Air- and water-tolerant rare earth guanidinium BINOLate complexes as practical precatalysts in multifunctional asymmetric catalysis

Article abstract of DOI:10.1021/ja502568g

Shibasaki's REMB catalysts (REMB; RE = Sc, Y, La-Lu; M = Li, Na, K; B = 1,1′-bi-2-naphtholate; RE/M/B = 1/3/3) are among the most enantioselective asymmetric catalysts across a broad range of mechanistically diverse reactions. However, their widespread us

Full text of DOI:10.1021/ja502568g

Article
pubs.acs.org/JACS
Air- and Water-Tolerant Rare Earth Guanidinium BINOLate
Complexes as Practical Precatalysts in Multifunctional Asymmetric
Catalysis
Jerome R. Robinson,† Xinyuan Fan,‡ Jagjit Yadav,‡ Patrick J. Carroll,† Alfred J. Wooten,†,∥
Miquel A. Pericas,*,‡,§ Eric J. Schelter,*,† and Patrick J. Walsh*,†
̀
†P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104,
United States
‡Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain
§Departament de Química Organ
̀
ica, Universitat de Barcelona (UB), 08028 Barcelona, Spain
S
* Supporting Information
ABSTRACT: Shibasaki’s REMB catalysts (REMB; RE = Sc,
Y, La−Lu; M = Li, Na, K; B = 1,1′-bi-2-naphtholate; RE/M/B
= 1/3/3) are among the most enantioselective asymmetric
catalysts across a broad range of mechanistically diverse
reactions. However, their widespread use has been hampered
by the challenges associated with their synthesis and
manipulation. We report here the self-assembly of novel
hydrogen-bonded rare earth metal BINOLate complexes that
serve as bench-stable precatalysts for Shibasaki’s REMB
catalysts. Incorporation of hydrogen-bonded guanidinium
cations in the secondary coordination sphere leads to unique
properties, most notably, improved stability toward moisture
in solution and in the solid state. We have exploited these properties to develop straightforward, high-yielding, and scalable open-
air syntheses that provide rapid access to crystalline, nonhygroscopic complexes from inexpensive hydrated RE starting materials.
These compounds can be used as precatalysts for Shibasaki’s REMB frameworks, where we have demonstrated that our system
performs with comparable or improved levels of stereoselectivity in several mechanistically diverse reactions including Michael
additions, aza-Michael additions, and direct Aldol reactions.
where simple modulation of RE, M, and BINOLate substitution
patterns produces a diverse library of catalysts. These privileged
frameworks catalyze the formation of C−C and C−E (E = N,
O, P, S) bonds2 with high levels of stereoselection and atom
economy. The products generated by these catalysts have been
used as key intermediates toward the synthesis of natural
products and biologically active compounds.2b,e,h−k,3 Despite
their exceptional performance, there are several challenges that
have prevented the widespread practical application of REMB
catalysts.
1. INTRODUCTION
Multifunctional asymmetric catalysts show marked improve-
ments in reactivity and selectivity over traditional catalysts, due
to cooperative activation of reaction partners within a single
catalyst framework.1 Shibasaki’s heterobimetallic complexes
[M3(THF)n][(BINOLate)3RE] (REMB; RE = Sc, Y, La−Lu;
M = Li, Na, K; B = 1,1′-bi-2-naphtholate; RE/M/B = 1/3/3;
Figure 1) are the most successful heterobimetallic catalysts,
One such challenge arises because both the structure and the
catalytic performance of the REMB frameworks are sensitive to
trace amounts of moisture.2c−e,i,k,l,4 As such, REMB syntheses
typically require the rigorous exclusion of water.2k−m,4a,5 This
restriction represents a significant synthetic impediment and
also increases the cost of the catalyst, because expensive
anhydrous functionalized RE starting materials must be
employed rather than inexpensive RE hydrates.1d,6 A key
Figure 1. Shibasaki’s REMB framework. RE = Sc, Y, La−Lu; M = Li,
Na, K; B = (S)-BINOLate; RE/M/B = 1/3/3.
