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Self-Assembly of Bidentate Ligands for Combinatorial Homogeneous Catalysis Based on an AЦT Base-Pair Model.

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Communications
Homogeneous Catalysis
Self-Assembly of Bidentate Ligands for
Combinatorial Homogeneous Catalysis Based on
an A?T Base-Pair Model**
Bernhard Breit* and Wolfgang Seiche
bidentate ligand, as exemplified in a highly regioselective
hydroformylation of n-alkenes into linear aldehydes.[2]
A potential advantage of this self-assembly approach is
the possibility to form libraries of heterodimeric bidentate
ligands (Scheme 2). Combining m different ligands equipped
with donor functions Dox with n different ligands equipped
with donor functions Doy should result in the formation of a
set of m n different bidentate ligands without the need for
additional synthetic steps.[3]
Selectivity control in homogeneous metal complex
catalysis relies in many cases on tailor-made bidentate ligands. The quest for the ultimate ligand which
gives a catalyst with optimal activity and selectivity
is difficult. Since rational design still does not allow
the ligand of choice for a given reaction and
substrate to be predicted, the combinatorial synthesis of ligand libraries and their subsequent use
has become an additional strategy.[1] However, the
rate-determining step in catalyst development is in
most cases the time-consuming ligand synthesis
required to generate the library.
Herein we report an alternative approach to
generate bidentate ligand libraries that relies on a
Scheme 2. An AT base-pair model (highlighted in red) as a platform for the selfassembly of monomeric to mixed bidentate ligands.
self-assembly of bidentate ligands from monodentate ligands and is based on hydrogen-bonding.
From a 4 4 library of self-assembling ligands a
catalyst that shows optimal activity and regioselectivity for
Unfortunately, this goal cannot be reached relying on the
the hydroformylation of terminal alkenes could be identified.
tautomeric pyridone/hydroxypyridine platform, since mixing
We recently showed that 6-DPPon (1) self-assembles
of two pyridones with different donor functions results in a
through hydrogen bonding of the hydroxypyridine 1 A with its
statistical mixture of the heterodimeric and the two homodipyridone tautomer 1 B in the coordination sphere of a latemeric ligands.[4] Our goal was to ensure the formation of
transition-metal center, such as platinum(ii) or rhodium(i)
single defined catalysts based on heterodimeric bidentate
(Scheme 1). Compound 1 displays the typical behavior of a
ligands, since this is a prerequisite for the delineation of
structure?activity and structure?selectivity relations. Hence, a
new template that assembles in a well-defined complementary mode giving exclusively heterodimers was required.
In fact, self-assembly of two complementary species
through hydrogen bonding is possible with high precision as
exemplified by DNA base pairing, for example, between
adenine and thymine (Scheme 2). The physical basis for this
specific complementary heterodimerization process relies on
the inherent ?fixation? of the adenine base as the lactim
tautomeric form, and the thymine base as the lactam
tautomeric form.[5] As a model system emulating these
properties
of the AT base pair the aminopyridine 3/isoquinoScheme 1. Self-assembly through hydrogen bonding of the 2-pyridone/
lone
4
system
was selected. Thus 2-aminopyridines exist in the
2-hydroxypyridine system 1 to generate bidentate ligand metal comlactim form[6] whereas the isoquinolone system strongly
plexes 2 for homogeneous catalysis.
prefers the lactam tautomeric form.[7] As a consequence,
homodimer formation similar to the 6-DPPon system should
be suppressed leaving heterodimer formation (!5) as the
[*] Prof. Dr. B. Breit, Dipl.-Chem. W. Seiche
Institut fr Organische Chemie und Biochemie
exclusive alternative.
Albert-Ludwigs-Universitt Freiburg
Synthesis of the phosphine functionalized aminopyridine
Albertstrasse 21, 79104 Freiburg (Germany)
derivatives 3 a?d is readily accomplished starting from 2,6Fax: (+ 49) 761-203-8715
dibromopyridine (Scheme 3). Reaction with aqueous ammoE-mail: bernhard.breit@organik.chemie.uni-freiburg.de
nia and protection of the amino function as the pivaloate gave
[**] This work was supported by the Fonds der Chemischen Industrie,
pyridine 7. The introduction of the pivaloate (Piv) protecting
the Alfried Krupp Award for young university teachers of the Krupp
group
proved to be beneficial for clean bromine?lithium
foundation (to B.B.), and BASF. We thank Dr. M. Keller for the X-ray
exchange with nBuLi in THF at 100 8C. Diversification was
crystal structure analysis and N. Stcks and G. Leonhardtachieved by trapping the thus generated lithiopyridine with
Lutterbeck for technical assistance.
