Chiral organic ion pair catalysts assembled through a hydrogen-bonding network

Article abstract of DOI:10.1126/science.1176758

Research to develop structurally discrete, chiral supramolecular catalysts for asymmetric organic transformations has met with limited success. Here, we report that a chiral tetraaminophosphonium cation, two phenols, and a phenoxide anion appear to self-a

Full text of DOI:10.1126/science.1176758

REPORTS
13. Material and methods are available as supporting
material on Science Online.
the same strong faceting is yet again observed relatively low density of grown tubes under
(Fig. 4F) (movie S1). This behavior was found Ar-supported ambient (fig. S1). Thus, it ap-
to be ubiquitous, with all particles in He show- pears that the addition of H2O not only assists
ing stronger faceting, and those in Ar exhibiting the super growth of nanotubes by etching the
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a more rounded shape.
amorphous carbon (27) and controls the particle
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We also investigated relatively larger catalyst diameter by retarding the effect of Ostwald
sizes to more clearly observe the evolution of ripening (28), but also alters the shape of the
catalyst shapes. Figure 4G indicates that the particle. Remarkably, Yamada et al. (29) ob-
equilibrium shape of the catalyst in the 500°C served the alteration of carbon coated Fe catalysts
and 500 mtorr of He/H2O environment has into flatter particles upon the removal of the
{111} facets with very sharp edges, which is ex- carbon coating by water treatment, under stan-
actly the same observation as in Fig. 4D. Figure dard water-assisted nanotube growth conditions.
4, H and I, (movie S2) tracks the gradual de- There are other adsorbates (such as CO and O2)
faceting of the hill-and-valley structure with in- that, in combination with the carbon source, may
creasing time at the same temperature after the be even more effective at altering the catalyst
removal of He/H2O and the addition of Ar/H2O. size and morphology. Our results indicate that,
All of the observations reflected in Fig. 4 are with further optimization, direct control over
consistent: Catalysts in the He/H2O environment nanotube structure during growth may well be
are strongly faceted. Additionally, the fact that feasible (30, 31).
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the particles are more faceted in H2/H2O and
He/H2O ambients is consistent with the obser-
vation of a reduced ripening rate.
Broadly, the adsorbent-induced variation of
the surface free energy Dg of the facets with Miller
indices (h, k, l) on the particle can be expressed by
References and Notes
31. P. Avouris, Acc. Chem. Res. 35, 1026 (2002).
32. We thank M. S. Dresselhaus and J. Kong for help in
Raman measurements, B. Weisman and A. Naumov for
photoluminescence measurements, B. I. Yakobson for
discussions, and B. Maruyama for assistance. A patent
related to this work has been submitted (U.S. patent
application no. 12/511,047). This research was supported
by the Honda Research Institute USA.
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Dgh,k,l ¼ Dgh,k,lðP, T, CS, qo, kd, SÞ
ð1Þ
Supporting Online Material
where P is the partial pressure of the adsorbate, T
is the temperature, CS is the surface concentration
of the particle surface atoms, qo is the saturation
coverage of the adsorbate, kd is the desorption
rate constant, and S is the sticking probability of
the adsorbate (21, 22). Our TEM data suggest
that the surface energy anisotropy of specific
facets of Fe is affected differently by the He/H2O
and Ar/H2O ambients, resulting in the observed
dynamic alterations in the particle shape. It is
unlikely that the inert gas itself causes a dif-
ference in surface free energy via adsorption at
high temperatures. Therefore, the entirety of
our data set indicates that it is most probable that
H2O adsorption is responsible for the induced
surface reconstruction (23). The adsorption
pathway can be either dissociative (with forma-
tion of adsorbed hydroxyl, atomic oxygen, and
atomic hydrogen species on the surface) or
molecular (24). However, analysis of the catalyst
structure (through the use of diffractograms
produced by fast Fourier transforms of the
images, shown as an inset to Fig. 4G, indicating
that the particles are g-Fe) and observation of
reversible shape changes (Fig. 4, D to F) suggest
that there is no strong oxygen binding, which
could have an impact on tube growth (25, 26).
