Host-guest crystal

Host-guest crystal Chiral host molecule Moderate-high enantiodifferentiation... 

Scheme 4 Enantioselective photocyclization via ylide intermediates in host-guest crystals.

Scheme 5 Enantioselective photocyclization of acrylanilides in their host-guest crystals.

Irradiation of host-guest crystals of the 2-[N-(2-propenyl) amino] cyclohex 2-enone derivatives 36 with chiral hosts 6a-c caused intramolecular [2 + 2]... 

Irradiation of the 2 1 host-guest crystals of cyclohex-2-enone 81 with the axle-wheel-type host compound (— )-5 as an aqueous suspension caused regio- and enantioselective [2 + 2] photodimerization to afford the (— )-anti-head-to-head dimer 82 of 48% ee in 75% chemical yield (Scheme 18) [86]. Similarly, solid-state photolysis of the 3 2 complex of cycloocta-2,4-dien-l-one 83 with (R,R)-( — )-4 gave the (-)-anti-head-to-head dimer 84 in moderate optical yield [87]. 

Irradiation of 1 1 host-guest crystals of coumarin 85a with (/ ,/ )-(— )-6a derived from tartaric acid gave the (— )-antvdtedd-to-head dimer 86a of 96% ee [88]. Enantiospecific photodimerization of thiocoumarin 85b gave optically pure (+ )-anti-head-to-head dimer 86b when the 1 1 complex with R,R)- — )-6b was used. X-ray structure analysis revealed that the distance between the two ethylenic double bonds was short enough (3.59 and 3.42 A for 85a and 3.73 and 3.41 A for 85b) for addition to occur and topochemically [89]. Further, both reactions were found to proceed via a single crystal-to-single crystal transformation. 

When the intermolecular forces are relatively strong and directional, crystalline molecular compounds (crystalline molecular complexes) are formed. They have fixed stoichiometries and ordered structures. These two-component molecular crystals are also called cocrystals or maybe adduct crystals. Hydrogen-bonded cocrystals, donor-acceptor crystals (charge transfer crystals), and inclusion crystals (host-guest crystals) are examples of crystalline molecular complexes. Crystalline organic salt is a special case of hydrogen-bonded cocrystal or donor-acceptor crystal, i.e., proton (or electron) transfer from the acid (or donor) to base (or acceptor) occurred. 

Poulsen L et al (2007) Three-dimensional energy transport in highly luminescent host-guest crystals a quantitative experimental and theoretical study. J Am Chem Soc 129 8585-8593... 

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. 

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