Synthesis of Enantiomerically Pure <i>N</i>-<i>tert</i>-Butanesulfinyl Imines (<i>tert</i>-Butanesulfinimines) by the Direct Condensation of <i>tert</i>-Butanesulfinamide with Aldehydes and Ketones

Experimental details for the first general methods for the one-step preparation of N-tert-butanesulfinyl imines (tert-butanesulfinimines) (2) from aldehydes and ketones is described. To effect the condensations of tert-butanesulfinamide (1) with aldehydes, the Lewis acidic dehydrating agents MgSO4, CuSO4, or Ti(OEt)4 are employed. Aldehyde condensations mediated by MgSO4 proceed in high yields (84−96%) when an excess of aldehyde is used. In contrast, only a slight excess of aldehyde (1.1 equiv) relative to tert-butanesulfinamide provides sulfinimines in high yields when the more Lewis acidic dehydrating agent CuSO4 is used. The CuSO4-mediated procedure is effective for a wide range of aldehydes, including sterically demanding aldehydes, such as isobutyraldehyde (90%), and electron-rich aldehydes, such as p-anisaldehyde (81%). The still more Lewis acidic Ti(OEt)4 and Ti(O-i-Pr)4 also afford N-tert-butanesulfinyl aldimines from especially unreactive aldehydes, such as pivaldehyde (82%). In addition, Ti(OEt)4 is effective for the condensation of 1 with ketones to afford a wide range of N-tert-butanesulfinyl ketimines in good yields (77−91%). For sulfinyl ketimines derived from methyl or n-alkyl phenyl ketones and methyl or n-alkyl isopropyl ketones, only the E isomer is detected by 1H and 13C NMR in CDCl3. For those cases where the difference in steric demand about the imine is very small, such as for 2-hexanone, high E/Z ratios are still observed (5:1).