Olefins photochemical reaction with

TeClFs and the rather unstable TeBrFs have been prepared in low yield by the reaction of F2, diluted with N2, with TeCU or TeBr4 at 25°C (242). Another convenient method condenses CIF onto TeF4, TeCU, or Te02 and warms the reaction mixture to room temperature (270). From NMR spectra (57) and the analysis of the microwave spectrum (272) a C4V structure was proposed, and from the vibrational spectrum a detailed normal coordinate analysis was undertaken (57). TeClFs is also formed if TeBrFs is reacted photochemically with CI2, which also converts PF3 to PCI2F3 and PCIF4. Both TeClFs and TeBrFs undergo photochemical reactions with olefins at 25°C (242). 

The reaction between the PtCh ion and alkanes can be induced by irradiation [26]. When a solution of hexachloroplatinic acid and n-hexane in acetic acid is irradiated by light (A > 300 nm) [26a,c,d] or y-quanta [26b], a 7t-coinplex of hex-1-ene with platinum(II) is formed in addition to isomeric chlorohexanes. This complex has been isolated in the form of the pyridine adduct, (hex-l-cne)PtCl2Py. The yield of the it-complex in the y-induced reaction reaches 17% based on Pt. The photochemical reaction is of first order with respect to hexane. It is interesting that the photochemical reaction with hex-2-ene affords the n-complex ofhex-l-ene (Scheme VII.5). This complex can be also prepared by the reaction ofhex-l-ene with PtCL, " under irradiation. It should be noted that the photoinduced reaction of PtCl6 or PtCU with olefins is a convenient method for the synthesis of various n-o efin complexes of platinum(II) [27]. The photostimulated reaction of PtCU with stilbene does not occur, possibly due to steric restrictions. 

These photochemical reactions with olefins can be considered a cationic analogue of the Meerwein arylation that occurs with nucleophilic rather than with electrophilic alkenes. The rapid cleavage of excited aryl halides and esters in polar solvent and the efficient trapping of the formed aryl cation render these arylations normally less-sensitive towards dissolved oxygen, in contrast to many other photochemical reactions. These characteristics, along with the mild reaction conditions and the simple experimental set-up, make the photochemical method a complementary and valuable alternative to metal-mediated or -catalyzed reactions. 

The add-catalyzed cyclodehydration of (Z)- and ( )-6-hydroxy-a,p-unsaturared ketones offers a mild synthesis of substituted furans. In the case of ( )-olefins, photochemical isomerisation was found to accelerate the reaction <96TL6065>. Reaction of alkynyl(phenyl)iodonium tetrafluoroborates with tropolone in the presence of a base yields 2-substituted furotropones (Scheme 16, <96TL5539>). 

PET reactions [2] can be considered as versatile methods for generating radical cations from electron-rich olefins and aromatic compounds [3], which then can undergo an intramolecular cationic cyclization. Niwa and coworkers [4] reported on a photochemical reaction of l,l-diphenyl-l, -alkadienes in the presence of phenanthrene (Phen) and 1,4-dicyanobenzene (DCNB) as sensitizer and electron acceptor to construct 5/6/6- and 6/6/6-fused ring systems with high stereoselectivity. 

A 1,2-diazetidine has been proposed as an intermediate in the reaction of pyridazine-3,6-dione (12) with styrene.87 The observed product was thought to arise from addition of water to the 1,2-diazetidine, although the alternative more likely explanation involving a dipolar intermediate (cf. Scheme 5) was apparently not considered. In the photochemical reaction of styrene with DEAZD, a 1,2-diazetidine structure was tentatively assigned to a minor product.88 Attempted photochemical [2 + 2] cycloaddition of DEAZD to other olefins failed to give any 1,2-diazetidines.88... 

The [2+2]-photocycloaddition of carbonyl groups with olefins (Paterno-Buchi reaction) is one of the oldest known photochemical reactions and has become increasingly important for the synthesis of complex molecules. Existing reviews have summarized the mechanistic considerations and defined the scope and limitations of this photocycloaddition73. Although this reaction likely proceeds via initial excitation of the carbonyl compound and not the excited state of the diene, the many examples of this reaction in natural product synthesis justify inclusion in this chapter. 

