Asymmetric gold-catalyzed lactonizations in water at room temperature

Article abstract of DOI:10.1002/anie.201404729

Asymmetric gold-catalyzed hydrocarboxylations are reported that show broad substrate scope. The hydrophobic effect associated with in situ-formed aqueous nanomicelles gives good to excellent ee's of product lactones. In-flask product isolation, along with

Full text of DOI:10.1002/anie.201404729

Angewandte
Chemie
DOI: 10.1002/anie.201404729
Micellar Catalysis
Asymmetric Gold-Catalyzed Lactonizations in Water at Room
Temperature**
Sachin Handa, Daniel J. Lippincott, Donald H. Aue, and Bruce H. Lipshutz*
Abstract: Asymmetric gold-catalyzed hydrocarboxylations are
reported that show broad substrate scope. The hydrophobic
effect associated with in situ-formed aqueous nanomicelles
gives good to excellent eeꢀs of product lactones. In-flask
product isolation, along with the recycling of the catalyst and
the reaction medium, are combined to arrive at an especially
environmentally friendly process.
metal-catalyzed enantioselective hydrocarboxylations of
allenes forming valuable nonracemic lactones, however, is
still an unsolved problem. Indeed, only one simple example is
currently known, done in the no longer acceptable solvent
benzene and with moderate ee.[11] By contrast, related
reactions such as intramolecular hydroalkoxylations[5] and
hydroaminations[12] of allenes are well established. These are
typically done in nonpolar media, and show promising levels
of asymmetric induction. Nonetheless, because these reac-
tions, carried out in organic solvents, oftentimes require low
temperatures[8,13] to maximize eeꢀs, reaction times can be
significantly increased. Moreover, none offers an opportunity
to recycle the metal, ligand, or counterion, thus making the
overall process both costly and environmentally unattractive.
On the other hand, synergistic binding between catalyst
and counterion might be expected due to the hydrophobic
effect found within nanomicelles. This could enhance
a defined chiral pocket leading to improved enantiocontrol
that otherwise is not always observed in traditional organic
solvents.[14] Despite these potential advantages to micellar
catalysis, no report on asymmetric gold catalysis within such
nanoreactors has been reported to date. Herein we describe
asymmetric gold-catalyzed intramolecular cyclizations of b-
allenic acids[15] to give enantioenriched lactones, carried out
within self-aggregated, tailor-made nanoparticles in water at
room temperature (Scheme 1).
M
icellar catalysis in water can play an important contribu-
ting role in green chemistry. Today, many valued reactions
take place within the lipophilic core of self-aggregated
nanomicelles, composed of “designer” surfactants tailor-
made to best accommodate the synthetic chemistry of
interest.[1] The hydrophobic interior of these nanoreactors
can function as far more than an alternative, albeit green,
solvent for a desired reaction; rather, it offers an opportunity
to enhance the binding between a charged catalyst and its
counterion, especially in such a characteristically nonpolar
environment. Synergistic binding between ions might be
leveraged, e.g., in asymmetric gold-catalyzed reactions, which
is particularly challenging due to the bicoordinate geometry
of AuI. This orientation places a nonracemic ligand distal to its
catalytic site,[2] thereby preventing the substrate from attain-
ing the highly biased mode of chelation desired for max-
imizing enantioselectivity. Nonetheless, a variety of factors
have been found leading to asymmetric induction.[2,3] These
include the specifics of the ligand–metal binding, such as with
Au–arene interactions as well as the nature of the counter-
ion,[4] reaction temperature,[5] and the solvent.[5,6] Ligands
such as phosphines,[7] acyclic diaminocarbenes (ADCs),[8] and
N-heterocyclic carbenes (NHCs)[9] function well for such Au-
catalyzed processes in organic media.
We started our investigations with allenic acid 1 as
a model substrate, looking to find a nonracemic catalyst
suitable for use in an aqueous micellar medium. An initial
reaction in the designer surfactant TPGS-750M/H2O with
5 mol% of catalyst R-BINAP-Au2Cl2/AgX afforded 2 in low
ee (Table 1, entry 1). Altering the axially chiral core of the
ligand to H8-BINAP (4) and SYNPHOS (5a), likewise led to
no significant improvement of the ee (entries 2 and 3).
