Microwave-Assisted Esterification of Phosphinic Acids

The usual synthesis of phosphinates involving the reaction of phosphinic chlorides with alcohols or phenols in the presence of a tertiary amine in an apolar solvent cannot be regarded as “green” and requires expensive P-reagents. We have found that in certain cases phosphinic acids can be esterified by simple alcohols under microwave (MW) conditions. Hence, cyclic phosphinic acids, such as 1-hydroxy-3- and 2-phospholene 1-oxide derivatives (A), 1-hydroxyphospholane 1-oxides (B) and 1-hydroxy-1,2,3,4,5,6- hexahydrophosphinine 1-oxide (C) could be converted to the corresponding esters by reaction with alcohols. Using a 15-fold excess of the alcohol at around 200 ° C, the preparative yields of the phosphinates amounted to 45-60%. The novel reaction is the consequence of the beneficial specific MW effect that was proved by comparative thermal experiments. Phosphinates may also be prepared by the alkylating esterification of phosphinic acids with alkyl halides. Under solventless and MW conditions in the presence of a phase transfer (PT) catalyst and K2CO3, cyclic phosphinic acids A, B and C mentioned above were converted to the corresponding phosphinates in 85-95% yields. It was found that the simultaneous use of the MW and PT catalytic techniques is beneficial when alkyl halides of normal or decreased reactivity are applied. Both methods described can be regarded as environmentally friendly and offer advantages over the traditional esterifications of phosphinic acids.