Transformations of the chiral diphosphine rhodium catalyst [(1,5-COD)Rh(-)R,R-DIOP]+CF3SO3- under conditions of hydrogenation

Article abstract of DOI:10.1023/A:1014338330802

Transformation products of the cationic rhodium complex [(1,5-COD)Rh(-)R,R-DIOP]⁺CF₃SO₃¹ (1) (COD is cycloocta-1,5-diene and DIOP is (±)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane), whic

Full text of DOI:10.1023/A:1014338330802

Russian Chemical Bulletin, International Edition, Vol. 50, No. 10, pp. 1855—1859, October, 2001
1855
Organic and Organometallic Chemistry
Transformations of the chiral diphosphine rhodium catalyst
+
3 3
–
[
(1,5-COD)Rh(–)R,R-DIOP] CF SO under conditions of hydrogenation
L. O. Nindakova and B. A. Shainyan
A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences,
1 ul. Favorskogo, 664033 Irkutsk, Russian Federation.
Fax: +7 (395 2) 39 6046. E-mail: bagrat@irioch.irk.ru
Transformation
(1,5-COD)Rh(–)R,R-DIOP] CF SO
products
of
the
(1) (COD is cycloocta-1,5-diene and DIOP is
±)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane), which were ob-
cationic
rhodium
complex
+
–
[
(
3
3
tained in its reactions with molecular hydrogen, base (NEt ), and solvents in the absence of a
3
substrate, were investigated by 1H and
31
P NMR spectroscopy. The solvate complexes
(Solv) Rh(–)R,R-DIOP] CF SO3–, which were generated from complex 1 in its reaction
+
[
2
3
with molecular hydrogen, underwent destruction of the diphosphine ligand with elimination of
benzene and were subjected to oxidation by traces of moisture and oxygen to form the DIOP
I
dioxide complex with Rh . In the absence of hydrogen, complex 1 in solutions produced the
diphosphine dioxide rhodium(I) complex and mono- and binuclear rhodium(I) solvate com-
plexes. The scheme of deactivation of the complex in the absence of the substrate was
proposed. The catalytic activity of the solvate complexes [(ArH)Rh(–)R,R-DIOP] CF SO3–,
+
3
which contain benzene, p-xylene, and mesitylene in the coordination sphere, was studied in
hydrogenation of Z-α-acetamidocinnamic acid.
Key words: chiral rhodium complexes, catalysts of enantioselective hydrogenation, 31P NMR
spectroscopy.
Recently, we have demonstrated1 that the activity
ible. Thus, the activity of the complex varied over a wide
of
[
the
cationic
triflate
rhodium
complex
range from 6—20 to 6000—6650 mol H (g-at. Rh h)–1
;
2
+
–
(1,5-COD)Rh(–)R,R-DIOP] CF SO
(1) (COD is
however, the optical yields were rather high (84—94%).
3
3
cycloocta-1,5-diene) in hydrogenation of α-acetamido-
cinnamic acid with molecular hydrogen is 2—3 times
higher than the activities of the rhodium complexes2
containing (±)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-
bis(diphenylphosphino)butane (DIOP). The optical yield
of N-acetyl-R(–)-phenylalanine reached 94%, the con-
version of the substrate being 100%. The kinetic results
for various specimens of complex 1 were poorly reproduc-
To gain an understanding of the nature of the pro-
cesses, which occur in the chiral catalytic system upon
losing its catalytic activity, there was a need to study the
transformation products of complex 1 in the reactions
with other components of the system, viz., with the
—4
solvent, base (NEt ), and hydrogen in the absence of
3
the substrate. In the present study, we examined this
problem by H and 31P NMR spectroscopy.
1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1772—1776, October, 2001.
066-5285/01/5010-1855 $25.00 © 2001 Plenum Publishing Corporation
1

1
856
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
Nindakova and Shainyan
Scheme 1
H
H
Ph2
Me Me
H
OMe
P
Ph P
O
O
2
MeOH
Me
Me
MeOH, Et O
2
Rh
H
OMe
Rh
3 +
O
O
+ 6
i
Ph P
2
P
Ph2
H
H
4
CF SO
3 3
1
Ph P
PPh2
MeOH, ArH
H2
2
+
ArH
O
Rh
Rh
Ph Ph
O
R1
H
H
Et O
HOR
2
P
O
O
Ph P
Me
Me
2
5
Rh
Rh
O
R2
P
O
Ph P
R4
2
R3
Ph Ph
2
a—c
3
Ph P
2
Rh
a: R1 = R2 = R3 = R4 = H
b: R1 = R3 = Me, R2 = R4 = H
c: R1 = R2 = R4 = Me, R3 = H
Solv
Ph P
2
6
i. 6 months.
