A chiral spiroborate anion from diphenyl-l-tartramide [B{l-Tar(NHPh)2}2]- applied to some challenging resolutions

Article abstract of DOI:10.1039/c8ce00855h

The chiral spiroborate anion [B{l-Tar(NHPh)₂}₂]^- has been prepared as simple salts K, NH₄, Na in high yield and purity. Its structural features have been examined by single crystal XRD and DFT calculations and indicate that conformational arrangements are dominated by intramolecular NH - -OC inter-amide hydrogen bonds. These confer predictable shape, as well as a clear binding site for ion-pair formation. The potential of this anion for resolution by diastereomeric salt formation was then tested using five challenging racemic amines of type NH₂CHR₁R₂ with high shape similarity between their enantiomers. Organo-ammonium salts from these were made directly from a simple 1-pot reaction from racemic amine, boric acid and 2 eq. N,N′-diphenyl-l-tartramide in MeOH. The products are single phase crystalline solids with moderate to excellent enantioexcesses up to 95% ee. They show conserved NH₃R⁺ binding and layered packing arrangements, all with short 5.5 ? axes. Based on chiral HPLC the initial [B{l-Tar(NHPh)₂}₂] salt from rac-phenylglycinol has [S-NH₃CH(CH₂OH)Ph]⁺ with 95% ee and the salt from rac-1-phenylpropylamine is also well resolved (>91% ee) in a single step. Three disorder modes that limit resolution in the other salts were identified at the cation site-H/R₁ site exchange, R₁/R₂ site exchange or C-H re-pyramidalization. Extension to a family of such aryltartramide anions may allow crystal engineering of the cation binding pockets to overcome the disorder inherent to such resolutions.

Full text of DOI:10.1039/c8ce00855h

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A chiral spiroborate anion from diphenyl-L-
tartramide [B{L-TarIJNHPh)2}2]− applied to some
challenging resolutions†
Cite this: DOI: 10.1039/c8ce00855h
Lawrence W.-Y. Wong,
Gemma S.-S. Tam, Xiaoyan Chen,
Frederick T.-K. So,
Aristyo Soecipto,
Zhenyang Lin
Fu Kit Sheong, Herman H.-Y. Sung,
*
and Ian D. Williams
The chiral spiroborate anion [B{L-TarIJNHPh)2}2]− has been prepared as simple salts K, NH4, Na in high yield
and purity. Its structural features have been examined by single crystal XRD and DFT calculations and indi-
cate that conformational arrangements are dominated by intramolecular NH---OC inter-amide hydro-
gen bonds. These confer predictable shape, as well as a clear binding site for ion-pair formation. The
potential of this anion for resolution by diastereomeric salt formation was then tested using five challenging
racemic amines of type NH2CHR1R2 with high shape similarity between their enantiomers. Organo-
ammonium salts from these were made directly from a simple 1-pot reaction from racemic amine, boric
acid and 2 eq. N,N′-diphenyl-L-tartramide in MeOH. The products are single phase crystalline solids with
moderate to excellent enantioexcesses up to 95% ee. They show conserved NH3R+ binding and layered
packing arrangements, all with short 5.5 Å axes. Based on chiral HPLC the initial [B{L-TarIJNHPh)2}2] salt
from rac-phenylglycinol has [S-NH3CHIJCH2OH)Ph]+ with 95% ee and the salt from rac-1-
phenylpropylamine is also well resolved (>91% ee) in a single step. Three disorder modes that limit resolu-
tion in the other salts were identified at the cation site – H/R1 site exchange, R1/R2 site exchange or C–H
re-pyramidalization. Extension to a family of such aryltartramide anions may allow crystal engineering of
the cation binding pockets to overcome the disorder inherent to such resolutions.
Received 23rd May 2018,
Accepted 12th July 2018
DOI: 10.1039/c8ce00855h
rsc.li/crystengcomm
Several factors can affect the effectiveness of resolving
ions, shape, size, functionality, degree of chirality. They
Introduction
The separation of enantiomers from a racemate via selective
diastereomeric crystallization1,2 is a classical optical resolu-
tion method of considerable value to pharmaceutical and
chemical industry3 that can benefit from the application of
modern crystal engineering approaches.4 In the case of salts a
pure enantiomer of one ion is introduced to a racemic mix-
ture of counterions with the idea that two diastereomeric salts
may be formed with differing physical properties and solubil-
ity. Ideally the less soluble salt will be selectively precipitated,
removing one hand of the racemic ion to the solid whilst leav-
ing the other in solution. By judicious selection of resolving
ion, solvent and other conditions highly efficient resolutions
can be achieved by this approach.4
should form crystalline solids with a wide range of counter-
ions and have low tendency for disorder. From a practical
viewpoint ideally they should also be inexpensive, non-toxic,
unreactive and readily available in both hands. Recently we
described the use of the chiral spiroborate anions
bisIJmandelato)borate [BIJR-Man)2]− and [BIJS-Man)2]− as prom-
ising and versatile resolving agents that meet the above
criteria.5 Resolutions could be carried out by one-pot proce-
dure, or they can be readily prepared and isolated as simple
salts, such as NaijBIJR-Man)2], that could then be used in a me-
tathesis crystallization.6 Examples included resolution of the
alkaloid base tetrahydropalmatine as its monoprotonated cat-
ion (THP-H)+, the simple diamine 1,2-diaminopropane as its
di-ammonium salt [1,2-dap-H2]2+ and the metal coordination
complex tris-phenanthroline cobaltIII [CoIJphen)3]3+.5
Department of Chemistry, Hong Kong University of Science and Technology, Clear
Water Bay, Kowloon, Hong Kong, China. E-mail: chwill@ust.hk;
Fax: +852 2358 1594; Tel: +852 2358 7384
A single anion such as [BIJMan)2]− cannot guarantee
success in a specific diastereomeric salt system, so a larger
collection of resolving agents7 is needed for improving the
† Electronic supplementary information (ESI) available: Single crystal structure
determination summaries, powder X-Ray Diffractograms, chiral HPLC chromato-
grams and other spectra. CCDC 145865–145868 and 1844600–1844603. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8ce00855h
chances of
a
successful, efficient resolution. Spiro-
compounds are privileged chiral architectures,8 so we sought
to build on our earlier [BIJMan)2]− results and develop a
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wider family of spiroborate anions. Whilst both enantiomers Results and discussion
of mandelic acid itself are inexpensive and readily available
Simple spiroborate salts from [L-TarIJNHPh)2-H2]
in bulk quantity, most of its derivatives are not. To form a
family of spiroborate anions we sought a set of chiral chelat-
ing oxyacid-ligands that could be readily prepared. They
should allow incorporation of functional groups to enable
crystal engineering of diastereomeric salts at the molecular
level.
Tartramides are a class of chiral diols that appear ideally
suited to this need.9 They can be readily prepared by a single
step from cheap L-tartaric acid (or the unnatural D-form
which is also inexpensive) with a wide range of amines and
their bis-chelation to boron (Fig. 1) should form a family of
chiral spiroborate anions.
Sharpless' asymmetric epoxidation method was based on
the use of titanium tartrate-ester and tartramide catalysts.10
Toda et al. also successfully applied chiral tartramides as
hosts for resolution by inclusion complexation.9 Furthermore
coordination of tartrate by antimonyIII affords dimeric anions
with good resolving power,11 so it is reasonable that
bisIJtartramido)borate anions [B{TarIJNR2)2}2]− might also have
promise as resolving agents.
In this paper we begin to explore this possibility through
synthesis of a prototype chiral anion [B{L-TarIJNHPh)2}2]− de-
rived from N,N′-diphenyl-L-tartramide, conveniently synthe-
sized from condensation of aniline with L-tartaric acid. First
the preparation and structure of simple salts of [B{L-
TarIJNHPh)2}2]− is described. (Fig. 1) The spiroborate anion is
then deployed in the attempted resolution of five racemic
amines of the type NH2CHR1R2, sec-butylamine (R1 = Me, R2 =
Et), 2-pentylamine (R1 = Me, R2 = Et), 2-phenylethylamine
(R1 = Me, R2 = Ph), 2-phenylpropylamine (R1 = Et, R2 = Ph)
and phenylglycinol (R1 = CH2OH, R2 = Ph). These systems
provide a challenge to resolution by the diastereomeric salt
method, since their enantiomers have considerable shape
similarity.12 This means they may form diastereomeric mixed
crystals13 or solid solutions14 – single phase products with
disorder of the enantiomers at the cation sites. These may
necessitate multistage recrystallizations to achieve acceptable
enantiopurity.15 The success and limitation of the
diphenyltartramide spiroborate anion to such resolutions
and the modes of disorder found in its diastereomeric chiral
ammonium salts will be discussed.
