Cyclic decapeptide gramicidin S derivatives containing phosphines: Novel ligands for asymmetric catalysis

Article abstract of DOI:10.1039/c2dt31782f

The cyclic peptide gramicidin S was used as a rigid template to provide novel peptide-based bisphosphine ligands for transition metal catalysis. Two bisphosphine-coordinated Rh(i) complexes allowed asymmetric hydrogenation with 10-52% ee and the corresponding Pd(ii) analogues catalysed asymmetric allylic alkylation with 13-15% ee. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt31782f

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Cyclic decapeptide gramicidin S derivatives containing
phosphines: novel ligands for asymmetric catalysis†‡
Cite this: Dalton Trans., 2013, 42, 1973
Received 3rd August 2012,
Gregorio Guisado-Barrios,a Bianca K. Muñoz,a Paul C. J. Kamer,*a Bas Lastdrager,b
Gijs van der Marel,b Mark Overhand,*b Marino Vega-Vázquezc and
Manuel Martin-Pastorc
Accepted 27th November 2012
DOI: 10.1039/c2dt31782f
www.rsc.org/dalton
The cyclic peptide gramicidin S was used as a rigid template to problems to overcome.20 Metallopeptides are presented as
provide novel peptide-based bisphosphine ligands for transition alternative hybrid catalysts, since they are readily accessible.16
metal catalysis. Two bisphosphine-coordinated Rh(I) complexes They are obtained by incorporating a metal complex into a
allowed asymmetric hydrogenation with 10–52% ee and the peptide.21 Gilbertson and co-workers as pioneers of this area
corresponding Pd(II) analogues catalysed asymmetric allylic alkyl- made use of a fruitful approach where oligopeptides are used
ation with 13–15% ee.
as scaffolds to create a unique asymmetric environment for
organo- or transition metal catalysis.22–25 Of key importance
for the design of peptide-based catalysts for bidentate metal
chelation is the structure of the oligopeptide and the spatial
orientation of its ligands amenable for transition metal com-
plexation,15 as well as the nonproteinogenic metal coordinat-
ing moieties used, which enhance the coordination to metals.
Examples with carboxylate-containing side chains,26 phos-
phines27 and pyridines28 have been described. Phosphines are
arguably the most successful ligands in transition metal
catalysis.15
The natural product gramicidin S (GS) is a promising
scaffold for the construction of metal complexes.29 New hybrid
chiral ligands incorporating achiral phosphines into the chiral
scaffolds of biomolecules may generate biomimetic molecules
with interesting catalytic properties once coordinated to active
metals such as palladium, ruthenium or rhodium. GS is a
cyclic decameric peptide ((Pro-Val-Orn-Leu-dPhe)2) with C2-
symmetry that adopts a rigid cyclic β-hairpin structure.
Because of the cyclic β-hairpin structure the side-chains of the
two ornithine (Orn) residues are on the same side of the mole-
cule and have been used for the attachment of ligands for
transition metals. Exemplary is the design and synthesis of a
GS-tetrakis(2-pyridylmethyl) dinuclear zinc complex that is
able to catalyse the hydrolysis of a phosphodiester bond.30
Several modified-phosphine gramicidin S derivatives I–III have
been recently reported by our laboratories and presented as
potential chiral ligands for catalysts (Fig. 1), maintaining the
general β-hairpin secondary structure of the parent GS mole-
cule as confirmed by X-ray crystal structure analysis. It was
concluded that there are significant differences between the
different GS derivatives, which are found in the shape and size
of the linkage between the oligopeptide backbone and the
phosphine.
Introduction
Catalysis is of vital importance for many industrial chemical
processes. Still, man-made catalysts do not meet the excep-
tional selectivity obtained by enzymes in living systems. On
the other hand, no enzymes are known to catalyse most of the
reactions of industrial interest such as hydroformylation,1–3
hydrogenation,4–9 and allylic substitution.10–14 The design of
functional molecules able to catalyse a variety of asymmetric
transformations is one of the important challenges in contem-
porary chemistry.15 The design of artificial metalloenzymes,
where a transition metal is anchored covalently or in a supra-
molecular fashion to an existing protein,16 has involved tre-
mendous effort.17 Several highly enantioselective hybrid
catalysts have been reported.18 Although these artificial
metalloenzymes can be further improved using the strength of
directed evolution methodologies,19 there are still practical
aEaStCHEM, School of Chemistry, University of St Andrews, St Andrews, Fife KY16
9ST, UK. E-mail: pcjk@st-andrews.ac.uk; Fax: +44 (0)1334463808;
Tel: +44 (0)1334467258
bFaculty of Science, Leiden Institute of Science, Eisteinweg55, 2333 CC Leiden,
Netherlands. E-mail: overhand@chem.leidenuniv.nl; Tel: +31 (0)71 527 4483
cLaboratorio Integral de Dinámica e Estructura de Biomoléculas José R. Carracido,
Unidade de Resonancia Magnética, Edificio CACTUS, RIAIDT, Univesidade de
Santiago de Compostela, 15706 Santiago de Compostela, Spain.