The 31P NMR spectrum, which was measured af-
ter treatment of a benzene—methanolic solution
and diphenylphosphide or phenylphosphinidene rhodium
complexes. Apparently, the broadened signal observed
in the 31P NMR spectrum at δ31P 22—28 belongs to the
last-mentioned complexes.
The addition of one equivalent of benzene to a
methanolic solution, which contained complex 4 pre-
pared in situ, led to a sharp decrease in the intensity of
(
C D —CD OD, 1 : 10) of complex 1 with molecular
6 6 3
hydrogen at –20 °C, had a doublet signal of the initial
complex (δ
11.6, 1JRh—P = 144.0 Hz) and a low-
31P
intensity singlet (δ
31P
33.5) along with a doublet at
1
δ
31P
28.0 ( J
= 200.5 Hz) belonging to solvate
Rh—P
complex 2a containing the benzene-d6 molecule in the
the signal at δ
intensity doublet at δ
The parameters of the latter are similar to those of
complex 2a, i.e., the conversion 4→2a takes place (see
31P
43.9 and the appearance of a low-
coordination sphere (Scheme 1). A complex of the same
31P
= 29.8 with 1JRh—P = 203.0 Hz.
5
type has been detected previously in the reaction of
+
[
(1,5-COD)Rh(–)R,R-DIOP] PPh4– with molecular
hydrogen. Heating of the resulting solution to room
temperature and treatment with H2 for 30 min led to
the disappearance of the signals of complexes 1 and 2a
and to a sharp increase in the intensity of the signal of
complex 3 at δ
31P
33.5.
In the 3 P NMR spectrum measured in CD OD, the
1
3
signal of complex 1 was observed at δ
31P
13.2 with the
1
c
constant JRh—P = 146.3 Hz. After the reaction of the
above-mentioned solution with H2 for 10 min, the
NMR spectrum showed a signal of the initial complex
as well as a doublet signal of complex 4 at δ
31P
= 43.9
31P
35.6
1
(
(
JRh—P = 200.0 Hz), a singlet of complex 3 at δ
it is shifted downfield by 2.1 ppm relative to the signal
b
a
in the benzene—methanolic solution), and a broadened
signal at δ 22—28. At the same time, signals for the
31P
1
protons of coordinated cyclooctadiene in the H NMR
spectrum disappeared, the singlet signal (at δ 1.48) of
cyclooctane appeared and then became more intense,
and the intensity of the signal of benzene at δ 7.3
gradually increased, the intensities of the signals of the
phenyl groups of DIOP decreasing in parallel. These
facts are indicative of destruction of the diphenyl-
phosphine fragment of DIOP to form benzene (Fig. 1)
7
.8
7.6
7.4
7.2
δ
Fig. 1. 1H NMR spectra of complex 1 in CD OD without
treatment with H2 (a) and after treatment with H2 for 15 (b)
and 40 min (c).
3

Transformations of chiral rhodium catalyst
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
1857
Scheme 1). In addition, the intensity of the singlet of
signal is shifted upfield as the ionizing ability of the
solvent decreases.
complex 3 at δ
31P
35.6 sharply increased. This signal
remained as the only signal after the additional treat-
ment of the reaction mixture with hydrogen, a gray
precipitate of metallic rhodium being formed. The addi-
tion of a threefold molar excess of triethylamine had no
effect on the spectral characteristics of complex 1. In
this case, the reaction with hydrogen resulted in the
same sequence of transformations with the only differ-
ence that the precipitate formed more rapidly. Complex
When one equivalent of itaconic acid and five equiva-
lents of NEt were added to the methanolic solution of
3
complex 1 prior to its treatment with hydrogen, the
31P NMR spectrum contained signals of complexes 1,
3, and 4 with the intensity ratio of 20 : 1 : 4 and the
1H NMR spectrum showed signals belonging to the
hydrogenation product of itaconic acid, viz., to
α-metylsuccinic acid. When the mixture was subse-
quently purged with hydrogen, the signal of complex 4
disappeared and the signals of complexes 1 and 3 were
observed with the intensity ratio of ∼ 3 : 1. Hence it
follows that the intermediate, which was generated from
methanol complex 4 and itaconic acid, reacted with
hydrogen more rapidly than the initial compound. Con-
sequently, the maximum activity of the catalyst can be
achieved when the initial complex is most completely
converted into the methanol complex prior to interac-
tion of the catalyst with the substrate.
