Methods for the Purification of Cucurbituril
Priority
This application claims priority to GB 0922623.4, filed on 23 December 2009, the
contents of which are hereby incorporated by reference in their entirety.
Field of the Invention
This invention relates to methods for the purification of a cucurbituril from a mixture,
preferably a mixture further comprising other cucurbituril forms.
Background
The cucurbit[n]uril (CB[n]) family has been extensively studied in recent years on
account of their scope within the fields of molecular recognition and aqueous
self-assembly. [1] CB[7], a member of the CB[n] family, has a substantially higher
water solubility than CB[6] and CB[8] and a larger binding cavity than its water
soluble brother, CB[5].[2]
Recent reports have shown that CB[7] is an excellent host for binding and stabilising
a wide range of small molecules, such as fluorescent dyes,[3] controlling aggregate
formation of guest molecules,[4] and for use in light-harvesting systems.[5] However,
a major drawback which has limited the study and application of CB[7] is its difficult
isolation and purification from the synthesised mixture of CB[n] homologues.
Traditional isolation methods of CB[7] are based on the different solubilities of CB[n]
in various solvents, such as acetone/water and methanol/water mixtures, limiting the
overall yield of CB[7] to just under 3%.[6, 7] Nau ei al. [8] has reported a more direct
approach to preparing CB[7] by controlling the direct reaction of formaldehyde,
glycoluril and sulfuric acid, followed by a similar isolation methodology based on
solubility of CB[7] in an acetone/water mixture. This method gives a substantially
better yield with higher purity, however, the separation and purification is still based
primarily on solubility differences in a variety of mixed solvents. For the standard
separation routes, it is difficult to obtain CB[7] in high purity as both CB[5] and CB[7]
have low solubility in the mixed solvent systems (either acetone/H 20 or MeOH/H20),
requiring multiple solvent-based separation cycles.
Another isolation method which has been reported by Isaacs et al. uses column
chromatography with a harsh acidic eluent HC0 2H (88%) : HCI (0.2 M) (1:1 v/v).[9]
This can be a highly toxic process which results in acidic products and may limit the
viability of scale-up.
The use of alkyl imidazolium salts to form complexes with cucurbituril compounds
has been described [18], although this disclosure was made after the priority date of
the present application.
Herein is reported a new cucurbituril isolation method which is convenient for
obtaining pure cucurbituril material, is inherently more environmentally friendly and
suitable for large scale purification by exploiting the reversible association and
dissociation of a host-guest complex in a controlled manner.
Summary of the Invention
The present invention provides a method for purifying a cucurbituril from a mixture.
The method comprises the step of forming a complex between the cucurbituril and a
cucurbituril guest. The complex has altered physical and/or chemical properties to
the cucurbituril which allow the complex to be separated from at least one other
component of the mixture.
Thus, in a first aspect of the invention there is provided a method for the purifying a
cucurbituril from a mixture, the method comprising the steps of:
providing a mixture comprising the cucurbituril and at least one other
component;
introducing to the mixture a cucurbituril guest, thereby to form a guest-host
complex between the guest and the cucurbituril;
separating the guest-host complex from at least one component of the
mixture thereby to purify the cucurbituril from the other component.
The mixture comprises at least a cucurbituril, at least one other component and a
solvent. One or more cucurbituril guests may be added to the mixture thereby to
form a guest-host complex between the guest or guests and the cucurbituril. Thus,
the method comprises the steps of:
providing a mixture comprising the cucurbituril and at least one other
component in a solvent;
introducing to the mixture a cucurbituril guest or guests, thereby to form a
guest-host complex between the guest or guests and the cucurbituril;
separating the guest-host complex from at least one component of the
mixture, and optionally the solvent also, thereby to purify the cucurbituril from the
other component.
The mixture is preferably an aqueous mixture.
The cucurbituril is preferably CB[7]. The other component is preferably CB[5]. In an
alternative embodiment, the cucurbituril is CB[8] and the other component is CB[6],
or the cucurbituril is CB[6] and the other component is CB[8].
The cucurbituril guest is preferably an ionic liquid, and most preferably a compound,
including a salt, comprising an imidazolium moiety.
The present invention also provides a cucurbituril obtained by the method of the
invention.
Also provided by the present invention is a mixture comprising a complex of a
cucurbituril with an ionic liquid, and further comprising another cucurbituril.
Summary of the Figures
Figure 1 is a schematic representation of a method for the purification a cucurbituril
from a mixture according to an embodiment of the invention. The mixture comprises
CB[7] and CB[5], and the CB[7] guest is a 1-alkyl-3-methylimidazolium.
Figure 2 shows the stepwise procedure for the purification a cucurbituril from a
mixture according to an embodiment of the invention. At each step, the components
were fully isolated. The starting material (2 g) contains CB[7] (80%) and CB[5]
(12%), and produces pure CB[7] 1.41 g. The left solution is collected and further
purified CB[5] by step (5) (yield of step (5) is 46%). [C„mim]X is extracted from waste
solvent produced by step (3) (4) (5).
Figure 3 shows the H NMR spectrum (in 20% DCI, CB[7]'s peaks (star) are at
3.7 and 2.4 pp ) of the dissociation of CB[7]-[C 2mim]Br complex (top) and of
CB[7]-[C mim]Br complex (bottom) over time. The dots (peak at 1.9 ppm) represent
the protons of the methyl group on imidazolium and others are binding peaks of
imidazolium in CB[7]. The squares represent the protons of the methyl group of
MeOH.
