Cavity-tailored, self-sorting supramolecular catalytic boxes for selective oxidation

Article abstract of DOI:10.1021/ja804014y

Using a steric self-sorting strategy, the assembly of highly ordered and rigid supramolecular boxes possessing catalytic properties has been achieved in one step. The formation of these assemblies, comprising up to 18 porphyrin centers, was readily confirmed by solution X-ray scattering in conjunction with fluorescent spectroscopy. Size-selective and enantioselective oxidation catalysis were both demonstrated. Copyright

Full text of DOI:10.1021/ja804014y

Published on Web 11/20/2008
Cavity-Tailored, Self-Sorting Supramolecular Catalytic Boxes for Selective
Oxidation
Suk Joong Lee,† So-Hye Cho,† Karen L. Mulfort,†,‡ David M. Tiede,‡ Joseph T. Hupp,*,† and
SonBinh T. Nguyen*,†
Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, and Chemistry
DiVision, Argonne National Laboratory, Argonne, Illinois 60439
Received June 3, 2008; E-mail: j-hupp@northwestern.edu; stn@northwestern.edu
Scheme 1. Schematic Representation of the Steric-Self-Sorting
Assembly of Supramolecular Catalytic Boxes, AMAT4 and CMCT4
The field of metal-meditated functional assembly has seen tremen-
dous growth,1 with continuous emergence of exciting applications in
catalysis,2 chemical sensing,3 and selective guest inclusion.4 Although
asymmetric catalysis has been demonstrated with abiotic supramo-
lecular assemblies,5 it has not yet been possible to achieve an
enantioselective outcome via indirect through-space control of the
chirality around the catalyst center. Such control should be feasible
given the ability of a variety of enzymes to exploit environmental (e.g.,
polypeptide superstructure), rather than active site, chirality to achieve
enantioselective catalysis. Specifically, if a remote chiral auxiliary could
be incorporated into a supramolecular system to afford a well-defined
enzyme-like chiral environment around an achiral catalyst center,
asymmetric events could be controlled in through-space fashion.
Previously, we have demonstrated the encapsulation of a manganese
porphyrin catalyst within a large tetraporphyrin cavity structure (Figure
1), leading to a highly enhanced catalyst lifetime in epoxidation
reactions as well as moderate size selectivity.2e While attempts to
modify this catalytic cavity postsynthetically using pyridylbis(men-
tholate ester) lead to further improvements in size selectivity, enanti-
oselectivity was not achieved. Subsequent modeling work6 pointed to
two factors that likely limited the selectivity of this supramolecular
encapsulation system: (a) ligand binding on the outside of, rather than
within, the cavity and (b) torsional freedom of the cavity walls and
the encapsulated catalyst (Figure 1). Knowing these limitations, we
set out to design porphyrin building blocks that can assemble into rigid,
cavity-tailored, catalytic supramolecular boxes via a high-yielding self-
sorting process.7 With these highly stable assemblies, asymmetric
catalysis via through-space control of cavity chirality should now be
possible.
space control of chirality in abiotic supramolecular catalysis at a
modularly assembled achiral site.8
As described previously,7 the free-base precursor to porphyrin dimer
A was readily obtained in high yield via Suzuki coupling of 2,5-
dibutoxy-1,4-phenyldiboronic acid and 5-(p-bromophenyl)-15-(2,6-
dibutoxyphenyl)-10,20-dipyridyl-porphyrin (available through a one-
pot synthesis in 18% yield; see Supporting Information (SI)). Subsequent
treatment with SnCl2 in air, followed by derivatization with tert-
butylbenzoic acid, generates the (porphyrin)SnIV(carboxylate)2 dimer
A in high yield (Scheme SI-1 in SI). This highly efficient synthetic
scheme facilitates the modification of dimer A with virtually any
available carboxylic acid, allowing for the ready construction of a wide
range of supramolecular environments (Vide infra).
When 2 equiv of A are combined with 4 equiv of the diacetylene-
linked (porphyrin)Zn trimer T, the rigid 16-porphyrin AAT4 “box”
results.7 The analogous combination of A with the phenylene-linked
(porphyrin)Zn trimer T′, ∼1.7 Å (Zn1 to Zn3) shorter than T, afforded
the corresponding AAT′4 in good yield. These observations suggest
that the self-assembly process responsible for formation of these boxes
is reasonably tolerant to size variations among the partners, presumably
due to cooperative binding between A and either T or T′.
Figure 1. Idealized vs more realistic representations of the chiral
Given the large axial steric bulk of the carboxylate ligand in A, the
middle zinc porphyrins in the T components of AAT4 remain unbound,
leaving an unoccupied cavity inside of the box.7 We hypothesized that
this space could be uniquely occupied by the manganese dimer M to
generate a supramolecular catalyst box AMAT4 (Scheme 1). Indeed,
AMAT4 can be obtained via the sequential addition of M to AAT4 or
in a one-pot fashion where A, T, and M are added together in one
step, illustrating the high selectivity and synthetic versatility of the
self-sorting9 process (Scheme 1).
environment inside a pentaporphyrin catalyst assembly.
Herein, we demonstrate the use of steric self-sorting strategy to
encapsulate catalytically active achiral manganese porphyrin dimers