E-mail: manuel.martin@usc.es; Tel: +34-981 56 31 00, Ext. 16231
†Dedicated to an inspiring scientist and friend, Prof. David Cole-Hamilton, for
his invaluable contribution to transition metal catalysis, on the occasion of his
retirement. “Onwards and upwards” as Geoffrey Wilkinson used to say!
‡Electronic supplementary information (ESI)
available. See DOI:
10.1039/c2dt31782f
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groups can be incorporated into gramicidin S by reaction of
the free amino groups in the two ornithine residues, which are
situated at one side of the β-sheet. For instance, two bis-
(pyridylmethyl)amino groups were introduced in place of free
amino groups, and it was demonstrated that this system accel-
erates the cleavage of phosphodiesters in the presence of
zinc(II).30 Most of the hybrid ligands reported30–33 contain a
phosphine modified amino acid which is protected prior to
incorporation into the peptide chain, requiring extra deprotec-
tion steps and cleavage from the solid support. To avoid extra
steps, we followed the method of Christensen and Meldal.34
Phosphine moieties were incorporated in solution, via
amide bond formation, to the amino residue of the
amino acid ornithine prior to complexation with rhodium(I) or
palladium(II).
We have prepared four new GS derivatives following a pub-
lished method by our laboratories, incorporating o-, m-,
p-diphenylphosphinobenzoic acids 1a, 1b, 1c and diphenyl-
phosphinopropanoic acid 1d, previously activated by reaction
with N-hydroxysuccinimide in dichloromethane (see Fig. 2).35
The reaction of the corresponding phosphines GS−PP (3a, 3b,
3c and 3d) with [Rh(nbd)2]BF4 leads to the formation of
rhodium(I) complexes [Rh(nbd)(3a–d)]BF4 (4a–d) which were
characterised by 1H, 31P NMR spectroscopy and MALDI-TOF
Fig. 1 Schematic representation of sulfur protected diphosphine-functiona-
lized gramicidin derivatives I–III.
Preliminary modelling studies indicated that these bipho-
sphine-functionalized gramicidin S derivatives were suitable
ligands for transition-metals, favouring the subtle trans-
bisphosphine coordination with metal over cis configuration.29
We here report the first asymmetric catalytic hydrogenation
and allylic substitution using four new GS derivatives contain-
ing phosphines (GS–PP) 3a, 3b, 3c and 3d (Fig. 2), for the con-
struction of bidentate phosphine metal complexes of
palladium and rhodium.
Results and discussion
The synthesis and characterisation of four new bisphosphine- MS (see ESI‡).
containing cyclic decapeptide derivatives 3a, 3b, 3c and 3d of
Phosphorus chemical shifts for complexes 4a–d and phos-
gramicidin S are reported (see Fig. 2, also ESI‡). Complexation phorus–rhodium coupling constants for 4a, 4b and 4c are in
with rhodium and palladium was studied by 31P NMR spec- the expected range (Table 1). The resonances were found
troscopy and the resulting complexes were evaluated in asym- around 30 ppm as expected with JRh–P of the order of 170 Hz,
metric hydrogenation and allylic amination. Several functional showing that phosphorus atoms were coordinated to the
Fig. 2 Synthesis of modified-phosphine GS derivatives, 3a–3d.
