Immobilization of Optically Active Rhodium-Diphosphine Complexes on Porous Silica via Hydrogen Bonding

Full text of DOI:10.1002/1615-4169(20010129)343:1<41::AID-ADSC41>3.0.CO;2-P

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Immobilization of Optically Active Rhodium-
Diphosphine Complexes on Porous Silica
via Hydrogen Bonding
Claudio Bianchini,*a Pierluigi Barbaro,a Vladimiro Dal Santo,b
Roberto Gobetto,c Andrea Meli,a Werner Oberhauser,a Rinaldo Psaro,b
Francesco Vizzaa
a ISSECC-CNR, Via J. Nardi 39, 50132 Firenze, Italy
Fax: +39 05 52 47 83 66; e-mail: bianchin@fi.cnr.it
b CSSCMTBSO-CNR, Via G. Venezian 21, 20133 Milano, Italy
c Laboratorio di Spettroscopia NMR, UniversitaÁ degli Studi di Torino, Via P. Giuria 7, 10125 Torino, Italy
Received June 20, 2000; Accepted August 3, 2000
Catalyst recovery is of
tiguous Rh±Rh sites, indi-
cating that the catalytic
active sites were isolated
rhodium complexes, as
in the homogeneous
phase.
Keywords: asymmetric catalysis; heterogeneous
catalysis; tethered chiral diphosphines; hydrogen-
bonded catalysts; olefin hydrogenation
paramount importance in
fine chemical production
when sophisticated li-
gands are used, whose
cost often exceeds that of
the noble metal employed. This is certainly the case
for the hydrogenation of prochiral olefins with rho-
dium catalysts containing chiral diphosphine ligands.
The heterogenization of rhodium-diphosphine com-
plexes is therefore a research topic of much current in-
terest. Common immobilization procedures of chiral
Rh-phosphine complexes involve either their covalent
grafting to both organic and inorganic supports or the
dissolution of the precursor in a hydrophilic solvent
which is then adsorbed as a thin layer onto a porous
support material with a hydrophilic surface.[1,2,3] The
use of heteropoly acids as anchoring agents for chiral
rhodium complexes has also been reported.[4]
Scheme 1.
We have recently introduced an alternative, facile
and clean method for preparing silica-tethered poly-
phosphine metal catalysts, denoted supported hydro-
gen-bonded (SHB) catalysts, for use in both solid-gas
and solid-liquid reactions in hydrocarbon sol-
vents.[5,6]
In this paper we describe, for the first time, the SHB
immobilization of various optically pure Rh(I)-di-
phosphine
complexes,
including
[((R)-(R)-
BDPBzPSO3)Rh(nbd)] (1), which contains the new
chiral anionic ligand (R)-(R)-3-(4-sulphonate)ben-
zyl-2,4-bis(diphenylphosphino)pentane, [((+)-DIOP)-
Rh(nbd)]OTf (2), and [((S)-BINAP)Rh(nbd)]OTf (3)
(nbd = norbornadiene). A preliminary study of the po-
tential of the resulting SHB catalysts in the enantiose-
lective hydrogenation of prochiral olefins is also re-
ported.
The procedure involves
a hydrogen bonding
interaction between the silanol groups of silica and
sulphonate groups from the phosphine ligands and,
possibly,
[OTf = O3SCF3]. Some of these SHB catalysts, namely
(sulphos)Rh(cod)/SiO2 (I) and [(sulphos)Ru-
also
from
triflate
counter-anions
(NCMe)3]OTf/SiO2 (II) (Scheme 1), were successfully
employed in the heterogeneous hydrogenation of al-
kenes, nitriles and α,b-unsaturated ketones [sul-
phos = ±O3S(C6H4)CH2C(CH2PPh2)3, cod = cycloocta-
1,5-diene].[5],[6] In all the cases investigated there
was no evidence whatsoever for the formation of con-
Scheme 2.
