Controlling rates and reversibilities of elimination reactions of hydroxybenzylammoniums by tuning dearomatization energies

Abstract

Hydroxybenzylammonium compounds can undergo a reversible 1,4- or 1,6-elimination to afford quinone methide intermediates after release of the amine. These molecules are useful for the reversible conjugation of payloads to amines. We hypothesized that aromaticity could be used to alter the rate of reversibility as a distinct thermodynamic driving force. We describe the use of density functional theory (DFT) calculations to determine the effect of aromaticity on the rate of release of the amine from hydroxybenzylammonium compounds. Namely, the aromatic scaffold affects the dearomatization reaction to reduce the kinetic barrier and prevent the reversibility of the amine elimination. We consequently synthesized a small library of polycyclic hydroxybenzylammoniums, which resulted in a range of release half-lives from 18 minutes to 350 hours. The novel mechanistic insight provided herein significantly expands the range of release rates amenable to hydroxybenzylammonium-containing compounds. This work provides another way to affect the rate of payload release in hydroxybenzylammoniums.

Fig. 1. (a) Benzyl carbamates for amine release. (b) Hydroxybenzylammoniums with electron-donating groups and intramolecular trapping arms. (c) Polycyclic azaquinone methide-mediated release of benzylic phenols. (d) Polycyclic aromatic hydroxybenzylammoniums. Star indicates a potential payload of interest (fluorophore, polymer, protein/peptide, etc.).

Fig. 2. (a) Free energy diagrams for polycyclic aromatic hydroxybenzylammonium release calculated at the M06-2X/aug-cc-pVTZ, CPCM(Water)//B3LYP-D3/6-31+G(d,p), CPCM(Water) level of theory. (b) TS1 geometries calculated for 1,4-benzene, 1,4-naphthalene, 1,4-anthracene, and 9,10-anthracene-based hydroxybenzylammoniums. (c) Calculated free energy of activation at the aforementioned level of theory plotted against the corresponding Polansky Aromaticity Index of the parent hydrocarbon scaffold.

Scheme 1. Synthesis of 1,4-anthracene hydroxybenzaldehyde 8.

Fig. 3. (a) Library of polycyclic aromatic–phenethylamine conjugates prepared and their experimentally determined release rate constants. (b) Phenethylamine release kinetics of 1, 2a, and 3a (n = 3, some error bars are smaller than markers) carried out at 5 mM of the benzene- or naphthalene-hydroxybenzylammoniums or 1 mM of the anthracene-hydroxybenzylammonium in a 1 : 1 mixture of methanol and 0.1 M Tris buffer (pH 7.4). (c) First-order plot of phenethylamine release kinetics of 2a and 3a (n = 3, some error bars are smaller than markers).

Fig. 4. (a) TS1 structures calculated at the B3LYP-D3/6-31+G(d,p), CPCM(water) level of theory for 5- and 8-hydroxyquinoline benzylammoniums and calculated free energies of activation for the forward and reverse reactions at the M06-2X/aug-cc-pVTZ, CPCM(Water)//B3LYP-D3/6-31+G(d,p), CPCM(Water) level of theory. (b) Library of pyridine-, quinoline-, and azanaphthalene-based phenethylamine conjugates synthesized and their experimentally determined release rate constants. (c) First-order plot of phenethylamine release kinetics (n = 3, some error bars are smaller than markers) carried out at 5 mM of the hydroxy- or azabenzylammonium in a 1 : 1 mixture of methanol and 0.1 M Tris buffer (pH 7.4).

Fig. 5. Release half-life spectrum of hydroxybenzylammonium molecules. Release half-lives displayed are from this report as well as our previous article.