Phospholium ions

All conversions of phosphines to derivatives with C.N.-4 are accompanied by sizeable downfield shifts, and this is true of phospholes. Phosphole oxides are rare species, since they tend to dimerize unless sterically protected (see Section 2.15.6.1). Nevertheless, some data are available and collected in Table 5. The magnitude of the downfield shifting that takes place is not unlike that known for simple phosphine to oxide conversions. Phosphole sulfides and phospholium ions (both Table 5) also give downfield shifts of expected size. The 3,4-dimethyl shielding effect is evident in all of these classes on examining the tables. 

The site of protonation of phospholes in nonaqueous solvents by dry HCl or CFjSOjH has been found to be the phosphorus atom when the reaction is conducted at — 90°C <86JCS(P2)629> in pyrroles it is a ring carbon that undergoes protonation. The 1 //-phospholium ions are not stable with Cl as counterion, and consequences of the P-protonation by HCl are discussed in Section 2.15.5.3. When the counterion is nonnucleophihc, the l//-phospholium ions are sufficiently stable to be directly observable <71JHC877, 72CJC1008>. 

Alkylphospholes have been shown to be much weaker bases than other tertiary phosphines a pA a of 0.5 was recorded for the 1-methyl derivative <69JA3308>. The site of proton attachment in nonaqueous systems has been investigated <86JCS(P2)629> and it was conclusively shown that if the protonation is allowed to occur at low temperature (—70°C) and with an acid whose anionic part is nonnucleophilic (e.g. CF3SO2OH) the proton attaches to phosphorus to form a lAf-phospholium ion. These ions are quite stable up to about — 30 °C, but decomposition occurs at room temperature (Equation (4)). 

A similar effect brought about by electronic factors can also be obtained by polymethylation of the dienic ring, as with cyclopentadienes . The 3,4-dimethylsiloles and -germoles are generally stable in the monomeric state, except for some functional derivatives . The close analogy between the stability of C-unsubstituted and C-methylated siloles on the one hand and that of isoelectronic phospholium ions on the other has been emphasized. ... 

Dimethyl-l-butynyl(phenyl)iodonium tosylate has been employed with bis-(diphenylphosphino)methane to give the (terr-butyl)phospholium tosylate shown in equation 7892. The initial formation of an alkynylphosphonium ion with a free phosphino group has been proposed. Intramolecular cyclization of this intermediate followed by a 1,3-prototropic shift would lead to the observed product. Evidence for the probability of such an intermediate is provided by the fact that the alkynyliodonium tosylate, like the... 

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