Allylic silylations, palladium-catalyzed

The Alder-ene cyclization of allylic silyl ethers represents a clever use of cycloisomerization chemistry, as the enol ether products can be easily unmasked to yield aldehydes. Palladium-catalyzed cycloisomerization of 1,6- and 1,7-enynes containing an allylic oxygen most often gives rise to 1,3-dienes (see Section 10.12.4.1). However, enynes of type 63 underwent facile Alder-ene cyclization to the corresponding five- or six-membered rings (Equation (40)) using both [CpRu(MeCN)3]PF6 41 and the Cp analog ([Cp Ru(MeCN)3]PF6, 64).53... 

Palladium-catalyzed cross-coupling of 19 with allyl bromide occurs exclusively at the vinylstannane moiety to give 1-ethoxy-1-silyl-1,4-diene 20. The following ether exchange with allyl alcohol causes the Glaisen rearrangement to give an acylsilane derivative 21 (Scheme 64).2... 

Ceric ammonium nitrate promoted oxidative addition of silyl enol ethers to 1,3-butadiene affords 1 1 mixtures of 4-(/J-oxoalkyl)-substituted 3-nitroxy-l-butene and l-nitroxy-2-butene27. Palladium(0)-catalyzed alkylation of the nitroxy isomeric mixture takes place through a common ij3 palladium complex which undergoes nucleophilic attack almost exclusively at the less substituted allylic carbon. Thus, oxidative addition of the silyl enol ether of 1-indanone to 1,3-butadiene followed by palladium-catalyzed substitution with sodium dimethyl malonate afforded 42% of a 19 1 mixture of methyl ( )-2-(methoxycarbonyl)-6-(l-oxo-2-indanyl)-4-hexenoate (5) and methyl 2-(methoxycarbonyl)-4-(l-oxo-2-indanyl)-3-vinylbutanoate (6), respectively (equation 12). 

Many other reports of ligand libraries for specific catalytic applications have been reported. Among them, Gilbertson and co-workers reported a chiral phosphine library, tested in the rhodium-catalyzed asymmetric hydrogenation of an enamide (158,159), and a similar library for the palladium-catalyzed allylation of malonates (160, 161) Hoveyda and co-workers (162, 163) reported a chiral Schiff base library, screened in the titanium-catalyzed opening of epoxides with (TMSCN) (trimethyl silyl cyanide) ... 

On the other hand, unsaturated aldehydes and ketones were obtained using allylic alcohols as alkene components [68]. Similarly, allyl f-butyldimethylsilyl ether and N-allylamides gave silyl enol ethers [69] and enamides [70], respectively. The ruthenium-catalyzed alkene-alkyne coupling was successfully combined with the palladium-catalyzed intramolecular asymmetric allylic alkylation [71] to provide a novel one-pot heterocyclization method [72]. 

Z)-2-butenylene dicarbonate with dimethyl malonate gives a low yield (20—40%) of 2-vinylcyclopropane-l,l-dicarboxylate with up to 70% ee (Scheme 2-38) [54], Reaction with methyl acetoacetate or acetylacetone takes place in a different manner to give a dihydrofuran derivative (59% ee), which results from nucleophilic attack of enolate oxygen at the cyclization step, (c) Asymmetric elimination of an acetyl-acetate ester gives (R)-4-rerr-butyl-l-vinylcyclohexene of up to 44% ee (Scheme 2-39) [55]. (d) Palladium-catalyzed allylic silylation is also applied to asymmetric synthesis... 

Palladium-catalyzed reductive silylation has also been reported. The cathodic reduction of allylic acetates in the presence of silylating agents and a catalytic amount of (Ph3P)4Pd [53] gives the corresponding allylsilanes. The initially formed rr-allyl-palladium(II) complex seems to be reduced at the cathode to generate the allyl anion intermediate, which reacts with chlorosilane to give the final product. 

