Imines reactions with enol silanes

Enol silanes (181), as reported by Pilli and Russowsky, add to arylimines (182) in the presence of catalytic amounts of TMS-OTf to afford (V-aryl- 3-amino ketones (183 equation 20).144 The reaction is carried out by adding the enol silane to the imine in the presence of 15 mol % of TMS-OTf at 0 C in CH2CI2, followed by aqueous work-up. Yields range from 50% to 98% in a series of benzylideneaniline derivatives using the enol silanes derived from acetophenone and f-butyl methyl ketone. The extent to which the reaction may be limited to nonenolizable imines, as in the analogous TMS-OTf-promoted reactions of silyl ketene acetals (see Section 4.1.3.2.2.iii),135 136 is not reported. The authors describe some spectral characteristics of an insoluble material formed by treating benzylideneaniline with TMS-OTf in CHCb. The data are consistent with the derived N-silyliminium salt of (182), the putative reactive intermediate. 

The TiCU-mediated reaction of enol silanes with imines was first introduced by Ojima and coworkers in 1977. The reaction was then extended to several similar substrates, i.e. nitrones, ot-methoxycarba-mates, aminals, 4-acetoxyazetidin-2-one, 40 anj to different Lewis acids, i.e. SnCU, TiCU-(0PH)2, catalytic ZnX2, catalytic TMSOTf, ° to give good yields of the addition products with low levels ( 80 20) or a complete lack of simple stereoselection. Moderate to good anti selectivities were reported in the addition of silyl ketene acetals to imines under particular reaction conditions (equation 9) significant results are summarized in Table 4. 

Imines, being neutral species, are less reactive than iminium salts. Consequently, active methylene compounds with pKn values greater than -15-16 must be preactivated as an enolate or enol silane derivative. Lewis acids are sometimes used in reactions with enolates to improve reactivity and stereoselectivity, but in contrast to the reactions of preformed iminium salts, Lewis acids are required in reactions with enol silanes. Additional activation of the active methylene compound or catalysis is not usually required in reactions with more acidic active methylene compounds (pATa values below -15-16). In these reactions, higher acidity not only gives higher enol concentrations but also serves to catalyze the reaction through protonation of the imine. 

V-Acyliminium ions, generated from imines by acylation, are trapped in a one-pot reaction with reactive ir-nucleophiles like allylstannanes and enol silanes (equation 88). Thus, 3-dihydrocarboline... 

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