The chiral diol N,N′-diphenyl-L-tartramide
1
(Fig. 1) [L-
TarIJNHPh)2-H2] was chosen as a starting ligand, since it was
readily prepared from L-tartaric acid and aniline and was pre-
viously used as a chiral host for guest-host complex forma-
tion by Toda et al.,9 to achieve the resolution of axially chiral
bis-phenols such as BINOL.16 Our initial goal was to show
that chiral spiroborate anions could be readily formed from 1
and crystallize them as salts to explore their structural chem-
istry and potential for resolution.
Ideally the [B{L-TarIJNHPh)2}2] anion could then serve as a
prototype for an extensive family of spiroborates, since a wide
range of tartramide diols are accessible by varying the aniline
or amine used in the condensation with tartaric acid.
As with bisIJmandelato)borate [BIJMan)2]−,5,6 our approach
was to prepare the spiroborate first as an alkali metal salt by
direct 1-pot reaction of KOH, boric acid and 2 eq. of the che-
lating oxy-acid, in this case the diol 1, as shown schematically
in Fig. 1. The use of lower temperatures and reaction times
and non-solvothermal conditions can be applied to this syn-
thesis, but the solvothermal conditions we use tend to give
suitably crystalline and phase pure products in a single step.
They also avoid the tendency to gel formation that have been
frequently encountered in such systems, perhaps due to 1D
coordination or H-bond chains in these salts.17,18
In the case of potassium, the desired salt KijB{L-
TarIJNHPh)2}2] 2 is formed in good yield and purity as platy
crystallites by solvothermal crystallization in MeOH at 110 °C
for 1 day. A small micro-crystallite of 2 was chosen for single
crystal structure determination. The phase is monoclinic with
chiral space group C2 and was found to be a non-solvated
form of the compound. Overall the asymmetric unit contains
one formula unit, but has two separate K+ cations and two
spiroborate [B{L-TarIJNHPh)2}2]− anions that all sit on crystallo-
graphic 2-fold axes. A symmetry generated ion-pair based on
K(1) and B(1) is depicted in Fig. 2a, a similar geometry is
found for the K(2) and B(2) ion-pair. In principle a [B{L-
TarIJNHPh)2}2]− anion should have four chemically equivalent
arms, however an intra-ligand NH---OC hydrogen bond
(NIJ1)–HIJ1)---OIJ4) = 3.061IJ3)Å) is formed between the two arms
of each ligand, rendering the chelating diolate asymmetric.
Fig. 1 Synthetic scheme for simple bisIJdiphenyltartramido)borate salts M = K, NH4, Na (2–4). Use of rac-amines as base allows extension of this to
preparation of organoammonium salts (5–9).
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Fig. 2 a) Structure of ion pair KijB{L-TarIJNHPh)2}2] in 2,and b) Na salt 4, 50% probability ellipsoids.
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Intriguingly this asymmetry of chelation then nominally in-
troduces a stereochemical center at boron. If the NH donor
side is taken as higher priority substituent this gives a BS-
stereochemical center (this still holds if the reverse priority is
assigned, since both ligands are affected by the change).
The projection of the spiroborate anion in Fig. 2a has the
Ph groups disposed in what could be loosely called a “Y-
shape”. Another conformational possibility chelating the sec-
ond TarIJNHPh)2 the other way around gives a BR-configuration
which has Ph group disposition more like an “X-shape” for the
same projection (for further discussion, see below and Fig. 11).
In our recent studies of bisIJmandelato)borate [BIJMan)2]− an-
ions diastereomeric conformations described as “Twisted” and
“V-shaped” resulted from BR- and BS-stereochemistry with
S-mandelate ligands respectively.5 The gas phase or solution
energies of the two quite different shapes were shown to be en-
ergetically similar and hence likely to have similar solution
populations, although only the “Twisted” conformational an-
ions were found in the solid state.
In the case of bisIJN,N′-diphenyltartramido)borate [B{L-
TarIJNHPh)2}2]− it is worth noting that the ‘asymmetry’ of the
ligand is not fixed as is the case of mandelate, but could dy-
namically switch with amide rotation with concomitant
breaking and possible re-formation of intramolecular NH---
OC amide H-bonds. This would scramble the ligand asymme-
try and hence the ‘boron stereochemistry’ without any need
for disruption of the actual boron BO4 tetrahedral arrange-
ment itself.
coordinates a tetradentate pocket for the B(2) anion and
makes single chelations to O(2) and O(3) for two B(1)-based
anions. Overall the K+ and [B{L-TarIJNHPh)2}2]− anions form a
1D molecular stack along the [001] direction, (see Fig. S1†).
The molecular stack interdigitates its Ph rings with those of
other neighboring stacks to provide the overall 3D crystal
structure. In addition to NH---O H-bonds between different
halves of the same ligand there are also incipient H-bond
contacts NIJ4)–HIJ4)---OIJ3) = 2.630 Å, although the angle at H
is just 113.9°. Despite the fact that [B{L-TarIJNHPh)2}2]− has a
tetrahedral arrangement at boron one effect of the intra-
molecular H-bonds is that the molecular anion is flattened
into a corrugated planar shape.
Using related solvothermal synthetic conditions to the for-
mation of 2, the substitution of NH4OH gave an isostructural
phase type [NH4]ijB{L-TarIJNHPh)2}2], 3. The X-ray structure
clearly showed the H-bond arrangement between the ammo-
nium and spiroborate ions and is instructive in seeing how
there is a similar cation recognition site for ammonium
H-bonding to the anion similar to the tetradentate coordina-
tion site for potassium.
The substitution of NaOH into the reaction scheme in
Fig. 1 gives the salt NaijB{L-TarIJNHPh)2}2] 4, this time as a di-
methanolate phase. Variation of conditions (time and tem-
perature) within the methanol system did not result in for-
mation of an unsolvated phase and a change of solvent typi-
cally afforded a hydrated phase instead. The di-methanolate
4
is quite reproducibly prepared, phase pure and the
As shown in Fig. 2a K(1) sits in a pocket of four contacts
to O(1), O(2), OIJ2)′ and OIJ1)′ where the primes are the 2-fold
related atoms. The K(1) also makes two separate chelations
to the B(2) based anions coordinating the diolate O(2) and
O(3) type-oxygen atoms, so that overall K(1) is eight coordi-
nate. A similar arrangement is found for K(2) and which
resulting crystal structure of reasonable quality. Crystals of 4
are orthorhombic with the P212121 space group. The asym-
metric unit contains one formula unit and the NaijB{L-
TarIJNHPh)2}2] ion pair and two methanols, as shown in
Fig. 2b.
The structure of 4 exemplifies how the structure of the
[B{L-TarIJNHPh)2}2]− anions can be modified by solvation. One
MeOH coordinates to the Na while the other MeOH is
‘inserted’ into an amide NH---O H-bond, thus slightly modi-
fying the anion's conformational shape. The Na+ cation again
sits in a tetradentate pocket of the anion formed by O(1) O(2)
O(12) and O(11) in a similar way to K ion in 2. The smaller
Na is coordinated to MeOH and a single bidentate chelation
to a neighboring [B{L-TarIJNHPh)2}2]− through the OIJ2)–OIJ13)
edge of the BO4 tetrahedron.
The NIJ1)–H---OIJ4) H-bond is still in place on one half of
the ion but on the second half the H-bond arrangement is
more extended so that a second uncoordinated methanol acts
as acceptor from NH and donor to amide CO, NIJ11)–H---
OIJ2S)–H---OIJ14). The result of this is that the fourth arm
swings around and forms a NIJ14)–H---OIJ3) hydrogen bond of
3.083 Å between the two ligands. The [B{L-TarIJNHPh)2}2]− an-
ion in 4 thus has a slightly modified shape from the K salt 2,
as seen in the overlay in Fig. 3. Despite the change intro-
duced by the smaller sodium and insertion of solvent in
modifying the NH---OC amide hydrogen bonding an overall
conformational preference for [B{L-TarIJNHPh)2}2]− anion be-
gins to emerge. A similar ‘semi-rigid’ structure was noted for
Fig.
