Epoxidation biphasic conditions

Table 1 Results of the alkene epoxidation reactions with fluorinated (salen)Mn complexes under biphasic conditions ...

Table 2. Epoxidation of Enones using Biphasic Conditions ...

The biphasic method, like the triphasic method, allows the epoxidation of a broad range of substrates accommodating both aliphatic and aromatic substituents at either end of the enone moiety. However, unlike the triphasic method, the biphasic conditions allow reaction of hydroxide-sensitive systems. 

The stereo- and chemoselective epoxidation a,j9-unsaturated ketones is best achieved employing the biphasic conditions (PLL/UHP/DBU/THF), (see above). 

One of the early examples for organocatalysis is the asymmetric Weitz-Scheffer epoxidation of electron-deficient olefins, which can be effected either by organic chiral phase transfer catalysts (PTC) under biphasic conditions or by polyamino acids. This reaction has gained considerable attention and is of great synthetic use. 

Rudler and coworkers reported that in the case of moderately acid-sensitive epoxides the use of biphasic reaction conditions (H2O/CH2CI2) proved to be sufficient in order to obtain the epoxides with good selectivity because under biphasic conditions the contact of epoxides with water is minimized. Because of the lability of pyridines under the reaction conditions employed, alternative and more stable cocatalysts such as pyrazole (12 mol%, biphasic conditions), bipyridine (6 mol%, biphasic conditions H20/CH2Cl2) and bipyridine-A,Af -dioxide (1.2mol%) were employed together with MTO (Scheme Pyrazole is stable against oxidation and with this additive... 

Herrmann and coworkers observed short reaction times (0.02-14 h, generally 1 h). Conversions (89 to >99%) as well as epoxide selectivities (90 to >99%) were very high. Bipyridine has been employed by Rudler and coworkers under biphasic conditions with good results. Later, in 1998, Nakajima and coworkers presented their results on this topic in which they proposed that it is not the bipyridine but the bipyridine-A,A -dioxide, which is formed during the reaction, that is responsible for the suppression of the acidity of the MTO/H2O2 system . With bipyridine-A, A -dioxide as additive, a variety of olefins could be oxidized to the corresponding epoxides at room temperature with yields ranging from 80 to 96%. 

Another oil used for epoxidation with MT0/H202 is the oil from Jatropha curcas L. also known as Barbados or Physic nut. As with palm oil, it mostly consists of oleic acid (50%) and linoleic acid (29%) and various saturated fatty acids (20%). With 0.5 mol% of MTO and 12 mol% of pyridine in biphasic conditions, it was found that Jatropha oil can be completely epoxidized within 1.5 h [78]. 

The use of isolated fatty acids as substrates for epoxidations has already been reported in 1990 with the epoxidation of methyl oleate and methyl linoleate by MT0/H202 in ferf-butanol. After respectively 24 and 2 h, good yields of either the corresponding diol (methyl oleate) or the monoepoxide (methyl linoleate) are obtained (respectively 92% and 80%) [48]. Under biphasic conditions, the MTO-catalyzed epoxidation of methyl linoleate yields a mixture of mono- and diepoxide (approximately 1 1) at complete conversion after 6 h [79]. Finally, a conjugated methyl linoleate is treated with the MT0/H202 system in biphasic conditions, but here, poor results are obtained after 24 h at room temperature, yielding 26% of the 11,12-monoepoxide and 20% of the 9,10-epoxide [80]. 

Good to excellent enantioselectivity was achieved in the epoxidation of mainly cyclic olefins with the chiral salen-catalyst 52 immobilised in [C4Ciim][PF6], but selectivity deteriorated upon catalyst recycling, see Scheme 5.6.[48] Relative to molecular solvents, higher reaction rates were observed even under biphasic conditions when the epoxidation reaction was carried out in the presence of an ionic liquid. UV-VIS spectroscopic1341 and cyclovoltammetric[49] studies suggest that the commonly observed superior reaction rates are a reflection of the solvent s ability to stabilise the active metalla-oxo intermediate. 

The possibility of asymmetric induction under the fluorous biphase conditions was first speculated upon by Horvath and Rabai [10], and this year has seen the first report of asymmetric catalysis in a fluorous biphase [69]. Two, C2 symmetric salen ligands (29a, b) with four C8Fi7 ponytails have been prepared (Scheme 5) and their Mn(II) complexes evaluated as chiral catalysts for the aerobic oxidation of alkenes under FBS-modified Mukaiyama conditions. Both complexes are active catalysts (isolated yields of epoxides up to 85%) under unusually low catalyst loadings (1.5% cf. the usual 12%). Although catalyst recovery and re-use was demonstrated, low enantioselectivities were observed in most cases. 

Denmark et al. reported a general protocol for the catalytic epoxidation of alkenes by in r// -generated reactive dioxiranes capable of epoxidizing a variety of alkenes under biphasic conditions <1995JOC1391>. The epoxide diastereoselectivity (Scheme 4) showed pronounced dependence on the solvent used since the ratio of diastereo-mers, as well as the distribution between epoxide and enone products, is dependent on the solvent <1995TL2437, 1999TL8023>. Selected examples are given in Table 2. 

Under the name Oxone an oxidation agent has been introduced, consisting of KHSO4-K2SO4-2KHSO5. Solid Oxone converts methylenic functions under anhydrous, biphasic conditions to carbonyl compounds under the catalytic influence of ligand-modified Mn porphyrins and phase-transfer catalysts (e. g., acetophenone is obtained from ethylbenzene). In the case of cyelohexane, e-caprolactone results as well as cyclohexanol and -one ([219 b, 241] cf. also Baeyer-Villiger oxidation). Biphasic oxidations with methyltrioxorhenium (e. g., to epoxides) are reviewed in Section 3.3.13 [244 i]. 

Denmark and co-workers have introduced a convenient protocol for the catalytic epoxidation of alkenes with in. v/tw-generated dioxiranes under biphasic conditions using phase-transfer catalysts bearing a carbonyl group [Pg.134]

In 1976, using the parent quinine as a catalyst and 30% aqueous hydroperoxide as an oxidant, the asymmetric epoxidation of cyclic enones, such as naphthoquinones, was explored by Wynberg and coworkers [13]. However, the resulting enantioselec-tivity was minimal. Their further attempt under biphasic conditions with N-benzyl-quininium chloride 1 as the catalyst and t-BuOOH as the oxidant also resulted in very low enantioselectivities (up to 20% ee for the epoxidation of various cyclohexe-nones) [14]. However, the use of the dimeric form of Wynberg s catalysts 6 and 7 resulted in somewhat better (up to 63% ee with cyclohexenone) asymmetric... 

The manganese catalyst is selectively soluble in perfluorocarbons and has been tested in the enantioselective epoxidation of styrene derivatives under flu-orous biphasic conditions (C8F18/CH2CI2) at 20°C. In most cases, good yields have been observed, however, only indene was epoxidized with high enantiose-lectivity (92% ee) while all other olefins gave low enantiomeric excess Eq. (24). 

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