Whereas tin(II) chloride, or zinc chloride, catalyzed reaction of trimethylsilyl azide (TMSA) with carbonyl compounds gave gem-diazides 3, a catalytic amount of sodium azide/15-crown-5 promoted an addition reaction of TMSA toward these compounds to give α-siloxy azides 2 exclusively. A stereoelectronic effect was found to be important for these reactions.