The π-Electron Delocalization in 2-Oxazolines Revisited: Quantification and Comparison with Its Analogue in Esters.

The single crystal X-ray analysis of the ester-functionalized 2-oxazoline, methyl 3-(4,5-dihydrooxazol-2-yl)propanoate, revealed π-electron delocalization along the N-C-O segment in the 2-oxazoline pentacycle to significant extent, which is comparable to its counterpart along the O-C-O segment in the ester. Quantum chemical calculations based on the experimental X-ray geometry of the molecule supported the conjecture that the N-C-O segment has a delocalized electronic structure similar to an ester group. The calculated bond orders were 1.97 and 1.10 for the N=C and C-O bonds, and the computed partial charges for the nitrogen and oxygen atoms of -0.43 and -0.44 were almost identical. In the ester group, the bond orders were 1.94 and 1.18 for the C-O bonds, while the partial charges of the oxygen atom are -0.49 and -0.41, which demonstrates the similar electronic structure of the N-C-O and O-C-O segments. In 2-oxazolines, despite the higher electronegativity of the oxygen atom (compared to the nitrogen atom), the charges of the hetero atoms oxygen and nitrogen are equalized due to the delocalization, and it also means that a cationic attack on the nitrogen is possible, enabling regioselectivity during the initiation of the cationic ring-opening polymerization of 2-oxazoline monomers, which is a prerequisite for the synthesis of materials with well-defined structures.

1. Introduction

Since the introduction of microwave reactors dedicatedly designed for usage in laboratories, considering the chemists’ and material scientists’ requirements [1,2,3,4], the class of poly(2-oxazoline)s has been reawakened from its hibernation (Scheme 1): discovered in 1966/67 by four research groups almost simultaneously [5,6,7,8], the versatility of this class of polymers, and its derived materials, has been under constant investigation, while the commonly-low polymerization rates have been the bottleneck for a long time in terms of applicability of these materials. With the exclusion of acceleration-limiting factors such as temperatures (boiling points), microwave reactors have helped to successfully accelerate the polymerizations of 2-oxazolines, and, currently, they are under thorough investigation, in particular, for usage in medical and sanitary applications [9,10,11,12,13,14,15] and as cross-linkable materials [16,17,18,19,20,21,22].

Scheme 1

Methyl tosylate-initiated polymerization of 2-oxazolines.

In particular for medical and medicinal applications, a precise knowledge of the materials’ structures is of key importance, which makes living or at least pseudo-living polymerizations and their inherent access to polymers with narrow molecular weight distributions favorite synthetic strategies. For the polymerization of 2-oxazolines, it has been shown that the highly reactive methyl tosylate is one of the initiators that can start pseudo-living polymerizations [23,24,25]. Due to its high reactivity, it has been argued whether the initiation by methyl cations occurs regioselectively at the nitrogen atom (Scheme 1) when the polymerization times experience accelerations by a factor of up to 400 [26].

In a precedent study [27], we could show that, due to π-electron delocalization, the partial negative charge at the oxygen atom of the 2-oxazoline ring is lessened (Scheme 2). The negative charge of the nitrogen atom, on the other hand, is enhanced and, hence, the nitrogen atom is an ideal reaction partner for the methyl tosylate. In order to expand the understanding of this π-electron delocalization in 2-oxazolines, we aimed for a correlation/comparison with its counterparts in esters, where the C–O “single” bond as well has been reported to show an intermediate value between that of a C–O double and single bond. In this study, we therefore present the single crystal X-ray analysis of an ester-functionalized 2-oxazoline and the corresponding ring-opened ester-functionalized amino acid.

Scheme 2

Delocalization of π-electrons in 2-oxazolines (left) and esters (right).

2. Experimental Section

2.1. Materials

All chemicals were used as received. Chloroethylamine hydrochloride, hydrochloric acid solution (0.1 mol/L), methyl chlorobutyrate, triethylamin, and calcium carbonate were purchased from Sigma Aldrich (Vienna, Austria), while ethanol, methanol, dichloromethane, and sodium hydroxide were bought from Carl Roth (Karlsruhe, Germany).

