Synthesis, Crystal Structures, and Skeletal Inversion of Dithienodiphosphorines

trans- and cis_endo-Diphenyl[c]dithienodiphosphorines 1a and 1b were successfully synthesized as phosphorus atoms are trivalent. Two stereoisomers, trans form 1a and cis_endo form 1b, were isolated by preparative gel permeation chromatography (GPC) and their butterfly structures were shown by X-ray crystallographic analyses of the single crystals from hexane/CHCl3 and CH3CN/CHCl3, respectively. The VT-P-31{H-1} NMR spectra in CD2Cl2 showed that the signal due to cis_endo form 1b appeared at (P) -45.0 with 9.5Hz of half-width at -90 degrees C, which is almost same as that at room temperature, while broadening of the signal due to trans form 1a was observed and finally the signal appeared at (P) -33.7 with 55.5Hz of half-width at -90 degrees C. The results indicated that the skeletal inversion of a tricyclic system in solution is a fast process on the NMR timescale. On the other hand, X-ray crystallographic analysis showed that trans-dimesityl[c]dithienodiphosphorine 2a has a planar structure, which is considered as the corresponding transition state of the skeletal inversion of trans form 1a. The P-31 signal due to 2a in CD2Cl2 did not change at all even at -90 degrees C, which was in a different manner from 1a at the same temperature, showing that its planar structure is also kept in solution because of the release of the steric repulsion between bulky mesityl groups and fused thiophene rings in the butterfly structure. The density functional theory calculations suggested the low-energy barriers of the skeletal inversion of trans-trans and cis_endo-cis_exo forms, which were estimated to be 3.79 and 3.31 kcal/mol by searching for transition state using the synchronous transit-guided quasi-Newton (STQN) methods.