Detailed Description of the Invention
The present inventors have identified a method whereby a guest molecule may be
used to generate a guest-host complex with a cucurbituril. The guest-host complex
may have altered physical and/or chemical properties compared to the uncomplexed
cucurbituril. This change in physical and/or chemical properties may be used to
purify the cucurbituril, as a guest-host complex, from a mixture that also comprises
another component. After purification, the guest-host complex may optionally be
decomplexed to obtain the cucurbituril.
The mixture comprises the cucurbituril, the other component and a solvent,
preferably water. Thus, the method of the invention particularly relates to the
separation of a cucurbituril from a non-solvent component. Nevertheless, the
methods described herein may also be used to separate the cucurbituril from the
other component and the solvent.
The methods of the invention encompass the use of two guest molecules to form a
guest-host complex between the guests and the cucurbituril. A reference in the text
to a cucurbituril guest may therefore be taken to be a reference to two guests also,
which are capable of forming a complex with a cucurbituril. The guests may be the
same or they may be different.
In a preferred embodiment, the solubility of the guest-host complex is different to that
of the cucurbituril.
The mixture may be an aqueous mixture.
In one embodiment, the mixture comprises another cucurbituril structure, and the
method relates to the separation of these structures.
A cucurbituril guest is used to generate a guest-host complex with the cucurbituril.
The cucurbituril guest is selective for the cucurbituril over the other component in the
mixture. For example, the cucurbituril guest is selective for the cucurbituril over
another cucurbituril present in the mixture.
Cucurbituril
In one embodiment, the cucurbituril is a CB[6], CB[7], CB[8], CB[10], or CB[12]
compound. Preferably the cucurbituril is a CB[6], CB[7], or CB[8] compound, most
preferably a CB[7] compound.
In one embodiment, the other component is another cucurbituril. The other
cucurbituril is preferably a compound having a different number of repeat glycoluril
units. In other embodiments, the cucurbituril and the other cucurbituril component
may be variants and/or derivatives of another. Thus, the present invention also
relates to the separation of a cucurbituril from another cucurbituril form.
CB[5] and/or CB[7] may be separated from CB[6] and/or CB[8] on the basis of their
solubility in water. CB[5] and CB[7] are soluble in water, CB[6] and CB[8] are not.
The methods described herein relate, in certain embodiments, to the separation of
CB[5] from CB[7], and CB[6] from CB[8]. The method of the invention may be used
in combination with those methods for the separation of CB[5] and/or CB[7] from
CB[6] and/or CB[8].
In one embodiment, the cucurbituril is a CB[7] compound, and the other component
is a CB[5] compound.
The separation of CB[7] from CB[5] is particularly difficult as the solubility of these
cucurbituril compounds in water is very similar (20-30 mM). Thus, the compounds
usually co-crystallise or drop out of solution together as a mixture. The present
inventors have therefore found a useful way of separating these particular cucurbituril
compounds on the basis of the selective formation of a complex of CB[7] with a guest
molecule. The guest molecule does not substantially interact with CB[5].
In one embodiment, the cucurbituril is a CB[6] compound, and the other component
is a CB[8] compound.
In one embodiment, the cucurbituril is a CB[8] compound, and the other component
is a CB[6] compound.
The present inventors have found a useful way of separating these particular
cucurbituril compounds on the basis of the selective formation of a complex of a
guest with either CB[6] or CB[8].
Cucurbit[7]uril (CB[7]) is a barrel shaped container molecule which has seven repeat
glycoluril units:
References to cucurbituril may also include a reference to a variant or derivative of a
cucurbituril.
For example, a variant of a cucurbiturii may also include a structure having one, two,
three, four or more repeat units that are structurally analogous to glycoluril. The
repeat unit may include an ethylurea unit. Where all the units are ethylurea units, the
variant is a hemicucurbituril. For example a variant of CB[12] is hemicucurbit [12]uril
(shown below).
In other aspects of the invention, cucurbiturii derivatives are provided and find use in
the methods described herein. A derivative of a cucurbiturii is a structure having one,
two, three, four or more substituted glycoluril units. A substituted cucurbiturii
compound may be represented by the structure below:
wherein:
n is an integer of at least 5 ;
X is O, S or NR3, and
-R and -R2 are each independently selected from -H and the following
optionally substituted groups: -R3, -OH, -OR3, -COOH, -COOR3, -NH2, -NHR3 and
-N(R )2 where -R3 is independently selected from C1-20alkyl, C 2ocarboaryl, and
C5-2oheteroaryl, or where -R and/or -R2 is -N(R3)2, both -R3 together form a C5-7
heterocyclic ring; or together -R1 and -R2 are C^alkylene forming a C .8carbocyclic
ring together with the uracil frame. It is preferred that -R and -R2 are not both -H.
Preferably, n is 5 , 6, 7, 8, 10 or 12.
Preferably, X is O.
