Chiral amide directed assembly of a diastereo- and enantiopure supramolecular host and its application to enantioselective catalysis of neutral substrates

Article abstract of DOI:10.1021/ja411631v

The synthesis of a novel supramolecular tetrahedral assembly of K ₁₂Ga₄L₆ stoichiometry is reported. The newly designed chiral ligand exhibits high diastereoselective control during cluster formation, leading exclusively t

Full text of DOI:10.1021/ja411631v

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Communication
Chiral Amide Directed Assembly of a Diastereo- and
Enantiopure Supramolecular Host and its Application
to Enantioselective Catalysis of Neutral Substrates
Chen Zhao, Qing-Fu Sun, William M. Hart-Cooper, Antonio G DiPasquale,
F. Dean Toste, Robert George Bergman, and Kenneth N. Raymond
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Nov 2013
Downloaded from http://pubs.acs.org on December 2, 2013
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Chiral Amide Directed Assembly of a Diastereo- and Enantiopure
Supramolecular Host and its Application to Enantioselective Cataly-
sis of Neutral Substrates
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Chen Zhao, QingꢀFu Sun, William M. HartꢀCooper, Antonio G. DiPasquale, F. Dean Toste,* Robert G.
Bergman,* and Kenneth N. Raymond*
Chemical Sciences Division, Lawrence Berkeley National Laboratory, and Department of Chemistry, University of Califorꢀ
nia, Berkeley, California 94720, United States
Supporting Information Placeholder
thesis of a new enantiopure supramolecular Ga L cluster that
ABSTRACT: The synthesis of a novel supramolecular tetraheꢀ
4 6
spontaneously self assembles. In addition to circumventing the
need for resolution, these new assemblies provide enhanced staꢀ
bility and catalytic reactivity required for asymmetric organic
transformations of neutral guests.
dral assembly of K Ga L stoichiometry is reported. The newly
12
4 6
designed chiral ligand exhibits high diastereoselective control
during cluster formation, leading exclusively to a single diastereꢀ
omer of the desired host. This new assembly also exhibits high
stability toward oxidation or a low pH environment, and is a more
robust and efficient catalyst for asymmetric organic transforꢀ
mations of neutral substrates.
Inspired by nature, recent work in supramolecular chemistry
has focused on the design and construction of assemblies that
1
imitate the properties of enzymes. Many such synthetic nanoꢀ
vessels can function in aqueous environments at physiological
2
pH, contain wellꢀdefined cavities for selective guest encapsulaꢀ
3
tion and recognition, and have been shown to stabilize otherwise
4
reactive and unstable species. Furthermore, many supramolecular
hosts have proven to be efficient catalysts that increase both the
5
rate and selectivity of a variety of chemical reactions. Raymond
and coꢀworkers have developed tetrahedral supramolecular asꢀ
Figure 1. Relationship of racemic 1 to diastereoꢀ and enantioenriched
sembly 1 of K Ga 2 stoichiometry, where 2 = N,Nꢀbis(2,3ꢀ
12
4 6
Ga4L supramolecular assembly
6
6
dihydroxybenzoyl)ꢀ1,5ꢀdiaminonaphthalene. The highly charged
anionic host 1 has been shown to encapsulate a variety of cationic
and neutral guests; however, to date, its use in enantioselective
Our strategy for achieving an enantiopure supramolecular M4L6
assembly without resolution involves the addition of an amideꢀ
containing chiral directing group at the vertex of ligand 2, as
shown in Figure 1. We envisioned that this chiral source would
control the helical configuration of the proximal metal center
during cluster formation and direct a highly diastereoselective
process in which the desired M4L6 supramolecular assemblies
would be formed enantioenriched rather than as a racemate. We
also suspected that this additional amide functional group would
stabilize the resulting assembly via hydrogen bonding with the
catecholates and could prevent ligand oxidation and decomposiꢀ
tion due to its electron withdrawing nature.
7
catalysis has been limited to the charged substrates of the Azaꢀ
8
Cope rearrangement. While Fujita and coworkers have reported
the [2+2] cycloaddition of neutral guests in stoichiometric chiral
9
hosts, the use of nanoscale molecular flasks possessing chiral
cavities as catalysts for asymmetric transformations of neutral
8
ꢀ10
guests remains elusive.