Allylsilanes, which are accessible by bis-silylation of 1,3-dienes and silylation of allylic substrates, are useful allylation reagents in organic synthesis [99, 100] ( , )-l,8-bis(Trimethylsilyl)octa-2,6-diene 63a, prepared by palladium-catalyzed bis-silylative dimerization of 1,3-butadiene, was successfully applied to the synthesis of dZ-muscone (Scheme 8) [58]. A key feature of the synthesis is regi-oselective reaction with acetaldehyde at the positions y to the silyl groups. 

Elimination of silyl groups with p-oxy groups, i.e., Peterson-type elimination, is a useful method for preparing stereodefined alkenes [101,102]. The synthetically useful allylsilanes are effectively synthesized in geometrically and enantio-merically pure forms through the Peterson-type elimination of organosilanes prepared by palladium-catalyzed bis-silylation (Eq. 57) [72, 73]. The intramolecular bis-silylation of optically active allylic alcohols in refluxing toluene af-... 

Cyclopropane formation was also observed as a side reaction (1 9 up to 1 1) in the palladium-catalyzed coupling of ketene alkyl silyl acetals with open-chain and cyclic allyl acetates. The reaction is interpreted as proceeding via nucleophilic central attack of a jr-allyl intermediate. Although cyclopropane formation proceeds only with low yields, a highly stereospecific pathway was observed with substituted 3-cyclohexenyl acetates. ... 

A-Glycosylimidazoles (related to nucleosides) are of interest in the preparation of novel chemotherapeutic agents, hence selective A -glycosylation techniques are of interest. The palladium-catalyzed addition of vinyl epoxides and allyl acetates to imidazoles provides one such route to imidazole nucleoside analogues <91JCS(Pi)2603, 9UOC4990>. The silyl-Hilbert-Johnson method is another in which the imidazole is silylated, then treated with a suitable peracetylated carbohydrate derivative in the presence of trimethylsilyl triflate catalyst. With 4-carbamoylimidazolium-5-olate, for example, the major product is the l-glycosyl-5-carbamoyl isomer with only low yields of the... 

Synthesis of neutral Ji-allylpalladium complexes having bisnitrogen ligands (mainly, those related to 2-(pyrazol-3-yl)pyridine, 2-(imidazol-2-yl)pyridine, and 2,2-bipyridine) and palladium-catalyzed cyclopropanation of ketene silyl acetals with allylic acetates 00YGK736. 

We sought to examine the enzymatic dioxygenation of aryl silanes using a number of different aromatic dioxygenases in order to determine if such transformations were possible and to define the substrate-specificity profile. We were also motivated by the rich chemistry of silicon-based materials, which includes the hydrosilylation of alkenes and ketones, the addition of electrophiles to vinyl and allyl silanes, and palladium catalyzed cross-coupling of vinyl silanes with aryl halides (13). As a result, silyl functional cw-diols have potential as chiral intermediates for drug development, as polymer precursors/modifiers and as elements in non-linear optical materials. 

Further examples of papers are 4-ferrocenyl-l,3-oxazoline ligands in asymmetric allylation reactions ortho-silylation of 2,2, -bis(oxazolinyl)-l,l -bis(diphenylphosphino)ferrocenes in palladium-catalyzed asymmetric allylic alkylations polymer-supported ferrocenyl oxazalines for asymmetric phenyl-transfer reactions 1 -substituted... 

Scheme 5.24 Enantioselective palladium-catalyzed allylic alkylation of silyl enol ethers 74 and 76 in the presence of f-butyl-PHOX ligand 42b and the activator Bu NPhjSiFj.

Scheme 5.26 Enantioselective palladium-catalyzed allylic alkylation of fluoro-substituted silyl enol ethers 80.

A remarkable kinetic resolution was observed recently by Feringa and coworkers when 2-silyloxyfuran was reacted with the racemic unsymmetrical allylic substrate 86, catalyzed by the palladium complex of Trost s ligand (/ ,/ )-14, as illustrated in Scheme 5.28. The acetate anion liberated upon oxidative addition of palladium(O) is assumed to cleave the silyl protecting group, so that the enolate anion forms aside from TMSOAc. After allylic alkylation of that enolate, double bond migration leads to butenolide (R)-87 isolated in 47%, if the reaction was run with 52% conversion. Not only the product 87 but also the recovered acetate... 

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