Received: March 18, 2014
© XXXX American Chemical Society
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attribute of the REMB catalysts is the tunability in reactivity
and selectivity by simply changing RE and M.
deprotonate the phenolic BINOLate hydrogens, pKa(ArOH)
= 10.0 in H2O.12 Guanidines are known H-bond donors for a
variety of anionic hosts.10c,13 Under anhydrous conditions,
addition of 3 equiv of TMG to a mixture of 1 equiv of
RE[N(SiMe3)2]3 and 3 equiv of (S)-BINOL in THF resulted in
instantaneous and quantitative formation of a new 1:3:3
complex, [TMG−H+]3[RE(BINOLate)3] (1−RE), RE = La,
Eu, Yb, Y. Removal of the volatiles followed by dissolution of
the residue in CH2Cl2 and layering with pentane furnished 1−
RE in excellent crystalline yields; 1−RE, RE = La, 91%; Eu,
92%; Yb, 93%; Y, 91% (Figure 2).
Current synthetic strategies to prepare these catalysts require
each RE/M combination to be prepared independently. Such
an approach is not attractive to high-throughpout experimenta-
tion (HTE) strategies,7 where ideally a single precatalyst could
be used to generate multiple catalysts to screen against a large
parameter space of reactions and conditions. To overcome
these challenges, we envisioned air- and water-tolerant REMB
precatalysts that could provide a rapid, simple, and user-friendly
entry into multiple heterobimetallic frameworks.
Herein, we report the self-assembly of novel hydrogen-
bonded rare earth metal BINOLate complexes that serve as
bench-stable precatalysts for Shibasaki’s REMB catalysts.
Incorporation of hydrogen-bonded guanidinium cations in the
secondary coordination sphere leads to unique properties, most
notably, markedly improved stability toward moisture in
solution and in the solid state. We have exploited these
properties to develop straightforward, high-yielding, and
scalable open-air syntheses that provide rapid access to
crystalline, nonhygroscopic complexes from inexpensive
hydrated RE starting materials. Using the precatalysts,
Shibasaki’s REMB M = Li+, Na+, K+ frameworks can be
quantitatively generated through either acid−base or cation-
exchange methods. Our approach provides a general strategy to
various RE/M combinations without the use of pyrophoric or
moisture-sensitive reagents. It also provides a general
precatalyst system for the REMB catalysts that can be applied
to mechanistically diverse reactions with comparable or
improved levels of stereoselectivity.
Figure 2. Synthesis of 1−RE from rigorously anhydrous conditions.
Single-crystal X-ray diffraction data for 1−La supported the
formation of a 1:3:3 complex (Figure 3B). The primary
coordination sphere at the La(III) cation formed a distorted
octahedron consisting of the six-BINOLate oxygen atoms.
RE−OBINOLate distances ranged from 2.3996(15)−2.4154(14)
Å, similar to the reported six-coordinate REMB frame-
works4a,5,8a,14 after accounting for differences in ionic radii of
the RE cations.15 As expected, the tetramethylguanidinium
cations were engaged in bifurcated H-bonding interactions,
where each guanidinium cation participated in two H-bonds
with neighboring anionic BINOLate oxygen atoms. The
NTMG−H···OBINOLate distances ranged from 2.782(2) to
2.811(2), and were consistent with reported charged
guanidinium N+−H···O− hydrogen bonds.13a,16
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of Rare Earth
BINOLate Ammonium Complexes. Recent work from our
laboratories5c,8 has demonstrated the importance of non-
covalent interactions in the secondary coordination sphere
with respect to tuning the reactivity and properties of REMB
frameworks. In these examples, the alkali metal cations
modulate the electronics at the RE cation and BINOLate
oxygen atoms, and are the primary determinant for the ability
of the RE cation to act as a Lewis acid. Given these
observations, we hypothesized that the isoelectronic replace-
ment of alkali metal cations with the appropriate ammonium
cations would result in the formation of complexes with
intramolecular ionic hydrogen-bonding networks.9 Hydrogen
bonds (H-bonds) are essential noncovalent interactions that
can direct self-assembly processes and stabilize reactive
fragments in nature and synthetic systems.9b,10 The strength
of H-bonding varies greatly with the directionality and charge
of the donor/acceptor pair, where bond strengths of up to ∼35
kcal mol−1 can be found for ionic/charged systems.9 We
expected that these relatively weak interactions should allow for
facile exchange of H-bonded ammonium cations for alkali metal
cations, which would provide a rapid and unified entry to
various REMB frameworks. With this approach in mind, we
embarked on the synthesis of REMB precatalysts supported by
hydrogen bonds.