2.2. Instrumentation

IR spectra were recorded with 48 scans per sample on a Bruker Alpha FT-IR spectrometer (Bruker Optics Inc., Billerica, MA, USA) equipped with the ALPHA’s Platinum attenuated total reflection (ATR) single reflection diamond ATR module. The spectral range was set from 500 to 4000 cm−1. 1H NMR spectra were measured in deuterated chloroform or deuterium dioxide on a Bruker 300 MHz spectrometer (Bruker BioSpin Corporation, Billerica, MA, USA) with 32 scans and relaxation delays of 5 s. The solvent residual peaks were used for referencing the spectra to 7.26 ppm and 4.80 ppm, respectively.

2.3. Single Crystal X-ray Diffraction Analyses

The crystalline samples were placed in inert oil, mounted on a glass pin and transferred to the cold gas stream of the diffractometer. Crystal data were collected and integrated with a Bruker APEX-II CCD system (Bruker AXS GmbH, Karlsruhe, Germany) with monochromated Mo-Kα (λ = 0.71073 Å) radiation at 100(2) K. The structures were solved by direct methods using SHELXS-97 [28] and refined by full matrix least squares calculations on F2 with SHELXL-97 [29]. The space group assignments and structural solutions were evaluated using PLATON [30]. Non-H-atoms were refined with anisotropic thermal parameters. All protons located on carbon atoms were calculated and allowed to ride on their parent atoms with fixed isotropic contributions; protons on nitrogen atoms were located and refined with isotropic contributions. Extinction corrections were applied for all compounds using SADABS [31]. A summary of the crystal data, experimental details and refinement results is listed in Table 1. Important interatomic distances and angles are given in the figure captions. Thermal parameters and complete tables of interatomic distances and angles have been deposited with the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1 EZ, UK. The data are available on request on quoting CCDS-1418758/1418759 and in the Supporting Information.

Table 1

Crystal data, data collection, absorption and structure refinement of methyl 3-(4,5-dihydrooxazol-2-yl)propanoate EstOx (ester-functionalized 2-oxazoline) and 4-(2-aminoethoxy)-4-oxobutanoic acid EstAA (ester-functionalized amino acid).

Crystal Structure Analysis	Methyl 3-(4,5-dihydrooxazol-2-yl) Propanoate, EstOx	4-(2-Aminoethoxy)-4-oxo-butanoic Acid, EstAA
Crystal data
CCDC No.	1418758	1418759
Crystal form	block	block
Crystal colour	colorless	colorless
Empirical formula	C7H11NO3	C6H11NO4
Formula weight	157.17	161.16
Crystal system	monoclinic	monoclinic
Space group	P21	P21/c
a (Å)	5.547 (2)	10.3132 (8)
b (Å)	6.765 (3)	9.0304 (7)
c (Å)	9.993 (4)	8.0012 (7)
α (°)	90	90
β (°)	91.583 (13)	96.688 (5)
γ (°)	90	90
V (Å3)	374.9 (3)	740.10 (10)
ρcalc (g·cm−3)	1.268	1.446
Z	2	4
F(0 0 0)	152	344
µ (Mo-Kα) (cm−1)	0.102	0.122
Data Collection
Measured reflections	4559	1786
Unique reflections	1278	1786
Rint	0.0659	0.000
Absorption
Tmin/Tmax	0.9690/0.9808	0.9574/0.9855
Refinement Results
Refined parameters	101	101
R1 a,b, wR2 a;c	0.0530; 0.1352	0.0630; 0.1432
a, b	0.0887; 0.0457	0,000; 2.2816
ρ (e·Å−3)	0.306; −0.219	0.362; −0.325

a I > 2σ(I); b R1 = Σ(||Fo| − |Fc||/Σ|Fo|; c wR2 = {[Σw(Fo2 − Fc2)2]/Σ[w(Fo2)2]}0.5; w = 1/[σ2(Fo2) + (ap)2 + bp]; p = (Fo2 + 2Fc2)/3; a and b: free variables.

2.4. Preparation of Methyl 3-(4,5-Dihydrooxazol-2-yl)propanoate EstOx

Ten millilitres (0.13 mol) of methyl 4-chloro-4-oxobutanoate and 15.41 g (0.13 mol) of chloroethylamine hydrochloride were dissolved in 130 mL of dichloromethane under inert conditions and cooled to 0 °C. Forty-one millilitres of triethylamine, dissolved in 20 mL of dichloromethane, were added dropwise within 1 h, and the reaction mixture was stirred overnight. The organic phase was extracted twice with deionized water and once with brine, prior to drying with sodium sulphate. Subsequently, the solvent was removed under reduced pressure. 17.40 g of the dry intermediate product were recovered (0.09 mol, 69% yield). 9.537 g (1 equiv.) of sodium carbonate were added and the mixture was stirred overnight under reduced pressure. The mixture was subsequently filtrated. 7.11 g (0.063 mol, 48% yield) of the final product were recovered as colorless liquid by distillation under reduced pressure. The product crystallized at 6 °C.

