Synthesis, X-ray structure, thermal stability and reactions of triaryl(3-oxoalkyl)bismuthonium salts

Abstract

Treatment of triarylbismuth difluorides 2 with siloxycyclopropanes 3 in the presence of trimethylsilyl trifluoromethanesulfonate (triflate) or boron trifluoride–diethyl ether gave the corresponding triaryl(3-oxoalkyl)bismuthonium salts 4 as crystals. X-Ray crystallographic analysis of the triflate 4a showed that the central bismuth atom has a distorted tetrahedral geometry with weak intramolecular coordination by the oxygen atoms of the carbonyl group and triflate anion. Whilst all onium salts 4 are thermally stable in a solid state, the triflate 4a slowly decomposed in chloroform to give triphenylbismuthane 6 and an ester 7 quantitatively. In nucleophilic solvents such as MeOH, DMSO and DMF, both triflate 4a and tetrafluoroborate 4d underwent decomposition in a complicated way to afford the bismuthane 6, 3-methoxypropanoyl compounds 8a,b, α,β-unsaturated carbonyl compounds 9a,b, diphenylbismuth triflate 10a or tetrafluoroborate 10b and alkoxysulfonium salts 11a,b. The formate 12 was an additional product from 4d in DMF. The bismuthonium salt 4d reacted with dimethyl sulfide 13 and sodium benzenesulfinate 15 to afford the sulfonium salt 14 and the sulfonyl ketone 16, respectively. Reaction of the salt 4d with potassium halides 17 in DMF gave the enone 9b, formate 12 and halogeno ketone 18 in varying yields depending on the nucleophilic nature of the halogen anions employed. When treated with KOBu^t, the salt 4d underwent facile β-elimination to yield the enone 9b. In all of these reactions, the triphenylbismuthane moiety behaved as a good leaving group.