Download Chapter 2 - University of Amsterdam

UvA-DARE (Digital Academic Repository)
Chiral supramolecular ligands in transition metal catalysis
Bellini, R.
Link to publication
Citation for published version (APA):
Bellini, R. (2012). Chiral supramolecular ligands in transition metal catalysis
General rights
It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s),
other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons).
Disclaimer/Complaints regulations
If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating
your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask
the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam,
The Netherlands. You will be contacted as soon as possible.
UvA-DARE is a service provided by the library of the University of Amsterdam (http://dare.uva.nl)
Download date: 18 Jun 2017
Chapter 2
Supramolecular Control of Ligand
Coordination and Implications in
Hydroformylation Reactions
Zn(II)
H
H
CO
CO
Rh
P
Rh
CO
CO
CO
CO
P
Zn(II)
N
N
Zn(II)-TPP
=
2
N Zn
N
N
P
=
O
N P
O
N
Abstract: The coordination mode of a monodentate phosphoramidite ligand in a
rhodium complex can be switched from equatorial to axial mode by a unique
supramolecular pseudo encapsulation. The supramolecular axial complex displays
higher activity and selectivity in the challenging asymmetric hydroformylation of
internal alkenes.
Part of this chapter has been published: R. Bellini, S. H. Chikkali, G. BerthonGelloz, J. N. H. Reek, Angew. Chem. Int. Ed. 2011, 50, 7342-7345.
Chapter 2
2.1 Introduction
Coordination chemistry and organometallic chemistry have intensely fuelled the
field of transition metal catalysis, and knowledge based ligand design is, next to
combinatorial and high throughput experimentation, a leading approach to develop
new catalytic systems. For the industrially important hydroformylation reaction,
ligand effects have been studied in detail, also with the aid of high-pressure
spectroscopy techniques, (e.g. HP NMR and HP IR).1 The active species in this
reaction is a trigonal bipyramidal complex, with a hydride on the axial position. The
classical Wilkinson’s dissociative mechanism, based on triphenyl phosphine as
ligand, is widely accepted and explains most observations made so far. Two
coordination complexes have been observed with either the ligands coordinated in
the equatorial-equatorial (eq-eq) or in the equatorial-axial (eq-ax) mode (Figure
2.1).
H
H
PPh3
OC
PPh3
Rh
OC
Rh
PPh3
CO
PPh3
CO
eq-eq
eq-ax
Figure 2.1 Different coordination mode of a tris-phenylphosphine rhodium catalyst: eq-eq (both
equatorial) and eq-ax (equatorial-axial).
Van Leeuwen and co-workers elegantly demonstrated that bidentate phosphorus
ligands with wide bite angles provide predominantly the formation of the eq-eq
rhodium complex, resulting in highly selective rhodium-catalysed hydroformylation
of 1-octene, promoting the formation of the linear aldehyde product.2 In the area of
asymmetric hydroformylation the most promising class of ligands are hybrid
phosphine and phosphite or phosphoramidite.3 A real breakthrough in this field was
achieved by Takaya and Nozaki with the discovery of Binaphos, giving ee’s up to
95 % for a wide range of substrates.4 The reason for the exceptionally high
enantioselectivity is attributed to the exclusive formation of a single active species,
in which the ligand coordinates in eq-ax mode to the transition metal centre.
Monodentate ligands have been much less applied in these transformations as lower
selectivities are anticipated due to lower control over coordination modes.5
However, due to their simple structure, their synthesis is generally much less
elaborate making these ligands potentially cheaper and enabling the preparation of
large ligand libraries, which are required to rapidly find new active and selective
catalysts by rapid screening technologies. Application of very bulky monodentate
phosphite (π-accepting) ligands demonstrated that these form very active catalysts
that are also active in the hydroformylation of internal alkenes, albeit with
40
Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions
significant isomerisation.6 Spectroscopy experiments on such systems indicate that
only one phosphite ligand coordinates to the metal centre in the equatorial plane.
Along the same lines, Breit and co-workers have reported the use of bulky
phosphabenezenes in hydroformylation catalysis.7 In situ high pressure NMR
investigations demonstrate the prevalent formation of a monophosphine rhodium
complex in which the ligand is located in equatorial position whereas the hydride
occupies the axial site, which accounts for the reported activity and selectivity.
Recently, we have introduced a ligand-template approach for the supramolecular
encapsulation of transition metal complexes.8 Ligand-template tris-3-pyridyl
phosphine coordinate zinc(II)-porphyrins exclusively to the nitrogen donor atom,
whereas the phosphorus atom coordinates to the catalytically active rhodium. The
rhodium complex formed under hydroformylation conditions has only one
phosphorus atom coordinated in the equatorial plane, and this catalyst system
displayed unprecedented regioselectivity in the hydroformylation of
unfunctionalized terminal and internal alkenes. Based on these results we explored
the use of chiral analogues, i.e. typically based on the bis-naphthol skeleton
(Scheme 2.1). In this chapter, we report the remarkable supramolecular control over
the coordination chemistry of these chiral pyridine-containing phosphoramidite
ligands and the effect in the asymmetric rhodium-catalysed hydroformylation of
unfunctionalized internal alkenes by this unusual coordination complex.
N
NZn
N
N
N
N
N
2
O
P N
O
N Zn N
N
O
N P
O
2
N
N
1a
N
N
NZn
N
1a-(2)2
Scheme 2.1 The assembly of supramolecular ligand based on ligand 1a and tetraphenyl zinc(II)porphyrin 2.
41
Chapter 2
2.2 Ligand synthesis
Ligands (S)-1a-b were synthesised starting from the commercially available (S)2,2'-binaphthol, which was reduced in the presence of Pd/C (Scheme 2.2).9 The diol
3 obtained was selectively brominated in 3,3'-position at low temperature (-30 °C)
to give compound 4.9 Suzuki coupling of 4 with the opportune boronic acid
provided the intermediates 5a-b. Diols 5a-b were treated with P(NMe2)3 and
DMAP in refluxing toluene to give in good yield the ligands (S)-1a-b.10-11 All
compounds were fully characterized by 1H NMR, 31P NMR and 13C NMR
spectroscopy and high-resolution mass spectroscopy.
Br
OH
OH
(S)-Binol
OH
R B
OH
R = p-pyridyl, phenyl
PEPPSI, K3PO4
dioxane/H2O
H2
OH
Pd/C
OH
(S)-3, 98 %
Br2
OH
CH2Cl2
OH
Br
(S)-4, 70 %
R
R
OH
OH
P(NMe2)3
DMAP
Toluene
O
P N
O
R
R
(S)-5a R = p-pyridyl, 85 %
(S)-5b R = phenyl, 95 %
(S)-1a R = p-pyridyl, 90 %
(S)-1b R = phenyl, 95 %
Scheme 2.2 Synthesis of phosphoramidites ligands (S)-1a-b.
