Phase transfer catalysis of chemical tetraoctyl phosphine bromide

As a quaternary phosphonium salt compound, the phase transfer catalysis of chemical tetraoctyl phosphine bromide is reflected in the interface regulation of water-organic two-phase reaction, which promotes the phase transfer of nucleophiles through ion pair formation. 
The molecular structure determines the phase transfer ability of chemical tetraoctyl phosphine bromide. The four octyl long chains (lipophilic groups) contained in it form an amphiphilic structure with bromide ions (hydrophilic groups), which can be directionally arranged at the water-organic interface, reducing the interfacial tension (from 30mN/m to below 15mN/m) and promoting the increase of the contact area between the two phases. The critical micelle concentration of tetraoctyl phosphine bromide is 0.5-1mmol/L, and micelles can be formed above this concentration, which provides a microscopic homogeneous environment for the reaction and improves the reaction rate by 3-5 times.
The formation of ion pairs is the core mechanism of the catalysis of chemical tetraoctyl phosphine bromide. In the water phase, it forms ion pairs with nucleophiles (such as cyanogen ions and phenol anions), and enters the organic phase through the package of lipophilic groups, so that the reactants that could not be contacted originally react, and the extraction constant (logP) of the ion pairs reaches 3-4, ensuring efficient phase transfer. The catalytic activity of tetraoctyl phosphine bromide changes with the polarity of the reaction system, and it has the highest activity in moderately polar solvents (such as toluene and chloroform), and its catalytic efficiency is 2-3 times that of tetrabutylammonium bromide.
Applying reaction types to expand the application range of chemical tetraoctyl phosphine bromide. In nucleophilic substitution reactions (such as alkylation and etherification), the yield can be increased from 50%-60% to over 80%; In the oxidation reaction, it can promote the transfer of oxidants (such as potassium permanganate and sodium hypochlorite) to the organic phase, and improve the oxidation selectivity (above 90%). The catalytic effect of tetra-octyl phosphine bromide is not strongly affected by the reaction temperature, and its activity remains stable (fluctuation ≤10%) in the range of 50-100℃.
The control of catalytic conditions affects the efficiency of chemical tetraoctyl phosphine bromide. The dosage is usually 1%-5mol% of the substrate, and excessive dosage will lead to increased side reactions (such as alkylation of quaternary phosphonium salt itself); The water content of the reaction system should be controlled at 5%-10%. If it is too high, it will reduce the solubility of the organic phase of the ion pair, while if it is too low, it will affect the dissociation of the nucleophile. Tetraoctyl phosphine bromide can be recovered by liquid separation after the reaction (recovery rate ≥80%), and it can be reused for 3-5 times after simple treatment, with no obvious decrease in catalytic activity.
The phase transfer catalysis of chemical tetra-octylphosphine bromide is the synergistic result of amphiphilic structure and ion-pair effect. This mechanism effectively solves the mass transfer obstacle of water-organic two-phase reaction and provides a practical catalytic scheme for the realization of efficient and highly selective reaction in pharmaceutical synthesis.