Preferably -R3 is C -20alkyl, most preferably C . alkyl. The C1-20alkyl group may be
linear and/or saturated. Each group -R3 may be independently unsubstituted or
substituted. Preferred substituents are selected from: -R4, -OH, -OR4, -SH, -SR ,
-COOH, -COOR4, -NH2, -NHR4 and -N(R )2, wherein -R4 is selected from C1-2oalkyl,
C6-2ocarboaryl, and C -2oheteroaryl. The substituents may be independently selected
from -COOH and -COOR4.
In some embodiments, -R4 is not the same as -R3. In some embodiments, -R4 is
preferably unsubstituted.
Where -R1 and/or -R2 is -OR3, -NHR3 or -N(R3)2, then -R3 is preferably C - alkyl. In
some embodiments, -R3 is substituted with a substituent -OR4, -NHR4 or -N(R )2.
Each -R4 is Ci.6alkyl and is itself preferably substituted.
Cucurbituril Guest
As noted above, the guest is a compound that is capable of forming a guest-host
complex with a cucurbituril. The resulting complex has different physical and/or
chemical properties to the cucurbituril.
In a broadest aspect, therefore, the guest is any compound capable of providing a
complex having different physical and/or chemical properties to the cucurbituril.
The guest is a compound, which includes a salt, that is capable of forming a complex
with the cucurbituril. Where the other component is another cucurbituril, the guest
selectively forms a complex with the cucurbituril rather than the other cucurbituril.
In principal, any compound having a suitable binding affinity and selectively may be
used in the methods of the present invention. However, the most preferred
compounds are ionic liquids as discussed in detail below.
In selecting a compound suitable for use as a guest, consideration may be given to
the selectivity of that guest for one cucurbituril over another cucurbituril form. The
compound used may be selected based on the size of the moieties that are thought
to interact with the cavity of the cucurbituril. The size of these moieties may be
sufficiently large to permit complexation only with larger cucurbituril forms.
The guest and the cucurbituril typically form a 1:1 complex. Preferably the guest is a
compound (which includes salts) capable of forming a complex which has a binding
affinity (Ka) of at least 103 M 1 , of at least 04 , of at least 105 M , of at least 106
of at least 107 M , of at least 108 of at least 109 M , of at least 101 M of at
least 1011 M , or of at least 1012 M .
In one embodiment, the guest is a compound (which includes salts) capable of
forming a complex which has a binding affinity in the range 104 to 10 M .
The term selective may be used to refer to the amount of guest-host complex formed,
where the cucurbituril (the first cucurbituril) and a second cucurbituril are present in
the mixture. The guest-host complex formed between the first cucurbituril and the
guest the may be at least 60 mol %, at least 70 mol %, at least 80 ol %, at least 90
mol %, at least 95 mol %, at least 97 mol %, at least 98 mol %, or at least 99 mol %,
of the total amount of guest-host complex formed (for, example taking into account
the amount of guest-host complex formed between the second cucurbituril and the
guest, if any).
In one embodiment, the guest-host complex formed from the (first) cucurbituril and
the guest has a binding affinity that is at least 100 times greater than the binding
affinity of a guest host complex formed from the second cucurbituril and the guest.
Preferably, the binding affinity is at least 103, at least 104, at least 10s, at least 106, or
at least 07 greater.
The guest is preferably an ionic liquid.
The ionic liquid typically comprises a cationic organic nitrogen heterocycle, which
may be an aromatic nitrogen heterocycle (a heteroaryl) or a non aromatic nitrogen
heterocycle. The ionic liquid also typically comprises a counter-anion to the cationic
organic nitrogen heterocycle. The nitrogen heteroaryl group is preferably a nitrogen
C .ioheteroaryl group, most preferably a nitrogen C -sheteroaryl group, where the
subscript refers to the total number of atoms in the ring or rings, including carbon and
nitrogen atoms. The non aromatic nitrogen heterocycle is preferably a nitrogen
C5-6heterocycle, where the subscript refers to the total number of atoms in the ring or
rings, including carbon and nitrogen atoms. A nitrogen atom in the ring of the
nitrogen heterocycle is quaternised.
The counter-anion may be a halide, preferably a bromide. Other counter-anions
suitable for use are those that result in a complex that is soluble in water.
The guest is preferably a compound, including a salt, comprising one of the following
groups selected from the list consisting of: imidazolium moiety; pyridinium moiety;
quinolinium moiety; pyrimidinium moiety; pyrrolium moiety; and quaternary pyrrolidine
moiety.
Preferably, the guest comprises an imidazolium moiety. An especially preferred
guest is 1-alkyl-3-alkylimidazolium, where the alkyl groups are optionally substituted.
1-Alkyl-3-alkylimidazolium compounds, where the alkyl groups are unsubstituted, are
especially suitable for use in the separation of CB[7] from CB[5]. Here, the
alkylimidazolium compound forms a complex with CB[7]. The complex is preferably
water soluble. The complex may be converted to an insoluble form (for example, by
ion exchange) to allow separation from CB[5], which is soluble in water.
1-Alkyl-3-alkylimidazolium compounds, where the alkyl groups are unsubstituted, are
especially suitable for use in the separation of CB[6] from CB[8]. Here, the
alkylimidazolium compound forms a complex with CB[6]. The complex is preferably
water soluble, thereby to allow separation from CB[8], which is insoluble in water.
