirred solution of 5 (27.0 g, 0.10 mol) in acetic acid (100 ml)
was added bromine (12 ml, 0.24 mol) with cooling in an ice-bath, and the
mixture was stirred at room temperature for 3 h. To the reaction mixture was
added saturated sodium bisulfite solution until the orange color of the re-
maining bromine disappeared. White precipitates were collected by filtra-
tion, washed with water (300 ml), and dried in vacuo. Recrystallization from
2-propanol gave pure (RS)-6 (33.4 g, 78%) as colorless prisms: mp 122—
1
123 °C. H-NMR d (CDCl3): 1.93 (6H, s, 2ꢄCH3), 2.32 (6H, s, 2ꢄCH3),
3.73 (6H, s, 2ꢄOCH3), 7.36 (2H, s). Anal. Calcd for C18H20Br2O2: C, 50.49;
H, 4.71. Found: C, 50.85; H, 4.73.
(RS)-6,6ꢀ-Bis(diphenylphosphoryl)-3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-tetra-
methyl-1,1ꢀ-biphenyl ((RS)-7a) To a stirred solution of (RS)-6 (4.28 g,

December 2004
1449
0.01 mol) in THF (90 ml) at ꢁ70 °C under an argon atmosphere was added
dropwise a solution of tert-butyllithium (1.5 M in pentane, 28 ml, 0.042 mol),
and the mixture was stirred at the same temperature for 1 h. To the stirred
and cooled reaction mixture was added a solution of diphenylphosphinic
chloride (10.4 g, 0.044 mol) in THF (10 ml). After stirring for 1 h, the reac-
tion temperature was slowly raised up to room temperature during more than
3 h. The solvent was removed in vacuo and dichloromethane (100 ml) was
added to the residue. The dichloromethane solution was washed successively
with aq. 1 M NaOH solution (100 ml), water (3ꢄ50 ml), and saturate brine
(30 ml), dried over MgSO4, and concentrated in vacuo. Purification by silica
gel column chromatography (eluent: chloroform/acetoneꢃ9/1) followed by
recrystallization from methanol gave pure (RS)-7a (3.42 g, 51%) as color-
less leaflets: mp ꢅ320 °C. IR (KBr) cmꢁ1: 1195 (PꢃO). 1H-NMR d
(CDCl3): 1.29 (6H, s, 2ꢄCH3), 2.20 (6H, s, 2ꢄCH3), 3.59 (6H, s,
2ꢄOCH3), 6.90 (2H, d, JHCCPꢃ14 Hz), 7.26—7.51 (12H, m), 7.60—7.76
(8H, m). Anal. Calcd for C42H40O4P2·1/2H2O: C, 74.21; H, 6.08. Found: C,
73.88; H, 5.93.
acid (2.42 g, 47%): mp 216—217 °C (decomp.), [a]D21 ꢂ20.0° (cꢃ0.63,
1
CHCl3). IR (KBr) cmꢁ1: 1730 (CꢃO). H-NMR d (CDCl3): 1.28 (6H, s,
2ꢄCH3), 2.15 (6H, s, 2ꢄCH3), 3.52 (6H, s, 2ꢄOCH3), 5.02 (2H, br,
2ꢄCOOH), 5.81 (2H, s, 2ꢄCH), 6.90 (2H, d, JHCCPꢃ15 Hz), 7.19—7.56
(26H, m), 8.00—8.02 (4H, m). Anal. Calcd for C60H54O12P2: C, 70.03;
H, 5.29. Found: C, 70.46; H, 5.29. The complex was dissolved in
dichloromethane (150 ml) and stirred with aq. 1 M NaOH (50 ml) for 0.5 h.
The organic layer was separated, washed successively with water (3ꢄ50 ml)
and saturated brine (30 ml), and dried over MgSO4. Removal of the solvent
in vacuo left (R)-7a as a white solid (1.58 g, 47% from (RS)-7a). An analyti-
cal sample of (R)-7a was obtained by recrystallization from methanol/iso-
propyl ether as colorless prisms, mp 268—269 °C, [a]D20 ꢂ77.9° (cꢃ0.72,
1
CHCl3). IR (KBr) cmꢁ1: 1195 (PꢃO). H-NMR spectral data were in good
agreement with those of (RS)-7a. Anal. Calcd for C42H40O4P2·1/2CH3OH:
C, 74.33; H, 6.17. Found: C, 74.63; H, 6.09.
