ELIMINATION-ADDITION WITH ALKYLIDINEOXOPHOSPHORANE (PHOSPHENE) INTERMEDIATES IN NUCLEOPHILIC-SUBSTITUTION AT P=O CENTERS - FLUOREN-9-YLPHOSPHONAMIDIC CHLORIDES WITH AMINE NUCLEOPHILES

Compared with Me2CHP(O)(NEt2)Cl, the fluorenyl compound R2CHP(O)(NEt2) Cl(R2CH = fluoren-9-yl) shows remarkably high reactivity in nucleophil ic substitution with Et2NH, Substitution is catalysed by base {1,8-dia zabicyclo[5.4.0]undec-7-ene (DBU)} and shows little discrimination bet ween competing Me2NH and Et2NH, These characteristics point to an elim ination-addition (EA) mechanism with a reactive phosphene intermediate [R2C=P(O)NEt2]. When Et2ND is the nucleophile, H-D exchange at the a carbon atom occurs much more quickly than substitution. This suggests that the elimination stage of the EA mechanism is reversible E1cB.