Destiny of transient phosphenium ions generated from the addition of electrophiles to phosphaalkenes: Intramolecular C-H activation, donor-acceptor formation, and linear oligomerization

The reaction of the phosphaalkenes MesP=CPh2 (Mes = 2,4,6-Me3C6H2) and Mes*P=CH2 acids has been (Mes* = 2,4,6-(Bu3C6H2)-Bu-t) with Lewis (AlCl3, GaCl3, InCl3) and protic (HOU) examined to evaluate the feasibility of cationic polymerization for P=C bonds. Addition of GaCl3 to Mes*P=CH2 generates the adduct Mes*(Cl3Ga)P=CH2, 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-Bu-t group in the P-Mes* substituent, resulting in a GaCl3-coordinated ylide that has been characterized crystallographically. The analogous reaction of GaCl3 with MesP=CPh2 gives stable Mes(Cl3Ga)P= CPh2, for which a crystal structure determination has been conducted. Significantly, treating a highly concentrated solution of Mes*P=CH2 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 Mes*PCH2P(Mes*)CH2GaCl3 derived from the reaction of Mes*PCH2GaCl3 with Mes*P= CH2 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 MesP=CPh2 with HOW gives both the diphosphiranium species [MesPCPh(2)P(Mes)CPh2H]OTf and the adduct Ph2C=(Mes)P-->P-(Mes)(CHPh2)]OTf. Remarkably, treating concentrated Mes*P=CH2 solutions with HOW 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 P=C bonds using cationic initiators and that the propagating species will be a cationic phosphenium moiety.