Complex 1 is a chiral species because the three catecholates
coordinate to a given gallium atom can form either a right (ꢁ)ꢀ or
a left (Λ)ꢀhanded helicity at each metal center. Enforced by meꢀ
chanical coupling that leads to chirality transfer between the four
11
vertices, complex 1 is formed as a racemic mixture of two hoꢀ
mochiral enantiomeric forms, namely ΛΛΛΛꢀ1 and ꢁꢁꢁꢁꢀ1.
Resolution of the racemate was realized using (ꢀ)ꢀN’ꢀ
methylnicotinium iodide, giving access to enantiopure ΛΛΛΛꢀ(Sꢀ
Ligand (R)-5 was prepared as shown in Scheme 1. The terephꢀ
thalate sodium salt was converted to the corresponding acyl chloꢀ
ride. This was followed by amide bond formation with commerꢀ
cially available chiral amine (R)ꢀ(ꢀ)ꢀ3,3ꢀdimethylꢀ2ꢀbutylamine
and subsequent saponification with KOH in methanol to afford
the desired intermediate (R)-3. Reaction between (R)-3 and 1.2
equiv. of HATU, (Oꢀ(7ꢀazabenzotriazolꢀ1ꢀyl)ꢀN,N,N′,N′ꢀ
tetramethyluronium hexafluorophosphate), in THF for 1h at room
temperature followed by addition of 1,5ꢀdiaminonapthalene gave
the desired methylꢀprotected chiral ligand (R)-4. Methyl group
12
nic ⊂ 1) and ꢁꢁꢁꢁꢀ(Sꢀnic ⊂ 1) stereoisomers. Sequential ion
exchange chromatography with large excess amounts of tetrameꢀ
thylammonium and potassium iodides salts then afforded “empty”
and enantiopure clusters. However, the instability of the isolated
+
cationic guestꢀfree or K ꢀfilled ΛΛΛΛꢀ1 and ꢁꢁꢁꢁꢀ1 clusters
1
2
warrant improvements. We describe herein the design and synꢀ
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deprotection of (R)-4 was achieved by treatment with BBr and
hydrolysis of the resulting borate to produce the desired terephꢀ
thalamideꢀbased chiral ligand (R)-5 in 52% yield over 5 steps.
The enantiomer (S)-5 was also synthesized according to the proꢀ
cedures shown in Scheme 1.
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Scheme 1 – Synthesis of ligand (R)-5
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*
It was reported previously that the UV π–π transitions of the
catechol moiety of assembly 1 produced a strong and distinct
14
exciton couplet.
This property enabled the determination of
absolute configuration of the resolved enantioenriched parent
12
assembly 1 by circular dichroism spectroscopy. When assemꢀ
blies 6-K Ga (R)-5 and 6-K Ga (S)-5 were examined by CD
12
4
6
12
4
6
spectroscopy, the spectra of the two enantiomers proved to be
perfect mirror images of each other and contain a shape and sign
of the Cotton effect similar to those of ꢁꢁꢁꢁꢀ1 and ΛΛΛΛꢀ1 (see
12
Supporting Information). Thus, we infer by comparison and
assign complex 6-K Ga (R)-5 as the ꢁꢁꢁꢁ stereoisomer and 6-
12
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6
K Ga (S)-5 as the ΛΛΛΛ stereoisomer.
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6
We next investigated whether ligand (R)-5 would form the deꢀ
sired tetrahedral supramolecular assembly. The initial reaction
The absolute stereochemical assignment of ꢁꢁꢁꢁꢀ6 was furꢀ
ther supported by Xꢀray crystallographic analysis. Single crystals
were obtained by slow diffusion of THF vapor into a water soluꢀ
tion of ꢁꢁꢁꢁꢀ6 without any strongꢀbinding and cationic guest
molecules under aerobic conditions. The structure conforms to the
chiral space group R3 with three molecules of the enantiopure
complex in the unit cell, each with crystallographic 3ꢀfold symꢀ
metry. As shown in Figure 2, all four gallium centers adopt the ꢁ
between 4 equiv. of Ga(acac) , 6 equiv. of ligand (R)-5, and 12
3
equiv. of KOH in methanol at room temperature, in the absence of
any cationic species as a template, gave a mixture of products as
1
analyzed by H NMR spectroscopy (see Supporting Information).
