Elimination–addition with alkylidineoxophosphorane (phosphene) intermediates in nucleophilic substitution at PO centres: fluoren-9-ylphosphonamidic chlorides with amine nucleophiles1

Abstract

Compared with Me₂CHP(O)(NEt₂)Cl, the fluorenyl compound R₂CHP(O)(NEt₂)Cl (R₂CH = fluoren-9-yl) shows remarkably high reactivity in nucleophilic substitution with Et₂NH. Substitution is catalysed by base {1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)} and shows little discrimination between competing Me₂NH and Et₂NH. These characteristics point to an elimination–addition (EA) mechanism with a reactive phosphene intermediate [R₂CP(O)NEt₂]. When Et₂ND is the nucleophile, H–D exchange at the α carbon atom occurs much more quickly than substitution. This suggests that the elimination stage of the EA mechanism is reversible E1cB.