Mechanism of the Nucleophilic Substitution Reaction

Several mechanistic pathways have been identified in the nucleophilic substitution reactions of chlorocyclophosphazenes [12]. Most of the kinetic experiments have been carried out in the reactions involving chlorocyclophosphazenes with amines. These experiments are suggestive of various 

Preference for nongeminal product is natural. Imagine the reaction of an amine with a PCI2 center or a sp(Cl)NRR center. The phosphorus atom in the former is more electrophilic and hence a nucleophile would prefer to attack this center in comparison to the less electrophilic P(C1)(NR2) site (Fig. 3.17). This means that nongeminal products should always be preferred. 

Although kinetic experiments have established different t) es of reaction mechanisms one of the most important pathways for the nucleophilic reactions in chlorocyclophosphazenes seems to be a bimolecular associative pathway (Sn2). Two types of transition states have been identified for this mechanism (Fig. 3.18). 

Either way, the reaction pathway envisages that the four-coordinate phosphorus is converted first into a five-coordinate state before reverting back to the four-coordinate state (Fig. 3.18). 

SnI pathways also occur in reactions where the chlorine from the P-Cl bond is heterolytically cleaved as a result of a reaction with a Lewis aeid such as AICI3. For example, in the Friedel-Crafts reaction of N3P3CI6 the first step may be the abstraction of the chloride. This is followed by the electrophilic attack of the phosphonium center on the aromatic reagent to afford N3P3Cl5Ph (see Eq. 3.28). 

---

Summary of the Mechanisms of the Nucleophilic Substitution Reactions in Chlorocyclophosphazenes... 

---