Stabilized bismuthonium ylides bearing a highly cross-conjugated ylidic carbon atom: synthesis, structures, and reactions

The reaction of iminotriaryl-λ5-bismuthanes (2; Ar3BiNR; R=COR″ or SO2R″) with dialkyl acetylenedicarboxylates (3; R′O2CCCCO2R′) has been found to afford highly stabilized bismuthonium ylides (4; Ar3BiC(CO2R′)C(CO2R′)NR) in 50–92% yield. The X-ray crystallographic analyses of two of these ylides demonstrated that their bismuth center possesses a distorted tetrahedral geometry with the highly cross-conjugated ylidic carbon atom. The observed BiCylide bond lengths of 2.178(7)–2.199(7) Å are close to ordinary BiCAr bond lengths, suggesting an appreciable single bond character of the BiCylide bond. The action of two equivalents of acetic acid, hydrogen chloride, or 4-nitrophenol on 4 cleaved the BiCylide bond to give the corresponding triarylbismuth(V) compounds of the type Ar3BiX2 and olefins (5; R′O2CCHC(CO2R′)NHR). 4-Nitrobenzenethiol was oxidized by 0.5 equivalents of 4 to give bis(4-nitrophenyl) disulfide with a good recovery of triarylbismuthane 6 and olefin 5. On thermolysis at 200°C, or when decomposed in the presence of a copper catalyst at room temperature, the ylides 4 bearing an N-aroyl group gave 2,4,5-trisubstituted oxazoles 7 and triarylbismuthanes 6 in moderate to excellent yields.