1-Alkyl-3-alkylimidazolium compounds, where an alkyl group is substituted with aryl
(preferably napthyl), are especially suitable for use in the separation of CB[8] from
CB[6]. Here, the alkylimidazolium compound forms a complex with CB[8]. The
complex is preferably water soluble, thereby to allow separation from CB[6], which is
insoluble in water.
The -alkyl and 3-alkyl substituents may the same or different. Preferably, they are
different.
In one embodiment, the 3-alkyl substituent is methyl, and is preferably unsubstituted.
In one embodiment, the 1-alkyl substituent is ethyl or butyl, and each is preferably
unsubstituted.
A preferred optional substituent is aryl, preferably C - aryl. Aryl includes carboaryl
and heteroaryl. Aryl groups include phenyl, napthyl and quinolinyl.
In one embodiment, the alkyl groups described herein are linear alkyl groups.
Each alkyl group is independently a C - alkyl group, preferably a C^alkyl group.
The aryl substituent may itself be another 1-alkyl-3-substituted-imidazolium moiety
(where the alkyl group is attached to the 3-position of the ring).
1-Alkyl-3-methylimidazolium salt ([C mim]X) ionic liquids have become popular in
variety of chemical research areas as "green" solvents.[10] Additionally, [C mim]X
species have received recent attention and have been studied by several groups as
good guests for both CB[6] and CB[7] and in some cases are significantly able to
increase the solubility of the CB host in neutral water.[1 1, 12, 13, 14]
CB[7]-[C mim]X complexes have a wide range of binding affinities (Ka) ranging from
104 to 10 M and whose water solubility is dependent upon both the alkyl chain
length and the counter ion. [13] As both the alkyl chain length and counter ion can
easily be altered from a synthetic point of view, they provide a handle for control and
an opportunity for use in the purification process.
In another embodiment, the compound preferably comprises a pyridinium moiety.
The guest-host complexes described herein are not limited to those complexes
having a ratio of guest to cucurbiturii of 1: . Complexes comprising two guest
compounds within a cucurbiturii host are also encompassed by the present methods.
Whether or not a complex formed comprises one or two guests depends on the
nature of the cucurbiturii and the nature the guest. For such complexes it is not
essential that each of the guests in the complex be the same, but it may be preferred
that they are.
Where the complex comprises two guests within the cucurbiturii cavity, the
association constant, Ka, for that complex is at least 106 M2, of at least 10 M'2, of at
least 108 M 2, of at least 109 M2, of at least 01 M2, of at least 1011 I 2, or of at least
1012 M2.
For CB[6] compounds it has been suggested that short chain 1-alkyl-3-
methylimidazolium compounds form 2:1 complexes, whilst longer chain 1-alkyl-3-
methylimidazolium compounds form 1:1 complexes.
Where a complex is formed having two guest molecules within a cucurbiturii host, it is
preferred that the guest molecules are the same. However, the present invention
also encompasses methods where the guest molecules are different. Complexes
having different guests may be used to finely tune the solubility characteristics of the
overall complex. The guest molecules may be chosen on the basis of a preferred
formation of a complex with one cucurbiturii host over another cucurbiturii compound.
Where two different guest molecules are used, such molecules are preferably
selected from the guest molecules described herein.
For example, the guest molecules may be a charge transfer pair comprising electron
rich and electron-deficient compounds. Examples of ternary guest-host complexes
formed from different guests are described in the applicant's earlier filing,
WO 2009/071899. An example complex is that of a CB[8] molecule hosting a
compound comprising a 2,6-dihydroxynapthalene moiety and a compound
comprising a N,N'-dimethyldipyridyliumylethylene diiodide.
In one embodiment, where the complex formed comprises two guests, each guest is
an ionic liquid. These ionic liquids may be the same or different. Each ionic liquid
may be selected from the ionic liquids described herein.
The present invention also encompasses methods whereby a single guest molecule
interacts with two identical cucurbituril compounds, thereby to form a guest-host
complex comprising two hosts. Such complexes may form where, for example, the
guest comprises two moieties that are capable of each independently interacting with
a cucurbituril compound.
Where the guest is an ionic liquid, the counter-anion may be used to alter the
solubility of the overall complex in a solvent, and preferably in water. Thus, in
certain embodiments a halide counter-anion, preferably a bromide counter-anion,
may be used to generate a complex that is water soluble. In other embodiments, a
PF counter-anion may be used to generate a complex that is water insoluble. In a
preferred method described herein, a water soluble complex having a halide counter
ion is generated, and the halide counter-anion is exchanged for a PF6 counter-anion,
thereby to generate a water insoluble complex that is separable from the other
component of the mixture, for example by filtration. In this way both the complex and
the other component may be separately collected.
The guest may be used to solubilise a cucurbituril that is insoluble in a solvent, for
example water. The guest may be used to form a complex with the cucurbituril that
is soluble in that solvent. A mixture comprising a cucurbituril that is insoluble in water
and another component that is insoluble in water may be purified by selectively
forming a water soluble complex of the cucurbituril and a guest. The water soluble
complex may be separated from the water insoluble other component, for example
by filtration. In this way both the complex and the other component may be
separately collected.
In both cases above, the recovered complex may be decomplexed to obtain the
cucurbituril and the guest, which may be suitable for use in a further purification step.