Adv. Synth. Catal. 2001, 343, No. 1
Ó WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2001
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41

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The ligand [(R)-(R)-BDPBzPSO3]± has been prepared
by treatment of (R)-(R)-3-benzyl-2,4-bis(diphenyl-
phosphino)pentane[7] with H2SO4 (96%) at 100 °C
and was isolated as its sodium salt. This reaction re-
presents a new protocol[8] for the regioselective sul-
phonation of a phosphine ligand in a remote benzyl
group. Previous examples involve, in fact, the sulpho-
nation of the ligand framework prior to introduction
of the phosphino groups.[9,10,11] The simple reaction
of [(R)-(R)-BDPBzPSO3]Na with [Rh(nbd)Cl]2 yields
1. Compounds 2 and 3 have similarly been prepared
by reaction of the corresponding diphosphine ligand
with [Rh(nbd)Cl]2, followed by addition of silver tri-
flate as chloride scavenger. This immobilization pro-
cedure of a chiral metal catalyst is the simplest ever
reported since it does not imply any modification of
the ligand structure.
The heterogenization of the Rh complexes
(Scheme 3) was carried out by the solvent impregna-
tion method using anhydrous dichloromethane as
solvent and activated Davison 62 silica as support.[5,6]
The grafting procedure was complete and reproduci-
ble up to ca. 1 wt % metal loading (determined by
ICP-AES). Once grafted to silica, the Rh complexes
were not extracted back into CH2Cl2 solutions even
after repeated washings. In contrast, stirring the
grafted complexes in MeOH or EtOH at room tem-
perature resulted in their complete delivery into solu-
tion (31P NMR experiments in CD3OD). No immobili-
zation whatsoever was observed when the triflate
counter-anion in either 2 or 3 was replaced by coun-
±
ter-anions such as BPh4 that are not capable of hy-
drogen-bonding interactions with silica.
Figure 1. CP MAS 31P NMR (109.4 MHz) spectra of 1/SiO2
(a) and 1 (b); 31P{1H} NMR (81.01 MHz, CD2Cl2) spectrum of
1 (c).
that of unsupported 1 (Fig. 1 b), which is consistent
with the immobilization of the complex on the silica
through a functional group away from the metal coor-
dination sphere.[5,6] For comparative purposes, the
31P{1H} NMR spectrum of 1 in CD2Cl2 is reported in
Figure 1 c.
In addition to CP MAS 31P NMR spectroscopy, 2/SiO2
and 3/SiO2 have been characterized by 31P and 19F
NMR experiments on slurries in CD2Cl2. The 31P
NMR spectra of 2/SiO2 (a) and 2 (b) are shown in Fig-
ure 2. The 31P signal of 2/SiO2 is apparently much
broader than that of the free complex due to the re-
stricted mobility of the complex cation on the silica
surface. Indeed, the electrostatic interaction with the
H-bonded triflate ion forces [((+)-DIOP)Rh(nbd)]+ to
remain close to the counter-anion even in the pres-
ence of a solvent. Consistent with this picture are the
following observations: i) The addition of (NBu4)BPh4
to a CD2Cl2 slurry of 2/SiO2 reduces the line-width of
Scheme 3.
All the SHB complexes have properly been character-
ized by DRIFT and CP MAS 31P NMR spectroscopy.[5,6]
The CP MAS 31P NMR spectrum of 1/SiO2 is shown in
Figure 1 a and consists of a broad signal centered at
39.9 ppm. The spectrum is substantially similar to
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Adv. Synth. Catal. 2001, 343, 41±45

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the 31P signal to the value observed for free 2 as a con-
sequence of the delivery of the complex cation into
solution; ii) The 19F NMR spectrum of 2 consists of a
sharp singlet at 79.6 ppm, while 2/SiO2 gives rise to
a very large resonance centered at ca. 79 ppm,
which does not sharpen upon addition of (NBu4)BPh4
to the slurry (thus indicating that the triflate ion re-
mains tethered to silica). A similar NMR behaviour is
observed for 3/SiO2.
Three olefins [dimethyl itaconate, ethyl trans-b-
(methyl)cinnamate, and methyl α-(acetamido)acry-
late] have been chosen at random from the prochiral
pool to compare the catalytic performance of the im-
mobilized and free rhodium precursors, especially in
terms of activity, enantioselectivity and ease of recy-
cling. Standard hydrogenation conditions have been
Figure 2. 31P NMR (81.01 MHz, CD2Cl2) spectra: 2/SiO2 (a), 2 (b).
