Reaction of (5S)‐1‐azabicyclo[3.1.0]hexane with boron trifluoride etherate. Formation of an adduct and polymerization

Abstract The ring‐opening polymerization of (5S)‐1‐azabicyclo[3.1.0]hexane (1) derived from L‐proline was attempted, by use of a catalytic amount of boron trifluoride etherate as an initiator, in order to obtain optically active polymers. Chiral polymers (2) were indeed easily formed when the above aziridine 1 was polymerized in bulk or in benzene. On the other hand, a stable BF3‐adduct was obtained in the form of colorless crystals when the dilute benzene solution (1%) of 1 was allowed to react with an excess of boron trifluoride etherate. The elemental analysis and the NMR (19F, 11B, and 13C) spectra of this adduct showed it to be a (5S)‐1‐azabicyclo[3.1.0]hexane‐boron trifluoride (1:1) complex (3). It was also found that the adduct initiated the polymerization of aziridine 1 more mildly than boron trifluoride etherate.