3
was isolated from the solution as a yellow-brown
powder whose 31P NMR spectrum in CDCl3 has a
singlet at δ 34.4. In the NMR spectrum in DMSO-d6,
31P
this signal is observed at δ
31P
28.3.
To confirm the validity of the assignment of the
signals at δ 35.6 (in CD OD) and 33.4 (in a
31P
3
CD OD—C D mixture) to complex 3, independent
3
6
6
oxidation of DIOP was carried out. After oxidation of
DIOP under mild conditions with two equivalents of
hydrogen peroxide in acetone for 15 min, the 31P NMR
spectrum of the reaction product in CDCl3 had two
Deactivation of the catalyst also occurred upon pro-
longed storage of complex 1 in solution. Red-brown
crystals of the artificially deactivated catalyst were iso-
lated after prolonged storage of complex 1 in a mixture
of methanol and diethyl ether. The 31P NMR spectrum
singlets at δ
31P
31.18 and 30.16 with the intensity ratio
of 4 : 1. After oxidation under more drastic conditions
(
δ
heating with an excess of H O ), an additional signal at
2 2
34.13 appeared in the 31P NMR spectrum. Appar-
31P
ently, the presence of several 31P signals at δ31P 30—35
is indicative of the formation of products in different
oxidation states, such as phosphinite 7, phosphine oxide
(Fig. 2) has the main singlet at δ
31P
34.7 corresponding
to complex 3 along with two doublets of equal intensi-
ties at δ
36.1 (1JP—Rh = 123.8 Hz) and 46.7
31P
, and phosphinate 9.6—8 Phosphinate 9 (δ31P = 34.13)
( J
1
= 121.0 Hz) and two doublets of doublets of
P—Rh
8
1
was not obtained upon oxidation under mild conditions.
equal intensities at δ
31P
24.9 ( J
= 136.6 Hz,
= 135.5 Hz,
P—Rh
2
1
JP—P = 41.6 Hz) and 62.6 ( J
P—Rh
O
1JP—P = 40.1 Hz). In our opinion, the former pair of
the doublets, which is characterized by the non-
equivalence of the phosphorus nuclei and the absence of
P—P splitting, can be assigned to rhodium complex 5
because the low 1JP—Rh constants are typical of bi-
nuclear bridged complexes,11 whereas the high-field
H
H
O
O
PPh2
PPh2
PPh2
O
O
O
O
Me
Me
Me
Me
PPh2
O
H
H
7
8
signal at δ
31P
36.1 belongs, apparently, to the phospho-
O
rus atom in the trans-axial position with respect to the
ether oxygen atom. The signals of the magnetically
H
O
O
PPh2
O
O
Me
Me
nonequivalent phosphorus nuclei at δ
31P
24.9 and 62.6
1
with the splitting constant JP—P ≈ 40 Hz can be as-
signed to the planar-square rhodium complex in which
two phosphorus atoms of DIOP are in the trans posi-
tions with respect to two ligands of different nature
(complex 6). Complex 6, which is analogous to that
suggested previously,5 can be generated in an insignifi-
cant amount due to partial hydrogenation of cyclo-
octadiene; the solvent MeOH can also serve as a donor
of hydrogen. It should be noted that the rate of hydro-
genation of α-acetamidocinnamic acid on the deacti-
vated catalyst was substantially lower than that on com-
plex 1 although the optical yields remained high.1 This
is, apparently, associated with low concentrations of
catalytically active complexes 5 and 6 in the specimen.
Hence, deactivation of complex 1 in the reaction
with hydrogen in the absence of the substrate is a
consequence of the formation of a coordinatively unsat-
PPh2
H
O
9
The chemical shifts of the protons of the CH, CH2,
and CH3 groups (see the Experimental section) in the
1H NMR spectra of dioxide 8 and DIOP differ substan-
tially, the signal for the methyl protons of the
isopropylidene group is identical with that reported in
the literature.9,10 Hence, the signal at δ31P 35.6 belongs,
apparently, to phosphine oxide complex 3, which is in
agreement with the low-field shift of this signal relative
to the signal of phosphine oxide 8. It should also be
noted that the position of the signal of complex 3 in the
31P NMR spectrum depends noticeably on the solvent.