3 Overlay of KijB{L-TarIJNHPh)2}2] 2 (pale green) and NaijB{L-
TarIJNHPh)2}2]·2MeOH, 4 (dark red) showing effect of MeOH ‘insertion’
on spiroborate conformation.
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[BIJMan)2] ions5 and deemed helpful in providing crystalline
products with a wide range of counterions.
In practice we have found that acceptable results were
obtained for [BMan2] by using a stoichiometric amount of ra-
cemic cations for the chiral anion used, so made initial reac-
tions in this way with the idea that ratios and conditions
could later be varied to optimize yield and % ee in the
resulting solids.
Application of chiral anion [B{L-TarIJNHPh)2}2]− to resolution
In order to test whether [B{L-TarIJNHPh)2}2]− could achieve a
similar chiral discrimination to [BIJMan)2]−, we next
attempted to employ the bisIJdiphenyltartramido)borate an-
ion in resolution of five racemic amines, sec-butylamine
(NH2CHMeEt) and 2-pentanamine (NH2CHMePr) as well as
1-phenylethylamineIJNH2CHMePh), 1-phenylpropylamineIJNH2-
CHEtPh) and 2-phenylglycinol (NH2CHIJCH2OH)Ph). These
were chosen to be challenging since there is a single chiral
center with a strong shape similarity between the enantio-
mers of each racemic pair. Consequently, the resolution
needs to discriminate not only through distinct solubilities
of pure diastereomeric salts, but also in limiting possible
cation site disorder in single phase solid-solutions. As will
be seen three main classifications of such site disorder can
be considered in these systems, as shown in Fig. 4.
Resolutions with varying degrees of success have been
achieved for sec-butylamine from diastereomeric salts of
deoxycholic acid19 and tartaric acid20 or by neutral molecule
cocrystallization.21 The case of rac-1-phenylethylamineIJα-
methylbenzylamine) is slightly less demanding and has been
previously resolved using mandelic22 and tartaric acids.23
Subsequently its enantiomerically pure forms have become
standard resolving cations in their own right24 and some of
its diastereomeric salts have been the subject of computa-
tional studies to aid rational design of resolving agents.25
Our previous efforts to employ [BIJMan)2]− in the resolution of
the two racemic bases sec-butylamine and 1-phenylethylamine
met with limited success,26 so they offer an exacting chal-
lenge for potential new chiral spiroborate anions.
Like the
K
salt 2, the product crystals [R/(S)-
NH3CHMeEt]ijB{L-TarIJNHPh)2}2] 5 are unsolvated and belong
to the orthorhombic system with chiral space group P212121.
The asymmetric unit consists of an ion pair as shown in
Fig. 5a. We use the R/(S)-nomenclature to indicate the cation
is predominantly R-configuration. The cation site is disor-
dered with a best fit indicating about a 67 : 33 ratio of the
R-cation to its S-enantiomer. Interestingly it is not C–H/C–Me
disorder at the chiral carbon center, but C–Me/C–Et disorder
that causes the loss of enantiospecificity. The packing in 5 is
shown in Fig. 10 and reveals the 21 screw of cations are in
close contact with each other. The disordering of R- and
S-cations may be partially due to the possibility of mutual cat-
ion disordering due to the fact that the cation neighbors are
in contact with each other in their aliphatic regions.
Whilst variations in crystallization conditions could proba-
bly improve on the initial 34% ee found, the packing implies
that for this phase a usefully high level of chiral discrimina-
tion is unlikely. Several other solvents were used to see
whether a change of phase type could be induced with supe-
rior resolution characteristics, but various other alcohols and
polar solvents such as acetonitrile and acetone yielded either
the same phase type 5, or poorly crystalline/gel like products.
The resolution for sec-butylamine of an approximately 2 : 1
ratio of R : S cations in the salt prepared from the racemic
amine in a 1-pot reaction still represents a modest success
given the extreme similarity of the enantiomeric organoamm-
onium ions. The observed R1/R2 disorder of the Me and Et
positions is hard to mitigate against. It was decided to probe
this issue further by perturbing the racemic amine and study
the related 2-pentylamine. In this case the Et group at the chi-
ral centre is replaced with slightly larger nPr. The chiral coef-
ficient of this amine is slightly larger and it seemed reason-
First the feasibility of resolution of racemic sec-butylamine
(NH2CHMeEt) was attempted through
a
direct 1-pot
solvothermal crystallization, similar to that of simple salts
2–4, only employing the rac-amine as the base. As a strategy
to optimize % ee in a precipitated diastereomeric salt Fogassy
suggests to use only 50 mol% of the racemic ion in a resolu-
tion,27 with a possible addition of a more soluble achiral
counter-ion to retain the unwanted enantiomer in solution.28
able that
a Me/Pr disorder would be less likely. The
crystallization of the corresponding organoammonium salt
[NH3CHMePr]ijB{L-TarIJNHPh)2}2] 6 was carried out in similar
Fig. 4 Three distinct disorder modes that have been identified at the cation sites in the organoammonium salts 5–8.
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Fig. 5 a) Ion pair in [R/(S)-NH3CHMeEt]ijB{L-TarIJNHPh)2}2] 5, with disordered cation due to Me/Et exchange. b) Ion pair [R/(S)-NH3CHMePr]ijB{L-
TarIJNHPh)2}2] 6, with CH pyramidal disorder. The minor S-components are shown in pink.
manner to 5 and the solid was isolated in reasonable yield
and phase purity. This time the space group was monoclinic
P21, although some conserved packing features are seen. A
molecular ion pair is shown in Fig. 5b and reveals that once
again a solid solution is formed which limits enantiopurity,
but this time the location of –Me, –nPr and –NH3 substituents
at the chiral centre remain roughly in place, whilst there is a
‘pyramidal’ disorder of the C–H group (Fig. 9b). The chiral
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carbon itself thus has two alternate orientations and can be
refined to approximately 61 : 39 ratio.
which confirmed (Fig. 8) the 88-R: 12-S ratio in excellent
agreement with the X-ray structure from an individual crystal.
This 76% ee for a first crystallization step is still very respect-
able. A single re-crystallization of this material from pure
methanol (80% yield) afforded a crystal 7b with no observ-
able peaks for the minor S-enantiomer. The largest other re-
sidual peak in the original crystal of 7 of ca. 0.3 eÅ−3 that lay
in the vicinity of the non-coordinated methanol also
disappeared, supporting the idea that this was in fact due to
a small component of water which was inserted into the am-
ide hydrogen bond instead of methanol, when the crystalliz-
ing solvent was a 3 : 1 methanol : water mixture.
Whilst the R- and S-cations are not perfectly discrimi-
nated in 7 the potential of [B{L-TarIJNHPh)2}2]− or related
bora–tartramide anions for chiral resolution is well demon-
strated. The reasonable success with the resolution of 7,
but with the –H/–Me disorder at the chiral centre led us to
next investigate the resolution of the ethyl analogue, for
which such disorder should be reduced or eliminated. Ac-
cordingly a similar 1-pot resolution was attempted using
rac-1-phenylpropylamine and the structure of the resulting
salt [S-NH3CHEtPh][B{L-Tar (NHPh)2}2]·MeOH 8 was deter-
mined and consistent with effective resolution in a single
step. The compound was not isostructural with 7, but crys-
tallizes in space group C2. This time a short 5.4 Å repeat
due to stacking of molecular layers was found for the
monoclinic b-axis. The asymmetric unit and ion pair are
shown in Fig. S2.† The disposition of Ph and Et substitu-
ents is reversed compared to 7 so that the predominant
enantiomer is now S-, rather than R-. No discernable peaks
for pyramidal or other disorder could be found and chiral
HPLC confirmed the cations in the isolated solid were close
to pure S-enantiomer.
The prevalence of three different disorder modes in the
four racemic amine systems tested indicated the inherent
difficulties in the diastereomeric salt resolution method
for such amines with high levels of molecular similarity
for their enantiomers. Our previous success with [BMan2]−
also included one example 1,2-diaminopropane with ex-
tremely similar enantiomeric cations. Given the partial
success of the boron tartramide [B{L-TarIJNHPh)2}2]− in the
resolution of α-methylbenzylamine, we decided to make
one further change by replacing the –Me group with –CH2
OH. It was anticipated that the introduction of an addiional
hydrogen bonding group to augment the –NH3 might also
lead to superior discrimination between enantiomers. Ac-
cordingly the 1-pot reaction of rac-2-phenylglycinol was
attempted which afforded [S-NH3CHIJCH2OH)Ph]ijB{L-Tar-
IJNHPh)2}2]·MeOH, 9.