1H and 13C{1H} NMR spectra were consistent with D3
1
symmetric 1−RE complexes in solution. The H NMR spectra
revealed six sharp BINOLate resonances and two resonances
belonging to the methyl and ammonium protons of TMG−H+
(Figure 3C). Given the importance of Lewis base coordination
at the central RE cation, binding studies were pursued with the
paramagnetic analogues, 1−Eu and 1−Yb. Contrary to RE/Li
frameworks, addition of cyclohexenone to 1−Eu and 1−Yb
resulted in negligible shifts (≤0.012 ppm) of the alkenyl
protons (Supporting Information, Figure S15 and Table S3),
which suggested that no binding of the cyclohexenone occurred
at the RE cation.
Intrigued by the inability of 1−RE to bind cyclohexenone,
we extended our investigations to a smaller Lewis base, H2O.
While H2O can coordinate to REMB systems,2c,4a partial ligand
hydrolysis occurs where the formation of polynuclear hydroxide
clusters has been observed and characterized in the solid
state.4b Addition of H2O (1−200 equiv) to 1−RE does not
Commercially available 1,1,3,3-tetramethylguanidine (TMG)
appeared as an ideal candidate for our synthetic investigation,
because, when protonated, it is a dual H-bond donor that could
replace the interactions of the main group metal with two
BINOLate ligands (see Figure 1) in REMB complexes. TMG is
sufficiently basic, pKa(TMG−H+) = 13.6 in H2O,11 to
1
result in the appearance of free protonated BINOL in the H
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Figure 3. (A) Generation of 1−RE using hydrated starting materials and conversion to REMB through cation-exchange. (B) Thermal ellipsoid plot
(30% probability) of 1−La. (C) 1H NMR spectra of 1−Eu (blue ★) in THF-d8. (D) 1H and 7Li NMR (inset) spectra of 1−Eu treated with excess
1
7
●
●
LiI in THF-d8. EuLB (orange ) and LiI (green ). (E) H and Li NMR (inset) spectra in THF-d8 of independently synthesized EuLB (orange
●
).
NMR, nor does it induce formation of multi-RE cation cluster
compounds as observed with the REMB frameworks.
We propose that the coordination preferences in 1−RE arise
from the unique intramolecular, ionic H-bonding interactions.
The H-bond donors, H−TMG+, assume geometries in the solid
state that maximize the strength of the directional H-bonding
interactions. Coordination of H2O or other Lewis bases at the
RE3+ cations would increase the energy of the system by
weakening those intramolecular H-bonding interactions,
disfavoring the seven-coordinate geometries for 1−RE.