2.3 Coordination studies
To ascertain that ligands (S)-1a bind to two zinc(II)-porphyrin 2, we measured the
binding constants and performed a Job plot analysis. The Uv-vis titration
experiment revealed a binding constant of K1(1a-2) = 1.8 x 103 M-1 and K2(1a-2) = 1.7 x
103 M-1, associated to the first and second porphyrin binding to (S)-1a. The
similarity between these constants indicate that the binding events are independent
and no cooperativity is observed as previously with the tris-3-pyridyl phosphine.8b
The Job-plot analysis of NMR spectroscopy experiments in [D8]toluene confirmed
the formation of a 1:2 complex.
High-pressure NMR studies
In order to investigate the coordination behaviour of (S)-1a to rhodium under
catalytically relevant conditions we studied the complexes formed by high-pressure
(HP) NMR spectroscopy. Rhodium complexes were prepared in situ using
42
Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions
[Rh(acac)CO2] as metal precursor in [D8]toluene under syngas (H2/CO 1:1) at a
pressure of 5 bar (Scheme 2.3).
Scheme 2.3 High Pressure NMR spectra of cis-[Rh(H)(CO)31a] and trans-[Rh(H)(CO)3(1a(2)2)].
In line with previous reports,8b the complex based on bulky ligand (S)-1a is
monoligated [Rh(H)(CO)31a] with the phosphorus ligand in the equatorial position;
an hydride signal centred at -11.01 ppm is observed with a small PH coupling
typical of such cis-[Rh(H)(CO)31a] complex. Much to our surprise, in the presence
of two equivalent of porphyrin 2, this signal disappears and the HP 1H NMR shows
new signals in the hydride region; a double doublet centred at –10.3 ppm with a
large phosphorus coupling (JP-H = 180 Hz and JRh-H= 6.1 Hz) indicating that in this
complex the phosphorus donor atom is located trans to the hydride. The 1H-{31P}NMR spectrum shows a signal at -10.3 ppm confirming the large coupling between
the phosphorus and the hydride. Also the 2D 1H-31P-NMR spectra support the
formation of a complex with a very large H-P coupling. To the best of our
knowledge this is the first example of a rhodium complex in which a bulky
monodentate ligand is coordinated trans to the hydride.
Interestingly, the high pressure NMR studies using the control ligand (S)-1b reveals
the presence of both complexes, the cis complex (31P NMR: σ = 162.1 ppm, JP-Rh =
225 Hz; 1H NMR: σ = - 11.0 ppm, JP-H = 11 Hz, JRh-H = 3.6 Hz) and, in minor
amount, the trans complex (31P NMR: 162.1 ppm and JP-Rh = 225 Hz; 1H NMR: σ =
-9.55 ppm, JP-H = 185.6 Hz, JRh-H = 4.7 Hz). Also in this case the formation of these
complexes was further confirmed by 1H-{31P}-NMR and 2D 1H-31P-NMR.
43
Chapter 2
Importantly, these experiment establish the control on the ligand coordination mode
by addition of a supramolecular template, which triggers the ligand coordination on
rhodium from the cis position to trans position with respect to the hydride.
High-pressure IR studies
Using high-pressure IR spectroscopy we studied the formation of complexes cis[Rh(H)(CO)31a] and trans-[Rh(H)(CO)31a(2)2] under actual catalytic conditions,
using 20 bar of syngas (H2/CO 1:1) and concentrations identical to those in catalysis
experiments.12 In the presence of [Rh(acac)CO2] and ligand (S)-1a the triscarbonyl rhodium hydride complex cis-[Rh(H)(CO)31a] was obtained, as was
evident from the three peaks in the carbonyl region, 2054, 2000 and 1982 cm-1
(Figure 2.2). The rhodium complex formed in presence of (S)-1a and two
equivalents of porphyrin 2 shows three absorption bands that are shifted to higher
wavenumbers (2055, 2022, 1998 cm-1), in line what we expected as the CO is a
stronger π-accepting ligand than the phosphoramidite. This change also suggests
that CO dissociation from the trans-[Rh(H)(CO)31a(2)2] complex might be faster,
enhancing the alkene coordination step and possibly increasing the reaction rate.
In line with the HP NMR experiments, the HP IR studies using ligand (S)-1b show
formation of both cis-[Rh(H)(CO)31b] and trans-[Rh(H)(CO)31b] complexes,
characterised by four carbonyl bands in the IR spectrum (2058, 2033, 1998 and
1982 cm-1).
Figure 2.2 HP IR spectra of the cis-[Rh(H)(CO)31a] and trans-[Rh(H)(CO)3(1a(2)2)] complexes.
44
Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions
2.4 Asymmetric hydroformylation
To find out if the change in coordination mode would affect the catalytic
performance we used both complexes cis-[Rh(H)(CO)31a] and trans[Rh(H)(CO)31a(2)2] as catalyst for the asymmetric hydroformylation of internal
alkenes.13-14 We have chosen this reaction as 1) it is very challenging to introduce
functional groups in non-functionalized alkenes so success will lead to new
enabling technology 2) we previously demonstrated that encapsulation can lead to
unusual regioselective reactions8 3) monodentate phosphoramidite ligands have
been demonstrated to provide selective rhodium catalyst for the asymmetric
hydroformylation of functionalized alkenes.13c
The ligand (S)-1a and (S)-1a(2)2 as well as some control ligands (PPh3, (R,S)Binaphos and (S)-1b were studied in the rhodium-catalysed asymmetric
hydroformylation (AHF) of trans-2-octene (6) under 20 bar of syngas pressure
(H2/CO = 1/1) at 25 °C in toluene (1 mM).
Table 2.1 Results of the hydroformylation of trans-2-octene.[a]
O
O
CO/H2
6
7a
*
[Rh]+L
7b
*
Entry
Ligand
Conv%[b]
Iso%[c] 7a/7b[d]
7a ee%[e, f]
1
PPh3
6
5
0.8
0
2
(R,S) Binaphos
55
4
0.7
0
3
(S)-1b
12
4
0.7
11 (R)
4
(S)-1b +2
11
4
0.6
10 (R)
5
(S)-1a
12
4
0.7
25 (R)
6
(S)-1a(2)2
56
0
1
45 (R)
[a] [Rh] = 1mM in toluene, ligand/rhodium = 9, trans-2-octene/rhodium = 200, 25 °C, 20 bar, 84 h. [b]
Percentage conversion calculated using GC method. [c] Percentage isomerisation. [d] Ratio of the
products 7a and 7b. [e] 7a ee % = enantiomeric ratio for product 7a. [f] The absolute configuration
was determined by comparing the GC traces with the one from the enantiopure aldehydes (R)-7a and
(S)-7a. For detailed synthesis of (R)-7a and (S)-7a see Experimental section.
