Oxazoles XXI. Three ester ammonium salts resulting from the rupture of an oxazoline ring

Article abstract of DOI:10.1007/s10870-009-9645-6

The synthesis and characterisation of three ammonium salts that are formed via a ring opening of a 2-oxazoline (i.e., a 4,5-dihydro-1,3-oxazole) are described. Compound 1, derived from 2-phenyl-2-oxazoline (Phox), was isolated as a major reaction product during an unrelated synthesis using Phox which was being carried out in the presence of NO₃ ^- ions. The isolated compound is identified as the nitrate salt of the terminal ammonium ester [H₃NCH₂CH₂OC(=O)Ph]⁺ (1). The X-ray crystal structure (C₉H₁₂N₂O₅) of 1 is revealed to be orthorhombic (space group Pna2₁) with a = 21.1304(8), b = 7.5975(2) and c = 6.8629(3)A. The analogous material 2a, obtained from the related oxazole 4,4-dimethyl-2-phenyl-2-oxazoline (MePhox) under identical conditions to that of 1, contains the [H₃NCMe ₂CH₂OC(=O)Ph]⁺ cation. The X-ray crystal structure (C₁₁H₁₆N₂O₅) of 2a is monoclinic with a = 6.1270(3), b = 6.3603(3), c = 16.1632(8)A and β = 90.662(3)° and of space group P2₁. Chiral derivative 2b, was isolated from a wet CH₂Cl₂/THF solution upon treatment of MePhox with (S)-mandelic acid. This material was shown to also contain the [H₃NCMe₂CH₂OC(=O)Ph]⁺ cation, a lattice water molecule and an (S)-mandalate anion. The X-ray crystal structure of 2b (C₁₉H₂₃NO₅?H₂O) is orthorhombic (space group P2₁2₁2₁) with a = 6.4733(3), b = 11.4661(5) and c = 24.5791(11)A. One proposed mechanism by which all three materials are formed was first presented in seminal work by Deslongchamps. The compounds reported herein are representatives of a class of rarely isolated intermediates in heterocyclic ring opening reactions and they are also the first examples of (ω-ammoniumethyl)(aryl) esters to be structurally characterised.

Full text of DOI:10.1007/s10870-009-9645-6

J Chem Crystallogr (2010) 40:272–277
DOI 10.1007/s10870-009-9645-6
ORIGINAL PAPER
Oxazoles XXI. Three Ester Ammonium Salts Resulting
from the Rupture of an Oxazoline Ring
•
Robert A. Gossage Hilary A. Jenkins
•
J. Wilson Quail
Received: 21 October 2008 / Accepted: 1 October 2009 / Published online: 21 October 2009
Ó Springer Science+Business Media, LLC 2009
Abstract The synthesis and characterisation of three
ammonium salts that are formed via a ring opening of a
2-oxazoline (i.e., a 4,5-dihydro-1,3-oxazole) are described.
Compound 1, derived from 2-phenyl-2-oxazoline (Phox),
was isolated as a major reaction product during an unrelated
synthesis using Phox which was being carried out in the
presence of NO3- ions. The isolated compound is identified
as the nitrate salt of the terminal ammonium ester
[H3NCH2CH2OC(=O)Ph]? (1). The X-ray crystal structure
(C9H12N2O5) of 1 is revealed to be orthorhombic (space
group Pna21) with a = 21.1304(8), b = 7.5975(2) and
(space group P212121) with a = 6.4733(3), b = 11.4661(5)
˚
and c = 24.5791(11)A. One proposed mechanism by which
all three materials are formed was first presented in seminal
work by Deslongchamps. The compounds reported herein
are representatives of a class of rarely isolated intermediates
in heterocyclic ring opening reactions and they are also the
first examples of (x-ammoniumethyl)(aryl) esters to be
structurally characterised.
Keywords Oxazole Á Oxazoline Á Ring-opening Á
Ester Á X-ray crystal structure
˚
c = 6.8629(3)A. The analogous material 2a, obtained from
the related oxazole 4,4-dimethyl-2-phenyl-2-oxazoline
(MePhox) under identical conditions to that of 1, contains the
[H3NCMe2CH2OC(=O)Ph]? cation. The X-ray crystal
structure (C11H16N2O5) of 2a is monoclinic with a =
Introduction
Beginning in the 1970’s, Deslongchamps and co-workers
published a series of seminal papers detailing the concept of
the ‘‘Stereoelectronic Effect’’ in organic syntheses [1–17].
The investigations of these effects, both synthetic and the-
oretical, lead to the publication of a well-known monograph
on the subject [18] and indeed such ideas are still discussed at
length today [19–24]. One class of reactions that was
investigated by Deslongchamps involved the exploration of
stereoelectronic effects on the classic ring-opening of five-
membered 2-oxazoline (i.e., 4,5-dihydro-1,3-oxazole) rings
[25–28] and their six-membered congeners: the 1,3-oxazines
[29–33]. These heterocycles readily form iminium salts
upon treatment with alkyl- and aryl-halides or strong acids
(R–X: Scheme 1). In some cases, these salts are stable
entities although, somewhat surprisingly, few have actually
been isolated. The properties of the resulting nucleophile
X- (derived from R–X) can predispose the material towards
ring-opening polymerisation (Pathway One: Scheme 1)
[34–39]. An alternative outcome (Two: Scheme 1) involves
nucleophilic attack of the oxazolium intermediate by a
˚
6.1270(3), b = 6.3603(3), c = 16.1632(8)A and b =
90.662(3)8 and of space group P21. Chiral derivative 2b, was
isolated from a wet CH2Cl2/THF solution upon treatment of
MePhox with (S)-mandelic acid. This material was shown to
also contain the [H3NCMe2CH2OC(=O)Ph]? cation, a lat-
tice water molecule and an (S)-mandalate anion. The X-ray
crystal structure of 2b (C19H23NO5ÁH2O) is orthorhombic
R. A. Gossage (&)
Department of Chemistry and Biology, Ryerson University,
350 Victoria Street, Toronto, ON M5K 2B3, Canada
e-mail: gossage@ryerson.ca
URL: http://oxazoles.ca
H. A. Jenkins
Department of Chemistry, MAX Diffraction Facility,
McMaster University, Hamilton, ON L8S 4M1, Canada
J. W. Quail
Department of Chemistry, University of Saskatchewan,
110 Science Place, Saskatoon, SK S7N 5C9, Canada
123

