Irradiation of crystals

The diffusion of H and D atoms in the molecular crystals of hydrogen isotopes was explored with the EPR method. The atoms were generated by y-irradiation of crystals or by photolysis of a dopant. In the H2 crystals the initial concentration of the hydrogen atoms 4x 10 mol/cm is halved during 10 s at 4.2 K as well as at 1.9 K [Miyazaki et al. 1984 Itskovskii et al. 1986]. The bimolecular recombination (with rate constant /ch = 82cm mol s ) is limited by diffusion, where, because of the low concentration of H atoms, each encounter of the recombinating partners is preceded by 10 -10 hops between adjacent sites. 

In late 1995, a team led by Vollhardt and Youngs reported their work on the strained PAM/PDM hybrid 80 [55]. Whereas the synthesis of 80 was not remarkable [Eq. (2)1, the solid-state behavior of the molecule was. X-ray crystallography revealed that the macrocycle was moderately strained, with the monoynes bent inward toward the center of the macrocycle by 3.9 -11.5° and the diyne unit bent outward by 8.6-11.2°. More importantly, crystal packing revealed that the diyne moieties were aligned in the prerequisite fashion for topochemical diacetylene polymerization to occur. Indeed, irradiation of crystals of 80 produced a violet... 

Irradiation of (+ )-crystals of 96 with a 400 W high-pressure Hg-lamp, with occasional grinding with an agate mortar and pestle for 40 h at room temperature gave ( + )-97 of 93 % ee in 74 % yield. Irradiation of (—)-crystals of 96 under the same conditions gave (—)-97 of 93 % ec in 75% yield48. Purification to 100% ee can easily be achieved by recrystallization from benzene. Although the photochemical conversion of 96 into 97 on irradiation in the solid state has been reported, enantioselectivity of the reaction has not been discussed 441. 

A second example of the use of ionic chiral auxiliaries for asymmetric synthesis is found in the work of Chong et al. on the cis.trans photoisomerization of certain cyclopropane derivatives [33]. Based on the report by Zimmerman and Flechtner [34] that achiral tmns,trans-2,3-diphenyl-l-benzoylcyclopropane (35a, Scheme 7) undergoes very efficient (0=0.94) photoisomerization in solution to afford the racemic cis,trans isomer 36a, the correspondingp-carboxylic acid 35b was synthesized and treated with a variety of optically pure amines to give salts of general structure 35c (CA=chiral auxiliary). Irradiation of crystals of these salts followed by diazomethane workup yielded methyl ester 36d, which was analyzed by chiral HPLC for enantiomeric excess. The results are summarized in Table 3. 

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. 

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... 

For instance, head-head photodimers are predicted from the crystal structures of 9-cyanoanthracene and 9-anthraldehyde, but the head-tail isomer is produced. Craig and Sarti-Fantoni and later others found that photoreactions of 9-cyanoanthracene and 9-anthraldehyde take place at defect sites [96,215], Systematic photochemical and crystallographic studies by Schmidt and co-workers uncovered many cases of substituted anthracenes which behave in an unexpected fashion (Scheme 40) [216,217]. Examples shown in Scheme 40 clearly illustrate that, unlike cinnamic acid derivatives, the stereochemistry of the product dimer from anthracenes cannot be predicted on the basis of crystal packing. An example from the laboratories of Venkatesan is noteworthy in this context [218], Irradiation of crystals of 7-... 

Finely powdered monomer crystals were dispersed in distilled water containing a few drops of surfactant and irradiated, with vigorous stirring, by a 500-W super-high-pressure mercury lamp set outside of the flask. Dimers 2 were prepared by the irradiation of crystals 1 with /.>410 nrn, and by successive purification by preparative TLC. [2.2]Paracyclophane 3a was obtained by the photoirradiation of la crystals with X >300 nm at -40 °C, followed by successive washing with ethyl acetate. 

CN or A, led to no measurable resolution. When heated in an optically active solvent, (-)-2-methyl-l-butanol, recovered BN remained racemic. On the other extreme, irradiation of crystals of optically active BN at room temperature produced no measureable racemization. When recrystallized very slowly at room temperature from (-)-2-methyl-l-butanol, BN displayed no measureable optical activity. 

