Active template synthesis of rotaxanes and molecular shuttles with switchable dynamics by four-component PdII-promoted Michael additions

Article abstract of DOI:10.1002/anie.200705859

(Chemical Equation). Taking the Michael: Rotaxanes (see structure) and molecular shuttles are prepared in up to 99% yield by successive Pd ^II-promoted 1,4-conjugate additions in a one-pot four-component assembly process. This process represents the first active template reaction in which the template motif is retained in the interlocked product.

Full text of DOI:10.1002/anie.200705859

Angewandte
Chemie
DOI: 10.1002/anie.200705859
Molecular Shuttles
Active Template Synthesis of Rotaxanes and Molecular Shuttles with
Switchable Dynamics by Four-Component PdII-Promoted Michael
Additions**
Stephen M. Goldup, David A. Leigh,* Paul J. Lusby,* Roy T. McBurney, and
Alexandra M. Z. Slawin
Active template synthesis,[1] in which a substrate acts as both
the template for a threaded complex and the catalyst for the
covalent bond forming step that captures the interlocked
product, is a new addition to the strategies available[2,3] for the
assembly of rotaxanes. Unlike traditional “passive template”
methods,[3] the intercomponent interactions responsible for
rotaxane formation in an active template reaction do not
normally “live on” in the final product.[4] Removing the need
for permanent recognition features increases the structural
diversity that can be introduced into interlocked architectures
but also loses one of the key features of rotaxanes that has
been so successfully exploited in the early generations of
artificial molecular machines.[5] Herein we report on the first
active template reaction in which the template interactions
between macrocycle and thread persist in the resulting
rotaxane. A macrocycle-coordinated palladium(II) center
promotes the formation of carbon–carbon bonds between
three other building blocks by successive Michael additions
while positioning the reactants so that the final covalent bond
forms through the macrocycle to produce [2]rotaxanes in up
to 99% yield (Scheme 1 and Table 1). The Pd–nitrile coordi-
nation bond that directs and controls the synthesis is retained
in the rotaxane product, enabling the preparation of both
degenerate station molecular shuttles (with controllable
dynamics, Scheme 2) and stimuli-switchable molecular shut-
tles (Scheme 1) through a simple one-pot multicomponent
assembly process.
[*]Dr. S. M. Goldup, Prof. D. A. Leigh, Dr. P. J. Lusby, R. T. McBurney
School of Chemistry, University of Edinburgh
The King’s Buildings
West Mains Road, Edinburgh EH9 3JJ (UK)
Fax: (+44)131-650-6453
E-mail: david.leigh@ed.ac.uk
paul.lusby@ed.ac.uk
Homepage: http://www.catenane.net
Scheme 1. Synthesis of metal-coordinated [2]rotaxane [(L3)Pd]by an
Prof. A. M. Z. Slawin
School of Chemistry
University of St. Andrews
active template PdII-promoted double Michael addition, and its decom-
plexation to hydrogen-bonded molecular shuttle H2L3. a) DIPEA,
CH2Cl2 (see Table 1 for the effects of reagent stoichiometry on the
Purdie Building, St. Andrews, Fife KY16 9ST (UK)
yield of rotaxane [(L3)Pd]); b) 1m HCl (aq):CH2Cl2 (1:1), reflux, 92%.
[**]We thank Juraj Bella, Mark D. Symes, and Diego Gonzµlez Cabrera
In CDCl3 the macrocycle in H2L3 shuttles rapidly on the NMR time-
for their assistance with the NMR experiments and the EPSRC
National Mass Spectrometry Service Centre (Swansea (UK)) for
accurate mass data. This work was supported by the EPSRC. P.J.L. is
scale between the two thread ketone groups, even at 250 K; c) Pd-
(OAc)2 (2.2 equiv), CH3CN:CH2Cl2 (1:1), 18 h, 313 K, 99%.
a Royal Society University Research Fellow. D.A.L. is an EPSRC
Senior Research Fellow and holds a Royal Society Wolfson Research
The basis for the recognition-element-persistent active
template reaction lies in a range of tridentate pyrroloimida-
zolone[6] and bisoxazoline[7] pincer PdII complexes that
Merit Award.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2008, 47, 3381 –3384
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3381

Communications
Table 1: Optimization of the active template PdII-promoted Michael
shows that the coordination mode[11] of L22ꢀ and the shape
of the ring holds the nitrile ligand occupying the fourth PdII
coordination site centrally within the macrocycle cavity. The
crystal structures suggested that the first alkylation of the a-
methylene group should result in one face of the macrocycle
being blocked such that the second Michael addition must
proceed from the other side, generating a [2]rotaxane.
