Recognition of atropisomerism and thermally labile prochirality, and measurement of barriers to rotation in sulfines (thione oxides) by the use of chiral shift reagents

Abstract

AbstractThe sulfines 1–9 in which the o‐positions of the aryl ring A are substituted by methyl groups, have a high rotational barrier about the aryl A to sulfine bond (bond α). Due to this α‐barrier, sulfines 1 and 2 exist as a pair of rotational enantiomers (atropisomerism). The symmetrically substituted sulfines 3,4,7,8 and 9 possess a prochiral centre, viz. the mesityl‐CSO part, provided rotation about bond α is restricted. With rapid rotation about bond α this prochirality disappears (thermally labile prochirality). Chiral lanthanide shift reagents are employed to uncover these chiral properties. The atropisomerism in the sulfines 1 and 2 and the thermally labile prochirality in the sulfines 3,4, 7, 8 and 9 are demonstrated by use of chiral shift reagent. Moreover, chiral shift reagents are successfully used to determine the α‐barrier in the sulfines 3, 4 and 9.