1640
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200462499
Angew. Chem. Int. Ed. 2005, 44, 1640 ?1643
Angewandte
Chemie
Scheme 3. Synthesis of monodentate aminopyridinyl phosphines 3 and
isoquinolones 4: 1) NH4OH, 190 8C, 5 h; 2) pivaloylchloride, NEt3,
CH2Cl2, 0 8C!RT; 3) n-BuLi (2 equiv), THF, 100 8C, then ClPR2,
100 8C!RT; 4) KOtBu (1.1 equiv), toluene, 80 8C, 1 h; 5) n-BuLi
(1 equiv), THF, 100 8C, then ClPR2, 100 8C!RT, then H2O (1 equiv)
and formic acid excess(50?75 %).
different chlorophosphines to furnish the aminopyridine type
ligand set 3 a?d.
The complementary isoquinolone system 4 was obtained
starting from 1,3-dibromoisoquinoline (8).[8] Nucleophilic
introduction of a tert-butoxy substituent gave bromide 9
which underwent clean bromine?lithium exchange with
nBuLi. Trapping of the resulting lithioisoquinoline with the
chlorophosphines furnished the isoquinolone ligands 4 a?d.
Mixing of one equivalent of 6-diphenylphosphino-Npivaloyl-2-aminopyridine (6-DPPAP; 3 a) with one equivalent
of 3-diphenylphosphinoisoquinolone (3-DPPICon; 4 a) in the
presence of [PtCl2(1,5-cod)] (cod = cyclooctadienyl) gave the
heteroleptic cis-complex 5 aa-PtCl2 in quantitative yield.
From the X-ray crystal structure[9] of 5 aa-PtCl2 (Figure 1) it
is clear that the two cis-coordinated phosphine ligands form
the expected hydrogen-bonding network reminiscent of the
Watson Crick base pairing of A and T in DNA.
NMR spectra indicate that a similar structure is found in
aprotic solvents, such as CDCl3. The 31P NMR spectrum
shows an AB system with a typical 2J(P,P) coupling of 13 Hz
which confirms the presence of two non-equivalent phosphine
ligands coordinated to the same platinum center.[10] Furthermore, the size of the 1J(P,Pt) coupling (3658 and 3484 Hz) is in
the order expected for a cis-platinum(ii) diphosphine complex
(Figure 2).[11] Additionally, 1H NOE experiments in CDCl3
show a distinct NOE between the amide NH of the pyridine
ligand and the NH function of the isoquinolone unit. This
result suggests that hydrogen bonding occurs in solution
(CDCl3) as well.
To explore whether the heterodimeric chelate bonding
mode that results through hydrogen-bonding of the 3/4 system
is operative throughout a catalytic reaction, the rhodiumcatalyzed hydroformylation of terminal alkenes was chosen as
Angew. Chem. Int. Ed. 2005, 44, 1640 ?1643
Figure 1. PLATON plot of the structure of cis-5 aa-PtCl2 in the solid
state (H atoms bound to carbon are omitted for clarity). Selected
interatomic distances [] and angles [8]: Pt-P1 2.2486(5), Pt-P2
2.2437(5), NHиииN 2.932(2), OиииHN 2.977(2); P1-Pt-P2 102.896(18), NHиииN, 163(2) OиииH-N 172(2). Green Pt, yellow Cl, orange P, blue N,
red O.
Figure 2.
31
P NMR spectrum of 5 aa-PtCl2 in CDCl3 solution.
a test reaction (see Table 1). A strong chelate effect on the
regioselectivity of this reaction is well established.[12]
Both monodentate ligands, the aminopyridine 3 a as well
as the isoquinolone 4 a, furnished active rhodium catalysts for
the hydroformylation of 1-octene (Table 1, entries 1 and 2).
However, as expected for a monodentate phosphine ligand,
regioselectivity for the linear aldehyde was rather low.[2, 13] On
mixing ligands 3 a and 4 a, hydroformylation occurred with a
significantly increased regioselectivity for the linear aldehyde
(entries 3?6). This result indicates that the heterodimeric 3 a/
4 a ligand system operates as a bidentate ligand 5 aa, which is
held together by hydrogen bonding, in the active rhodium(i)
catalyst.
Hence, the stage was set for the generation of the first selfassembled bidentate-ligand library based on hydrogen bonding to identify a catalyst that operates with optimal activity
and regioselectivity.
www.angewandte.org
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1641
Communications
Table 1: Results of rhodium-catalyzed hydroformylation of terminal
alkenes with 3 a and 4 a.
