Main Chain Noncentrosymmetric Hydrogen Bonded Macromolecules Incorporating Aniline, Alkanol, and Alkanoic Acid Hydrogen Bond Donors

The syntheses and characterization of three noncentrosymmetricmain chain hydrogen bondedmacromolecules which incorporate aniline, alkanoic acid, and alkanol hydrogen bond donor units are reported. These macromolecules participate in weak intermolecular hydrogen bonding as demonstrated using attenuated total reflectance (ATR) FTIR. The phase transitions of these macromolecules depend on the identity of the hydrogen bond donor.

In several of our studies, we found that replacing a benzoic acid hydrogen bond donor group with a weaker hydrogen bond donor (such as a OH group) in our macromolecules resulted in a large melting point depressions and increased solubility in organic solvents [19,20] presumably due to the weaker donor-acceptor hydrogen bonding.Alkanoic acids, alkanols, and aniline are all weaker hydrogen bond donors than their benzoic acid or phenol analogs [21], therefore incorporating these hydrogen bond donors into our noncentrosymmetric hydrogen bonded macromolecules should result in lower melting points and increased solubility.In this paper, we report the synthesis and characterization of main chain noncentrosymmetric hydrogen bonded macromolecules incorporating alkanoic acid, alkanol, and aniline hydrogen bond donors and stilbazole acceptors.

Experimental Section
2.1.Materials.All chemicals were purchased from Fisher or Aldrich chemical company and used as received.Tetrahydrofuran (THF) was distilled over sodium-benzophenone under an argon atmosphere.Chromatography was performed using Sorbent Technology 60 angstrom, 63-200 m mesh silica (10940-25).Thin layer chromatography was performed using Whatman flexible plates with 250 m layer of fluorescent silica gel (UV 254 ) or EM Science glass TLC plates (60 F 265 ).All final products were dried at appropriate temperatures (below their melting and/or decomposition temperatures) in a Napco E-series 5831 vacuum oven prior to analysis.

Model Mixtures.
A standard procedure for making model mixtures was as follows.One component was weighted into a glass vial.The required amount of the other compound (to obtain the desired mole ratio) was weighted into the same vial.The content of the vial was heated in a silicon oil bath until both components were visibly melted.The vial was removed from the oil bath and the mixture was allowed to crystallize (at room temperature).The melting/crystallization sequence was repeated two additional times and the resulting mixture was analyzed.

Measurements.
Proton (300 MHz) and carbon 13 (75 MHz) nuclear magnetic resonance spectra were acquired using a Varian VXR 300 wide bore instrument and processed using either VNMR or Mestrec software.Infrared spectroscopy was performed on Thermo-Nicolet Nexus 670-FTIR using an Avatar multibounce HATR accessory; spectra were processed using OMNIC software.Mass spectra were acquired by the staff at the Mass Spectroscopy Lab at University of Illinois, Urbana, IL.Differential scanning calorimetry was performed on a Mettler Toledo DSC 821e equipped with a Julabo FT 900 cooling unit using heating and cooling rates of 10 ∘ C/min unless otherwise stated; all reported transition temperatures are from the second cycle of a DSC scan unless otherwise stated.All DSC transition temperatures reported are the midpoints of the transition; the enthalpies of the transitions are reported in parentheses following the transition temperature.Polarized optical microscopy was performed on an Olympus BXP polarizing microscope equipped with and Instec HCS400 heating stage; heating and cooling rates used varied between 1 and 5 ∘ C/min.
Anhydrous DMF (20 mL) was added via syringe.Solid compound 6 (3.002 g, 14.3 mmol) was slowly added to the flask; this mixture was stirred for 30 minutes.1,10-Dichlorodecane (9.0 mL, 42.6 mmol) was added to the flask via syringe and the mixture was stirred at room temperature for 30 minutes.Water (50 mL) was added to the reaction mixture and the solution was extracted with ethyl acetate (5 × 50 mL).The extract was washed with brine and dried over MgSO 4 , and the crude product was adsorbed onto silica.The product was purified via column chromatography (85/15 Hex/EtOAc); evaporation of the solvent yielded 0.751 g (14%) of 7 as a white solid:   = 0.39 (85/15 Hex/EtOAc);