Pleasingly, the four component assembly of [(L2)Pd-
(CH3CN)], 1 (” 2), and L1 at room temperature in CH2Cl2 in
the presence of DIPEA led to [2]rotaxane [(L3)Pd], initially
in 47% yield (Scheme 1 and Table 1, entry 1). The assignment
of [(L3)Pd] as a rotaxane was initially made by mass
spectrometry (no dissociation of the components observed
in the fragmentation pattern without cleavage of a stopper)
and 1H NMR spectroscopy (for a stack plot of the spectra of
[(L3)Pd] and the corresponding thread showing upfield shifts
of the encapsulated regions of the rotaxane axle,[12] see the
Supporting Information). Confirmation of the mechanically
interlocked structure came from demetalation of [(L3)Pd]
(1m HCl (aq):CH2Cl2 (1:1), 92%, Scheme 1, step b), which
gave the weakly hydrogen bonded rotaxane H2L3 rather than
the dethreaded components. The 1H NMR spectrum (see the
Supporting Information) shows that the macrocycle in H2L3
preferentially resides over, and rapidly shuttles between, the
two thread ketone groups in CDCl3, even at 250 K. Finally,
single crystals of [(L3)Pd] were grown from a saturated
solution in butanone/methanol. The solid-state structure[10]
(Figure 1c) confirms that the Pd–nitrile coordination bond is
maintained in the metalated rotaxane and is responsible for
holding the macrocycle and thread in the well-defined co-
conformation observed in solution.
The rotaxane-forming reaction shown in Scheme 1 was
optimized in terms of reagent stoichiometry and conditions
(Table 1). The amount of base was decreased to 10 mol% to
minimize the background non-palladium promoted reaction
(Table 1, entry 2).[13] A concomitant increase in reaction
temperature (to 313 K,[14] Table 1, entry 3) was then required
for the reaction to achieve significant conversion within a
reasonable timeframe (76% yield of double Michael addition
products after 7 days). Under these conditions and using a
modest excess of the thread building blocks (2.5 versus
1.0 equiv), the yield of [2]rotaxane [(L3)Pd] was increased to
over 99% (Table 1, entry 6). Thus, this active template
reaction is among the most efficient, albeit rather slowly
proceeding, rotaxane-forming procedures reported to date.
Since the nitrile functionality is preserved in the product
of the active template reaction, it can in principle be used as a
binding site or “station” for the macrocycle–Pd component in
a molecular shuttle. To incorporate two such units into a
rotaxane, the reaction pattern for formation of the thread was
reversed, so that shuttle [(L4)Pd] was synthesized by Michael
additions of two “mono-stoppered” a-cyano esters (L5) to
bis(vinyl ketone) spacer 2 in the presence of [(L2)Pd-
(CH3CN)] (Scheme 2, step a).
addition synthesis of rotaxane [(L3)Pd][a]
Entry Thread components DIPEA
Total conver- Yield of
[equiv][b]
[equiv]
sion [%][c]
[(L3)Pd][%]
1[d]
2[e]
3[f]
4[f]
5[f]
6[f]
1.0
1.0
1.0
1.5
2.0
2.5
1.0
0.1
0.1
0.1
0.1
0.1
69
70
76
77
71
67
47
53
60
85
96
>99
[a]Reaction conditions: [( L2)Pd(CH3CN)], thread components 1 (”2)
and L1, DIPEA, CH2Cl2. [b]2:1 stoichiometry of 1:L1. [c]For L1+
[(L2)Pd(CH3CN)]+2”1![(L3)Pd]+noninterlocked thread. [d]5 days
at room temperature. [e]19 days at room temperature. [f]7 days at
313 K.
catalyze asymmetric 1,4-conjugate additions of a-cyano esters
to alkyl vinyl ketones in the presence of N,N-diisopropyle-
thylamine (DIPEA).[8] It seemed that a suitably constructed
tridentate macrocycle–PdII complex[9] might promote the
double Michael addition between ethyl cyanoacetate (L1)
and two suitably derivatized vinyl ketone “stoppers” (1) to
generate [2]rotaxanes with the intercomponent recognition
motif intact (Scheme 1).