1974 | Dalton Trans., 2013, 42, 1973–1978
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rhodium centre.36 The 31P NMR spectrum of 4a indicates that
We tested the catalytic performance of rhodium(I) com-
the metal complex keeps C2 symmetry, since a doublet was plexes of 4a–d in the asymmetric hydrogenation of substrates
observed. The 31P NMR spectrum of complex 4b shows a broad A, B and C (Fig. 3). The results are summarised in Table 2.
signal probably due to coalescence between two different
It can be observed that only metal complexes 4b and 4c
complex structures or two non-equivalent phosphorus atoms, show catalytic activity. Rhodium complex 4b containing the
on the other hand, only one species was detected by MALDI- phosphine in the meta position was found to be more active
TOF MS.
and selective than 4c (phosphine in para position) for all sub-
At the same time, the 31P NMR spectrum of complex 4c strates studied, probably due to the closer proximity of the two
shows two different doublets at 30.9 and 29.1 ppm, which phosphines to the chiral scaffold. However, the unexpected
suggests either the presence of two different conformers or inactivity of 4a breaks this trend. At the same time, metal
one metal complex with two non-equivalent phosphorus complex 4d showed no activity for asymmetric hydrogenation.
nuclei. However, this last possibility was discarded since Preliminary modeling studies comparing three different grami-
MALDI-TOF MS of 4c shows only one species. Similar to 4c, cidin S complexes 4a–c (containing triphenylphosphine substi-
the 31P NMR spectrum of complex 4d also shows two doublets tuted at the o-, m-, p- position) predicted higher enantiomeric
almost overlapped, indicating that two conformers are present, induction in the order o- > m- > p- based on the generally
since only one species was detected by MALDI-TOF MS.
accepted quadrant model4 as the phosphine moieties coordi-
The coordination chemistry with the palladium dimer [Pd- nated to metal that were attached to gramicidin S were
(η3-allyl)(μ-Cl)]2 was studied for the ligands meta- and para-GS directed towards the peptide backbone. Interestingly, 4b and
−PP, 3b and 3c respectively. Reactions were performed by 4c produced enantiomers R and S respectively, showing that
mixing the ligand and the metal precursor in dichloromethane the simple variation on the phosphine position has a direct
under argon.
The new palladium(II) complexes 5b and 5c showed phos-
phorus resonances at 23.1 and 22.2 ppm, respectively. These
chemical shifts are in the typical region for palladium phos-
phine complexes indicating the coordination of the ligand to
the palladium centre. Rh and Pd complexes 4b and 5b were
fully characterised by including 31P–1H HSQC and TOCSY (see
ESI‡).
Table 2 Catalytic hydrogenation results using GS derivativesa
Entry
Complex
Substrate
% Conv.
% ee
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
A
A
A
A
A
B
B
B
C
C
C
C
0
—
57
30
0
15 (R)
13 (S)
—
0
—
99
>99
0
43 (R)
7 (S)
—
Table 1 31P{1H} chemical shifts for the complexes 4a–d
9
0
—
10
11
12
73
36
0
52 (R)
10 (S)
—
Complex
δ (ppm), [mult.]
J(Rh–P) Hz
[Rh(nbd)(3a)](BF4)
[Rh(nbd)(3b)](BF4)
[Rh(nbd)(3c)](BF4)
[Rh(nbd)(3d)](BF4)
(4a)
(4b)
(4c)
(4d)
29.7 [d]
30.7 [broad]
30.9 [d], 29.1 [d]
26.0 [d], 25.9 [d]
170
—
a Reaction conditions: 48 nmol catalyst [Rh(nbd)(3a–d)]BF4 (4a–d),
176.8, 161.7 substrate : Rh : L = 30 : 1 : 1, solvent: DCM 0.5 mL, P = 20 bar, T = 25 °C,
172.8, 175.9 t = 18 h.
Fig. 3 Substrates utilised in hydrogenation reactions catalysed by Rh complexes of phosphine containing GS derivatives, 3a–d.
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configuration of products was observed for 4b to 4c in asym-
metric hydrogenation and allylic alkylation, which indicates
that small changes in the ligand influence tremendously the
outcome of the reaction. We are currently expanding the range
of phosphorus containing GS derivatives to find active and
selective systems for several catalytic reactions.
Shortening the side-chains of the amino acid residue would
place the metal closer to the chiral scaffold, which may
improve the enantiomeric excess of the reactions.
Fig. 4 Asymmetric allylic alkylation of 1,3-diphenylallyl acetate.
