Removing Volatile Butyl Bromide by Vacuum Transfer

Why physical removal works

n-Butyl bromide is a small, volatile primary halide (bp ≈ 101 °C at atmospheric pressure, and readily pumped at reduced pressure). The aryllithium and the metal carbonyl are far less volatile and stay in the flask. That volatility gap is the whole basis of this method: you do not need a reagent to destroy nBuBr if you can simply take it out of the mixture.

Procedure logic

1. Perform the Li–Br exchange as usual at low temperature.
2. Keep the mixture cold so the aryllithium does not decompose.
3. Apply vacuum (or a short bulb-to-bulb distillation) to strip nBuBr and the volatile solvent fraction.
4. Redissolve or top up with dry solvent, then add the metal carbonyl.

What changes in the mixture

• Before: ArBr, nBuLi, ArLi, nBuBr, LiBr, solvent.
• After removal: ArLi, LiBr, solvent — the butyl electrophile is gone.

Cautions

• The aryllithium must survive the operation; keep it cold and avoid prolonged exposure to vacuum.
• Do not let the mixture warm, or the aryllithium itself may decompose or attack the solvent.
• This removes the electrophile, not the aryllithium, so the desired carbene pathway is left intact.

Predicted effect on yield

Removing nBuBr before the carbene step eliminates the butyl–carbene adduct pathway, so the oxy-carbene yield should rise relative to the untreated reaction.