Alkylation of Benzothiazolines and the Stevens Rearrangement of the Resulting 2,3,3-Trisubstituted Benzothiazolinium Salts

Article abstract of DOI:10.1246/bcsj.55.2976

Alkylation of 2-substituted 3-methyl- or 3-ethylbenzothiazolines with Meerwein reagents gave 2-substituted 3,3-dialkylbenzothiazolinium tetrafluoroborates (3).The configuration of two alkyl groups on the nitrogen was assigned by NMR spectra and NOE measurement.In the Stevens rearrangement of 3 with lithium diisopropylamide ethyl group showed a much larger migratory aptitude (Et:Me>20:1) than methyl group irrespective of the configuration of 3, and cyclic ammonium ylide with planar ?-type carbanion was proposed as an intermediate. 3 suffered nucleophilic attack at the ring sulfur atom by butyllithium to afford a ring-opened ammonium ylide, which collapses to a radical pair to give unusual Stevens rearrangement product, where o-alkylthiophenyl group migrated selectively in preference to alkyl group, because of stabilization by participation of o-alkylthio group.