Lewis Acid-catalyzed Aldol Reactions of Silicon Enolates

Lewis Acid-catalyzed Aldol Reactions of Silicon Enolates 

Initially, the titanium(IV) chloride-mediated aldol reaction of silyl enolates tvith aldehydes tvas investigated [20] and a catalytic amount of trityl salt 60 (e.g. trityl perchlorate) vas found to promote the aldol reaction (Eq. (27)) [35]. Whereas the original reaction is performed by using a stoichiometric amount of titanium(IV) chloride, 5-10 mol% trityl salt is sufficient to drive the aldol reaction to completion. One interesting finding in this catalytic reaction is that the silicon enolate reacts vith aldehydes to give the corresponding aldol adducts as their silyl ethers 61. 

The aldol reaction of a variety of silyl enolates and acetals in the presence of a catalytic amount (1-10 mol%) of trityl perchlorate proceeded efficiently to afford /i-methoxy ketone in high yield. In the presence of trityl tetra-fluoroborate catalyst, the reaction of dithioacetal 62 vith silyl enol ethers affords /i-ethylthio ketones 63 (Eq. (28)) [36]. 

A specific combination of t vo veak acids, tin(II) chloride and chloro-trimethylsilane, is found to serve as an effective catalyst for the aldol reaction [37]. Neither chlorotrimethylsilane or tin(II) chloride has any accelerating effect at —78 °C even vhen more than one equivalent is added. In the combined presence of catalytic amounts of chlorotrimethylsilane and tin(II) chloride, hovrever, the aldol reaction gives the desired product in more than 90% yield (Eq. (29)). It is supposed that the cationic silyl species generated by coordination of the chloride to the tin(II) atom catalyzes the aldol reaction. Because a similar cationic silicon Le vis acid, trimethylsilyl trifiate, is kno vn to catalyze aldol reactions of silicon enolates [38], and several other unique Le vis acid catalysts have also been reported (e.g. lanthanide catalysts [39]). Water-stable Le vis acids such as lanthanide trifiates, vhich catalyze the aldol reaction in aqueous solvent or pure vater, have received much attention in the development of economical and environmentally benign synthetic methods [40]. 

Silicon enolate-mediated aldol reactions are performed by other activation methods. The aldol reaction is performed by using a catalytic amount of tetrabutylammonium fiuoride (TBAE) or tris(diethylamino)sulfonium di-fiuorotrimethylsiliconate (TASE) (Table 3.4) [41]. Aldehydes undergo this 

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Transition-state models for Lewis acid-catalyzed aldol reaction of silicon enolates. 

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