Table 1. Asymmetric catalytic hydrogenations of olefins using free and anchored Rh(I)-chiral diphosphine complexes.
Precursor
T [°C]
t [h]
Substrate
Yield [%][a], ee [%][a]
homogeneous[b]
73, 4 (S)
heterogeneous[c]
1
60
4
100, 3 (S)
1
1
r. t.
r. t.
24
3
100, 57 (S)
100, 16 (S)
100, 53 (S)
100, 14 (S)
2
2
60
60
4
4
24, 16 (R)
56, 11 (R)
100, 41 (R)
100, 24 (R)
2
3
r. t.
60
3
4
100, 55 (R)
9, 22 (S)
62, 20 (R)
41, 20 (S)
3
3
60
4
3
100, 33 (S)
100, 25 (S)
99, 32 (S)
r. t.
100, 30 (S)
[a] Reaction mixture, GC. Product absolute configuration in brackets.
[b]
Reaction conditions: Rh = 0.0052 mmol, H2 pressure = 20 bar, solvent (MeOH) volume = 20 mL, 350 rpm, substrate/
Rh = 100.
[c]
Reaction conditions: Rh = 0.0078 mmol, H2 pressure = 20 bar, solvent (n-heptane) volume = 30 mL, 1500 rpm, substrate/
Rh = 100.
Adv. Synth. Catal. 2001, 343, 41±45
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rotors with sample volume of 120 lL were employed with
spinning speed in the range from 4.5 to 5.5 kHz. For all sam-
ples the magic angle was carefully adjusted from the 79Br
MAS spectrum of KBr by minimizing the line-width of the
spinning side band satellite transitions. Optical rotations
were measured with a Perkin-Elmer 341 polarimeter using
10 cm cells. GC analyses were performed either on a Shi-
madzu GC-14A gas-chromatograph equipped with a flame
ionization detector and a 30 m (0.25 mm i. d., 0.25 lm FT)
SPB-1 Supelco fused silica capillary column and coupled
with a Shimadzu C-R6A Chromatopac operating in the cor-
rected area method or with a Shimadzu GC-17A gas chroma-
tograph equipped with a flame ionization detector and a
40 m (0.25 mm i. d.) Chiraldex G-TA capillary column and
coupled with a Shimadzu C-R7A Chromatopac.
employed for both homogeneous and heterogeneous
reactions which have been carried out in methanol
and n-heptane, respectively (Table 1). Although the
use of solvents with different polarity rules out a reli-
able comparison between the tethered and homoge-
neous catalysts, one may draw out some general con-
clusions: i) The hydrogen-bonding immobilization of
the [(R)-(R)-BDPBzPSO3] and BINAP complexes on
silica does not reduce the enantioselectivity (this lat-
ter is intrinsically low for the catalyst/substrate sys-
tems employed in this work).[12,13,14] ii) The sup-
ported DIOP complex is less enantioselective than
the homogeneous derivative, which may tentatively
be related to the presence of ether oxygen atoms in
the ligand backbone and hence to possible H-bonding
interactions with the support leading to less efficient
enantio-discrimination. However, a different hydro-
genation mechanism in the two-phase systems can-
not be ruled out. iii) The reduction of ethyl trans-b-
(methyl)cinnamate is generally faster in the hetero-
geneous phase than in the homogeneous phase,
which may again be due to the reasons given above.
iv) In all cases, no rhodium leaching (< 1 ppm) was
observed within three consecutive heterogeneous
runs, while an effective catalyst recycling, with no
loss of activity or enantioselectivity, was accom-
plished by using a dip pipe with a sintered metal piece
at its dipping end to remove the fluid reaction mixture
and leave the catalyst in the reactor. Dichloro-
methane cannot be used to carry out heterogeneous
reactions due to appreciable rhodium leaching under
catalytic conditions.