However when the reaction was repeated at 50°C for 1h, highlyꢀ
1
symmetric complex 6, as suggested by the simplicity of its H
NMR spectrum (see Supporting Information), was isolated as a
yellow solid in 78% yield. Analysis of 6 by ESI mass spectromeꢀ
try confirmed its stoichiometry as K Ga (R)-5 . Furthermore,
configuration, with an average Ga–Ga distance of 12.6 Å, similar
6,12
1
2
4
6
to that found in the resolved parent assembly 1.
The chiral
when 5 equiv. of PEt I was added to a D O solution of 6, encapsuꢀ
4
2
directing groups bury the metal vertices of the cage with additionꢀ
al intramolecular hydrogen bonds between the amide proton and
catecholate oxygen, which could be responsible for the observed
stability of this new cluster. By crystal packing, each cage is part
of a larger network of 12 neighboring cages, forming a 3ꢀ
dimensional molecular organic framework. A huge solvent accesꢀ
+
lation of PEt4 was observed as indicated by the proton resonancꢀ
es at δ = ꢀ1.45 and ꢀ1.78 ppm (see Supporting Information). This
observation can also be taken as an indication of the successful
6
formation of the desired tetrahedral assembly 6. Furthermore, 6ꢀ
K Ga (R)-5 was synthesized without the use of any cationic
12
4
6
species as a template, whereas enantiopure 1 could only be obꢀ
3
sible void of 25000 Å is calculated for the unit cell (65% of total
tained as a stable species after treatment with excess amount of
unit cell volume), as a result of the large channels found along
both the a and b axes of the crystal.
+
NMe4 as the template and counterion. Complex 6ꢀK Ga (S)-5 ,
12
4
6
the enantiomer of 6ꢀK Ga (R)-5 , was also synthesized by using
1
2
4
6
ligand (S)-5, Ga(acac) and KOH following a procedure directly
3
analogous to that outlined in Scheme 2.
Complex 6 was also found to be benchꢀtop stable in the solid
state and in solution state at elevated temperature, whereas comꢀ
plex 1 was sensitive to oxidation and relatively less stable at 40
°C in the absence of a strong binding guest in solution over time.
More importantly, complex 6 proved to be stable in aerobic D O
2
+
at pD 5 and readily encapsulates PEt4 even after heating at 70 °C
for 6h, while complex 1 and (NEt ) 1 dissociate in anaerobic
4 12
D O immediately at the same pD (see Supporting Information).
2
This property is a consequence of the lower basicity of the terephꢀ
1
3
thalamide functionality relative to catecholate.
Scheme 2 – Synthesis of supramolecular assembly 6 and its encapsulation
+
4
of PEt cation
Figure 2. Xꢀray structure of ꢁꢁꢁꢁꢀ6.
As a further probe of the stereochemistry of ꢁꢁꢁꢁꢀ6 and
ΛΛΛΛꢀ6, we investigated their hostꢀguest chemistries individualꢀ
ly with both enantiomers of ammonium salt 8. As illustrated in
Figure 3, hostꢀguest complex 9, or ꢁꢁꢁꢁꢀ[(S)ꢀ8 ⊂ 6], should
have different and distinguishable properties from complex 10,
1
ꢁꢁꢁꢁꢀ[(R)ꢀ8 ⊂ 6], due to their diastereomeric relationship.
H
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NMR spectroscopy (Figure. 3) reveals that the two complexes are
time to give the desired product mixture in 92% yield and 65% ee
of 14b.
17,18
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indeed different, most notably in the encapsulation region of the
spectra. On the other hand, complex 11, ΛΛΛΛꢀ[(R)ꢀ8 ⊂ 6], and
complex 9 are enantiomers, and exhibit exactly the same spectroꢀ
Table 1 – Enantioselective and chemoselective monoterpeneꢀlike cyclizaꢀ
tion of neutral substrates catalyzed by ꢁꢁꢁꢁꢀ6
1
scopic behaviors when analyzed by H NMR; the same result was
also observed for complexes 10 and 12. This evidence, combined
with results from Xꢀray crystallography and CD spectroscopy,
demonstrate that complex 6 is highly enantioenriched. The chiral
group of ligand 5 exhibits strong control during cluster formation
to give the desired supramolecular K Ga 5 cluster as a single
1
2
4 6
15
diastereomer.