(R)-6,6ꢀ-Bis(dicyclohexylphosphoryl)-3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-tetra-
methyl-1,1ꢀ-biphenyl ((RS)-7b) A mixture of (RS)-7b (6.95 g, 0.01 mol)
and (2R,3R)-(ꢁ)-2,3-O,Oꢀ-dibenzoyltartaric acid monohydrate (4.14 g,
0.011 mol) in methanol (170 ml) was heated to form a clear solution, and
then left at room temperature. Precipitated crystals were collected by filtra-
tion and repeatedly recrystallized from methanol (80 ml), affording a pure
complex of (R)-7b and O,Oꢀ-dibenzoyltartaric acid (2.11 g, 40%): mp 213—
214 °C (decomp.), [a]D21 ꢁ63.5° (cꢃ1.02, CHCl3). IR (KBr) cmꢁ1: 1730
(RS)-6,6ꢀ-Bis(dicyclohexylphosphoryl)-3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-
tetramethyl-1,1ꢀ-biphenyl ((RS)-7b) To a stirred solution of (RS)-6
(4.28 g, 0.01 mol) in THF (90 ml) at ꢁ70 °C under an argon atmosphere was
added dropwise a solution of tert-butyllithium (1.5 M in pentane, 28 ml,
0.042 mol), and the mixture was stirred at the same temperature for 1 h. To
the stirred and cooled reaction mixture was added a solution of chlorodicy-
clohexylphosphine (8.38 g, 0.036 mol) in THF (10 ml). After stirring for 1 h,
the reaction temperature was slowly raised up to room temperature during
more than 6 h. The solvent was removed in vacuo and dichloromethane
(100 ml) and water (10 ml) were added to the residue. The two-phase solu-
tion was ice-cooled and a solution of 31% hydrogen peroxide (11 ml,
0.1 mol) was added. The mixture was stirred at room temperature for 3 h.
The organic layer was separated, washed successively with aq. 1 M NaOH
(50 ml), water (3ꢄ50 ml), and saturated brine (30 ml), dried over MgSO4,
and concentrated in vacuo. Purification of the residue by silica gel column
chromatography (eluent: toluene/ethyl acetateꢃ9/1) gave pure ((RS)-7b)
(4.38 g, 63%) as a white solid: mp 249—250 °C. IR (KBr) cmꢁ1: 1182
1
(CꢃO). H-NMR d (CDCl3): 0.82—1.90 (40H, m, 4ꢄ(CH2)5), 1.75 (6H, s,
2ꢄCH3), 2.24 (6H, s, 2ꢄCH3), 2.12—2.38 (4H, m, 4ꢄCH), 3.65 (6H, s,
2ꢄOCH3), 5.72 (2H, s, 2ꢄCH), 6.86 (2H, d, JHCCPꢃ12 Hz), 7.37—7.43
(4H, m), 7.52—7.57 (2H, m), 7.47 (2H, br, 2ꢄCOOH), 8.04—8.07 (4H, m).
Anal. Calcd for C60H78O12P2: C, 68.42; H, 7.46. Found: C, 68.37; H, 7.75.
The complex was dissolved in dichloromethane (150 ml) and stirred with aq.
1 M NaOH (50 ml) for 0.5 h. The organic layer was separated, washed succes-
sively with water (3ꢄ50 ml) and saturated brine (30 ml), and dried over
MgSO4. Removal of the solvent in vacuo left (R)-7b as a solid (1.32 g, 38%
from (RS)-7b): mp 99—100 °C, [a]D21 ꢁ42.6° (cꢃ1.10, CHCl3). IR (KBr)
cmꢁ1: 1182 (PꢃO). 1H-NMR spectral data were in good agreement with
those of (RS)-7b. Anal. Calcd for C42H64O4P2·1/2H2O: C, 71.66; H, 9.31.