J Chem Crystallogr (2010) 40:272–277
273
second nucleophile (e.g., water: Scheme 1), which eventu-
ally leads to an ammonium intermediate (‘‘I’’) following the
ring opening reaction. This latter reaction pathway leads
exclusively to the ester-ammonium presumably under the
influence of stereo-electronic control [1–17, 29–33] and
other factors [18–24]. Deslongchamps has provided spec-
troscopic evidence for the formation of salt materials such as
I during the conversion of a number of heterocyclic ring
systems into their respective open chain forms. To our
knowledge, such an ester-ammonium intermediate has never
been crystallographically studied. In this manuscript, we
report the formation, characterisation (NMR, X-ray dif-
fraction) and isolation of three such species. These com-
pounds are formed in the presence of a strong or a weak acid
[40, 41] which independently initiate oxazolium formation
that allows for subsequent nucleophilic attack by opportu-
nistic water and subsequent rupturing of the ring system.
were also used as purchased from commercial sources.
Samples of (?)-(S)-Mandelic acid were kindly supplied by
Sepracor Canada Ltd. (Windsor, NS). [Bi(NO3)3(dmso)3]
1
was produced by a published method [42]. H NMR data
were recorded on a Bruker Avance 300 NMR spectrometer
operating at 300 MHz at room temperature. Elemental
analyses were carried out by L.U.C.A.S. (Thunder Bay,
ON) and the analytical services department of the Uni-
versity of Saskatchewan.
Synthesis of Compound 1
A sample of [Bi(NO3)3(dmso)3] (0.3403 g: 0.5406 mmol)
and 2-phenyl-2-oxazoline (0.24 g: 1.6 mmol) were dis-
solved together in MeOH (7 mL) and stirred at RT over-
night. The mixture was thereafter filtered and the MeOH
solution treated with Et2O (25 mL), sealed and cooled in a
freezer. However, no solid was obtained. The mixture was
then allowed to slowly evaporate after warming to RT in an
open 200 mL beaker. The resulting crystalline solid that
formed over a period of approximately 21 days (0.30 g:
82%) was collected by filtration. mp 81–85 °C (decomp.).
Anal. Calc. for C9H12N2O5 (%): C: 47.37; H: 5.30; N:
12.28; found: C: 47.27; H: 5.11; N: 12.43. NMR: 1H NMR
(MeOH-d4): d = 8.12 (d, J = 5.1 Hz, 2H, ArH), 7.63 (m,
1H, ArH), 7.52 (m, 2H, ArH), 4.58 (t, J = 3.6 Hz, 2H,
CH2), 3.42 (t, 2H, J = 3.6 Hz, CH2).
Experimental
General
All manipulations were carried out in air using reagent
grade solvents that were used as received. The oxazolines
Synthesis of Compound 2a
An analogous procedure as that listed above for compound
1 was employed. mp 147–149 °C (decomp.). Anal. Calc.
for C11H16N2O5Á‘H2O (%): C: 49.81; H: 6.46; N: 10.56;
1
found: C: 49.45; H: 6.15; N: 11.07. H NMR (dmso-d6):
d = 8.19 (s, br, 3H, -NH3), 8.13 (d, 2H, J = 7.5 Hz,
ArH), 7.69 (t, 1H, J = 7.8 Hz, ArH), 7.56 (t, 2H, ArH),
4.27 (s, 2H, OCH2), 1.36 (s, 6H, CH3).
Synthesis of Compound 2b
A sample of (?)-(S)-mandelic acid (0.50 g: 3.3 mmol) and
4,4-dimethyl-2-phenyl-2-oxazoline (0.5 mL: 3 mmol)
were dissolved together in a mixture of 1:1 CH2Cl2 and
THF (50 mL total volume). The resulting colourless mix-
ture was allowed to slowly evaporate in open air at RT in a
150 mL beaker. The resulting crystalline solid that formed
over a period of several days was collected by filtration. mp
151–153 °C. 1H NMR (MeOH-d4): d = 8.13 (m, 2H,
ArH), 7.64 (m, 1H, ArH), 7.49 (m, 4H, ArH), 7.28 (m, 3H,
ArH), 4.92 (s, 4H, -OH ? MeOH), 4.84 (s, 1H, CH), 4.33
(s, 2H, OCH2), 1.42 (s, 6H, CH3).
Scheme 1 Possible reaction pathways for protonated/alkylated
oxazolines
123

