Bulky phosphazenium cation catalysis for dehydrative condensation of phosphoric acid with alcohols{{§

Phosphoric acid monoesters are among the most important substances in materials chemistry, medicinal chemistry, and so on. Many phosphoric acid monoesters have been synthesized on an industrial scale and are used as necessities in our daily life. From the perspective of green chemistry, the direct catalytic condensation of equimolar amounts of phosphoric acid and alcohols is attractive for the synthesis of phosphoric acid monoesters, especially for industrial-scale synthesis, since the reaction produces only water as a byproduct. Although various methods for the direct catalytic condensation of carboxylic acids with equimolar amounts of alcohols have been reported, there are only a few successful methods for the direct catalytic condensation between phosphoric acid and alcohols. Previously, we reported that a catalytic amount of nucleophilic bases such as N-butylimidazole promoted the dehydrative condensation of phosphoric acid and alcohols. However, this condensation is not a true catalytic process, since the reaction requires one equivalent of tributylamine (n-Bu3N). Very recently, we reported that perrhenic acid (1 mol%) showed excellent catalytic acitivity in the condensation between phosphoric acid and alcohols in the presence of dibutylamine (20 mol%). Since rhenium is a rare metalandthe oxorhenium(VII) complexes are very expensive, the recovery and reuse of the oxorhenium compounds is strongly needed, especially for industrial-scale synthesis. However, we haven’t been able to establish it yet. The contamination of phosphoric acid monoesters with oxorhenium compounds makes purification of the products very difficult. From the perspective of green chemistry, a metal-free catalytic method is more desirable for preparation of phosphoric acid monoesters. We report here a phosphazenium salt as a metal-free catalyst for the dehydrative condensation between phosphoric acid and alcohols. In our previous work, a 1 : 1 (v/v) mixture of DMF and EtNO2 was used as a solvent. Since phosphoric acid can not be dissolved in that solvent, 100 mol% of n-Bu3N is required to dissolve phosphoric acid and promote the reaction. We considered that some organic bases might catalyze the condensation in N-methyl2-pyrrolidon (NMP)–o-xylene (1 : 1 v/v) in the absence of any auxiliary base, since phosphoric acid dissolves well in NMP– o-xylene (1 : 1 v/v) even without an auxiliary base. Therefore, we examined the catalytic activities of several organic bases in the condensation of an equimolar mixture of phosphoric acid and stearyl alcohol in NMP–o-xylene (1 : 1 v/v) (Table 1). When the reaction was conducted in the presence of n-Bu3N (100 mol%), stearyl phosphate was obtained in 87% conversion yield (entry 1). P NMR analysis indicated that the crude products included the phosphoric acid diester (10%) and pyrophosphoric acid esters (17%) as byproducts and that the purity of stearyl phosphate was 73% aside from phosphoric acid. We then tried to reduce the catalyst loading, but unfortunately the use of 10 mol% of n-Bu3N decreased reactivity to give the product in 46% yield and 94% purity (entry 2). While a catalytic amount of n-Bu3N did promote the present reaction (entry 2 versus entry 10), the catalytic activity