Kinetics of the reaction of trimethyl phosphite with cyclic α-diketones

Abstract

The kinetics of the reaction of trimethyl phosphite with aliphatic α-diketones, 9,10-phenanthrenequinones, and acenaphthenequinone to form a cyclic adduct has been studied in anhydrous dioxan. The rates are expressed as v=k[(MeO)₃P][α-diketone]. The rate constant k decreases with an increase in the size of the α-diketone ring in the order 5 (cyclopentane-1,2-dione) 6 (cyclohexane-1,2-dione) <10 (cyclodecane-1,2-dione), while acenaphthenequinone reacts more quickly than 9,10-phenanthrenequinone which has a condensed ring system. This suggests that the reaction proceeds with releasing strain by changing a carbonyl C atom of cyclopentane-1,2-dione from sp² to sp³, and that a β-ketophosphonium intermediate, i.e., an attack of carbonyl carbon atom by the phosphorus atom, is involved. The Hammett equation for the phenanthrenequinones gives a ρ value of +1·5 for the reaction.