Cucurbituril may find use in cosmetic products or domestic cleaning products and the
like. Therefore it is preferred that the guest compounds used in the present methods
are not toxic.
Preferred Guest Compounds
Preferred guests for use in the present invention are of the form H+X , where H+ is
one of the following cations,
and X is a suitable counter-anion, as defined above. A preferred counter
anion is a halide anion, preferably Br".
In a preferred embodiment, cation A or cation B may be used to form a complex with
CB[7] or CB[6].
In a preferred embodiment, cation D or cation E may be used to form a complex with
CB[8].
Cations A and B may be referred to as 1-ethyl-3-methylimidazolium and 1-butyl-3-
methylimidazolium respectively.
Cations D and E may be referred to as 1-naphthalenylmethyl-3-methylimidazolium,
where D is 1-naphthalen-2-ylmethyl-3-methylimidazolium and E is 1-naphthalen-1-
ylmethyl-3-methylimidazolium.
Additional Guest Compounds
Alternatively or additionally, the guest compounds may be an imidazolium salt of
formula (I):
wherein X is a counter anion;
R1 is independently selected from H and saturated Ci-6 alkyl;
R2 is independently C 0 alkyl which may optionally contain one or more
double or triple bonds, and may be optionally interrupted by a heteroatom selected
from -0-, -S-, -NH-, and -B-, and may be optionally substituted.
In one embodiment, X is independently selected from the group consisting of CI , Br ,
I , BF , PF6 , OH , SH-, HS0 4 , HC0 3 , NTf2, C2N50 , AICI , Fe3CI 2 , N0 , N eS2
' ,
MeS0 3 , SbF ' , PrCBuHn , AuCI4 , HF2 , N0 2 , Ag(CN)2
", and NiCI . In one
embodiment, X is selected from CI , Br , and I .
In one embodiment, R1 is selected from H and linear saturated C alkyl.
In one embodiment, R2 is linear C -10 alkyl, which may optionally contain one or more
double bonds, and may be optionally interrupted by a heteroatom selected from -0-,
-S-, -NH-, and -B-, and may be optionally substituted.
In one embodiment, R2 is linear .ioalkyl, which may optionally contain one or more
double bonds, and may be optionally substituted.
In one embodiment, where a double or triple bond is present, it may be conjugated to
the imidazolium moiety. Alternatively, the double or triple bond may not be
conjugated to the imidazolium moiety.
In one embodiment, the optional substituents are independently selected from the
group consisting of halo, optionally substituted C5- 0 aryl, -OR3, -OCOR3, =0, -SR3,
=S, -BR3, -NR3R4, -NR COR3, -N(R )CONR R4, -COOR3, -C(0)R 3, -C(=0)SR 3,
-CONR3R4, -C(S)R3, -C(=S)SR3, and -C(=S)NR R4,
where each of R3 and R4 is independently selected from H and optionally
substituted saturated C -6 alkyl, C5-20 aryl and C -6 alkylene-C5-20 aryl.
or R3 and R4 may together may form an optionally saturated 5-, 6- or
7-membered heterocyclic ring which is optionally substituted with a group -R3.
In one embodiment, the optional substituents are independently selected from the
group consisting of halo, optionally substituted C5-2 aryl, -OR3, -OCOR3, -NR3R4,
-NR COR 3, -N(R 3)CONR R4 , -COOR 3, -C (0)R3, and -CONR R4 , where R 3 and R4 are
defined as above.
Each C5-2o aryl group may be independently selected from a C6- o carboaryl group or
a C5.20 heteroaryl group.
Examples of C .2o carboaryl groups include phenyl and napthyl.
Examples of C .2o heteroaryl groups include pyrrole (azole) (C5) , pyridine (azine) (C6) ,
furan (oxole) (C ) , thiophene (thiole) (C5) , oxazole (Cs) , thiazole (C ) , imidazole
(1,3-diazole) (C5) , pyrazole ( 1 ,2-diazole) (C5) , pyridazine (1,2-diazine) (C ) , and
pyrimidine ( 1,3-diazine) (C ) (e.g., cytosine, thymine, uracil).
Each Cs-20 aryl is preferably selected from optionally substituted phenyl, napthyl and
imidazolium.
Each C5- o aryl group is optionally substituted. The optional substituents are
independently selected from halo, C 1-6 alkyl, -OR 3, -OCOR 3 , -NR R4 , -NR COR 3 ,
-N(R )CONR R 4 , -COOR 3, - C (0)R3, and -CONR 3R 4 , where R 3 and R 4 are defined as
above.
In one embodiment, each C5-20 aryl group is optionally substituted with C -6 alkyl.
Where the C .2o aryl group is an imidazolium, such is preferably substituted at
nitrogen with a group R (thereby forming a quaternary nitrogen).
The compound of formula (I) comprises an imidazolium moiety having a substituent
R2 at the 1-position and a substituent R at the 3-position. In a further aspect of the
invention, the compound of formula (I) may be optionally further substituted at the 2-,
4- or 5-positon with a group R , wherein R has the same meaning as R .
The embodiments above are combinable in any combination, as appropriate.
Complex
In a preferred embodiment, the complex formed from the guest and the cucurbituril is
soluble in water.
