Opposite regioselectivity in the sequential ring-opening of 2-(alkanoyloxymethyl)aziridinium salts by bromide and fluoride in the synthesis of functionalized β-fluoro amines

1-Arylmethyl-2-(bromomethyl)aziridines were converted into the corresponding 2-(alkanoyloxymethyl)aziridines upon treatment with potassium 2-methylpropanoate or potassium 2-methylbutyrate in DMSO in excellent yields, following regio-selective ring-opening towards N-(2-bromo-3-alkanoyloxypropyl)amines using allyl bromide or an arylmethyl bromide in acetonitrile. Treatment of the latter p-bromo amines with tetrabutylammonium fluoride in acetonitrile afforded 2-amino-1-fluoropropanes as the major compounds (72-86%) besides the isomeric 1-amino-2-fluoropropanes in minor quantities (14-28%). The ring-opening of the intermediate aziridinium salts by bromide and fluoride in acetonitrile resulted in a different regioselectivity with a preferential attack of bromide at the more hindered carbon atom and of fluoride at the less hindered carbon atom of the aziridinium ion.