Table 3 Catalytic results of asymmetric allylic alkylationa
Entry Complex
Substrate Nucleophile % Conv. % ee
Experimental
General procedure for hydrogenation reactions
13
14
[Pd(allyl)(3b)]Cl (5b)
[Pd(allyl)(3c)]Cl (5c)
D
D
DMM
DMM
14
12
15 (R)
13 (S)
a Reaction conditions: 58 nmol catalyst [Pd(allyl)(3b–c)]Cl (5b–c),
nucleophile : BSA : allyl acetate : Pd : L 120 : 120 : 40 : 1 : 1, catalytic
amount of KOAc, solvent: DCM 0.5 mL, T = 25 °C, t = 18 h.
The reactions were carried out in a GC vial, reaction volume
0.5 mL, due to the very small amounts of catalyst required; it
was observed that reproducibility of the results critically
depends on the care taken in the preparation of the catalytic
samples. Dichloromethane was distilled over CaH2, and
degassed prior to use.
=
influence on the absolute configuration of the products. The
best results in terms of activity were found for substrate B
using complexes 4b and 4c, where more than 99% conversion
to the desired product was detected by GC. For substrates A
and C, the conversions observed were from moderate to good
(30–73%).
This may explain why metal complexes 4b and 4c, in which
the phosphorus atoms are much closer than in 4d, yielded
higher catalytic activity and enantioselectivity, while metal
complex 4a containing the most sterically hindered phosphine
is not active.
Complex 4b gives the best ee both for methyl 2-acetamido-
acrylate B (43%), and for methyl-Z-α-acetamidocinnamate C
(52%). These results show that the design of ligands using a
biological chiral scaffold is a promising approach.
The palladium(II) complexes 5b and 5c have been evaluated
in allylic substitution of 1,3-diphenylallyl acetate (D) with the
anion from dimethyl malonate (DMM) (see Fig. 4). Both com-
plexes gave low conversions and 13 and 15% ee respectively.
The results are summarised in Table 3.
Catalytic runs were prepared in GC vials, which were pre-
viously purged and flushed with argon three times in a carrou-
sel or a Schlenk vessel under argon. The metal complex stock
solution was obtained by adding 0.1 mL of [Rh(nbd)2]BF4
(2.21 mg, 5.82 μmol) previously dissolved in 1 mL of DCM to a
solution of 0.5 mL of DCM of the appropriate ligand (1 mg,
0.582 μmol). The final volume of the metal complex stock solu-
tion was 0.6 mL. The solution was stirred at room temperature
for 20 min. Stock solutions of the different substrates,
dimethyl itaconate, methyl acetamidoacrylate, methyl acetamido-
cinnamate and decane (internal standard) were prepared in
Schlenk vessels which were previously purged and flushed
with argon three times, 14.6 μmol of substrate was dissolved
in 5 mL of DCM, 7.3 μmol of decane was dissolved in 5 mL of
DCM. The GC vials in the carrousel were filled with 1.46 μmol
of substrate, 0.73 μmol of decane, and 58.2 nmol of the metal
complex. The GC vial was filled with 0.35 mL of DCM to
obtain a final reaction volume of 0.5 mL. All GC vials
were placed in a stainless steel insert and introduced in a
stainless steel autoclave; under argon, all GC vials were
punched with a needle. The autoclave was flushed 3 times
with 5 bar of H2 and finally pressurised with 20 bar of H2 and
stirred at room temperature for 18 hours. After the reaction,
the 0.5 mL reaction volume of each run was passed through a
silica pad, washed with 1 mL of DCM–MeOH (9 : 1) solution
prior to GC analysis.
An inversion of absolute configuration of the products was
observed, as occurred for hydrogenation, since complexes
bearing ligands 3b m- and 3c p produced the opposite enantio-
mers, S and R respectively.
Conclusions
In summary, unprotected phosphine moieties can be directly
incorporated into the cyclic decapeptide gramicidin S via
amide bond formation avoiding extra steps. The phosphine-
General procedure for allylic alkylation
containing m- and p-GS derivatives were found to be active in Catalytic runs were prepared in GC vials, which were previously
Rh-catalysed asymmetric hydrogenation with up to 52% ee and purged and flushed with argon three times in a carrousel or a
in Pd-catalysed allylic substitution with up to 15% ee. Complex Schlenk vessel under argon containing a catalytic amount of
4b is the most active of the whole series, for all substrates in KOAc, and the stirring bar. The GC vials were placed in the car-
asymmetric hydrogenation giving the highest ee, probably due rousel. The metal source [Pd(allyl)Cl]2 (1 mg, 2.73 μmol) was
to the angle between the phosphines and the metal along with dissolved in 1 mL of DCM and 0.273 μmol of this solution was
steric environment. Interestingly, an inversion of the absolute added to the stock solution 0.5 mL of DCM of the appropriate
1976 | Dalton Trans., 2013, 42, 1973–1978
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ligand (1 mg, 0.582 μmol). The final volume of the stock solu- 14 G. Helmchen and A. Pfaltz, Acc. Chem. Res., 2000, 33,
tion containing the metal complex was 0.6 mL. 336–345.
