Stereochemical Behavior of Pairs of P-stereogenic Phosphanyl Groups at the Dimethylxanthene Backbone

The P-stereogenic bis(phosphanes) 7 and 9, featuring pairs of P(Mes)-ethynyl or vinyl substituents at the dimethyl xanthene backbone show rather low barriers of stereochemical inversion at phosphorus. pi-Conjugative effects are probably causing these low inversion barriers. Compound 7 reacted with B(C6F5)(3) to form the nine-membered heterocyclic product 10, featuring a [P] C=C B(C6F5)(3) substituent. Compound 7 was converted to the bis[P-(Mes)vinyl] xanthene derivative 9, which gave the zwitterionic P(H)(Mes) CH=CH B(C6F5)(3) containing product 16 upon treatment with B(C6F5)(3). Thermally induced epimerization barriers at phosphorus of ca. 20 to 27 kcal mol(-1) were calculated by DFT for the alkenyl- and alkynyl-P derived systems 6 to 9, 15 and 16 and experimentally determined for the examples 7 and 16.