Hydrogen bonding as a construction element for bidentate donor ligands in homogeneous catalysis: Regioselective hydroformylation of terminal alkenes

Article abstract of DOI:10.1021/ja0348997

A new concept for the construction of bidentate ligands for homogeneous metal complex catalysis is described. The concept relies on the self-assembly of monodentate ligands through hydrogen bonding. As a prototype of such systems, 6-diphenylphosphanyl-2-pyridone (6-DPPon) was shown to form a chelate in the coordination sphere of a transition metal center through unusual pyridone/hydroxypyridine hydrogen bonding (X-ray). This hydrogen bonding stays intact in a catalytic reaction as proven upon highly regioselective hydroformylation of terminal alkenes. Regioselectivities and reactivities observed rank the 6-DPPon/rhodium system among the most active and regioselective catalysts for n-selective hydroformylation of terminal alkenes. Copyright

Full text of DOI:10.1021/ja0348997

Published on Web 05/07/2003
Hydrogen Bonding as a Construction Element for Bidentate Donor Ligands in
Homogeneous Catalysis: Regioselective Hydroformylation of Terminal
Alkenes
Bernhard Breit* and Wolfgang Seiche
Institut fu¨r Organische Chemie und Biochemie, Albert-Ludwigs-UniVersita¨t, Freiburg, Germany
Received February 27, 2003; E-mail: bernhard.breit@organik.chemie.uni-freiburg.de
Bidentate donor ligands are essential components of many
transition metal catalysts in the homogeneous phase. In contrast to
their monodentate counterparts, they often lead to higher levels of
regio- and enantioselectivities, which is generally due to the
formation of a more rigid microenvironment at the catalytically
active metal center and, hence, to a better energetic discrimination
of competing reaction pathways.1 However, in many cases, the
covalent connection of the two donor atoms to a suited ligand
backbone necessitates a number of synthetic steps which often
render the ligand more expensive than the noble metal source.
We herein report on a new concept for the in situ generation of
bidentate donor ligands based on the self-assembly of a monodentate
ligand in the coordination sphere of a metal center through hydrogen
bonding. These new ligands provide highly active and regioselective
catalysts for the n-selective hydroformylation of terminal alkenes.
The 2-pyridone 1A/2-hydroxypyridine 1B tautomer system is
known to dimerize in aprotic solvents to form predominantly the
symmetrical pyridone dimer 3 through hydrogen bonding (Scheme
1).2 If X would be a donor atom capable of binding a metal center,
a bidentate binding mode would be impossible for dimer 3 for
geometric reasons. Alternatively, formation of the mixed dimer
between the 2-pyridone and the 2-hydroxypyridine tautomer would
generate a ligand architecture suited for a bidentate binding mode
4. Although this nonsymmetrical dimerization mode is unusual,
recent calculations in the gas phase have shown that it is just 4.8
kcal/mol less favorable (X ) H) as compared to the commonly
observed 2-pyridone dimer.3 We speculated that if X would be an
appropriate donor atom the chelation effect exhibited through
coordinative binding to a metal center might overcome this energy
penalty to provide the bidentate binding mode 4 as shown in
Scheme 1.
Figure 1. X-ray plot of cis-[PtCl2(6-DPPon)2] 5.
cis-coordinated 6-DPPon ligands form the expected hydrogen
bonded nonsymmetrical dimer 4 and, hence, act as a chelating ligand
for platinum. The significantly different C/N and C/O bond
distances within the nitrogen heterocycles confirm that one 6-DPPon
ligand exists as the pyridone and the other as the hydroxypyridine
tautomer.4
To see whether such a chelating binding mode through hydrogen
bonding is operative throughout a catalytic reaction, hydroformyl-
ation of olefins was chosen as a first test case because a strong
chelation effect upon hydroformylation of terminal alkenes is well
established.1a Thus, only a few tailor-made bidentate ligands such
as BISBI,5 BIPHEPHOS,6 and XANTPHOS7 are known to provide
good levels of regioselectivity in favor of the linear aldehyde isomer.
As a first test system, hydroformylation of 1-octene was studied.
To calibrate the results, the industrially employed rhodium/PPh3
system was selected as the prototype for a monodentate phosphine/
rhodium catalyst. As a reference for a bidentate ligand, t-Bu-
XANTPHOS, one of the best ligands for n-selective hydroformyl-
ation of terminal alkenes, was chosen.4,7 As expected, regioselectivity
for the rhodium/PPh3 system was rather low (Table 1, entries 1
and 2). A significantly higher regioselectivity is observed for the
t-Bu-XANTPHOS system. However, catalyst activity is significantly
lower (entries 3,4). When employing the 6-DPPon ligand 2, a
catalyst system that performed with both excellent regioselectivity
and activity was obtained (entries 5,6). These results indicate that
6-DPPon acts as a chelating ligand. The significantly enhanced
activity as compared to that of the t-Bu-XANTPHOS system may
be due to electronic effects exerted by the electron-withdrawing
nature of the pyridone nucleus. Similar activating effects are well
documented in hydroformylation.8
