Communications
harder tpy unit into 1 should allow for selective coordination
in toluene at a hydrogen pressure of 40 atm (Table 1).
Encouraging results were obtained using catalysts bearing
weakly coordinating anions (3c–g), wherein the enantiose-
of the phosphine and tpy domains by discrete metal species, a
property valuable for exemplification of the aforementioned
strategy.
Ligand 1 was prepared in good overall yield (60%)
through a copper-free Sonogashira coupling reaction of 4’-(4-
bromophenyl)-2,2’:6’,2’’-terpyridine[16] and bis(methoxy-
methyl)-protected (S)-6-ethynyl-1,1’-binaphthyl-2,2’-diol[5a]
with Ph2PNiPr2/Pd(OAc)2 as the catalyst,[17] followed by
acidic deprotection and further reaction with hexamethyl-
phosphorous triamide (HMPT) in toluene heated at reflux
(see the Supporting Information). While many metal ions
could have been chosen as the “glue” for linking two ligand 1
molecules via formation of the [M(tpy)2]n+ unit, FeII was used
herein owing to its strong binding affinity towards tpy,[18] low
toxicity, cheap availability, and wide use in tpy-containing
supramolecular systems.[19]
Table 1: Enantioselective hydrogenation of compound 4a under the
catalysis of 3a–g or MonoPhos/Rh.[a]
Entry
Catalyst
Conv. [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
3a
3b
3c
3d
0
>99
>99
>99
>99
>99
>99
>99
–
88
97
94
96
96
95
97
3e
3 f
3g[d]
Thus, treatment of a dichloromethane solution of 1 with
MonoPhos/Rh[e]
0.5 equiv of an FeII salt bearing different counterions (ClÀ,
À
SO42À, PF6À, ClO4À, BF4À, or SO3CF3 ) immediately afforded
[a] Conditions: [4a]=1.0m, [3]=1 mol% (with respect to 4a), T=298 K,
P(H2)=40 atm, t=11 h, toluene solvent. [b] Determined by 1H NMR
spectroscopy. [c] Determined by GC on a Supelco BETA-DEX 120
column. [d] t=9 h. [e] Molar ratio of MonoPhos/Rh=2:1, t=2 h.
a dark blue-violet solution or suspension (Scheme 1). Further
addition of diethyl ether resulted in the gradual precipitation
of FeII-bridged ligands 2a–f as violet-purple powders, which
were characterized by UV/Vis and IR spectroscopy, elemen-
tal analyses, and/or HRMS. UV/Vis spectra clearly show an
overall similar molecular structure for 2a–f (see the Support-
ing Information), wherein the FeII ion binds two tpy moieties
from two ligand 1 molecules while leaving the MonoPhos sites
untouched, thus leading to the effective formation of [Fe-
(tpy)2]2+-expanded bis-MonoPhos ligands with a chemical
reactivity expected to resemble that of MonoPhos.[20] Indeed,
self-assembly of 2a–f with a rhodium salt in dichloromethane
immediately affords 3a–g as violet-purple precipitates having
compositions consistent with the expected structures
(Scheme 1). Scanning electron microscopy (SEM) images
showed that solids 3a–g are composed of micrometer-sized
particles (see Figure 2a), while powder X-ray diffraction
patterns indicated that they are amorphous (see the Support-
ing Information, Figure S2).
lectivity for 5a (94–97% ee) was comparable to that of their
homogeneous counterpart MonoPhos/Rh (97% ee) under
otherwise identical conditions (Table 1).
However, catalysts 3a and 3b prepared from chloride and
sulfate salts of FeII, respectively, are the exception. While
under the catalysis of 3b full conversion could still be reached
with a slightly lowered ee value (88%) of 5a (Table 1,
entry 2), no reaction occurred at all after 11 h at room
temperature in the case of 3a (Table 1, entry 1). This is not
surprising, considering that the anions in 2a–f are situated at
the outer coordination sphere of these [Fe(tpy)2]2+(XÀ)2-type
complexes, which are well known to undergo facile anion
exchange when another electrolyte is present in the solution.
It is likely that during the assembling of 3a, ClÀ ions in 2a
À
interchange rapidly with BF4 ions in [Rh(cod)2]BF4 to give
3a, wherein the ClÀ anions stay nearby or bond directly with
the RhI centers by virtue of higher affinity, thus leading to
inhibition of the catalysis.[21] Such an anionic scrambling may
also occur in catalysts 3b–g, and lead to a variation in their
chiral induction capabilities, albeit the weakly or noncoordi-
nating nature of the anions does not prohibit the catalysis
under the somewhat forcing conditions.
A shift to milder conditions clearly revealed that the
anions in 3 exert influence on the catalytic activity as well.
Compounds 3c–e and 3g were effective in the catalytic
hydrogenation of 4a even under an ambient pressure of
hydrogen, conducted in a Schlenk tube with a hydrogen gas
balloon. The reactions using catalysts 3c–e, 3g, and their
homogeneous counterpart (MonoPhos)2/RhI were monitored
by GC analysis, and the reaction profiles are shown in
Figure 3. While 3d and 3e exhibited a reactivity higher than
that of (MonoPhos)2/RhI, the reactions with 3c and 3g were
somewhat slower. It is conceivable that such an activity
difference may reflect the subtle variation in catalyst struc-
Figure 2. a) SEM image of 3g (scale bar: 1 mm); b) catalyst 3g in
toluene (solid at the bottom of the reactor); c) supernatant of the
reaction mixture filtered after hydrogenation of 4c using catalyst 3g.
The RhI-containing solids 3 were found to be completely
insoluble in toluene (see Figure 2b), thus fulfilling one of the
prerequisites for heterogeneous catalysis. Accordingly, 3a–g
were initially examined in the hydrogenation of methyl a-
acetamidoacrylate (4a), under a catalyst loading of 1 mol%
3628
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 3627 –3630