.
Angewandte
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tethered. The composite fibers were prepared by the well-
established dry-jet, wet-quench, nonsolvent-induced phase-
separation spinning method.[32] For Lewis base catalysis, 3-
aminopropyl (APS) groups were grafted to the silica particles
by a recently developed post-spinning amine infusion tech-
nique after spinning the hollow fibers.[35] Similarly, an
analogue of the well-known [Rh2(S-DOSP)4] catalyst was
grafted to the silica particle surface after fiber formation by
using previously developed techniques, in which a [Rh2(S-
DOSP)4] derivative containing a styryl group on a prolinate
ligand was tethered to styryl-functionalized silica in the fiber
wall through a radical coupling initiated by 2,2’-azobisisobu-
tyronitrile (AIBN).[47] [Rh2(S-DOSP)4] was chosen because it
is stable, and a highly active catalyst for a wide range of
reactions of donor–acceptor diazo compounds, including
[48]
À
stereoselective C H functionalizations.
Scheme 1. Homogeneous [Rh2(S-DOSP)4] and its immobilized ana-
logue on silica. The other catalyst structures are shown in the
Supporting Information.
The reactants are fed through the bore of the hollow fiber
reactor (Figure 1). The fiber walls, composed of cellulose
acetate and inorganic particles, are highly permeable. The
walls are approximately 300–400 mm thick and can be
considered an ultrashort catalyst bed through which the
reactants pass. Outside the fiber is a sweep flow of an organic
solvent that facilitates product collection. In this initial study,
reactant pulses were flowed through the fiber, although
continuous flow operation can also be conducted. The fiber
was capped at the opposite end to ensure the reactants flow
through the walls and contact the catalyst (tubular dead-end
flow). To evaluate the viability of the reactor system, the
Brønsted acid catalyzed deprotection of benzaldehyde
dimethyl acetal (1) to benzaldehyde (2) was examined using
hollow fibers containing the proton-exchanged form of zeolite
ZSM-5. Scheme 2 demonstrates the conditions used to drive
powerful [Rh2(S-DOSP)4] catalyst is the centerpiece of this
investigation. While cyclopropanations/cyclopropenations
have been investigated using flow systems,[18–20,26,28] no such
investigations have occurred using hollow fiber reactors, nor
have there been studies of dirhodium(II)-catalyzed asym-
À
metric intermolecular C H functionalization reactions in
flow, to the best of our knowledge.
Polymeric hollow fibers are commercially used for
industrial gas separations, namely separation and purification
of CO2, and provide high surface areas at low cost.[32] The
polymeric hollow fibers used here were prepared in a con-
tinuous process whereby the solid oxide particles were
incorporated into the porous polymer matrix during the
fiber spinning, thereby allowing fibers to be made rapidly at
low cost using commercially available materials.[32] The small
inner (ca. 300 mm) and outer (ca. 1100 mm) diameters of the
fibers ensure that the walls are thin, which imparts excellent
heat and mass-transfer properties.[33] In this case, the thin wall
presents an extremely short reaction bed length when radial
flow through the fiber wall is used. Recently, composite
hollow fibers (organic polymers and inorganic fillers such as
zeolites,[34] silicas,[35] and metal–organic frameworks
(MOFs)[36]) have emerged as separation membranes and
sorbents that offer highly tunable material properties. Pre-
vious work on hollow fiber reactors focused on Pd-catalyzed
reactions and alkene/alkyne hydrogenations,[37–46] although, to
the best of our knowledge, no examples of asymmetric
reactions using well-defined organometallic complexes
embedded in hollow fiber reactors in flow have been
reported. In general, the few examples of flow catalysis
using hollow fiber catalysts/reactors are for simple catalysts
and reactions, with no examples in organic synthesis using
well-defined asymmetric organometallic catalysts.
Scheme 2. Acid-catalyzed deprotection of 1 using zeolite ZSM-5
embedded in the walls of hollow fibers. Conversions were determined
using GC and are the averages of three different fibers.
the reaction to completion. Water-saturated hexanes were
used as the solvent to provide the trace amount of water
needed for deprotection. Control experiments using bare
fibers without ZSM-5 exhibited almost no conversion, thus
demonstrating the ability of the catalytic fibers to achieve
substantial conversion, even over a short catalyst bed.
Next, APS-functionalized silica particles embedded
within the fiber walls were used as catalysts for the
Knoevenagel condensation of benzaldehyde (2) and malono-
nitrile (3). Scheme 3 demonstrates that the reaction can be
driven to completion by using an excess of malononitrile.
Control experiments using bare fibers without APS-function-
alized silica showed that the conversion decreased by a factor
of about two in the absence of the amine catalyst, thus
suggesting that the bare cellulose acetate/silica fibers may
provide some background activity for this reaction as well.
The polymer/oxide hollow fibers used herein were com-
prised of cellulose acetate, a common polymer used in
commercial hollow fibers for gas separation. Two inorganic
fillers were also used, an aluminosilicate MFI zeolite (ZSM-5)
for Brønsted acid catalysis as well as commercially available
porous amorphous silica on which molecular catalysts were
2
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!
Moschetta, Eric G.