The challenging character of this reaction is clear from the results obtained with the
rhodium catalyst derived from triphenylphosphine, which only gave low conversion
and significant amount of isomerisation (Table 2.1, entry 1). The rhodium catalyst
based (R,S)-Binaphos (Table 2.1, entry 2) gave useful conversion (55 %) but no
45
Chapter 2
significant ee %. Complex [Rh(H)(CO)31a] provided the product with an ee of 26
%, albeit at relatively low conversion (Table 2.1, entry 5). Interestingly, in the
presence of the template 2, thus with complex [Rh(H)(CO)31a(2)2] an increase in
both conversion (54 %) and enantioselectivity (45 %) is observed (Table 2.1, entry
6). This indicates that the change of coordination mode improves the catalyst
performance in this challenging reaction both in terms of activity and selectivity.
Control experiments using ligand (S)-1b (Table 2.1, entry 3) that lacks the pyridyl
group shows that these complexes give rise to low activity and the product 7a is
produced in low ee, regardless of the presence of zinc(II)-porphyrin 2 (Table 2.1,
entry 4). Further indications of the absence of any effects of the zinc(II)-template in
the presence of ligand (S)-1b were given by IR studies and high-pressure NMR
experiments. No changes in the spectra were observed after addition of the zinc(II)template on the rhodium catalyst binding ligand (S)-1b. This demonstrates that
template 2 therefore does not interfere directly with the rhodium-catalysed
hydroformylation.
In further studies we explored a small series of sterically and electronically different
templates15-16 (Figure 2.3) in combination with ligand (S)-1a, aiming to further finetune the properties of the supramolecular ligands.
R
R
OAc
N
Br
B
B
[Zn] =
N
Zn
N
R=
N
10a
R
R
8
10c
10b
9
Cl
Cl
N
CO
N
Ru
N
N
N
t-Bu
O
N
Zn
Zn
N
t-Bu t-Bu
O
O
O
t-Bu
N
t-Bu
t-Bu
t-Bu
t-Bu
12b
12a
11
Figure 2.3 Boron-, zinc(II)- and ruthenium(II)-templates used in this study.
Remarkably, the properties of the template had very little effect on the catalyst
properties; in all cases the ee increases from 25 % (Table 2.2, entry 1), observed for
the catalyst based on non-templated ligand, to 35-42 % ee for the various templated
ligands (Table 2.2, entries 2-9). As we even observed this increase in ee for the
smallest boron based templates 8 and 9 (Table 2.2, entries 2 and 3), we conclude
that the dominant effect on enantioselectivity is associated with this change in
coordination mode of the ligand, switching from equatorial to axial upon
templation. In terms of regioselectivity (7b/7a ratio) and conversion also small
effects were noted upon changing the template.
46
Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions
Table 2.2 Hydroformylation trans-2-octene: template variation.[a]
Conv%[b] Iso%[c]
7a/7b[d]
7a ee%[e, f]
4
0.7
25 (R)
30
0.6
0.9
35 (R)
(S)-1a(9)2
30
1
0.8
41 (R)
4
(S)-1a(10a)2
43
0.6
0.9
37 (R)
5
(S)-1a(10b)2
52
0.6
1
41 (R)
6
(S)-1a(10c)2
52
0.6
1
41 (R)
7
(S)-1a(11)2
30
1
1
37 (R)
8
(S)-1a(12a)2
51
0.7
1
42 (R)
9
(S)-1a(12b)2
53
0
1
35 (R)
Entry
Ligand
1
(S)-1a
12
2
(S)-1a(8)2
3
[a] [Rh] = 1mM in toluene, ligand/rhodium = 9, trans-2-octene/rhodium = 200, 25 °C, 20 bar, 84 h. [b]
Percentage conversion calculated using GC method. [c] Percentage isomerisation. [d] Ratio of the
products 7a and 7b. [e] 7a ee % = enantiomeric ratio for product 7a. [f] The absolute configuration
was determined by comparing the GC traces with the one from the enantiopure aldehydes (R)-7a and
(S)-7a. For detailed synthesis of (R)-7a and (S)-7a see Experimental section.
To see if the unusual template effect is more general, we investigated the AHF of a
series of trans-2-alkenes and cis-2-alkenes.
Table 2.3 Asymmetric hydroformylation of E-2-alkenes using ligand (S)-1a and template 2.[a]
O
R
14(a-d)
R
*
[Rh]+L
O
CO/H2
13(a-d)
*
R
15(a-d)
R = C2H5, C3H7, C4H9, C5H11
Conv%[b] Iso%[c]
14/15[d] 14 ee%[e]
Entry
Substrate
Ligand
1
E-2-hexene
(S)-1a
23
2
0.7
21
2
E-2-hexene
(S)-1a(2)2
65
0
0.9
46
3
E-2-heptene
(S)-1a
22
1
0.7
26
4
E-2-heptene
(S)-1a(2)2
43
0
1.0
44
5
E-2-nonene
(S)-1a
40
0
0.7
23
6
E-2-nonene
(S)-1a(2)2
52
0
1.0
47
[a] [Rh] = 1mM in toluene, ligand/rhodium = 9, trans-2-alkene/rhodium = 200, 25 °C, 20 bar, 84 h. [b]
Percentage conversion calculated using GC method. [c] Percentage isomerisation. [d] Ratio of the
products 14 and 15. [e] 14 ee % = enantiomeric ratio for product 14.
47
Chapter 2
In Table 2.3 we report the catalytic results obtained using complexes cis[Rh(H)(CO)31a] and trans-[Rh(H)(CO)31a(2)2] in the presence of different trans-2alkenes. As observed for trans-2-octene, these substrates are also produced with
higher enantioselectivity if the templated ligand 1a(2)2 is used (Table 2.3, entries 2,
4 and 6) compared to the non-templated analogue (Table 2.3, entries 1, 3 and 5).
Complexes cis-[Rh(H)(CO)31a] and trans-[Rh(H)(CO)31a(2)2] were also evaluated
in the asymmetric hydroformylation of a small series of cis-2-alkenes (Table 2.4).
