Solid-state photodimerisation

Stoddart has employed the anion-directed [3]pseudorotaxane assemblies described above to control the outcome of solid-state photodimerisation reactions of olefins [116]. A combination of supramolecular interactions (one of them being hydrogen-bonding to PF,) has been employed to pre-organize bis(dialky-lammonium) salts containing fraws-stilbenoid units (89) into the [3]pseudoro-taxane assembly 90 shown in Scheme 45. 

Figure 8.27 (a) Topotactic, single crystal solid-state photodimerisation of 5-benzyl-2-benzylidene-... 

Figure 8.27 (a) Topotactic, single crystal solid-state photodimerisation of 5-benzyl-2-benzylidene-cyclopentanone. (b) Superposition of the structures of the starting material and dimersied product (reprinted with permission from [33] 1981 American Chemical Society). 

Solid state photodimerisation has been reported for a series of unsymmetrical... 

The solid state [2+2] photochemical reaction of olefins is an attractive transformation for the generation of C-C bonds. However, this type of reaction can only take place when the olefins to be dimerized crystallise in the appropriate relative orientation. For several decades chemists have strived to design molecules that will predictably crystallize in such orientations. In spite of these efforts, to date there are not general and reliable methods to align olefins in the solid state so that the photodimerisation reactions can take place. In this context, an approach that has started to emerge as a potentially useful alternative to orient olefins in the solid state is by templated processes. Some examples where hydro gen-bonding templates have been used in the photodimerisation of olefins have appeared and are discussed herein. 

A crystalline sample of this supramolecular assembly was irradiated with UV light and the formation of the corresponding cyclobutane 91 with syn-anti-syn stereochemistry was observed. In contrast, the photodimerisation of trans-stilbenoid-bis(dialkylammonium) salts does not take place in the absence of the macro cycle, indicating the importance of pre-organizing the stilbenoid units (which requires the presence of the anion) for this solid-state reaction to occur. 

Schmidt, G. M. J. (1971). Photodimerisation in the solid state. Pure Appl. 

The most recent phenomenon observed to occur within the confines of 24 is the stereoselective photodimerisation of olefins [68]. Without the presence of the cage, the dimerisation of 28 results in a mixture of the syn- and anti- isomers (29 and 30 respectively, Scheme 8). With the cage present, two molecules of 28 are encapsulated before the photodimerisation occurs. Within the cavity the syn isomer is the only one capable of forming for steric reasons. This type of selectivity has been observed for several related compounds. Recently, photodimerisation has also been observed to be controlled by supramolecular templation in the solid state [69]. 

Benzo[a]quinolizinium (ions), methyl-, reactivity, 55, 349 BenzoIWquinolizinium (ions/salts) calculated electron densities, 55, 275 calculated electronic spectrum, 55, 324 charge transfer complexation, 55, 352 halogenation, 59, 331 photodimerisation in solid state, 55, 320, 352... 

High resolution NMR has been used to investigate the photo-dimerisation of 9-methylanthracene. In the solid state, only the trans dimer is formed whereas in benzene solution both the trans and cis dimers are produced. Studies show that the maximum domain size of the minor component is about 0.3 pm, and that the reaction occurs at crystal defects in the monomer. Photodimerisation of the 9-substituted anthracenes An-CHzN- MejBr- (166), AnCH2C02 Na-" (167), AnCH20H (168), AnCOMe... 

Irradiation of some 1,4-dihydropyridine derivatives leads in some cases to the formation of cw-dimers and in others to oxidation products, and an enhanced regioselectivity has been observed for the [4 + 4] photodimerisation of 9-aminoacridizinium perchlorate (171 R = NH2, X = C104 ) as compared with (171 R = H, X = Br ). The head-to-tail products syn (172) and anti (173) are produced exclusively. Styryldicyanopyrazines undergo a selective topo-chemical photodimerisation in the solid state by a process whose reactivity and stereochemistry are controlled by differences in their molecular stacking. 

