Phenylcyclohexene, epoxidation

REGIO- AND STEREO-CONTROLLED OXIDATIONS AND REDUCTIONS Table 6.9 Recycling of KG-60-FT in the 1-phenylcyclohexene epoxidation. 

The use of the ionic liquid [bmim][BF4] further improved the Burgess epoxidation system [70]. Chan and coworkers found that replacement of sodium bicarbonate for tetramethylammonium bicarbonate and performing the reaction in [hmim][BF4] allowed for efficient epoxidation of a number of different olefins, including substrates affording acid-labile epoxides (such as dihydronaphthalene (99% yield) and 1-phenylcyclohexene (80% yield)). 

Sherrington et al. were the first to attempt the synthesis of chiral polymeric metal complexes by the chemical modification route,78,177,178 whereby the [Mn(salen)Cl] units are attached in a pendant fashion, by only one of the aromatic rings, to poly(styrene) or poly(methacrylate) resin beads of various morphology. Epoxidation of 1-phenylcyclohexene gave enantioselectivity values between 61% and 91%. 

Fig. 12 The competing transition states for the epoxidation of 1-phenylcyclohexene with ketone 26 and ketone 55...

In 1996, Aggarwal and coworkers synthesized binaphthyl-based iminium salt 76 via oxidation and methylation from binaphthylamine (Scheme 15) [147], Catalyst loading of 5 mol% is sufficient to catalyze the epoxidation of a number of olefins in good yield. Up to 71% ee can be obtained for 1-phenylcyclohexene oxide using this catalytic system (Table 7, entry 8). 

While most of the iminium salts studied are cyclic, several acyclic iminium salts have also been investigated. In 1997, Armstrong and coworkers reported the use of acyclic iminium salt 83 as chiral epoxidation promoter (Fig. 27) [156, 157]. 1-Phenylcyclohexene oxide could be obtained in 100% conversion and 22% ee with stoichiometric amounts of 83. In 2002 acyclic iminium salt 84, prepared from L-prolinol, was investigated by Komatsu and coworkers, and cinnamyl alcohol was epoxidized in 70% yield and 39% ee (Fig. 27) [158]. 

Goncalves et have compared the amine (V) and the iminium salt (W) for the enantioselective epoxidation of some prochiral olefins in acetonitrile/water and found that the yields and ees are nearly the same for the epoxidation of a selection of olefins. The amines of type (X) are less well developed. Armstrong has summarized the developments in this field and suggested mechanisms based on hydrogen bonded species, one of which is shown in Figure 1.49. Typical yield and ee data for the epoxidation of 1-phenylcyclohexene for these catalysts are also shown in Figure 1.49. 

Table 7.8 Asymmetric epoxidation of 1-phenylcyclohexene utilizing protonated ammonium salts.

The key intermediate, 1-phenylcyclohexene, is both a proper precursor to the desired epoxide and readily available from the given starting materials. A reasonable synthesis is... 

In 1996, Aggarwal reported a chiral binaphthyl iminium 16 which gave 31% ee for the epoxidation of F-stilbene and 71% ee for the epoxidation of 1-phenylcyclohexene [40]. Several other classes of chiral iminium salt have since... 

A one-sided attachment of a vinyl-functionalized salen monomer to an MCM-41 material was reported by Janssen (95). In the epoxidation of 1-phenylcyclohexene with PhIO in acetonitrile, the Mn-functionalized structure 7g gave an ee of 75%, which is the same as for the soluble Jacobsen complex and considerably higher than that obtained with 7f. Morever, the chemoselectivity, the olefin conversion, and the enantioselectivity remained unchanged over four consecutive cycles. 

Figure 3.19 Epoxidation of 1-phenylcyclohexene by the MTO/pyridine/hydrogen peroxide system.

A further series of ketone catalysts containing spiro ethers and lactones have been examined in the asymmetric epoxidation of phenylcyclohexene (cf Table 5, entry 2 94% yield, 98% ee) (Figure 11). The substituents on the spiro ring appeared to effect enantioselectivity both sterically and electronically <2005T6409>. 

Armstrong has developed a-fluoro-W-ethoxycarbonyltropinone 54 as an asymmetric catalyst for the enantio-selective epoxidation reaction 1-phenylcyclohexene oxide is produced in 97% yield and 69% ee (Scheme 20) <1998CC621>. Analogously, the use of a-acetoxy-8-oxabicyclo[3.2.1]octan-3-one 55 results in 71% yield and 98% ee (Scheme 20) <2001TA2779>. Tropinone catalyst 54 gives only low levels of enantioselectivity in the asymmetric epoxidation of dihydronaphthalene and an enol benzoate (Scheme 20) <2002JOC8610>. 