The complex may be made insoluble in water to aid separation from the other
component in the mixture. For example, where the guest compound is a salt, such
as an ionic liquid, the complex formed is also a salt. The counter ion of the guest
compound in the complex may be exchanged for another counter ion. That counter
ion may cause a change in the solubility of the overall complex. Preferably, the ion
exchange causes a change in the water solubility of the complex. In one
embodiment, the ion exchange results in a complex having lower water solubility.
The complex may be converted to a substantially insoluble form. In another
embodiment, the ion exchange results in a complex having higher water solubility.
The timescale for complex formation may be at least 6 hours, at least 12 hours, at
least 24 hours, at least 48 hours, or at least 72 hours.
The decomplexation of the complex to separate guest and cucurbituril components is
described in more detail in the Separation section below.
The formation of the complex may be monitored using those techniques familiar to
one skilled in the art. For example, NMR (including 1H and 2D NMR techniques),
UV/vis, fluorescence, IR, calometric titration and specific viscosity measurements
may be used to monitor the formation of a guest-host complex in the mixture, and
may be used to indentify the complex after isolation. Such techniques may also be
used to determine the purity of the complex after separation.
Separation
The guest-host complex is purified from the other component of the mixture. This
may be referred to as the separation of the cucurbituril, or the complex, from the
other component.
The methods described herein relate to the purification of a cucurbituril from a
mixture. The cucurbituril may be purified as a guest-host complex, or may be
obtained as the cucurbituril after decomplexation.
The methods of purification refer to the separation of the cucurbituril from at least one
other component of the mixture. The separated cucurbituril, which may be in a
complex, is substantially free of the other component.
In one embodiment, the separated cucurbituril is at least 50% by weight, e.g., at least
60% by weight, e.g., at least 70% by weight, e.g., at least 80% by weight, e.g., at
least 90% by weight, e.g., at least 95% by weight, e.g., at least 97% by weight, e.g.,
at least 98% by weight, e.g., at least 99% by weight.
In one embodiment, the other component represents no more than 50% by weight,
e.g., no more than 40% by weight, e.g., no more than 30% by weight, e.g., no more
than 20% by weight, e.g., no more than 10% by weight, e.g., no more than 5% by
weight, e.g., no more than 3% by weight, e.g., no more than 2% by weight, e.g., no
more than 1% by weight.
The method comprises a separation step whereby the guest-host complex is
separated from the other component of the mixture. The separation of the complex
from the other component may be achieved using those purification techniques
familiar to those of skill in the art. The separation may optionally involve the
additional separation of the complex from the mixture solvent.
In one embodiment, the complex formed from the guest and the cucurbituril is soluble
in water. In the purification step, the complex may be made insoluble for separation
from the other component, which is soluble in water.
In a preferred embodiment, the purification comprises the step separating a complex
from the other component by one or more of filtration, chromatography, separation,
and crystallisation.
The purification step may also comprise one or more ion exchange step. The ion
exchange may be before or after any filtration, chromatography, separation, or
crystallisation step.
As is apparent from the detailed description of the invention above, the method
described herein may also be used to purify the other component from the mixture.
The present method is of particular advantage in purifying mixtures where the
cucurbituril and the other component (which may be another cucurbituril) have similar
physical and/or chemical properties thereby preventing or complicating the isolation
of one from the other. The formation of a complex of the cucurbituril with a guest
alters the physical and/or chemical properties of the cucurbituril, which allow the
cucurbituril (in a complex) to be separated from the other component. It is apparent
that this allows the other component to be purified from the cucurbituril. Thus, the
methods as described herein as relating to methods for the purification of a
cucurbituril from a mixture (the mixture further comprising another component), may
also be read as referring to methods for the purification of a component from a
mixture (the mixture further comprising a cucurbituril).
In a preferred embodiment, the complex is decomplexed by way of a solid state
metathesis (SSM) reaction. The SSM reaction may be performed in a solvent in
which the complex is insoluble. The SSM reaction may be performed in a solvent in
which the at least on of the guest or the cucurbituril soluble.
Other decomplexation methods may be used, such as soxhlate extraction and reflux
with various solvents, such as MeOH, DCM, and diethyl ether. However, these
techniques may not be suitable for all cucurbituril complexes, including for example
the preferred CB[7]-[C mim] complexes described herein.
The timescale for decomplexation may be at least 6 hours, at least 12 hours, at least
24 hours, at least 48 hours (2 days), at least 72 hours (3 days), or at least 168 hours
( 1 week).
The purification methods described herein permit recycling of the guest for use in
further cycles of purification. In the decomplexation step, the guest may be isolated
and retained for further use. In this way, the present method is particularly
advantageous as it requires only a limited amount of guest material.
In a preferred embodiment, if required, the isolated guest is converted to a form
suitable for use in a further purification step.
In the preferred embodiments described herein, the guest may be obtained in a salt
form that, if used in a purification method of the invention, would not yield a soluble
complex. The salt from may therefore be converted to a form that would yield a
soluble complex, when used. For example, the counter-anion may be exchanged for
a counter-anion that would yield a soluble complex. In a preferred embodiment, an
isolated guest having a PF counter-anion is converted to a guest having a Br
counter-anion.