The water tolerance of 1−RE is exceptional, especially when
considering the moisture sensitivity observed for Saa and co-
worker’s RE−BINOLAM system (BINOLAM = 3,3′-dieth-
ylaminomethyl-1,1′-bi-2-naphthol; RE:BINOLAM = 1:3, Fig-
ure 4).17 In contrast to 1−RE, RE−BINOLAM contains neutral
Encouraged by the moisture stability of 1−RE, we pursued a
modified, open-air, benchtop synthesis using inexpensive
hydrated RE starting materials. Taking advantage of the rapid
kinetics associated with complex formation and the low
solubility of 1−RE in polar solvents, a convenient and
expedient synthetic procedure was identified. Addition of 6
equiv of TMG to concentrated stirring solutions of RE(NO3)3·
6H2O/(S)-BINOL (1:3 ratio) resulted in the immediate
precipitation of 1−RE, which could be crystallized from
CH2Cl2/pentane in 70−85% yield. Using these conditions,
1−La was easily prepared on a 25 g scale (Figure 3A). Other
early REs (La−Eu) were accessible following this procedure,
with 1−Eu reported as a representative, fully characterized
example obtained in 79% crystalline yield.
The successful synthesis of 1−RE from hydrated starting
materials was surprising, because of the high hydration
enthalpies associated with RE3+ cations18a,19 and the aqueous
speciation of RE(NO3)3 that tend to form RE(NO3)x(OH)y−x
compounds at neutral or basic pH following acid hydrolysis.20
In this context, the increased Lewis acidity of the late
lanthanides (Gd−Lu) and Y proved problematic for their
Figure 4. Saa and co-worker’s RE−BINOLAM framework (RE = Sc,
Y, La−Lu; BINOLAM = 3,3′-diethylaminomethyl-1,1′-bi-2-naphthol).
intramolecular H-bonding pairs that consist of phenolic OH
donors and alkyl amine acceptors. The RE−BINOLAM
complexes are highly sensitive to ligand hydrolysis; synthesis
of RE−BINOLAM complexes require rigorous exclusion of
water, while the generation of free ligand from a hydrolysis
event can be observed even in dry CD3CN.17c
We attribute the water tolerance of 1−RE to the strong
preference for a six-coordinate geometry at the RE cation. Both
RE−BINOLAM and REMB complexes will coordinate H2O to
adopt seven-coordinate geometries.2c,4a,17c The acidity of H2O
coordinated to RE cations is increased by ∼5−6 orders of
magnitude,18 resulting in enhanced rates of ligand hydrolysis.
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open-air syntheses of 1−RE, where unlike the early lanthanides,
the formation of an inseparable byproduct (∼30%) was
observed even upon changing the order or rate of addition of
the reactants. Suppression of this byproduct, likely a mixed
hydroxide species, was possible by lowering the pH of the
RE(NO3)3·xH2O solution with 3 equiv of acetic acid. Addition
of a CH3CN solution of 3 equiv of (S)-BINOL and 6 equiv of
TMG to the acidified RE(NO3)3·xH2O solution, followed by
neutralization with an additional 3 equiv of TMG resulted in
the rapid formation and precipitation of 1−RE. Crystallization
from CH2Cl2/pentane furnished 1−RE in similarly high yields;
1−RE: Y = 85%, Yb = 80%, where 1−Y was synthesized on a
10 g scale.
synthetic utility,1c,22 and the sensitivity of the Lewis-acid/
Brønsted-base mechanism to catalyst structure, especially in
REMB frameworks.2c While we demonstrated REMB can be
generated from 1−RE and MX, the optimal combination of
MX source, solvent, and additives necessary to ensure the best
catalytic performance was unclear at the onset. Given the large
number of available combinations of RE and the main group
metal, we used microscale high-throughput experimentation
(HTE) techniques7 to identify conditions for 1−La/NaX as a
precatalyst for LaNaB. A variety of NaX sources were screened
with THF or toluene as solvent using cyclohexenone (2a) and
dibenzylmalonate (3c) as model substrates. The optimization
results for this study are presented in Table 1.
Notably, the synthesis of 1−RE from either method could be
carried out using technical-grade solvents without additional
drying, and provided anhydrous, crystalline products following
mild drying conditions (∼50 °C, 200 mTorr, 2 h). Unlike the
REMB or RE−BINOLAM complexes, no coordinated or
interstitial H2O crystallized with 1−RE synthesized from
benchtop methods.2c,4a,17c In addition to the strong preference
for a six-coordinate geometry of the RE cation in 1−RE, we
propose that the hydrophobic methyl substiutents of TMG−H+
contribute to the nonhygroscopic properties of 1−RE.