Palladium(II)–macrocycle complex [(L2)Pd(CH3CN)]
was prepared in five steps from commercially available
materials (see the Supporting Information). Treatment of
[(L2)Pd(CH3CN)] with ethyl cyanoacetate in CH2Cl2 and
subsequent removal of the solvent and acetonitrile under
reduced pressure afforded complex [(L2)Pd(L1)], which
crystallized from a saturated solution in CH2Cl2/Et2O in a
form suitable for investigation by X-ray diffraction.[10] The
solid-state structure of both [(L2)Pd(L1)] (Figure 1a and b)
and [(L2)Pd(CH3CN)] (see the Supporting Information)
Figure 1. X-ray crystal structures[10] of [(L2)Pd(L1)]from a) face-on and
b) side-on views and c) rotaxane [(L3)Pd]. Selected bond lengths [Š]
and angles [8] : [(L2)Pd(L1)]: N1–Pd 2.02, N2–Pd 1.92, N3–Pd 2.04,
N4–Pd 2.02; N1-Pd-N3 161.7, N2-Pd-N4 177.9. [(L3)Pd]: N1–Pd 2.02,
N2–Pd 1.92, N3–Pd 2.03, N4–Pd 2.00; N1-Pd-N3 161.8, N2-Pd-N4
178.9.
There are few examples[15] of metal-complexed molecular
shuttles, and the investigation of their dynamics is of interest.
At room temperature in CDCl3, the exchange of the
palladium-coordinated macrocycle between the thread nitrile
groups in [(L4)Pd] is slow on the NMR timescale. Ligand
3382
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 3381 –3384

Angewandte
Chemie
rate in [D7]DMF (kobs = 3 sꢀ1) to be 600 times faster than in
CDCl3, corresponding to a rate kbi for the bimolecular process
involving solvent participation of 0.2mꢀ1 sꢀ1.
A bimolecular mechanism could also be made to operate
in CDCl3 by simply adding pyridine (Scheme 2, step b).
EXSY experiments showed that at 300 K addition of just
one equivalent of pyridine led to a 200-fold increase in the
rate of macrocycle shuttling in [(L4)Pd] to 1 sꢀ1, correspond-
ing to a bimolecular rate constant kbi = 95mꢀ1 sꢀ1. In other
words, a molecule of pyridine is around 500 times more
effective at accelerating the shuttling rate in [(L4)Pd] than a
molecule of DMF.[18]
Subsequently neutralizing the added pyridine with one
equivalent of TsOH (Scheme 2, step c) returned the shuttling
rate to its original value in CDCl3, providing powerful and
reversible control over the macrocycle dynamics by reagent
addition.
Scheme 2. Active template synthesis of a metal-coordinated molecular
shuttle, [(L4)Pd], with stimuli-responsive dynamics. a) L5 (2 equiv), 2
(1 equiv), [(L2)Pd(CH3CN)](1 equiv), DIPEA (0.1 equiv), CH Cl2,
2
313 K, 5 days; 27% [2]rotaxane [(L4)Pd]+32% of the corresponding
[3]rotaxane; b) pyridine (1 equiv); c) TsOH (1 equiv).
The PdII-catalyzed Michael addition of a-cyano esters to
vinyl ketones provides an exceptionally high yielding multi-
component active metal template reaction in which the
template motif is retained in the rotaxane product. The
reaction is operationally simple to perform and can generate
[2]rotaxanes in essentially quantitative yields with only a
modest excess of the thread-forming components. The
reaction can be applied to the construction of metal-
complexed molecular shuttles in which the dynamics of the
shuttling can be controlled by the addition of external
reagents. The search for other active template reactions that
offer different or improved rotaxane design features com-
pared with existing strategies is ongoing in our laboratory.
