ALKENES silylations

Photolysis of /V-(diphcnylainmo)-2.4.6-tnmcthylpyridinium tetrafluoroborate (37) generates diarylnitrenium ion (38) as revealed by its reaction with electron-rich alkenes (silyl enol ethers and allylsilanes) to give products of substitution at para- and ortho-positions (39) and (40) (Scheme ll).47... 

In the ISOC process alkenic silyl nitronates (649) are the reactive intermediates which undergo cycloaddition. This species is generated by reacting the nitroalkene (647) (Scheme 86) or nitroalkyne (651) (Scheme 87) with triethylamine and trimethylchlorosilane in benzene at room temperature for 48 h to give the fused isoxazole systems (648) or (652). A cis relationship between the less bulky C-7 and the hydrogen at C-3a in the products (648) is preferred. The yields of the reactions average... 

The platinum catalyst is effective in very small amounts, and can be introduced as H2PtCl6 or as elemental platinum on an inert support. A particularly active catalyst is the soluble platinum complex of divinyltetramethyldisilox-ane, CH2=CHSiMe2-0-SiMe2CH=CH2. The hydrosilyla-tion reaction operates through the Chalk-Harrod mechanism or one of its variants. bz jn these mechanisms, the first step involves the conversion of a metal alkene complex to a metal alkene silyl hydride complex. In addition to platinum, recently ruthenium, rhodium, palladium, copper, and zinc complexes are being studied as hydrosilation catalysts. " ... 

Chlorosulfides are often used for vicinal functionalization of alkenes. Silyl enol ethers can react with a-chloromethyl phenyl sulfides (5) in the presence of ZnBr2 or TiCU to give the corresponding -keto sulfide, which could be easily transfonned into the a-methylene ketone via sulfide oxidation to sulfoxide, followed by pyrolytic elimination (Scheme 23). 

Alkenes silylated at a remote carbon atom have been converted... 

Marciniec B (2005) Catalysis by transition metal complexes of alkene silylation - recent progress and mechanistic implications. Coord Chem Rev 249 2374-2390... 

The oxidation of higher alkenes in organic solvents proceeds under almost neutral conditions, and hence many functional groups such as ester or lac-tone[26,56-59], sulfonate[60], aldehyde[61-63], acetal[60], MOM ether[64], car-bobenzoxy[65], /-allylic alcohol[66], bromide[67,68], tertiary amine[69], and phenylselenide[70] can be tolerated. Partial hydrolysis of THP ether[71] and silyl ethers under certain conditions was reported. Alcohols are oxidized with Pd(II)[72-74] but the oxidation is slower than the oxidation of terminal alkenes and gives no problem when alcohols are used as solvents[75,76]. 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

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],... 

An Q-arylalkanoate is prepared by the reaction of aryl halide or triflate with the ketene silyl acetal 74 as an alkene component. However, the reaction is explained by transmetallation of Ph - Pd—Br with 74 to generate the Pd eno-late 75, which gives the a-arylalkanoate by reductive elimination[76]. 

Pd(II)-catalyzed cyclization of the siloxyhexatriene 34 offers a cyclohexe-none annulation method. The Pd enolate 35, formed by transraetallation of the silyl enol ether with Pd(II), is an intermediate which undergoes intramolecular eWo-alkene insertion. Then Pd(II) is regenerated to give 36, and finally cyclohexenone is formed[38]. 

A newer method for the preparation of nitronic esters, namely utilizing the (9-trimethyl-silyl ester, has been reported and these are prepared by the reaction of alkylnitro compounds and (V,(V-bis(trimethylsilyl)acetamide. These nitronic esters also undergo cycloaddition with alkenes to produce isoxazolidines (equation 54) (74MIP41601, 74DOK109, 78ACS(B)ll8). 

SynttMSH ol alkenes from a silyl carbanions and cartMnyl compounds In cases where separation ol sitylaloohol diastereomers (e g 4] can be achieved, pure Z or E oleflns can be Isolaled... 

Reaction of triethylsilyl hydrotrfoxide with electron-rich olefins to gh/e dioxetanes that react IntrarTMlecularly with a keto group in the presence of t-txrtyidimethyl silyl triflateto afford 1,2,4 Inoxanes also oxydatnre cleavage ol alkenes Also used in cleavage ol olefins... 

Computational investigations of vinylsilanes indicate that there is a groimd-state interaction between the alkene n oibital and the carbon-silicon bond which raises the energy of the n HOMO and enhances reactivity. Furthermore, this stereoelectronic interaction favors attack of the electrophile anti to the silyl substituent. 

Systems usually fluonnated by electropositive fluorine reagents include acti-vated alkenes (enol ethers, enol acetates, silyl enol ethers, and enamines), activated aromatic systems, certain slightly activated carbon-hydrogen bonds, and selected organometallics. 

FITS reagents), has undergone considerable development recently [141,142,143, 144, 14S. These compounds, available fromperfluoroalkyhodides (equation 76), are very effective electrophilicperfluoroalkylating agents They react with carban-lons, aromatic compounds, alkenes, alkynes, silyl enol ethers, and other nucleophiles under mild conditions to introduce the perfluoroalkyl moiety mto organic substrates (equation 77) (see the section on alkylation, page 446). 

Examine the eleetrostatic potential map of eaeh nueleophile (enamine, silyl enol ether, lithium enolate and enol) with emphasis on the face of the nucleophilic alkene carbon. Rank the nucleophiles from most electron rich to least electron rich. What factors are responsible for this order (Hint For each molecule, consider an alternative Lewis structure to that given above that places a negative charge on the nucleophilic carbon.)... 

Ironically, auxiliary-induced control via the alkene failed to generate synthetically useful selectivities, but direct substrate-induced control did. In particular, chiral silyl enol ethers with stereocenters in the y-position allowed the synthesis of enantiomerically... 

More recently, further developments have shown that the reaction outlined in Scheme 4.33 can also proceed for other alkenes, such as silyl-enol ethers of acetophenone [48 b], which gives the endo diastereomer in up to 99% ee. It was also shown that / -ethyl-/ -methyl-substituted acyl phosphonate also can undergo a dia-stereo- and enantioselective cycloaddition reaction with ethyl vinyl ether catalyzed by the chiral Ph-BOX-copper(ll) catalyst. The preparative use of the cycloaddition reaction was demonstrated by performing reactions on the gram scale and showing that no special measures are required for the reaction and that the dihydro-pyrans can be obtained in high yield and with very high diastereo- and enantioselective excess. 

The Peterson olefination can be viewed as a silicon variant of the Wittig reaction, the well-known method for the formation of carbon-carbon double bonds. A ketone or aldehyde 1 can react with an a-silyl organometallic compound 2—e.g. with M = Li or Mg—to yield an alkene 3. 

By application of the most common procedure—i.e by using an a-silylated organolithium or magnesium reagent—the /3-hydroxysilane 5a/5b can be isolated. However in the case of M = Na or K, the alkoxide oxygen in 4a/4b is of strong ionic character, and a spontaneous elimination step follows to yield directly the alkene 3. 

In a formal synthesis of fasicularin, the critical spirocyclic ketone intermediate 183 was obtained by use of the rearrangement reaction of the silyloxy epoxide 182, derived from the unsaturated alcohol 180. Alkene 180 was epoxidized with DMDO to produce epoxy alcohol 181 as a single diastereoisomer, which was transformed into the trimethyl silyl ether derivative 182. Treatment of 182 with HCU resulted in smooth ring-expansion to produce spiro compound 183, which was subsequently elaborated to the desired natural product (Scheme 8.46) [88]. 

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