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. π‐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. [ABSTRACT FROM AUTHOR]