exchange at PdII centers proceeds predominately via associa-
tive pathways,[16] possibly through folding at the low concen-
trations employed herein, and no assistance for this mecha-
nism is intrinsically available from noncoordinating solvents
such as CDCl3. Two-dimensional exchange spectroscopy (2D-
EXSY)[17] gave the room-temperature pseudo-first-order rate
constant kobs = 5 ” 10ꢀ3 sꢀ1, which is equivalent to an activa-
tion free energy of approximately 20 kcalmolꢀ1 (details of the
kinetic measurements are given in the Supporting
Information). In [D7]DMF, the separated signals in the
1H NMR spectrum caused by slow shuttling of the macrocycle
at room temperature (Figure 2b) coalesced at 390 K (Fig-
ure 2c). EXSYexperiments at 300 K determined the shuttling Experimental Section
Experimental procedure for the active template synthesis of rotaxane
[(L3)Pd]: DIPEA (0.7 mL, 0.004 mmol) was added to a solution of
[(L2)Pd(CH3CN)] (30 mg, 0.040 mmol), 1 (120 mg, 0.20 mmol), and
L1 (10.7 mL, 0.10 mmol) in CH2Cl2 (0.4 mL). The solution was stirred
for 7 days at 313 K after which time the solvent was removed under
reduced pressure and the crude residue was subjected to column
chromatography on silica gel (0–1% MeOH in CH2Cl2, gradient
elution) to yield [(L3)Pd] as a yellow solid (81 mg, 99%). Full details
and compound characterization are provided in the Supporting
Information.
Received: December 20, 2007
Published online: March 20, 2008
Keywords: Lewis acid catalysts · molecular shuttles · palladium ·
.
rotaxanes · template synthesis
[1] a) V. Aucagne, K. D. Hänni, D. A. Leigh, P. J. Lusby, D. B.
Walker, J. Am. Chem. Soc. 2006, 128, 2186 – 2187; b) S. Saito, E.
Takahashi, K. Nakazono, Org. Lett. 2006, 8, 5133 – 5136; c) J.
Bernµ, J. D. Crowley, S. M. Goldup, K. D. Hänni, A.-L. Lee,
D. A. Leigh, Angew. Chem. 2007, 119, 5811 – 5815; Angew.
Chem. Int. Ed. 2007, 46, 5709 – 5713; d) V. Aucagne, J. Bernµ,
J. D. Crowley, S. M. Goldup, K. D. Hänni, D. A. Leigh, P. J.
Lusby, V. E. Ronaldson, A. M. Z. Slawin, A. Viterisi, D. B.
Walker, J. Am. Chem. Soc. 2007, 129, 11950 – 11963; e) J. D.
Crowley, K. D. Hänni, A.-L. Lee, D. A. Leigh, J. Am. Chem. Soc.
2007, 129, 12092 – 12093.
Figure 2. Partial 1H NMR (500 MHz, [D7]DMF) spectra of a) the free
thread at 298 K; b) molecular shuttle [(L4)Pd]at 298 K; and c) molec-
ular shuttle [(L4)Pd]at 390 K. The assignments correspond to the
lettering shown in Scheme 2. Signals labeled * are parts of sets of
diastereotopic protons.
Angew. Chem. Int. Ed. 2008, 47, 3381 –3384
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3383

Communications
[2] a) D. B. Amabilino, J. F. Stoddart, Chem. Rev. 1995, 95, 2725 –
complexes [M. S. Taylor, D. N. Zalatan, A. M. Lerchner, E. N.
Jacobsen, J. Am. Chem. Soc. 2005, 127, 1313 – 1317].
2828; b) MolecularCatenanes, Rotaxanes and Knots: A Journey
Through the World of Molecular Topology (Eds.: J.-P. Sauvage,
C. Dietrich-Buchecker), Wiley-VCH, Weinheim, 1999.
[9] The PdII complex of a benzylic amide macrocycle previously
successfully employed [a) A.-M. Fuller, D. A. Leigh, P. J. Lusby,
I. D. H. Oswald, S. Parsons, D. B. Walker, Angew. Chem. 2004,
116, 4004 – 4008; Angew. Chem. Int. Ed. 2004, 43, 3914 – 3918;
b) A.-M. L. Fuller, D. A. Leigh, P. J. Lusby, A. M. Z. Slawin,
D. B. Walker, J. Am. Chem. Soc. 2005, 127, 12612 – 12619] in the
passive template synthesis of interlocked architectures effi-
ciently catalyzes the double Michael addition of 1 with L1 but
only affords noninterlocked thread. Molecular modeling and X-
ray crystallography show that the benzylic methylene group
kinks the tridentate macrocyclic ring away from the nitrile ligand
bound to the fourth coordination site of the PdII center such that
Michael addition can occur out of the plane of the macrocycle
rather than through it. Replacing the benzylic groups with
anilide spacers changes the shape of the macrocycle (Figure 1)
such that alkylation of the Pd-coordinated ethyl cyanoacetate is
directed through the macrocycle cavity.
