Silyl enol ethers alkenes

As shown in Figure 3-2, titanium is coordinated with the oxygen from both the aldehyde and the alkene enol silyl ether. When aldehyde approaches the enol species, intermediate A is favored to B, and anti-aldol is obtained as the major product. Table 3-4 presents some results of these reactions. 

Since enol silyl ethers are readily accessible by a number of methods in a regioselective manner and since the trialkylsilyl moiety as a potential cationic leaving group facilitates the termination of a cyclization sequence, unsaturated 1-trialkylsilyloxy-1-alkenes represent very promising substrates for radical-cation cyclization reactions. Several methods have been reported on the synthesis of 1,4-diketones by intermolecular oxidative coupling of enol silyl ethers with Cu(II) [76, 77], Ce(IV) [78], Pb(IV) [79], Ag(I) [80] V(V) [81] or iodosoben-zene/BFa-etherate [82] as oxidants without further oxidation of the products. 

When substituted silanes are used instead of hydrogen, the process is referred to as silylformylation or silylcarbonylation. Only rhodium complexes catalyze the transformation of unsaturated compounds to silylaldehydes via the silylformylation reaction. Iridium complexes also are able to catalyze the simultaneous incorporation of substituted silanes and CO into unsaturated compounds, although during the reaction other types of product are formed. In the presence of [ IrCl(C03) ] and [Ir4(CO)i2]) the alkenes react with trisubstituted silanes and CO to give enol silyl ethers of acyl silanes [58] according to Scheme 14.10. 

In 1991, the Co2(CO)8-catalyzed reaction of terminal olefins was reported by Murai and co-workers. As two reaction sites are available in a terminal alkene, four isomers of enol silyl ethers are obtained, each having one additional carbon atom arising from CO incorporation [Eq. (37)].106... 

When iridium complexes, [IrCl(CO)3]n and Ir4(CO)12, are used to catalyze the reaction of terminal alkenes with HSiR3 and CO, good yields of enol silyl ethers of acylsilanes are obtained. One molecule of CO and two molecules of silane are incorporated regioselectively at the terminal carbon atom of the alkene to form a siloxy(silyl)methylene unit [Eq. (38)].107... 

This reaction is also a transfer dehydrogenative reaction, as two reactant hydrogen atoms are not incorporated into the enol silyl ether product but instead serve to hydrogenate another molecule of starting alkene. For example, in the reaction of vinylcyclohexane, ethylcyclohexane is obtained in equal amounts to the silylated product. Both iridium complexes effectively catalyze the reaction. Various silanes can be used, including di-ethylmethyl-, triethyl-, and dimethylphenylsilane. The reaction is successful for a range of terminal alkenes, even those bearing cyano, acetal, and epoxide functionalities. The E isomer of the product is predominantly formed. 

Carbolithiation reactions of ketone a,-dianions, generated by the above amine-free method with several alkenes, such as styrenyl derivatives, vinyl sulfides and vinylsilanes, can lead to the generation of ketone a,5-dianions (Scheme 15)14. For example, when one equivalents of triphenylvinylsilane was treated with a ketone a,-dianion, in THF, at 0 C for 1 h and the resulting reaction mixture was quenched by 2.2 mol equivalents of trimethylchlorosilane, the corresponding bis-silylated enol silyl ether was obtained. Substituted styrenyl derivatives, such as 1,1-diphenylethylene and cinnamyl alcohol, also underwent a smooth carbolithiation to give the corresponding ketone a,5-dianions. Similar addition reactions of ketone a,f)-dianions to vinyl phenyl sulfide took place smoothly to give a,5-dianions with a sulfur attached in the 5-position. 

The sUylformylation of alkenes to produce enol silyl ethers by [lrCl(CO)3] or lr4(CO)i2 has been reported by Murai and coworkers (equation 9). ... 

In this case, alkene insertion into the h-H bond is likely to occur first, producing a linear alkyl species. CO insertion would produce an acyl species, which would then be followed by reductive elimination of the acylsilane product. Enohzation, followed by rapid reaction of HSiR3 with the OH group, traps out the enol silyl ether. The Hy produced Irom this sUylation step is used to hydrogenate some of the starting alkene. Thus, the maximum yield will generally be only 66% of the enol silyl ether product. The enol silyl ethers can be readily converted into silyl ketones (equation 10). 

The synthesis of bicyclo[3.2.2]non-6-en-3-one (192) and the spiro-substituted derivatives (195) can be achieved via similar methodology. Thus, thermolysis of the enol silyl ether (190), which is readily derived from the ketone (189), gives the bicyclic substance (191). Mild acid hydrolysis of (191) affords (192 Scheme 27). In a similar fashion the exo-ketones (193) have been converted, via the enol silyl ethers (194), into the tricyclic keto alkenes (195). ... 

