Ionic liquids rearrangement

Beckmann rearrangements of several ketoximes were performed in room-temperature ionic liquids based on l,3-dialkylimida2olium or alkylpyridinium salts containing phosphorus compounds (such as PCI5) by Deng and Peng [59] (Scheme 5.1-31, BP = 1-butylpyridinium). Turnover numbers of up to 6.6 were observed, but the authors did not mention whether the ionic liquid could be reused. 

In addition, a Lewis acid behaviour was proposed in the cyclopropyl carbonyl rearrangement catalyzed by [pmim][Br] as depicted in Scheme 78 [200]. The products were obtained in good yields up to 95% when stirred at rt in the ionic liquid. By the application of sonication, the reaction time was decreased to 0.75 h. 

Lee and colleagues studied the ionic-liquid-mediated Beckmann rearrangement under microwaves (equation 96). They screened several ionic liquids and investigated the amount of sulfuric acid needed as catalyst. The microwave methodology was found to be superior to analogous conventional heating. All the ionic liquids tested worked well in the microwave and only 5 mol% sulfuric acid was needed. The BMIM ionic liquids ([BMIM]PFg, [BMIM]Bp4, [BMIMJSbFg and [BMIMJOTf) were found to be thermally stable. 

To explain the difficulties encountered in the reuse of sulfonyl chloride functionalized ionic liquids during Beckmann rearrangement, Deng and colleagues proposed a mechanism for rearrangement of cyclohexanone oxime (equation 97). 

Kitazume and Zulfiqar have investigated the Claisen rearrangement of several aromatic allyl ethers in ionic Hquids, catalyzed by scandium(III) trifluoromethane-sulfonate [72]. The reaction initially gave the 2-aUylphenol but this reacted further to give 2-methyl-2,3-dihydrobenzo[b]furan (Scheme 5.1-41). The yields in this reaction were highly dependant on the ionic liquid chosen, with [EDBU][OTf giving the best yields (e.g., 91 % for R = 6-CH3). Reactions in [BMIMjlBFJ and [BMIM][PF j gave low yields (9-12 %). 

Alternative purification protocols are available. Zhaolin Sun of Lanzhou University reports (Tetrahedron Lett. 45 2681,2004) that the ionic liquid TISC was specifically designed to promote Beckmann rearrangement. TISC is not soluble in water, so the product caprolactam was easily removed from the ionic liquid by extraction with water. 

Olefin isomerization, with Claisen rearrangement, 1, 365 Olefin metathesis with alkyllead, 9, 415 in aqueous media, 1, 834 ESI—MS studies, 1, 812 in high-throughput catalyst discovery, 1, 365 in ionic liquids, 1, 869 for polymerization characteristics, 1, 149 Grubbs catalysts, 1, 151 Schrock catalysis, 1, 150... 

Rapid Claisen rearrangement of allyl phenyl ether and meta-substituted derivatives has been reported to occur with poor regioselectivity at 250 °C in dicationic ionic liquids, especially (24).23 Claisen rearrangement of resorcinol allyl ethers (25) was found to have poor regioselectivity under thermal or microwave conditions, but it was further found that selectivity for the 6-substituted product (26) over (27) could be improved to 13 1 using boron trichloride and an appropriately protected ether (25 ... 

Scheme 2. Claisen rearrangement catalysed by Sc(OTf)3 in a trifluoromethansulfonate ionic liquid.

A -Butylpyridinium tetrafluoroborate, containing dissolved phosphorus pentachloride, allows catalytic Beckmann rearrangement of cyclohexanone oxime giving e-caprolactam with good conversion and selectivity <2001TL403>. The same ionic liquid containing dissolved ytterbium(m) trifluoromethanesulfonate was used to perform Friedel-Crafts acylation of furan and thiophene <2005JIG398>. 

The DPM rearrangement of 16a in the presence of a soluble ionic liquid sensitizer in the ionic liquid [bmin]BF4 gives 17a in 87% yield (upper part in Scheme 4.8) [10]. Chiral ionic liquids have been evaluated as chiral induction solvents for the DPM rearrangement of 16b (bottom part in Scheme 4.8) [11]. 

Various l-aryl(hetaryl)-4-nitro-l,2,3-triazoles are obtained in result of heterocyclic rearrangements of uncondenced furoxans with the use of ionic liquids as reaction media [559-564],... 

Not much attention has been directed to isomerisation and rearrangement reactions in ionic liquids. Beckmann rearrangements of ketoximes have been performed in neat ionic liquids catalysed by phosphorus(V) chlorides and oxides1 301 and Fries rearrangements of phenylbenzoates have been carried out in chloroaluminate ionic liquids with moderate to good selectivity.1 311... 

Another example describes the Claisen rearrangement of allylphenyl ethers in ionic liquids mainly based on the l,8-diazabicyclo[5.4.0]-7-undecenium cation. Under the chosen conditions, with scandium(III) triflate as catalyst, these substrates were found the undergo a subsequent cyclisation reaction.[134]... 

HCl-HOAc-AcjO, POCls, BiCla, neat with FeCls, and polyphosphoric acid. The reaction has been done in supercritical water and in ionic liquids. A polymer-bound Beckman rearrangement has been reported. Sim-ply heating the oxime of benzophenone neat leads to A-phenyl benzamide. The oximes of cyclic ketones give ring enlargement and form the lactam,as in the formation of caprolactam (83) from the oxime of cyclohexanone. Heating an... 

---