Cycloaddition, 1,3-anionic photochemical

Phosphoryl-stabilized anions, 25, 2 Photochemical cycloadditions, 44, 2 Photocyclization of stilbenes, 30, 1 Photooxygenation of olefins, 20, 2 Photosensitizers, 20, 2 Pictet-Spengler reaction, 6, 3 Pig liver esterase, 37, 1 Polonovski reaction, 39, 2... 

All the other cycloadditions, such as the [4+2] cycloadditions of allyl cations and anions, and the [8+2] and [6+4] cycloadditions of longer conjugated systems, have also been found to be suprafacial on both components, wherever it has been possible to test them. Thus the trans phenyl groups on the cyclopentene 2.65 show that the two new bonds were formed suprafacially on the rrans-stilbene. The tricyclic adducts 2.61, 2.77, 2.79, and 2.83, and the tetracyclic adduct 2.82, show that both components in each case have reacted suprafacially, although only suprafacial reactions are possible in cases like these, since the products from antarafacial attack on either component would have been prohibitively strained. Nevertheless, the fact that they have undergone cycloaddition is important, for it is the failure of thermal [2+2], [4+4] and [6+6], and photochemical [4+2], [8+2] and [6+4] pericyclic cycloadditions to take place, even when all-suprafacial options are open to them, that is significant. 

In all examples discussed up to now the radical cation of Qo is involved in the reaction mechanism. However, due to the electronic features reduction of the fullerenes leading to radical anions should be much easier performed. For example, a useful method to synthesize 1-substituted l,2-dihydro-[60]fullerenes is the irradiation of Q0 with ketene silyl acetals (KAs) first reported by Nakamura et al. [216], Interestingly, when unstrained KAs are used, this reaction did not yield the expected [2 + 2]-cycloaddition product either by the thermal, as observed by the use of highly strained ketene silyl acetals [217], or by the photochemical pathway. In a typical reaction Q0 was irradiated for 10 h at 5°C with a high pressure mercury lamp (Pyrex filter) in a degassed toluene solution with an excess amount of the KA in the presence of water (Scheme 11). Some examples of the addition of KAs are summarized in Table 11. 

The cationic site is of the immonium type, and is relatively iinreactive, but the radical site is reactive toward electron deficient alkenes such as acrylonitrile, yielding a 50 50 mixture of the diastereoisomeric pyrrolines, after being neutralized by back electron transfer from the sensitizer anion radical. The net result is an interesting example of a net 1,3-dipolar cycloaddition which, in the Huisgen method of classification of cycloadditions, is of the [3 -l- 2] type. The same general reaction had previously been carried out by Padwa, using direct photochemical excitation, a procedure which, in contrast, was highly diastereoselective (90 10, in favor of the trans isomer) [92]. 

The starting material can be made by a photochemical [2 + 2] cycloaddition of acetylene and rr. anhydride. Treatment with butanol and base leads to the monoester because, after the butane attacked once, the product is the anion of the carboxyhc acid and cannot be attacked again b. nucleophile. DMAP is a base whose structure appears in the margin. 

Acetone-sensitized irradiation of uracil (107) with ethene affords the adduct (108) in 75% yield. This compound can be transformed into the cyclobutane derivative (109) in an overall yield of 52%. The intramolecular cyclization of the dinucleotide model (110) has been investigated. The reaction affords the cycloadduct (111) by irradiation using wavelengths > 300 nm. The use of an anionic template for photochemical dimerization of a thymine system has been demonstrated. The thymine forms an assembly (112) with pyrophosphate. Irradiation of this brings about syn- 2 + 2)-cycloaddition of the thymine units. ... 

The photochemical substitution reactions of 2-naphthoxide anion and o-halophenol (X = Br, I) in an aqueous sodium sulfite solution were also reported Photosubstitution of the sulfo group for hydrogen is observed under irradiation of sulfonated derivatives of hydroquinone. Benzo[a]phenanthridine-5-ones (156) (159) were synthesized by photo-induced substitution and cycloaddition of 3-chloroisoquinolin-l-ones... 

Photochemical cycloaddition for several unsaturated carboxylates has been studied in the presence of hydrotalcite (LDH) (223-225). In addition to the antiparallel packing of the guest, the intermolecular distances of two double bonds of adjacent carboxylates were found to affect the stereoselectivity of the photochemical reactions. While cinnamate yielded head-to-head dimers exclusively, stilbenecaroboxylates (Fig. 33) led to a significant amount of head-to-tail dimer in addition to a head-to-head dimer. This difference was explained by molecular packing of the dye anions in the interlayer space of LDH. The proposed molecular packing of stilbenecarboxylate and p-phenylcinnamate ions in the interlays space of Mg/Al LDH is shown schematically in Figure 34. 

Unsaturated organic anions can undCTgo photochemical cycloaddition within the LDH interlayer, provided that the distance between them is suitable. Tagaki et al. examined the cycloaddition of two kinds of unsaturated carboxylates intercalated in the MgAl-LDH (701-703). They found that (a) the conversion is higher than that of reaction without LDH (b) the addition is very stereoselective for head-to-head isomers. In conttast, photoaddition without LDH is nonselective, giving both head-to-head and head-to-tail isomers in approximate amounts. 

Wang X-Y, Wang Z-M, Gao S (2007) A pillared layer MOF with anion-tunable magnetic properties and photochemical [2+2] cycloaddition. Chem Commun 11 1127-1129... 

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