2.2. Generation of REMB from Rare Earth BINOLate
Ammonium Complexes and Catalytic Investigations.
Generation of REMB from 1−RE. After establishing a practical
synthetic protocol for the generation of 1−RE, we investigated
methods to access Shibasaki’s heterobimetallic catalysts using
1−RE as starting materials. While the ionic H-bonding
interactions in 1−RE evidently conferred stability against
hydrolysis, we envisioned that the large enthalpic contribution
from forming new M−OBINOLate bonds should provide a strong
thermodynamic driving force for the formation of the REMB
complexes from 1−RE. Indeed, installation of M+ was possible
through either acid−base or cation-exchange methods (Figure
3a), which produced REMB along with 3 equiv of TMG or
[TMG−H+][X−] (Figure 3a and Supporting Information). A
representative example is shown in Figure 3C−E, where
addition of excess LiI to 1−Eu immediately generated EuLB as
the single observable Eu-containing product. Notably, the
presence of coordinated water to the REMB was not observed
by 1H NMR using 1−RE synthesized from rigorously
anhydrous or benchtop methods, supporting the anhydrous
and nonhygroscopic nature of 1−RE (Figure 3C−E,
Supporting Information S16).
While syntheses of RE heterobimetallic frameworks have
been achieved through acid−base, redox, ligand-exchange, or
metathetical synthetic routes,21 to the best of our knowledge,
there have been no reports using cation-exchange from a RE/
ammonium precursor. Our method provides a new and
complementary approach that offers several potential advan-
tages as compared to traditional synthetic strategies. A large
variety of inexpensive MX salts and amine bases of varying pKa
are commercially available, which should expedite the
identification of new heterobimetallic frameworks. Moreover,
operational simplicity is also greatly improved by avoiding the
use of strong bases that are typically moisture sensitive.
Catalytic Investigations of 1−La/MX Precatalyst System.
Given the rapid and clean conversion of 1−RE to various
REMB products through cation metathesis, we turned our
attention to identifying conditions where 1−RE could be used
as a general precatalyst for REMB reactivity. As an initial trial,
the asymmetric Michael addition was chosen due to its
Table 1. Optimization of 1−La/MX in the Asymmetric
Michael Addition
entry La source
Na−X
Cl
H2O (X mol %) temp (°C) ee (%)
1
2
3
4
5
6
7
8
9
1−La
1−La
1−La
1−La
1−La
LaNaB
1−La
1−La
1−La
1−La
1−La
LaNaB
0
0
25
25
25
25
25
25
25
25
25
25
0
14
8
I
0
42
33
50
62
70
69
75
78
88
88
a
BAr4
0
N(SiMe3)2
0
0
I
10
10
20
30
30
30
b
I
I
I
I
10
c
11
c
12
0
a
b
Ar = 3,5-(CF3)2-C6H3. 60 mol % NaI was used instead of 30 mol %.
c
Malonate added portionwise over 20 min.
As a control experiment, we screened the hydrogen-bound
complex 1−La in the asymmetric Michael addition. 1−La was
not a competent catalyst for the formation of Michael adduct
4c (entry 1). As revealed in our binding studies, 1−La is
sterically saturated and would not be expected to provide dual
activation of the electrophile and nucleophile required for the
Lewis-acid/Brønsted-base-mediated mechanism. Inorganic salts
such as NaCl (entry 2) were ineffective in generating an active
catalyst; however, use of more soluble salts provided 4c in
moderate levels of enantiomeric excess (entries 3−5). Using
independently prepared LaNaB, we discovered that rigorously
anhydrous conditions resulted in only moderate ee’s, suggesting
the reactions conducted in the original reports on the activity of
LaNaB contained trace amounts of water (entry 6). Use of
varying amounts of water as an additive (entries 7, 9, 10; Figure
S1 and Table S2 in the Supporting Information) afforded
identification of an optimal [La]:[H2O] ratio of 1:3. Addition
of excess NaI did not negatively impact selectivity (entry 8);
however, selectivities were lower than the original report.2c
Ultimately, we found that slow addition of malonate was critical
to obtain high levels of enantioselectivity, a key observation that
was made by Shibasaki and co-workers for more reactive
Michael partners.23 At 0 °C, the use of 10 mol % 1−La/NaI or
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LaNaB (entries 11 and 12) provided ee’s identical to those
observed in the original report of LaNaB.
