Irradiation in the Solid State

Experiments in the solid state, especially EPR studies, have supplemented our knowledge in the area of the free-radical chemistry of nucleobases and related compounds considerably. Here, only some salient points can be mentioned, and for more detailed information the reader is referred to the excellent reviews that have appeared on this topic (Wyard and Elliott 1973 Bernhard 1981 Hiitter-mann 1982,1991 Close 1993 Sevilla and Becker 1994 Becker and Sevilla 1997) and to the book by Box (1977). 

Whenever H--adducts are observed, they must not necessarily have H- as precursor. Protonation of the radical anion must always be considered as an alter-native/additional route. In crystals, the radical cations may serve as the proton source, while in frozen aqueous solutions the solvent will provide the proton. 

Frozen solutions. In frozen aqueous solutions, the additive may not precipitate but accumulate in ice-free areas in a rather uncontrolled way. Upon irradiation, there are only a few radicals from the radiolysis of ice that reach the solute, and radical formation can occur by direct absorption of the energy of ionizing radia- 

Single-crystals and powders. In Ura, the N(l)-centered radical is observed (Zeh-ner et al. 1976). Based on our present knowledge, one may suggest that it arises most likely from the deprotonation of the radical cation. The radical anion is protonated at 0(4). The C(5) -TP-adduct primarily formed is converted with light of A 400 nm into the thermodynamically more stable C(6)-H -adduct [reaction (303)]. This is also observed with other pyrimidines (Flossmann et al. 1976). 

In IMeUra, a predominating radical is the -CH2 radical (Flossmann et al. 1973, 1975a,b). A -CH2 radical (here, allylic) is also observed with Thy and 5MeCyt (Hiittermann 1970 Hiittermann et al. 1971 Dulcic and Herak 1973). Radical cations are likely precursors, while the precursor of the Thy C(6)-H--adduct that is commonly observed (Henriksen and Snipes 1970) could be the Thy radical anion (Symons 1990 see above). 

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One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiating in the solid state and at very low temperatures. Then, the intermediates can be measured at the end of the irradiation by optical absorption spectroscopy or ESR. 

Similarly, fumaronitrile yields cyclobutane (115) when irradiated in the solid state<120> ... 

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

Although 26b-n (Table 3) and 27a are photoinactive 15), irradiation in the solid state of complexes 30 of N-methylpyridones 29 (Table 4) gives the corresponding... 

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. 

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. 

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. Then, the intermediates can be detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR study of radicals in the solid state is done on single crystals, polycrystalline samples or frozen aqueous solution. In case of polycrystalline samples or frozen aqueous solution the identification of the radicals from the ESR spectra is difficult in many cases and, for better identification, the ESR experiment should be conducted on irradiated single crystals. Later, the method of spin trapping, developed for the liquid phase5, was extended to polycrystalline solids. In this technique the polycrystalline solids are /-irradiated and subsequently dissolved in a solution containing the spin trap. 

POLYAMINO ACIDS Aliphatic polyamino acids irradiated in the solid state have been reported to undergo N-Ctf, main-chain, bond scission on gamma radiolysis (9) and the stable radical intermediates formed following radiolysis at 303 K are alpha carbon radicals, as observed in the N-acetyl amino acids. 

When thioamides 24f-h were irradiated in benzene with a high-pressure mercury lamp, iV-isopropyl-P-thiolactam 26 and 1,3,5-dithiazine 28 were obtained (Scheme 15 and Table 10, entries 1,3, and 5). Considerably different photochemical behavior was observed between that in solution and in the solid state. Powdered thioamide 24f was irradiated in the solid state at 0 °C until 19% conversion, because the solid changed to amorphous at around 20% conversion. In this case, only dithiazine 28f was obtained as the sole photoproduct (entry 2). On the contrary, photolysis of 24g gave a new type of p-lactam 27g as a main product in 88% yield in addition to dithiazine 28g (12%) the P-lactam 26g was not detected at all (entry 4). Photochemical ( ,Z) isomerization of (Z)-24g was also observed in the early stage of the reaction, where the ratio of the photostationary state was ZIE=1.9. In the case of 24h, P-thiolactam 27h was ob-... 

With most solid polymers the G values for radical production, as determined by ESR, lie in the range 2 to 5. With biological macromolecules irradiated in the solid state comparable G values are obtained. 

