Coordination of a chiral tin(ii) cation bearing a bis(oxazoline) ligand with tetrahydrofuran derivatives

Abstract

The reaction of SnCl₂ with the lithio derivative of a bis(oxazoline) ligand precursor afforded the enantiomeric chlorostannylene whose chloride ion can be substituted by several neutral or anionic Lewis donors. Abstraction of the chloride ion from the chlorostannylene with silver salts gave the corresponding tetrahydrofuran (THF) complexes of a chiral tin(II) cation in 1,2-dimethoxyethane (DME) containing THF. That is, the reaction with silver hexafluoroantimonate (AgSbF₆) afforded the THF complex without interaction with the counteranion. In contrast, reaction with silver triflate (AgOTf) gave the THF complex whose tin center had a pseudo-trigonal bipyramidal structure with two nitrogen atoms of a bidentate ligand and a lone pair at the equatorial positions and one of the oxygen atoms of triflate and an oxygen atom of THF at the apical positions in the solid state. Use of 3-methyltetrahydrofuran (3-MeTHF) instead of THF afforded the 3-MeTHF complexes, where the R-enantiomer of 3-MeTHF predominantly coordinates to the tin center. The previously reported germanium(II) analogue of the tin(II) cation indicated a similar enantioselectivity for the coordination of 3-MeTHF on the germanium center.