Synthesis and complexation properties of allenic bipyridines, a new class of axially chiral ligands for transition metal catalysis

Article abstract of DOI:10.1002/ejic.200701049

The allenic bipyridines 3, which represent a new class of axially chiral ligands for transition metal catalysis, were synthesized and their complexation properties were studied. The synthesis was achieved by copper-mediated or palladium-catalyzed S_N2′-substitution of the tertiary propargyl acetates 2. The allenic bipyridines form 1:1 complexes with Cu⁺ and Ag⁺ ions that were characterized by electrospray mass spectrometry and temperature-dependent NMR spectroscopy. In the case of the C ₂-symmetrical allene 3b, a crystalline silver complex was isolated that exists as a 2:2-(L₂M₂) complex in the solid state. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701049

FULL PAPER
DOI: 10.1002/ejic.200701049
Synthesis and Complexation Properties of Allenic Bipyridines, a New Class of
Axially Chiral Ligands for Transition Metal Catalysis
Sandra Löhr,[a] Jan Averbeck,[a] Markus Schürmann,[b] and Norbert Krause*[a]
Dedicated to Prof. Gerard van Koten on the occasion of his 65th birthday
Keywords: Allenes / Axial chirality / Bipyridines / Catalysis / Transition metal complexes
The allenic bipyridines 3, which represent a new class of axi-
ally chiral ligands for transition metal catalysis, were synthe-
sized and their complexation properties were studied. The
synthesis was achieved by copper-mediated or palladium-
catalyzed SN2Ј-substitution of the tertiary propargyl acetates
2. The allenic bipyridines form 1:1 complexes with Cu+ and
Ag+ ions that were characterized by electrospray mass spec-
trometry and temperature-dependent NMR spectroscopy. In
the case of the C2-symmetrical allene 3b, a crystalline silver
complex was isolated that exists as a 2:2-(L2M2) complex in
the solid state.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
reasons for the absence of allenic structures in axially chiral
ligands might be the high reactivity of the allenic π-system,
as well as the tendency of allenes to racemize in the pres-
ence of transition metals.[8] Based on our experience with
the synthesis and application of functionalized allenes,[7e,9]
we now report on the preparation and the complexation
properties of allenic bipyridines A (Figure 1). Depending
on the presence or absence of bulky substituents and the
coordination geometry of the metal, we expect the forma-
tion of 1:1- (B) or 2:1-chelate complexes (C), as well as 2:2
Transition metal catalysis is of pivotal importance for
synthetic chemistry. The fine-tuning of the catalytic activity
of transition metal complexes relies strongly on the proper-
ties of the ligands; this is particularly evident in enantiose-
lective catalysis since the chirality of the ligand controls the
preferred formation of one enantiomeric product over the
other.[1] Privileged ligands that afford high enantioselectivi-
ties in different transformations are often bidentate and
C2-symmetrical, e.g., chiral bisoxazolines,[2] salens,[3]
TADDOLs,[4] bipyridines,[5] or binaphthyls.[6] In spite of re-
cent progress in the field, it is still necessary to develop
novel ligand systems since many established catalysts have
serious limitations in terms of substrate scope, catalyst
loading, etc.
With regard to the chirality elements, ligands with
stereogenic centers (diols, diamines) and axial chirality
(BINOL, BINAP) are predominant. However, it seems that
the prototype of an axially chiral molecule, an allene, has
never been introduced into a ligand for stereoselective tran-
sition metal catalysis. This is all the more surprising in view
of the multitude of (frequently stereoselective) methods for
the synthesis of allenes which are known today.[7] Possible
[a] Organic Chemistry II, TU Dortmund,
Otto-Hahn-Strasse 6, 44227 Dortmund, Germany
Fax: +49-231-7553884
E-mail: norbert.krause@uni-dortmund.de
[b] Inorganic Chemistry, TU Dortmund,
Otto-Hahn-Strasse 6, 44227 Dortmund, Germany
Supporting information for this article is available on the
WWW under http://www.eurjic.org/ or from the author.