Table III. Stability of complexed and adsorbed flavors to UV/365nm/ irradiation in the solid state at room temperature...

Similar effects are observed in y-irradiated n-C H (530 kGy) in the molten state. Three new structures are identified as a) one-bond crosslinks (H-structure), b) trans-vinylene groups and c) long branches (T- or Y-structure)144). However, highly crystalline polyethylene y-irradiated in the solid state at low doses (up to 40 kGy) yields predominantly the branched Y-structure. A failure to detect the cross-linked H-structure could arise from a) insufficient abundance of crosslinks to give a detectable signal and b) insufficient mobility of crosslinked chains in the polyethylene gel which results in very broad resonance lines, not observable during normal data acquisition in the solution 13C NMR experiment145). 

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... 

Only the centrosymmetric syn head-to-tail dimer 21 is formed upon irradiation of trans-pentafluorostilbene (15) in benzene solution or the solid state (62). Irradiation of the trans-2,4-dichlorostilbenes 16-18 in the solid state yields the mirror-symmetric syn head-to-head dimers 23, while in benzene solution 16 yields both 23 and the syn head-to-tail dimer 21 in similar yield (59-61). trans-3,4-Dichlorostilbene (lg) fails to react upon irradiation in the solid state, but yields a mixture of the four photodimers 21-24 upon irradiation in benzene solution (59). Decafluorobenzene (20) fails to dimerize either in the solid state or in solution (62). 

The j5-chlorobenzenesulfonate salt of N-octadecyl-trans-4-stil-bazole (27) fails to dimerize in solution or when incorporated into CTAB micelles (66a,b). However, irradiation in the solid state or in monolayer assemblies yields the syn head-to-head dimer 23 accompanied by long-wavelength fluorescence attributed to the excimer. Evidently, aggregation of the hydrophobic hydrocarbon tails is more important than like-charge repulsion in determining the stereochemistry of photodimerization in the case of 27. Whitten and co-workers (66c) recently reported the observation of strong red-shifted fluorescence attributed to... 

The solid-state photoreaction of thioamide 1 was done under an atmosphere purged with dry argon. The solid sample was irradiated for 4 h as a powder prepared by grinding, and placed inside a Pyrex slide. When powdered thioamide 1 was irradiated in the solid state at 0 °C, at up to 19% conversion, optically active /i-thiolactam 2 of 31% ee was isolated. The optical purity was determined by HPLC using a chiral cell OD column (Daicel Chemical Ind.). 

It should be pointed out that the nature of the primary photochemical step(s) is still obscured and can depend, even for the same complex, on experimental conditions. Thus, Fen,(Por)N3 converts under irradiation in the solid state at low temperatures [162,163] into FevN(Por) in some solution systems [133] the formation of azidoradicals N3 has been detected by spin-trapping EPR no information on the heterolytic splitting of the Fe-N3 bond yielding NJ anion has been described in the literature (for azido complexes of some other central atoms the photosubstitution of the coordinated N3 ligand is a dominant chemical deactivation mode [1]). In addition, at particular conditions, the... 

Cinnamic acid derivative 36 crystallizes in the chiral space group P2t and gives the optically pure dimer 37 upon irradiation in the solid state [22], Chiral crystals of 38 gave, upon irradiation, the optically active dimer 39 of 90% ee, whereas the corresponding methyl ester gave a highly crystalline linear polymer through a typical [2 + 2] topochemical photopolymerization [23],... 

The intramolecular [2 + 2] photocycloaddition of cyclohexadienone derivative 51, which is substituted with chiral piperidine ring in the solid state, proceeds enantioselectively and optically active products 52 and 53 were obtained [28], Two crystal modifications of 51, the a-form (mp 102-104°C) and (3-form (mp 127-128°C), gave optically active 52 and 53, respectively, upon irradiation in the solid state. 

Intramolecular [2 + 2] photocyclization reactions of 2- /V-(2-propcnyl)amino]cy-clohex-2-enones (67) are also controlled enantioselectively by carrying out irradiation in inclusion complex with a chiral host compound. When inclusion crystals of 67 with 12 are irradiated in the solid state, optically active 9-azatricyclo[5.2.1.0]-decan-2-ones (68) were obtained in the chemical and optical yields indicated in Table 3 [30],... 

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. 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. 

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