THE ARBUZOV REACTION OF ALKYL DIPHENYLPHOSPHINITES WITH 10-METHYLACRIDINIUM ION - KINETIC-STUDY ON THE FORMATION AND THE DECOMPOSITION OF PHOSPHONIUM INTERMEDIATES

Alkyl diphenylphosphinites (1) react rapidly with 10-methylacridinium iodide (2a) to afford the corresponding phosphonium ions (3), which gradually decompose to phosphine oxide, the expected Arbuzov product. Large difference in the rate of the first (phosphonium-forming) and the second (phosphonium-decomposing) steps enables us independent kinetic investigation for both steps, with which it has been found that the first step obeys second-order kinetics with first-order with respect to 1 and 2a, respectively. The second step proceeds according to either the S(N)2 mechanism when alkyl substituent in 1 is primary or secondary, in which iodide ion acts as a nucleophile, or the S(N)1 mechanism when the substituent is tertiary. Closer examination on the first step with activation parameters reveals that the transition state of this step becomes more reactant-like as the substituent in 1 becomes bulkier. For the second step, survey of the activation parameters shows that the breaking of the carbon-oxygen bond in 3 predominates over the formation of the oxygen-phosphorus double bond.