Bis(benzylamine) monomers: One-pot preparation and application in dendrimer scaffolds for removing pyrene from aqueous environments

Summary Bisimine and bisamine AB2 monomers have been synthesized from 3,5-diaminobenzoic acid and benzaldehyde derivatives without the need for protective groups or purification. This monomer preparation is universal for various electron-donating and electron-withdrawing benzaldehyde substrates. To demonstrate the versatility of these previously unreported AB2 monomers in the formation of high molecular weight structures, novel first-generation dendrimers and hybrid second-generation dendrimers have been synthesized. Using fluorescence spectroscopy, pyrene was shown to be removed from an aqueous environment upon exposure to thin dendrimer films, with the first-generation dendrimer removing 70% of the pyrene within 30 min and the hybrid second-generation dendrimers removing 38–52%. Inclusion formation constants were calculated to be on the order of 109–1011 M−1 and are comparable to the values of previously reported macromolecules. These results illustrate that size may not influence pyrene removal as effectively as composition.


3,5-Bis(benzylamino)benzoic acid (4a)
To a heterogeneous solution of 3,5-diaminobenzoic acid (0.50 g, 3.29 mmol) in MeOH (10 mL) was dropwise added benzaldehyde (0.70 mL, 6.89 mmol). After stirring at room temperature for 20 min, the reaction flask was cooled in an ice water bath and NaBH 4 (0.20 g, 5.55 mmol) was added slowly. The reaction mixture was then stirred at room temperature overnight. After cooling the reaction in an ice water bath, distilled water (3 mL) was added and the reaction mixture was acidified with 2N HCl until the product precipitated. The resulting product was filtered and washed with cold distilled water to

3,5-Bis(4-methoxybenzylamino)benzoic acid (4b)
To a heterogeneous solution of 3,5-diaminobenzoic acid (0.25 g, 1.64 mmol) in MeOH (5 mL) was dropwise added p-anisaldehyde (0.43 mL, 3.53 mmol). After stirring at room temperature for 20 min, the reaction flask was cooled in an ice water bath and NaBH 4 (0.12 g, 3.17 mmol) was added slowly. The reaction mixture was then stirred at room temperature overnight. After cooling the reaction in an ice water bath, distilled water (3

3,5-Bis(4-methylbenzylamino)benzoic acid 4c
To a heterogeneous solution of 3,5-diaminobenzoic acid (0.50 g, 3.29 mmol) in MeOH (5 mL) was dropwise added p-tolualdehyde (0.78 mL, 6.62 mmol). After stirring at room temperature for 20 min, the reaction flask was cooled with an ice water bath and NaBH 4 (0.20 g, 5.29 mmol) was added slowly. The reaction mixture was then stirred at room temperature overnight. After cooling the reaction in an ice water bath, distilled water (3 mL) was added and the reaction mixture was acidified with 2N HCl until the product precipitated. The resulting product was filtered and washed with cold distilled water to give 0.42 g (35%) of the desired product. Mp 155-157 ˚C; R f = 0.81 (CH 2 Cl 2 :MeOH, 3:1).

3,5-Bis(4-chlorobenzylamino)benzoic acid (4e)
To a heterogeneous solution of 3,5-diaminobenzoic acid (0.25 g, 1.64 mmol) in MeOH (16 mL) was added 4-chlorobenzaldehyde (0.46 g, 3.30 mmol). After stirring at room temperature for 20 min, the reaction flask was cooled with an ice water bath and NaBH 4 (0.12 g, 3.15 mmol) was added slowly. The reaction mixture was then stirred at room temperature overnight. After cooling the reaction in an ice water bath, distilled water (3 mL) was added and the reaction mixture was acidified with 2N HCl until the product

3,5-Bis(4-bromobenzylamino)benzoic acid (4f)
To a heterogeneous solution of 3,5-diaminobenzoic acid (0.25 g, 1.64 mmol) in MeOH (15 mL) was added 4-bromobenzaldehyde (0.65 g, 3.52 mmol). After stirring at room temperature for 20 min, the reaction flask was cooled with an ice water bath and NaBH 4 (0.13 g, 3.49 mmol) was added slowly. The reaction mixture was then stirred at room temperature overnight. After cooling the reaction in an ice water bath, distilled water (3 mL) was added and the reaction mixture acidified with 2N HCl until the product precipitated. The resulting product was filtered and washed with cold distilled water to