This disorder mode is possible for many chiral molecules
possessing a simple –H substituent at the chiral carbon. In-
deed, the larger or more highly functional the three other
substituents, the more prevalent it can become, since pre-
serving their relative packing arrangement would be increas-
ingly important. Once again given the relatively poor direct
resolution further recrystallizations to enhance the enantio-
purity of salt 6 were not attempted.
It was however encouraging to note that various structural
features of 6 were preserved from 5. The spiroborate anion
had a similar overall geometry, with Y-shape conformation,
intra-molecular amide H-bonds and an organoammonium
‘binding pocket’ with two NH---OC H-bonds within an ion
pair and the third NH hydrogen bonding to an anion trans-
lated along a short 5.5 Å axis, which again was the crystallo-
graphic [100] direction. Hence both in both structures in-
register layered stacking is found along the common short
axis.
A further perturbation of the amine structure was then
made by replacing the aliphatic Et or nPr groups with a Ph
substituent. Once again a 1-pot solvothermal crystallization
in methanol–water was carried out to effect a possible resolu-
tion of the racemic amine. This time the resulting crystalline
solid produced the product phase [R-NH3CHMePh]ijB{L-
TarIJNHPh)2}2] 7 which belongs to the monoclinic system with
space group P21. Like the Na salt 4 it is solvated, with one
methanol disrupting the amide hydrogen bonding. The asym-
metric unit contains one formula shown in Fig. 6a. A speci-
men of 7 gave 88 : 12 ratio of R- to S- within the crystal. A dif-
ference electron density map gave peaks of 3.26 and 0.56
eÅ−3 respectively for the major and minor components of the
–CH3 group, shown in Fig. 7. The lower level of disorder/solid
solution in 7, compared to the sec-butylammonium analog 5,
can be traced to the different chiral environment found
around the cation, as shown in Fig. 10 as viewed along the
short a-axis. Each cation is surrounded by five neighbors in
the bc plane. One is the [B{L-TarIJNHPh)2}2]− anion to which
the –NH3 head forms two hydrogen bonds, two other [B{L-
TarIJNHPh)2}2]− make weak aryl-H contacts and the other two
neighbors are other cations, which form the 21-screw arrange-
ment along the b-axis.
It should be noted that the ghost peak for the Me of the
S-cation does not lie along the C–H vector for the R-cation,
but is rotated so that C–H---O interaction is still maintained.
The ellipsoid of the chiral carbon is clearly elongated indicat-
ing it should have a split position, although the geometry of
the NH3 and Ph groups are not greatly perturbed. The de-
scription of the disorder as H/Me exchange is hence a best
approximation and may be seen as a starting point from
which the minor component makes a best fit to the sur-
rounding structure.
Interestingly the powder XRD pattern of 9 indicated a
striking similarity to that of 7 and single crystal structure
analysis confirmed the isomorphic nature of the two phases.
In 9 however the hydroxymethyl –CH2OH group appears close
to fully ordered in replacing the disordered –Me.
Recovery of α-methylbenzylamine from the salt 7 and de-
rivatization of the amine group to form the benzamide
PhCONHCHMePh allowed for chiral HPLC chromatography
The arrangement shown for the ion-pair of the asymmetric
unit in Fig. 6b is essentially identical to that for 7. The work
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Fig. 6 a) Structure of ion pair in 7 [R-NH3CHMePh]ijB{L-TarIJNHPh)2}2]·MeOH, and b) the isostructural 9 [S-NH3CHIJCH2OH)Ph] [B{L-TarIJNHPh)2}2]
·MeOH.
up of the isolated solids from 9 gave chiral HPLC with 94%
ee. The X-ray structure of 9 has very weak ghost peaks at the
cation sites which in view of the chiral HPLC result can be
seen as a 97S : 3R disorder. This indicates the practical limit
for X-ray identification of the diastereomeric solid-solutions
is around 95% ee.
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Fig. 7 Difference electron density map of disordered cation in 7.
Showing minor-CH3 position C(51a) lying between major-CH3 C(51)
and major-H H(52).
Fig. 8 Chiral HPLC chromatogram of PhCONHCHMePh enantiomers
derived from 7.
Cation disorder modes and environments
The disorder of the cations in salts 5–7 can be considered
first in terms of the three possible modes of disorder
presented earlier, namely H/R1 exchange, R1/R2 exchange and
CH re-pyramidalization. These were considered and com-
puted in the model chiral system CHFClBr as ways of com-
paring shape similarity based on overlap of molecular
wavefunctions.29 The overlay of major and minor cation com-
ponents in 5–7 are shown in Fig. 9.
In 5 the sec-butyl-ammonium ion has a minor component
(34%) in which the Me and Et group positions are clearly re-
versed – an R1/R2 site exchange. This is accompanied by a
slight splitting of the ammonium N position (0.20 Å) and a
change of the N–C bond vector (19°). This leads to a more
pronounced splitting of the chiral carbon position (0.52 Å)
and exchange of the Me and Et group positions, such that
the Me carbon in each orientation is just 0.54 Å from the ter-
minal C of the Et groups of the other hand. The two compo-
nents thus form an almost perfect mirror image with each
other.
Fig. 9 Cation disorders in a) [NH3CHMeEt] in 5, b) [NH3CHMePr] in 6
and c) [NH3CHMePh] in 7.
position is split by 0.78 Å and the C–H are oriented away
from each other (2.73 Å). The n-propyl arms comprise
overlapped zig-zag chains that share a common site for the
central CH2 group. The terminal CH3 of the propyl chain of
the major component is also split, which is apparently
linked to orientational disorder of a Ph ring in a neighbor-
ing anion.
In the [NH3CHMePh] salt 7 the NH3 and Ph retain com-
mon sites, whilst the Me position is clearly split (1.48 Å).
Modelling indicates there should be a slight spitting of the
chiral center by 0.29 Å and the vectors for the major and mi-
nor C–H directed about 145° apart. This disorder is best de-
scribed as
a
hybrid of C–H/Me disorder and CH
repyramidalization. Pure CH/Me disorder would have com-
mon H/Me vectors for major/minor components and pure
repyramidalization a common Me position for both hands.
In the case of salt 9 the [S-NH3CHIJCH2OH)Ph] cation oc-
cupies a similar crystal site to that in the isomorphous 7,
In 6 [NH3CHMePr] the extension of Et to Pr changes the
disorder mode in order to fit to the cation environment to a
C–H re-pyramidalization for the minor 36% component. In
this the NH3 and Me retain common sites but the chiral C
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Fig. 10 Cation environments and self-associations in 5–8, (a–d respectively) all viewed along short 5.5 Å axis.
however the conversion of CH3 to CH2OH forms an addi-
tional H-bond that helps lock in the major component. It
should be noted that switch to S-configuration in 9 from R in
the major component of 7 is due to the higher priority of
CH2OH compared the Ph in the Cahn–Ingold–Prelog priority
rules.
Whilst the origin of the preference in the case of 9 may be
clearly traced, this is harder for cases 5–7, for which the dual
orientations of R- and S- are required to be similar in energy.
All structures have similar short 5.5 axis repeats that derive
from the NH3 binding and roughly planar anion shape. The
‘tetradentate’ binding pockets use two N–H in binding to the
anion and the other one to the next anion along the 5.5 Å
stack. The packing environments of the cations may be exam-
ined by viewing along the 5.5 Å axis, i.e. along (100) direction
for 5–7 and (010) for 8.
The arrangements and metrics in these projections show
unique packing and cation environments in each case. In 5 a
pocket with two cations surrounded by parts of four different
anions is found. The cations make no strong interactions
that would clearly orient the Me and Et groups, but the
pairwise arrangement may facilitate Me/Et orientational dis-
order. In 6 the cations form isolated stacks along the 5.5 Å
axis, which are surrounded by parts of three anions. The Pr
group position is clearly differentiated from the Me, but the
disorder is now CH re-pyramidaliztion. This has chiral
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Fig. 11 “Y-shape” [BS{L-TarIJNHPh)2}2] anion – labelling scheme refers to torsion angles in Table 2 and hypothetical “X-shape” conformation for
the [BR{L-TarIJNHPh)2}2]− anion.
carbon CH oriented closer to a keto O (H---O 2.94 Å) for the
major R-component, which may help explain its preference
for incorporation in the structure.
may also be considered to form corrugated sheets, but in this
case Et---Et and Ph---Ph make ordered close contacts.
