Carbocations silyl enol ethers

Silyl enol ethers and silyl ketene acetals also offer both enhanced reactivity and a favorable termination step. Electrophilic attack is followed by desilylation to give an a-substituted carbonyl compound. The carbocations can be generated from tertiary chlorides and a Lewis acid, such as TiCl4. This reaction provides a method for introducing tertiary alkyl groups a to a carbonyl, a transformation that cannot be achieved by base-catalyzed alkylation because of the strong tendency for tertiary halides to undergo elimination. 

In these reactions boron trifluoride assisted ring opening occurs to give a carbocation (1 to the silyl group and then rearrangement gives the product silyl enol ether (Figure Si5.16 illustrates the pathway taken by the reaction depicted in Equation Si5.24.)... 

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. 

You may protest that these last two species are not carfoo-cations at all but rather sulfonium ions, and you would be right. However, they can be used in place of carbocations as they are electrophilic at carbon so it is useful to think of them as modified carbocations as well as sulfonium ions. Sulfur-stabilized a-cations are easily made from a-chlorosulfides and are useful in alkylation of silyl enol ethers. 

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. 

Cyclobutadiene iron tricarbonyl complexes also stabilized carbocations on an adjacent carbon. The cation reacts with silyl enol ethers to afford alkylated complexes such as (127) (Scheme 187). A samarimn diiodide -mediated intermolecular radical cychzation of iron tricarbonyl complex (128) is depicted in Scheme 188. An excellent stereocontrol at three contiguous centers is observed. 

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. 

The coupling of an allyl or acyl moiety onto carbon atoms is achieved by anodic oxidation of a-heteroatom substituted organostannanes or Oj -acetals in the presence of allylsilanes or silyl enol ethers. The reaction probably involves carbocations as intermediates that undergo electrophilic addition to the double bond [245c]. 

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. ... 

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. 

Silyl enol ethers react with carbocations to form a carbon-carbon bond upon the release of the trimethyl silyl group. Verma et prepared ester end-functionalized poly(vinyl... 

The / -silicon effect on carbocation stability has been widely exploited in organic synthesis, perhaps most notably in the reactions of allylsilanes toward electrophiles. In this, allylsilanes behave remarkably like silyl enol ethers, as shown by the following generic transformations ... 

The stability of carbocations flanked by n-coordinated organic moieties is dramatically enhanced so that they react easily with nucleophiles [125]. Magnus et al. [126] applied this principle to the synthesis of the core structure (208) of the diynene antibiotics esperamicim and calicheamicins. As shown in Scheme 68, diynene 205 was converted to enol silyl ether which was treated with Co2(CO)g to give dicobalt hexacarbonyl adduct 206. Exposure of 206 to 3 equiv of TiCU and 1 equiv of DABCO at —43 to —35 °C gave macrocycle 207 in 50% yield. 

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