Preferred Method
In a preferred embodiment there is provided a method of purifying CB[7] from a
mixture, the method comprising the steps of:
providing a mixture comprising CB[7] and CB[5];
introducing to the mixture a CB[7] guest, thereby to form a guest-host
complex between the guest and the CB[7], wherein the CB[7] guest is an ionic liquid;
separating the guest-host complex from CB[5] thereby to purify the CB[7]
from the mixture.
The ionic liquid may be a 1-alkyl-3-alkylimidazolium salt. The salt is preferably a
bromide salt.
The complex is preferably soluble in water. The separation may comprise the step of
converting the water soluble complex to a water insoluble complex. This step may
include ion exchange. Where the 1-alkyl-3-alkylimidazolium salt is a bromide salt,
the corresponding complex may be made insoluble by replacement of the bromide
anion with a PF anion. The insoluble complex may be separated from the soluble
CB[5], for example by filtration. CB[5] may be recovered from the aqueous solution.
It may be further purified, for example by recrystalisation.
The complex may be decomplexed to yield CB[7] and the guest. In a preferred
embodiment, the complex is decomplexed by way of a solid state metathesis
reaction.
The complex may be first resolubilised in water, for example by ionic exchange, then
precipitated in a polar organic solvent. The precipitated complex may then be
allowed to undergo a solid sate metathesis reaction to yield CB[7] and guest.
In another preferred embodiment there is provided a method of purifying CB[6] from
a mixture further comprising CB[8], the method comprising the steps of:
providing a mixture comprising CB[6] and CB[8];
introducing to the mixture a guest for CB[6] or CB[8], thereby to form a guesthost
complex between the guest and the CB[6] or CB[8], wherein the CB[6] or CB[8]
guest is an ionic liquid;
separating the guest-host complex thereby to purify the CB[6] or CB[8] from
the mixture.
The preferred methods relate to the use of an ionic liquid, although in principle any
compound that is capable of forming a complex selectively with one cucurbituril form
over another may be suitable for use in these methods.
The mixtures described above include a solvent as well as the cucurbituril
compounds described. The solvent is preferably water.
In each of the methods above, a reference to the formation of a guest-host complex
is also a reference to complex comprising two guest molecules that may be the same
of different. Where appropriate, a reference to a guest may be construed as a
reference to two different guest molecules.
Other Preferences
Each and every compatible combination of the embodiments described above is
explicitly disclosed herein, as if each and every combination was individually and
explicitly recited.
Various further aspects and embodiments of the present invention will be apparent to
those skilled in the art in view of the present disclosure.
"and/or" where used herein is to be taken as specific disclosure of each of the two
specified features or components with or without the other. For example "A and/or B"
is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if
each is set out individually herein.
Unless context dictates otherwise, the descriptions and definitions of the features set
out above are not limited to any particular aspect or embodiment of the invention and
apply equally to all aspects and embodiments which are described.
Certain aspects and embodiments of the invention will now be illustrated by way of
example and with reference to the figures described above.
Experimental
The separation of a mixture of CB[5] and CB[7] is described below, with particular
reference to Figures 1 and 2 . For reference, [C mim] refers to 1-alkyl-3-
methylimidazolium, where n is the number of carbon atoms in the 1-alkyl group.
The starting material (2 g) contained CB[7] (80%) and CB[5] (12%).
[Cnmim]Br can be employed to increase the solubility of CB[7] in water as shown in
step 1 of Figure 2. The CB[7]-[Cnmim] complex formed can be isolated from CB[5] by
adding NH PF salt into the system resulting in a CB[7]-[Cnmim]PF6 complex which
has poor solubility and precipitates from water. The CB[5] remains in solution: its
cavity is too small to bind the [Cnmim] guest, and addition of the PF6 salt does not
reduce CB[5]'s solubility.
In fact, addition of NH PF6 to the system is believed to produce NH Br which forms
CB[5]-NH4
+ complexes through portal binding. This may actually enhance the
solubility of CB[5] in water. Pure CB[5] can be isolated by recrystallisation from the
filtered solution.
Once CB[5] has been removed from the mixture, a final key step in the purification
method for CB[7] is the dissociation of the CB[7]-[C mim] complex. As CB[7] has a
reasonable solubility in water, and [Cnmim]PF6 is soluble in a wide variety of
common organic solvents, it was reasoned that decomplexation might be afforded by
either precipitation or successive extraction. Attempts to decomplex the system by
exhaustive soxhlate extraction and reflux with various solvents, such as eOH,
DCM, and diethyl ether all failed and none of these techniques liberated the ionic
liquid guest from the CB[7] cavity.
As a solution-based method proved difficult to achieve, a dissociation based on a
solid-state ion exchange was therefore attempted.
After successful removal of CB[5], CB[7]-[Cnmim]PF is converted back to the
bromide counter ion in step 3 (Figure 2), followed by dissociation of the
CB[7]-[Cnmim]Br complex through a solid state metathesis (SSM) reaction.[15, 16,
17] The reaction used NH PF in refluxing DCM. The SSM reaction was shown to
be useful in this particular case as both starting materials were not soluble in DCM.
However, as the reaction proceeded, [C mim]PF6 formed and immediately partitioned
into DCM leaving NH4Br and CB[7] in the solid state. The selective removal of
[C mim]PF from the reaction state provided the required driving force for quantitative
decomplexation. The reaction was followed by 1H NMR and was stopped when all
the complex was observed to have dissociated. The resulting CB[7] solid was then
filtered and washed with MeOH. The free [Cnmim]PF6 may be extracted from the
DCM solution for later re-use.
