Zeolite chirally modified

The Use of Chirally Modified Zeolites in Photochemical Asymmetric Synthesis... 

Chirally modified zeolites can also be used as a chiral host for enantiose-lective photoreaction. For example, a mixture of 62a, (-(-norephedrine and zeolite (NaY) in CH2Cl2-hexane was stirred for 12 hr and filtered to give the zeolite... 

Joy, A., Scheffer, J.R. and Ramamurthy, V. (2000) Chirally modified zeolites as readion media photochemistry of an achiral tropolone ether. Organic Letters, 2, 119-121. 

Asymmetric photoisomerization of cyclo-octene was also investigated in supramolecular systems such as native [153] and modified cyclodextrins [154,155], chirally-modified zeolite [156], and DNA grooves [157]. 

II. SUPRAMOLECULAR PHOTOCHIROGENESIS WITH CHIRALLY MODIFIED ZEOLITES... 

Chirally modified zeolite Norrish/Yang Type II Reaction Direct excitation Enantiodifferentiation 49-53... 

Pioneering work on the photochemical diastereocontrol in zeolite supercages was reported by Turro and coworkers in 1991 [48]. They investigated the diastereoselective photodecarbonylation of 2,4-diphenyl-3-pentanone (DPP) adsorbed in various cation-exchanged X and Y zeolites to find that the diastereo-selectivity of d9l- over mestf-2,3-diphenylbutane increases in the order LiX NaX < LiY NaY < KY. In 1996, Ramamurthy and coworkers reported the first example of photochemical asymmetric induction in chirally modified zeolites [49], where they employed the Norrish/Yang type II reaction of cis-4-tert-butyl-cyclohexyl aryl ketones to the corresponding cyclobutanols. Since then, a variety of asymmetric photoreactions in zeolite supercages have been reported as reviewed below. 

A. Enantiodifferentiating Norrish/Yang Type II Reactions in Chirally Modified Zeolites... 

Ramamurthy, Scheffer and coworkers and other investigators reported the enantiodifferentiating Norrish/Yang photocyclization [49-53] of aryl cis-4-tert-buty -cyclohexyl ketones 1 to cyclobutanols 2 (Scheme 1) in chirally modified zeolite supercages [49]. 

Achiral cis-1,2-diphenylcyclopropane photoisomerizes to the chiral trans isomer upon singlet- or triplet-photosensitized irradiation [64-67], It is expected that the reactant and chiral inductor immobilized in a zeolite supercage interact intimately with each other to afford more efficient photochirogenesis. Ramamurthy and coworkers reported that the enantio- and diastereodifferentiating photoisomeriza-tions of ris-2p,3p-diphenyl-la-cyclopropanecarboxylates 20 (Scheme 7) in chirally modified zeolite supercages lead to the corresponding chiral trans isomer 21 [68]. 

In this study, they demonstrated for the first time that chirally modified zeolites not only function as supramolecular photosensitizing media but also enhance the original enantiodifferentiating ability of the chiral photosensitizer. This newly developed methodology should not be restricted to this particular system but be readily expandable to a wide variety of supramolecular photochirogenesis reactions. 

Irradiation of tropolone alkyl ether 22 (Scheme 14) led to a 4ir-disrotatory ring closure to yield bicyclo[3.2.0]heptadienone 23 with two chiral centers, while prolonged irradiations led to the formation of a secondary product 24 [76-78]. As the same photocyclization was performed in chirally modified zeolites, it is interesting to compare the asymmetric photochemical behavior of 22 in the distinctly different chiral confined media of zeolites and cyclodextrins. Even in the... 

Ramamurthy and coworkers have utilized zeolites modified with chiral organic compounds [17,18]. Zeolites are crystalline aluminosilicates with open framework structures. In this approach, the zeolite is first loaded with a chiral inductor and the compound to be photolyzed is then added in a second, separate adsorption step. Asymmetric induction ensues as a result of the close proximity enforced between reactant and chiral inductor in the confined space of the zeolite supercage. The zeolite method has the disadvantage that the size of the substrate is limited by the pore size of the zeolite being used. Most of the work using the chirally modified zeolite approach was compared with the ionic chiral auxiliary method by Scheffer and coworkers. The enantiodifferentiations by the zeolites are usually low to moderate. 

In contrast, the chirally modified zeolite approach led to poor results. For instance, irradiation of the methyl ester of 2-benzoyladamantane-2-carboxylic acid 44, included in the NaY zeolite, which had been preloaded with optically pure ephedrine as a chiral inductor, gave the cyclobutanols 45 and 46 in 35 and 5% ee, respectively (Scheme 11) [65]. Similarly, the methyl ester of benzoylcyclohexane carboxylic acid 47 afforded the cyclobutenol 48 in 30% ee [63]. 

Salt crystals of the carboxylic-acid-containing a-oxoamide 59a with opti j cally pure amines were irradiated under nitrogen. (S)-Prolinamide was revealed to be the best chiral auxiliary, giving essentially optically pure photoproduct (+ M (3-lactam 60a in 99% ee and 94% chemical yield at almost complete conversion [76]. In contrast, the achiral oxoamide 59b, included within chirally modified zeolites, which had been preloaded with optically pure chiral inductors like ephed ... 

Chirally modified zeolites were also applied to the asymmetric oxa-di-TT-methane photorearrangement. Irradiation of a hexane slurry of compound 74 included in dry (— )-ephedrine-modified NaY zeolite gave product 75 enantiomer-ically enriched to around 30% [83]. However, photolysis of 74 in solution, in the presence of optically pure ephedrine, gave only a racemic product mixture. 

Scheme 16 Enantioselective oxa-di-Tr-methane rearrangement using the ionic chiral auxiliary and chirally modified zeolite method.

R = H ionic chiral auxiliary R = Me chirally modified zeolite... 

Photobehavior of N-alkylpyridones within zeolites provides support to the claim that chirally modified zeolite is a useful chiral medium (Scheme 42) [292]. Of the three examples provided, two with phenyl substitution yield cyclized products in moderate enantioselectivity. 

The results discussed above show that chirally modified zeolite could serve as a chiral medium to achieve low to moderate enantiomeric excess in photochemical reactions. Given that most examples thus far examined give less than moderate ee, one tends to get discouraged. At the same time there are a few examples that give respectable ee, which provides hope for the success of this approach. 

The two ketones (28) and (29) are known to undergo the Norrish Type II hydrogen abstraction process, and their photochemical reactivities have now been studied in chirally modified zeolites. The zeolites were modified by stirring them with known amounts of ( —)-ephedrine. Irradiation of the ketones in the zeolites brought about some enantiomeric enhancement. However, the various zeolites studied behaved differently and the NaX zeolite favoured the (+)-isomer of the product (30) while the NaY favoured the ( —)-isomer. The other ketone (29) showed only low enantiomeric enhancement and gave both the cis and the trans cyclobutanols (31) and (32) in a ratio of 4 1. ... 

They further investigated the enantio- and diastereodifferentiating Yang photocyclizations of achiral 2-benzoyladamantanes 3 and optically active menthyl and isomenthyl 2-benzoyl-2-adamantanecarboxylates 7 in chirally modified zeolites (Scheme 2 and Scheme 3) [54]. 

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