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In conclusion, a new enantiopure supramolecular K Ga L asꢀ
1
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4 6
sembly has been synthesized, fully characterized, and applied as a
rare example of chiral host catalyzed enantioselective transforꢀ
mations of neutral guests. The chiral amide in the terephꢀ
thalamideꢀbased ligands (R)ꢀ5 and (S)ꢀ5 direct cluster formation
to afford highly diastereoꢀ and enantiomerically enriched comꢀ
plexes. Remarkably, cationic guestꢀfree variants of complexes
ꢁꢁꢁꢁꢀ6 and ΛΛΛΛꢀ6, which in comparison to 1 vary only in
modification to the exterior of the assembly, show increased staꢀ
bility towards air oxidation in both the solid and solution states,
and to low pH in solution. These features allow complexes
ꢁꢁꢁꢁꢀ6 and ΛΛΛΛꢀ6 to serve as efficient catalysts for chemoꢀ,
diastereoꢀ and enantioselective carbonylꢀene cyclization.
1
Figure 3. H NMR spectra (encapsulation region) of complexes from
hostꢀguest chemistry of ꢁꢁꢁꢁꢀ6 and ΛΛΛΛꢀ6 individually with chiral
ammonium salts (S)ꢀ8 and (R)ꢀ8
One challenge to the development of asymmetric organic reacꢀ
tions catalyzed by enantiopure host ꢁꢁꢁꢁꢀ1 is the requirement
for cationic starting material or substrates that are more tightly
ASSOCIATED CONTENT
Supporting Information
+
bound than is NMe4 to the cavity of ꢁꢁꢁꢁꢀ1. Since ꢁꢁꢁꢁꢀ6 was
Experimental procedures and spectroscopic data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
synthesized without the use of any templates or cationic species,
this new supramolecular host makes possible the enantioselective
transformations of neutral compounds.
We recently reported the chemoselective carbonylꢀene cyclizaꢀ
tion of compounds 13a and 13b catalyzed by complex 1 to give
exclusively products 14a,b and 15a,b respectively as compared to
AUTHOR INFORMATION
Corresponding Author
1
6
reaction performed in bulk solution.
When the reaction was
fdtoste@berkeley.edu; rbergman@berkeley.edu; raymond@
socrates.berkeley.edu
repeated with 10 mol% of (NMe ) 1 at 60 °C in D O buffered at
4
12
2
pD 8 for 14h, no desired products were observed. On the other
hand, when compound 13a was treated with 2.5 mol% of ꢁꢁꢁꢁꢀ6
in a solvent mixture of CD OD and D O buffered at pD 8 at room
Notes
3
2
The authors declare no competing financial interests.
temperature, the desired products 14a and 15a were obtained in
92% NMR yield with a trans:cis ratio of 8:1 and 60% ee for 14a
over two days (Table 1, entry 1). Compared to reaction with comꢀ
plex 1 as the catalyst at the same pD, cyclization of 13a in the
presence of a catalytic amount of ꢁꢁꢁꢁꢀ6 proved to be faster by
ACKNOWLEDGMENT
This research was supported by the Director, Office of Science,
Office of Basic Energy Sciences, and the Division of Chemical
Sciences, Geosciences, and Biosciences of the U.S. Department of
Energy at LBNL (DEꢀAC02ꢀ05CH11231. The authors thank
Amela Drljevic, Kristen Burford, and Rebecca Triano for helpful
discussions.
7
ꢀfold (see Supporting Information). Since complex ꢁꢁꢁꢁꢀ6 is
stable at low pD, attempts to effect the cyclization of 13a at pD 5
led to faster conversion compared to reaction at pD 8 (Table 1,
entry 2). The stability and turnover capability of catalyst ꢁꢁꢁꢁꢀ6
was further illustrated as only 0.3 mol% of the complex is reꢀ
quired to achieve 33% yield of 14a and 15a with no loss in enanꢀ
tiomeric excess of 14a (Table 1, entry 4), representing 98 TON of
the catalyst. Interestingly, carbonylꢀene cyclization of 13b proꢀ
ceeded with complex 6 at pD 8 over 16h at 60 C to give the deꢀ
sired products in only 12% yield (Table 1, entry 5), whereas reacꢀ
tion at pD 5 led to much better conversion over the same reaction
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(
(
2
(15) Molecular mechanics calculations of ꢁꢁꢁꢁ-[Ga
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the two diastereomers is approximately 19 kcal/mol, favoring the
4 6
(R)-5 ] and
4
6
(
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