Named after Hideki Sakurai | Reaction type Addition reaction | |
Organic Chemistry Portal hosomi-sakurai-reaction | ||
The Sakurai reaction (also known as the Hosomi–Sakurai reaction) is the chemical reaction of carbon electrophiles (such as a ketone shown here) with allylic silanes catalyzed by strong Lewis acids. It is named after the chemists Akira Hosomi and Hideki Sakurai.
Contents
Lewis acid activation is essential for complete reaction. Strong Lewis acids such as titanium tetrachloride, boron trifluoride, tin tetrachloride, and AlCl(Et)2 are all effective in promoting the Hosomi reaction. The reaction is a type of electrophilic allyl shift with formation of an intermediate beta-silyl carbocation. Driving force is the stabilization of said carbocation by the beta-silicon effect.
Various reactions
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.
Mechanism
The Hosomi-Sakurai reactions are allylation reactions which involve use of allyl silanes as allylmetal reagents. This section demonstrates examples of allylation of different ketone groups. 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 allyl ketone. In figure 2, the Hosomi- Sakurai reaction has been shown using a cinnamoyl-group containing ketone. This reaction follows the same mechanism as the previous reaction shown here.
β-Silicon effect 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.