Palladium-catalyzed reactions allylic silylations

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... 

Recently, hydrosilanes are also employed for the silylation of aryl halides in the presence of a palladium,platinum, or rhodium catalyst. In view of atom efficiency, this procedure has an advantage. Palladium-catalyzed reactions are suitable for the silylation of electron-rich aryl halides, whereas rhodium-catalysis works well with a wide range of aryl halides (Scheme 3-17). Silylation of allyl halides with trichlorosilane proceeds in the presence of a copper catalyst. ... 

Palladium(0)-catalyzed a-allylation of silyl ethers is a reaction which can be carried out with ketones as well as with aldehydes91. It is highly regiospecific when applied to ketones. a-Allylations can also be performed with enol acetates using allyl carbonates in the presence of catalytic amounts of palladium(O) complexes and (tributyl)methoxytin92,93. The steric course of the reaction has not been studied systematically but a high level of diastereoselectivity is expected and possibilities for asymmetric induction by the use of chiral auxiliaries are envisaged. 

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(0)-catalyzed a-allylations of TMS enol ethers can be carried out cleanly with allylic carbonates. These reactions are highly regioselective, e.g. the mtne- and less-substituted TMS enol derivative of 2-methylcyclohexanone cf. Scheme 37) gave 2-allyl-2-methylcyclohexanone and 2-methyl-6-allylcyclohexanone, respectively. Allylations of aldehyde silyl enol ers occur similarly. Allylations of enol acetates occur with allyl carbonates in the presence of catalytic amounts of palla-dium(0) complexes and tri-n-butyltin methoxide. ... 

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. 

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. ... 

The importance of the torquoselective olefination is illustrated in Fig. 34 for the particular case in which a multisubstituted alkenylsilane is converted to various kinds of multisubstituted olefins. The silyl-substituted allyl alcohol 93 is allylated to give the 1,4-diene 94, and the iodoalkene 95, prepared by desilyliodination of 93, is subjected to palladium-catalyzed cross-coupling reactions (the Heck reaction, Stille coupling) to afford the dienes 96 and 97 without ElZ isomerization. 

The third reaction was related to the hydrocarboxylation of allenes with C02, catalyzed by a tridendate silyl pincer-type palladium complex [108]. In this reaction, a a-allyl palladium species (via hydropallation of allene) was formed, and its trigonal bipyramidal geometry allowed the facile coordination of C02 (presumably in a side-on fashion) and a following nucleophilic addition to realize the carboxylation of allene (Figure 4.16). This reaction proved to be very attractive not only as a C02 fixation reaction, but also as a general method for the synthesis of Py-unsaturated carboxylic acids. 

The first gold-catalyzed addition reactions of carbon nucleophiles to allenes were only first disclosed in 2006, and the number of examples is still small. Toste and co-workers showed that allenic silyl enol ethers undergo a 5-endo- ng cyclization to hexahydroindenone derivatives in the presence of a cationic gold catalyst (Scheme 4-10). In these transformations, water or methanol is used as an external proton source for protodeauration of an intermediate vinylgold species. In an analogous manner, cyclopentenes were obtained in good yields from allenic P-ketoesters. In the presence of a palladium catalyst and an allylic halide, these substrates afford functionalized 2,3-dihydroflirans. 

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... 

---