Although, higher conversions could, also in this case, be achieved with the
supramolecular system the enantioselectivities were not always higher for the
templated complex (Table 2.4, entry 4 vs 3, entry 8 vs 7).
Table 2.4 Asymmetric hydroformylation of Z-2-alkenes using ligand (S)-1a and template 2.[a]
O
R
16(a-d)
*
[Rh]+L
17(a-d)
R
O
CO/H2
*
R
18(a-d)
R = C2H5, C3H7, C4H9, C5H11
Entry
Substrate
Ligand
Conv%[b]
Iso%[c] 17/18[d]
17 ee%[e]
1
Z-2-hexene
(S)-1a
24
2
0.7
5
2
Z-2-hexene
(S)-1a(2)2
69
0
1.0
17
3
Z-2-heptene
(S)-1a
30
1
0.7
27
4
Z-2-heptene
(S)-1a(2)2
49
0
1.0
20
5
Z-2-octene
(S)-1a
40
1
0.7
9
6
Z-2-octene
(S)-1a(2)2
58
0
1.0
10
7
Z-2-nonene
(S)-1a
40
1
0.7
10
8
Z-2-nonene
(S)-1a(2)2
53
1
1.0
8
[a] [Rh] = 1mM in toluene, ligand/rhodium = 9, cis-2-alkene/rhodium = 200, 25 °C, 20 bar,
84 h. [b] Percentage conversion calculated using GC method. [c] Percentage isomerisation.
[d] Ratio of the products 17 and 18. [e] 17 ee % = enantiomeric ratio for product 17.
The substrate scope of cis-[Rh(H)(CO)31a] and trans-[Rh(H)(CO)31a(2)2]
complexes was also evaluated in the asymmetric hydroformylation of functionalised
alkenes, such as styrene, vinyl acetate and allyl cyanide (Table 2.5). For all
substrates, higher conversions were achieved with the supramolecular complex
compared to the non-template analogue. However, only in the hydroformylation of
vinyl acetate the trans-[Rh(H)(CO)31a(2)2] displayed higher enantioselectivity than
the cis-[Rh(H)(CO)31a] complex.
48
Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions
Table 2.5 Asymmetric hydroformylation of functionalised alkenes using ligand (S)-1a and template
2.[a]
Styrene
Vinyl Acetate
Allyl Cyanide
Entry
Ligand
Conv%[b]
b:l[c]
ee
%[d]
Conv%[b]
b:l[c]
ee
%[d]
Conv%[b]
b:l[c]
ee
%[d]
1
(S)-1a
30
97:3
4
61
99:1
11
20
70:30
14
2
(S)1a(2)2
50
99:1
2
72
99:1
40
25
70:30
2
[a] [Rh] = 1mM in toluene, ligand/rhodium = 9, alkene/rhodium = 200, 25 °C, 20 bar, 84 h. [b]
Percentage conversion calculated using GC method. [c] Ratio branched to linear aldehyde. [d]
Enantiomeric ratio determined by chiral GC.
2.5 Conclusions
In this chapter we have reported a new class of monodentate phosphoramidite
ligands of which the coordination mode to rhodium can be controlled in a unique
supramolecular fashion, providing a new tool to control the activity and selectivity
of a transition metal catalyst. In situ high-pressure NMR and IR studies under
hydroformylation conditions, demonstrate the formation of the first rhodiumhydride complex in which the phosphorus donor atom of the ligand is trans to the
hydride, but only after coordination of boron or zinc(II) templates to the pyridyl
moieties of the ligand. In absence of these templates the common monoligated
rhodium-hydrido complexes are formed with the ligand in the equatorial plane, in
cis orientation to the hydride. Interestingly, the supramolecular change to the
unusual coordination is reflected in higher activity and selectivity when these
complexes are applied to the very challenging asymmetric hydroformylation of
unfunctionalized internal alkenes.
2.6 Experimental section
Unless stated otherwise, reactions were carried out under an atmosphere of argon using
standard Schlenk techniques. NMR spectra (1H, 31P and 13C) were measured on a Bruker
DRX 400 MHz and Inova 500 MHz; CDCl3 was used as a solvent, if not further specified.
High-resolution mass spectra were recorded on a JEOL JMS SX/SX102A four sector mass
spectrometer; for FAB-MS 3-nitrobenzyl alcohol was used as matrix. UV-vis spectroscopy
experiments were performed on a Cary UV/Visible System. Gas chromatographic analyses
were run on a Shimadzu GC-17A apparatus (split/splitless injector, J&W Scientific, DB-1
J&W 30 m column, film thickness 3.0 µm, carrier gas 70kPa He, FID Detector). Chiral GC
separations were conducted on Interscience HR GC apparatus with a Supelco β-dex 225
(0.25 mm x 30 m) capillary column.
49
Chapter 2
With exception of the compounds given below, all reagents were purchased from
commercial suppliers and used without further purification. The following compounds were
synthesized according to published procedures:
(S)-5,6,7,8,5,6,7,8-octahydro-(1,1'-binaphtalene)-2,2'-diol 3,9
(S)-3,3'-Dibromo-5,6,7,8,5',6',7',8'-octahydro-(1,1'-binaphtalene)-2,2'-diol 4,9
(S)-3,3'-Diphenyl-5,5',6,6',7,7',8,8'-octahydro-1,1'-binaphtyl-2,2'-diol 5b,10
(S)-3,3'-Diphenyl-5,5',6,6',7,7',8,8'-octahydro-1,1'-binaphtyl-phosphoroamidite 1b,10
Zinc(II)-porphyrins 2 and 10a-c, 15
Zinc(II)-salphens 12a-b.16
Synthesis of (S)-3,3'-4pyridin-2,2'-dihydroxy-1,1'-dinaphthyl (S)-5a: In a flame dried
Schlenk (452 mg, 1 mmol) of (S)-4 and p-pyridinyl-boronic acid (307 mg, 2.5 equiv) were
suspended in degassed dioxane (10 mL, 0.1 M) and an aqueous degassed solution of K3PO4
(1 M 3.5 mL) was added. The mixture was heated to 100 °C until a clear solution was
obtained. After, [Pd(iPr)Cl2(3-ClPy)] (14 mg, 2 mol % ) was added under inert atmosphere
and the resulting mixture was warmed at 100 °C. (16 h, TLC control, EtOAc)
The reaction mixture was cooled to room temperature and the product precipitated out as
zwitterions salt was isolated by filtration, washed with Et2O, and dried under reduced
pressure. The resulting solid was used for the next step without any further purification.