The TEM measurements were performed under
conditions of dynamic adsorption/desorption equi-
librium with both H2O and either He or Ar gas
www.sciencemag.org/cgi/content/full/326/5949/116/DC1
Materials and Methods
Figs. S1 to S6
Table S1
References
Movies S1 and S2
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11 June 2009; accepted 6 August 2009
10.1126/science.1177599
12. L. Qu, F. Du, L. Dai, Nano Lett. 8, 2682 (2008).
Chiral Organic Ion Pair
Catalysts Assembled Through
a Hydrogen-Bonding Network
Daisuke Uraguchi, Yusuke Ueki, Takashi Ooi*
Research to develop structurally discrete, chiral supramolecular catalysts for asymmetric organic
transformations has met with limited success. Here, we report that a chiral tetraaminophosphonium
cation, two phenols, and a phenoxide anion appear to self-assemble into a catalytically active
supramolecular architecture through intermolecular hydrogen bonding. The structure of the resulting
molecular assembly was determined in the solid state by means of x-ray diffraction analysis. Furthermore,
in solution the complex promotes a highly stereoselective conjugate addition of acyl anion equivalents
to a,b-unsaturated ester surrogates with a broad substrate scope. All structural components of the catalyst
cooperatively participate in the stereocontrolling event.
ature harnesses weak interactions, par- ular recognition (1). Despite important advances
ticularly hydrogen bonds, to construct in the elaboration of self-assembled molecular
biologically active supramolecular archi- receptors and catalyst systems, however, ratio-
N
present. Both the observation of an adsorbent- tectures, as demonstrated by the three-dimensional nal design of chiral supramolecular catalysts for
induced reversible shape reconstruction and structures of enzymes and nucleic acids. Inspired stereoselective bond-forming reactions by the use
severe particle ripening under the Ar/H2O envi- by these biological systems, research for the de-
ronment (fig. S6) are consistent with the adatom’s velopment and application of supramolecular
coverage of the Fe particles being greater in the catalysts assembled through noncovalent in-
Department of Applied Chemistry, Graduate School of En-
gineering, Nagoya University, Nagoya 464-8603, Japan.
He versus the Ar ambient [q(He+H2O) > q(Ar+H2O)]. teractions has attracted interest in the fields of
*To whom correspondence should be addressed. E-mail:
This would also be correlated to the resulting, both selective chemical synthesis and molec- tooi@apchem.nagoya-u.ac.jp
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Fig. 1. (A) Structures of chiral tetraaminophosphonium cations. (B) Oak Ridge thermal ellipsoid plot diagram of 1a·(OPh)3H2. All calculated hydrogens were
omitted for clarity. (C) Plausible modes of assembly in catalyst and enolate.
of discrete molecular associations remains a great
challenge (1–5). Described here are systems in performed with 0.22 mmol of 2 and 0.2 mmol of 3a in the presence of catalyst (1 mol %) in 2.0 mL of
Table 1. Effect of each component of the catalyst assembly. Unless otherwise noted, the reactions were
toluene at −40°C under argon atmosphere. The catalyst concentrations are indicated. The isolated yields
which chiral and achiral small molecules spon-
taneously assemble through a well-defined array
of intermolecular hydrogen bonds to produce
structured chiral organic ion pairs. The distinc-
tive feature of the supramolecular ion pair as-
sembly as a chiral molecular catalyst is that its
stereocontrolling ability can be fine-tuned by
structural modification of both chiral and achiral
components. We developed a catalyst for the
highly enantioselective conjugate addition of
acyl anion equivalents to a,b-unsaturated ester
surrogates.
1
are reported. All diastereomeric ratios (dr) and ee were determined by means of H nuclear magnetic
resonance (NMR) (500 MHz) analysis of crude aliquots and chiral stationary phase high-performance liquid
chromatography (HPLC), respectively. The relative stereochemistry was not determined.
We initially prepared chiral tetraaminophos-
phonium phenoxides as part of our broader ex-
ploration of chiral organic ion pairs for cooperative
asymmetric catalysis (6). The requisite cation
framework 1a (Fig. 1A) was readily synthesized
from the parent a-amino acid, L-valine, as its
chloride salt (7). Then, the anion was exchanged
with hydroxide, and subsequent neutralization
with phenol (8) yielded 1a·(OPh)3H2. The solid-
state structure of 1a·(OPh)3H2 was unambiguous-
ly determined by means of single-crystal x-ray
diffraction analysis (Fig. 1B). Surprisingly, the
analysis revealed a molecular assembly in which
an aminophosphonium cation, two phenols, and
a phenoxide anion were aligned through a 10-
membered cyclic network of intermolecular
hydrogen-bonding interactions. The hydrogen-
bonding donor capability of the phenols seems
to be enhanced by an additional hydrogen bond
*Two mol % of catalyst was used.
of toluene was used as solvent.
†Five mol % of catalyst was used.
‡Ten mol % of catalyst was used.