within rigid porphyrin boxes possessing cavities that can be tailored
with carboxylate ligands. These assemblies readily effect size-
selective olefin epoxidation and, when the cavity is modified with
a chiral amino acid ligand, enantioselective sulfoxidation. To the
best of our knowledge, this constitutes the first example of through-
Porphyrin box AMAT4 was characterized by UV-vis absor-
bance and fluorescence spectroscopy as well as solution-phase small-
angle X-ray scattering (SAXS) (see SI). Fluorescence spectra of a
† Northwestern University.
‡ Argonne National Laboratory.
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16828 J. AM. CHEM. SOC. 2008, 130, 16828–16829
10.1021/ja804014y CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
solution of AAT4, with increasing amounts of M (Figure S6), showed
nearly complete quenching when 1 equiv of M was added. Data
suggest the formation of a 1:1 AAT4 and M complex, which readily
quenches the ns-time-scale emissions from the surrounding (porphy-
rin)Zn and, thus, is highly diagnostic of binding of M by the box.
attributed to the small size of N-acetyl-phenylalanine. Clearly, this
strategy can be readily extended to other chiral ligands.
In conclusion, we have demonstrated the utility of the self-sorting
strategy for the spontaneous assembly of highly ordered, rigid
supramolecular boxes possessing catalytic properties. The formation
of these assemblies, comprising up to 18 porphyrins, was readily
confirmed by solution-phase X-ray scattering in conjunction with
fluorescence spectroscopy. The resulting catalytically active boxes
readily effect both size-selective and enantioselective oxidation
catalysis. We anticipate that this powerful modular construction
strategy will be of more general value for creating functional
metallo-supramolecular assemblies. Current work in our laborato-
ries with catalytic salen complexes and related porphyrin boxes is
centered precisely on this notion.
Table 1. Modeled and Experimentally Obtained Rg Values for T
and the Box Assemblies
model Rg (Å)
expt Rg (Å)
T
14.8
20.6
19.8
15.1 ( 0.4
20.7 ( 0.3
19.9 ( 0.3
AAT4
AMAT4
While NMR characterization of AMAT4 was not possible (due to
the paramagnetic nature of M), SAXS data (Table 1) proved par-
ticularly instructive in confirming its formation. We note that solution-
phase X-ray scattering has become an increasingly powerful charac-
terization strategy for supramolecular assemblies10 whose structures
cannot easily be determined by crystallographic and/or mass spectro-
metric methods. SAXS is well-suited for determining the overall sizes
of large assemblies in solution as the electron-density-weighted radius
of gyration (Rg) can be extracted from the scattering at very low angles
by application of the Guinier analysis.11 Formation of specific
supramolecular assemblies can then be readily confirmed by comparing
the experimental Rg to the value extracted from the scattering of
proposed model structures.10 For AMAT4, the experimental Rg is 19.9
( 0.3 Å, slightly smaller than that for the “empty” AAT4 box (20.7
( 0.3 Å) and consistent with the increased electron density at the
“center” of the box upon complexation of the Mn dimer. Rg values
for both AAT4 and AMAT4 are significantly larger than those for T
alone, clearly indicating intact large assemblies in solution.
Acknowledgment. We gratefully acknowledge the U.S. DOE
(Grant No. DE-FG02-ER15244 for initial support at NU and
Contract DE-AC02-06CH11357 for work at ANL). AFOSR and
DTRA via ARO provided subsequent support at NU. S.H.C. was
an AAUW fellow. K.L.M. is an Argonne Laboratory-Grad Fellow.
Supporting Information Available: Complete experimental details
of syntheses, compound characterization, and X-ray experimental
methods and data analysis. This material is available free of charge
via the Internet at http://pubs.acs.org.
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As mentioned above, we anticipated that encapsulation of M by
the rigid supramolecular boxes AAT4 and AAT′4 would impart both
size selectivity and enantioselectivity if the boxes could be made chiral.
To assess the ability of the resulting supramolecular boxes AMAT4
and AMAT′4 to effect size-selective catalysis, we first employed them
in the epoxidation of cis-stilbene vs cis-3,3′,5,5′-tetra(tert-butyl)stilbene
(Figure 2).2e Due to access inhibition of the larger olefin to the metal
center by the cavity modifier, we anticipated that the smaller olefin
would react preferentially. This is indeed the case: cis-stilbene is 5.5-
fold more reactive with the catalytic boxes than the sterically bulky
cis-3,3′,5,5′-tetra(tert-butyl)stilbene. Most gratifyingly, when CDM-
CDT4, the chiral version of AMAT4, was assembled and used in the
catalytic sulfoxidation of methyl p-tolyl sulfide (Figure 2), methyl
p-tolyl sulfoxide was obtained with 12% ee! (CD is the (porphyrin)SnIV
dimer ligated with N-acetyl-(D)-phenylalanine.) The enantiomeric
excess increased to 14% when the smaller catalytic box CDMCDT′4
was used. Importantly, the sense of the chiral excess was reversed
when N-acetyl-(L)-phenylalanine was used and no enantioselectivity
resulted when only free M was used, with or without CD. To the best
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