Abstract
Two stereoisomers, the trans form 1a and the cis form 1b, of 2,3,7,8-tetramethyl-5,10-diphenyl-5,10-dihydrophosphanthrene dioxide (1) were formed by spontaneous oxidation in air of the corresponding 5,10-dihydrophosphanthrenes 2 and were separated by recrystallization. Both 1a and 1b were characterized by X-ray crystallographic analysis, which revealed configurations at the phosphorus and conformations of the central tricyclic system. The 31P NMR of the reaction mixture showed at first two signals at −15.3 and −15.1 ppm, which shifted to 9.11 and 10.3 ppm after purification of the reaction mixture under aerobic conditions by oxidation of phosphorus atoms. The reaction of 1b with HSiCl3 in toluene-d8 at 100 °C gave no change, whereas 1a reacted with HSiCl3 to give reduced compounds 2a and 2b under the same conditions. The trans–cis isomerization of 2 occurred via vertex inversion at phosphorus atom. VT-NMR, X-ray analysis, and density functional theory (DFT) calculation showed that the conformational exchange of the cis form 1b proceeded via skeletal inversion of the tricyclic ring.
Acknowledgments
This study was partially supported by a Grant-in-Aid for Encouragement of Young Scientists (B) to Y. U. (No. 16750041 and No. 18750037) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and a Kitasato University Research Grant for Young Researchers.