(S)-5a: Yield: 381 mg 85 % (white solid); 1H-NMR (400 MHz, (CD3)2SO): δ = 8.55 (d, 4H,
J = 5.6 Hz), 7.57 (d, 4H, J = 5.6 Hz), 7.08 (s, 2H), 3.36 (m, 4H), 2.24 (m, 2H), 2.07 (m, 2H),
1.65 (m, 8H); 13C-ATP (101 MHz): 149.9 (C), 147.7 (CH), 147.2 (C), 137.7 (C), 130.4
(CH), 129.1 (C), 124.6 (C), 124.4 (C), 124.3 (CH), 29.2 (CH2), 27.4 (CH2), 23.1 (CH2), 23.0
(CH2). HRMS (FAB+): m/z calcd. for C30H29N2O2 [M+H]+: 449.2229; obsd.: 449.2225.
Synthesis of (S)-3,3'-4pyridin-1,1'-dinaphthyl-phosphoramidite (S)-1a: Diol (S)-5a (300
mg, 0.66 mmol) and (Me2N)3P (0.24 ml, 1.32 mmol), DMAP (10 mol %) were dissolved in
toluene (0.1 M) and refluxed for 16 h during which time the reaction mixture became
homogeneous. After, the reaction mixture was cooled to room temperature and a saturated
solution of NH4Cl was added and the organic layer was extracted with EtOAc. The organic
phase was washed with water and dried over MgSO4. The solvent was removed and the
residue was purified by crystallization in ethyl acetate.
Ligand (S)-1a: Yield: 309 mg 90 % (white solid); 1H-NMR (400 MHz, CDCl3): δ = 8.62
ppm (m, 4H), 7.60 (m, 2H), 7.53 (m, 2H), 7.28 (s, 1H), 7.24 (s, 1H), 2.93 (m, 4H), 2.73 (m,
2H), 2.40 (m, 2H), 1.99 (s, 3H), 1.95 (s, 3H), 1.8 (m, 8H); 13C-ATP (101 MHz): 149.6 (CH),
146.0 (C), 139.2 (C), 130.3 (C), 129.4 (CH), 27.6 (C), 128.1 (CH), 124.4 (CH), 123.7 (CH),
34.4 (CH3), 29.1 (CH2), 27.9 (CH2), 22.6 (CH2); 31P NMR (162 MHz): δ = 140.6 ppm.
HRMS (FAB+): m/z calcd. for C32H33N3O2P [M+H] +: 522.2232; obsd.: 522.2305.
General Procedure for the Rhodium-Catalysed Hydroformylation Reactions: A typical
experiment was carried out in a stainless steel autoclave (150 mL) charged with an insert
suitable for 14 reaction vessels (equipped with Teflon mini stirring bars) for performing
parallel reactions. Each vial was charged with zinc-template 2 (16 mg, 18 µmol), ligand (S)1a (4.7 mg, 9 µmol), [Rh(acac)CO2] (0.26 mg, 1 µmol), substrate (0.03 mL, 20 µmol) and
toluene (1 mL). The substrate was filtered over basic alumina to remove possible peroxide
impurities. The toluene was distilled from sodium prior to use. Before starting the catalysis,
50
Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions
the charged autoclave was purged three times with 10 bar of syngas (H2/CO = 1/1) and then
pressurized to 20 bar. After the catalytic reaction, the pressure was reduced to 1.0 bar and a
few droops of tri-n-butyl-phosphite were added in each reaction vessels to prevent any
further reaction. The reaction mixtures were not filtered over basic alumina to remove
catalyst residues, because filtration may cause retention of the aldehydes and thus influence
the GC-results. The mixtures were diluted with CH2Cl2 for GC-analysis.
The enantiomeric ratio was analysed by chiral GC (Supelco β-dex 225) isothermal at 80 °C,
1.0 °C/min to 105 °C for 10 min, 40 °C/min to 200 °C. 2-ethylheptanal (7a): (S)-enantiomer
t = 14.8 min, (R)-enantiomer t = 14.9 min; 2-methyloctanal (7b): 1st enantiomer t = 19.4
min, 2nd enantiomer t = 19.5 min. We did not calculate the ee % for the compound 7b
because the GC-separation of the two enantiomers was not optimal.
The absolute configuration was determined by comparing the chiral GC traces of the
reaction mixture with the enantiopure aldehydes
(S)-2-ethylheptanal and (R)-2ethylheptanal obtained by organic synthesis.
Synthesis of (R)-2-ethylheptanal (Scheme 2.4): (R)-7a17
MeO
MeO
O
H
18
N
RAMP
CH2Cl2,
4 A MS
N
N
LDA, EtI
N
O
O3
H
H
H
THF
CH2Cl2
(R,R)-20
(R)-19
(R)-7a
Scheme 2.4 Synthesis of (R)-2-ethylheptanal, (R)-7a.
Hydrazone (R)-19: To a flame dried molecular sieves (250 mg) were added (R)-(+)-1amino-2-(methoxymethyl)pyrrolidine (RAMP) (489 mg, 3.75 mmol) and CH2Cl2 (1.35
mL). After cooling the mixture to 0 °C heptanal (0.58 mL, 4.17 mmol) was added and the
reaction was left stirring for 24 h at RT. The resulting mixture was filtered over a plug of
celite and purified by Kugelrohr distillation to afford (R)-19 as a yellow oil (510 mg, Y=
60%). [α]22D = +107.2° (c = 1.00, CH2Cl2). 1H-NMR (400 MHz, CDCl3): δ = 6.66 (t, 1H, J
= 5.5 Hz), 3.56 (dd, 1H, J = 8.5, 3.4 Hz), 3.44-3.33 ppm (m, 6H), 2.75-2.69 (q, 1H, J = 8.5
Hz), 2.23-2.18 ppm (m, 2H), 1.99-1.85 ppm (m, 5H), 1.81-1.75 ppm (m, 1H), 1.50-1.42 (m,
2H), 1.37-1.26 ppm (m, 4H), 0.88 (t, 3H, J = 6.5 Hz); 13C-ATP (101 MHz): 139.5 (CH),
74.8 (CH2), 63.5 (CH3), 59.1 (CH), 50.5 (CH2), 33.1 (CH2), 31.8 (CH2), 28.8 (CH2), 27.8
(CH2), 26.5 (CH2), 22.5 (CH2), 22.1 (CH2), 14.0 (CH3). HRMS (FAB+): m/z calcd. for
C13H27N2O [M+H]+: 227.2045; obsd.: 227.2123.