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J Chem Crystallogr (2010) 40:272–277
X-ray Crystallography1
Compound 1
Data was collected at -100 °C on a Nonius Kappa CCD
diffractometer, using the COLLECT program [43]. Cell
refinement and data reductions used the programs DENZO
and SCALEPACK [44]. SIR97 [45] was used to solve the
structure and SHELXL97 [46] was used to refine the structure.
ORTEP-3 for Windows [47] was used for molecular graphics
and PLATON [48] was used to prepare material for publica-
tion. H atoms were placed in calculated positions with Uiso
constrained to be 1.2 times Ueq of the carrier atom for all
hydrogen atoms.
Scheme 2 General reaction scheme for the compounds isolated
herein
In these two cases, it is likely that HNO3 has protonated the
oxazoline which has then undergone water induced ring-
opening to yield the resulting ammonium-ester salts 1 or 2a
[56]. In contrast, compound 2b was produced as a result of
a direct attempt at acid–base (i.e., mandelic acid-oxazoline)
complex formation between the weak acid and the oxazo-
line; likewise a similar ring-opening reaction as described
above has occurred [56]. The syntheses of compounds 1, 2a
and 2b are shown schematically in Scheme 2. These
materials are the first examples of (x-ammoniumeth-
yl)(aryl) esters to be structurally characterised and repre-
sent rare isolated examples [56] of the spectroscopically
characterised intermediates in oxazoline ring-opening
chemistry [25–28].
N2 is the central atom in a nitrate ion, and sitting at the
centre of gravity of the moiety. The oxygen atoms can
vibrate or move without moving the N atom. As a result,
the N has a much lower Ueq than the attached O atoms.
As the molecule contains only C, H, N and O atoms,
Friedel mates were merged before refinement. The mole-
cule has no asymmetry, but happens to crystallise in an
acentric space group.
The structure of 1 consists of a discrete nitrate (formal)
mono-anion and the ammonium cation (Ph–CO2–
CH2CH2NH3)? (Fig. 1). The crystal system is determined to
be orthorhombic and of space group Pna21 (see the Table 1).
The anion and cation are in relatively close proximity with an
Compound 2a
Crystals of compound 2a were examined as described
above for 1 with the following additional comments. A
long H-bond from H1B to N2 is flagged in the checkCIF
ALERTs. It is indeed long, but on the edge of observed
H-bonds. A similar H-bond is also observed in 1.
˚
O5ÁÁÁH1BN1 distance of less than 1.93 A (see CCDC
#705773); in addition one of the nitrate oxygen atoms (O3) is
in close contact with the H2 atom on aromatic carbon C2
˚
(H2ÁÁÁC2 = *2.67 A). There are also complimentary
Compound 2b
intermolecular interactions between the carbonyl oxygen
The crystallographic examination of compound 2b was
carried out as described in our previous publications
[49–53].
Results and Discussion
Bismuth salts are known to produce traces of acid in the
presence of potential donor ligands. This propensity often
leads to the formation of multi-valent bismuth anion
complexes which are in turn supplemented by protonated
ligand counterions [42, 54, 55]. Compounds 1 and 2a were
isolated as the major products (serendipitously) during the
attempted syntheses of this class of Bi-oxazoline materials.
1
CCDC #705773-705775 (inclusive) contains the supplementary
crystallographic data for this manuscript. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif
Fig. 1 ORTEP representation of 1; thermal ellipsoids are shown at the
50% probability level
123

