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

The reaction of iminotriaryl-lambda(5)-bismuthanes (2; Ar3Bi=NR; R = COR " or SO2R ") with dialkyl acetylenedicarboxylates (3; R'O2CC=CCO2R') has been found to afford highly stabilized bismuthortium 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. T he observed Bi-C-ylide bond lengths of 2.178(7)-2.199(7) Angstrom are close to ordinary Bi-C-Ar bond lengths, suggesting an appreciable single bond ch aracter of the Bi-C-ylide bond. The action of two equivalents of acetic aci d, hydrogen chloride, or 4-nitrophenol on 4 cleaved the Bi-C-ylide 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 e quivalents of 4 to give bis(4-nitrophenyl) disulfide with a good recovery o f triarylbismuthane 6 and olefin 5. On thermolysis at 200 degrees C, or whe n 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. (C) 2000 Elsevier Sci ence S.A. All rights reserved.