Fluorination allyl silanes

Shibata successfully adapted the asymmetric transfer fluorination to cyclic silyl enol ethers, cyclic allyl silanes and oxindoles, illustrated in Schemes 13.1-13.3, as a catalytic method (Scheme 13.6) [16]. Similar reaction conditions were identified for all three substrates, including the use of stoichiometric NFSI as the electrophilic fluorine source and a stoichiometric inorganic base additive. It was observed that bis-Cinchona alkaloid (DHQ)2PHAL was best for cyclic silyl enol ethers (X = 0), (DHQ)2PYR (Scheme 13.2) was best for cyclic allyl silanes (X = CH2), while (DHQD)2AQN was best for oxindoles. A similar method was applied to cyclic enol ethers, providing products in modest ee s [17]. 

In 2008, Shibata et al. disclosed the first successful catalytic enantioselective fluorination based on the use of cinchona alkaloids. Therefore, it was demonstrated that allyl silanes and silyl enol ethers underwent efficient enantioselective fluorodesilylation with NFSI and a catalytic amount of a bis-cinchona alkaloid in the presence of an excess of base to provide the corresponding fluorinated compounds with an F-substituted quaternary carbon centre with enantioselectivities of up to 95% ee (Scheme 5.3). Furthermore, the authors showed that this catalytic system could be applied to the catalytic... 

Scheme 5.3 Fluorinations of allyl silanes, silyl enols and oxindoles.

Ishimaru T, Shibata N, Horikawa T, Yasuda N, Nakamura S, Tom T, Shiro M. Cinchona alkaloid catalyzed enantioselective fluorination of allyl silanes, silyl enol ethers, and oxindoles. Awgew. Chem. Int. Ed. 2008 47 4157-4161. 

Indium-mediated allylation of trialkyl(difluoroacetyl)silane 70 in aqueous media gives homoallylic alcohol 71 exclusively (Scheme 60). Both water and THF are essential for the allylation reaction. It is worth noting that homoallylic alcohol 71 is formed exclusively under these reaction conditions. On the contrary, enol silyl ether 72 is a major product of the fluorinated acylsilanes reaction with other organometallic compounds than indium via a Brook rearrangement and defluorination. Indium-mediated allylsilylation of carbonyl compounds provides a facile route to 2-(hydroxyethyl)allylsilanes. The allene homologs are similarly prepared (Scheme 61).244,244a... 

The selected examples by Studer et al. [97-99] reported the synthesis of two small, discrete libraries of isoxazolines (eight compounds) and of amides derived from a Ugi 4CC reaction (10 compounds). The synthesis of the tagging silicon derivative and the scheme for isoxazoline synthesis are reported in Figure 16. A known silane [100] was coupled quantitatively with bromine in the fluorinated solvent FC-72 [101] and the tagging reagent was used to protect and label the allyl alcohols in THF. After the cycloaddition of the fluorinated dipolarophiles with the nitrile oxides, the protected isoxazolines were finally cleaved with HF/pyridine complex in ethyl ether. The scheme of the purification for each synthetic step is depicted in Figure 17. 

Allyl derivatives of hydrocarbons with functional group were often used as starting materials. The hydrosilylation of allylperfluoroethers with methyl(chloro)silanes is a frequently used method for synthesis (after hydrolysis and polycondensation) of fluorine-containing polymers (162). Unsaturated esters of unsaturated carboxylic acids in the reaction of hydrosilylation give silyl derivatives of carboxylic acid (e.g., 3-methacryloxypropyltriethoxysilane). 

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