Found: C, 71.43; H, 9.43.
1
(PꢃO). H-NMR d (CDCl3): 1.10—1.91 (40H, m), 1.81 (6H, s, 2ꢄCH3),
1.97—2.11 (4H, m, 4ꢄCH), 2.33 (6H, s, 2ꢄCH3), 3.74 (6H, s, 2ꢄOCH3),
6.99 (2H, d, JHCCPꢃ11 Hz). Anal. Calcd for C42H64O4P2: C, 72.59; H, 9.28.
Found: C, 73.07; H, 9.52.
(R)-6-Dicyclohexylphosphoryl-6ꢀ-diphenylphosphoryl-3,3ꢀ-di-
methoxy-2,2ꢀ,4,4ꢀ-tetramethyl-1,1ꢀ-biphenyl ((R)-7c) Optically pure (R)-
7c was isolated in 80% yield by preparative HPLC with a chiral column
(Chiralpack OT(ꢂ), Daicel) using hexane/2-propanol (4/1): [a]D20 ꢂ25.7°
(cꢃ1.00, CHCl3). 1H-NMR spectral data were in good agreement with those
of (RS)-7c.
(RS)-6-Dicyclohexylphosphoryl-6ꢀ-diphenylphosphoryl-3,3ꢀ-di-
methoxy-2,2ꢀ,4,4ꢀ-tetramethyl-1,1ꢀ-biphenyl ((RS)-7c) To a stirred solu-
tion of (RS)-6 (1.00 g, 2.3 mmol) in THF (30 ml) at ꢁ70 °C under an argon
atmosphere was added dropwise a solution of tert-butyllithium (1.5 M in
pentane, 2 ml, 3.5 mmol), and the mixture was stirred at the same tempera-
ture for 0.5 h. To the reaction mixture was added a solution of diphenylphos-
phinic chloride (0.51 g, 2.3 mmol) in THF (5 ml). After 0.5 h the reaction
temperature was gradually raised up to room temperature during more than
3 h. The reaction mixture was chilled again at ꢁ70 °C and a solution of tert-
butyllithium (1.5 M in pentane, 2 ml, 3.5 mmol) was added dropwise. After
0.5 h a solution of chlorodicyclohexylphosphine (0.55 g, 2.3 mmol) in THF
(5 ml) was added, and the mixture was stirred at ꢁ70 °C for 0.5 h. Then the
temperature was gradually raised up to room temperature during more than
6 h, and the solvent was removed by evaporation. The residue was dissolved
in a mixed solvent of dichloromethane (30 ml) and water (5 ml). To the solu-
tion with ice-cooling was added 31% hydrogen peroxide (5 ml, 45 mmol),
and the mixture was stirred at room temperature for 3 h. The organic layer
was separated, washed successively with aq. 1 M NaOH (10 ml), water
(3ꢄ20 ml), and saturated brine (20 ml), dried over MgSO4, and concentrated
in vacuo. Isolation by silica gel column chromatography (eluent: ethyl ac-
etate) gave pure (RS)-7c in 17% yield as a white solid: mp 225—227 °C. IR
(R)-6,6ꢀ-Bis(diphenylphosphino)-3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-tetra-
methyl-1,1ꢀ-biphenyl ((R)-BIMOP, (R)-8a) In a sealed tube were placed
(R)-7a (671 mg, 1.00 mmol), degassed chlorobenzene (30 ml), and triethyl-
amine (4.86 g, 48 mg), and cooled in an ice bath under argon. To the mixture
was added trichlorosilane (5.42 g, 40 mmol). After the tube was sealed, the
whole mixture was stirred and heated at a bath temperature of 140 °C for
5 h. The reaction mixture was ice-cooled, and a degassed solution of aq. 30%
NaOH was added slowly enough to decompose white precipitates. The mix-
ture was stirred and heated at 70 °C for 0.5 h under argon, and cooled to
room temperature. After chlorobenzene (70 ml) was added, the organic layer
was separated, washed successively with water (3ꢄ50 ml) and saturated
brine (30 ml), dried over MgSO4, and concentrated in vacuo. Purification of
the residue by silica gel column chromatography (eluent: chloroform) gave
pure (R)-8a (428 mg, 67%) as a white solid: mp 265—266 °C, [a]D19 ꢂ37.4°
(cꢃ0.50, CHCl3); CD l nm: 240 (neg. max.), 278 (pos. max.) (cꢃ0.002,
1
EtOH/CH2Cl2ꢃ22/3). H-NMR d (CDCl3): 1.24 (6H, s, 2ꢄCH3), 2.21 (6H,
s, 2ꢄCH3), 3.48 (6H, s, 2ꢄOCH3), 6.84 (2H, s, 2ꢄCH), 7.24—7.27 (20H,
m, 4ꢄC6H5). Anal. Calcd for C42H40O2P2: C, 78.98; H, 6.31. Found: C,
78.73; H, 6.25.
