Aromatic heterocycles containing arsenic and sulfur: The synthesis of 1,2- and 1,3-thiaarsole and the gas-phase molecular structure of 1,2-thiaarsole

The reaction of chloromethyl ethynyl sulfide with Bu2SnH2/LDA affords 2,3-dibutyl-2,3-dihydro-1,3-thiastannole which on reaction with AsBr3 followed by DBU affords 1,3-thiaarsole in 22% yield. The reaction of allyl mercaptan with 2 equiv of BuLi followed by 1 equiv of Me2SnCl2 affords an 80% yield of 2,3-dihydro-2,2-dimethyl-1,2-thiastannole which on reaction with AsBr3 followed by DBU gives 1,2-thiaarsole in 56% yield. The NMR, UV and mass spectra are consistent with aromaticity of both thiaarsoles. The structure of 1,2-thiaarsole, measured by electron diffraction, suggests that it is an aromatic compound. The thermal average bond lengths (r(g)/Angstrom) and bond angles (angle(alpha)/deg) with estimated 2 sigma uncertainties an r(C-H) = 1.078(14), r(C-3-C-4) = 1,414(10), r(C-4-C-5)= 1.382(9), r(As-C) = 1.829(4), r(S-C) = 1.711(5), angle(CCC) = 115.0(12), angle(AsCC) = 117.7(10), angle(SCC) = 117.9(9), angle(SAsC) = 89.8(5), angle(AsSC) = 96.6(4). Ab initio molecular orbital calculations at the B3LYP/6-31G* level were performed for both thiaarsoles.