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.[1][2][3][4][5][6] It is named after the chemists Akira Hosomi and Hideki Sakurai.

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 three different functional groups that can be used in the Hosomi–Sakurai reaction.

Mechanism

The proposed mechanism for the Hosomi–Sakurai reaction using a ketone is displayed above. The mechanisms for all Hosomi-Sakurai reactions follow the same general principle where a strong Lewis Acid activates an electrophilic carbon, which then undergoes nucleophilic attack from electrons on the allylic silane.

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.

See also

References

  1. ↑ Hosomi, A.; Sakurai, H. (1976). "Syntheses of γ,δ-unsaturated alcohols from allylsilanes and carbonyl compounds in the presence of titanium tetrachloride". Tetrahedron Letters 17 (16): 1295. doi:10.1016/S0040-4039(00)78044-0.
  2. ↑ Hosomi, Akira; Endo, Masahiko; Sakurai, Hideki (1976). "Allylsilanes As Synthetic Intermediates. Ii. Syntheses of Homoallyl Ethers from Allylsilanes and Acetals Promoted by Titanium Tetrachloride". Chem. Letters (9): 941. doi:10.1246/cl.1976.941.
  3. ↑ Hosomi, Akira; Sakurai, Hideki (1977). "Conjugate addition of allylsilanes to α,β-enones. A New method of stereoselective introduction of the angular allyl group in fused cyclic α,β-enones". J. Am. Chem. Soc. 99 (5): 1673. doi:10.1021/ja00447a080.
  4. ↑ Hosomi, A. (1988). "Characteristics in the reactions of allylsilanes and their applications to versatile synthetic equivalents". Acc. Chem. Res. 21 (5): 200–206. doi:10.1021/ar00149a004.
  5. ↑ Fleming, Ian; Dunoguès, Jacques; Smithers, Roger (1989). "The Electrophilic Substitution of Allylsilanes and Vinylsilanes". Org. React. 37: 57–575. doi:10.1002/0471264180.or037.02. ISBN 0471264180.
  6. ↑ Fleming, I. (1991). Comp. Org. Syn. 2: 563–593. Missing or empty |title= (help)

External links