Hi!
Recently SWIA wondered about why the method outlined in
https://www.thevespiary.org/rhodium/Rhodium/pdf/bd2gbl.cuo.pdf
(and translated by GC_MS,
Post 441898 (missing)
(GC_MS: "1,4-Butanediol to gamma-Butyrolactone in English", Methods Discourse)) wasn't tried on 1,4-BDO here at the hive - and decided to give the dehydrogenation a try.
SWIA first measured out 87ml 1,4-butanediol (1 mol) and placed it in a clean, dry 500ml 2-necked BRF together with a mag.stirbar and 0.79g black copper(II)-oxide (CuO; 0.01 molar eq.). A thermometer and dean-stark trap were attached and the trap fitted with a big vigreux column as air-cooled conderser, then everything was heated to >200°C with good stirring.
As soon as 180°C rxn temp. were reached, gas evolution started (bubbles evolved). After ~0.75 hours of stirring/heating, the black CuO suspension had cleared up almost completely, and a tiny copper "mirror" could be seen at the side walls (elemental copper precipitated). At this point, the internal temperature was noted to be 204°C.
During the next hours, some few ml's of distillate were collected in the trap, they were dried over some 4A sieves and stored to be added to the workup later.
After 12 hours of reflux, the mixture was cooled down, then the dean-stark was replaced with a distillation setup and the reaction mixture was distilled at atmospherical pressure, collecting distillate until the temp. neared 230°C, then dist. was stopped.
The crude product and unreacted diol were then diluted with d.H2O (1/10th of the total volume), heated to reflux in other clean RBF and a solution of 32g NaOH in 80ml d.H2O was added dropwise until pH 8 was reached. After addition was complete, reflux was maintained for 5 minutes, then everything was poured into a sep. funnel, cooled down to room temp. and the upper BDO layer was separated and further extracted with Et2O. The remaining NaGHB/H2O solution was then concentrated on a hot plate (160°C) until no more water boiled off, then everything was cooled very slowly to -7°C and the precipitated GHB*Na collected, dried on hotplate (110°C) and finally over anhydrous 4A sieves in a desiccator.
The yield of dried NaGHB amounted to ~27 grams, corresponding to a molar 35% yield calculated from 1,4-butanediol.
After redissolving the Na salt in H2O and adding 1.01 eq. H2SO4, followed by steam dist. and extracting the distillate with EtOAc and solvent removal, the yield was 17ml (or ~19g) of purified
gamma-butyrolactone, which resulted in very clean product when reacted with ACS grade KOH to pH 7.5 (no soap taste whatsoever, slight licorice taste/smell, quite agreeable).
Nice procedure, dare I say. And with longer rxn times, yields could still be improved a bit I think (the original paper recommends 15h rxn time for Bu-1,4-OH)
As the reaction proceeds through removal of formed water: does anybody know if 4A mol. sieves will react with CuO at these high temperatures? If not, adding enuff sieves to absorb all formed water would increase yield (or at least rxn speed). But I'm afraid that the catalyst may react with the sieves and become unreactive... (?)
Greetz, A