Vinyl iodide, photolysis

Atom transfer cyctization (14, 173-174). This method of cyclization can be more useful than reductive tin hydride cyclization in the case of 6-substituted 5-hexynyl iodides such as 1 and has the further advantage of resulting in a vinyl iodide, which can be used for further functionalization.1 The reaction is best conducted by photolysis in benzene at 80° with catalytic amounts of hexabutylditin. 

Process (2) is favoured at short wavelengths. Irradiation of tetra-iodoethylene with A 2537 A in hexane solution at 0 °C produces diiodoacetylene but the mechanism is not clear. The primary process involving carbon-halogen rupture is important in the liquid phase photolysis of 3-bromopropyne at A 2537 A. Studies have been made on the photolysis of allyl iodide" and vinyl iodide in solution. The photochemistry of cis- and Irani-1-iodopropene has also been reported ... 

Photolysis of vinyl iodides is a convenient method for the generation of vinyl cations, highly reactive intermediates that are difficult to generate by thermal processes. Irradiation of 1-iodocyclohexene (478, X = I) in methanol in the presence of zinc as an iodine... 

Cationic t 3-allyltetracarbonyliron complexes are generated by oxidative addition of allyl iodide to pentacarbonyliron followed by removal of the iodide ligand with AgBF4 under a carbon monoxide atmosphere [35]. Similarly, photolysis of vinyl epoxides or cyclic vinyl sulfites with pentacarbonyliron or nonacarbonyldiiron provides Jt-allyltricarbonyliron lactone complexes. Oxidation with CAN provides by demetallation with concomitant coupling of the iron acyl carbon to one of the termini of the coordinated allyl moiety either [3- or 8-lactones (Scheme 1.12) [36, 37]. In a related procedure, the corresponding Jt-allyltricarbonyliron lactam complexes lead to P- and 8-lactams [37]. 

A regioselective iodoperfluoroalkylation of terminal alkynes (R—C = CH) has been reported, and is based on photolysis ofthe C—I bond in perfluoroalkyl iodides (Rp-I). Addition of the thus-formed RF" radical onto the alkyne afforded a vinyl radical that in turn abstracts an iodine atom from the starting Rp—I to form the end olefin R-C(I)= CH-Rf. A xenon lamp through Pyrex (hv > 300 nm) was used for the reaction, where aliphatic alkynes gave a better alkylation yield with respect to phenylacetylene [81],... 

An excellent alternative to the classical Hunsdiecker reaction and its variants, which totally avoids the use of heavy metal salts and potent electrophilic reagents, consists of the simple photolysis or thermolysis of Barton esters in refluxing bromotri-chloromethane for the bromides or tetrachloromethane for the chlorides [4], The analogous decarboxylative iodination can also be achieved using iodoform as the reagent in a benzene/cyclohexene solvent system (Scheme 5). For the cases of vinylic and aromatic acids, where the usual problems of chain efficiency are encountered, the addition of azobisisobutyronitrile (AIBN) is also required [10]. Nevertheless, since this method can operate on both electron-rich and electron-poor aromatic systems, and moreover does not suffer from the competitive electrophilic aromatic bromination found with electron rich aromatics under normal Hunsdiecker conditions, this route to synthetically useful aryl iodides and bromides should find widespread application. 

The initially formed adduct radicals were scavenged by an excess of methyl iodide. A study of the addition of methyl radicals, from the photolysis of biacetyl in the presence of isobutane, to hexafluoropropene has indicated that it is rather unselective, reacting at a rate similar to that with tetrafluoro-ethylene, but a further study, employing octane as scavenger, has indicated that tetrafluoroethylene is some six times as reactive (in the latter work the vinyl ethers CFa O CFiCFs and n-C8F, O CF CF2 were also studied). 

The well-known principal example in solution is the homolysis of the C-X bond, affording a radical pair composed of a vinyl radical and a halogen atom, as represented by the photolysis of alkenyl iodides (Equation 11.1). ... 

Some general rules can be used to predict the outcome of irradiation of alkenyl haHdes. Thus, the photolysis of a-arylvinylhalides leads to stable a-arylvinyl cations, and the vinyl cation route is followed predominantly. A nucleophilic reagent can capture such intermediates. Conversely, in the cases that produce unstable vinyl cations, vinyl radicals are formed preferentially, or else the formation of radical and cation intermediates competes. Clearly, the nature of the substituent is important in predicting the reactions of the vinyl cations. Results indicate that the nucleophilic trapping of the vinyl cations follows the order d H << alkyl << Ph < p-MeC( H4 < p-MeOC( H4. The halogen employed is also important, with chlorides and bromides better than iodides at undergoing hssion. Fluorides, as mentioned previously, are inactive in photolysis. In aliphatic alkenyl halides, conversely, iodides afford ionic intermediates more preferentially than do bromides or chlorides. However, capture by nucleophiles is restricted to the nucleophilic solvent employed. 

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