1
(KBr) cmꢁ1: 1190 (PꢃO). H-NMR d (CDCl3): 1.36 (3H, s, CH3), 1.51—
2.07 (22H, m), 1.85 (3H, s, CH3), 2.33 (3H, s, CH3), 2.35 (3H, s, CH3), 3.68
(3H, s, OCH3), 3.75 (3H, s, OCH3), 6.98 (1H, d, JHCCPꢃ13 Hz), 7.03 (1H, d,
Jꢃ8 Hz), 7.58—7.71 (4H, m), 7.26—7.47 (6H, m). Anal. Calcd for
C42H52O4P2: C, 73.88; H, 7.68. Found: C, 73.57; H, 8.05.
(R)-6,6ꢀ-Bis(dicyclohexylphosphino)-3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-tetra-
methyl-1,1ꢀ-biphenyl ((R)-Cy-BIMOP, (R)-8b) Reduction of (R)-7b (695
mg, 1.00 mmol) with trichlorosilane was carried out in a similar manner as
described for the preparation of (R)-8a. Purification by silica gel column
chromatography (eluent: toluene) gave pure (R)-8b (424 mg, 64%) as a
white solid: mp 185—186 °C, [a]D19 ꢁ56.5° (cꢃ1.16, CHCl3); CD l nm:
(R)-6,6ꢀ-Bis(diphenylphosphoryl)-3,3ꢀ-dimethoxy-2,2ꢀ,4,4ꢀ-tetra-
methyl-1,1ꢀ-biphenyl ((R)-7a) A mixture of (RS)-7a (6.71 g, 0.01 mol)
and (2R,3R)-(ꢁ)-2,3-O,Oꢀ-dibenzoyltartaric acid monohydrate (4.14 g,
0.011 mol) in chloroform (10 ml) was heated to form a clear solution. To the
hot solution was added hot ethyl acetate (20 ml), and the solution was cooled
to room temperature. After addition of a small amount of pure (R)-7a as a
seed, additional hot ethyl acetate (10 ml) was added, and the whole solution
was allowed to stand for 15 h. Precipitated crystals were collected by filtra-
tion, and repeatedly recrystallized from hot chloroform (5 ml) and ethyl ac-
etate (10 ml), affording a pure complex of (R)-7a and O,Oꢀ-dibenzoyltartaric
1
230 (neg. max.), 266 (pos. max.) (cꢃ0.002, EtOH). H-NMR d (CDCl3):
0.90—1.90 (44H, m, 4ꢄC6H11), 1.80 (6H, s, 2ꢄCH3), 2.35 (6H, s, 2ꢄCH3),
3.72 (6H, s, 2ꢄOCH3), 7.09 (2H, s, 2ꢄCH). Anal. Calcd for C42H64O2P2: C,
76.10; H, 9.73. Found: C, 76.04; H, 9.59.
