Enol silanes with enones

Michael Reactions of Enol Silanes with Enones Catalyzed hy Al-Mont... 

In the stannic chloride catalysed reactions of enol silanes with enones a preference for anti-addition is observed which is... 

Cyclohexanones can be dehydrogenated to enones by treatment of the corresponding enol silanes with DDQ in the presence of a catalytic amount of the bis-silyl derivative of acetamide. However, the yields fall off drastically when the reaction is applied to cyclopentanones and cyclo-heptanones. Another new method for dehydrogenation of carbonyl compounds consists of conversion to their pyridine-2-sulfIde derivatives, followed by oxidation (MCPBA), and by mild heat. ... 

Q Chiral racemic y-alkyl-substituted enones the titanium(IV) chloride mediated addition of enol silanes and silylketene acetals to 7 shows high induced diastereoselection (diastereomeric ratios from 89 11 to more than 97 3) and the major isomer 8 results from addition of the enolsilane with ul topicity288. Re face attack on the S enantiomer of 7.)... 

Regioselective deprotonation of a ketone regioselective resolution. Treatment of the (R,R)-ketone 2 with (S)-l provides the A -enol silane 3, whereas a similar reaction of the (S,S)-isomer of 2 provides the A -enol silane 4. Since 3 and 4 are difficult to separate, they were identified from their corresponding enones. In contrast treatment... 

In this context, lipshutz et al. reported in 2000 a catalytic reductive aldol reaction of enones and aldehydes with [PhsPCuHjs (5 mol%) and PhMe2SiH (150mol%) [46]. The two-step reaction was carried out in one pot, without isolation of the intermediate sUyl enol ethers, efficiently providing the b-hydroxyketones in high yield. Lewis acids such as BF3 or TiCLj are used to promote the second step involving aldol reaction of the enol silane. In place of hydrosilanes, dialkylboranes could be employed as hydride sources, circumventing the need to introduce additional Lewis acids. Here, the aldol products are formed via intermediacy of the boron-enolates, with 5y -selectively for acychc enones and antz-selectively for cycHc enones [47-50]. 

Enolization of racemic ketone 122 with the chiral lithium amide 123 produced a mixture of regioisomeric enol silanes 124 and 125 [78]. A treatment with PhSeCl and subsequent oxidation with dimethyldioxirane afforded the enones 126 and 127. This result shows a regiodivergent KR of racemic ketone 122. 

Allyl silanes are rather like silyl enol ethers they react with electrophiles, provided they are activated, for example by a Lewis acid. Titanium tetrachloride is widely used but other successful Lewis acids include boron trifluoride, aluminium chloride, and trimethylsilyl triflate. Electrophiles include acylium ions produced from acid chlorides, carbocations from tertiary halides or secondary benzylic halides, activated enones, and epoxides all in the presence of Lewis acid. In each case the new bond is highlighted in black. 

In order to explain this DKR, it was proposed that a copper enolate intermediate was formed on asymmetric conjugate reduction of the enone. Subsequent a-bond metathesis with silane yielded the silyl enol ether and regenerated the copper hydride catalyst. Under basic conditions, a rapid... 

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