The Patemo-Buchi reaction is the photocycloaddition of an alkene with an aldehyde or ketone to form oxetanes. This transformation has been shown to proceed through a biradical intermediate, and up to three new stereocenters can be formed as a result of this reaction. A general mechanism for the reaction between an aldehyde and a chiral enol silyl ether is shown in Eq. (13.7) [18]. Allylic 1,3-strain is cited as the control element in reactions of this type, and diastereomeric ratios of >95 5 are reported for products 30 containing four contiguous stereocenters. Examples of photocyclizations of amino acid derivatives proceeding through biradical intermediates have been repotted [19]. 

Acyloin condensations of esters conducted with TMS-Cl and sodium in toluene give 1,2-bis(trimethyl-silyloxy)alkenes. In general, the silylacyloin reaction provides higher yields than the conventional acyloin reaction and is particularly useful for the preparation of cyclic enol silyl ethers. " The synthesis of compound (47) serves as an illustration. 

Carbonylative hydrosilylation. IrCl(CO)3 [or lr4(CO)i2] catalyzes this reaction with terminal alkenes to form the enol silyl ethers of acylsilanes. Acetal, epoxide, and cyano groups are not affected. 

Hydrosilylation in the presence of a carbon electrophile is often accompanied by C-C bond formation. For example, three-component coupling of hydrosilane, alkyne, and y unsaturated aldehyde is suggested to proceed via oxanickelacycle intermediate to give (Z)-enol silyl ether (Scheme 3-28). Hydrosilylation of alkenes under a carbon monoxide atmosphere allows carbonyl incorporation, giving silyl enol ethers by using a cobalt or iridium catalyst (Schemes 3-29 and 3-30). Under similar reaction conditions in the presence of a rhodium catalyst, alkynes are converted to y silyl-substituted acroleins (Scheme 3-31). ... 

EtAlCb catalyzes the Friedel-Crafts acylation of alkenes with acid chlorides, the formal [3 + 2] cycloaddition of alkenes with cyclopropane-1,1-dicarboxylates (eq 21), the Friedel-Crafts alkylation of anilines and indoles with ct-aminoacrylate esters, and the formation of allyl sulfoxides from sulfinyl chlorides and alkenes. EtAlCU induces the Beckmann rearrangement of oxime sulfonates. The cationic intermediates can be trapped with enol silyl ethers (eq 22). EtAlC is the preferred catalyst for addition of the cation derived from an a-chloro sulfide to an alkene to give a cation which undergoes a Friedel-Crafts alkylation (eq 23). ... 

In the prostaglandin synthesis shown, silyl enol ether 216, after transmetaJ-lation with Pd(II), undergoes tandem intramolecular and intermolecular alkene insertions to yield 217[205], It should be noted that a different mechanism (palladation of the alkene, rather than palladium enolate formation) has been proposed for this reaction, because the corresponding alkyl enol ethers, instead of the silyl ethers, undergo a similar cyclization[20I],... 

Coupling with silyl enol ethers. Alkenes can be prepared by coupling of Grignard reagents with silyl enol elbers. Nickel aectylacetonate is the most active... 

Diels-Alder reactionsBoth 1,4-dicyanonaphthalene (DCN) and 2,6,9,10-te-tracyanoanthracene (TCA) have been used as sensitizers to effect photochemical [4 + 2]cycloadditions of electron-rich dienes and electron-rich dienophiles, which do not normally undergo thermal cycloadditions. These cycloadditions are known as triplex Diels-Alder reactions because they are postulated to involve as an intermediate a three-membered complex of sensitizer, dienophile, and diene. This reaction is useful for synthesis of bicyclo[2.2.2]octenes from some silyl enol ethers, alkenes, or arylalkynes. 

Nitrosolysis of camphor ethyl acetal with ethanolic ethyl nitrite in sulphur dioxide yields the orthoester oxime (205) which is rapidly dehydrated by excess acetal to the orthoester nitrile which then reacts with sulphur dioxide to form the ester nitrile and diethyl sulphite.Further papers in this section include the full paper on ozonolysis of silyl ethers (Vol. 5, p. 33), another synthesis of camphor-enol trimethylsilyl ether (cf. Vol. 6, p. 41)/°° the conversion of camphor oxime with Grignard reagents into the corresponding imine with no aziridine formation/° the preparation of (206) by treating bornylene with trichloroacetyl isocyanate/ the oxidation of thiocam-phor to the 5-oxide and alkylation in the presence of thallium(i) ethoxide to a/S-unsaturated sulphoxides/ and the free-radical C-3 alkylation of camphor with alkenes. " ... 

In addition to alkenes or compounds with strained double bonds, 1,2,4-triazines react readily with electron-rich dienophiles such as enol ethers, enamines, enaminones, vinyl silyl ethers, vinyl sulfides, vinyl acetate, and ketene derivatives (see also Ilouben-Weyl, Vol. E7b, p 471 ff). It has been shown195,403,404 that oxo compounds and catalytic amounts of amines can be used instead of enamines however, water formed in the reaction can result in the formation of side products.403... 

Section 1.1.1 described the formation of enolates, silyl enol ethers and enamines and their alkylation reactions. An alternative type of alkylation occurs on addition of these nucleophiles to electrophilic alkenes, such as a,p-unsaturated ketones, esters... 

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