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

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 15% 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 was decreased further (R = Es = 2,4,6-triethylphenyl, R = Mes = 2,4,6-trimethylphenyl), the substitution products RP(Cl)N(Pri)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 6awith bromine gave an E/Z mixture of the C-bromo-C-iodophosphaalkene (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).