Enantioselective dimerization inclusion complex

Enantioselective photodimerization of coumarin (97) was accomplished in the inclusion complex (100) with 12a. For example, irradiation of the 1 1 inclusion compound of 97 with 12a gave the anti-head-to-head dimer (—)-98 of 96% ee in 96% yield [52], This photochemical reaction of 100 to the complex (101) of 98 was also found to proceed by a single-crystal to single-crystal manner. However, 1 2 inclusion compound of 11 and 97 gave the. vw-head-to-head dimer (99) upon irradiation in 75% yield [53],... 

The efficient enantioselective dimerization reaction of 97 in the inclusion complex (100) with 12a suggests that two coumarin molecules are arranged in chiral positions in 100 and their [2 + 2] photodimerization occurs by keeping the chirality in 100. The chiral arrangement of 97 was detected by CD spectral measurement of the inclusion complex in Nujol mulls. The 1 1 complex of 97 with 12a and 13a showed CD spectra with a mirror-imaged relation (Fig. 17). After photoirradiation, the CD absorptions of the complex (100) at 225, 275, 300, and 330 nm disappeared, and the new CD absorption due to the inclusion complex (101) of 98 at 240 nm appeared. The photodimerization of 97 was also followed by measurement of infrared (IR) spectra as Nujol mulls. Upon photoirradiation, the nCO absorption of 97 in 100 at 1700 cm 1 decreased gradually and finally disappeared after 4 hr, and new vCO absorption due to 98 in 101 appeared at 1740 cm-1. 

The enantioselective photodimerization of thiocoumarin (102) to optically pure (+)-a ft -head-to-head dimer (103) in the 1 1 inclusion complex of 102 with 12b was also found to proceed in a single-crystal to single-crystal manner. For example, when a mixture of thiocoumarin (102) and optically active host compound 12b in butyl ether was kept at room temperature for 12 hr, a 1 1 inclusion complex of 103 with 12b was obtained as colorless needles. Photoirradiation of a 1 1 inclusion complex in the solid state (400-W high-pressure Hg lamp, Pyrex filter, room temperature, 2 hr) gave a 2 1 complex of 12b with 103,... 

The steric course of the photodimerization reactions of coumarin (37a) and thiocoumarin (37b) succeeded in being controlled almost perfectly by carrying out the reaction in inclusion complexes using various host compounds. Furthermore, enantioselective dimerization reactions of 37a and 37b were found to proceed through a single-crystal to single-crystal process. 

Tanaka, K., Kakinoki, O., and Toda, F. (1992) Regio- and Enantioselective Photodimerisation of Cyclohex-2-enone as an Inclusion Complex with a New Optically Active Host, (-)-l,4-Bis[3-(o-chlorophenyl)-3-hydroxy-3-phenyl- l-propynyl]benzene Preparation of the Optically Pure (-)-symtram-Dimer of Cyclohex-2-enone, J. Chem. Soc., Perkin Trans. 1, 307. 

Similarly, irradiation in the solid state of a 1 1 inclusion complex of coumarin with 241a led to the cis-anti-cis head-to-head dimer 244 as an almost pure enantiomer [187]. The selectivity observed was very sensitive to the nature of the solvent used for crystallization. Replacing a mixture of ethylacetate-hexane with toluene-hexane led to a different 1 1 complex and different photodimers. Furthermore, among the numerous inclusion complexes tested, only the preceding example using 241a as the host led to an enantioselective photodimerization of coumarin. 

The solid-state chemistry of coumarin has also received considerable attention, mainly due to the biological importance of coumarin derivatives, and has been summarized in several reports. " In a comprehensive study of nearly 30 coumarin derivatives in the solid state, Venkatesan, Ramamurthy, and co-workers discuss the criteria for topochemical reactivity in the solid set forth 20 years earlier by Schmidt. Their results, though not invalidating previous guidelines for solid-state reactivity, indicate that caution, rather than rigor, is recommended in their application. The irradiation of coumarin within chiral inclusion complexes has also been used as a means to achieve enantioselective dimer synthesis. " ... 

The [2-1-2]-cycloaddition reactions in inclusion crystals, or host-guest crystals, have been thoroughly reviewed. Coumarin 61a, thiocoumarin 61b, and cyclohex-2-enone 62 efficiently undergo single-crystal-to-single-crystal enantioselective photodimerization in inclusion complexes with chiral host compounds (R,R)-(-)-fra s 63a, (R,R)- -)-trans 63b, and (-)-64, respectively. The products are (-)-anri-HH 65a (100% EE), +)-anti-HH 65b (100% EE), and -)-syn-trans 66 (48% EE) dimers. An example for enantioselective intramolecular [2+2]-photocycloadditions in inclusion crystals is the reaction of guest 67 in chiral host 68. Irradiation of the powdered inclusion crystals in water suspension affords the optically active photocyclization product 69 with 100% EE in 90% yield. 

Nakajima and co-workers have carried out extensive investigations into the influence of different chiral diamine-copper complexes on the oxidative dimerization of naphthols [146-148]. As emerged from Smrcina s work, the inclusion of an ester moiety on the naphthol precursor is an important factor for optimizing the enantioselectivity. After establishing a catalytic cycle with TMDA as the base and showing that sparteine gave promising results (Scheme 57), they focused their work on other chiral diamines (Table 39) [147]. 

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