Figure 1. Structure of allenic bipyridines A and possible metal com-
plexes.
552
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 552–556

Allenic Axially Chiral Ligands for Transition Metal Catalysis
complexes D or larger aggregates of this type. Evidently, clinic unit cell which contains two molecules. Interestingly,
the complexation properties will strongly influence possible the pyridine nitrogen atoms are facing away from each
applications in transition metal catalysis.
other.
Results and Discussion
The hitherto unknown allenic bipyridines were synthe-
sized using the copper-mediated or palladium-catalyzed
SN2Ј-substitution of propargyl acetates (Scheme 1).[7] The
propargyl alcohols 1[10] (which are readily accessible by ad-
dition of lithium acetylides to the appropriate ketones) were
converted in good yield into the corresponding tertiary ace-
tates 2 by Steglich esterification[11] or (in case of the steri-
cally hindered substrates 1b/c) by treatment with acetic an-
hydride and catalytic amounts of zinc perchlorate.[12] In ac- Figure 2. Solid state structure of allenic bipyridine 3b (hydrogen
atoms were omitted for clarity).[17]
cordance with previous observations[13] the attempted intro-
duction of a methyl group by reaction of propargyl acetates
2 with methylmagnesium cuprates was not successful; in
contrast to this, the tert-butyl-substituted allenes 3a–c were
obtained in good yields using the highly reactive lower-or-
der cyanocuprate tBuCu(CN)Li.[14] The doubly phenyl-sub-
stituted allene 3d, on the other hand, was synthesized by
palladium-catalyzed SN2Ј-substitution of propargyl acetate
2c with phenylzinc chloride. Allene 3a was also synthesized
in enantiomerically pure form by HPLC separation of the
enantiomers of precursor 2a (DIACEL Chiracel OD) and
reaction of the enantiomerically pure propargyl acetates
with tBuCu(CN)Li.[15] The allenes (+)-3a (99% ee) and
(–)-3a (Ͼ99% ee) thus obtained have unstructured CD
spectra with a single band at 310 nm (see Supporting Infor-
mation);[16] the absolute configuration could not be deter-
mined so far.
The complexation properties of the allenic bipyridines 3
in the presence of Cu+ or Ag+ ions was examined by means
of electrospray (ESI) mass spectrometry and temperature-
dependent NMR spectroscopy. We treated a solution of the
ligand in CD2Cl2 with 0.5 or 1.0 equiv. of a copper (CuI or
[Cu(MeCN)4PF6]) or silver salt (AgOTf or AgBF4). In all
cases, the ESI spectra show the presence of the 1:1 com-
plexes of type B (Table 1), regardless of the ratio of ligand
to metal. The isotopic pattern of the molecular ions clearly
excludes the presence of 2:2-(L2M2) complexes D (see Sup-
porting Information) and 2:1-(L2M) complexes C were not
observed as well.
Table 1. Molecular ions detected by ESI mass spectrometry for
complexes formed from allenic bipyridines 3 and copper or silver
salts.[a]
Allene
Cation
m/z (exp.)[b]
m/z (1:1)[c]
m/z (2:1)[d]
3a
3a
3b
3b
3c
3c
3d
3d
Cu+
Ag+
Cu+
Ag+
Cu+
Ag+
Cu+
Ag+
355.2
399.1
397.2
441.2
417.2
461.2
437.3
481.3
355.1
399.2
397.2
441.2
417.1
461.1
437.1
481.1
647.3
691.3
731.4
775.4
771.4
815.3
811.3
855.3
[a] Solvent: CD2Cl2. [b] Determined experimentally for 1:1 and 2:1
mixtures of ligand and metal salt; see text. [c] Calculated for the
1:1 complex. [d] Calculated for the 2:1 complex.
Insight into the dynamics of complexation of the allenic
ligands 3 by copper and silver ions was obtained by tem-
perature-dependent NMR spectroscopy. In many cases, ob-
servation of the line broadening caused by slow ligand ex-
change during cooling was hampered by the limited sym-
metry of the ligand (3a/c) and/or insufficient solubility of
the metal complex at low temperature. An exception is the
complex formed from the C2-symmetrical allene 3b and
Scheme 1. Synthesis of allenic bipyridines 3.