The generality of the 1−La/NaI precatalyst system was
investigated by exploring the scope of Michael donors (Table
2). While the performance of the precatalyst system, 1−La/
NaI, matched LaNaB in the Michael addition of 3c to 2a (entry
5), examination of other symmetrical malonates (3a, 3b, 3d)
resulted in improved levels of stereoselectivity (94−96% ee,
entries 1, 4, 6) as compared to literature reports (Table 2,
values in parentheses).2c We propose that the increased
selectivity was due to the high purity of LaNaB generated
from the 1−La/NaI system (Figure S5, Supporting Informa-
tion).
Table 2. Asymmetric Michael Addition of 1,3-Dicarbonyls to
Enones with the 1−La/NaI Precatalyst System
In light of the excellent levels of activity and selectivity for 1−
La in the optimized Michael addition, we sought to improve
the practicality of the system by reducing catalyst loading and
examining the scalability of this reaction to produce 4a (Table
2, entries 1−3). Compound 4a has been used as a key
intermediate in the enantioselective syntheses of diverse
products24 including strychnine alkaloids,3d,25 (−)-Gilbertine,26
Haouamine B,27 and (+)-2-deoxyolivin.28 Decreased catalyst
loadings were possible from the 1−La precatalyst (entries 2 and
3), where 2.5 mol % loading furnished 4a on an 8.7 mmol scale
under highly concentrated reaction conditions29 with minimal
losses in enantioselectivity (entry 3). The original levels of
selectivity could be restored by a single recrystallization of 4a in
87% isolated yield and 94% ee. While LaNaB is not as effective
for this particular transformation as Shibasaki’s ALB catalyst,
[Li(THF)3][(BINOLate)2Al],30 the 1−La/NaI system is an
operationally simple complement, because no pyrophoric
materials are necessary for the catalyst synthesis.
While a number of highly enantioselective catalysts for the
asymmetric Michael addition of malonates to cyclic enones
have been identified,22a−c,31 the corresponding addition of β-
ketoesters to acyclic enones still remains challenging.32
Shibasaki and co-workers reported high levels of stereo-
selectivity for the addition of β-ketoesters to acyclic enones
in CH2Cl2 with as little as 5 mol % LaNaB as a catalyst.23
Employing the 1−La/NaI precatalyst system under similar
conditions, addition of cyclic (3e) and acyclic (3f) Michael
donors to methyl vinyl ketone (2b) furnished Michael adducts
4e and 4f in 98% and 99% ee, respectively (entries 7 and 8). A
similar ∼10% improvement in ee was observed over the original
report,23 suggesting that this phenomenon could be observed in
other Lewis-acid/Brønsted-base reactions.