§0.2 mL
from the N-H protons of the phosphonium cation, the chiral environment around the remotely lo- pose, we chose 2-unsubstituted oxazol-5(4H)-one,
thus facilitating the construction of an antidromic cated phenoxide anion. namely azlactone, 2 (Fig. 1C and Table 1) as a
circular network (9–11). Another salient feature
On the basis of these serendipitous obser- pro-C1-nucleophile (12) with the expectation
of the assembly was that the stereochemical infor- vations, we explored the use of 1·(OAr)3H2 as that the enolate 2′ generated through deprotona-
mation in the chiral cation moiety was effectively a chiral molecular catalyst for synthetically val- tion by 1·(OAr)3H2 would behave as a suitable
relayed by the two phenol molecules, extending uable stereoselective transformations. For this pur- phenoxide anion equivalent; as such, it would
www.sciencemag.org SCIENCE VOL 326 2 OCTOBER 2009
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REPORTS
dichlorophenol, induced substantial increases
Table 2. Scope of a,b-unsaturated acylbenzotriazole 3. The reactions were performed with 0.22 mmol
of 2 and 0.2 mmol of 3 under the influence of 1b·(3,5-Cl2−C6H3O)3H2 (1 mol %) in 0.2 mL of toluene in enantioselectivity (entries 5 to 9) (table S1 and
at −40°C under argon atmosphere. The isolated yields are reported. All dr and ee were determined by
SOM text). Furthermore, we confirmed the effect
of the catalyst concentration on the propensity
for molecular association in solution. As we
had assumed, both increased catalyst loading
and decreased solvent volume improved enan-
tioselectivity (entries 10 to 12 and 13), which
also argues for the role of a requisite molecular
assembly (fig. S1 and SOM text). Structural
modification of the chiral cationic moiety by
using L-isoleucine–derived 1b (see Fig. 1A) also
appeared to be crucial for achieving the highest
selectivity (entry 14). Consequently, all the struc-
tural components of the self-assembled catalyst
cooperatively participate in realizing the rigorous
stereocontrol.
means of 1H NMR (500 MHz) analysis of crude aliquots and chiral stationary phase HPLC, respectively.
With the optimal catalyst structure and reaction
conditions in hand, we conducted further experi-
ments to probe the scope of a,b-unsaturated acyl-
benzotriazole 3, and the representative results are
summarized in Table 2. Generally, use of 1 mole
percent (mol %) of 1b·(3,5-Cl2−C6H3O)3H2 and
1.1 equivalents of 2 was sufficient for a smooth
reaction, giving 4 in high yield with excellent
enantioselectivity. With aryl-substituted 3, this
method tolerated the incorporation of both electron-
withdrawing and electron-donating substituents
at an arbitrary position on the aromatic scaffold
(entries 1 to 4). In addition, fused- and hetero-
aromatic b–substituents had no influence on the
stereochemical outcome (entries 5 and 6). A range
of primary and secondary alkyl groups were also
nicely accommodated (entries 7 to 10).
*Absolute configuration of the product 4h was determined, after conversion to the known compound 6 (Fig. 2), by comparison
of the optical rotation with the literature value (7).
The acylbenzotriazole and azlactone moie-
ties of product 4 can be selectively converted to
various useful functional groups, thus highlight-
ing the synthetic potential of the present highly
stereoselective conjugate addition protocol. For
example, 4h (96% ee) was successfully derivatized
Fig. 2. Conversion of 4h to optically active methylsuccinic acid 6. DBU, 1,8-diazabicyclo[5.4.0]
undec-7-ene; Ac, acetyl; THF, tetrahydrofuran.
be captured into a similar hydrogen-bonding net- through the generation of chiral aminophos- into optically active methylsuccinic acid (6) in
work without negatively affecting the preorgani- phonium enolate 1a·2′, the expected, highly or- four steps without loss of the enantiomeric excess
zation [1a·(OPh)2H2·2′] (Fig. 1C). If this held ganized 1a·(OPh)2H2·2′ complex could be (Fig. 2).
true, the structures of not only the chiral amino- involved in the 1a·(OPh)3H2-catalyzed system,
We believe these results encourage further
phosphonium cations but also the achiral phenols and the two phenols may play a key role in research efforts to determine the minimal chiral
would affect the stereo-determining step of the creating an attractive chiral environment around or achiral structural bias necessary to induce spon-
addition reaction of the enolate 2′. To verify this the azlactone enolate 2′.
taneous yet discrete associations between simple
hypothetical synergistic effect of the structural Treatment of iminophosphorane 1a′ with organic molecules through weak noncovalent in-
components of the supramolecularly assembled phenol (3 equiv), followed by the sequential teractions. The resultant catalytically active and
ion-pair catalyst, we examined the conjugate addition of 3a and 2, resulted in the formation selective structures may greatly expand the pos-
addition of 2 to a,b-unsaturated acylbenzotriazole of 4a in 98% yield with 62% ee after 10 hours sibilities for designing functional supramolecular
3 (13), a reactive and readily derivatizable Michael of stirring (entry 3). The reactivity and selec- catalysts.
