Chemistry of sulphines. Part XXVIII. Barriers to rotation in highly substituted arylthio, alkyl- and aryl-sulphinyl, and arylsulphonyl E- and Z-sulphines

Abstract

The barrier to rotation about the aryl–sulphine bond (α-barrier) in the E- and Z-sulphines (I) and (II) was determined by means of coalescence of the two diastereotopic o-methyl groups of the mesityl ring A. In the sulphinyl E-sulphine (III), in which the ortho-positions of ring A are unsubstituted, no barrier was observed. To determine the barrier about bond α in the arylthio and sulphonyl sulphines, a diastereotopic marker, the isopropyl group, was introduced in the meta-position of the ring A. Coalescence of the two methyl groups of Prⁱ in arylthio sulphines (V) and (VI) and sulphonyl sulphines (IX) and (X) affords a method to measure the α-barrier in these sulphines. For comparison the α-barrier in the corresponding S-aryl thiocarboxylates (XI) and dithiocarboxylates (XII) were also determined. In the sulphinyl E- and Z-sulphines (VII) and (VIII), two rotational diastereoisomers are observed in the n.m.r. At higher temperatures there is a fast exchange between these diastereoisomers by rotation about α, resulting in coalescence of the two different sets of signals and thus providing an alternative method to determine the α-barrier in these sulphinyl sulphines. In the E-sulphines (VIIa—c) and in the Z-sulphine (VIIIc) the α-barrier is > 23 kcal mol^–1 and therefore the two rotational diastereoisomers could be isolated at room temperature. For these sulphines the direct equilibration method was also used to determine the α-barrier. The activation parameters of the α-barrier in these latter sulphines were calculated by means of an Arrhenius plot. In the sulphines with an S-mesityl group [(Ie), (IIe), (IV), (Vc)—(Xc)] in most cases a rotational barrier about the mesityl–sulphur bond was observed (β-barrier). This barrier was obtained by means of coalescence of the two o-methyl groups of the S-mesityl group. The α- and β-barriers are discussed in terms of steric interactions encountered in the preferred conformations for the different types of sulphines.