Recently, Armstrong and Tsuchiya have prepared the chiral tetrahydropyranone 56 and examined its use in asymmetric epoxidation reactions. Phenylcyclohexene oxide was formed in excellent yield and high ee however, low enantioselectivity was observed with a trimethylsilyl enol ether (Scheme 21) <2006T257>. 

Denmark and Matsuhashi achieved only moderate success in the asymmetric epoxidation of phenylcyclohexene and indene catalyzed by novel chiral a-fluoroketones 57 and 58 (Scheme 22) <2002JOC3479>. 

Matsumoto and Tomioka examined chiral ketone 59 containing the l-aza-7-oxabicyclo[3.5.0]decane skeleton and its C2-symmetric analog 60 as catalysts in the asymmetric epoxidation of phenylcyclohexene and phenyldihydro-naphthalene (Scheme 23) in both cases, excellent yields of the oxiranes were obtained with good ee s <2002TL631>. 

Shing et al. have prepared a number of arabinose (commercially available in both enantiomeric forms) derived 4-uloses and examined their potential as chiral dioxirane precursors. The most successful of these ketones was found to epoxidize phenylcyclohexene in 92% yield and 85% ee (Scheme 24) <2003TL9225>. 

Yang ( / /. have investigated a series of C 2-symmetric chiral ketones based on the 2,2-bis(diphenyl-phosphanyl)-l,l-binaphthyl (BINAP) skeleton. The asymmetric epoxidation of phenylcyclohexene (and in one example dihydronaphthalene) was achieved in good yields and levels of enantioselectivity (Figure 12) <1996JA491, 1998JA5943, 2004ACR497>. 

The first use of an enantiomerically pure oxaziridinium salt to catalyze asymmetric epoxidation (trans stilbene oxide produced with 33% ee using 61 (Figure 13)) was reported by Lusinchi and co-workers in 1993 <1993TL7271> Subsequently, it was reported that phenylcyclohexene is converted to the corresponding epoxide with just 5% ee using stoichiometric quantities of 61 <1999T141>. 

Aggarwal and Wang have investigated binaphthyl-based iminium salt 62 (Figure 13) 5 mol% of the catalyst is sufficient to mediate the asymmetric epoxidation of phenylcyclohexene (80% yield, 71% ee), whereas the induction was not as efficient for methylcyclohexene (80% yield, 39% ee) <1996CC191>. 

Armstrong a al. found that stoichiometric use of chiral pyrrolidine-derived iminium salt 63 (Figure 13 could only be isolated in impure form) in the asymmetric epoxidation of phenylcyclohexene gave the oxirane with 22% ee < 1999T2341 >. 

In 2002, Bulman Page etal. reported the dibenzazepinium salt analogs of catalysts 64e and 641,65 and 66, respectively. These were examined in the asymmetric epoxidation reactions of phenylcyclohexene, phenyldihydronaphthalene, and indene, with very similar results being observed with 65 and 66 as with 64e and 641 (Figure 15) <2002SL580>. 

Yang and co-workers have suggested that the inherent difficulties in the preparation/isolation of unstable exocyclic iminium salts can be overcome by in situ formation of the catalytic species from chiral pyrrolidines and aldehydes. The catalytic asymmetric epoxidation of phenylcyclohexene and dihydronaphthalene mediated by the iminium salt derived from pyrrolidine 67 and aldehyde 68 has been examined (Scheme 27) <20010L2587>. 

Chan and co-workers have developed a novel ruthenium complex (81, Figure 16) which catalyses the epoxidation of cyclic alkenes with molecular oxygen in excellent yield under mild reaction conditions. Cyclopentene (90% yield), cyclohexene (up to 90% yield), methylcyclohexene (90% yield), phenylcyclohexene (78% yield), and cyclooctene (97% yield) are all readily epoxidized <2003CC1058>. 

Denmark and Wu used 0-labeled ketone 81 in their studies of epoxidation of 1-phenylcyclohexene, yielding the conclusion that dioxiranes are the reactive oxidants in monophasic epoxidations with Oxone <1997JOC8964>. 

ASYMMETRIC EPOXIDATION OF trans-p-METHYLSTYRENE AND 1-PHENYLCYCLOHEXENE USING A D-FRUCTOSE-DERIVED KETONE (R,R)-trans-P-METHYLSTYRENE OXIDE AND (R,R)-l-PHENYLCYCLOHEXENE OXIDE... 

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