Increasing the steric bulk of the aromatic residue on
Hydrocarboxylations of olefins and allenes have been
found to be highly valuable in the synthesis of target bioactive
molecules.[10] Transition metal catalysts, in particular coinage
[*] Dr. S. Handa, D. J. Lippincott, D. H. Aue, Prof. B. H. Lipshutz
Department of Chemistry & Biochemistry, University of California
Santa Barbara, CA 93106 (USA)
E-mail: lipshutz@chem.ucsb.edu
[**] We acknowledge financial support for our programs in green
chemistry, provided by the NIH (GM86485), the Center for Scientific
Computing at the CNSI and MRL [NSF MRSEC (DMR-1121053) and
NSF CNS-0960316], and NSF support from the National Center for
Supercomputing Applications (NSF TG-CHE100123) utilizing the
NCSA Gordon and Blacklight systems. We thank Prof. L. Slaughter
(U North Texas) for sharing experimental procedures leading to
some of the allenic acids utilized in this work, and Eric D. Slack for
technical assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404729.
Scheme 1. Cationic gold binding A) within a nanomicellar core in water
versus B) in many organic solvents.
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Table 1: Optimization of Au-catalyzed asymmetric intramolecular
hydrocarboxylations of allenic acids.
Entry
Ligand L
XÀ
t [h]
Yield 2
ee [%]
1
2
3
4
5
6
7
8
3
4
5a
5b
7
8
9
9
10
11a
11b
11c
11d
12a
12b
12c
12d
12e
12 f
BF4
78
74
72
78
29
32
67
70
60
70
75
42
72
48
49
40
30
38
48
98
95
95
96
93
92
67
70
85
90
91
94
95
95
91
85
97
70
97
3
11
18
30
1
SbF6
SbF6
SbF6
SbF6
SbF6
PMB
PNB
SbF6
SbF6
SbF6
SbF6
PNB
SbF6
SbF6
SbF6
PNB
PNB
PNB
7
40
50
29
6
36
7
51
12
18
31
37
30
55
9
10
11
12
13
14
15
16
17
18
19
Figure 1. Ligands and counterions used for optimization studies.
*LAu2Cl2 =LAuCl (5 mol%) when L=7, 8, and 9. PNB=p-nitroben-
zoate, PMB=p-methoxybenzoate. For ligands, see Scheme 2. For
a detailed optimization Table, see SI.
phosphorus contributes toward the observed enantioselectiv-
ity, as 3,5-Xyl-SYNPHOS (5b) increased the ee to 30%
(entry 4). The improvement in ee due to steric effects near the
donor atom of the ligand suggested the use of monodentate
ligands, including phosphoramidite 7 and ADCs 8 and 9.
Interestingly, an increased ee to 50% was only seen with the
larger ADC 9. The choice of counterion also affected the
selectivity of the reaction (entries 7 and 8). No significant
improvements in terms of ee were observed even with further
alterations in the axially chiral core of several bis-phosphine
ligands (entries 9–12). A 51% ee was obtained using SEG-
PHOS ligand 11d when accompanied by p-nitrobenzoate as
a counterion (entry 13). The best results were obtained by the
fine-tuning of BIPHEP ligands (entries 14–19), with 12 f
being identified as the best choice so far (entry 19). In the
absence of surfactant there was essentially no reaction.
The impact of counterions on both eeꢀs and yields in
aqueous micelles suggested that further tuning of a catalyst
system with focus on the chiral counterion might be produc-
tive. Initial match/mismatch studies were performed on both
R and S enantiomers of methoxy-BIPHEP ligand 12 f, with
counterions S-13a, R-13a, S-13b, R-13b, S-14a, and R-14a
(Figure 1). A significant mismatch was found between ligand
R-12 f and counterion R-13a giving only 27% ee of product 2,
whereas the enantiomeric counterion S-13a led to an ee of
51% (Scheme 2). Using a far bulkier BINOL-based counter-
Scheme 2. Venn diagram showing match/mismatch combinations
between ligand 12 f and counterions.
ion S-13b, an insignificant match/mismatch existed, as only
a 5% difference in ee was obtained in product 2 (75% vs.
70%). An ee of 66% was found in the reaction of 1 using S-
12 f-Au2 and counterion R-13b-Ag. Spirocyclic phosphate
counterion 14 provided slightly better results in terms of ee
than did counterion 13a. In order to find the best match for an
active catalyst, additional reactions were performed using S-
12 f-Au2, along with both enantiomers of silver salts of 13a,b
and 14. Interestingly, no general trend of match/mismatch
between catalyst 12 f and different counterions was observed.