A key attribute of the REMB system is the diversity observed
in the catalytic reactions upon changing RE and M
combinations. To establish that our precatalyst was amenable
to different RE/M combinations, we investigated the Lewis-
acid/Lewis-acid-mediated aza-Michael addition of O-methylhy-
droxylamine to α,β-unsaturated ketones.2h,27 Shibasaki and co-
workers accessed optically active β-amino carbonyl compounds
using low catalyst loadings of YLB (0.5−3.0 mol %). In
addition, β-amino carbonyl compounds are important structural
motifs in many biologically active compounds.33 They also
demonstrated that their products could be further transformed
to other useful chiral building blocks such as aziridines or β-
amino alcohols with no loss in ee.2k,34
Application of the optimized 1−RE/MI precatalyst system to
generate YLB from 1−Y/LiI proved general. At 3 mol %
loading of 1−Y, comparable selectivities were obtained for
various substitution patterns (Table 3, 7a−e) including
examples of an electron-donating group (7b), electron-
withdrawing group (7c), heterocycle (7d), and extended
conjugation (7e). The scalability of this reaction was also
maintained, where 7a could be obtained in 93% isolated yield
and 93% ee on a 1 g scale (entry 2). Catalyst loading could be
a
Reactions conducted on a 1.0 mmol scale using 1 equiv of 2a and 1
equiv of 3a−d, or 1.2 equiv of 2b and 1 equiv of 3e,f unless otherwise
specified. Values in parentheses are from refs 2c and 23 using LaNaB
as a catalyst. Isolated yield after chromatographic purification.
b
c
Michael donor added portionwise over ∼20 min. Reported reaction
d
performed at room temperature. Michael donor added slowly over 8
h. Performed on 1 g scale. A single recrystallization furnished product
in 87% yield with 94% ee.
e
E
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a
Table 3. Asymmetric aza-Michael Addition of O-
Methylhydroxylamine to Chalcone Derivatives with the 1−Y
/LiI Precatalyst System
Scheme 1. Asymmetric Direct Aldol Reaction of
Acetophenone and Pivaldehyde Catalyzed by Second
Generation LaLB, LaLB·KOH
a
b
Isolated yield after chromatographic purification. Results from ref 2i.
Scheme 2. Catalytic Activity of 1−RE and REMB Systems
after Exposure to Open-Air for 6 months in the Asymmetric
(1) Michael Addition, (2) aza-Michael Addition, and (3)
a
Direct Aldol Reaction
a
Reactions conducted on a 0.5 mmol scale using 1 equiv of 7a−e and
1.2 equiv of 8 in THF ([enone] = 1.6 M) unless otherwise specificed.
Values in parentheses are from using YLB (refs 2k and 34). Isolated
yield after chromatographic purification. 1 g scale. 80 h, [enone] =
b
c
d
2.05 M. 60 h, [enone] = 1 M.
a
b
Isolated yield after chromatographic purification. Not determined.
further reduced to 0.5 mol % (entry 3), albeit with slightly
decreased levels of enantioselectivity (88% versus 93% ee).
Additives have played an important, and at times poorly
understood, role in improving the performance of the REMB
catalysts.1a,2i,l−n,35 For example, the addition of MOH and H2O
to REMB solutions can generate highly active second
generation catalysts for aldol and nitroaldol reactions.1a,2i To
test the compatibility of additives in our precatalyst system, we
chose the direct aldol reaction catalyzed by second generation
LaLB (LaLB·KOH). Addition of 8 mol % KOtBu and 16 mol %
H2O to 1−La/LiI (8/24 mol %) generated LaLB·KOH, which
catalyzed the direct aldol reaction between pivaldehyde (8) and
acetophenone (9) to furnish 3-hydroxy-4,4-dimethyl-1-phenyl-
pentan-1-one (10) in 74% isolated yield and 95% ee (Scheme
1). Interestingly, our preliminary results revealed an improve-
ment (∼7% ee) in enantioselectivity using our precatalyst
system in a second Lewis-acid/Brønsted-base-catalyzed reac-
tion, which supports that our system was amenable to additives
similar to those of the REMB framework.
open air for 6 months and then employed in each of the
mechanistically distinct reactions (Scheme 2). 1−RE/MX
precatalysts maintained excellent catalytic activity, whereas the
performance of REMB were signficantly reduced due to the
decomposition associated with prolonged exposure to ambient
atmosphere. These experiments further supported the tolerance
of 1−RE to benchtop conditions, and highlight their suitability
as robust precatalysts.