Entry[a]
Ligand
FG
1
2
3
4
5
6
3a
4a
3a+4a
3a+4a
3a+4a
3a+4a
CH3(CH2)5
CH3(CH2)5
CH3(CH2)5
HO(CH2)9
MeO2C(CH2)8
AcO(CH2)4
R
l:b[b]
Conversion[b]
72:28
76:24
94:6
95:5
94:6
93:7
quant.
quant.
quant.
quant.
quant.
quant.
Table 2: 4 4 ligand matrix of aminopyridine(3 a?d)/isoquinolone(4 a?d)
derived self-assembled bidentate ligands in the [Rh]-catalyzed hydroformylation of 1-octene.[a]
L(3)
L(4)
4a
4b
[a] Conditions: [Rh(CO)2(acac)]:ligand:substrate 1:20:1000, 10 bar CO/
H2 (1:1), toluene (c0(alkene) = 0.7 m), 70 8C, 20 h. [b] Linear versus
branched (l:b). Determined by GC analysis and or 1H NMR spectroscopy.
4c
4d
3a
3b
1[b]
2425 h
94:6[c]
2033 h1
93:7
3537 h1
94:6
7439 h1
96:4
3c
1
1040 h
94:6
1058 h1
92:8
1842 h1
93:7
2695 h1
95:5
3d
1
2732 h
96:4
1281 h1
96:4
1808 h1
96:4
7465 h1
94:6
2559 h1
95:5
1772 h1
94:6
2287 h1
94:6
8643 h1
96:4
[a] Reaction conditions: [Rh(CO)2(acac)], [Rh]:L(3):L(4):1-octene
= 1:10:10:7500, 10 bar CO/H2(1:1), toluene (c0(1-octene) = 2.91 m),
5 h. Catalyst preformation: 5 bar CO/H2 1:1 , 30 min, RT!80 8C.
[b] Turnover frequency (TOF) was calculated as (mol aldehydes) (mol
catalyst)1 (t h1)1 at 20?30 % conversion, determined by GC analysis.
[c] Regioselectivity: linear to branched determined by GC analysis. The
best TOF and regioselectivity are highlighted in bold.
3 a?d had the greatest impact on catalyst performance.
Second, the increasing donor capability of the phosphine
donors of both 3 and 4 decreased catalyst activity; correspondingly, acceptor substituents at the phosphine donors
increased the catalyst activity. The most active catalyst was
the 3 d/4 d combination. For this catalyst a turnover frequency
of 8653 h1 was noted which is exceptional for a bidentate
phosphine/rhodium catalyst.[14] Furthermore, an excellent
regioselectivity of 96:4 in favor of n-nonanal was observed.
In conclusion, the first bidentate-phosphine-ligand library
for homogeneous metal-complex catalysis, based on selfassembly through hydrogen bonding was realized. The basis
for the success of this concept was the use of an AT base-pair
analogous system?the aminopyridine 3/isoquinolone 4
system. The heterodimeric ligands based on this system
operate as bidentate ligands throughout the hydroformylation
of terminal alkenes. From a 4 4 library generated by selfassembly a catalyst operating with outstanding activity and
regioselectivity upon hydroformylation of terminal alkenes
was identified. Application of this general principle and
related libraries to asymmetric catalysis is a logical step.
Received: November 2, 2004
Published online: February 9, 2005
From four aminopyridyl phosphines 3 a?d and four
phosphinoisoquinolone 4 a?d a set of 16 bidentate-ligand
combinations was generated by simple mixing of the components with the catalyst precursor [Rh(CO)2(acac)] (acac
= 2,4-pentanedione). The resulting catalysts were explored
with respect to their potential to catalyze the hydroformylation of 1-octene. Table 2 summarizes the results of these
experiments. Some noteworthy trends are apparent. First,
phosphine-substituent modification of the aminopyridines
1642
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
.
Keywords: combinatorial chemistry и homogeneous catalysis и
hydroformylation и rhodium и self-assembly
[1] C. Gennari, U. Piarulli, Chem. Rev. 2003, 103, 3071 ? 3100.
[2] B. Breit, W. Seiche, J. Am. Chem. Soc. 2003, 125, 6608 ? 6609.
[3] For alternative approaches to ligand/catalyst libraries through
self-assembly see: J. M. Takacs, D. S. Reddy, S. A. Moteki, D.
Wu, H. Palencia, J. Am. Chem. Soc. 2004, 126, 4494 ? 4495; V. F.
Slagt, M. Rder, P. C. J. Kamer, P. W. N. M. van Leeuwen,
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 1640 ?1643
Angewandte
Chemie
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
J. N. H. Reek, J. Am. Chem. Soc. 2004, 126, 4056 ? 4057; V. F.
Slagt, P. W. N. M. van Leeuwen, J. N. H. Reek, Angew. Chem.