(R)-6-Dicyclohexylphosphino-6ꢀ-diphenylphosphino-3,3ꢀ-dimethoxy-
2,2ꢀ,4,4ꢀ-tetramethyl-1,1ꢀ-biphenyl ((R)-MOC-BIMOP, (R)-8c) Reduc-
tion of (R)-7c (110 mg, 0.16 mmol) with trichlorosilane (800 mg, 6.5 mmol)

1450
Vol. 52, No. 12
was carried out in a similar manner as described for the preparation of (R)-
8a. Purification by silica gel column chromatography (eluent: toluene) gave
pure (R)-8c (72 mg, 67%) as a white solid: mp 226—229 °C, [a]D20 ꢁ2.1°
(cꢃ1.00, benzene); CD l nm: 238 (neg. max.), 274 (pos. max.) (cꢃ0.001,
7) Morimoto T., Takahashi H., Fujii M., Chiba M., Achiwa K., Chem.
Lett., 1986, 2061—2064 (1986).
8) Chiba M., Takahashi Hit., Takahashi His., Morimoto T., Achiwa K.,
Tetrahedron Lett., 28, 3675—3678 (1987).
9) Takahashi H., Morimoto T., Achiwa K., J. Synth. Org. Chem., Jpn., 48,
29—42 (1990).
1
EtOH). H-NMR d (CDCl3): 1.11—2.12 (22H, m, 2ꢄC6H11), 1.56 (3H, s,
CH3), 1.81 (3H, s, CH3), 2.23 (3H, s, CH3), 2.35 (3H, s, CH3), 3.52 (3H, s,
OCH3), 3.71 (3H, s, OCH3), 6.86 (1H, s, CH), 6.88 (1H, s, CH), 7.18—7.29 10) Inoguchi K., Sakuraba S., Achiwa K., Synlett, 1992, 169—178 (1992).
(10H, m, 2ꢄC6H5). Anal. Calcd for C42H52O2P2: C, 77.51; H, 8.05. Found: 11) Takahashi H., Hattori M., Chiba M., Morimoto T., Achiwa K., Tetra-
C, 77.04; H, 8.23.
hedron Lett., 27, 4477—4480 (1986).
Catalytic Asymmetric Hydrogenation of Itaconic Acid (9) and Its 12) Takeda H., Tachinami T., Aburatani M., Takahashi H., Morimoto T.,
Analog (11) A rhodium(I) complex catalyst solution was prepared by mix-
Achiwa K., Tetrahedron Lett., 30, 363—366 (1989).
13) Sakuraba S., Takahashi H., Takeda H., Achiwa K., Chem. Pharm.
Bull., 43, 738—745 (1995).
ꢁ
ing bis(norbornadiene)rhodium(I) perchlorate [Rh(nbd)2]ꢂClO4 (0.8 mg,
0.002 mmol) and a chiral ligand (R)-8a—c (0.0022 mmol) in degassed
dichloromethane (2 ml) at room temperature for 3 h under an argon atmos- 14) Morimoto T., Chiba M., Achiwa K., Tetrahedron Lett., 29, 4755—
phere followed by concentration in vacuo and dissolution in methanol (2 ml). 4758 (1988).
To a glass tube equipped with a stirring bar were placed itaconic acid (9) 15) Morimoto T., Chiba M., Achiwa K., Tetrahedron Lett., 30, 735—738
(260 mg, 2.0 mmol) (or a-piperonylidenesuccinic acid monomethyl ester (1989).
(11) (528 mg, 2.0 mmol)), degassed methanol (2 ml), and the above catalyst 16) Morimoto T., Chiba M., Achiwa K., Chem. Pharm. Bull., 41, 1149—
solution. The hydrogenation was carried out with stirring in an autoclave
under 5 atm of hydrogen at 30 °C for 20 h. According to the procedures re-
ported previously,15,16) the hydrogenation product 10 or 12 was isolated and
the enantiomeric excess was determined. All the results are summarized in
Table 1.
1156 (1993).
17) Yoshikawa K., Inoguchi K., Morimoto T., Achiwa K., Heterocycles,
31, 1413—1416 (1990).
18) Yamamoto N., Murata M., Morimoto T., Achiwa K., Chem. Pharm.
Bull., 39, 1085—1087 (1991).
Catalytic Asymmetric Hydrogenation of (Z)-a-Acetamidocinnamic 19) Murata M., Morimoto T., Achiwa K., Synlett, 1991, 827—829 (1991).
