Luche reaction

What is Luche reaction?

The Luche reaction is a modification of the aqueous allylation of carbonyl compounds via the intermediacy of tin, which was initially reported in 1983. The Luche reaction uses ultrasonic irradiation in saturated aqueous NH4Cl/THF solution, and ammonium chloride is often added to activate the metal surface and enhance the yield. Despite zinc’s tendency to reduce protons to hydrogen and its expected reaction with water, the Luche reaction proceeds smoothly.

Luche reaction - general reaction scheme
Luche reaction
  • R1 = alkyl, aryl
  • R2 = H, alkyl, aryl

When 1,3-dichloropropene or 3-iodo-1-chloropropene is used in the reaction, 1,3-butadienes are obtained. When interrupted by base treatment after the initial allylation to the carbonyl group, vinyloxiranes are obtained as final products. Aldehyde reacts in preference to ketone when both groups exist, and aromatic aldehyde prevails in the presence of an aliphatic aldehyde. The diastereomer mixture obtained from the reaction between aldehyde and bromopropenyl methylcarbonate to give the rearranged allyl methylcarbonate depends on the nature of the aldehyde.

Nickel acetylacetone catalyzes the conjugate addition of allylzinc bromide with α,β-unsaturated carbonyl compounds, but the corresponding branched allylzinc chloride adds only to the carbonyl group of α,β-unsaturated compounds. Alkyl halide reacts with α,β-unsaturated carbonyl compounds and nitriles via a conjugate addition under the conditions of the Luche reaction when a zinc-copper couple is used.

In addition to zinc, other metals have also been utilized to facilitate the aqueous allylation of carbonyl compounds. One such metal combination is manganese and copper, with even a small amount of copper in any oxidation state (e.g. Cu(0), Cu(I), or Cu(II)) serving as a highly regioselective mediator for the allylation of aromatic aldehydes in the presence of aliphatic aldehydes. This reaction is particularly effective when combined with a catalytic amount of acetic acid or in aqueous NH4Cl, with the high regioselectivity attributed to the differing reductive potentials of the two types of aldehydes. On the other hand, indium is considered an ideal mediator due to its lower first ionization potential compared to zinc, tin, and even magnesium, and its insensitivity to boiling water or alkali.

In aqueous solution, indium has been found to be particularly effective for the allylation of carbonyl compounds, with the reaction proceeding smoothly at room temperature without the need for any other promoters. Furthermore, only a catalytic amount of indium in combination with zinc is required.