The allenic bipyridines were characterized by the usual [Cu(MeCN)4PF6]. In this case, addition of 1.0 equiv. of the
spectroscopic methods. In case of the racemic bis-tert-butyl- copper salt to a solution of the ligand in CD2Cl2 caused a
substituted allene 3b the solid-state structure could also be distinct shift of the resonances for the tert-butyl and meth-
determined (Figure 2).[17] The allene crystallizes in a tri- ylpyridine protons from δ = 1.35/2.50 (free ligand) to δ =
Eur. J. Inorg. Chem. 2008, 552–556
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S. Löhr, J. Averbeck, M. Schürmann, N. Krause
FULL PAPER
Figure 3. 1H NMR spectra (400 MHz, CD2Cl2) of a 2:1 mixture of bipyridine 3b and [Cu(MeCN)4PF6] at different temperatures between
236 and 296 K (3b: free ligand, C: 1:1 complex).
1.29/2.74 (296 K). At 236 K, the resonances were observed
at δ = 1.29/2.47 (free ligand 3b) and δ = 1.23/2.65 (1:1 com-
plex; see Supporting Information). The observation of only
one resonance for the tert-butyl and methylpyridine pro-
tons, respectively, proves the formation of a symmetric com-
plex and excludes a monodentate complexation.
In contrast to this, a 2:1-mixture of 3b and [Cu(MeCN)4-
PF6] in CD2Cl2 at 296 K shows a broadened tert-butyl reso-
nance at δ = 1.31 and a very broad signal for the pyridine
methyl protons at δ ≈ 2.62 (Figure 3). By variation of tem-
perature, it was shown that this is not a 2:1 complex but
rather a slowly exchanging mixture of a complex with 1:1-
stoichiometry and the free ligand. Hence, cooling induces a
splitting of the signals (Figure 3), and the resonances ob-
served at 236 K correspond to the 1:1 complex (δ = 1.23/
2.65) and the free ligand 3b (δ = 1.29/2.47). Using the co-
alescence temperature and the chemical shift difference be-
tween the resonances of the tert-butyl and pyridine methyl
protons at slow exchange, the free activation enthalpy of
Figure 4. Solid state structure of the complex [3b·AgBF4]2 (sym-
the ligand exchange was estimated to be ∆G϶ = 55–57 kJ/
metry transformations used to generate equivalent atoms: a =
mol (see Supporting Information).[18]
0.25 – x, y, 0.25 – z; b = x, 0.25 – y, 0.25 – z; c = 0.25 – x, 0.25 –
y, z; d = 0.75 – y, 0.25 – y, z; hydrogen atoms and 2 CH2Cl2
molcules were omitted).[19] Selected bond lengths [Å] and bond
angles [°]: Ag1–N1 2.150(2), Ag1–N1A 2.150(2), Ag1–F1 2.942(4),
Ag1–F1A 2.942(4), Ag1–Ag1C 3.5098(5), N1–N1B 4.953(3),
N1–N1C 3.753(3), C22–C21b 1.314(2), C21–C22–C21B 173.3(3),
Since the NMR experiments were carried out under sim-
ilar conditions (solvent, concentration) as the measurement
of the ESI mass spectra, we assume that the observed com-
plex with 1:1 stoichiometry is actually a 1:1-(LM) chelate
complex of type C rather than a 2:2-(L2M2) complex D or C1–C21–C7 118.7(2), C7–C21–C21B–C7B 89.9.
554 Eur. J. Inorg. Chem. 2008, 552–556
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Allenic Axially Chiral Ligands for Transition Metal Catalysis
reduced pressure, the crude product was purified by column
chromatography (SiO2, cyclohexane/ethyl acetate/triethylamine
66:33:1). Yield 22 mg (85%) of 3a as a colorless solid, m.p. 39 °C.