Synthesis of [(R)-(R)-BDPBzPSO3]Na
A solution of (R)-(R)-3-benzyl-2,4-bis(diphenylphosphino)-
pentane (2.65 g, 5 mmol) in conc. H2SO4 (96%, 10 mL) was
heated with stirring for 20 h at 100 °C. After cooling to 0 °C,
water (50 mL) was added dropwise. The solution was neu-
tralized by addition of 10% aqueous NaOH, leading to the
precipitation of a white solid. The solid compound was fil-
tered and washed with cold water (20 mL). The solid ob-
tained was dissolved in CH2Cl2 (50 mL), dried over MgSO4
and the solvent was evaporated in vacuo. Elemental analysis
calcd for C36H35NaO3P2S: C 68.34, H 5.58; found: C 67.97,
H 5.55. 31P{1H} NMR (81.01 MHz, CDCl3): d = ±1.1 (s), ±
10.8 (s); 1H NMR (200.13 MHz, CDCl3): d = 7.7±6.5 (m, 24 H;
Ph), 3.32 (m, 1 H; CH3CH), 3.08 (dd, J = 15.1, 4.2 Hz, 1 H;
CHH), 2.71 (t, J = 15.1 Hz, 1 H; CHH), 2.33 (m, 1 H; CH3CH),
1.89 (m, 1 H; CH), 1.08 (dd, J = 15.4, 7.1 Hz, 3 H; CH3CH),
0.88 (dd, J = 14.2, 6.9 Hz, 3 H; CH3CH); [a]2D5 = +58.5 (c = 3.7
in CHCl3).
Synthesis of [((R)-(R)-BDPBzPSO3)Rh(nbd)] (1)
The results reported in this work define neutral and
cationic metal complexes containing groups with hy-
drogen bond acceptor properties as viable precursors
for preparing efficient and easily recyclable enantio-
selective catalysts supported over materials with a
hydrophilic surface.
Solid [Rh(nbd)Cl]2 (0.09 g, 0.19 mmol) was added to a solu-
tion of [(R)-(R)-BDPBzPSO3]Na (0.24 g, 0.38 mmol) in
CH2Cl2 (15 mL). After stirring for 30 min at room tempera-
ture, the red-orange solution was washed with water
(30 mL). The addition of diethyl ether (30 mL) to the organic
phase caused the precipitation of an orange oil which solidi-
fied on standing. The solid compound was washed with
diethyl ether (50 mL) and dried in vacuo. Yield 82%. Ele-
mental analysis calcd for C43H43NaO3P2RhS: C 62.40, H 5.24;
found: C 62.47, H 5.35; 31P{1H} NMR (81.01 MHz, CD3OD):
Experimental Section
2
1
d = 32.6 (dd, J(P,P) = 49.3 Hz, J(P,Rh) = 145.9 Hz), 29.8 (dd,
1J(P,Rh) = 151.4 Hz); H NMR (200.13 MHz, CD3OD): 7.7±6.7
1
[15]
The compounds [Rh(nbd)Cl]2
and (R)-(R)-3-benzyl-2,4-
bis(diphenylphosphino)pentane[7] were synthesized as pre-
viously reported. The chiral ligands (+)-DIOP and (S)-BINAP
were obtained from Aldrich. High resolution 31P solid state
NMR spectra were performed on a Jeol GSE 270 (6.34 T)
spectrometer operating at 109.4 MHz under conditions of
1H ® 31P cross-polarization, high power proton decoupling
and magic angle spinning. The spectra were recorded at a
spinning rate of 5 kHz. The 90° pulse was 6.0 ls and the con-
tact pulse was 5 ms. The spectra of the unsupported com-
plexes were collected after 400 scans using a recycle delay
of 10 s, while the spectra of the supported complexes were
acquired with 6000 transients and a relaxation delay of 10 s.
The line broadening was set to be 100 Hz for pure complexes
and 150 Hz for the silica-grafted derivatives. 85% H3PO4 was
used as a reference (d = 0). Cylindrical 6 mm o. d. zirconia
(m, 24 H), 4.82 (bs, 2 H), 4.62 (bs, 1 H), 4.08 (bs, 1 H), 3.83
(bd, 2 H), 2.9±2.6 (m, 3 H), 2.49 (t, 1 H), 2.03 (m, 1 H), 1.52
(bs, 1 H), 1.40 (dd, 3 H), 1.22 (m, 1 H), 1.11 (dd, 3 H); CP MAS
31P NMR (109.4 MHz): 1, d = 39.8 (s, w1/2 = 1.4 kHz); 1/SiO2,
d = 39.9 (s, w1/2 = 1.9 kHz).