Solid state NMR spectroscopy has been used to analyse the dimerisation of the 1,4-dihydropyridine derivative (107) that yields the two dimers (108) and (109) on irradiation. This report complements an earlier account of the same dimerisation. The reaction is claimed as a useful synthetic entry into derivatives of 3,9-diazatetraasteranes. The photodimerisation of the thymine derivative (110) does not take place to any great extent on direct irradiation in solution but when acetone-sensitisation is used all four possible (2+2)-cycloadducts are obtained. Only one of these dimers is obtained from the irradiation in the crystalline phase and this was identified as the cis,anti,cis dimer (111). Interestingly, when the thymine (110) is crystallised from acetonitrile both needles and plates are obtained but only the needle form of the crystalline compound gives the dimer (111). ... 

Irradiation of 3-amino-2-oxo-4-hydroxyquinoline leads to its photochemical condensation to a dimer,and photodimerisation of acridizinium bromide in both solution and in the solid state forms four photodimers, two of which may be enantiomeric. In anionic micelles, a different regioselectivity is observed as reflected in a higher yield of those dimers showing higher dipole moments. 

Contrary to previous reports, acridizinium bromide yields all four isomeric [4+4]photodimerisation products both in solution and in the solid state. 2-Cyano-6,6-dimethyl-2-cyclohexenone undergoes photoaddition to 2,3-dimethyl-2-butene to give tricyclic[c,d]fused isoxazole (146) in 92% yield, possibly involving the triplet biradical (144) and the nitrene (145). With 2-methyI-2-butene and 2-methylpropene, isoxazoles (147) and (148) respectively are formed, though products from competitive formation of triplet biradical (149) are also found.The crystalline 1 2 adducts of 1,2,4,5-tetracyanobenzene with benzyl cyanide yield the coupling product (150) on irradiation this involves electron and proton transfer, radical coupling and tautomerisation steps. In solution (150) is not formed. 

K-bond in a small ring. Irradiation of dimethyl 9-anthrylmethyl phosphite in benzene yields the corresponding dimethyl 9-anthrylmethylphosphonate which, on further irradiation either in solution or in the solid state, undergoes HT 9,10-anthracene-type photodimerisation. ... 

A first example that illustrated this concept of directing a photochemical [2 + 2] dimerisation in the solid state was reported by Ito and Sheffer/ in which diamines were used to form salts from a,p unsaturated carboxylic acids. The transference of the acidic proton in carbo g lic acids by strong base leads to the molecular salts where photodimerisation occurs between two anionic moieties. For example, two cinnamate anions are positioned in parallel form suitable for a [2 -I- 2] photocycloaddition using ethylenediamine via charge-assisted hydrogen bonds 0 ). 

Concomitant [2 - - 2] photodimerisation reactions in the solid state from supramolecular H-bonded arrays self-assembled via mechanochemistiy... 

The photodimerisation of cinnamic acid can be controlled by irradiation of its double salts with certain diamines in the solid state. Thus, the double salt crystal of cinnamic acid and o-diaminocyclohexane gave upon irradiation in the solid state, P-truxinic acid as the major product (Scheme 129)P ... 

Alcock NW, de Meester P, Kemp TJ (1979) Solid-state photochemistry. Part 1. Nature of the stereocontrol in the photodimerisation of dibenzylideneacetone by U022-I- ion Crystal and molecular structure of trans-dichlorobis(trans, trans-dibenzylideneacetone)dioxouranium(VI) and of its acetic acid solvate. J Chem Soc Perkin Trans 2(7) 921-926... 

Priscic T, MacGillivray LR (2005) Reversing the code of a template-directed solid-state synthesis a bipyridine template that directs a single-crystal-to-single-crystal [2 -l- 2] photodimerisation of a dicarboxylic acid. Chem Commun 5748... 

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