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CYCLODEXTRIN CAVITIES

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Tetrahedron Letters,Vo1.30,No.35,pp 4641-4644,1989 oo4o-4o39/89 $3.00 + .oO Printed in Great Britain Pergamon Press plc ASYMMETRIC INDUCTION IN BENZOIN BY PHOTOLYSIS OF BENZALDEHYDE ADSORBED IN CYCLODEXTRIN CAVITIES V. Pushkara Rao and Nicholas J. Turro* Chemistry Department, Columbia University New York, New York 10027. Abstract Irradiation of solid 5-cyclodextrin complexes of benzaldehyde produces optically active benzoin as a major photo-product with an enantiomeric excess of up to 15%. Introduction The utilization of inclusion complexes of cyclodextrins in the modification of chemical reactivity of organic molecules has been of much interest,’ but so far their use in photochemistry and in asymmetric synthesis has been limited.2 We report the utility of a cyclodextrin as a tool in asymmetric induction in the simple synthesis of optically active benzoin from the photochemistry of benzaldehyde by solid complex formation with the chiral template p- cyclodextrin. The photolysis of benzaldehyde in benzene solution3y4 produced benzoin as the major primary photolysis product via a free radical process. Extended photolysis produced, as secondary products, almost equal amounts of deoxybenzoin, benroin and 1,2-diphenylethane-l,2-diol. On the other hand, irradiation of benzaldehyde in ethanol yielded i,2- diphenylethane-1,2-diol with only a trace of benzoin. Photo-CIDNP studies in perdeuterobenzene confirm the free radical nature of the photolysis by showing that excited triplet benzafdehyde abstracted hydrogen from the ground state benzaldehyde to form a radical pair, which collapsed to give benzoin.4 Results Benzaldehyde readily forms solid inclusion complexes with a, 3 and rcyclodextrin: To a saturated solution of each of the cyclodextrins in distilled water, equimolar amounts of benzaldehyde were added and magnetically stirred at room temperature for 12 h. The white precipitate which was formed was filtered and washed with diethyl ether and dried at 50’ C for 5 h to yield solid complexes. The presence of an inclusion complex in the solid state was inferred by comparing the X-ray powder diffractograms of the solid complexes with those of the pure cyclodextrins. Comptexation was also evident from their FT-IR spectra and 13CCPMAS solid slate NMR spectra of p and r-complexes of benzaldehyde.10 Additional support for the presence of an inclusion complex in the solid state comes from the observation that a known weight of the complex, when extracted with diethyl ether, yielded one equivalent of guest in the cases of a and p- cyclodextrin complexes and two equivalents of benzaldehyde in the case of’the r-cyclodextrin complex. Thus, a stoichiometric 1 :l complex was formed between. P-CD/a-CD and benzaldehyde, and a 1:2 stoichiometric complex was formed between y-CD and benzaldehyde. These complexes were stable to air and oxygen. Photolyses of the solid cyclodextrin complexes were carried out with a Hanovia 450-W Hg medium-pressure lamp for 3 h at room temperature in a quartz vessel under vacuum. The photolysis vessel was tumbled continuously during the irradiation to ensure homogeneous phototysis of the sample. Conversions were limited to less than 20%. After photolysis,. the solid complexes were dissolved in excess water and extracted with diethyl ether and chromatographed with hexane/ ethyl acetate (53) to isolate the products in pure form. 46414642 Irradiation of solid jI-cyclodextrin complexes of benzaldehyde resulted in an intermolecular reaction to give benzoin and 4-benzoylbenzaldehyde (7:3, 80%; Scheme 1). The latter product, which is not formed in the photolysis of benzaldehyde in organic solvents, was characterized by spectral characteristics and by comparison with its authentic sample.3 The formation of this rearranged product in the solid state irradiation of benzaldehyde contrasts its photo-reactivity in isotropic media and in aqueous solutions containing &cyclodextrin (vide infra). Another interesting feature of this solid state irradiation is that the benzoin formed in 5-cyclodextrin cavity is optically active and the enantiomeric excess calculated from the optical rotation, is 15~1 %.6 Similar irradiation of the a- cyclodextrin complexes did not result in any detectable reaction even after prolonged irradiation (10 h). Irradiation of the y-cyclodextrin complexes (3 h) in the solid state undergo reaction resulting both benzoin and 4-benzoyl benzaldehyde in 78% yield (55:45). However, enantiomeric excess observed for the benzoin formed in y- cyclodextrin cavity is negligible (cl %). hu 3h Scheme 1 (313) , j3-cyclodextrin solid state 0 ZWX2 R-(-)-Benzoin [a]” -18.8 f 0.4 (c = 1.5, acetone) % E.e. = 15 f 1% Aqueous cyclodextrin complexes were prepared by sonicating a mixture of cyclodextrin (5 mmol) and benraldehyde (0.5 mmol) in water (100 ml). In aqueous medium, the photolysis of either a- or p-cyclodextrin complexes of benzaldehyde under argon atmosphere, resulted in photoreduction to give meso and d&l ,2-diphenylethane-1,2-diois as major products (60-70%, meso/d,l = 0.9); benzoin and 4-benzoylbenzaldehyde (-3%, ~1% respectively) are formed in only trace amounts in these aqueous irradiations and therefore, optical rotation of benzoin could not be measured. In the absense of substantial amount of benzoin formation in the solution photolysis of benzaldehyde, it may be interesting to look at the optical activity of benzoin formed in a cyclodextrin aqueous solution experiment using cyanide as a catalyst.7 However, such an experiment is out of scope of the present investigation. Discussion The observed photochemical behavior of benzaldehyde-cyclodextrin complexes in the sotid state is unique and different from that of these complexes in aqueous solution and


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