1H NMR (500 MHz, CDCl3): δ = 7.45 (t, J = 7.6 Hz, 1 H), 7.40
(t, J = 7.6 Hz, 2 H), 7.12 (d, J = 7.6 Hz, 1 H), 6.92 (dd, J = 2.1/
7.6 Hz, 2 H), 2.55 (s, 6 H), 2.25 (s, 3 H), 1.37 (s, 9 H) ppm. 13C
NMR (125 MHz, CDCl3): δ = 207.7 (ϫ), 157.6 (ϫ), 156.9 (ϫ),
156.2 (ϫ), 155.6 (ϫ), 135.9 (+), 135.9 (+), 121.1 (+), 120.3 (+),
120.2 (+), 118.3 (+), 35.7 (ϫ) 30.1 (+), 24.6 (+), 15.8 (+) ppm. IR:
a larger aggregate. Interestingly, we were able to obtain the
X-ray structure of a complex formed from ligand 3b and
silver tetrafluoroborate; this crystallizes in an orthorhombic
unit cell that contains eight molecules of a 2:2 complex of
the type D (Figure 4; see Supporting Information).[19] The
silver ions are coordinated by pyridine nitrogen atoms of
two ligand molecules in a linear fashion. Each pyridine ring
of one ligand is parallel to the distal pyridine ring of the
other ligand. The dihedral angle at the allenic bridges is
ν = 3045 (m), 1930 (w), 1570 (s), 1448 (s) cm–1. MS (FAB): m/z
˜
89.9°, the length of the allene double bonds is 1.314 Å. The (%) = 293 (100) [M + H]+, 237 (30). HRMS (FAB): C20H24N2:
calcd. 292.1939 (M+), found 292.1916.
allene moieties of the two ligand molecules are in parallel
planes that are twisted to each other by 65.7°. The discrep- 2-Methyl-6-[2,2,6,6-tetramethyl-5-(6-methylpyridin-2-yl)hepta-3,4-
dien-3-yl]pyridine (3b): According to the synthesis of 3a, reaction
of CuCN (297 mg, 3.3 mmol) in THF (5 mL) with tBuLi (2.2 mL,
3.3 mmol, 1.5  solution in pentane) and propargyl acetate 2b
(280 mg, 0.65 mmol) in THF (5 mL) afforded 202 mg (93%) of 3b
ancy between this solid-state structure and the results ob-
tained in solution may be due to a preferential crystalli-
zation of the 2:2 complex.
1
as a colorless solid, m.p. 59 °C. H NMR (400 MHz, CDCl3): δ =
7.43 (t, J = 7.7 Hz, 2 H), 7.26 (d, J = 7.8 Hz, 2 H), 6.93 (d, J =
7.6 Hz, 2 H), 2.55 (s, 6 H), 1.38 (s, 18 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 206.1 (ϫ), 156.8 (ϫ), 156.2 (ϫ), 135.9 (+),
120.4 (+), 120.1 (+), 118.4 (ϫ), 35.2 (ϫ), 30.1 (+), 24.6 (+) ppm.
Conclusions
In this paper, we report on the first synthesis of the all-
enic bipyridines 3 and their complexation properties. The
axially chiral allenes were efficiently obtained by copper-
mediated or palladium-catalyzed SN2Ј-substitution of the
tertiary propargyl acetates 2. The allenic bipyridines form
1:1 complexes with Cu+ and Ag+ ions that were charac-
terized by electrospray mass spectrometry and temperature-
dependent NMR spectroscopy. In the case of the C2-sym-
metrical allene 3b, a crystalline silver complex was isolated
that exists as a 2:2-(L2M2) complex in the solid state. We
are now engaged in the development of efficient procedures
for the synthesis of enantiomerically pure allenic bipyr-
idines, as well as in their application in transition metal ca-
talysis.[20]
IR: ν = 3054 (m), 2961 (s), 1920 (w), 1571 (s) cm–1. MS (FAB): m/z
˜
(%) = 335 (100) [M + H]+, 223 (10). HRMS (FAB): C23H30N2:
calcd. 335.2482 ([M + H]+), found 335.2482.