The timescale of dissociation of the CB[7]-[Cnmim]Br complex through the SSM
reaction depends highly on the binding constant of the complex. As shown in
Figure 3, the imidazolium (which is indicated by the peak of the methyl group at
2 ppm) has been removed from the complex in both systems (top spectra in both
stack plots). The binding constant of CB[7]-[C mim]Br (Ka > 107 M" ) is much greater
than that of CB[7]-[C2mim]Br (Ka > 105 M ),[1 3] and therefore required a longer
dissociation time. With [C2mim]Br the decomplexation procedure required only two
days, however, using [ mim Br required an entire week under the same reaction
conditions.
Solvent choice is an important component in designing this separation route via an
SSM reaction. When MeOH was used instead of DCM, no reaction took place as
NH PF is also soluble in MeOH, thereby preventing any ion exchange in the solid
state. Whilst DCM does not solubilise any of the starting materials, it solubilises one
of the products, which in turn drives the equilibrium to favour the dissociation of the
complex:
+ NH4PF6 (solid)
CB[n]-tCnmim]Br CB[n] + Cn im]PF6 + N Br
(solid) reflux in DCM (solid) (solution) (solid)
In summary, both CB[7] and CB[5] can be easily isolated in very good yields: CB[7] >
70%, and in excellent purity in comparison with previously published separationbased
routes. Moreover, the "waste" material from each step can be recycled, thus,
leading to a near quantitative recovery loop. The imidazolium ionic liquids were
isolated from the solution waste through a straight forward extraction process and
were recycled for subsequent use.
[Cnmim]Brwas synthesised from methyl imidazole and the appropriate 1-bromo alkyl.
The starting CB[5]/[7] mixture used in these experiments was obtained from a
standard CB[n] synthesis.[6, 7] The overall yield of CB[7] and CB[5] is 19.9% and
6.4%, respectively, using the procedure disclosed in this paper, produces both
greater isolated yield and higher purity of both CB[7] and CB[5] than in previously
published routes for all preparing all CB[n] species in a single reaction.
The waste from each step has also been carefully controlled. The non-recycled salt
(NH Br) was filtered from the solution after the ion exchange process. The waste
solvents (DCM and methanol) were collected from the isolation process and 90% of
both of these solvents could be recycled for further use in this procedure after
distillation. Finally, 80% of the [Cnmim]PF was isolated from the solvent and was
reused after a simple ion exchange form PF back to the original Br counterion.
References
All documents mentioned in this specification are incorporated herein by reference in
their entirety.
[1] J. Lagona, P. Mukhopadhyay, S. Chakrabarti and L. Isaacs, Angew. Chem. Int.
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2003, 36, 621-630.
[3] A. Hennig, H. Bakirci and W. Nau, Nat. Methods, 2007, 4, 629-632.
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Claims
1. A method for the purifying a cucurbituril from a mixture, the method
comprising the steps of:
providing a mixture comprising the cucurbituril and at least one other
component in a solvent;
introducing to the mixture a cucurbituril guest or guests, thereby to form a
guest-host complex between the guest or guests and the cucurbituril;
separating the guest-host complex from at least one component of the
mixture thereby to purify the cucurbituril from the other component.
2. The method of claim , wherein the cucurbituril is cucurbit[7]uril.
3. The method of claim 1 or claim 2, wherein the other component is
cucurbit[5]uril.
4. The method of claim 1, wherein the cucurbituril is cucurbit[6]uril or
cucurbit[8]uril.
5. The method of claim 4, wherein the other component is cucurbit[8]uril, where
the cucurbituril is cucurbit[6]uril, or the other component is cucurbit[6]uril, where the
cucurbituril is cucurbit[8]uril.
6. The method of any one of the preceding claims wherein a cucurbituril guest is
introduced, and the cucurbituril guest is an ionic liquid.
7. The method of claim 6, wherein the separating step includes the step of ion
exchange.
8 . The method of claim 6 or claim 7, wherein the ionic liquid is a compound
comprising one of the following groups selected from the list consisting of:
imidazolium moiety;
pyridinium moiety;
quinolinium moiety;
pyrimidinium moiety;
pyrrolium moiety; and
quaternary pyrrolidine moiety.
9. The method of claim 8, wherein the ionic liquid is a compound comprising an
imidazolium moiety.
10. The method of claim 9, wherein the compound is a l-alkyl-3-alkylimidazolium
salt, wherein each alkyl group is independently optionally substituted.
1. The method of claim 9, wherein the compound is a 1-alkyl-3-
methylllimidazolium salt, wherein the 1-alkyl group is independently optionally
substituted.
12. The method of claim 9 , wherein the compound is a 1-ethyl-3-
methylimidazolium salt, a 1-butyl-3-methylimidazolium salt, or a
1-naphthalenylmethyl-3-methylimidazolium salt.
13. The method according to any one of the preceding claims wherein the mixture
is an aqueous mixture.
14. The method according to any one of the preceding claims further comprising
the step of decomplexing the separated guest-host complex, and purifying the
cucurbituril from the guest.
15. A mixture comprising a complex of a cucurbituril with an ionic liquid, and
further comprising another cucurbituril.