2003, 115, 5777 ? 5781; Angew. Chem. Int. Ed. 2003, 42, 5619 ?
5623; V. F. Slagt, P. W. N. M. van Leeuwen, J. N. H. Reek, Chem.
Commun. 2003, 2474 ? 2475; K. Ding, H. Du, Y. Yuan, J. Long,
Chem. Eur. J. 2004, 10, 2872 ? 2884.
M. T. Reetz, T. Sell, A. Meiswinkel, G. Mehler, Angew. Chem.
2003, 115, 814 ? 817; Angew. Chem. Int. Ed. 2003, 42, 790 ? 793.
J. D. Watson, F. H. C. Crick, Nature 1953, 171, 964 ? 967; M. D.
Topal, J. R. Fresco, Nature 1976, 263, 285 ? 289.
H. I. Abdulla, M. F. El-Bermani, Spectrochim. Acta Part A 2001,
57, 2659 ? 2672.
G. Pfister-Guillouzo, C. Guimon, J. Frank, J. Ellison, A. R.
Katritzky, Liebigs Ann. Chem. 1981, 366 ? 375.
A. R. Osborn, K. Schofield, L. N. Short, J. Chem. Soc. 1956,
4191 ? 4206.
CCDC-254720 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Crystal
data
for
5 aa-PtCl2
C47H49Cl2N3O3P2Pt; Mr = 1031.82; T = 100(2) K; l = 0.71073 ;
crystal system: triclinic; space group: P1?; a = 11.4445(2), b
= 12.6925(3),
c = 16.1777(3) ,
a = 92.8636(12),
b
= 95.4611(11), g = 109.5054(10)8; V = 2196.66(8) 3 ; Z = 2;
1calcd = 1.560 Mg m3 ; absorption coefficient: 3.433 mm1;
F(000) = 1036; crystal size: 0.3 0.2 0.1 mm; q range for data
collection: 1.71?27.498; limiting indices: 14 h 14, 16 k 16, 20 l 21; 25 457 reflections collected, 10 036 reflections
unique (Rint = 0.0312); completeness to q = 25.008:99.9 %;
absorption correction: semi-empirical from equivalents; max.
and min. transmission: 0.711 and 0.640; refinement method: fullmatrix least-squares on F2 ; data/restraints/parameters: 10 036/0/
536; GoF on F2 : 1.076; final R indices [I > 2s(I)]: R1 = 0.0207,
wR2 = 0.0467; R indices (all data): R1 = 0.0239, wR2 = 0.0477;
largest diff. peak and hole: 0.919 and 1.430 e 3.
?31P and 13C NMR of Transition Metal Phosphine Complexes?:
P. S. Pregosin, R. W. Kunz in NMR Basic Principles and Progress,
Vol. 16 (Eds.: P. Diehl, E. Fluck, R. Kosfeld), Springer, Heidelberg, 1979, pp. 115 ? 122, and references therein; P. S. Pregosin,
S. N. Sze, Helv. Chim. Acta 1978, 61, 1848 ? 1855.
see ?31P and 13C NMR of Transition Metal Phosphine Complexes?: P. S. Pregosin, R. W. Kunz in NMR Basic Principles and
Progress, Vol. 16 (Eds.: P. Diehl, E. Fluck, R. Kosfeld), Springer,
Heidelberg, 1979, pp. 94 ? 95; H. G. Alt, R. Baumgartner, H. A.
Brune, Chem. Ber. 1986, 119, 1694 ? 1703.
P. W. N. M. van Leeuwen, C. P. Casey, G. T. Whiteker in Rhodium Catalyzed Hydroformylation (Eds.: P. W. N. M. van Leeuwen, C. Claver), Kluwer Academic Publishers, Dordrecht, 2000,
chap. 4, pp. 76 ? 105.
see P. W. N. M. van Leeuwen, C. P. Casey, G. T. Whiteker in
Rhodium Catalyzed Hydroformylation (Eds.: P. W. N. M. van
Leeuwen, C. Claver), Kluwer Academic Publishers, Dordrecht,
2000, chap. 4, pp. 63 ? 75; B. Breit, W. Seiche, Synthesis 2001, 1 ?
36.
Under identical conditions Rh/triphenylphosphine (TOF:
1312 h1, 76:24), Rh/6-DPPon (1; TOF: 3284 h1, 98:2), for
tBu-Xanthphos see ref. [2]. See also ref. [12].
Angew. Chem. Int. Ed. 2005, 44, 1640 ?1643
www.angewandte.org
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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