In 7 the cations form a corrugated sheet, which have
closer contacts between CH3 and Ph of neighboring cations
for the major R-component. Each cation is contacted by parts
of just three anions. In the [S-NH3CHEtPh] salt 8 the cations
Geometry of [B{L-TarIJNHPh)2}2]− spiroborate anions
The bisIJN,N′-diphenyltartramido)borate ions are readily pre-
pared from L-tartaric acid in two facile steps. Since a similar
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synthesis can be extended to using many aromatic amines in
the place of aniline the [B{L-TarIJNHPh)2}2]− anion can be
viewed as the parent in a large family of chiral aromatic
spiroborates derived from aryltartramide ligands. These
might have considerable potential for resolution, so what fea-
tures supporting this idea can be gained from these studies
of [B{L-TarIJNHPh)2}2]− salts 2–9?
Firstly, all salts are quite crystalline and apart from the is-
sues associated with R/S-cation disorder most structures show
well-ordered arrangements with quite strongly defined con-
formational preferences. The geometric features of [B{L-
TarIJNHPh)2}2]− anions in selected salts are summarized for
comparison in Table 2 with torsion angles labeled as in
Fig. 11.
geometry from the K salt 2 to the sec-butylammonium salt 5,
and a similar preservation from the solvated Na salt 4 to the
methanol-solvated α-methylphenylammonium salt 7.
The phenyl ring orientations at the top of the ‘Y-shape’
are reasonably close to co-planar with the amide (torsion an-
gles 11 & 12) due to the reinforcing effect of C–H---O interac-
tions. The phenyl rings at the stem or bottom of the ‘Y-
shape’ are a little more variable in their orientation (torsions
13 & 14) with values from +20.0 to −29.0°. In short the
bisIJtartramido)borate anions have some fairly well defined
conformational preferences, especially when cation binding
through coordination or hydrogen bonding is involved, but
with some built-in degrees of flexibility.
Since all structures are well determined at low tempera-
ture of 100 K, even the B–O bond lengths are to reasonable
precision of 0.002–3 Å. The range found in the sec-
butylammonium salt 5 of 1.450–1.485IJ3)Å encompasses the
narrower range of values found in the other salts too. Since
all four arms of the anion are in principle chemically identi-
cal this is anticipated, some variation is introduced primarily
due to coordination to K, Na or N–H hydrogen bonding to
the B–O oxygen atoms. The B–O bond lengths in these
diolates are intermediate to the B–O of 1.43–1.45 Å for
DFT calculations on [B{L-TarIJNHPh)2}2]−
We have investigated the relative energetics of spiroborate
anions and some ion pairs for the ‘Y-shape’ [BS{L-
TarIJNHPh)2}2]− and ‘X-shape’ [BR{L-TarIJNHPh)2}2]− (Fig. 11)
using DFT calculations32 and the Gaussian09 package.33
Since solvent effects on the related [BIJMan)2]− system indi-
cated only minor perturbation in the relative energies from
the gas phase to solution, this was not carried out for the
[B{L-TarIJNHPh)2}2]− anions. Interestingly the isolated gas
phase ions indicate that the individual ‘X-shape’ ions are no-
tably more stable, (7.2 kcal mol−1) however introduction of a
chelating K+ ion in an isolated ion-pair reverses the stability,
once again with a considerable energetic difference (20.3 kcal
mol−1). Neither case is a fair indication of the situation found
for either alkali metal or organoammonium salts in the solid
state however, since in the crystal the anions interact with
several counterions, rather than just one. However the ‘tetra-
dentate pocket’ found in the solid state for the ‘Y-shape’ in
phenoxy and 1.49–1.51
Å
for carboxy groups in
bisIJmandelato)borate, [BMan2] anions.6 An even wider dispar-
ity of B–O lengths of 1.345 to 1.525IJ7)Å was found in an
open-framework vanadoborate possessing both 3- and
4-coordinate borate centres.30
The five membered spiroborate rings are roughly planar
but all exhibit slight puckering. This in line with the situa-
tion for the 5-membered ring chelate rings in [BMan2]−,5
but contrasts with strongly puckered 6-membered rings
found in bisIJsalicylato)borate [BIJSal)2]−.31 The root-mean
square displacements are less than 0.1 Å. In general the C
and O atoms tend to be more displaced than the boron.
The syn O–C–C–O torsion of -56° found in the parent diol 1
is thus reduced to less than 10° for the diolate torsion an-
gles (3 and 4) in the unsolvated anions, though the
methanolated structures have an increase to about 28° on
the solvated side. The tartramido ligand O–B–O ‘bite angles’
lie in a narrow range bound by the value of 104.2 and
105.6° found in 4. These bite angles are close enough to
ideal tetrahedral values to induce little angular strain and
help explain why the five membered rings (which require
108° angles on average) are close to planar.
Ligand asymmetry has already been described as resulting
from the tendency to form intra-ligand NH---OC H-bonds
between two amide groups within each ligand. This is the
case for 2, 3, 5 and 6, whilst in 4, 7, 8 and 9 the arrangement
is just slightly modified by inserting a MeOH solvent (or
water) molecule into the hydrogen bond arrangement. This
maintains the asymmetry whilst introducing a perturbation
to the preferred geometry of the anion. The overlay of anion
geometries for the K and Na salts was shown in Fig. 3, in fact
as Table 2 indicates, there is also fairly close preservation of
+
coordination binding to metals, or for H-bonding to RNH3
cations, is clearly a stabilizing feature that can explain the ex-
clusive observation of this conformer in the structures of 2–9,
despite the supposed energetic favorability of the less polar
‘X-shape’. As further salts of [B{L-TarIJNHPh)2}2]− are isolated,
especially with non-coordinating counter cations, it may be
anticipated that the ‘X-shape’ structure and other arrange-
ments will eventually be encountered. In both X- and Y- cases
the DFT calculations faithfully reproduce the pair of intra-
molecular NH–OC amide hydrogen bonds found in the
unsolvated crystal structures 2 and 5, which break the poten-
tial D2 symmetry of the anions, by introducing ligand asym-
metry. These will undoubtedly remain a feature, unless
disrupted by the insertion of a H-bonding solvent, such as
MeOH or H2O as found in structures 4 and 7. Tartramides
without N–H functionalities, or tartrate esters would be
expected to have greater scope for individual freedom of the
substituent arms without the internal N–H---OC hydrogen
bonds, which may or may not be better for general crystalliza-
tion purposes.
Whilst the exclusive solid state favoring of the ‘Y-shape’ in
the salts studied here can be rationalized by the preferred co-
ordination or H-bonding, in solution it is probable that no
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1
major conformational preference is found, since the H NMR
indicates a singlet for the tartramide C(2)–H–C(3)–H protons,
due to rapid equilibration between X- and Y- and other
forms. Either X- or Y- alone should give two doublet signals
due to coupling of inequivalent protons. However in solution
they are rapidly interconverting through amide H-bond
breaking, amide rotation and amide H-bond reformation. A
mixed population of X-, Y- and other conformers in solution
may assist resolution crystallizations in a similar manner that
we have speculated may operate for [BMan2]−. Since one half
of the molecular shape is retained whilst the other half is
not, the presence of the X-conformer may disrupt nucleation
to differing extents for the two diastereomeric Y-type salts.
This is believed to be the basis for the ‘Dutch method’34 in
which the presence of a related resolving ion may consider-
ably improve the efficiency of a resolution, despite the fact it
is not incorporated in the final solid.
disfavored.45 Regarding the resolution potential of
spiroborate anions, a successful resolving agent must crys-
tallize effectively with the organoammonium cation with
sufficient overall difference in stability and hence solubility,
between the potential diastereomeric salts formed. It should
also minimize the likely contamination of a given phase
with the cation of opposite chirality through disorder and
solid-solution formation. As we have found for [BMan2] vari-
ation in solvent can have a profound impact, especially if
solvates are formed, as for [1,2-dap-H2]ijBMan2]·MeOH.5 This
clearly applies to the tartramide spiroborates since several
methanolates have been found in the chiral resolution sys-
tems studied herein, screening of many solvents may pro-
vide superior resolutions to those we present here, which
are not meant to represent optimal resolutions using these
anions, but establish their generic potential as resolving
agents.