3. CONCLUSIONS
In summary, Shibasaki’s REMB catalysts are among the most
enantioselective asymmetric catalysts across a broad range of
mechanistically diverse reactions. The widespread utility of
these catalysts, however, has been hampered by their
challenging syntheses and manipulation. Even for those
proficient in their use, this family of catalysts is not readily
amenable to high throughput screening, because each catalyst
requires independent preparation and use due to the lack of
suitable precatalysts. To address these challenges, we have
designed a novel class of self-assembled H-bonded RE
tris(BINOLate) complexes, which have immediate application
as air-stable precatalysts for Shibasaki’s REMB frameworks.
Incorporation of H-bonding interactions in the secondary
coordination sphere resulted in improved moisture tolerance as
While the REMB catalysts can be stored at room
temperature for extended periods of time under a dry N2
atmosphere with no significant loss in catalytic activity, we were
interested in performing a side-by-side comparison of the
stability of 1−RE and REMB as solids stored on the benchtop.
Crystals of 1−RE and REMB were stored in vials exposed to
F
dx.doi.org/10.1021/ja502568g | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
compared to related REMB and RE−BINOLAM systems,
which facilitated the development of an operationally simple
open-air synthesis of 1−RE. The 1−RE precatalysts can be
synthesized using conventional benchtop methods and
hydrated RE starting materials to provide high yields of
crystalline, nonhygroscopic 1−RE on large scales. Use of
hydrated RE sources provided a significant cost reduction;
RE(NO3)3·xH2O are ∼100-fold less expensive than commonly
employed functionalized RE materials such as RE(OiPr)3 or
RE[N(SiMe3)2]3.36 Because of these properties, 1−RE were
identified as excellent precursors for the generation of
anhydrous heterobimetallic complexes by acid−base or
cation-exchange methods with a variety of RE/M combinations.
Furthermore, we have demonstrated that 1−RE/MI could be
applied as a general precatalyst system for Shibasaki’s REMB
framework using both traditional bench-scale and HTE
techniques. This precatalyst system showed performance
comparable to or improved over the reported REMB systems,
and was amenable to different RE/M combinations, different
reaction types (Lewis-acid/Brønsted-base, Lewis-acid/Lewis-
acid), and the presence of additives. We attribute the success of
this particular system to the use of MI, which cleanly generates
REMB through cation-exchange while producing an innocent
guanidinium iodide spectator ion. We expect that this system
will provide a convenient and complementary synthetic strategy
to well-known and, as of yet, unidentified heterobimetallic
frameworks. Further investigations on the self-assembly of ionic
H-bond pairs, identification of new heterobimetallic frame-
works through cation-exchange, and their applications in
catalysis are underway.
Library, for helpful discussion and assistance with searches
related to literature precedent.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, NMR spectra, HPLC chromatograms of
racemic and enantioenriched product, and crystallographic data
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
pwalsh@sas.upenn.edu
schelter@sas.upenn.edu
mapericas@iciq.es
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Present Address
∥Batelle Memorial Institute, 2900 Fire Road, Suite 201, Egg
Harbor Township, New Jersey 08234, United States.
Notes
The authors declare the following competing financial
interest(s):The authors declare competing financial interests:
intellectual property pertaining to the technology described in
this Report is covered by U.S. Patent Application No. 61/
837,833.
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ACKNOWLEDGMENTS
■
J.Y. thanks MINECO (Spain) for a postdoctoral contract. X.F.
thanks CSC (Chinese Scholarship Committee) and ICIQ
Foundation for a fellowship. This work was supported by the
NSF (NSF-ICC:CHE-1026553) to P.J.W. and E.J.S. and by
MINECO (Grants PIB2010US-00616 and CTQ2012-38594-
C02-01), AGAUR (Grant 2009SGR623), and ICIQ Founda-
tion to M.A.P. E.J.S. and P.J.W. acknowledge the University of
Pennsylvania. We thank Judith Currano, Penn Chemistry
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detailed comparison.
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