Stock solutions of the substrate, 1,3-diphenylallyl acetate, 15 P. W. N. M. van Leeuwen, Homogeneous Catalysis: Under-
dimethyl malonate, BSA and diphenyl ether (internal standard) standing the Art, 2004.
were prepared in Schlenk vessels, which were previously 16 (a) T. Heinisch and T. R. Ward, Curr. Opin. Chem. Biol.,
purged and flushed with argon three times. To the GC vial
containing 300 μL KOAc was added 58.2 nmol of the metal
2010, 14, 184–199; (b) J. Steinreiber and T. Ward, Coord.
Chem. Rev., 2008, 252, 751–766.
complex, 14.6 μmol of substrate, and the reaction mixture was 17 (a) G. Roelfes, ChemCatChem, 2011, 3, 647–648;
stirred for 20 min, after this time 6 μmol of dimethyl malo-
nate, 6 μmol of BSA and 7.3 μmol of diphenyl ether in 5 mL of
(b) F. Rosati and G. Roelfes, ChemCatChem, 2010, 2,
916–927.
DCM were added. The final reaction volume was 0.5 mL. All 18 (a) M. E. Wilson and G. M. Whitesides, J. Am. Chem. Soc.,
GC vials were stirred at room temperature for 18 hours.
To work up the reactions, the mixtures were filtered over a
plug of silica and washed with 1 mL of DCM. The solvent was
evaporated, and HPLC grade hexane was added. The samples
were analysed by HPLC.
1978, 100, 306–307; (b) J. Collot, J. Gradinaru, N. Humbert,
M. Skander, A. Zocci and T. R. Ward, J. Am. Chem. Soc.,
2003, 125, 9030–9031; (c) M. T. Reetz and N. Jiao, Angew.
Chem., Int. Ed., 2006, 45, 2416–2419; (d) A. Pordea,
M. Creus, J. Panek, C. Duboc, D. Mathis, M. Novic and
T. R. Ward, J. Am. Chem. Soc., 2008, 130, 8085–8088;
(e) A. J. Boersma, D. Coquiere, D. Geerdink, F. Rosati,
B. L. Feringa and G. Roelfes, Nat. Chem., 2010, 2, 991–995.
19 (a) M. T. Reetz, J. Org. Chem., 2009, 74, 5767–5778;
(b) M. T. Reetz, Proc. Natl. Acad. Sci. U. S. A., 2004, 101,
5716–5722.
20 M. T. Reetz, J. J.-P. Peyralans, A. Maichele, Y. Fu and
M. Maywald, Chem. Commun., 2006, 4318–4320.
21 S. Bajusz, T. Janaky, V. J. Cernus, L. Bokser, M. Fekete,
G. Srkalovic, T. W. Redding and A. V. Schally, Proc. Natl.
Acad. Sci. U. S. A., 1989, 86, 6313–6317.
Acknowledgements
We acknowledge J.M. Otero for helping with NMR sample
preparation, and the EU for funding: NEST Adventure STREP
Project artizymes, contract no. FP6-2003-NEST-B3 15471; Marie
Curie Excellence Grants, artizyme catalysis, contract number
MEXT-2004-014320; Network of Excellence Idecat (Idecat-
CT-2005-011730); COST action CM0802 PhoSciNet.
22 B. R. Linton, M. H. Reutershan, C. M. Aderman,
E. A. Richardson, K. R. Brownell, C. W. Ashley, C. A. Evans
and S. J. Miller, Tetrahedron Lett., 2007, 48, 1993–1997.
23 S. R. Gilbertson, S. E. Collibee and A. Agarkov, J. Am. Chem.
Soc., 2000, 122, 6522–6523.
Notes and references
1 Rhodium Catalyzed Hydroformylation, ed. P. W. N. M. Van
Leeuwen and C. Claver, Kluwer, Dordrecht, 2000.