acceptor, under the influence of 1·(OAr)3H2.
tivity were scarcely affected by the order of
References and Notes
The initial attempt entailed stirring a mixture the addition of phenol and azlactone 2 (entry
of 2, 3a, and 1a·(OPh)3H2 in toluene at −40°C, 4). These results suggest that the preorganiza-
giving rise to essentially diastereomerically pure tion of the catalyst is not a prerequisite for the
adduct 4a in a quantitative yield with 60% enan- generation of 1a·(OPh)2H2·2′, which could be
tiomeric excess (ee) (Table 1, entry 1). In con- self-assembled from 1a′, phenols, and 2 by way
trast, use of the independently prepared conjugate of either 1a·(OPh)3H2 or 1a·2′. These observations
base of 1a (triaminoiminophosphorane 1a′) (Fig. implied that the selectivity could be tunable
1C) (14) as a catalyst led to the formation of 4a through structural modification of the achiral
with 34% ee (entry 2). These results clearly phenolic component. Indeed, although the selec-
indicate the importance of the phenolic com- tivity decreased when phenols of 1a·(OPh)3H2
ponent of 1a·(OPh)3H2 in enhancing enantio- were replaced by 4-methylphenol, a series of
selectivity. Whereas the reaction with 1a′ proceeds chloro-substituted phenols, particularly 3,5-
1. P. W. N. M. van Leeuwen, Ed., Supramolecular Catalysis
(Wiley-VCH, Weinheim, Germany, 2008) and references
therein.
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material on Science Online.
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REPORTS
8. T. Tozawa, H. Nagao, Y. Yamane, T. Mukaiyama, Chem.
Asian J. 2, 123 (2007).
Concerto Catalysis” from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, the
Global Centers of Excellence Program in Chemistry of
Nagoya University, Grants of Japan Society for the
Promotion of Science for Scientific Research, and the
Kurata Memorial Hitachi Science and Technology
Foundation. We gratefully acknowledge S. Hiroto and
H. Shinokubo (Nagoya University) for kindly allowing us
access to high-resolution mass spectroscopy facilities.
Structural parameters for 1a·(OPh)3H2 are available free
of charge from the Cambridge Crystallographic Data
Centre under reference number CCDC 741902.
Supporting Online Material
www.sciencemag.org/cgi/content/full/1176758
Materials and Methods
SOM Text
Figs. S1 to S3
Tables S1 to S3
9. W. Saenger, Nature 279, 343 (1979).
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15. Financial support was provided by a Grant-in-Aid for
Scientific Research on Priority Areas “Chemistry of
References
26 May 2009; accepted 18 August 2009
Published online 27 August 2009;
10.1126/science.1176758
Include this information when citing this paper.
depleting substance on the basis of the extent of
ozone depletion it causes. Indeed, current anthro-
pogenic ODP-weighted N2O emissions are the
largest of all the ODSs and are projected to re-
main the largest for the rest of the 21st century.
We have calculated the ODP of N2O by using
the Garcia and Solomon two-dimensional (2D)
model [(11) and references therein], which is
similar to models used previously for such cal-
culations (12, 13). The ODP of N2O under cur-
rent atmospheric conditions is computed to be
0.017. This value is comparable to the ODPs of
many hydrochlorofluorocarbons (HCFCs) (3) such
as HCFC-123 (0.02), -124 (0.022), -225ca (0.025),
and -225cb (0.033) that are currently being
phased out under the MP. We conclude that
Nitrous Oxide (N2O): The Dominant
Ozone-Depleting Substance Emitted
in the 21st Century
A. R. Ravishankara,* John S. Daniel, Robert W. Portmann
By comparing the ozone depletion potential–weighted anthropogenic emissions of N2O with those
of other ozone-depleting substances, we show that N2O emission currently is the single most important
ozone-depleting emission and is expected to remain the largest throughout the 21st century.
