Rhodium catalyzed asymmetric hydroformylation of vinylarenes with a diphosphite ligand forming a large chelating ring

Article abstract of DOI:10.1039/b105208j

A rhodium complex containing a sixteen membered chelated diphosphite, with the appropriate combination of stereogenic centers, produces ee's above 70% in the hydroformylation of vinylarenes, while a related diastereo-isomeric ligand renders very low ee's because it does not form a chelated species.

Full text of DOI:10.1039/b105208j

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Rhodium catalyzed asymmetric hydroformylation of vinylarenes
with a diphosphite ligand forming a large chelating ring†
Zoraida Freixa and J. Carles Bayón*
Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona,
Spain. E-mail: bayon@cc.uab.es
Received 13th June 2001, Accepted 14th June 2001
First published as an Advance Article on the web 3rd July 2001
A rhodium complex containing a sixteen membered
chelated diphosphite, with the appropriate combination
of stereogenic centers, produces ee’s above 70% in the
hydroformylation of vinylarenes, while a related diastereo-
isomeric ligand renders very low ee’s because it does not
form a chelated species.
ligands have been previously observed in asymmetric hydro-
formylation.3b,4b,9 In order to get some insight into the nature
of this effect in ligands 1 and 2, the structure of the catalytic
species formed by reacting them with [Rh(µ-OMe)(cod)]2 under
1
CO/H2 was studied by HPNMR. For ligand 1, H- and 31P-
NMR spectra show the formation of the species [RhH(CO)2(1)]
3, where the diphosphite coordinates the metal in equatorial
positions.10 Very broad spectra, which could not be resolved,
were obtained with ligand 2 in the explored temperature range
(Ϫ40 to 90 ЊC). These spectra likely correspond to a fluxional
species or a mixture of species in dynamic equilibrium.
Asymmetric hydroformylation catalyzed by transition metal
catalysts is a method for the synthesis of homochiral alde-
hydes.1 Both, platinum and rhodium catalysts, containing
bidentate P-donor ligands, have been extensively used in this
reaction. However, while Pt–SnCl2 catalysts produce the best
stereoselectivities with diphosphine ligands containing four
carbon atoms in the backbone (i.e. seven membered chelates),2
in the case of rhodium catalysts, phosphine–phosphite3 or
diphosphite4 ligands forming eight membered chelates render
the best results. Catalysts forming larger chelate rings are
reported to produce poor results in enantioselective hydro-
formylation,5 although they have been used successfully in other
asymmetric reactions.6 We report here the first results on the
enantioselective rhodium catalyzed hydroformylation of vinyl-
arenes using a diphosphite ligand forming a very large, sixteen
membered chelate.
Ligands (R)-phtabinphos 1 and (S)-phtabinphos 2 were
prepared by reaction of (2S)-hydroxypropyl isophthalate7 with
(R)- and (S)-binaphthol phosphorochlorhidites and NEt3.8
More conclusive results were obtained by NMR analysis of
the reactions of ligands 1 and 2 with [RhH(CO)(PPh3)3] in a
ligand/rhodium ratio of 0.5 : 1. Ligand 1 forms the expected
species [RhH(CO)(PPh3)(1)] 4, with the characteristic 16
(phosphite) plus 8 (phosphine) lines spectrum, again with the
diphosphite occupying equatorial positions.11 However, the
reaction with ligand
2
produces
a
binuclear species
[Rh2H2(CO)2(PPh3)4(2)] 5, in which the diphosphite is acting as
a bridge between two metal atoms, as indicated by the 8 (phos-
phite) plus 16 (phosphine) lines 31P-NMR spectrum.12 These
NMR results reveal that the low enantioselectivity observed
with diphosphite 2 is due to the tendency of this ligand to act in
a bridging or monodentate fashion, which creates a very loose
chiral environment on the catalysts. Species of this type are
known to be poorly enantioselective in asymmetric hydroform-
ylation.13 Furthermore, the remarkably different tendency of
diastereoisomeric ligands 1 and 2 to form chelating species can
be considered as an extreme case of the matching–mismatching
effect.
In the case of the catalyst containing ligand 1, an increase in
the temperature (entries 3 and 1 in Table 1) produces an improve-
ment in the activity of the system, but with a drop in the
regio- and enantio-selectivity. By increasing the syngas (CO/H2)
pressure (entries 3 and 6) a decrease in the activity of the system
was observed with almost no change in the selectivity. A sig-
nificant increase in the activity (TOF = 31 hϪ1) and a slight
improvement of the ee was achieved by running the reaction at
a higher concentration of substrate (entries 3 and 7). Finally,
the catalyst Rh/(R)-phtabinphos shows higher activity and
stereoselectivity in the hydroformylation of vinylnaphtha-
lene than in styrene (entries 5 and 3). However, a decrease in
Table 1 collects selected catalytic experiments on the hydro-
formylation of vinylarenes with rhodium catalysts formed with
ligands 1 and 2. Entries 1 and 2 reveal the different behavior
of the diastereoisomeric ligands in the hydroformylation of
styrene. Reaction with ligand 1 is slower than that of ligand
2, but the latter shows very low stereoselectivity. Matching–
mismatching effects between the stereogenic centers of the
† Electronic supplementary information (ESI) available: experimental
details, NMR data for 1 and 5 and NMR spectra of 3, 4 and 5. See
http://www.rsc.org/suppdata/dt/b1/b105208j/
DOI: 10.1039/b105208j
J. Chem. Soc., Dalton Trans., 2001, 2067–2068
This journal is © The Royal Society of Chemistry 2001
2067