[3] For some recent innovations in passive template strategies to
rotaxanes, see: a) M. S. Vickers, P. D. Beer, Chem. Soc. Rev.
2007, 36, 211 – 225; b) S. J. Loeb, Chem. Soc. Rev. 2007, 36, 226 –
235; c) B. Champin, P. Mobian, J.-P. Sauvage, Chem. Soc. Rev.
2007, 36, 358 – 366; d) P. C. Haussmann, S. I. Khan, J. F. Stoddart,
J. Org. Chem. 2007, 72, 6708 – 6713; e) A. B. Braunschweig,
ˇ
´
W. R. Dichtel, O. S. Miljanic, M. A. Olson, J. M. Spruell, S. I.
Khan, J. R. Heath, J. F. Stoddart, Chem. Asian J. 2007, 2, 634 –
647; f) J. Wu, K. C.-F. Leung, J. F. Stoddart, Proc. Natl. Acad. Sci.
USA 2007, 104, 17266 – 17271; g) S.-Y. Kim, Y. H. Ko, J. W. Lee,
S. Sakamoto, K. Yamaguchi, K. Kim, Chem. Asian J. 2007, 2,
747 – 754; h) C.-W. Chiu, C.-C. Lai, S.-H. Chiu, J. Am. Chem.
Soc. 2007, 129, 3500 – 3501; i) H.-B. Yang, K. Ghosh, B. H.
Northrop, Y.-R. Zheng, M. M. Lyndon, D. C. Muddiman, P. J.
Stang, J. Am. Chem. Soc. 2007, 129, 14187 – 14189; j) A. I.
Prikhodꢀko, F. Durola, J.-P. Sauvage, J. Am. Chem. Soc. 2008,
130, 448 – 449; k) A.-M. L. Fuller, D. A. Leigh, P. J. Lusby,
Angew. Chem. 2007, 119, 5103 – 5107; Angew. Chem. Int. Ed.
2007, 46, 5015 – 5019; l) J. R. Johnson, N. Fu, E. Arunkumar,
W. M. Leevy, S. T. Gammon, D. Piwnica-Worms, B. D. Smith,
Angew. Chem. 2007, 119, 5624 – 5627; Angew. Chem. Int. Ed.
2007, 46, 5528 – 5531; m) S. Nygaard, B. W. Laursen, T. S.
Hansen, A. D. Bond, A. H. Flood, J. O. Jeppesen, Angew.
Chem. 2007, 119, 6205 – 6209; Angew. Chem. Int. Ed. 2007, 46,
6093 – 6097; n) H. W. Daniell, E. J. F. Klotz, B. Odell, T. D. W.
Claridge, H. L. Anderson, Angew. Chem. 2007, 119, 6969 – 6972;
Angew. Chem. Int. Ed. 2007, 46, 6845 – 6848; o) T. Sato, T.
Takata, Tetrahedron Lett. 2007, 48, 2797 – 2801; p) K. Nakazono,
T. Oku, T. Takata, Tetrahedron Lett. 2007, 48, 3409 – 3411; q) Y.
Tokunaga, N. Kawai, Y. Shimomura, Tetrahedron Lett. 2007, 48,
4995 – 4998; r) Y. Makita, N. Kihara, T. Takata, Chem. Lett. 2007,
36, 102 – 103; s) Y. Makita, N. Kihara, N. Nakakoji, T. Takata, S.
Inagaki, C. Yamamoto, Y. Okamoto, Chem. Lett. 2007, 36, 162 –
163; t) L. Li, G. J. Clarkson, Org. Lett. 2007, 9, 497 – 500; u) K.
Hirose, K. Nishihara, N. Harada, Y. Nakamura, D. Masuda, M.
Araki, Y. Tobe, Org. Lett. 2007, 9, 2969 – 2972; v) Y. Suzaki, K.
Osakada, Dalton Trans. 2007, 2376 – 2383.
[10] The crystal data and experimental details of the structural
refinement for [(L2)Pd(CH3CN)], [(L2)Pd(L1)], and [(L3)Pd]
are provided in the Supporting Information. CCDC-670786
([(L2)Pd(CH3CN)]), CCDC-670785 ([(L2)Pd(L1)]), and
CCDC-670784 ([(L3)Pd]) contain the supplementary crystallo-
graphic 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.