Hydrazone (R,R)-20: To a stirred solution of i-Pr2NH (0.35 mL, 2.55 mmol) in dry THF
(3.2 mL) was added n-BuLi ( 1.3 mL, 1.6 M in hexanes) at -78 °C. After 1 h a solution of
(R)-19 (503 mg, 2.22 mmol) in dry THF (2.0 mL) was added. The reaction mixture after 1.5
h was slowly warmed to -40 °C and 30 min later to -20 °C. After 30 min the reaction was
warmed to 0 °C and kept at this temperature for 1.5 h. Subsequently the reaction mixture
was cooled to -115 °C and EtI (0.27 mL, 3.37 mmol) was added. After 45 min the reaction
51
Chapter 2
was allowed to warm to RT and quenched with a saturated solution of NaHCO3 (5 mL).
Water (50 mL) and Et2O (25 mL) were added and the water layer was extracted with Et2O
(2x 50 mL). The combined organic layers were washed with brine (25 mL), dried over
MgSO4, filtered and concentrated in vacuo. The crude product was purified by flash
chromatography (silica gel, EtOAc/Hept. 1:20 to 1:3) to give (R,R)-20 (338.6 mg, Y= 60%)
as a colorless oil. [α]22D = +92.9° (c = 1.00, CH2Cl2). 1H-NMR (400 MHz, CDCl3): δ = 6.47
(d, 1H, J = 7.4 Hz), 3.58 (dd, 1H, J = 9.2, 3.2 Hz), 3.47-3.34 ppm (m, 6H), 2.74 (q, 1H, J =
8.3 Hz), 2.14-2.06 ppm (m, 1H), 2.01-1.86 ppm (m, 3H), 1.84-1.78 ppm (m, 1H), 1.54-1.23
(m, 13H), 0.88 (m, 3H); 13C-ATP (101 MHz): 144.4 (CH), 74.7 (CH2), 63.5 (CH3), 59.2
(CH), 50.7 (CH2), 44.1 (CH), 33.4 (CH2), 32.0 (CH2), 26.7 (CH2), 26.6 (CH2), 26.5 (CH2),
22.6 (CH2), 22.0 (CH2), 14.1 (CH3), 11.6 (CH3). HRMS (FAB+): m/z calcd. for C15H31N2O
[M+H]+: 255.2358; obsd.: 255.2437.
2-ethyl-heptanal (R)-7a: Through a stirred solution of (R,R)-20 (129.2 mg, 0.51 mmol) in
CH2Cl2 (15 mL) at -78 °C was bubbled ozone until there was no starting material left
according to TLC (EtOAc/ Hept. 1:3). Nitrogen was bubbled through the solution for 30
min and subsequently the reaction mixture was allowed to warm to RT and dimethyl sulfide
(0.75 mL, 10.2 mmol) was added. The mixture was stirred overnight at RT. Water (25 mL)
was added and the reaction mixture was extracted with CH2Cl2 (2x 25 mL). The solvent was
evaporated in vacuo and the residue was purified by flash chromatography (silica gel,
CH2Cl2/Pent. 1:1) to afford (R)-7a (50 mg, Y= 68%) as a colorless oil [α]22D = +90.4° (c =
1.00, CH2Cl2). 1H-NMR (400 MHz, CDCl3): δ = 9.59 (d, 1H, J = 3.1 Hz), 2.19 (m, 1H),
1.66 (m, 2H), 1.58 (m, 2H), 1.30 (m, 6H), 0.93 (m, 6H); 13C-ATP (101 MHz): 203.6 (CH),
51.4 (CH3), 29.7 (CH2), 28.7 (CH), 26.3 (CH2), 25.0 (CH2), 20.7 (CH2), 19.7 (CH2), 0.81
(CH3). HRMS (EI): m/z calcd. for C9H18O [M]+: 142.1358; obsd.: 142.1363. The
enantiomeric ratio was analysed by GC (Supelco β-dex 225 capillary column, 80 °C, 1.0
C/min up to 105 °C (10 min), 40 °C/min up to 200 °C (2.0 min): (S)-7a t = 14.8 min, (R)-7a
t = 14.9 min. ee = 94 % R-(+).
Synthesis of (S)-2-ethylheptanal (Scheme 2.5): (S)-7a17
MeO
MeO
O
H
18
N
SAMP
CH2Cl2,
4 A MS
N
N
LDA, EtI
N
O
O3
H
H
H
THF
CH2Cl2
(S)-21
(S,S)-22
(S)-7a
Scheme 2.5 Synthesis of (S)-2-ethylheptanal, (S)-7a.
Hydrazone (S)-21: Y= 88% [α]22D = -115.0° (c = 1.00, CH2Cl2). 1H-NMR (400 MHz,
CDCl3): δ = 6.66 (t, 1H, J = 5.5 Hz), 3.56 (dd, 1H, J = 8.5, 3.4 Hz), 3.44-3.33 ppm (m, 6H),
2.75-2.69 (q, 1H, J = 8.5 Hz), 2.23-2.18 ppm (m, 2H), 1.99-1.85 ppm (m, 5H), 1.81-1.75
ppm (m, 1H), 1.50-1.42 (m, 2H), 1.37-1.26 ppm (m, 4H), 0.88 (t, 3H, J = 6.5 Hz); 13C-ATP
(101 MHz): 139.5 (CH), 74.8 (CH2), 63.5 (CH3), 59.1 (CH), 50.5 (CH2), 33.1 (CH2), 31.8
52
Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions
(CH2), 28.8 (CH2), 27.8 (CH2), 26.5 (CH2), 22.5 (CH2), 22.1 (CH2), 14.0 (CH3). HRMS
(FAB+): m/z calcd. for C13H27N2O [M+H]+: 227.2045; obsd.: 227.2123.
Hydrazone (S,S)-22: Y= 64% [α]22D = -104.9° (c = 1.00, CH2Cl2). 1H-NMR (400 MHz,
CDCl3): δ = 6.47 (d, 1H, J = 7.4 Hz), 3.58 (dd, 1H, J = 9.2, 3.2 Hz), 3.47-3.34 ppm (m, 6H),
2.74 (q, 1H, J = 8.3 Hz), 2.14-2.06 ppm (m, 1H), 2.01-1.86 ppm (m, 3H), 1.84-1.78 ppm (m,
1H), 1.54-1.23 (m, 13H), 0.88 (m, 3H); 13C-ATP (101 MHz): 144.4 (CH), 74.7 (CH2), 63.5
(CH3), 59.2 (CH), 50.7 (CH2), 44.1 (CH), 33.4 (CH2), 32.0 (CH2), 26.7 (CH2), 26.6 (CH2),
26.5 (CH2), 22.6 (CH2), 22.0 (CH2), 14.1 (CH3), 11.6 (CH3). HRMS (FAB+): m/z calcd. for
C15H31N2O [M+H]+: 255.2358; obsd.: 255.2436.
