Synthesis of 2-azetidiniminium salts .1. Diastereoselectivity in keteniminium triflate/imine cycloadditions

A systematic study of the synthesis of 2-azetidiniminium triflates, by annulation of aldimines with iminium salts derived from tertiary carboxamides and trifluoromethanesulfonic anhydride, has been carried out. The stereochemical output of a number of 2-azetidiniminium triflates is compared with that of the corresponding chloride salts synthesized by reaction of the same imines with a-chloro iminium chlorides. As a general rule, the stereochemical output of the reactions involving a-chloro iminium chlorides is in stark contrast to that of the corresponding triflates: while the chloride salts are trans stereoselective, the triflates show a preference for the cis products. The stereochemistry of the reactions involving the triflates has been examined in light of the structure of the reagents. Clear trends for a preferential formation of cis or trans products with the steric and electronic demand of the imine have been observed. By contrast, no correlation of the product distribution with the steric demand of the amide could be made. The transient formation of a keteniminium triflate intermediate has been suggested. According to this model, the annulation of the imine with the keteniminium triflates occurs with a mechanism closely similar to that observed in the Staudinger reaction. A comparison between the reactions involving the bona fide keteniminium triflates, and the corresponding isoelectronic ketenes with the same, or structurally closely related, imines has also been made. This comparison is performed in light of Georg's stereochemical rules that are used to explain, or predict, the stereochemical output of the Staudinger reaction.