Basicity Addition of Alkyl or Acyl Ions

In contrast to X -phosphorins, X -phosphorins can be protonated. The basicity is very much influenced by the nature of the substituents R and R at the phosphorus. 1.1-Dialkyl or 1.1-diaryl-X -phosphorins are even protonated by aqueous HCl the salts are deprotonated by aqueous NaOH. Strong acids in organic solvents, e. g. trifluoroacetic acid in hexane or benzene, (see p. 106), are required to proto-nate 1.1-dialkoxy-X -phosphorins. Addition of tert-butoxide deprotonates the salt. By studying the NMR spectra of 1. l-dimethoxy-2.4.6-tris-pentadeuterophenyl-X -phosphorin 185 in benzene solutions containing H and D-trifluoroacetic acid Stade could show that two different protonation products are formed in a ratio of 3 1. One product is the result of C—2 protonation 186 the other of C-4 protonation 187 (Fig. 38). Similar results were observed in the case of 1.1-bis-dimethylamino-2.4.6-triphenyl-X -phosphorin 

Only those 1.1-diphenyl-X -phosphorins in which the phosphorin ring is unsubstituted could be alkylated or acylated at the ring (see p. 77). The 2.4.6-tri-phenylated 1.1-disubstituted A -phosphorins cannot be alkylated by oxonium salts or acylated by acylchlorides under normal conditions. 

According to method K (p. 87), one of the amino groups in 1.1-bis-diarylamino-or l.l-bis-dialkylamino-2.4.6-triphenyl-A -phosphorins can be replaced by an alkoxy group by treatment with alcohols in the presence of trifluoroacetic acid. If thio-alcohols are used, both amino groups are exchanged by alkylthio groups. Compounds of type 153 or 154 (p. 85) are also accessible to these exchange reactions. 

At present nothing definite can be said about the mechanism of these reactions. Protonation occurs at C-2 and C-4 of the ring. This has the effect of widening the C-P—C angle of the phosphorin ring of about 105—108°. In the transition state the C-2 and C—6 atoms remain in the equatorial position. The more bulky (or more negative) substituent B at the P atom should be axial The smaller (or less eletro negative) substituent A should occupy the equatorial position. 

Only the axial substituent should be displaced by the incoming nucleophile. 

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