Recently, a novel oxetene 12 has been isolated from the irradiation of crystals of a vinylogous cinnamophane 13 (Equation 3) <2005EJ0558>. The structure was elucidated by NMR spectroscopy and mass spectrometry and interestingly, for a thermodynamically unstable compound, was isolated at room temperature after column chromatography. The cycloreversion of 12 to 13 was also observed at ambient temperature. 

Several solid-state photochemical reactions have been investigated with polycrystalline samples suspended in solvents. Solvents such as water, where the reactant and the product are likely to be insoluble, are usually chosen and a surfactant is added to maintain the suspension. There are at least two apparent advantages to this method. First of all, photochemical equipment commonly used for fluid samples can be readily adopted to solid-state reactions. Secondly, it is expected that all microcrystals in a powdered sample will be homogeneously exposed to the incident light in a well-stirred reactor. Interestingly, while several examples of solid-to-solid reactions in suspended crystals have been documented, there are some cases where the solvent is incorporated into the phase of the final product. In a report by Nakanishi et al. [134] it was shown that p-formyl cinnamic acid (51, Scheme 33) forms mirror-symmetric dimers. While irradiation of crystals suspended in hexane gave amorphous cyclobutanes in 85% yield, suspension of the crystals in water gave a 100% yield of a crystalline photodimer with one water molecule of crystallization. 

In agreement with this suggestion, irradiation of crystals of racemic R,S) (2) yielded chiral dimers with the same stereochemistry as from (+)-(S) (2), but as a mixture of all possible diastereoisomers represented in Scheme 4. 

Photocyclisation of 2-arylthiocyclohexenones (414) yields racemic dihydrobenzo-thiophenes (416) via a thiocarbonyl ylide (415). Theprochiral compounds (414) also form 1 1 inclusion complexes with the homochiral host (63) and irradiation of crystals of these complexes yields the dihydrobenzothiophenes (416) in 32-83% enantiomeric excess, optically pure samples being easily accessible by further recrystallisation. The decay kinetics for ylides (415), produced by laser flash photolysis of the aryl vinyl sulfldes (414), indicate the presence of more than one ylide species in each case. ... 

Asymmetric induction in the di-jr-methane rearrangement is also of interest and studies on this have examined the influence of chiral esters. Thus the irradiation of 370 yields a cyclooctatetraene 371 and a diastereoisomeric mixture of the semibullvalenes 372 and 373 in a ratio of 60 40 in solution and 20 80 in the solid phase. The position of attachment is vital and the diastereoselectivity shown by the barrelene 374 is very poor and yields a 1 1 mixture of products . Further examples of the control exercised when dibenzobarrelenes are irradiated in the crystalline phase have used the derivatives 375 and 376 as the acid salts formed with chiral amines. Irradiation affords the products 377 and 378 respectively, obtained after esterification with MeOH, with an ee of >95% . The influence of a chiral crystal lattice on the outcome of the di-jr-methane reaction of achiral 379 has been studied. The irradiation in the crystalline phase gives two chiral di-TT-methane products 380 and 381. The former of these is racemic but the latter is obtained in high enantiomeric excess which, under the best conditions, i.e. at —20 °C, approaches 100% . The irradiation of crystals of 345, as an ethanol complex, affords 382 with an ee of 94%. Carrying out the irradiation at temperatures lower than ambient enhances the specificity of the reaction . 

Irradiation of crystals of a-santonin (6 ) yields a 2,4-cyclopentadienone whereas in solution lumisantonin (8) is a photoproduct (eq 4) (27). The observed selective rearrangement to... 

Direct irradiation of crystals of a P, y-unsaturated keto acid (Scheme 39) led primarily to the 1,3-acyl shift product. Irradiation of its Li, Na, K, Rb, and Cs salts in the crystalline state, however, produced increasing amounts of... 

Coppens and coworkers observed that laser irradiation of crystal samples of [Ru(bpy)2(N0)(N02)](PF6)2, at 90 K produces a double isonitrosyl-nitrito linkage isomer (Figure 15) and structurally identified the photogenerated hnkage isomers by a combination of X-ray crystallography and infrared spectroscopy (112). 

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