NMR (300 MHz, CDCl3): δ (ppm) = 2.55 (2 H, t, 3JH–H = 6.6 Hz, H4a and H4b), 2.64 (2 H, t, 3JH–H = 6.6 Hz, H5a and H5b), 3.67 (3 H, s, H7a, H7b and H7c), 3.78 (2 H, t, 3JH–H = 9.3 Hz, H3a and H3b), 4.21 (2 H, t, 3JH–H = 9.3 Hz, H2a and H2b) (for atomic labelling, see Figure 1).

Figure 1

Asymmetric unit in the crystalline structure of EstOx (ORTEP drawing [33] with 50% probability ellipsoids). Selected bond lengths (Å): C1–N1: 1.263(4), C1–O1: 1.376(3), C2–O1: 1.458(3) C3–N1: 1.483(4), C2–C3: 1.529(4), C1–C4: 1.497(3), C4–C5: 1.515(4), C5–C6: 1.507(3), C6–O2: 1.211(3), C6–O3: 1.338(3), C7–O3: 1.448(3). Selected bond angles (°): N1–C1–O1: 118.6(2), N1–C1–C4: 128.9(2), O1–C1–C4: 112.5(2), O1–C2–C3: 104.0(2), N1–C3–C2: 105.1(2).

IR (ATR, cm−1): ν = 2985 m, 2948 m, 2906 m, 2884 m, νstr (CH); 1734 s, νstr (C=O); 1669 s, νstr (N=C); 1438 m, νstr (CO); 1364 m, 1350 m, νdef (CH3); 1204 m, 1161 s, νasym str (COCH3); 657 w, νbend (COCH3); 583 w, νdef (CH2COCH3).

2.5. Preparation of 4-(2-Aminoethoxy)-4-oxobutanoic Acid EstAA

A solution of 0.5 g (0.0032 mol, 1 equiv.) of EstOx in 50 mL of methanol and 32 mL of an aqueous solution of sodium hydroxide (0.1 mol/L) were mixed and stirred for 1 h. The solvents were subsequently removed under reduced pressure. The crude product was dissolved in methanol, and 0.0032 mol of hydrochloric acid (aqueous 0.1 M solution) were added. After 10 min of stirring under reflux conditions, the mixture was stored overnight at 6 °C. The solvents were removed under reduced pressure, and 0.467 g (0.0029 mol, 91% yield) of the final product were recovered by recrystallization from ethanol.

1H NMR (20 °C, CDCl3, 300 MHz): δ (ppm) = 2.50 (2 H, t, 3JH–H = 6.5 Hz, H3a and H3b), 2.66 (2 H, t, 3JH–H = 6.5 Hz, H2a and H2b), 3.34 (2 H, t, 3JH–H = 5.1 Hz, H6a and H6b), 4.38 (2 H, t, 3JH–H = 5.1 Hz, H5a and H5b) (for atomic labelling, see Figure 3).

Figure 3

Asymmetric unit in the crystalline structure of EstAA (ORTEP drawing [33] with 50% probability ellipsoids). Selected bond lengths (Å): C1–O1: 1.251(3), C1–O2: 1.268(3), C4–O3: 1.205(4), C4–O4:1.354(3), C5–O4: 1.447(3), C6–N1: 1.489(3). Selected bond angles (°): O1–C1–C2 118.1(2), O3–C4–O4: 122.8(3), C1–C2–C3: 113.2(2).

IR (ATR, cm−1): ν = 3438 m, νstr (NH2); 2986 w, 2961 w, 2925 w, 2848 w, νstr (CH); 2524 m νstr (OH); 1729 s, νstr (C=O); 1611 s, 1571 s, νdef (NH2); 1312 s, νdef (OH); 1248 s, 1155 s, νasym str (COC); 1013 s, νstr(NH2); 957 m, ν(COOH).