[11] a) D. A. Leigh, P. J. Lusby, A. M. Z. Slawin, D. B. Walker,
Angew. Chem. 2005, 117, 4633 – 4640; Angew. Chem. Int. Ed.
2005, 44, 4557 – 4564; b) D. A. Leigh, P. J. Lusby, A. M. Z.
Slawin, D. B. Walker, Chem. Commun. 2005, 4919 – 4921.
[12] E. R. Kay, D. A. Leigh, Top. Curr. Chem. 2005, 262, 133 – 177.
[13] In the absence of [(L2)Pd(CH3CN)], only 4% thread was
produced after 7 days (other conditions as for Table 1, entry 6),
indicating that the uncatalyzed background reaction is slow.
[14] Extended reaction times at higher temperatures (e.g. 333 K for
24 h) led to some decomposition of the PdII–macrocycle com-
plex.
[15] a) P. Gaviꢁa, J.-P. Sauvage, Tetrahedron Lett. 1997, 38, 3521 –
3524; b) N. Armaroli, V. Balzani, J.-P. Collin, P. Gaviꢁa, J.-P.
Sauvage, B. Ventura, J. Am. Chem. Soc. 1999, 121, 4397 – 4408;
c) M. C. Jimꢂnez, C. Dietrich-Buchecker, J.-P. Sauvage, Angew.
Chem. 2000, 112, 3422 – 3425; Angew. Chem. Int. Ed. 2000, 39,
3284 – 3287; d) M. C. Jimꢂnez-Molero, C. Dietrich-Buchecker,
J.-P. Sauvage, Chem. Eur. J. 2002, 8, 1456 – 1466; e) F. Durola, J.-
P. Sauvage, Angew. Chem. 2007, 119, 3607 – 3610; Angew. Chem.
Int. Ed. 2007, 46, 3537 – 3540; f) B. A. Blight, X. Wei, J. A.
Wisner, M. C. Jennings, Inorg. Chem. 2007, 46, 8445 – 8447;
g) J. D. Crowley, D. A. Leigh, P. J. Lusby, R. T. McBurney, L.-E.
Perret-Aebi, C. Petzold, A. M. Z. Slawin, M. D. Symes, J. Am.
Chem. Soc. 2007, 129, 15085 – 15090.
[16] a) J. D. Atwood, Inorganic and Organometallic Reaction Mech-
anisms, 2nd ed., Wiley-VCH, New York, 1997. For a discussion
of the mechanisms involved in the exchange of the fourth ligand
in analogous Pd pincer complexes, see: b) W. C. Yount, D. M.
Loveless, S. L. Craig, J. Am. Chem. Soc. 2005, 127, 14488 – 14496.
[17] C. L. Perrin, T. J. Dwyer, Chem. Rev. 1990, 90, 935 – 967.
[18] However, in neat [D5]pyridine, the palladium is sequestered
from the rotaxane.
[4] The triazole rings introduced during active template CuI-
catalyzed alkyne–azide cycloaddition (CuAAC) reactions (ref-
erences [1a] and [1d]) can be used as ligands for metals
coordinated to the macrocycle.
[5] E. R. Kay, D. A. Leigh, F. Zerbetto, Angew. Chem. 2007, 119,
72 – 196; Angew. Chem. Int. Ed. 2007, 46, 72 – 191.
[6] a) K. Takenaka, Y. Uozumi, Org. Lett. 2004, 6, 1833 – 1835; b) K.
Takenaka, M. Minakawa, Y. Uozumi, J. Am. Chem. Soc. 2005,
127, 12273 – 12281.
[7] M. A. Stark, G. Jones, C. J. Richards, Organometallics 2000, 19,
1282 – 1291.
[8] Other ligands that promote this reaction include bisoxazoline
pyrrole PdII complexes [C. Mazet, L. H. Gade, Chem. Eur. J.
2003, 9, 1759 – 1767], bidentate phospino-oxazoline PtII [A. J.
Blacker, M. L. Clarke, M. S. Loft, M. F. Mahon, J. M. J. Williams,
Organometallics 1999, 18, 2867 – 2873] and IrI [D. Carmona, J.
Ferrer, N. Lorenzo, F. J. Lahoz, I. T. Dobrinovitch, L. A. Oro,
Eur. J. Inorg. Chem. 2005, 1657 – 1664] catalysts, and (salen)Al
3384
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 3381 –3384