N2O is unregulated by the Montreal Protocol. Limiting future N2O emissions would enhance the
recovery of the ozone layer from its depleted state and would also reduce the anthropogenic forcing
of the climate system, representing a win-win for both ozone and climate.
he depletion of the stratospheric ozone The primary source of stratospheric NOx is surface the value of the ODP of N2O is robust because
layer by human-made chemicals, referred N2O emissions [(7) and references therein]. N2O (i) our similarly calculated ODPs for CFC-12
to as ozone-depleting substances (ODSs), has been thought of as primarily a natural atmo- (1.03) and HCFC-22 (0.06) agree with the
T
was one of the major environmental issues of the spheric constituent, but the influence of its changes accepted values (3); (ii) ozone depletion by NOx
20th century. The Montreal Protocol on Sub- on long-term changes in ozone concentrations has from N2O dominates the chemical control of
stances That Deplete the Ozone Layer (1), MP, also been examined (8–10).
ozone in the mid-stratosphere (13), a region well
emerged from the Vienna Convention for the Pro- Nitrous oxide shares many similarities with represented with 2D models; and (iii) ozone
tection of the Ozone Layer (2). The MP has been the CFCs, historically the dominant ODSs. The reductions by enhanced N2O have been reported
highly successful in reducing the emissions, growth CFCs and N2O are very stable in the troposphere, in other studies (8, 10, 14), although no pub-
rates, and concentrations of chlorine- and bromine- where they are emitted, and are transported to lished study, to the best of our knowledge, has
containing halocarbons, the historically dominant the stratosphere where they release active chem- previously presented an ODP for N2O.
ODSs (3), and has limited ozone depletion and icals that destroy stratospheric ozone through
We examine here a few important factors that
initiated the recovery of the ozone layer. chlorine- or nitrogen oxide–catalyzed processes. influence the ODP of N2O. At mid-latitudes,
The relative contributions of various ODSs to They both have substantial anthropogenic sources. chlorine-catalyzed ozone destruction contributes
ozone layer depletion are often quantified by the Unlike CFCs, N2O also has natural sources, akin most to depletion in the lowest and upper strato-
ozone depletion potential (ODP) (4). An ODP re- to methyl bromide, which is another important spheres, that is, below and above the ozone max-
lates the amount of stratospheric ozone destroyed ODS. Assigning an ODP for N2O and separating imum. Nitrogen oxides contribute most to ozone
by the release of a unit mass of a chemical at out the natural and anthropogenic emissions are depletion just above where ozone concentrations
Earth’s surface to the amount destroyed by the therefore no more conceptually difficult than they are the largest. This leads to efficient ozone
release of a unit mass of chlorofluorocarbon 11, are for methyl bromide.
destruction from NOx (13). The ODP of N2O is
CFC-11 (CFCl3). ODPs are widely used for pol- In spite of these similarities between N2O lower than that of CFCs primarily because only
icy formulation because of their simplicity in quan- and previously recognized ODSs and in spite of ~10% of N2O is converted to NOx, whereas the
tifying the relative ozone-destroying capabilities the recognition of the impact of N2O on strato- CFCs potentially contribute all their chlorine.
of compounds.
spheric ozone, N2O has not been considered to
There are important interconnections be-
Through the work of Crutzen (5) and Johnston be an ODS in the same sense as chlorine- and tween the roles of nitrogen oxides with chlorine
(6), nitrogen oxides (NOx = NO + NO2) are also bromine-containing source gases. The signatories such that the N2O ODP may be different from
known to catalytically destroy ozone via
to the Vienna Convention (2) have agreed in Ar- the calculated value in the past and future. It is
ticle 2 (General Obligations) to “Adopt approp- well known that nitrogen oxides dampen the
riate legislative or administrative measures … to effect of chlorine-catalyzed ozone destruction
control, limit, reduce or prevent human activities via the formation of ClONO2, which ties up
under their jurisdiction or control should it be some of the chlorine in a benign form. However,
found that these activities have or are likely to as shown by Kinnison et al. (9), other reactions,
have adverse effects resulting from modification such as the conversion of ClO to Cl by NO, can
or likely modification of the ozone layer.” Yet offset the damping.
NO + O3 → NO2 + O2
O + NO2 → NO + O2
net: O + O3 → 2O2
Chemical Sciences Division, Earth System Research Labora-
tory, National Oceanic and Atmospheric Administration, 325
Broadway, Boulder, CO 80305, USA.
N2O remains unregulated by the MP (1).
We quantify the dependence of the ODP of
Here, we present the ODP of N2O to be pos- N2O on atmospheric concentrations of chorine
itive and nonzero and show that N2O is an ozone- by calculating it for 1959 concentrations of strato-
*To whom correspondence should be addressed. E-mail:
A.R.Ravishankara@noaa.gov
www.sciencemag.org SCIENCE VOL 326 2 OCTOBER 2009
123

Chiral Organic Ion Pair Catalysts Assembled Through a
Hydrogen-Bonding Network
Daisuke Uraguchi et al.
Science 326, 120 (2009);
DOI: 10.1126/science.1176758
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