3. Results and Discussion

3.1. Synthesis of the Compounds EstOx and EstAA

Methyl 3-(4,5-dihydrooxazol-2-yl)propanoate EstOx can be prepared from the reaction of methyl 4-chloro-4-oxobutanoate and chloroethylamine hydrochloride and subsequent ring-closure under alkaline conditions (Scheme 3, top), following a literature protocol [32]. Hydrolyses of the ester bond and the 2-oxazoline ring yield the ester-functionalized amino acid 4-(2-aminoethoxy)-4-oxobutanoic acid EstAA (Scheme 3, bottom). Single crystals from both compounds were grown at 6 °C.

Scheme 3

Synthesis of EstOx from methyl 4-chloro-4-oxobutanoate and chloroethylamine hydrochloride (top) as well as hydrolyses of the 2-oxazoline under alkaline and acidic conditions yielding EstAA (bottom).

3.2. Crystal Structure of EstOx

EstOx crystallizes in the monoclinic space group P21 with Z = 2 formula units in the unit cell. The asymmetric unit contains 1 formula unit (Figure 1). A detailed analysis of the dihedral angles reveals that the 2-oxazoline C3N1O1-pentacycle is almost planar [O1–C2–C3–N1: −4.2(3)°, N1–C1–O1–C2: −1.8(4)°, C3–C2–O1–C1: 3.6(3)°, O1–C1–N1–C3: −1.1(4)°, C2–C3–N1–C1: 3.3(3)°], and the carbon atoms of the side-chain are in trans alignment [C1–C4–C5–C6: −174.0(2)°]. Minor deviations of the overall trans alignment can only be observed around the ester group [C4–C5–C6–O2: 12.7(4)°, C4–C5–C6–O3: −168.4(2)°, C7–O3–C6–C5: −178.4(2)°].

Notably, like in the crystalline structures of 2-phenyl-2-oxazoline, 2-nonyl-2-oxazoline and 2,2'-tetramethylenebis(2-oxazoline) [27], the two C–O bonds in the 2-oxazoline pentacycle differ significantly: While the C2–O1 bond with a length of 1.458(3) Å has the expected length of a C–O single bond [34,35], the C1–O1 bond with a length of 1.376(3) Å is significantly shorter and exhibits a value intermediate between the expected bond lengths of a C–O single and double bond. This phenomenon can be explained by the delocalization the π-electrons along the N–C–O segment of the C3N1O1-pentacycle. Along the O–C–O segment of the ester group of the side-chain, comparable delocalization can be observed: while the O2–C6 bond with a length of 1.211(3) exhibits a bond length typical for a C=O double bond, the O3–C6 bond with a length of 1.338(3) Å shows a value intermediate between the expected bond lengths of a C–O single and double bond.

Hence, the lengths of the C1–O1 bond and the O3–C6 bond are of very comparable value. While the potential (hetero) keto-enol tautomerism of the ester bond cannot be elucidated from the C–C bond lengths [C4–C5: 1.515(4) Å, C5–C6: 1.507(3) Å], it can be stated that the extent of π-electron delocalization along the N–C–O segment in 2-oxazolines is very comparable to that along the O–C–O segment in esters. The π-electron delocalization in esters is less pronounced than in amides (Scheme 2), but nonetheless significant: In 2-oxazolines, it renders the partial charge of the oxygen atom less negative, and the partial negative charge of the nitrogen atom more negative.

Packing of the EstOx molecules in the crystalline phase seems to be controlled by steric factors only (Figure 2): The EstOx molecules are aligned in parallel fashion, with molecule-to-molecule distances of 5.547 Å; for comparison: Distances of adjacent molecules of 2,2'-tetramethylenebis(2-oxazoline) in the crystalline phase (that showed a packaging very similar to that of EstOx) exhibited a value of 5.084 Å [27].

Figure 2

Arrangement of EstOx molecules in the crystalline phase.

3.3. Crystal Structure of EstAA

EstAA crystallizes in its zwitterionic form in the monoclinic space group P21/c with Z = 4 formula units in the unit cell. The asymmetric unit contains 1 formula unit (Figure 3). A detailed analysis of the dihedral angles shows that the C6–C5–O4–C4(O3)–C3 segment of the formula unit is in trans alignment [C4–O4–C5–C6: 179.6(2)°, C5–O4–C4–C3: 176.2(2)°, C5–O4–C4–O3: 0.2(4)°, while the ammonium group and the C2–C1(O1O2) segment deviate from that alignment [C1–C2–C3–C4: 68.7(3)°, O4–C5–C6–N1: −57.6(3)°]. In addition to its ester group with C–O bond lengths of 1.205(4) and 1.354(3) Å (which are almost identical to the C–O ester bond lengths of 1.211(3) and 1.338(3) Å in EstOx), EstAA also contains a carboxylate group that exhibits C–O bond lengths of 1.251(3) and 1.268(3) Å, which correspond in close proximity to the bond lengths of C–O double bonds [34,35]. The very minor difference among the two C–O bond lengths in the carboxylate group is assumed to originate from a different involvement of the oxygen atoms O1 and O2 atoms in the formation of hydrogen bonds (Table 2).