This signal is observed at δ
31P
35.6 (MeOH), 33.5
MeOH—C D , 10 : 1), and 28.2 (DMSO-d ), i.e., the
6 6 6
(

1
858
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
Nindakova and Shainyan
d
c
b
a
6
4
60
56
52
48
44
40
36
32
28
24
20
16
12
δ
Fig. 2. 31P NMR spectra of mixtures of complexes 3, 5, and 6 (the deactivated catalyst) (a) and solvate complexes containing
benzene 2a (b), p-xylene 2b (c), and mesitylene 2c (d).
urated species after hydrogenation of cyclooctadiene. In
the absence of the strong ligand (COD), solvate com-
plexes 2a and 4 were formed through coordination of
the solvent molecules, and the phenyl groups of DIOP
were coordinated to the rhodium atom followed by the
destruction of the diphenylphosphine fragment due to
the ortho-addition of the phenyl group and its reduction
to benzene. Analogous processes have been described
Table 1. Hydrogenation of (Z)-α-acetamidocinnamic acid on
complex 1a
Arene
Conversion
%)
Optical
yield (%)
Rspecb
(
Benzene
p-Xylene
Mesitylene
100
100
17.3
84.3±0.5
88.4±0.5
77.8±1.3
2220
2340
346
previously for phosphine complexes of transition met-
a
als.12,13 Due to the low (∼ 0.001 mol L–1) concentration
Conditions: an arene—MeOH solvent mixture (1
= 1 atm, t = 20 °C. b Rspec/mol H2 (g-at. Rh h)–1
Í2
: 3),
p
.
of the catalyst, traces of moisture and oxygen, which
were accumulated in the solution upon purging with
hydrogen over a long period, were sufficiently large for
of the reaction with hydrogen. Apparently, the forma-
tion of the solvate p-xylene complex, which is more
stable than the benzene complex, leads to the shift of
the ratio of the diastereomeric olefinic complexes to-
ward the minor isomer resulting in an increase in the
proportion of the R(–) enantiomer of the product.
The initial specific rate of hydrogenation of (Z)-α-acet-
amidocinnamic acid in a mesitylene—MeOH mixture is
an order of magnitude lower than in those with benzene
and p-xylene, and hydrogenation ceased when the con-
version of the substrate reached ∼ 17%; the optical yield
of R(–)-N-acetylphenylalanine was only 78%.
the water and O molecules to be coordinated to the Rh
2
atom and, as a consequence, for the formation of
complex 3 and phosphine oxide 8.
One would expect that an appropriate solvent, which
produces a more stable complex of type 2, will allow
one to improve the characteristics of the catalyst. We
studied hydrogenation of (Z)-α-acetamidocinnamic acid
in the presence of complex 1 in mixtures of solvents
containing such arenes as benzene, p-xylene, and mesi-
tylene (Table 1).
The enhancement of the optical yield in hydrogena-
tion in p-xylene does not contradict Halpern's mecha-
nism14 according to which one of the diastereomeric
olefinic intermediates is kinetically selected at the stage
With the aim of obtaining additional information on
the nature of the complexes, we studied the re-
action of complex 1 with molecular hydrogen in

Transformations of chiral rhodium catalyst
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
1859
p-xylene—CD OD and mesitylene—CD OD mixtures
was isolated and dried and a finely crystalline white compound
3
3
was obtained, m.p. 182—188 °C. 1H NMR (CDCl ), δ: 1.14 (s,
3
1
(
the arene content was 5%) by P NMR spectroscopy.
3
6
H, CH ); 2.63 and 2.87 (both m, 2 H each, CH ); 4.18 (m,
3 2
The spectra of the solvate complexes with various arenes
as well as the spectrum of the deactivated catalyst are
shown in Fig. 2. In the case of p-xylene, the 31P NMR
spectrum has signals of complexes 1 and 3 as well as a
2
H, CH); 7.43—7.78 (m, 20 H, Ph). For comparison, the
1
H NMR spectrum of DIOP has signals of these groups at
δ 1.33, 2.30, 2.42, 3.89, and 7.24—7.43, respectively. 31P NMR
CDCl ), δ: 31.18, 30.16. IR (KBr pellets), ν/cm–1: 530, 690,
(
3
doublet at δ
31P
31.01 (1JRh—P = 201.2 Hz) belonging to
715, 1108, 1180, 1435.
complex 2b, which contains the p-xylene molecule in
the coordination sphere. In the case of mesitylene,
arene complex 2ñ was formed along with complexes 1,
The specimen of the artificially deactivated catalyst was
obtained upon storage of complex 1 in an MeOH—Et2O sol-
vent mixture (1 : 1) for 6 months.
The substrates were hydrogenated with intense stirring in a
glass temperature-controlled long-necked reactor connected
with a manometer and a system for supplying hydrogen. The
3
, and 4. Complex 2ñ gives a doublet signal at δ
31P
30.3
1
with JRh—P = 204.2 Hz. Judging from the positions of
the signals of the solvate complexes, the bond between
p-xylene and the rhodium atom is the weakest one,
which facilitates coordination of the substrate and the
interconversion of olefinic intermediates resulting in the
maximum optical yield (see Table 1).
gas was purified and dried according to a standard procedure.
