4
J. Potier et al. / Catalysis Communications xxx (2014) xxx–xxx
Scheme 3. Possible dipolar interactions resulting from inclusion of 1 within the β-CDNH2
cavity from the primary (left) or the secondary (right) face.
of 2 was found unchanged in solution either after 24 h at room temper-
ature (ca. 60%) or after 5 h heating to 80 °C (ca. 42%), respectively.
A different behaviour was observed upon addition of β-CDNH2 (2
equiv.) to a solution of 2, as witnessed by the corresponding 31P{1H}
NMR spectrum. The initial doublet centred at δ −33.1 ppm (1JRhP
=
160 Hz) corresponding to 2 was replaced by 6 doublets falling in the
1
range between −28 and −37 ppm, with JRhP in the range 129–
Fig. 3. Partial 2D T-ROESY NMR spectrum (tBu protons) of a 2:1 mixture of β-CDNH2 and 2
at room temperature in D2O.
158 Hz (Fig. 2). Attempts to separate the different components of the
mixture or grow crystals suitable for X-ray diffraction data collection
failed to date.
as expected, both the solubility and the stability in water of the com-
plex were increased by the addition of either native β-CD or β-
CDNH2.
Different Rh-phosphine species can be hypothesized to form in solu-
tion, including chloride complexes (higher 1JRhP) and aquo complexes
(lower 1JRhP), keeping a coordinated Rh(η4-cod) moiety, with mixed
first and second sphere coordination giving P,N and P,N,N complexes,
derived from various substitution degrees on the metal centre, as
shown in Scheme 2.
3.2. In-situ formation of supramolecular complexes
In order to enhance water solubility, inclusion of 2 in cyclodextrins can
be expected upon recognition of the tBu-benzyl group, which is known to
be well recognized by the β-CD cavity [25,26]. At first, addition of 1 equiv.
of native β-CD (8.0 mg, 7.0 × 10−3 mmol) to a suspension of 2 (5 mg, 7.0 ×
10−3 mmol) in D2O (2 mL) was tested, showing however that this ratio
was clearly insufficient to solubilise 2 completely at 25 °C. Conversely,
when 2 equiv. of native β-CD (16 mg, 1.4 × 10−2 mmol) were used
under the same conditions, a clear yellow solution was obtained. We
reasoned that under these conditions both the hydrophobic character
of cod ligand and tBu-benzyl group might be masked into the hydro-
phobic CD cavity. Interestingly, also the stability in water of complex 2
was increased. 31P{1H} NMR spectra showed that a significant amount
To confirm that the tBu-phenyl group was to a large extent included
into the β-CDNH2 cavity, 2D T-ROESY NMR experiments have been car-
ried out. This particular NMR sequence, commonly used to identify the
recognition process between the CD “host” and the phosphine ligand
“guest”, evidenced the existence of dipolar contacts between the pro-
tons of the tBu-phenyl group of 2 and the β-CDNH2 inner protons H-3
and H-5, hence the expected inclusion within the β-CD cavity (Figs. 3
and 4). Moreover, weak correlations were also observed between the
aromatic protons and the H-6 and H-5 CD protons (Fig. 4), indicative
of a weak penetration of the ligand into the CD cavity. Accordingly, in-
clusion of 2 took place through the β-CDNH2 primary face confirming
that the nitrogen and the phosphorus atoms were located on the same
side of the CD. As no cross-peaks were observed between the aromatic
protons and H-3, inclusion through the CD secondary face can be ex-
cluded (Scheme 3).
3.3. Catalytic unsaturated and allylic alcohol hydrogenation tests
The possible use of the supramolecular β-CD/rhodium complex as-
semblies as homogeneous aqueous-phase catalysts was at first evaluat-
ed in the proof-of-concept hydrogenation of a water soluble substrate,
namely 2-methyl-3-buten-2-ol (Scheme 4). The choice of this model
substrate was motivated not only by its high solubility in water, but
also because no C_C isomerization can take place, thus limiting the
number of products to analyse. The effect of the CD nature on the activ-
ity and stability of the catalytic system was assessed, comparing the
combination of 2/native β-CD and 2/β-CDNH2 under 10 and 20 bar H2
Fig. 4. Partial 2D T-ROESY NMR spectrum (aromatic protons) of a 2:1 mixture of β-CDNH2
and 2 at room temperature in D2O.
Scheme 4. Rh-catalysed hydrogenation of 2-methyl-3-buten-2-ol.
Please cite this article as: J. Potier, et al., Cyclodextrins as first and second sphere ligands for Rh(I) complexes of lower-rim PTA derivatives for use