2-ethyl-heptanal (S)-7a: Y= 50% [α]22D = - 99.7° (c = 1.00, CH2Cl2). 1H-NMR (400 MHz,
CDCl3): δ = 9.59 (d, 1H, J = 3.1 Hz), 2.19 (m, 1H), 1.66 (m, 2H), 1.58 (m, 2H), 1.30 (m,
6H), 0.93 (m, 6H); 13C-ATP (101 MHz): 203.6 (CH), 51.4 (CH3), 29.7 (CH2), 28.7 (CH),
26.3 (CH2), 25.0 (CH2), 20.7 (CH2), 19.7 (CH2), 0.81 (CH3). HRMS (EI): m/z calcd. for
C9H18O [M]+: 142.1358; obsd.: 142.1360. The enantiomeric ratio was analysed by GC
(Supelco β-dex 225 capillary column, 80 °C, 1.0 C/min up to 105 °C (10 min), 40 °C/min up
to 200 °C (2.0 min): (S)-7a t = 14.8 min, (R)-7a t = 14.9 min. ee = 94 % S-(-).
Chiral GC data for hydroformylation products.
The enantiomeric ratio was analysed by chiral GC (Supelco β-dex 225): isothermal at 80 °C,
1.0 °C/min to 105 °C for 10 min, 40 °C/min to 200 °C.
Hydroformylation trans-2-hexene and cis-2-hexene. 2-ethylpentanal: 1st enantiomer t =
6.16 min, 2nd enantiomer t = 6.29 min; 2-methylhexanal: 1st enantiomer t = 8.30 min, 2nd
enantiomer t = 8.30 min. The ee % for 2-methylhexanal was not calculated because the GCseparation of the two enantiomers was not optimal.
Hydroformylation trans-2-heptene and cis-2-heptene. 2-ethylhexanal: 1st enantiomer t =
9.40 min, 2nd enantiomer t = 9.57 min; 2-methylheptanal: 1st enantiomer t = 12.77 min, 2nd
enantiomer t = 12.81 min. The ee % for the 2-methylheptanal was not calculated because the
GC-separation of the two enantiomers was not optimal.
Hydroformylation cis-2-octene. 2-ethylheptanal: 1st enantiomer t = 14.8 min, 2nd
enantiomer t = 14.9 min; 2-methyloctanal: 1st enantiomer t = 19.4 min, 2nd enantiomer t =
19.5 min. The ee % for the 2-methyloctanal was not calculated because the GC-separation
of the two enantiomers was not optimal.
Hydroformylation trans-2-nonene and cis-2-nonene. 2-ethyloctanal: 1st enantiomer t =
22.80 min, 2nd enantiomer t = 23.11 min; 2-methylnonanal: 1st enantiomer t = 26.37 min, 2nd
enantiomer t = 26.40 min. The ee % for the 2-methylnonanal was not calculated because the
GC-separation of the two enantiomers was not optimal.
Hydroformylation styrene. The ee’s were calculated by chiral GC analysis (Supelco
BETA DEX 225). Initial temperature = 100 °C for 5 min, then 4 °C/min to 160 °C.
tR (S) = 11.5 min. and tR (R) = 11.8 min.
Hydroformylation vinyl acetate. The ee’s were calculated by chiral GC analysis (Supelco
BETA DEX 225). Initial temperature = 100 °C for 5 min, then 4 °C/min to 160 °C.
tR (S) = 6.1 min. and tR (R) = 7.8 min.
Hydroformylation allyl cyanide. The ee’s were calculated by chiral GC analysis (Astec
Chiraldex A-TA). Initial temperature = 90 °C for 7 min, then 5 °C/min to 180 °C.
53
Chapter 2
tR (S) = 16.3 min. and tR (R) = 16.7 min.
Preparation of the Hydride Complexes for high-pressure NMR: A solution of ligand
(S)-1a-b (1 equiv.), zinc(II)-template 2 (2 equiv.), [Rh(acac)CO2] in toluene-d8 (20 mM)
was stirred at 40 °C for 3 h. After this time the mixture was transferred into 5 mm HP NMR
tube, pressurized with 5 bar of syngas H2/CO 1:1 (5 bar) and left at 40 °C for 16 h. After this
time the high-pressure NMR spectra were recorded.
Preparation of the Hydride Complexes for high-pressure IR: High-pressure IR
experiments were performed in an SS-316 50 mL autoclave equipped with IRTRAN
windows (ZnS, transparent above 700 cm-1, ø = 10 mm, optical path length = 0.4 mm), a
mechanical stirrer, temperature controller, and a pressure device. In a typical experiment,
the high-pressure IR autoclave was filled with (S)-1a (33 mg, 0.063 mmol), zinc(II)porphyrin 2 (85 mg, 0.126 mmol) and 13 mL of dichloromethane. The autoclave was purged
three times with 15 bar of H2/CO (1:1) and pressurized to 20 bars. The anti-chamber was
charged with a solution of [Rh(acac)CO2] (2.7 mg, 0.01 mmol) in dichloromethane (2 mL)
and pressurized to 30 bar. The HP-IR autoclave was placed into a Nicolet 510 FTIR
spectrometer and the temperature was set up to 40 °C. When the desire temperature was
reached the anti-chamber was opened and the catalyst precursor was injected into the
solution. The series of IR spectra was recorded for 24 h while the samples were stirred.
2.7 References
1.
2.
3.
4.
5.
6.
a) P. W. N. M. van Leeuwen and C. Claver, in Rhodium-catalysed Hydroformylation,
Kluwer Academic Publishers, Dordrecht 2000; b) B. Breit, W. Seiche, Synthesis 2001;
c) P. W. N. M. van Leeuwen, Homogeneous Catalysis, Understanding the Art, Kluwer
Academic Publishers, Dordrecht 2004.
a) M. Kranenburg, Y. E. M. van der Burgt, P. C. J. Kamer, P. W. N. M. van Leeuwen,
Organometallics 1995, 14, 3081-3089; b) L. A. van der Veen, P. H. Keeven, G. C.
Schoemaker, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. van Leeuwen, M. Lutz, A. L.