Table 2

Hydrogen bonds in the crystalline phase of EstAA.

Atom Names	Bond Length N–H	Distance H···O	Distance N···O	Angle NHO
N1–H1A···O2 A	0.91 Å	1.84 Å	2.739(3) Å	171.1°
N1–H1B··O2 B	0.91 Å	1.86 Å	2.741(3) Å	162.7°
N1–H1C··O1 C	0.91 Å	1.84 Å	2.747(3) Å	172.2°

A,B,C: Symmetry operations used to generate equivalent atoms; A: −x + 2, y − 0.5, −z + 1.5; B: x, −y + 1.5, z − 0.5; C: −x + 2, −y + 1, −z + 2.

All acidic protons, namely the protons of the ammonium group, are involved in hydrogen bonds jointly with the oxygen atoms of the carboxylate group. The hydrogen bonds are likely to cause the deviation of the ammonium group and the carboxylate group from the overall trans alignment of the carbon chain of EstAA. The oxygen atoms of the ester group do not participate in the formation of hydrogen bonds. Correspondingly, packing of the EstAA molecules in the crystalline phase (Figure 4) is controlled by the formation of hydrogen bonds and large molecule-to-molecule distances of 8.001 Å.

Figure 4

Arrangement of EstAA molecules in the crystalline phase. The orientation of the ammonium groups towards the carboxylate groups is indicative of hydrogen bond formation.

3.4. Quantum Chemical Calculations of EstOx

In order to interpret the experimental findings, quantum chemical calculations were performed for the EstOx model system using the MRCC program [36,37]. Mulliken atomic charges [38] and Mayer bond orders [39] were computed with the density-fitting Hartree-Fock method using the correlation-consistent valence quadruple-zeta (cc-pVQZ) basis set [40] and the corresponding auxiliary basis sets [41]. The calculations were carried out at the experimental X-ray geometry of the molecule.

The theoretical results support the conjecture that the N–C–O segment has a delocalized electronic structure similar to an ester group. The calculated bond orders are 1.97 and 1.10, respectively, for the N=C and C–O bonds indicating that the former bond order is lower than a double bond, while the latter bond order is higher than a typical single bond. The computed partial charges for the nitrogen and oxygen atoms, namely −0.43 and −0.44, respectively, are comparable: despite the higher electronegativity of the oxygen atom (compared to the nitrogen atom), the charges of the hetero atoms are equalized due to the delocalization, and it also means that a cationic attack on the nitrogen atom is possible. It is interesting to compare the above numbers with the corresponding results for the ester group. The bond orders are 1.94 and 1.18 for the O=C and C–O bonds, while the partial charges of the carbonyl and other oxygen atom are −0.49 and −0.41, respectively, which demonstrate the similar electronic structure of the N–C–O and O–C–O segments.

4. Conclusions

The single crystal x-ray analysis of EstOx reveals that the π-electron delocalization along the N–C–O segment in 2-oxazolines occurs at a content comparable to the π-electron delocalization in esters along the O–C–O segment: while the observed C=N bond length in 2-oxazolines and the measured C=O bond length in the ester group are in good agreement with the literature values for the corresponding C=X double bonds [33,34], the C–O “single” bonds are similar to each other and exhibit a value intermediate between a single and double C–O bond. Quantum chemical calculations revealed calculated bond orders of 1.97 and 1.10 for the N=C and C–O bonds of the 2-oxazoline ring, and almost identical partial charges for the nitrogen and oxygen atoms of −0.43 and −0.44. As the charges of the hetero atoms are equalized due to the delocalization, the cationic attack on the nitrogen atom during the initiation of the cationic ring-opening polymerization is possible. The ester group exhibits bond orders of 1.94 and 1.18 for the O=C and C–O bonds, which demonstrates the similar electronic structure of the N–C–O and O–C–O segments.