Propargylmagnesium Bromide

Uses

Propargylmagnesium Bromide is used to propargylation and propadienylation of electrophilic centers.

Preparation

Propargylmagnesium Bromide was prepared from propargyl bromide and amalgamated magnesium, generated in situ from Magnesium and Mercury(II) Chloride. Without HgCl2, only a mixture of acetylenic and allenic hydrocarbons is obtained. Reaction at reflux temperature in ether using a recycling reactor results in a greater than 90% yield of Grignard reagent consisting of a 4:1 ratio of allenic and acetylenic forms. Low temperature and slow addition of a dilute solution of a mixture of propargylic bromide and substrate to a reacting mixture, initiated with a small amount of the bromide, is a technique that reduces equilibration of the Grignard reagent before its reaction with the substrate. The same slow addition of a dilute ether solution of propargyl bromide to an initiated reaction is said to lead to the pure allenic Grignard reagent. The Grignard reagent prepared from propargyl bromide has been converted into a solid powder by complexation with tris(3,6- dioxaheptyl)amine.

storage

Propargylmagnesium Bromide should be prepared and used when needed. Solutions undergo disproportionation to propyne, allene, the acetylenic Grignard reagent, and di-Grignard reagent over days. The reagent is easily hydrolyzed to gaseous hydrocarbons and must be handled and used in a dry and inert atmosphere. Pure propargyl bromide, the compound required for preparing the Grignard reagent, is shock sensitive. Propargyl bromide is also supplied as an 80% by wt solution in toluene, which does not show this hazardous property.