Synthesis and Reactions of P ‐Supermesityl‐ C ‐halophosphaalkenes

C‐Halophosphaalkenes ( 1, 2 ) were prepared in high yield by a one‐pot synthesis from HCX 3 or CI 4 and Mes* PCl 2 (Mes*  supermesityl  2,4,6‐tri‐tert‐butylphenyl). The C‐iodophosphaalkenes Mes* P  Cl 2 ( 1 C ) and Mes*P  CHI ( 2c ) undergo halogen‐metal exchange with n ‐BuLi at low temperatures. The resulting carbenoids Mes*P  CILi ( 11 c ) and Mes*P  CHLi ( 6 ) reacted with ClMMe 3 (M  Si, Ge, Sn) to give Mes*PCIMMe 3 [(Z)‐ 13 : M  Ge; (Z)‐14: M  Sn] or Mes*P  CHMMe 3 ( 8 – 10 ), respectively. Further reaction of (Z)‐ 13 and (Z)‐ 14 with n BuLi and ClMMe 3 gave Mes*PC(MMe 3 ( 18 : MGe; 19 : M  Sn). The carbenoid (Z)‐ 11 c decomposed at −85°C; instead of the expected “phosphaisonitrile” Mes*P  C : ( 21 ), only Mes*CP ( 15 ) was obtained which lends experimental support to the theoretically predicted instability of 21 .