C-Halophosphaalkenes: probing the range of stability and reactivity towards bromine

Article abstract of DOI:10.1002/recl.19941130503

The importance of steric protection for the stability of phosphaalkenes RP=CI2 (6) was investigated by varying the size of group R.The phosphaalkene IsP=CI2 (6b) (Is = 2,4,6-triisopropylphenyl) could be prepared in 15percent isolated yield by reaction of IsPCl2 and HCI3 with two equivalents of lithium diisopropylamide, in analogy to the synthesis of the stable, sterically more protected, Mes^*P=CI2 (6a) (Mes^* = 2,4,6-tri-tert-butylphenyl).If the steric protection on the phosphorus eas decreased further (R = Es = 2,4,6-triethylphenyl, R = Mes = 2,4,6-trimethylphenyl), the substitution products RP(Cl)N(i-Pr)2 (R = Es (9c) or Mes (9d)) were formed as main products, in addition to thermally unstable phosphaalkenes EsP=CI2 (6c) and MesP=CI2 (6d).The structures of 9c-d were corroborated by independent synthesis from RPCl2 and two equivalents of diisopropylamine.The reaction of 6a with bromine gave an E/Z mixture of the C-bromo-C-iodophoaphaalkene (EZ)-Mes^*P=CBrI (E/Z-10).Further reaction with bromine proceeded via Mes^*P=CBr2 (5a) and finally led to Mes^*P(Br)(CHBr2) (12).