Dibismuthanes in catalysis: from synthesis and characterization to redox behavior towards oxidative cleavage of 1,2-diols†

A family of aryl dinuclear bismuthane complexes has been successfully synthesized and characterized. The two bismuth centers are bonded to various xanthene-type backbones, which differ in ring-size and flexibility, resulting in complexes with different intramolecular Bi⋯Bi distances. Moreover, their pentavalent Bi(v) analogues have also been prepared and structurally characterized. Finally, the synergy between bismuth centers in catalysis has been studied by applying dinuclear bismuthanes 5–8 to the catalytic oxidative cleavage of 1,2-diols. Unfortunately, no synergistic effects were observed and the catalytic activities of dinuclear bismuthanes and triphenylbismuth are comparable.

The reaction mixture was quenched with a saturated aqueous solution of NaHCO3 and diluted with Et2O, whereupon it was extracted twice with Et2O (2 × 8 mL). The combined organic phases were dried over MgSO4, filtered and concentrated under reduced pressure (not to dryness!) [a] . The crude reaction mixture was then purified by flash chromatography (SiO2, 20:1 hexane:EtOAc). The obtained solid was washed further with cold pentane (2 × 5 mL) to yield the desired complex 7 as an off-white solid (234 mg, 45% yield). X-ray quality crystals were obtained from slow evaporation of a solution of complex 7 in CH2Cl2:hexane (1:5) at 23°C.
[a] Note: To avoid dismutation, which results in lower yields, it is advised to not to reach dryness. [4] [b] Note: One quartenary carbon signal was not observed in the 13 C NMR spectra. To avoid a misassignment, the observable quaternary carbons were assigned as Cq. S12
The mixture was placed in an oil bath and heated at 70 °C for 3 h. Then, the solution was cooled to room temperature, additional 15 mL of anhydrous THF were added and the mixture was cooled to -10 °C. Finally, Ph2BiOTs [4] (455.8 mg, 0.85 mmol, 2 equiv.) was added in one portion and the solution was left to stir for 1.5 h at -10 °C. The mixture was quenched with a saturated aqueous solution of NaHCO3 and diluted with Et2O, whereupon it was extracted twice with Et2O (2 × 10 mL X-ray quality crystals were obtained a phase transfer diffusion (5:1) of pentane into a concentrated solution of complex 8 in Et2O at 23 °C.

S13
[a] Note: To avoid dismutation, which results in lower yields, it is advised to not to reach dryness. [4] [b] Note: One quartenary carbon signal was not observed in the 13 C NMR spectra. To avoid a misassignment, the observable quaternary carbons were assigned as Cq. S14
[b] Note: One quartenary carbon signal was not observed in the 13 C NMR spectra. To avoid a misassignment, the observable quaternary carbons were assigned as Cq. S17
[b] Note: To avoid a misassignment, the observable quaternary carbons were assigned as Cq.

Barton´s proposed mechanism
Barton and co-workers proposed a mechanism for the Bi-catalyzed oxidative cleavage of 1,2-diols based on NMR spectroscopy and experimental evidences. [6] In the first step, the glycol reacts with NBS to form a hypobromite species, which acts as an oxidant of BiPh3 to form a pentavalent Bi-alcoxy intermediate. The last step is a base-induced reductive elimination with cleavage of the C-C bond to the carbonyl derivatives and regenerating triphenylbismuth.

Xray single crystal analysis
Single crystal structure analysis of 5 (13712)

INTENSITY STATISTICS FOR DATASET
A resolution cut off (SHEL 99 0.7) was applied to suppress poorly measured intensities at higher diffraction angles. Complete .cif-data of the compound are available under the CCDC number CCDC-2063973.   (7) Bi (1)