11-[4-(2-Pyridin-4-yl-vinyl)-phenoxy]-undecanoic Acid (14).
Compound 13 (0.579 g, 1.5 mmol), KOH (1.636 g, 32.7 mmol),  5N NaOH (22 mL), THF (30 mL), and MeOH (20 mL) were placed in a round-bottomed flask equipped with a condenser; this mixture was refluxed for 2 days.The mixture was cooled to room temperature and neutralized (pH = 7) with 5 N HCl; the mixture was cooled in the refrigerator.The solution was vacuum filtered and the solid so obtained was washed with water and acetone; this solid was dried in vacuum to afford 0.495 g (89%) of 14 as an off-white solid:   = 0.33 (EtOAc);  (16).Compound 8 (1.504 g, 7.6 mmol), K 2 CO 3 (2.618g, 18.9 mmol), decylbromide (1.60 mL, 7.8 mmol), and dry DMF (20 mL) were combined in an oven dried round bottomed flask equipped with a condenser and an argon purge; this mixture was heated to 120 ∘ C for 18 hours and then cooled to room temperature.Water (100 mL) was added and the solution was extracted with EtOAc (5 × 100 mL).The combined organic layers were washed with brine and dried over MgSO 4 .The crude product was purified via column chromatography using elutions from 60/40 to 50/50 (Hex/EtOAc).Evaporation of the solvent afforded an yellow solid, which was recrystallized twice from hexanes to afford 0.902 g (35%) of 16 as a pale yellow solid:   = 0.37 (50/50 Hex/EtOAc);  Compound 12 was alkylated with stilbazole 8 to afford compound 13.Basic hydrolysis of 13 afforded macromolecule 14, which precipitated upon neutralization of the basic cleavage medium.Macromolecule 11 is soluble in DMSO, DMF, and alcohols at room temperature, whereas the solubility of 14 is similar to that of macromolecule 10.

Hydrogen Bonding in Our
Macromolecules.Solid state FTIR was used to analyze the hydrogen bonding in our noncentrosymmetric macromolecules.Select FTIR stretches for model compounds, model mixtures (which mimic the hydrogen bonding in our macromolecules), and our macromolecules are shown in Table 2.The amine N-H peaks in model mixture A and macromolecule 10 are red shifted with respect to the N-H shifts in 4-decyloxyaniline which suggest strong intermolecular hydrogen bonding in 10.In macromolecule 10, the asymmetric and symmetric N-H stretches merge together into one broad peak which can be attributed to the numerous possible orientations of the N-H (i.e., donor-donor or donor-acceptor) hydrogen bonds in the macromolecule.The O-H stretches in macromolecule 11 and mixture B are red shifted by 10-20 cm −1 with respect to those in 1-decanol.The magnitude of this red shift is small compared to the approximately 250 cm −1 red shift in the O-H stretch observed when phenols form hydrogen bonds with stilbazoles [19,20], which suggests relatively weak intermolecular hydrogen bonding in macromolecule 11 compared to its phenol analogue (compound 5).The C=O stretch in macromolecule 14 is blue shifted 14 cm −1 relative to that in lauric acid.This shift is smaller than the 43 cm −1 shift between the C=O between 13 and macromolecule 2 which suggests that the intermolecular hydrogen bonds in macromolecule 14 are weaker than those of inmacromolecule 2.

Phase Transitions of Noncentrosymmetric Hydrogen
Bonded Macromolecules.The phase transitions of the macromolecules synthesized in this paper were characterized using Polarized Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC) and are summarized in Figure 1.The melting point of macromolecule 10 is 31 ∘ C lower than that of the phenol analog (macromolecule 5) and 87 ∘ C lower than that of its benzoic acid analogue (macromolecule 2).This melting point depression can be attributed to the relatively weaker amine-stilbazole hydrogen bonding.Macromolecules 11 and 14 have crystal-crystal transitions on the heating and cooling cycles but no liquid crystalline phases.The melting points of 11 and 14 are comparable to that of 10 suggesting that (like macromolecule 10) macromolecules 11 and 14 have relatively weak intermolecular hydrogen bonding (relative to those in macromolecules 2 and 5).

Conclusions
The synthesis of three new main chain noncentrosymmetric hydrogen bonded macromolecules incorporating alkanol, alkanoic acid, and aniline donors is described.Relatively weak intermolecular hydrogen bonding is present in macromolecules 10, 11, and 14.Macromolecules both possess enantiotropic crystal → crystal transitions in addition to melting and crystallization temperatures, while macromolecule 10 only has melting and crystallization phase transitions.The phase transition temperatures of 10, 11, and 14 are lower than that of the phenol and benzoic acid analogues due to weaker hydrogen bonding.

3. 1 .
Synthesis of Macromolecules.The synthesis of macromolecule 10 is shown in Scheme 1. Compound 6 was reacted with 1,10-dichlorodecane to afford O-alkylated compound 7.The identity of compound 7 (as the O-alkylated product) was confirmed using a nuclear overhauser effect (NOE) NMR experiment, while the yield of compound 7 was low, extending the reaction time or increasing the temperature during the reaction resulted in an inseparable mixture of Nalkylated and O-alkylated products which contained little to no compound 7. Alkylation of 7 with stilbazole 8 afforded compound 9 which was deprotected using HCl to afford macromolecule 10 which precipitated from solution upon neutralization of the cleavage medium.Macromolecule 10 is soluble in hot DMSO (120 ∘ C) and hot (120 ∘ C) DMF but insoluble in less polar organic solvents.The syntheses of macromolecules 11 and 14 are shown in Scheme 2. Macromolecule 11 was synthesized in one step via basic coupling of compound 8 and 10-chlorodecanol.

Table 1 :
First generation noncentrosymmetric main chain hydrogen bonded macromolecules.
*Polymer possesses a liquid crystalline phase.

Table 2 :
FTIR stretches of model compounds, model mixtures, and macromolecules.