Spek, Organometallics 2000,19, 872-883.
a) S. Deerenberg, P. C. J. Kamer, P. W. N. M. van Leeuwen, Organometallics 2000,
19, 2065-2072; b) O. Pàmies, G. Net, A. Ruiz, C. Claver, Tetrahedron: Asymmetry
2001, 12, 3441-3445; c) M. Rubio, A. Suárez, E. Álvarez, C. Bianchini, W.
Oberhauser, M. Peruzzini, A. Pizzano, Organometallics 2007, 26, 6428-6486; d) Y.
Yan, X. Zhang, J. Am. Chem. Soc. 2006, 128, 7198-7201; e) S. Chikkali, R. Bellini, G.
Berthon-Gelloz, J. I. van der Vlugt, B. de Bruin, J. N. H. Reek, Chem. Commun. 2010,
46, 1244-1246; f) X. Zhang, B. Cao, Y. Yan, S. Yu, B. Ji, X. Zhang, Chem. Eur. J.
2010, 16, 871-877; g) T. Robert, Z. Abiri, J. Wassenaar, A. J. Sandee, S. Romanski, J.
M. Neudörfl, H. G. Schmalz, J. N. H. Reek, Organometallics 2010, 29, 478-483; h) J.
Wassenaar, B. de Bruin, J. N. H. Reek, Organometallics 2010, 29, 2767-2776; i) G.
Francio, F. Faraone, W. Leitner, Angew. Chem. Int. Ed. 2000, 39, 1428.
a) K. Nozaki, N. Sakai, T. Nanno, T. Higashijima, S. Mano, T. Horiuchi, H. Takaya, J.
Am. Chem. Soc. 1997, 119, 4413-4423; b) K. Nozaki, T. Matsuo, F. Shibahara, T.
Hiyama, Organometallics, 2003, 22, 594-600; c) D. A. C. Molina, C. P. Casey, I.
Müller, K. Nozaki, C. Jäkel, Organometallics 2010, 29, 3362-3367.
Z. Hua, V. C. Vassar, H. Choi, I. Ojima, Proc. Natl. Acad. Sci. U. S. A. 2004, 101,
5411.
P. W. N. M. van Leeuwen, C. F. Roobeek, J. Organomet. Chem. 1983, 258, 343-350.
54
Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
B. Breit, R. Winde, T. Mackewitz, R. Paciello, K. Harms, Chem. Eur. J. 2001, 7, 31063121.
a) V. F. Slagt, J. N. H. Reek, P. C. J. Kamer, P. W. N. M van Leeuwen, Angew. Chem.
Int. Ed. 2001, 40, 4271-4274; b) V. F. Slagt, P. C. J. Kamer, P. W. N. M. van
Leeuwen, J. N. H. Reek, J. Am. Chem. Soc. 2004, 126, 1526-36; c) M. Kuil, T. Solter,
P. W. N. M. van Leeuwen, J. N. H. Reek, J. Am. Chem. Soc. 2006, 128, 11344-5; d) A.
W. Kleij, M. Kuil, D. M. Tooke, A. L. Spek, J. N. H. Reek, Inorg. Chem. 2005, 44,
7696-7698; e) A. W. Kleij, M. Lutz, A. L. Spek, P. W. N. M. van Leeuwen, J. N. H.
Reek, Chem. Commun. 2005, 3661-3663; f) A. W. Kleij, J. N. H. Reek, Chem. Eur. J.
2006, 12, 4219-4227.
A. Korostylev, V. I. Tararov, C. Fischer, A. Monsees, A. Borner, J. Org. Chem. 2004,
69, 3220-322.
a) M. Bartosek, M. Beller, J. Deutsch, M. Klawonn, A. Kockritz, N. Nemati, A. PewsDavtyan, Tetrahedron 2008, 64, 1316-1322; d) G. Erre, K. Junge, S. Enthaler, D.
Addis, D. Michalik, A. Spannenberg, M. Beller, Chem. Asian J. 2008, 3, 887-94.
a) N. Kudo, M. Perseghini, G. C. Fu, Angew. Chem. Int. Ed. 2006, 45, 1282-4; b) C. J.
O’Brien, E. A. B. Kantchev, C. Valente, N. Hadei, G. A. Chass, A. Lough, A. C.
Hopkinson, M. G. Organ, Chem. Eur. J. 2006, 12, 4743-8.
For high-pressure IR studies see: a) P. C. J. Kamer, A. van Rooy, G. C. Schoemaker, P.
W. N. M. van Leeuwen, Coordination Chemistry Reviews 2004, 248, 2409-2424; b) C.
Kubis, R. Ludwig, M. Sawall, K. Neymeyr, A. Börner, K. D. Wiese, D. Hess, R.
Franke, D. Selent, ChemCatChem 2010, 2, 287-295.
For Hydroformylation see: a) B. Breit, W. Seiche, Synthesis 2001, 1-36; b) B. Breit,
Top. Curr. Chem. 2007, 279, 139-172; c) B. Breit, E. Fuchs, Chem. Commun. 2004,
694-695; d) E. Fuchs, M. Keller and B. Breit, Chem. Eur. J. 2006, 12, 6930-6939; e)
M. L. Clarke, Current Org. Chem. 2005, 9, 701-718; f) N. Sakai, K. Nozaki, and H.
Takaya, Chem. Commun. 1994, 395-396; g) R. I. McDonald, G. W. Wong, R. P.
Neupane, S. S. Stahl, C. R. Landis, J. Am. Chem. Soc. 2010, 132, 14027-14029.
For alternative supramolecular control see: a) T. Smejkal, B. Breit, Angew. Chem. Int.
Ed. 2008, 47, 311-315; Angew. Chem. Int. Ed. 2008, 47, 3946-3949; b) P. Dydio, W.
Dzik, M. Lutz, B. de Bruin, J. N. H. Reek, Angew. Chem. Int. Ed. 2011, 50, 396-400.
a) A. D. Adler, F. R. Longo, J. D. Finarelli, J. D. Goldmacher, J. Assour, L. Korsakoff,
J. Org. Chem. 1967, 32, 467; b) A. D. Adler, F. R. Longo, F. Kampas, J. Kim, J. Inorg.
Nucl. Chem. 1970, 32, 2443-2445.
A. W. Kleij, M. Kuil, D. M. Tooke, M. Lutz, A. L. Spek, J. N. Reek, Chem. Eur. J.
2005, 11, 4743-4750.
D. C. J. Waalboer, H. A. van Kalkeren, M. C. Schaapman, F. L. van Delft, F. P. J. T.
Rutjes, J. Org. Chem. 2009, 74, 8878-8881.
55