The promising results for resolution from both our earlier
studies5 on bisIJmandelato)borate [BMan2]− and the new find-
ings for [B{L-TarIJNHPh)2}2]− anions presented here are en-
couraging. The relatively facile preparation of the chiral
tartramide diol 1 using aniline can be readily extended to
other aromatic amines. The resulting aryltartramides will
provide a family of related chiral spiroborate anions. It may
be reasonably expected that chiral resolutions, such as those
presented here, may then be more fully explored and opti-
mized, since both CH–pi interactions46 and the influence of
substituent groups47 on resolutions are both well established.
Future prospects of chiral spiroborates
For diastereomeric salt formation, the development of new
synthetic anions with high chiral discriminating power for
resolution or catalysis is a desirable objective.35 One notable
system developed by Lacour involves the TRISPHAT anions
[PIJO2C6Cl4)3]− which are based on non-labile tris-chelation of
substituted catecholates.36 These have good efficiency in a va-
riety of chiral resolutions, but remain relatively expensive,
limiting their attractiveness and bulk industrial use. Other
anions have also been proposed for use in recent times,37 in-
cluding some applied to similar amines to those described
here.38
Experimental
Synthesis and characterization
Chelation of boron by diols, acid-alcohols, catechols and
salicylic acids can result in spiroborate anions which can
have a variety of uses.39 Some such as bisIJcatecholato)borate
[BCat2]−, have been demonstrated to be effective crystallizing
anions.40 Their use in resolution was first reported by
Periasamy who made an improved resolution of BINOL based
on forming a diastereomeric salt of its borate ester.41 This ap-
proach was then used in resolution or purification of other
diols.42 Conversely the use of chiral [BIJBINOL)2]− anions was
then applied to resolution of chiral amines and amino-alco-
hols.43 This established the effectiveness of such anions, but
its use by others has been limited since the cost of resolved
BINOL remains very high. Despite this promising work the
use of spiroborates for resolution has not received much fur-
ther attention, though effective chiral resolution using a chi-
ral diborate has been demonstrated.44 One factor is that the
chiral borate centres are labile and prone to racemization.
However we have recently demonstrated that B-chiral
spiroborate anions from salicylate [BIJSal)2] can be success-
fully isolated in enantiopure forms and that they are stable
in aprotic solution.31
All chemicals were reagent grade (99%+) (Sigma-Aldrich, TCI
or Acros). Elemental combustion analyses were carried out by
Medac Ltd., Surrey, U.K.
N,N′-Diphenyl-L-tartramide [L-TarIJNHPh)2-H2] (1). The syn-
thesis of 1 followed an adaptation of the published procedure
of Chen et al.48 L-Tartaric acid was directly amidated by
refluxing in a two stage approach i) a mixture of xylenes (25
mL) aniline (C6H5NH2 Mw 93.1, d = 1.02 g mL−1, 5.0 mL, 55
mmol) and L-tartaric acid (C4H6O6, Mw 150.1, 2.25 g, 15.0
mmol) were refluxed for 2 h. ii) 2.5 mL dimethylformamide
(DMF) was then added and the mixture refluxed for a further
3 h. Upon cooling a white solid (1) was formed which was fil-
tered and washed with water and then 95% EtOH to yield
crystalline (1) as colorless transparent plates (C16H16N2O4,
Mw 300.3, 4.05 g, 13.5 mmol, 90% yield). CHN: found (calc.)
%C = 64.17 (63.99); %H = 5.31 (5.37); %N = 9.41 (9.33).
KijB{L-TarIJNHPh)2}2] (2). The potassium salt KijB{L-
TarIJNHPh)2}2] 2 of the spiroborate anion derived from chiral
diol [L-TarIJNHPh)2-H2]
1 was synthesized by a 1-pot
solvothermal reaction/crystallization approach. 300 mg (1
mmol) of [L-TarIJNHPh)2-H2], 31 mg (0.5 mmol) of boric acid
and 28 mg (0.5 mmol) of KOH were heated in 0.6 mL MeOH
at 110 °C for 1d. Colorless block crystals were filtered and
washed by minimal amount of water and acetone affording
Fogassy et al. have noted not only the intuitive result
that diastereomeric salts with higher melting points will
tend to precipitate in chiral resolutions, but also that sol-
vates can be favored, whereas amorphous solids are strongly
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284 mg of 2 (C32H28BKN4O8, Mw = 646.5, 88% yield). CHN:
found (calc.) %C = 58.98 (59.45); %H = 4.65 (4.37); %N = 8.52
(8.67). A single crystal specimen for X-ray structure analysis
of 2, was selected directly from the solvothermally crystallized
solid.
NH4ijB{L-TarIJNHPh)2}2] (3). 300 mg (1 mmol) of
[L-TarIJNHPh)2-H2], 31 mg (0.5 mmol) of boric acid and 40 μL
(0.6 mmol) of 30% aq. NH4OH were heated in 0.6 mL MeOH
at 110 °C for 1d. Colorless needle crystals were filtered and
washed by minimal amount of water and acetone affording
256 mg of 3 (C32H32BN5O8, Mw = 625.4, 82% yield). CHN:
found (calc.) %C = 61.39 (61.45); %H = 5.14 (5.16); %N =
11.03 (11.20).
[S-NH3CHIJCH2OH)Ph]ijB{L-TarIJNHPh)2}2]MeOH (9). 300
mg (1 mmol) of [L-TarIJNHPh)2-H2], 31 mg (0.5 mmol) of boric
acid and 140 mg (1 mmol) of solid rac-2-phenylglycinol
(NH2CHIJCH2OH)Ph, Mw = 137.2) were heated in 1 mL MeOH
at 110 °C for 2d.; colorless needles were filtered and washed
affording 305 mg of 9 as MeOH solvate (C40H40BN5O9.CH4O
Mw = 777.6, 78% yield). CHN: found (calc.) %C = 63.0
(63.33); %H = 5.44 (5.70); %N = 9.23 (9.01).
X-Ray structure determinations
The X-ray structures of compounds 2–9 were determined after
suitable single crystal specimens were grown, either directly
through solvothermal synthesis, or by recrystallization as de-
scribed in the synthesis section. All specimens are reasonably
stable and the methanolates do not desolvate rapidly at am-
bient temperature, though they were handled quickly before
transferring to the cold stream. The specimens were fixed in
a cryoloop using Paratone™ and diffraction intensity data
were collected at low temperature (100 K) on a Rigaku-Oxford
Diffraction Supernova diffractometer operating with a copper
micro-focus source. The structures were solved (SHELXS) and
refined (SHELXL) using SHELX software49 embedded in the
Olex2 software platform,50 which was also used to generate
the molecular figures and graphics. The key structural pa-
rameters are given in Table 1, with fuller crystallographic de-
tails in the ESI.† All non-hydrogen atoms were refined with
anisotropically excepting those of minor components which
were isotropic and with bond length restraints applied. Pro-
tonation states of alkaloid cations were clearly established by
identification of sensibly located electron density peaks for H
associated with N heteroatoms on ammonium and amide
centres. Where practicable these were refined isotropically, or
for disordered cases, these were placed and treated with geo-
metric riding constraints (dN–H = 0.88 Å) as were all C–H hy-
drogen atoms. For the disordered cations the separate parts
were refined with group occupancy factors that summed to
unity and then fixed in the final refinement. In general, the
values from S-XRD were in good agreement with values later
obtained from chiral HPLC. In the case of 8 a solvent channel
was identified with no well-defined peaks and the Squeeze
functionality (Platon51) was applied.
NaijB{L-TarIJNHPh)2}2]·2MeOH (4). 300 mg (1 mmol) of
[L-TarIJNHPh)2-H2], 31 mg (0.5 mmol) of boric acid and 20 mg
(0.5 mmol) of NaOH in
1 mL MeOH were heated
solvothermally at 80 °C for 2d. Colorless crystal platelets were
filtered and washed by minimal amount of water and acetone
affording 257 mg of 4, which was a di-methanol solvate
(C32H28BN4NaO8, Mw = 694.4, 74% yield). CHN found (calc.
best fit for 4 with 1.5MeOH) %C = 58.66 (59.48), %H = 4.62
(4.77); %N = 8.78 (8.28).
