Crystal photolysis, solid state

Many reactions that proceed with elimination of a gas (N2, COz, etc.) and formation of radical-pair intermediates show considerable stereochemical differences in solution and in the crystal. Thus, solid-state photolysis of the diaza compound 176 yields, among other products, cis-1,2-diphenylcyclopentane, 177 no trans isomer, 178, has been detected. Irradiation of 176 in solution leads to mixtures in which the trans product 178 predominates (244). 

Irradiation of the M-crystals caused solid-state photodecarboxylation, and then enantioselective condensation occurred, to give the optically active condensation product (S)-( —)-151 as the main product with [a] = —30 in 35% ee and in 37% chemical yield (Scheme 35). Conversely, irradiation of the P-crystals resulted in formation of the opposite handed condensation product (/ )-(+ )-151 with [a]r = +30 in 33% ee and in 38% chemical yield. For a comparison, solution phase photolysis of acridine 150 and diphenylacetic acid DPA in acetonitrile did not produce chiral product 151 but rather gave the achiral condensation product 153 in 74% as the major product at complete conversion of DPA. 

Chiral induction to obtain the /J-lactam 169 by photolysis of the chiral crystal 168 in the solid state is possible (equation 112). 

Fig. 2 X-ray crystal structure of salt 39 prior to (left) and following (right) photolysis in the solid state...

Photolysis of cycloheptenecarbothioamide 24d in the solid state also gave the corresponding p-thiolactam in specific yield (entry 7). As the X-ray crystallographic analysis revealed, the crystal is racemic (Fig. 9), and the isolated p-thiolactam was obtained as racemate. 

Photolysis of cobalt(III) ammines in the solid state and in various organic media have been reported. Irradiation of crystals of potassium cobaltioxalate gives Co(II) and C02. Study of this reaction by ESR techniques has been reported.69 Photoreduction of Co(NH3)5(H20)l3 gives CoI42- as the cobalt-containing product similar results were obtained with the corresponding bromide and chloride. Solid-state photoreduction of Co(en)3Cl3 has also been reported.70... 

Since intermolecular isotope effects were unprecedented in solid-state chemical reactions, a number of experiments were required to show that the results are reproducible and that differences could not be attributed to the history or condition of the crystals, the temperature or extent of photolysis, or differences in the orientation, coupling, absorptivity, or state of com-plexation of C02 dimers between labeled and unlabeled crystals. Success in excluding these alternative interpretations [5] underlines the power of the infrared method, and the special virtues of studying single crystals. 

Dimerizations of alkenes bearing heteroaromatic substituents such as pyridine have been widely reported. 2-(/ -Styryl)quinoline (239), for example, yields on irradiation in the solid state a dimer which can be represented either as the head-to-head dimer (240) or the head-to-tail dimer (241).225 The stereochemistry of ring fusion in this and many other related dimers is not known. The structures of the dimers obtained by solid state photolysis of /3-2-furyl-, jg-2-thienyl-, and /9-3-pyridylacrylic acid have been interpreted in terms of crystal packing.226... 

Sakamoto et al. provided an example of absolute asymmetric synthesis involving hydrogen abstraction by thiocarbonyl sulfur (Scheme 6). [24] Achiral A -diphenylacetyl-iV-isopropylthiobenzamide 33 and Y-diphenylacetyl-A-isopropyl(p-chloro)thio-benzamide 33 crystallize in chiral space group P2 2 2. Photolysis of the chiral crystals in the solid state gave optically active azetidin-2-ones whereas achiral thioketones were obtained as main products. When 33a was irradiated in the solid state at -45°C followed by acetylation (at -78°C), 2-acetylthio-3,3-dimethyl-l-diphenylacetyl-2-phenylaziridine (34a 39% yield, 84% ee), 4-acetylthio-5,5-dimethyl-2-diphenylmetyl-4-phenyloxazoline (35a 10% yield, 50% ee), 3,3-diphenyl-1-isopropy 1-4-... 

Sakamoto et al. reported an intramolecular [2+2] thietane formation in the solid state (Scheme 7). [26] Achiral A-(thiobenzoyl)methacrylamide 39 formed (E,Z)-conformation of the imide moiety, crystallized in a chiral space group Phhh, and the photolysis of single homochiral crystals at room temperature resulted in the formation of an optically active thietane-fused 3-lactam (40,75%) with 10% ee. The solid-state photoreaction proceeded even at -45°C to give higher ee value, 40% ee (conv. 30%, yield 70%). 

Similar absolute asymmetric synthesis was demonstrated in the solid-state photoreaction of A-(P,y-unsaturated carbonyl)thiocarbamate 41. [27] Achiral 0-methyl AT-(2.2-dmeth ibut-3-enoyl)-iV-phenylthiocarbarnate 41 crystallized in chiral space group P2i, and irradiation of these crystals gave optically active thiolactone in 10-31% ee. A plausible mechanism for the formation of 42 is rationalized on the basis that photolysis of 41 undergoes [2 + 2] cyclization to thietane and is subsequently followed by rearrangement to thiolactone 42. 

Sakamoto et al. also demonstrated an absolute oxetane synthesis in the solid-state photolysis of Y-( ,(3-unsaturatcd carbonyl)benzoylformamides 43. [28] The X-ray analysis of Y-isopropyl substituted imide 43a revealed that the crystal system was monoclinic and the space group P2. Crystals of 43a were powdered and photolyzed at 0°C. The imide undergoes the [2+2] cycloaddition to afford the bicyclic oxetane 44a, which is a mixture of diastereomers, namely, syn- and anh-isomers at the C-7 position. In this reaction optically active. syn-oxctanc 44a with 37% ee (84% chemical yield) and racemic anti-44a were obtained. The solid-state photoreaction proceeded even at -78°C, and optically active syn-44n which showed ee value as high as >95% ee, (conv 100%, chemical yield 89%) was formed in a higher diastereomeric ratio (syn/anti = 6.5). Under identical conditions A-bcnzyl substituted 43b was irradiated in the solid state. 

Sakamoto et al. found that. S -phenyl Y-benzoylfomiyl-A-tp-tolyljthiocarbamatc 54 crystallized in chiral space group P2i.[33] Photolysis of the chiral crystals in the solid-state gave optically active l-benzyl-4-phenyl-4-phenylthiooxazolidine-2,4-dione (55, 16% chemical yield, 21% ee) and cw-3,4-diphenyl-3-hydroxy-l-(thiophenylcarbonyl)-azetidin-2-one (56, 18% chemical yield, 23% ee) in 62 % conversion yield. Better optical purities were observed at low conversion (in 17% cov.), 46% ee for 55, and 32% ee for 56, respectively. 

The solid-state photolysis of powdered 59b gave a quantitative amount of 3-(IV-methylanilino)-3-phenylphthalide 60b, whereas 59a was inert toward topochemical reaction upon irradiation. As expected, the asymmetric induction in 60b was observed. As a result of the suppression of the reaction conversion yield and by decreasing the reaction temperature to -50°C, the enantiomeric purity rose up to 87% ee (at 25% conversion with quantitative yield). Furthermore, irradiation of single crystals of 59b gave 97% ee of 60b when the reaction conversion reached 46%. 

Ohashi also reported the asymmetric induction by the irradiation of chiral crystals of achiral (2-cyanoethyl)(pyrrolidine)cobaloxime complex 62. [37] In this case, the achiral cobaloxime complex crystallized in P2 2 2 space group, and the solid-state photolysis... 

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