Abstract
Because of the poor solubility of the commercially available bisacylphosphine oxides in dental acidic aqueous primer formulations, bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide (WBAPO) was synthesized starting from 3-(chloromethyl)-2,4,6-trimethylbenzoic acid by the dichlorophosphine route. The substituent was introduced by etherification with 2-(allyloxy)ethanol. In the second step, 3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoic acid was chlorinated. The formed acid chloride showed an unexpected low thermal stability. Its thermal rearrangement at 180 °C resulted in a fast formation of 3-(chloromethyl)-2,4,6-trimethylbenzoic acid 2-(allyloxy)ethyl ester. In the third step, the acid chloride was reacted with phenylphosphine dilithium with the formation of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine, which was oxidized to WBAPO. The structure of WBAPO was confirmed by 1H NMR, 13C NMR, 31P NMR, and IR spectroscopy, as well as elemental analysis. WBAPO, a yellow liquid, possesses improved solubility in polar solvents and shows UV–vis absorption, and a high photoreactivity comparable with the commercially available bisacylphosphine oxides. A sufficient storage stability was found in dental acidic aqueous primer formulations.
Highlights
Self-etching enamel-dentin adhesives (SEAs) are used in restorative dentistry to achieve a strong bond between the filling composites and dental hard tissues
We report the detailed synthesis of bis(3-{[2(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide WBAPO (Scheme 1) and its use as a tailor-made PI in SEAs
The synthesis of WBAPO started from 3-(chloromethyl)-2,4,6trimethylbenzoic acid, which was synthesized by chloromethylation of 2,4,6-trimethylbenzoic acid with a mixture of paraformaldehyde and hydrochloric acid, followed by treatment of the formed hydroxymethyl compound with concentrated hydrochloric acid in a one-pot reaction [15]
Summary
Self-etching enamel-dentin adhesives (SEAs) are used in restorative dentistry to achieve a strong bond between the filling composites and dental hard tissues. The solubility of BAPO in polar solvents and aqueous formulations is insufficiently low, which limits the use of BAPO in water-based SEAs. In this context, we were able to synthesize a number of new substituted bisacylphosphine oxides, which show improved solubility in aqueous compositions [12]. We report the detailed synthesis of bis(3-{[2(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide WBAPO (Scheme 1) and its use as a tailor-made PI in SEAs. Scheme 1: Bisacylphosphine oxide with improved solubility in polar solvents.
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