2-Methyl-6-[4,4-dimethyl-1-(6-methylpyridin-2-yl)-1-phenylpenta-
1,2-dien-3-yl]pyridine (3c): According to the synthesis of 3a, reac-
tion of CuCN (376 mg, 4.2 mmol) in THF (10 mL) with tBuLi
(2.7 mL, 4.1 mmol, 1.5  solution in pentane) and propargyl ace-
tate 2c (250 mg, 0.70 mmol) in THF (5 mL) afforded 193 mg (78%)
1
of 3c as a colorless oil. H NMR (500 MHz, CDCl3): δ = 7.56 (m,
3 H), 7.43 (t, J = 7.7 Hz, 1 H), 7.35 (m, 4 H), 7.28 (m, 1 H), 7.05
(d, J = 7.6 Hz, 1 H), 6.96 (d, J = 7.5 Hz, 1 H), 2.59 (s, 3 H), 2.58
(s, 3 H), 1.48 (s, 9 H) ppm. 13C NMR (125 MHz, CDCl3): δ =
208.9 (ϫ), 157.9 (ϫ), 157.0 (ϫ), 155.5 (ϫ), 155.4 (ϫ), 136.5 (+),
136.0 (+), 135.6 (ϫ), 128.1 (+), 128.0 (+), 126.8 (+), 121.2 (+),
121.1 (+), 120.6 (+), 120.5 (+), 120.0 (ϫ), 111.9 (ϫ), 36.0 (ϫ), 30.1
(+), 24.6 (+), 24.5 (+) ppm. IR: ν = 3057 (s), 2959 (s), 1920 (m),
˜
Experimental Section
1584 (s) cm–1. MS (FAB): m/z (%) = 355 (100) [M + H]+, 299 (30).
HRMS (FAB): C25H26N2: calcd. 354.2096 (M+), found 354.2098.
General Information: All reactions were performed in oven-dried
glassware under argon. Diethyl ether and THF were distilled from
sodium/benzophenone. tBuLi was titrated according to the pro-
cedure of Kofron and Baclawski.[21] Column chromatography was
carried out with Merck silica gel F60 (70–230 mesh). 1H and 13C
NMR spectra were recorded with Bruker DRX 400 and DRX 500
spectrometers at room temperature in CDCl3 as solvent and in-
ternal standard (1H NMR: δ = 7.27 ppm. 13C NMR: δ =
77.0 ppm). Signals of carbon atoms were assigned with APT ex-
periments; abbreviations: (+) for CH3, CH; (–) for CH2; (ϫ) for
Cquat. IR spectra were measured with a Nicolet Avatar 320 FT-IR
as a liquid film between NaCl plates or in the case of solids as
KBr pellet. FAB mass spectra (HRMS) were measured with a Jeol
SX102A spectrometer, ESI spectra with a LTQ ORBITRAP spec-
trometer.
2-Methyl-6-[3-(6-methylpyridin-2-yl)-1,3-diphenylpropa-1,2-dienyl]-
pyridine (3d): A mixture of PhMgCl (5.9 mL, 8.8 mmol, 1.5  solu-
tion in THF) and ZnCl2 (8.8 mL, 8.8 mmol, 1.0  solution in Et2O)
in THF (5 mL) was stirred for 40 min at room temperature and
then cooled to –40 °C. After addition of [Pd(PPh3)4] (104 mg,
0.09 mmol) and propargyl acetate 2c (628 mg, 1.76 mmol) in THF
(5 mL), the mixture was warmed up to –10 °C within 16 h. Satd.
