Scrambling of the label in a fluorenylphosphonamidic [18O]-sulfonate during dissociative nucleophilic substitution (elimination–addition): a measure of the importance of preassociation

Abstract

When R₂CHP(O)(NEt₂)OS¹⁸O₂Ar (R₂CH = 9-fluorenyl, Ar = p-tolyl) undergoes nucleophilic substitution (elimination–addition) with Et₂NH (0.4 mol dm⁻³ in CHCl₃) the phosphene intermediate R₂CP(O)NEt₂ recombines with the sulfonate leaving group (internal return), causing scrambling of the ¹⁸O label, more quickly than it diffuses away; efficient conversion into R₂CHP(O)(NEt₂)₂ therefore depends on preassociation between the substrate and the nucleophile.