2 M. L. Clarke, Curr. Org. Chem., 2005, 9, 701–718.
3 E. D. Daugs, W.-J. Peng and C. L. Rand, Hydroformylation 24 R. R. Cesati, J. De Armas and A. H. Hoveyda, J. Am. Chem.
process for pharmaceutical intermediate, Patent Application Soc., 2004, 126, 96–101.
WO/2005/110986, Application Number: PCT/US2005/014349, 25 A. G. Doyle and E. N. Jacobsen, Chem. Rev., 2007, 107,
Dow Global Technologies, 2005. 5713–5743.
4 Handbook of Homogeneous Hydrogenation, ed. J. G. de Vries 26 (a) B. V. Popp and Z. T. Ball, Chem. Sci., 2011, 2, 690–695;
and C. J. Elsevier, Wiley-VCH Verlag GmbH & Co. KGaA,
Weinheim, Germany, 2007.
5 B. D. Vineyard, W. S. Knowles, M. J. Sabacky, G. L. Bachman
and D. J. Weinkauff, J. Am. Chem. Soc., 1977, 99, 5946–5952.
6 S. J. Roseblade and A. Pfaltz, Acc. Chem. Res., 2007, 40,
1402–1411.
7 H. U. Blaser and E. Schmidt, Asymmetric Catalysis on Indus-
trial Scale: Challenges, Approaches and Solutions, 2004.
8 Comprehensive Asymmetric Catalysis I–III, ed. E. N. Jacobsen,
A. Pfaltz and H. Yamamoto, 1999, vol. 1.
(b) R. Sambasivan and Z. T. Ball, J. Am. Chem. Soc., 2010,
132, 9289–9291; (c) A. N. Zaykov, K. R. MacKenzie and
Z. T. Ball, Chem.–Eur. J., 2009, 15, 8961–8965.
27 A. C. Laungani, J. M. Slattery, I. Krossing and B. Breit,
Chem.–Eur. J., 2008, 14, 4488–4502.
28 D. Coquiere, J. Bos, J. Beld and G. Roelfes, Angew. Chem.,
Int. Ed., 2009, 48, 5159–5162.
29 S. Burck, S. G. A. van Assema, B. Lastdrager, J. C. Slootweg,
A. W. Ehlers, J. M. Otero, B. Dacunha-Marinho,
A. L. Llamas Saiz, M. Overhand, M. J. van Raaij and
K. Lammertsma, Chem.–Eur. J., 2009, 15, 8134–8145.
9 R. Noyori, Angew. Chem., Int. Ed., 2002, 41, 2008–2022.
10 B. M. Trost, K. Lehr, D. J. Michaelis, J. Xu and A. K. Buckl, 30 K. Yamada, Y.-I. Takahashi, H. Yamamura, S. Araki,
J. Am. Chem. Soc., 2010, 132, 8915–8917. K. Saito and M. Kawai, Chem. Commun., 2000, 1315.
11 B. M. Trost, M. R. Machacek and A. Aponick, Acc. Chem. 31 S. R. Gilbertson and R. V. Pawlick, Angew. Chem., Int. Ed.,
Res., 2006, 39, 747–760. 1996, 35, 902–904.
12 B. M. Trost and M. L. Crawley, Chem. Rev., 2003, 103, 32 S. R. Gilbertson, X. Wang, G. S. Hoge, C. A. Klug and
2921–2943. J. Schaefer, Organometallics, 1996, 15, 4678–4680.
13 B. M. Trost and D. L. Van Vranken, Chem. Rev., 1996, 96, 33 S. R. Gilbertson and G. W. Starkey, J. Org. Chem., 1996, 61,
395–422.
2922–2923.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 1973–1978 | 1977

View Article Online
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34 C. A. Christensen and M. Meldal, Chem.–Eur. J., 2005, 11,
4121–4131.
J. H. Van Boom, G. A. Van der Marel, H. S. Overkleeft and
M. Overhand, J. Org. Chem., 2004, 69, 7851–7859.
35 G. M. Grotenbreg, M. Kronemeijer, M. S. M. Timmer, 36 A. Agarkov, S. J. Greenfield, T. Ohishi, S. E. Collibee
F. ElOualid, R. M. Van Well, M. Verdoes, E. Spalburg,
P. A. V. VanHooft, A. J. De Neeling, D. Noort,
and S. R. Gilbertson, J. Org. Chem., 2004, 69, 8077–
8085.
1978 | Dalton Trans., 2013, 42, 1973–1978
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