The Sakurai reaction (also known as the Hosomi–Sakurai reaction) is the

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries ...

s (such as a

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

shown here) with

allyltrimethylsilane Allyltrimethylsilane is the organosilicon compound Organosilicon compounds are organometallic compounds containing carbon–silicon bonds. Organosilicon chemistry is the corresponding science of their preparation and properties. Most organo ...

catalyzed by strong

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

Lewis acid activation is essential for complete reaction. Strong Lewis acids such as

titanium tetrachloride Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds o ...

boron trifluoride Boron trifluoride is the inorganic compound with the formula BF3. This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and bondin ...

tin tetrachloride Tin(IV) chloride, also known as tin tetrachloride or stannic chloride, is an inorganic compound with the formula Sn Cl4. It is a colorless hygroscopic liquid, which fumes on contact with air. It is used as a precursor to other tin compounds. I ...

, and AlCl(Et)₂ are all effective in promoting the Hosomi reaction. The reaction is a type of

electrophilic allyl shift An allylic rearrangement or allylic shift is an organic reaction in which the double bond in an allyl chemical compound shifts to the next carbon atom. It is encountered in nucleophilic substitution. In reaction conditions that favor a SN1 react ...

with formation of an intermediate beta-silyl carbocation. Driving force is the stabilization of said carbocation by the

beta-silicon effect The beta-silicon effect also called silicon hyperconjugation in organosilicon chemistry is a special type of hyperconjugation that describes the stabilizing influence of a silicon atom on the development of positive charge at a carbon atom one posi ...

. The Hosomi-Sakurai reaction can be performed on a number of functional groups. An electrophilic carbon, activated by a Lewis acid, is required. Below is a list of different functional groups that can be used in the Hosomi–Sakurai reaction. The reaction achieves results similar to the addition of an allyl

Grignard reagent A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide ...

to the carbonyl.

Mechanism

The Hosomi-Sakurai reactions are allylation reactions which involve use of allyl silanes as allylmetal reagents. This section demonstrates examples of allylation of diverse ketones. In figure 1, allylation of a carbonyl ketone (compound containing a ketone group and two different functional groups) has been shown. In the given reaction, the electrophilic compound (carbon with a ketone group) is treated with titanium tetrachloride, a strong Lewis acid and allyltrimethylsilane. According to the general principle, the Lewis acid first activates the electrophilic carbon in presence of allyltrimethylsilane which then undergoes nucleophilic attack from electrons on the allylic silane. The silicon plays the key role in stabilizing the carbocation of carbon at the β-position. Hosomi-Sakurai reaction is also applicable for other functional groups such as enones, where conjugate addition is usually seen. In figure 2, the Hosomi- Sakurai reaction has been shown using a cinnamoyl ketone. This reaction follows the same mechanism as the previous reaction shown here.

Beta-silicon effec] stabilization

As displayed in the mechanism, the Hosomi–Sakurai reaction goes through a secondary carbocation intermediate. Secondary carbocations are inherently unstable, however the β-silicon effect from the silicon atom stabilizes the carbocation. Silicon is able to donate into an empty p-orbital, and the silicon orbital is shared between the two carbons. This stabilizes the positive charge over 3 orbitals. Another term for the β-silicon effect is silicon-hyperconjugation. This interaction is essential for the reaction to go to completion.

Literature of historic interest

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References

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External links

* Hosomi-Sakurai reaction @ www.organic-chemistry.or
Link

Akira Hosomi HP
Addition reactions Carbon-carbon bond forming reactions Name reactions