Hosomi-Sakurai Allylation

• General Characteristics

The allylation reactions using allylsilanes are known as the Hosomi-Sakurai allylation. Allylsilanes are less toxic than allylstannanes and more stable than allyl Grignard and allyl lithium reagents.

Lewis acids and fluoride salts are used to promote the reaction. Lewis-acid promoted Sakurai reaction is γ-selective in contrast to the reactions of allyl lithium and allyl Grignard. Highly stereoselective allyl additions are possible. The allylation of α,β-unsaturated ketones occurs in 1,4-fashion.

• General References

・Hosomi, A.; Endo, M.; Sakurai, H. Chem. Lett. 1976, 941. doi:10.1246/cl.1976.941
・Hosomi, A.; Sakurai, H. Tetrahedron Lett. 1976, 17, 1295. doi:10.1016/S0040-4039(00)78044-0
・Hosomi, A.; Sakurai, H. J. Am. Chem. Soc. 1977, 99, 1673. DOI: 10.1021/ja00447a080
・Wilson, S. R.; Price, M.F. J. Am. Chem. Soc. 1982, 104, 1124. DOI: 10.1021/ja00368a049
・Review: Fleming, I. et al. Org. React. 1989, 37, 57.
・Review: Fleming, I. Comprehensive Organic Syntheis 1991, 2, 563.

• Reaction Mechanism

The following mechanism has been proposed based on the beta-silicon effect (the effect of silicon stabilizing the carbocation at the β-position). The allyl addition is considered to proceed through acyclic transition state since the Lewis acidity of silicon is low. (Tetrahedron Lett. 1983, 24, 2865.)

• Examples

Denmark achieved asymmetric allylation of aldehydes using the chiral Lewis base developed in his lab.^[1]

Yamamoto achieved enantioselective allylation of ketones using silver fluoride and a chiral diphosphine ligand.^[2]

An application to the synthesis of furaquinocin A.^[3]

Another example in the synthesis of halichlorine.^[4]

The allylation of cyclic oxonium cation is known to be highly stereoselective. The selectivity is strongly influenced by the substituent at the 4-position for 6-membered rings and the 3-position for 5-membered rings.^[5]

• Experimental Procedure

Conjugate allylation of an α,β-unsaturated ketone.^[6]

• Experimental Tips

• References

[1] Denmark, S. E.; Coe, D. M.; Pratt, N. E.; Griedel, B. E. J. Org. Chem. 1994, 59, 6161. DOI: 10.1021/jo00100a013
[2] Wadamoto, M.; Yamamoto, H. J. Am. Chem. Soc. 2005, 127, 14556. DOI: 10.1021/ja0553351
[3] Trost, B. M.; Thiel, O. R.; Tsui, H.-C. J. Am. Chem. Soc. 2003, 125, 13155. DOI: 10.1021/ja0364118
[4] (a) Trauner, D.; Schwartz, J. B.; Danishefsky, S. J. Angew. Chem. Int. Ed. 1999, 38, 3542. [abstract] (b) Trauner, D.; Danishefsky, S. J. Tetahedron Lett. 1999, 40, 6513. doi:10.1016/S0040-4039(99)01170-3
[5] (a) Woerpel, K. A. et al. J. Am. Chem. Soc. 1999, 121. 12208. DOI: 10.1021/ja993349z (b) Woerpel, K. A. et al. J. Am. Chem. Soc. 2003, 125, 14149. DOI: 10.1021/ja0375176 (c) Woerpel, K. A. et al. J. Am. Chem. Soc. 2003, 125, 15521. DOI: 10.1021/ja037935a (d) Woerpel, K. A. et al. J. Am. Chem. Soc. 2005, 127, 10879. DOI: 10.1021/ja0524043
[6] Org. Synth. 1984, 62, 84.

• Related Books

allyl silanes, allylation