[R/(S)-NH3CHMeEt]ijB{L-TarIJNHPh)2}2] (5). 300 mg (1
mmol) of [L-TarIJNHPh)2-H2], 31 mg (0.5 mmol) of boric
acid and 75 mg (1 mmol) of rac-sec-butylamine (NH2-
CHMeEt) were heated in 1 mL of a 3:1 MeOH:H2O mix-
ture at 110 °C for 1d. 273 mg of 5 were isolated as color-
less crystal plates (C36H40BN5O8, Mw = 681.6, 80% yield).
CHN found (calc.) %C = 63.42 (63.44); %H = 5.84 (5.92);
%N = 10.25 (10.28).
[R/(S)-NH3CHMePr]ijB{L-TarIJNHPh)2}2] (6). 300 mg (1
mmol) of [L-TarIJNHPh)2-H2], 31 mg (0.5 mmol) of boric acid
and 90 mg (1 mmol) of rac-2-pentylamine (NH2CHMePr) were
heated in 1 mL MeOH at 110 °C for 2d. 288 mg of unsolvated
6 were isolated as colorless needles (C37H42BN5O8, Mw =
695.6, 82% yield).
[R/(S)-NH3CHMePh]ijB{L-TarIJNHPh)2}2]·MeOH (7). 300 mg
(1 mmol) of [L-TarIJNHPh)2-H2], 31 mg (0.5 mmol) of boric
acid and 118 mg (1 mmol) of rac-1-phenylethylamineIJNH2-
CHMePh Mw = 121.2, d = 0.94 g mL−1, 0.125 mL) were heated
in 1 mL MeOH at 110 °C for 5d.; 324 mg colorless platy crys-
tals of 7 isolated as a MeOH solvate (C40H40BN5O8.CH4O Mw
= 761.6, 85% yield). CHN found (calc. For 7 with 1.0MeOH)
%C = 63.96 (64.66), %H =5.14 (5.82), %N = 9.17 (9.20). A sin-
gle crystal XRD and chiral HPLC were also run on a sample
(7b) obtained by recrystallization (80% yield) of product 7
from pure MeOH and these compared well with the com-
pound directly obtained from commercial R-α-methyl-
benzylamine (99% ee).
Powder X-ray diffraction
The phase purity of products was established after X-ray
structure determination by recording diffractograms (2θ = 5–
40°) on PanAlytical Aeris, Empyrean or X'Pert powder diffrac-
tometers. These were then compared to simulated powder
XRD profiles from the Mercury software package52 and in
some cases unit cells were derived from the fitting of
d-spacings of experimental pXRD peaks by the programs
TREOR53 or DICVOL.54 Generally experimental P-XRD were in
good agreement with the patterns simulated from single crys-
tal structure determinations (see ESI†) consistent with single
phase products.
[S-NH3CHEtPh]ijB{L-TarIJNHPh)2}2]·MeOH (8). 300 mg (1
mmol) of [L-TarIJNHPh)2-H2], 31 mg (0.5 mmol) of boric acid
and 135 mg (1 mmol) of rac-1-phenylpropylamineIJNH2CHEtPh
Mw = 135.2, d = 0.94 g mL−1, 0.145 mL) were heated in 1 mL
MeOH at 110 °C for 2d.; 326 mg colorless needle crystals of
8
isolated as a mixed MeOH/H2O solvate (C41H42BN5-
O8.0.25CH4O·H2O, Mw = 767.1, 85% yield).
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Table 1 Structure determination summaries for [B{TarIJNHPh)2}2] salts 2–9
Compound
2
3
4
5
Formula
K [B{Tar(NHPh)2}2]
Gem81a
145865
C32H28BKN4O8
646.5
100(2)
Monoclinic
C2
30.9006(5)
13.4265(2)
6.99592 (11)
90
NH4 [B{Tar(NHPh)2}2]
Gem57d
1844600
C32H32BN5O8
625.43
100(2)
Monoclinic
C2
30.9663(9)
13.3119(4)
7.0955(3)
Na [B{Tar(NHPh)2}2] 2MeOH
Gem84
145866
C34H36BN4NaO10
694.5
100(2)
Orthorhombic
P212121
7.6104(2)
15.1976(3)
29.4158(5)
90
[R/(S)-NH3CHMeEt][B{Tar(NHPh)2}2]
Gem71a
145867
C36H40BN5O8
681.5
100(2)
Orthorhombic
P212121
5.4928(2)
21.2421(5)
29.4393(7)
90
Code name
CSD number
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
90
β/°
γ/°
94.2992IJ14)
90
2894.34(8)
4, 1
1.484
Cu Kα, 2.137
1344
93.965(3)
90
2917.92IJ17)
4, 1
1.424
Cu Kα, 0.855
1312
0.10 × 0.05 × 0.03
135
−38 ≤ h ≤ 38, −16 ≤
k ≤ 16, −8 ≤ l ≤ 8
8914 (100.0%)
[Rint = 0.044,
Rsig = 0.0400]
8014/1/422
1.043
R1 = 0.0425,
wR2 = 0.1147
R1 = 0.0485,
wR2 = 0.1170
0.23/−0.23
−0.09IJ16)
90
90
90
90
Volume/Å3
Z, Z′
3402.21IJ11)
4, 1
1.356
Cu Kα, 0.940
1456
0.20 × 0.05 × 0.05
136
−7 ≤ h ≤ 9, −11 ≤ k ≤ 18,
−35 ≤ l ≤ 34
9737 (98.1%)
[Rint = 0.0308,
Rsigma = 0.0401]
6079/0/477
1.017
R1 = 0.0328,
wR2 = 0.0784
R1 = 0.0368,
wR2 = 0.0803
0.17/−0.17
−0.04IJ5)
3434.97IJ13)
4, 1
1.318
Cu Kα, 0.769
1440
0.5 × 0.01 × 0.01
136
−6 ≤ h ≤ 4, −23 ≤ k ≤
25, −29 ≤ l ≤ 35
9855 (98.3%)
[Rint = 0.0305,
Rsigma = 0.0571]
6178/30/506
1.006
R1 = 0.0404,
wR2 = 0.0865
R1 = 0.0535,
wR2 = 0.0906
0.22/−0.20
−0.05IJ14)
ρcalc g cm−3
Radiation, μ mm
FIJ000)
Crystal size per mm3
2Θ maximum/°
Index ranges
0.05 × 0.03 × 0.02
136
−37 ≤ h ≤ 36, −15 ≤
k ≤ 16, −7 ≤ l ≤ 8
11 280 (98.3%)
[Rint = 0.0203,
Rsig = 0.0248]
5070/1/417
1.027
R1 = 0.0336,
wR2 = 0.0873
R1 = 0.0348,
wR2 = 0.0884
0.95/−0.22
0.015(4)
Total reflections (%)
Data quality
Data/restr./params.
