Chlorination of p-substituted triarylpnictogens by sulfuryl chloride: Difference in the reactivity and spectroscopic characteristics between bismuth and antimony

Competitive oxidative chlorination of p-substituted triarylstibines 3 [(p-XC6H4)3Sb; a: X = OMe, c: Cl, d: CO2Et, e: CF3, f: CN, g: NO2] by sulfuryl chloride was carried out against 3b (X = H) and the electronic effect of these substituents on the chlorination of 3 was compared with that of homologous triarylbismuthanes 1. The relative ratios 4/4b (Ar3SbCl2/Ph3SbCl2) decreased with increasing electron-withdrawing ability of the substituents (a: 53/47, c: 49/51, d: 46/54, e: 44/56, f: 40/60, g: 37/63), but the tendency was not so pronounced as observed in the chlorination of 1. A Hammett plot of the 4/4b ratios against the σp constants exhibited a good linear relationship with a negative slope, the value of which was almost half of that deduced from the 2/2b (Ar3BiCl2/Ph3BiCl2) ratios. The difference in the reactivity between 1 and 3 may be explained by the effect of the electron-withdrawing substituents in the aromatic rings, which affects the p-character of the lone pair on the pnictogen atoms by increasing the positive metal charge and appears more remarkably in 1 than in 3. The 13C NMR study of 3 revealed that the chemical shifts of the ipso carbons (C1) attached to the antimony show a linear relationship against the σp constants with a positive slope (14.5). The value was smaller than that deduced from 1 (17.0), suggesting that the antimony center of 3 is less sensitive to the substituent effect. This is in accord with the tendency of the chlorination.