S12
NaBH 4 (0.12 g, 3.17 mmol) was added slowly. The reaction was then allowed to stir at room temperature overnight. After cooling the reaction in an ice water bath, distilled water (3 mL) was added and the reaction mixture was acidified with 2N HCl until the product precipitated. The resulting product was filtered and washed with cold distilled water to give 0.27 g (38%) of the desired product. Mp: 168-169 ˚C; R f = 0.88 (CH 2 Cl 2 :MeOH, 3:1). 1
The reaction stirred at 0 °C for one hour, after which it was quenched with saturated aqueous Na 2 S 2 O 3 (310 mL). The solution was extracted with ethyl acetate (3 x 200 mL) and the organic layers combined and dried over MgSO 4 . Evaporation of the solvent afforded an oil which was purified by silica gel column chromatography in CH 2 Cl 2 to give 9.2 g (93%) of the desired product. The spectra was similar to data reported previously [3].

Hybrid dendron 8
To a heterogeneous solution of 3,5-diamino benzoic acid (0.23 g, 1.50 mmol) in MeOH (2 mL) was added a solution of aldehyde 7 (1.00 g, 3.15 mmol) in MeOH (20 mL). After stirring at room temperature for 20 min, the reaction flask was cooled in an ice bath and NaBH 4 (0.096 g, 2.53 mmol) was added. The reaction was allowed to stir at room temperature overnight, after which TLC in 5% MeOH, 1% Et 3 N in CH 2  solution was neutralized with 2N HCl (6.4 mL) and extracted with CH 2 Cl 2 (3X, 30 mL).
The organic layers were combined and dried over MgSO 4 and rotary evaporated to afford a green foam. The crude product was purified by silica gel column in 5% MeOH and 1% Et 3 N in CH 2 Cl 2 to afford 0.98 g (92%) of a gold foam product. R f = 0.45 (5% MeOH in CH 2 Cl 2 with 1% Et 3 N). 1

Methyl 3,5-bis(benzylamino)benzoate (14)
To a solution of methyl 3,5-diaminobenzoate (0.5 g, 3.01 mmol) in MeOH (10 mL) was added benzaldehyde (0.61 mL, 6.02 mmol). The solution stirred at room temperature for 20 min, after which the flask was cooled to 0 ˚C before adding NaBH 4  Films were protected from dust and other contaminants by placing parafilm over the opening.

Preparation of saturated aqueous pyrene standard
A 1 mL aliquot of a 3.7 x 10 -2 M solution of pyrene in CH 2 Cl 2 was added to a 250 mL volumetric flask. The solvent was evaporated and distilled water was added. The flask was then sonicated and allowed to sit for 1 d prior to use.

Encapsulation study procedures
An aliquot of saturated aqueous pyrene standard prepared above was filtered through a cotton plug into a cuvette. The cuvette was excited at a wavelength of 325 nm and the fluorescence emission spectrum obtained from 340-500 nm.
To each beaker containing a film was added 25 mL of filtered saturated aqueous pyrene. After time points of 30 min, 60 min, and 2 d, an aliquot was removed, filtered, and its fluorescence measured. Two studies were performed for each sample to ensure consistency. As a control, the films were exposed to distilled water and the fluorescence obtained. The data was analyzed and graphed as shown.

Encapsulation study results
Comparisons of the fluorescence signal at 370 nm between the dendrimers and the dendrons were graphed.

VI. Calculations of inclusion formation constants and Gibbs free energies
The following equilibrium equation was utilized for the determination of inclusion formation constants (K) and its corresponding Gibbs free energy (ΔG°) [4]: It was assumed that the dendrimer and the dendrimer-pyrene complex exist in the solid state and therefore their values were taken to be 1. The concentration of pyrene was calculated using the pyrene fluorescence intensity at 370 nm at 2 days and the pyrene calibration curve ( Figure S2). The pyrene molarity from the two trials was averaged for determination of the inclusion formation constants.
Based on the above results, the inclusion formation constant was simplified to K = 1/[pyrene].

Estimation of dendrimer capacity for pyrene
Taking the aqueous solubility of pyrene to be 6.27 x 10 -7 M, the capacity of dendrimer films for pyrene was calculated by determining the moles of pyrene removed from 25 mL of a saturated aqueous solution by each dendrimer film after 2 days [5].