Solid-state photoreaction

Solid-state photoreactions of heterocycles in two-component crystals 98S1. 

Solid-state photoreactions in two-component crystals with formation of heterocycles 98S1. 

Control and Acceleration of Photoreactions via Freeze of an Equilibrium by Complexation and Irradiation in the Solid State... 

When the complex formation by solid-solid reaction is combined with a photoreaction in the solid state, a stereoselective photoreaction can be carried out continu-... 

Not only the photoreaction but also a pinacol rearrangement (Scheme 3) and a m-chloroperbenzoic acid oxidation (Table 7) occurred in the solid state, selectively and acceleratively. 

An enantioselective photoreaction of a guest compound is expected when an inclusion complex of the guest with an optically active host compound is irradiated in the solid state. 

When 8 a is used instead of 2a for complexation with 50a, two kinds of 1 1 inclusion complexes 57 were obtained one (57a) is photoreactive and the other (57b) is photostable. Thus when a solution of 8a and an equimolar amount of 50a in benzene was kept at room temperature for 10 h, a 1 1 inclusion complex between both (57a) was obtained as colorless fine needles of mp 110 to 123 °C which upon irradiation in the solid state for 49 h gave (1 R,5S)-(+)-52a of 3.6% ee in 30% yield 13). However, recrystallization of 57a from benzene gave a different kind of 1 1 inclusion complex (57b) as colorless needles of mp 123-125 °C which is photostable. Photocyclization of 50a in 57b is probably prevented by a steric hindrance. The different behavior of 57a and 57b may be clarified in future by X-ray structural studies. 

Inclusion complexation of racemic 63 with the meso isomer of 2 was also examined. The host compound included racemic 63 as did 2a, and photoreaction of the complex in the solid state gave racemic 65 again 23). 

The exploitation of the reactivity of molecular crystals lies close to the origins of crystal engineering and is at the heart of the pioneering work of Schmidt [47a]. The idea is that of organizing molecules in the solid state using the principles of molecular recognition and self-assembly. Successful results have been obtained with bimolecular reactions, particularly [2+2] photoreactivity and cyclisation [47b,c]. Another important area is that of host-guest chemistry. 

Sakamoto et al. reported an interesting example involving the solid-state photoreaction of AT-(a,p-unsaturated carbonyl)thiobenzamides 1 leading to thietane-fused p-lactam 2 (Scheme 2) [29-31]. 

Sakamoto et al. also reported the solution photochemistry for la-d, which gave the corresponding thietane-fused -lactams 2a-d as shown in Table 2 [30,31]. For tigloyl derivatives Ic and Id, P-lactams were obtained as a mixture of two stereoisomers (entries 6 and 8). The anti isomers were preferably formed, in ratios of 0.8 and 0.7, respectively. In the solid-state photoreaction of... 

In contrast to the solution photochemistry, irradiation of the crystals of le provided remarkably high diastereoselectivities in the formation of 2e. The temperature at which the reaction occurs is important in determining dia-stereoselectivity. At 15 °C, the de of syn-2e was 61 % and the ratio of syn/anti was 8.7, which was the reverse of that in the solution reaction (syn/anti=0.9). The solid-state photoreaction proceeded even at -78 °C, and with higher diastereo-selectivity. At low conversion (9%), only syn isomers (de=93%) were obtained, and 71% de was observed even at 90% conversion. 

The stereoselective generation of the chiral center is exemplified by the formation of 5b at the C4 position, and optically active 4b was obtained in 10% ee. The solid-state photoreaction also proceeded at -78 °C and an optically active compound which showed a better ee value was formed, 20% ee at 84% conversion (entry 6) and 31% ee at 15% conversion (entry 7). The space group of the crystal of 3a could not be determined because 3a did not afford single crystals suitable for X-ray crystallography however, the production of racemic 4a shows that the crystals are achiral (entries 2 and 3). 

Irradiation of the powdered crystals of 6a-d with a high-pressure mercury lamp under argon at 0 °C showed a different photochemical behavior from that in benzene solution. Contrary to the photochemical results in solution media in which azetidin-2-ones 8 (y-hydrogen abstraction products) were formed as major products, thioketones 7 (P-hydrogen abstraction products) were obtained as major components in the solid-state photoreactions (Scheme 7). The... 

Table 6 Solid-state photoreaction of 6 followed by acetylation ...

Sakamoto et al. reported the X-ray crystallographic data and the solid-state photoreaction of eight Ar,AT-disubstituted a,p-unsaturated thioamides 23a-h, which involves hydrogen abstraction by the alkenyl carbon atom conjugated with thiocarbonyls (Fig. 9) [44-46]. 

In the solid-state photoreaction of 24c, a more chemoselective reaction occurred and only p-thiolactam 25c was obtained almost quantitatively. Of particular importance is the finding that the solid-state photoreaction of 24c involves a crystal-to-crystal nature where the optically active p-thiolactam 25c is formed in specific yield. Furthermore, the X-ray crystallographic analysis revealed that the crystals of 24c are chiral, and the space group is P2j. Irradiation of crystals at 0 °C exclusively gave optically active P-thiolactam 25c, in 81% yield at 100% conversion (entry 5). As expected, the thiolactam 25c showed optical activity (81% ee). This reaction exhibited good enantioselectivity throughout the whole reaction, where a small difference was observed in the ee value from 97 to 81% ee with increasing conversion from 20 to 100% (entries 5 and 6). The solid-state photoreaction also proceeded without phase separation even after 100% reaction conversion. The crystal-to-crystal nature of the transformation was confirmed by X-ray diffraction spectroscopy. 

Table 10 Photoreaction of thioamides 24f-h in both solution and the solid-state...

In the solid-state photochemical reaction of N,N-disubstituted a,(3-unsatu-rated thioamides 24, a crystal-to-crystal nature was observed in 24c furthermore, absolute asymmetric transformation in the chiral crystalline environment was performed in the photoreaction of 24b, 24c, and 24e. 

Solid-state reactions are known from thermal intracrystalline conversions (isomerizations or loss of volatile fragments), photoreactions, gas-solid reactions, and solid-solid reactions. As all of these relate strictly to the crystal packing (unifying solid-state mechanism) and are not separated in the various sections. Also, nontopotactic (normal) and topotactic (very rare) reactions are not separated in different sections. 

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