J Chem Crystallogr (2010) 40:272–277
275
Table 1 Crystallographic data of 1, 2a and 2b
Complex 1
Complex 2a
Complex 2b
CCDC number
705773
705774
705775
Empirical formula
Formula weight
C9H12N2O5
228.21
C11H16N2O5
256.26
C19H25NO6
363.40
Temperature (K)
173 (2)
173 (2)
198 (2)
Crystal system, space group
Calculated density (g/cm-3
Orthorhombic, Pna21
1.376
Monoclinic, P21
1.351
Orthorhombic, P212121
1.323
)
Absorption coefficient (mm-1
)
0.114
0.107
0.098
˚
a (A)
21.1304(8)
7.5975(2)
6.8629(3)
6.1270(3)
6.4733(3)
11.4661(5)
24.5791(11)
˚
b (A)
6.6303(3)
˚
c (A)
16.1632(8)
b (8)
˚
V (A
90.662(3)
-3
)
1101.76(7)
629.83(5)
1824.35(14)
Z
4
2
4
Crystal size (mm3)
0.22 9 0.18 9 0.10
480
0.22 9 0.18 9 0.04
272
0.25 9 0.2 9 0.18
776
F (000)
˚
Wavelength (A)
0.71073
Full matrix least-squares on F2
0.71073
0.71073
Full matrix least-squares on F2
Refinement
Full matrix least-squares on F2
3.33–27.570
-7 B h B 7
-8 B k B 8
-20 B l B 21
1572/1449
h range for data collection (8)
Index ranges
2.85–26.96
1.66–27.49
-26 B h B 26
-9 B k B 9
-8 B l B 8
-8 B h B 8
-13 B k B 14
-31 B l B 28
Reflections collected/unique
Absorption correction
Completeness to h
Rint
2237/1276
12793/3847
None
None
Semi-empirical from equivalents
h = 27.498 (99.0%)
0.0249
h = 26.968 (98.4%)
0.0288
h = 27.578 (99.4%)
0.0208
Final R indices [Iobs [ 2r(Iobs)]
R indices (all data)
R1 = 0.0376; wR2 = 0.0881
R1 = 0.0467; wR2 = 0.0947
1276/1/146
R1 = 0.0327; wR2 = 0.0823
R1 = 0.0377; wR2 = 0.0858
1572/1/167
1.073
R1 = 0.0282; wR2 = 0.0699
R1 = 0.0310; wR2 = 0.0726
3847/0/335
Data/restraints/parameters
Goodness-of-fit on F2
1.096
1.074
3
˚
Largest diff. peak and hole, e/A
0.154 and -0.185
0.164 and -0.149
0.297 and -0.141
(O2) and aromatic the hydrogen (H6) located on C6
˚
extensive 2-D network of intermolecular nitrate–ammo-
nium interactions, again separated by the hydrophobic
aromatic/aliphatic layers.
(O2ÁÁÁH6 = *2.54 A). The nitrate group is essentially pla-
nar as expected. The torsion angle between the aromatic ring
and the carbonyl group (C2–C1–C7–O1) is a narrow 1.348
forming a virtually planar PhCO2-unit with close O1–H2
Such cation–anion attractive forces appear to be com-
pletely absent in 2b, however. The mandalate and the
ammonium-ester ions are far removed from one another,
perhaps due to the steric bulk of the large anion (Fig. 3). A
lattice H2O molecule is also found in the structure of 2b
which is shown to be orthorhombic and of space group
P212121 (CCDC #705775). The water molecule forms a
strong H-bond with the mandalate –OH group in which the
˚
contacts (*2.42 A). Other bond lengths and angles are
typical [57]. The global structure appears to have extended
2-D structure consisting of H-bonded ammonium groups and
nitrate anions which are separated by a layer containing the
aromatic and aliphatic portion of the cations.
In material 2a (monoclinic; space group P21: CCDC
#705774), planarity of the PhCO2-group is not observed
(torsion[248) presumably due to the presence of sterically
demanding methyl groups on the aliphatic chain (Fig. 2).
Close contacts of the nitrate and ammonium groups are still
˚
intermolecular H4DÁÁÁO7 separation is *1.99 A. There are
also close cation–anion interactions (e.g., H1BÁÁÁO3:
˚
*1.93 A). The structures of several related mandalate-
containing materials have been previously reported and the
˚
present (O4–H1C distance of *2.04 A) as is a similar
results here are unsurprisingly in this regard [58–63].
123

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J Chem Crystallogr (2010) 40:272–277
donation of (?)-(S)-mandelic acid. The authors were greatly assisted
by the anonymous reviewers of this manuscript who provided a very
helpful critique of this work.
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