From all combinations studied using the model reaction, the
combinations R-12 f-Au2/R-13b-Ag or S-12 f-Au2/S-13b-Ag
(75% ee) was found to be the best pair between ligand 12 f
and counterions 13a, 13b, and S- or R-14.
Using the catalyst system containing ligand R-12 f and
a counterion R-13b, further optimization studies were under-
taken. These revealed a dependence of the ee on several
variables, including 1) Au-nanoparticle-free conditions;
2) the nature of acidic or basic additives; 3) the presence of
AgCl; 4) the ratio of (L-AuCl)2 to AgX; 5) removal of both
chloride ions attached to gold; 6) protection from ambient
light; 7) the purity of the catalyst and counterion; 8) the
2
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surfactant concentration, and 9) the reaction temperature
(see the Supporting Information (SI) for details). Optimal
reaction conditions were found to be: 3 wt% TPGS-750M as
surfactant in water as the reaction medium, breaking of the
crystallinity of solids by the addition of minimal amounts of
solvent additives, use of 3 mol% of AgCl-free, pure active
gold-catalyst R,R-15, 0.5m global concentration, extraction of
the product with a minimum amount of 10% ether in hexanes
to maximize the amount of catalyst that remains in the
aqueous medium in the reaction vessel, and ambient temper-
ature.
Under these optimized conditions, including the “trick” of
softening highly crystalline solid substrates with a drop or two
of an organic solvent (DMSO, benzene, or toluene) as
“additive”, reaction times were decreased significantly; in
the case of starting material 1, from 72 to 16 h. The ee could be
increased from 75% to 88% by using isolated AgCl-free
catalyst R,R-15. The X-ray structure of R,R-15 shows the
defined chiral pocket with weak Au–arene interactions
(Figure 2). The binding between the cationic gold and the
counterion through the phosphate group supports the match
between them, which had already been determined exper-
imentally (see above).
Scheme 3. Substrate scope (additive=trace amounts of organic sol-
vent).
of 90%. The other diastereomer underwent secondary
processes leading to unidentifiable material.
To confirm that the solvent additive is acting solely to
soften the highly crystalline nature of the substrates and the
catalyst, and neither impacting the role of the micellar
environment nor merely acting as the reaction medium, the
highly crystalline substrate 33 was subjected to the reaction
conditions in different reaction media including neat DMF,
DMSO, and toluene as well as in TPGS-750-M with these
same solvent additives (Scheme 4). Similar eeꢀs were observed
when reactions were run in aqueous TPGS-750-M with
solvent additives, whereas different eeꢀs were obtained in
each of the neat organic solvents. Poorer eeꢀs in polar organic
solvents and better eeꢀs in nonpolar organic solvents and
Figure 2. X-ray structure of R,R-15 in ball and stick. Color code:
C, gray; O, purple; P, green; Au, yellow.
As illustrated in Scheme 3, a variety of precursor allenic
acids were cyclized to afford high yields of product lactones.
This new technology is clearly applicable to many substitution
patterns associated with both the allenic group as well as the
a-carbon of the carboxylic acid. Unsubstituted as well as
dialkyl-substituted allenic termini appear to give eeꢀs that are
among the highest observed (81–92%). Allenes bearing
saturated rings at the terminus ranging from five- to eight-
membered in size afforded eeꢀs that were more variable, and
seemingly dependent on the nature of the substituent(s) at the
a-carbon. Most eeꢀs are in the 80–90% range, and hence, it is
unclear why spirocyclic lactone 23 was formed in 96% ee.
With a single substituent a to the acid, diastereomers are
formed; the major isomer 32 was isolated (48%) and has an ee
Scheme 4. Impact of the reaction medium on the ee.
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TPGS-750-M are indicative of the surfactant serving as the
reaction medium, providing the hydrophobic effect inside the
micellar core (for details, see SI).
that the addition of catalytic triethylamine or quaternary
ammonium hydroxides substantially accelerated the reactions
in the micelle, suggests that the reaction with base may
proceed via the deprotonated complex B to form C. After
deprotonation of the carboxylic acid group in the gold–allene
complex, the cyclization occurs with little or no barrier in our
theoretical modeling. Reaction trajectory modeling is con-
sistent with the observed preference for 5-exo-trig cyclization.