Goodness-of-fit F2
Final R indexes
[I > = 2σ(I)]
Final R indexes
[all data]
Diff. peak/hole eÅ−3
Flack parameter
Compound
Formula
6
7
8
9
[R/(S)-NH3CHMePr]
[B{Tar(NHPh)2}2]
Lawr435
[R/(S)-NH3CHMePh]
[B{Tar(NHPh)2}2] MeOH
Gem64b
[S-NH3CHEtPh]
[B{Tar(NHPh)2}2] MeOH
Lawr437
[S-NH3CH(CH2OH)Ph]
[B{Tar(NHPh)2}2] MeOH
Lawr434
Code name
CSD number
1844601
145868
1844602
1844603
Empirical formula C37H40BN5O8
C41H44BN5O9
761.72
100(2)
Monoclinic
P21
5.52108(7)
15.5073(2)
22.1422(3)
90
C42H46BN5O9
775.67
100(2)
Monoclinic
C2
28.6200IJ11)
5.25819IJ14)
28.6090(9)
90
C41H44BN5O10
777.62
100(2)
Monoclinic
P21
5.51895(7)
15.5145(2)
22.2271(3)
90.00
Formula weight
Temperature/K
Crystal system
Space group
a/Å
695.56
100(2)
Monoclinic
P21
5.56240(9)
19.1848(3)
16.7363(3)
90
b/Å
c/Å
α/°
β/°
γ/°
95.1248IJ15)
90
1778.85(5)
2, 1
1.299
Cu Kα, 0.752
736.0
94.8402IJ12)
90
1888.99(4)
2, 1
1.339
Cu Kα, 0.779
804
113.834(4)
90
3938.2(3)
4, 1
1.308
Cu Kα, 0.770
1640.0
94.4499IJ12)
90.00
1897.44(4)
2, 1
1.361
Cu Kα, 0.808
820.0
Volume/Å3
Z, Z′
ρcalc g cm−3
Radiation, μ mm
FIJ000)
Crystal size per
2 × 0.05 × 0.05
0.30 × 0.08 × 0.05
0.15 × 0.05 × 0.03
0.10 × 0.07 × 0.05
mm3
2Θ maximum/°
Index ranges
136
136
136
136
−6 ≤ h ≤ 6, −22 ≤ k ≤ 22, −5 ≤ h ≤ 6, −18 ≤ k ≤ 18,
−34 ≤ h ≤ 32, −6 ≤
k ≤ 6, −22 ≤ l ≤ 34
10 916 (99.4%)
−3 ≤ h ≤ 6, −18 ≤ k ≤ 18,
−16 ≤ l ≤ 20
−26 ≤ l ≤ 19
−25 ≤ l ≤ 26
Total reflections
(%)
10 205 (98.1%)
10 786 (98.7%)
10 612 (97.9%)
Data quality
[Rint = 0.0245,
Rsigma = 0.0417]
[Rint = 0.0159,
Rsigma = 0.0261]
6743/2/545
[Rint = 0.0238,
Rsigma = 0.0371]
7008/2/564
[Rint = 0.0292,
Rsigma = 0.0444]
6701/1/518
Data/restr./params. 6296/7/569
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Table 1 (continued)
Goodness-of-fit F2 1.011
1.010
1.039
1.042
Final R indexes
[I > = 2σ(I)]
Final R indexes
[all data]
R1 = 0.0300,
R1 = 0.0281,
wR2 = 0.0711
R1 = 0.0291,
wR2 = 0.0719
0.35/−0.17
R1 = 0.0364,
wR2 = 0.0950
R1 = 0.0396,
wR2 = 0.0971
0.22/−0.16
R1 = 0.0409,
wR2 = 0.1026
R1 = 0.0434,
wR2 = 0.1045
0.34/−0.24
wR2 = 0.0702
R1 = 0.0322,
wR2 = 0.0714
0.15/−0.18
Diff. peak/hole
eÅ−3
Flack parameter
0.10(8)
−0.17IJ6)
0.19(15)
0.08(15)
Table 2 Key structural parameters in selected [B{L-TarIJNHPh)2}2]− anions
2 (K)
4 (Na)·2MeOH
1.466–1.478IJ2)
5 (NH3CHMeEt)
7 (NH3CHMePh)·MeOH
B–O lengths
1.460–1.479IJ3)
−4, 0, 4, −6, 6: 5
4, 0, −4, 7, −7: 5
9.3/10.0
1.450–1.485IJ3)
−7, 9, −7, 2, 3: 6
−7, 9, −7, 3, 2: 6
8.6/9.5
1.467–1.474IJ2)
5, 3, −9, 12, −11: 9
9, −1, −6, 12, −14: 10
19.0/−17.0
5MR dIJÅx100)
B–O–C–C–O: rms
Tors OCCO 1/2
Tors OCCO 3/4
Tors OCCO 5/6
Tors OCNC 7/8
Tors OCNC 9/10
Tors CCNC 11/12
Tors CCNC 13/14
Chelate OBO
−4, −5, 11, −14, 12: 10
−3, −7, 14, −17, 13: 12
23.4/−28.2
6.2/9.3
7.7/27.1
5.5/7.0
3.5/28.1
−175.7/173.6
157.9/−179.0
174.8/172.2
−5.7/5.1
166.3/176.6
3.6/−6.1
−7.1/−8.8
8.3/−0.1
9.6/−3.1
0.0/4.0
1.7/1.1
2.1/0.2
−3.3/9.8
−9.7/−8.7
−12.6/−5.8
−7.7/−10.0
20.4/−2.0
−21.2/−7.7
−29.0/−12.2
104.5/105.4
110.9 NH/110.4
−19.3/−26.2
104.7/104.4
112.8 NH/111.5
105.1/105.0
109.8, 110.0 (K)/111.5, 112.8
105.6/104.2
111.7 (Na)/112.7
Jaw OBO
Chiral HPLC
and energy calculations were then carried out with 2-fold
symmetry constraints (C2) for both “X- and Y-forms” using
Becke3LYP functional32 within Gaussian09 software pack-
age.33 A 6-31G* basis set was used for all atoms.56
The 1-phenylethylamine free base were isolated from the salt
7 by a similar procedure to that published for organoamm-
onium salts of [BMan2].5 The salt was treated with aq. NaOH
(25%) and extracted (3×) into chloroform. The combined ex-
tracts were evaporated and recrystallized with methanol
affording ca. 90% of the R-/S-1-phenylethylaminefree bases. A
derivatization of these to their benzamide derivatives
PhCONHCHMePh was carried out to allow chiral column
chromatography and was by modification of a published pro-
cedure through reaction of the free base with benzoyl chlo-
ride (reaction in CH2Cl2 2 h, 0 °C with NEt3).55
Chiral chromatography was carried out on the phenyl-
amide derivatives of R- and S-1-phenylethylamines obtained
from compound 7, the corresponding phenylamides of
1-phenylpropylamines derived from compound 8 and finally
the doubly derivatized forms of phenylglycinol obtained from
compound 9.
Conclusions
In summary the N,N′-diphenyl-L-tartramide derived bis-
(tartramido)borate anions [B{L-TarIJNHPh)2}2]− have been
shown to be readily prepared and crystallized. The anions
have reasonably well-defined conformational preferences in
several salts within the solid state. The anions in the
unsolvated potassium salt 2 and its isostructural NH4 salt 3
differ slightly from those in the methanol-solvated sodium
analogue 4 through the intervention of methanol into the
intra-molecular H-bonds between amide functionalities on
each ligand arm. The anions in 2–4, have a tetradentate cat-
ion binding site.
Accurate determination of enantiomeric excess (% ee) was
carried out using CHIRALPAK® AD-H or IC columns (Daicel
Chemical Industries Ltd. and Chiral Technologies, Europe) –
a 5 μm silica gel coated with cellulose or amylose trisIJ3,5-
dimethylphenyl carbamate). Elutions were ‘normal phase’
with either 80:20 or 85:15 mixtures of hexane:i-propanol.
Similar ‘Y-shaped’ anion conformations are preserved in
the organoammonium salts 5–9, which indicate such anions
have considerable promise for resolution. The salts isolated
from several racemic amines exhibited high levels of enantio-
purity. Salts from rac-1-phenylpropylamine (91% ee) and
phenylglycinol (95% ee) were essentially directly resolved in
one step. Three disorder modes H/Me site exchange, Me/Et
site exchange and C–H repyramidalization at the cation sites
limited the resolution efficiency in other cases. However con-
siderable enantiopreference was found even in these solid so-
lutions. Such promising results demonstrate the potential of
bisIJaryltartramido) borate anions to resolution of even chal-
lenging cases involving cations with high enantiosimilarity.
Computation
DFT calculations on [B{L-TarIJNHPh)2}2]−
Approximate starting molecular conformations were derived
directly from the single crystal structure of 2 and constructed
by application of 2-fold symmetry operations. Optimization
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Conflicts of interest
12 D. Horvath, G. Marcou and A. Varnek, J. Chem. Inf. Model.,
2013, 53, 1543–1562; P. Willett, Mol. Inf., 2014, 33, 403–413;
W.-H. Shin, X. Zhu, M. G. Bures and D. Kihara, Molecules,
2015, 20, 12841–12862.
There are no conflicts to declare.
Acknowledgements
13 A. I. Kitaigorodsky, Mixed crystals, Springer Science
&
Business Media, 2012, vol. 33.
The authors gratefully acknowledge funding of this work by
the Research Grants Council of Hong Kong (grant numbers
604305, 605511 and 16306515).
14 A. K. S. Romasanta, D. Braga, M. T. Duarte and F. Grepioni,
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