Aminals reaction with enol silanes

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. 

Asynunetric Deprotonation/Protonation of Ketones. Lithium amides of chiral amines have been used for performing asymmetric deprotonations of symmetrically substituted (prochiral) ketones. The resulting optically active enols orenol derivatives (most frequently enol silanes) are highly versatile synthetic intermediates. Particularly useful for this purpose are chiral amines possessing Cj symmetry, such as (1). For example, reaction of 4-r-butylcyclohexanone with the lithium amide of (R,R)-(1) (readily prepared in situ by treatment of (1) with n-Butyllithium) is highly stereoselective the resulting enol silyl ether possesses an 88% ee (eq 4). ... 

Trimethylsilylation has been accomplished with a large number of reagents most of which are commercially available. The cheapest (chlorotrimethylsilane) and the most reactive (trimethylsilyl triflate) rapidly silylate hydroxyl groups in the presence of a suitable base such as pyridine, triethylamine, i-Pr2NEt, imidazole, or DBU but an aqueous workup is required to ensure complete removal of the resultant amine hydrochloride or triflate whence hydrolysis of the nascent TMS ether may occur. In some cases the insoluble salt may be removed by filtration without aqueous workup. A wide range of solvents can be used for the reaction such as dichloromethane, acetonitrile, THF, or DMF. Care must be taken with trimethylsilyl triflate (TMSOTf) since it will convert aldehydes and ketones to the corresponding enol silanes and it will open epoxides in a reaction that has preparative significance [Scheme 4.6]. Similar transformations can be accomplished with tert-butyldimelhylsilyl triflate (TBSOTf) or triethylsilyl triflate (TESOTf). ... 

The Stephenson s work undoubtedly stimulates a rapid development of visible-light-driven C(sp )-H bond functionalization of tertiary amines. Soon afterward, numerous nucleophiles were employed under the principle of iminium ions chemistry by visible-light photoredox catalysis (Scheme 3.4). The oxidative Mannich reaction of Af-Aryl tetrahydroisoquinoUnes with ketones [17] and enol silanes [18] were developed by Rueping and Xia, respectively. As a follow-up work. Rueping et al. [19] reported an efiicient alkynylation of a-C(sp )-H bond... 

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