Destiny of Transient Phosphenium Ions Generated from the Addition of Electrophiles to Phosphaalkenes: Intramolecular C−H Activation, Donor−Acceptor Formation, and Linear Oligomerization

Abstract

The reaction of the phosphaalkenes MesPCPh2 (Mes = 2,4,6-Me3C6H2) and Mes*PCH2 (Mes* = 2,4,6-tBu3C6H2) with Lewis (AlCl3, GaCl3, InCl3) and protic (HOTf) acids has been examined to evaluate the feasibility of cationic polymerization for PC bonds. Addition of GaCl3 to Mes*PCH2 generates the adduct Mes*(Cl3Ga)PCH2, which can be detected spectroscopically at 193 K. At higher temperatures, GaCl3 migrates from phosphorus to carbon to afford the fleeting phosphenium zwitterion Mes*PCH2GaCl3. This undetected transient species immediately oxidatively adds to a C−H bond of an o-tBu group in the P-Mes* substituent, resulting in a GaCl3-coordinated ylide that has been characterized crystallographically. The analogous reaction of GaCl3 with MesPCPh2 gives stable Mes(Cl3Ga)PCPh2, for which a crystal structure determination has been conducted. Significantly, treating a highly concentrated solution of Mes*PCH2 with substoichiometric quantities of GaCl3 leads to linear dimerization following a cationic chain growth mechanism; however, the oligomerization is terminated by intramolecular C−H activation. The novel coordinated linear dimer (C−H activated Mes*)PCH2PH(Mes*)CH2GaCl3 has been characterized crystallographically. Interestingly, mechanistic studies reveal that the diphosphiranium ring derived from the reaction of Mes*PCH2GaCl3 with Mes*PCH2 is an intermediate in this transformation. The reaction of phosphaalkenes with phosphenium species appears to be a general method to prepare diphosphiranium ions. In one case, NMR spectroscopic data suggests that treating MesPCPh2 with HOTf gives both the diphosphiranium species and the adduct Ph2C(Mes)P→P(Mes)(CHPh2)]OTf. Remarkably, treating concentrated Mes*PCH2 solutions with HOTf results in oligomers of up to six repeat units, as determined by ESI mass spectrometry. These results suggest that it may be possible to initiate the polymerization of PC bonds using cationic initiators and that the propagating species will be a cationic phosphenium moiety.