Titanium tetrachloride, reaction allyl silanes

Allyl silanes react with a wide variety of electrophiles, rather like the ones that react with silyl enol ethers, provided they are activated, usually by a Lewis acid. Titanium tetrachloride is widely used but other successful Lewis acids include boron trifluoride, aluminium chloride, and trim ethyls ilyl tri-flate. Electrophiles include the humble proton generated from acetic add. The regiocontrol is complete. No reaction is observed at the other end of the allylic system. All our examples are on the allyl silane we prepared earlier in the chapter. 

Silicon has also been used to control the acylation of 1,3-dienes in a manner analogous to the situation with alkenes. Isoprenylation with 2-trimethylsilylmethylbuta-1,3-diene follows the course expected of acylation of the allylic silane. In these very rapid reactions, titanium tetrachloride seems to be one of the more efficient catalysts, as is aluminum chloride. The method was used in synthetic approaches to the terpenes ipsenol (14) and ipsdienol (15 Scheme 16). Of particular interest is the comparison of this iso-prenylating agent with isoprene itself. The examples of Friedel-Crafts acylations cited show the regio-control that can be achieved by suitable choice of substrate. 

Titanium tetrachloride is a moisture-sensitive, highly flammable liquid reacting violently with water (34). It is a strong Lewis acid capable of promoting Diels-Alder reactions (35) and induces the addition of silyl enol ethers and allyl silanes to carbonyl compounds and derivatives (34r-36). It is a less commonly used catalyst in Friedel-Crafts reactions but very useful for the acylation of activated alkenes and in the Fries rearrangement. 

In 1976, Akira Hosomi and Hideki Sakurai of Tohoku University in Sendai, Japan, published a letter entitled Syntheses of y,S-Unsaturated Alcohols From Allylsilanes And Carbonyl Compounds In The Presence of Titanium Tetrachloride"The letter describes the reaction depicted in equation 1, in which allyl silanes react with aldehydes or ketones to provide homoallylic alcohols. The following year, Hosomi and Sakurai extended their finding to ketals, which provide homoallylic ethers (eq. 2)," and to a,P-unsaturated ketones (eq. 3), in which case the addition occurs in a 1,4-fashion and becomes a valuable method to generate quaternary centers. 

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