Some work [5] has been performed on the photochemical reaction between sulfur dioxide and hydrocarbons, both paraffins and olefins. In all cases, mists were found, and these mists settled out in the reaction vessels as oils with the characteristics of sulfuric acids. Because of the small amounts of materials formed, great problems arise in elucidating particular steps. When NO and 02 are added to this system, the situation is most complex. Bulfalini [3] sums up the status in this way The aerosol formed from mixtures of the lower hydrocarbons with NO and S02 is predominantly sulfuric acid, whereas the higher olefin hydrocarbons appear to produce carbonaceous aerosols also, possibly organic acids, sulfonic or sulfuric acids, nitrate-esters, etc. ... 

A new methodology for the construction of novel and uniquely shaped 3-azabicyclo[4.2.0]octan-4-one derivatives 166 by combining the Ugi multi-component reaction with [2 + 2] enone-olefin photochemical transformations was recently reported [132] (Fig. 32). The additional functional groups are in this case an enone (165) and a C=C double bond. Although the overall sequence is capable of creating up to five stereocentres, in most cases only two diastereomers are observed, which are epimeric at the exocyclic stereogenic centre. 

There are several other photochemical reactions which ketones can undergo in the absence of olefins. Here again, the products obtained depend upon the relative rates of the competing reactions. Some of these reactions are shown in Table XI. Since it is beyond the scope of this review to discuss these reactions in detail, only a few interesting examples germane to the competition with photocycloaddition will be mentioned. 

Since the migration reaction is always toward the end of a chain, terminal olefins can be produced from internal ones, so the migration is often opposite to that with the other methods. Alternatively, the rearranged borane can be converted directly to the olefin by heating with an alkene of molecular weight higher than that of the product (7-15). Photochemical isomerization can also lead to the thermodynamically less stable isomer.72... 

More recently, photochemical reactions of 138 with cyclic and acyclic olefins have been described. When 138 is irradiated (Pyrex) with cyclo-heptatriene, [4 + 4]- and [4 + 6]-adducts (247-250, Scheme 16) are obtained in addition, photodimer 242 and o-dibenzoylbenzene (140) were isolated. The ratio of the [4 + 6]-adducts to the [4 + 4]-adducts [(249 + 250)/(247 + 248)] is increased in air-saturated benzene solutions compared with oxygen-free benzene solutions, and enhanced in heavy atom solvents (e.g., chloroform compared with cyclohexane) furthermore, this ratio is decreased in the presence of the triplet quencher azulene. These observations suggest that [4 + 6]- and [4 + 4]-adducts are formed by different mechanisms. 

Another variant of the photochemical reaction between A-hetero-cyclic o-quinones and olefins has been described by Mustafa et al.196 200 Stilbene reacts with CXLIX and with CL to give the photoproducts CLVIII and CLIX, respectively. Similar photoaddition products were obtained by the interaction of phenanthraquinone with a-stilbazole245 and with l,2-di-(4 -pyridyl)ethylene.241 Although the process has been suggested as involving diradicals, it is not clear whether the quinone or the olefin undergoes photoexcitation. 

The synthesis of thiols by the photochemical reaction of an olefinic compound with hydrogen sulphide is possible in the presence of an initiator which contains the phos-phorane group76. 

Photolysis of nitrogen dioxide as a source of oxygen atoms has been employed by Sato and Cvetanovic (87-90) to study their reactions with several olefins. They found a long-wavelength threshold for the oxidation of 1-butene by N02 at around 4047 A. The reaction occurred at this wavelength but not on irradiation with the very intense line at 4358 A. In the same spectral region Dickinson and Baxter (34) and Norrish (78) had found previously a threshold for the photochemical decomposi-... 

An additional field where the NO2 technique offers particular advantages is in the study of oxygen atom reactions with olefins in the presence of molecular oxygen (90). This topic is outside the scope of the present article since it deals with secondary interactions. It is a most challenging field which is of great importance for the understanding of photochemical air pollution phenomena (67). 

There has been a study of photo-induced intramolecular cyclization of some o-haloarylheterylamines which may lead to pyridof 1,2-a]benzimidazole derivatives.51 Several studies have been reported of photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reactions with fluoride,52 cyanide,53 or acetonitrile54 acting as the nucleophile, hi the example illustrated in Scheme 3,... 

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