The reaction of the phosphaa
lkenes MesP=CPh
2 (Mes = 2,4,6-Me
3C
6H
2) and Mes*P=CH
2(Mes* = 2,4,6-
tBu
3C
6H
2) with Lewis (A
lC
l3, GaC
l3, InC
l3) and protic (HOTf) acids has beenexamined to eva
luate the feasibi
lity of cationic po
lymerization for P=C bonds. Addition ofGaC
l3 to Mes*P=CH
2 generates the adduct Mes*(C
l3Ga)P=CH
2, which can be detectedspectroscopica
lly at 193 K. At higher temperatures, GaC
l3 migrates from phosphorus tocarbon to afford the f
leeting phosphenium zwitterion Mes*PCH
2GaC
l3. This undetectedtransient species immediate
ly oxidative
ly adds to a C-H bond of an
o-
tBu group in the
P-Mes*substituent, resu
lting in a GaC
l3-coordinated y
lide that has been characterized crysta
llographica
lly. The ana
logous reaction of GaC
l3 with MesP=CPh
2 gives stab
le Mes(C
l3Ga)P=CPh
2, for which a crysta
l structure determination has been conducted. Significant
ly, treatinga high
ly concentrated so
lution of Mes*P=CH
2 with substoichiometric quantities of GaC
l3leads to
linear dimerization fo
llowing a cationic chain growth mechanism; however, theo
ligomerization is terminated by intramo
lecu
lar C-H activation. The nove
l coordinated
lineardimer (C-H activated Mes*)PCH
2PH(Mes*)CH
2GaC
l3 has been characterized crysta
llographica
lly. Interesting
ly, mechanistic studies revea
l that the diphosphiranium ring
Mes*PCH2P(Mes*)CH2GaCl3 derived from the reaction of Mes*PCH
2GaC
l3 with Mes*P=CH
2 is an intermediate in this transformation. The reaction of phosphaa
lkenes withphosphenium species appears to be a genera
l method to prepare diphosphiranium ions. Inone case, NMR spectroscopic data suggests that treating MesP=CPh
2 with HOTf gives boththe diphosphiranium species
[MesPCPh2P(Mes)CPh2H]OTf and the adduct Ph
2C=(Mes)P
P(Mes)(CHPh
2)]OTf. Remar
kab
ly, treating concentrated Mes*P=CH
2 so
lutions with HOTfresu
lts in o
ligomers of up to six repeat units, as determined by ESI mass spectrometry.These resu
lts suggest that it may be possib
le to initiate the po
lymerization of P=C bondsusing cationic initiators and that the propagating species wi
ll be a cationic phospheniummoiety.