Scheme 1
For that reason, 6-diphenylphosphanyl-2-pyridone (6-DPPon) 2
was chosen as a first test system. Reaction of 2 equiv of 6-DPPon
with [PtCl2(1,5-COD)] gave the cis-complex 5. From the X-ray
plot of 5 depicted in Figure 1, it is obvious that the two
To study the stability of the intramolecular hydrogen bonding
of the rhodium/2 catalyst, the influence of temperature upon
regioselectivity of the hydroformylation of 1-octene was investigated
(see Figure 2). Thus, at reaction temperatures between 50 and 110
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10.1021/ja0348997 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Regioselective Hydroformylation of Functionalized
Table 1. Hydroformylation of 1-Octenea
Terminal Alkenes with the Rhodium/6-DPPon (2) Catalyst in
Comparison to the Standard Industrial Rhodium/PPh3 Catalyst
entry
ligand
T (°C)
conv. (%)b
isom. (%)b
l:bb
1
2
3
4
5
6
PPh3
PPh3
t-Bu-XANTPHOS
t-Bu-XANTPHOS
2
2
65
80
65
80
65
80
22
98
6
31
56
96
0.3
9
1
2
3
73:27
72:28
98:2
98:2
97:3
96:4
8
a Reaction parameters: Rh:L:1-octene (1:20:7000), c (1-octene) ) 1.4
M, 4 h, toluene, 10 bar CO/H2 (1:1). b Determined by GC analysis.
a Reaction parameters: Rh:L:alkenic substrate (1:20:1000), c (alkene)
) 0.698 M, toluene, 10 bar CO/H2 (1:1), 70 °C. Full conversion was reached
in every case after 20 h. b Determined by GC and NMR analysis of crude
reaction mixture after 20 h. c Isolated as the corresponding γ-lactol.
Figure 2. Temperature dependence of n-selectivity upon hydroformylation
of 1-octene with the 6-DPPon (2)/rhodium catalyst.
°C, linearity stays high in the range of 95-97%. Above 110 °C,
this regioselectivtiy breaks down to values as low as 80%, which
is close to the regioselectivity range observed for the monodentate
PPh3/rhodium catalyst. These results indicate a continuous erosion
of the pyridone hydrogen bonding at temperatures above 110 °C.
The 6-DPPon/rhodium catalyst is operative as a chelation system
in the presence of functional groups such as bromides, acetates,
esters, and ketones (Table 2, entries 1-4) to give the linear
aldehydes in excellent yield (quantitative) and regioselectivity. We
next looked at functional groups possessing significant hydrogen
bonding capability to learn whether the ligand’s hydrogen bonding
system may be disrupted. Interestingly, neither a carbamate, a
salicylate, nor a free hydroxyl function was able to diminish the
high regioselectivity of the rhodium/6-DPPon catalyst (entries 5-8).
However, use of an alcoholic solvent or addition of acetic acid
resulted in low regioselectivity, which indicated a cleavage of the
intramolecular hydrogen bond of the rhodium/6-DPPon catalyst
under these conditions (entries 9,10).
rank the 6-DPPon/rhodium system among the most active and
regioselective catalysts for the n-selective hydroformylation of
terminal alkenes.
Acknowledgment. We thank DFG, the Fonds of the Chemical
Industry, and the Krupp Foundation (Krupp Award for young
university teachers to B.B.) for financial support, as well as Dr.
M. Keller for solving the X-ray crystal structure analysis of 5.
Supporting Information Available: Experimental details (PDF);
crystallographic information (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) For selected examples, see: (a) Regioselectivity: van Leeuwen, P. W.
N. M.; Casey, C. P.; Whiteker. In Rhodium Catalyzed Hydroformylation;
van Leeuwen, P. W. N., Claver, C., Eds.; Kluwer Academic Publishers:
Dordrecht, 2000; Chapter 4, pp 76-105. (b) Enantioselectivity: Ohkuma,
T.; Kitamura, M.; Noyori, R. Asymmetric Hydrogenation. In Catalytic
Asymmetric Synthesis; Ojima, I., Ed.; Wiley-VCH: New York, 2000;
Chapter 1, pp 1-110.
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(3) Chou, P.-T.; Wei, C.-Y.; Hung, F.-T. J. Phys. Chem. B 1997, 101, 9119-
9126.
(4) For details, see Supporting Information.
(5) Devon, T. J.; Phillips, G. W.; Puckette, T. A.; Stavinoha, J. L.; Vanderbilt,
J. J. U.S. Patent 4,694,109, 1987; Chem. Abstr. 1988, 108, 7890.
(6) Billig, E.; Abatjoglou, A. G.; Bryant, D. R. (UCC) U.S. Patent 4,769,498,
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(7) (a) XANTPHOS: Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P.
C. J.; van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J. Organometallics
1995, 14, 3081. (b) t-Bu-XANTPHOS: van der Veen, L. A.; Kamer, P.
C. J.; van Leeuwen, P. W. N. M. Organometallics 1999, 18, 4765-4777.
(8) See: Breit, B.; Winde, R.; Mackewitz, T.; Paciello, R.; Harms, K. Chem.-
Eur. J. 2001, 7, 3106-3121 and references cited therein.
In summary, we herein realized a new concept for the construc-
tion of bidentate ligands employing self-assembly through hydrogen
bonding. Thus, the 6-diphenylphosphino-2-pyridone system 2 forms
a chelate in the coordination sphere of a transition metal center
through unusual nonsymmetrical pyridone/hydroxypyridine hydro-
gen bonding. This hydrogen bonding stays intact in a catalytic
reaction as proven upon highly regioselective hydroformylation
of terminal alkenes. Regioselectivities and reactivities observed
JA0348997
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J. AM. CHEM. SOC. VOL. 125, NO. 22, 2003 6609