Introduction In the Hive there is currently going on some research on the preparation of 2-propanones synthesised from the propenylbenzenes via the epoxides. This small write-up gives some details on the physical properties of two epoxides, namely the anethol-epoxide and the asarone-epoxide. The preparation is based on an rxn proposed by Osmium, publish at Rhodiums site (see
https://www.thevespiary.org/rhodium/Rhodium/chemistry/safrolepoxide.html
. Uemura modified the precedure to allow re-use of the MeCN used in this synthesis. A more OTC method for the epoxide synthesis has been provided by Chromic.
Epoxide Synthesis (described for anethole) [N1] 0.13mol anethole (approx 20ml) are dissolved in 200ml of a MeCN/MeOH (1:1) mixture. 2g NaCO3 are added and the mixture is stirred under room temperature with slow stirring. [N2]
Within 10-15 minutes the molar amount of 30% H2O2 is added dropwise under stirring. [N3] Stirring is continued at room temp for 18-24Hrs. After the time has elapsed, the MeCN/MeOH solvent is destilled off on a waterbath. [N4] An oily water-epoxide suspension remains. The crude epoxide is extracted with ether or another non-polar solvent. The ether is washed with Brine and dried over NaSO4. [N5] After drying the ether is destilled off on a water bath and the last traces of the ether are removed in vaccuum leaving the crude epoxide in the flask.
Crude Anethole Epoxide: This is a clear yellow pleasant smelling oil. (Yield about 90%)
Crude Asarone Epoxide: This is a brown-gold clear oil with smell of asarone. Not too much pleasant. (Yield 80%) [N6] The following picture shows the crude asarone epxoide.
Purification of the epoxides To purify the crude epoxides slow vaccuum destillation has been applied.
Anethole Epoxide Destillation: The yellow crude epoxide was destilled at 13Torr and 15.4g (70%) of a water-clear, very pleasant smelling oil came over
in the range between 118 to 126 DegC. Höring reports 132DegC at 11Torr.[N7] In the destillation flask a deeper yellow oil with a higher bp. remains. [N8]
Pictures of the purified anethole epoxide and the high-boiling residue.
Asarone Epoxide Destillation: The golden-brown oil (17g) are destilled under maximum vacuum (2-4 Torr). Within the range of 125-135Deg a clear light yellow oil destilles over. The yellow oil has the typical epoxide smell, very similar to the anthole one, but very different from the crude asarone epoxide. Yield 8.3g In the destillation flask a red thick oil remains (weight 4,3g) which after cooling solidifies but does not cristallize. [N9]
Pictures of the purified asarone epoxide and the high-boiling residue.
Discussion As the 'law of difficulty conservation' predicted, the epoxidation of asarone is much more picky than the one of anethole. In case of anethole consistent results and clean products have been received. In case of asarone, the yield of the desired epoxide is low (8.3 gram out of 20ml asarone) and it seems to bee esential to purify already the crude epoxide before going ahead. [N10] A first thermal re-arrangment run on anethole epoxide went smoothly [N11], the similar attempt for the asarone epoxide should be worth a try as soon as sufficient clean epoxide is available.
Notes [N1] Same procedure has been applied for asarone.
[N2] Not all of the NaCO3 will dissolve, but this has no effect on the performance of the rxn. It keeps the ph in the alkine area (p.h. about 8.5)
[N3] Do not use more than 1.1
molar amount of the H2O2. In one run Uemura 'overshoot' the H2O2 and some of the Acetonitrile has been oxidized to Acetamide, which cristallised during destillation and has been identified by its smell.
[N4] Uemura found the evap of the solvent mix must not be done under reduced pressure. The destilled aceotrop of the MeOH and the MeCN can be reused for further runs, which is more advantegous than Osmiums description. Uemura found the yield do not differ significantly. Remove the last traces of the solvent by applying vaccuum (may help to minimise the emulsion later on).
[N5] Emulsion
forming may appear and the seperation becomes sometimes a bit a pain-in-the-neck. Be patient.
[N6] 'Low' yield of 80% was caused by problems separating the solvent mix.
[N7]
(1) Höring, 'Über die Dibromide aromatischer Propenylverbindungen', Ber 38, 1905, pg. 3464 [N8] Yields of destilled epoxides have been between 70-75% percent in all three runs.
[N9] Like glas or molten plastic, may indicated some kind of polymerisation took place?
[N10] Uemura reported already earlier on the failure from asarone -> ketone in one step.
[N11]
succesful to some extend, a write-up may follow later on.
Carpe Diem