This mechanism is in stark contrast to the mechanism
proposed for the gold-catalyzed cyclization of allenic alcohols,
with reversible cyclization and rate-determining protodeau-
ration,[16] but the reactions in the absence of added base may
occur via E rather than B. The calculated transition-state
energies for cyclization by this pathway also show a preference
for the 5-exo-trig pathway. Such a change of mechanism
would be consistent with our observation that a low ee is
observed in reactions with a chiral counterion in the presence
of added triethylamine.
The progress of a representative reaction of allenic acid 33
leading to lactone 18 under the influence of the cationic gold
catalyst R,R-15 was followed over time. As the reaction
progressed, the ee of 18 remained constant, as determined by
HPLC analyses of several aliquots. The absolute stereochem-
istry of the newly created center in the product was
determined by single-crystal X-ray diffraction analysis, clearly
indicating S-stereochemistry (Figure 3).
Studies were also conducted to assess the potential for
recycling of the aqueous medium. Given that our portfolio of
“designer” surfactants are engineered to remain in the
water,[16] the more intriguing question concerned the fate of
the gold catalyst with respect to both its recyclability as well as
its efficacy. Upon extraction with minimal amounts of 10%
ether in hexanes, more than 50% of the catalyst remained in
the aqueous phase such that only 1 mol% of fresh R,R-15
need be added at each cycle. As illustrated in Scheme 6, the
eeꢀs for product 2 remained essentially constant, as did the
yields, throughout six successive recycles. The associated
E Factor based on organic solvent usage,[17,18] as a measure of
the “greeniness” of this protocol was calculated to be only 4.9,
which compares very well with values typically seen from
chemistry done in the fine chemicals and pharma arenas.[17]
In summary, the hydrophobic effect characteristic of
nanomicelles has been utilized advantageously to enhance
tight ion-pair binding between a nonracemic cationic gold
catalyst and its nonracemic counterion. This combination is
highly effective in achieving especially challenging asymmet-
ric intramolecular hydrocarboxylation of allenes in high
yields and good to excellent eeꢀs. The aqueous micellar
reaction medium obviates the use of organic solvents, and
Figure 3. X-ray structure of S-19.
The mechanism by which these Au-catalyzed reactions
might proceed has been investigated with DFT calculations,
using a PH3 ligand and the carboxylic acid precursor of 19
without phenyl substituents. The calculations showed that
direct 5-exo-trig cyclization of A to form a protonated lactone
E was substantially uphill, DG8 = 13.2 kcalmolÀ1 in toluene
(Scheme 5). The 6-endo-dig complex was uphill by 10.7 kcal
Scheme 5. Plausible mechanism with DFT energy diagram at the
PBE1PBE/6-31+G(d,p)/SDD(Au)/SMD(toluene) level of theory with
relative free energies in toluene at 298 K in kcalmolÀ1. The free energy
difference between A and B depends on the base used and is not well
known in the micellar media (see SI).
molÀ1e, whereas the 6-exo-dig and 7-endo-trig products were
uphill by 22.6 and 18.8 kcalmolÀ1, respectively [PBE1PBE/
6-31 + G(d,p)/SDD(Au)/SMD(toluene)]. On the other hand,
cyclization reactions of the deprotonated complex B were
downhill energetically by 22.1 kcalmolÀ1 for the 5-exo-trig
cyclization and 21.9 kcalmolÀ1 for the 6-endo-dig cyclization
in dichloromethane (see SI). In toluene, these free energies
were considerably larger, 34.6 and 35.2 kcalmolÀ1, respec-
tively, as expected. These observations, together with the fact
Scheme 6. Studies on recycling and E Factor.
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Received: April 26, 2014
Revised: July 12, 2014
Published online: && &&, &&&&
Keywords: asymmetric catalysis · gold catalysis ·
.
micellar catalysis · surfactants
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Communications
Micellar Catalysis
S. Handa, D. J. Lippincott, D. H. Aue,
B. H. Lipshutz*
&&&& — &&&&
Asymmetric Gold-Catalyzed
Lactonizations in Water at Room
Temperature
With tight quarters come more interac-
tions! The high concentrations within
nanomicelles can be used to force a close
interaction between a nonracemic cat-
ionic metal complex and its nonracemic
counterion, thereby maximizing the cata-
lyst’s impact on a given transformation. A
chiral gold dicationic species and its
associated phosphate anion are used to
control absolute stereochemistry in newly
formed g-lactone products, derived from
allenic carboxylic acids.
6
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