The products were isolated and analyzed as described previously.1
References
Hence, the nature of the arene ligand exerts a sig-
nificant effect on the activity and enantioselectivity of
complex 1 in hydrogenation of prochiral substrates.
1. L. O. Nindakova, B. A. Shainyan, A. I. Albanov, and M. V.
Ustinov, Zh. Org. Khim., 2000, 35, 1660 [Russ. J. Org.
Chem., 2000, 35 (Engl. Transl.)].
2
3
4
. H. B. Kagan and T.-P. Dang, J. Am. Chem. Soc., 1972,
4, 6429.
. D. Sinou and H. B. Kagan, J. Organomet. Chem., 1976,
14, 325.
. J. Reiss and J. Hetflejs, Coll. Czech. Chem. Commun.,
986, 51, 340.
9
Experimental
1
The IR spectra were recorded on a Specord IR 75 instru-
ment. The NMR spectra were measured on a Bruker DPX-400
1
spectrometer operating at 400 (1H) and 162 MHz (31P) in the
5. J. M. Brown, P. A. Chaloner, A. G. Kent, B. A. Murrer,
P. N. Nicholson, D. Parker, and P. J. Sidebottom,
J. Organomet. Chem., 1981, 216, 263.
6. A. J. Kirby and S. G. Warren, The Organic Chemistry of
Phosphorus (Reactions in Organic Chemistry), Elsevier, New
York, 1967.
1
range from +15 to –25 ppm ( H) and from +300 to –150 ppm
(31
P) with HMDS as the internal standard. The chemical shifts
are given relative to Me Si ( H) and H PO (31P). The specific
1
4
3
4
optical rotation of the products was determined on a Polamat A
instrument at 546 nm and was scaled to the wavelength of 58 m
using the coefficient of 1.17543. The solvents used in the
experiments were thoroughly purified and degassed. All synthe-
ses were carried out under an atmosphere of argon with the use
of a system combining the supply of the purified gas and the
possibility to produce rarefaction.
Complex 1 was synthesized according to a procedure simi-
lar to that described15 for the preparation of the perchlorate
complex [Rh(S,S-chiraphos)(NBD)]+ClO4–.
7
. D. Purdela and R. Vylchanu, Khimiya organicheskikh
soedinenii fosfora [Chemistry of Organophosphorus Compounds],
Khimiya, Moscow, 1972, 752 pp. (in Russian).
8
. 31P Nuclear Magnetic Resonance, in Topics in Phosphorus
Chemistry, Vol. 5, Eds. M. Grayson and E. J. Griffith, Wiley
and Sons, New York—London—Sidney, 1967, 489 p.
9. K. Yamamoto and S.-U. Rehman, Chem. Lett., 1984, 1603.
10. E. Dunach and H. B. Kagan, Tetrahedron. Lett., 1985,
2
6, 2649.
Complex 3 was prepared by purging hydrogen through a
solution of complex 1 (26.3 mg, 3.2•10–5 mol) in methanol
11. A. R. Sanger, J. Chem. Soc., Chem. Commun., 1975, 893.
1
1
2. G. W. Parshall, Acc. Chem. Res., 1970, 3, 139.
3. T. Nishiguchi, K. Tanaka, and K. Fukuzumi, J. Organomet.
Chem., 1980, 193, 37.
4. A. S. C. Chan and J. Halpern, J. Am. Chem. Soc., 1980,
102, 838.
(
1.5 mL) for 30 min. The solvent was removed from the
resulting red-brown solution and the yellow-brown residue was
dried by heating in vacuo for 1 h. H NMR (CDCl ), δ: 1.24 (s,
6
2
(
1
3
1
H, CH ); 2.62 and 2.87 (both m, 2 H each, CH ); 4.30 (m,
3 2
H, CH); 7.30—7.75 (m, 20 H, Ph). 3 P NMR, δ: 34.40
1
–
1
15. M. D. Fryzuk and B. Bosnich, J. Am. Chem. Soc., 1977,
9, 6262.
CDCl ); 28.33 (DMSO-d ). IR (KBr pellets), ν/cm : 505,
3
6
9
5
20, 625, 690, 745, 1023, 1156, 1236, 1259, 1435.
Phosphine oxide 8 was synthesized by the reaction of a
solution of DIOP (50 mg, 1•10–3 mol) in acetone (50 mL)
Received January 17, 2001;
in revised form March 26, 2001
with two equivalents of H O2 for 15 min. The reaction product
2