aq. NH4Cl (50 mL) was added, the mixture was washed with di-
ethyl ether, and the combined organic layers were dried with
Na2SO4. After removal of the solvent under reduced pressure, the
crude product was purified by column chromatography (SiO2, cy-
clohexane/ethyl acetate/triethylamine 79:20:1). Yield 519 mg (79%)
1
of 3d as a colorless oil. H NMR (500 MHz, CDCl3): δ = 7.65 (d,
J = 7.4 Hz, 4 H), 7.57 (t, J = 7.7 Hz, 2 H), 7.48 (d, J = 7.7 Hz, 2
H), 7.39 (t, J = 7.4 Hz, 4 H), 7.32 (t, J = 7.7 Hz, 2 H), 7.09 (d, J
= 7.7 Hz, 2 H), 2.62 (s, 6 H) ppm. 13C NMR (125 MHz, CDCl3):
δ = 212.4 (ϫ), 158.0 (ϫ), 154.5 (ϫ), 136.6 (+), 134.8 (ϫ), 128.3 (+),
128.1 (+), 127.4 (+), 121.6 (+), 121.1 (+), 113.7 (ϫ), 24.5 (+) ppm.
2-Methyl-6-[2,2-dimethyl-5-(6-methylpyridin-2-yl)hexa-3,4-dien-3-
yl]pyridine (3a): To a suspension of CuCN (40 mg, 0.45 mmol) in
THF (2 mL) was added dropwise at –40 °C tBuLi (0.3 mL,
0.44 mmol, 1.5  solution in pentane). The mixture was cooled to
–80 °C and a solution of propargyl acetate 2a (26 mg, 0.088 mmol)
in THF (1 mL) was added. The mixture was stirred at –80 °C for
2 h, treated with satd. aq. NH4Cl (5 mL) and washed with diethyl
ether. The combined organic layers were washed with aq. NH3/
NH4Cl and dried with Na2SO4. After removal of the solvent under
IR: ν = 3057 (s), 2850 (s), 1921 (m), 1570 (s) cm–1. MS (FAB): m/z
˜
(%) = 375 (100) [M + H]+, 374 (40) [M+]. HRMS (FAB): =
C27H22N2: calcd. 375.1856 ([M + H]+), found 375.1852.
Supporting Information (see also the footnote on the first page of
this article): Copies of CD, mass, and NMR spectra of representa-
Eur. J. Inorg. Chem. 2008, 552–556
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555

S. Löhr, J. Averbeck, M. Schürmann, N. Krause
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N. Krause, Eur. J. Org. Chem. 2006, 4634–4641; g) B. Gockel,
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gand exchange in the 1:1 complex prepared from 3b and [Cu-
(MeCN)4PF6]; color figure of the X-ray structure of complex
[3b·AgBF4·CH2Cl2]2.
[10]
[11]
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Acknowledgments
We thank the Deutsche Forschungsgemeinschaft (DFG) and the
Fonds der Chemischen Industrie for financial support.
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a
colorless crystal of
[3b·AgBF4·CH2Cl2]2 (0.30ϫ0.15ϫ0.13 mm3) was performed
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1228.19 gmol–1, orthorhombic, space group Fddd,
a =
13.5824(8), b = 24.3346(8), c = 32.7072(16) Å, V = 10810.5(9)
Å3, Z = 8, µ(Mo-Kα) = 0.986 mm–1, 23363 independent reflec-
tions (θmin = 2.99, θmax = 27.48), 1961 with IՆ2σ(I), R1
=
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0.0283, wR2 (all data) = 0.0574; residual electron density
(max./min.): 0.497/–0.274 e Å3. After merging symmetry-equiv-
alent reflections and absorption correction with the program
SCALEPACK (Tmin = 0.751, Tmax = 0.883) an internal R value
of 0.044 was obtained. F (AgBF4) and Cl atoms (CH2Cl2) are
disordered over two positions with occupancies of 0.4 [F(1Ј),
F(2Ј)], 0.5 [Cl(1), Cl(1Ј)] and 0.6 [F(1), F(2)]. CCDC-644737
(for 3b·AgBF4) 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.
Initial experiments on the aziridination of styrene with [N-(p-
toluolsulfonyl)imino]phenyliodinane show the high catalytic
activity of a copper complex formed from allenic bipyridine 3b.
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Received: September 27, 2007
Published Online: December 10, 2007
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Eur. J. Inorg. Chem. 2008, 552–556