Trapp solvent mixture

Hoffmann has shown that diastereoselective bromine-lithium exchange may be achieved even in acyclic systems.118 Treatment of 138 with BuLi in the Trapp solvent mixture at -120 °C in the presence of acetone generates the epoxides 139 and 140 in a 94 6 ratio of diastereoisomers.119 120 The selectivity was found to depend on the organolithium used for the bromine-lithium exchange, and it must therefore be under kinetic control. 

Solvent plays an important role in these reactions. A highly basic solvent, usually THF, is used to provide stabilization of the lithium halocarbenoid, due to coordination which disrupts the internal Li-Br interaction which would otherwise lead to a-elimination (equation 20). The extreme low temperature of many of the reactions requires special solvent systems, commonly a mixture of THF, diethyl ether, and petroleum ether or pentane in a ratio of 4 1 1 (Trapp mixture). Solvent has also been noted to alter the mechanistic course of the reaction for example, reaction of ketones with lithiobromomethane in hexane was observed to give rise to alkenes rather than epoxides. ... 

A series of authors has defined the solvent relative strength for polar adsorbents in the form of eluotropic series, grouping them in order of their chromatographic elution strength, with both pure solvents and mixtures of solvents included. The most familiar of these is, of course, the one set up by Trappe, and variations of this have been published from time to time. Trappe gave the... 

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