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Table 1 Hydroformylation of vinylarenes using (R)- and (S)-phtabinphos (ligands 1 and 2) and [Rh(µ-OMe)(cod)]2
Conv. (%)
regiod
(%)
ee (%)
Entry
La
Substrateb
PhCH᎐CH2
T /ЊC
P/bar
(t/h)c
(conf )e
1
2
3
4
5
6
7f
1
2
1
1
1
1
1
50
50
40
40
40
40
40
15
15
15
15
15
30
15
78 (15)
99 (21)
30 (36)
53 (115)
89 (23)
25 (69)
17 (16)
75
83
80
66
81
80
80
62 (R)
11 (S)
70 (R)
72 (R)
75 (R)
73 (R)
76 (R)
᎐
PhCH᎐CH2
᎐
PhCH᎐CH2
᎐
p-tBuC6H4CH᎐CH2
᎐
NaphCH᎐CH2
᎐
PhCH᎐CH2
᎐
PhCH᎐CH2
᎐
Reaction conditions: 1.25 × 10Ϫ2 mmol Rh, 2.5 × 10Ϫ2 mmol ligand and 5.0 mmol substrate (substrate/catalyst = 400) in 7.5 ml of toluene;
P(CO)᎐P(H2). a Diphosphite. b Substrates: styrene; 4-tert-butylstyrene, and vinylnaphthalene. c Substrate consumed in the time indicated in paren-
᎐
theses. d Regioselectivity in the branched aldehyde. e Enantiomeric excess of the isomer indicated in parentheses. f 2.5 × 10Ϫ2 mmol Rh, 5.0 × 10Ϫ2
ligand and 45 mmol substrate (substrate/catalyst = 1800) in 4.0 ml of toluene; TOF is 31 hϪ1
.
3 (a) K. Nozaki, N. Sakai, T. Nanno, T. Higashijima, S. Mano,
T. Horiuchi and H. Takaya, J. Am. Chem. Soc., 1997, 119, 4413;
(b) N. Sakai, S. Mano, K. Nozaki and H. Takaya, J. Am. Chem. Soc.,
1993, 115, 7033.
4 (a) J. E. Babin and G. T. Whitecker, PCT Int. Appl., WO 93/03839,
for Union Carbide, 1993; (b) G. J. H. Buisman, L. A. van der Veen,
A. Klootwijk, W. G. J. Lange, P. C. J. Kamer, P. W. N. M.
van Leeuwen and D. Vogt, Organometallics, 1997, 16, 2929; (c) M.
Diéguez, O. Pagamies, A. Ruiz, S. Castillón and C. Claver, Chem.
Commun., 2000, 1607.
activity as well as in the regioselectivity was observed in the
hydroformylation of 4-tert-butylstyrene with respect to styrene
(entries 4 and 3).
In conclusion, diphosphite 1 provides the first example of a
ligand forming a chiral macrochelate, which produces a fairly
good stereoselective catalyst for asymmetric hydroformylation.
Furthermore, the modular structure of this ligand allows
an easy modification of its stereochemical properties. This
approach is currently under investigation.
5 G. J. H. Buisman, E. J. Vos, P. C. J. Kamer and P. W. N. M.
van Leeuwen, J. Chem. Soc., Dalton Trans., 1995, 409.
6 L. David, D. L. Van Vranken and B. M. Trost, Chem. Rev., 1996, 96,
395.
7 Z. Freixa, E. Martin, S. Gladiali and J. C. Bayón, Appl. Organomet.
Chem., 2000, 14, 66.
8 Selected data: 1 δP (CDCl3) 145.8; 2 δP (CDCl3) 148.6.
9 D. Gleich, R. Schmid and W. A. Herrmann, Organometallics, 1998,
17, 2141.
Acknowledgements
We thank the Spanish MEC (PB98-0913-C02-01) and DGE-
CIRIT of Catalonia for financial support and a scholarship
for Z. F.
10 NMR data: 3 δP (CDCl3, 121.6 MHz) 177.6 (d, JRh-P = 231 Hz). δH
(CDCl3, 300 MHz) Ϫ9.90 (hydride, q br, JP-H = JRh-H = 4Hz).
11 NMR data: 4 δP (CDCl3, 101.3 MHz) 180.6 (P1 phosphite, ddd,
JRh-P1 = 251 Hz, JP2-P1 = 271 Hz, JP3-P1 = 172 Hz); 175.3 (P2 phos-
phite, ddd, JRh-P2 = 237 Hz, JP3-P2 = 109 Hz); 37.7 (P3 phosphine,
ddd, JRh-P3 = 134 Hz). δH (CDCl3, 250 MHz) Ϫ10.25 (hydride,
dddd, J = 13.0 Hz, 6.5 Hz, 2.7 Hz).
12 NMR data: 5 δP (CDCl3, 101.3 MHz) 37.8 (P1 phosphine, ddd,
JRh-P1 = 143 Hz, JP2-P1 = 86 Hz, JP3-P1 = 167 Hz); 40.8 (P2 phos-
phine, ddd, JRh-P2 = 148 Hz, JP3-P2 = 191 Hz); 174.7 (P3 phosphite,
ddd, JRh-P3 = 258 Hz).
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J. Chem. Soc., Dalton Trans., 2001, 2067–2068