Acid (13) The hydrogenation of 13 was carried out in a similar manner as
described above except hydrogen pressure (10 atm), temperature (50 °C),
and reaction time (24 h). According to the procedure reported previ-
ously,32,33) the hydrogenation product 14 was isolated and the enantiomeric
excess was determined. All the results are summarized in Table 2.
20) Murata M., Yamamoto N., Yoshikawa K., Morimoto T., Achiwa K.,
Chem. Pharm. Bull., 39, 2757—2769 (1991).
21) Yoshikawa K., Yamamoto N., Murata M., Awano K., Morimoto T.,
Achiwa K., Tetrahedron: Asymmetry, 3, 13—16 (1992).
22) Schmid R., Cereghetti M., Heiser B., Schönholzer P., Hansen H.-J.,
Helv. Chim. Acta, 71, 897—928 (1988).
23) Miyashita A., Karino H., Shimamura J., Chiba T., Nagano K., Nohira
H., Takaya H., Chem. Lett., 1989, 1849—1852 (1989).
24) Sakuraba S., Morimoto T., Achiwa K., Tetrahedron: Asymmetry, 2,
597—600 (1991).
Catalytic Asymmetric Hydrogenation of 2-Aminoacetophenone Hy-
drochloride (15) The hydrogenation of 15 (685 mg, 5.0 mmol) was carried
out in methanol (10 ml) with a cationic rhodium(I) catalyst prepared from
ꢁ
[Rh(nbd)2]ꢂClO4 (1.9 mg, 0.005 mmol) and (R)-8c (3.6 mg, 0.0055 mmol)
or a neutral catalyst prepared in situ from [Rh(nbd)Cl]2 (0.005 mmol) and
(R)-8a, b (0.011 mmol) in the presence of triethylamine (0.025 mmol) under 25) Saitoh A., Achiwa K., Tanaka K., Morimoto T., J. Org. Chem., 65,
the conditions described in Table 3. According to the procedure reported
previously,32,33) the hydrogenation product 16 was isolated and the enan-
tiomeric excess was determined by HPLC after conversion to its N-benzoyl
derivative. All the results are summarized in Table 3.
4227—4240 (2000).
26) Miyashita A., Yasuda A., Takaya H., Toriumi K., Ito T., Souchi T.,
Noyori R., J. Am. Chem. Soc., 102, 7932—7934 (1980).
27) Ozawa F., Kubo A., Matsumoto Y., Hayashi T., Nishioka E., Yanagi K.,
Moriguchi K., Organometallics, 12, 4188—4196 (1993).
28) Chiba T., Miyashita A., Nohira H., Takaya H., Tetrahedron Lett., 32,
4745—4748 (1991).
29) Miyashita A., Takaya H., Souchi T., Noyori R., Tetrahedron, 40,
1245—1253 (1984).
30) Morimoto T., Nakajima N., Achiwa K., Tetrahedron: Asymmetry, 6,
23—26 (1995).
31) Landis C. R., Halpern J., J. Am. Chem. Soc., 109, 1746—1754 (1987).
32) Inoguchi K., Fujie N., Yoshikawa K., Achiwa K., Chem. Pharm. Bull.,
40, 2921—2926 (1992).
33) Morimoto T., Yamazaki A., Achiwa K., Chem. Pharm. Bull., 52,
1367—1371 (2004).
References
1) Seyden-Penne J., “Chiral Auxiliaries and Asymmetric Synthesis,” John
Wiley & Sons, Inc., New York, 1995.
2) Brunner H., “Topics in Stereochemistry,” Vol. 18, ed. by Eliel E. L.,
Wilen S. H., John Wiley & Sons, New York, 1988, pp. 129—247.
3) Brown J. M., “Comprehensive Asymmetric Catalysis,” Vol. 1, ed. by
Jacobsen E. N., Pfaltz A., Yamamoto H., Springer, Berlin, 1999, pp.
121—182.
4) Noyori R., Angew. Chem., Int. Ed., 41, 2008—2022 (2002).
5) Knowles W. S., Angew. Chem., Int. Ed., 41, 1998—2007 (2002).
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(2002).