Phosphorus–carbon exchange promoted by AlCl3: reactivity of heterocyclic systems containing the S–P–P unit

The reaction under mild conditions of fused cis-1,2,3-benzothiadiphospholes (1) with RCOCl–AlCl3(R = Me, Et, Pri, CH2(CH2)2CH2Cl, hexyl, CH2Ph) gave fused cis-6-R-1,3-benzothiaphospholes (2)via a phosphorus carbon exchange, in good yields. Use of a variety of acyl chlorides showed that the reaction does not occur when the R group is very bulky. Following the reaction by g.l.c.–m.s., it was noted that, initially, trans-(2) is formed and this isomerizes to cis-(2). A mechanism for this unusual phosphorus–carbon exchange is reported.