Synthesis of [((+)-DIOP)Rh(nbd)]OTf (2)
Solid [Rh(nbd)Cl]2 (0.07 g, 0.15 mmol) was added to a solu-
tion of (+)-DIOP (0.15 g, 0.30 mmol) in CH2Cl2 (10 mL). Sil-
ver trifluoromethanesulphonate (0.08 g, 0.30 mmol) was
then added and the resulting suspension was stirred for
15 min. After AgCl was filtered off, the addition of n-pentane
(30 mL) to the organic phase caused the precipitation of an
orange oil which solidified on standing. The solid compound
was washed with n-pentane and dried in vacuo. The com-
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Adv. Synth. Catal. 2001, 343, 41±45

COMMUNICATIONS
pound obtained was recrystallized from CH2Cl2/n-pentane.
Yield 77%. Elemental analysis calcd for C39H40F3O5P2RhS:
C 55.59, H 4.78; found: C 55.63, H 4.69; 31P{1H} NMR
(81.01 MHz, CD2Cl2): d = 18.5 (d, 1J(P,Rh) = 153.5 Hz); 1H
NMR (200.13 MHz, CD2Cl2): d = 7.9±7.3 (m, 20 H), 4.51 (m,
4 H), 4.02 (m, 2 H), 3.80 (m, 2 H), 3.77 (m, 4 H), 1.55 (s, 2 H),
1.24 (s, 6 H); CP MAS 31P NMR (109.4 MHz): 2, d = 15.4 (s,
Acknowledgments
Thanks are due to MURST (legge 95/95) and CNR/RAS pro-
ject.
w
1/2 = 1.2 kHz); 2/SiO2, d = 37.1 (s, w1/2 = 3.1 kHz).
References
Synthesis of [((S)-BINAP)Rh(nbd)]OTf (3)
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Solid [Rh(nbd)Cl]2 (0.04 g, 0.08 mmol) was added to a solu-
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ver trifluoromethanesulphonate (0.04 g, 0.16 mmol) was
then added and the resulting suspension was stirred for
15 min. After AgCl was filtered off, diethyl ether (30 mL)
was added causing the precipitation of orange microcrys-
tals. The product obtained was recrystallized from CH2Cl2/
diethyl ether. Yield 82%. Elemental analysis calcd for
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52H40F3O3P2RhS: C 64.60, H 4.17; found: C 64.52, H 4.22;
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NMR
(81.01 MHz,
CD2Cl2):
d = 28.6
(d,
1J(P,Rh) = 156.8 Hz); 1H NMR (200.13 MHz, CD2Cl2): d = 7.7±
6.4 (m, 32 H), 4.20 (m, 4 H), 4.16 (m, 2 H), 1.61 (m, 2 H); CP
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Heterogeneous Hydrogenation Reactions
In a typical hydrogenation reaction, the silica-supported cat-
alyst precursor (0.008 mmol of rhodium) was placed under
argon into a Parr 4565 autoclave equipped with a dip pipe
with a sintered (2 lm) metal piece at its dipping end. A solu-
tion of the prochiral substrate (0.8 mmol) in the appropriate
solvent (30 mL) was then transferred via a Teflon capillary
into the autoclave under argon. Argon was then replaced by
hydrogen with three cycles at 5 bar/normal pressure. The
autoclave was finally charged with the desired pressure of
H2 and then heated with stirring at 1500 rpm. After the de-
sired time, the reactor was cooled to room temperature, the
reaction mixture was filtered through the sintered metal
pipe. The filtrate was analyzed by GC, GC/MS. After the sol-
vent was removed in vacuo, the residue was analyzed by
ICP-AES (inductively coupled plasma atomic emission) as
well as NMR spectroscopy. The solid catalyst left in the reac-
tor was re-used for a second run.
The procedures for carrying out the homogeneous reac-
[15] G. Giordano, R. H. Crabtree, Inorg. Synth. 1990, 28,
88±90.
tions have been described previously.[5,6,7]
Adv. Synth. Catal. 2001, 343, 41±45
45