Silyl enol ethers reactions with carbocations

Enamines are among the most powerful neutral nucleophiles and react spontaneously with alkyl halides. Silyl enol ethers are less reactive and so require a more potent electrophile to initiate reaction. Carbocations will do, and they can be generated in situ by abstraction of a halide or other leaving group from a saturated carbon centre by a Lewis acid. 

The best alkylating agents for silyl enol ethers are tertiary alkyl halides they form stable carbocations in the presence of Lewis acids such as TiCLj or SnCLj. Most fortunately, this is just the type of compounds that is unsuitable for reaction with lithium enolates or enamines, as elimination results rather than alkylation a nice piece of complementary selectivity. 

In the electrochemical oxidation, similar reaction was observed (Scheme 12). Cation radical CR26 generated by electrochemical oxidation of a-stannyl sulfides cleaves to give carbocation C26, which react with allyltrimethylsilane or the silyl enol ether of cyclohexanone to give the usual addition products. In this electrochemical reaction, stannyl derivatives also afforded the desired product 27 or 28 in better yield compared with the corresponding silyl derivatives. 

Coupling of silyl enol ethers or boron enolates with Co2(CO)6-stabilized carbocations, generated via Lewis acid treatment of the appropriate propargyl ethers or aldehydes (aldol reaction), via the Nicholas reaction has been used to obtain large, highly strained, ring ketones. 

Competition experiments between two ir-bonded nucleophiles within the same molecule were studied in an attempt to identify reaction parameters and factors responsible for regioselectivity. These experiments, summarized in heme 12, demonstrated that the dithioacetal (initiating carbocation) is in competition with two nucleophilic functional groups within the same molecule, a silyl enol ether and a vinylsilane. In this instance, when the thioacetal (29) was treated with DMTSF, complete chemoselectiv-... 

A second, less used, strategy encompasses the Lewis acid catalyzed intramolecular reaction of a silyl enol ether with a propargyl cation. The latter can be conveniently generated by a cobalt complexed propargyl ether. This complexation strongly helps the carbocation formation. By using cobalt complexation, intramolecular aldol type reactions (for R = OR ) have been accomplished. ... 

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