2.2- Dimethylchromenes, enantioselective epoxidation

Interest in enantioselective epoxidation continues and 2,2-dimethylchromenes appear to be particularly suitable substrates for the evaluation of the catalytic system <96JCS(P1)1757, 96SL1079, 96TL3895>. 

Freire and coworkers described the intercalation of a chiral phosphonic acid derivatised [Mn (sa/e )Cl] complex within KlO-Montmorillonite layers by ion-exchange with the interlayer potassium cations. The catalytic performance of the heterogeneous catalyst (denoted as [Mn (sa/en)Cl] K10) was evaluated in the enantioselective epoxidation of styrene, a-methylstyrene and 6-cyano-2,2-dimethylchromene, at 0 °C in dichloromethane or acetonitrile, using three different oxidant systems m-CPBA/NMO, PhIO and NaOCl the results are summarised in Table 11.1. 

Kureshy and coauthors [82] also reported that chiral Mn(Salen) catalyst immobilized in the nanopores of MCM-41 and SBA-15 (Scheme 10.14) showed higher enantioselectivity (70% ee) than its homogeneous counterpart (45% ee) for the enantioselective epoxidation of styrene with aqueous NaOCl as the oxidant. In addition, the immobilized chiral Mn(Salen) could smoothly catalyze the epoxidation of bulkier alkenes such as 6-cyano-2,2-dimethylchromene into their epoxides with enanatioselectivity (up to 92% ee) comparable to those of the homogeneous counterparts. The heterogeneous catalyst could be recycled four times without notable loss of activity and enantioselectivity. 

Dimethylchromene has also proven to be a useful substrate for the assessment of various transition metal complexes as epoxidation catalysts. Chiral Mn(III)-salen complexes are efficient <00CC615 00T417> and can be recycled when used in an ionic liquid <00CC837>. The enantioselective aziridination of a chromene has been achieved using a chiral biaryldiamine-derived catalyst (Scheme 22) <00JA7132>. 

These catalysts, 11-13, show good enantioselectivity ranging from 80 to 95% ee in the epoxidation of conjugated cfs-di- and tri-substituted olefins. Epoxidation of "good substrates such as 2,2-dimethylchromene derivatives proceeds with excellent enantioselectivity (>95% ee). Since the results obtained with these first-generation Mn-salen catalysts have been reviewed [21,33], only typical examples are shown in Table 6B.1. These reactions are usually carried out in the presence of donor ligand [34] such as 4-phenylpyridine A -oxide with terminal oxidants such as iodosylbenzene and sodium hypochlorite as described above. However, the use of some other terminal oxidants under well-optimized conditions expands the scope of the Mn-salen-... 

Catalyst ( Bu-4.33) is effective in the epoxidation of many ds-alkenes, such as the acyclic alkenes (4.42) and also cychc alkenes, especially 2,2-dimethylchromenes such as (4.43). Enhanced enantioselectivities are often observed using Katsuki s second generation catalysts (4.34) where the phenyl substituents of the BINAP moiety undergo greater steric interactions with the alkene substrate. Dihydronaphthalene (4.44) has been epoxidised with record ees using this catalyst system. ... 

Kureshy s group [83] immobilized the Mn(Salen) catalyst axially in the nanopores of MCM-41 via pyridine N-oxide (Scheme 10.15). These immobilized catalysts showed higher enantioselectivity (69% ee) than their homogeneous counterparts (51% ee) for the asymmetric epoxidation of styrene and were also effective for the asymmetric epoxidation of bulkier substrates such as indene and 2,2-dimethylchromene (conversion 82-98% ee 69-92%). The catalysts could be recycled for at least four times without loss in performance. The increase in ee values was attributed to the unique spatial environment constituted by the chiral Salen ligand and the surface of the support. 

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