Allyl iodide, photolysis

Photolysis of allyl iodide in thiophene gives a mixture of 2-allyl- (63.8%) and 3-allyl-(36.2%) thiophenes (77JOC1570). This is in contrast to homolytic phenylation, where almost exclusive 2-phenylation takes place (Section 3.14.2.9). It has been suggested that the rate-determining step in the allyl substitution reaction has a small but definite charge-transfer character. 

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]. 

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 ... 

Alternative methods for the deprotection of the allyl group at the anomeric position include Pd(PPh3)4/AcOH [402], in which the reaction proceeds by the formation of a 7r-allyl complex, or PdCl2/CuCl/02 followed by photolysis in the presence of triethylamine [403]. Per-fluoroalkylation with perfluoroalkyl iodide under sodium dithionite and sodium bicarbonate followed by elimination in the presence of zinc powder and ammonium chloride has also been disclosed as an efficient procedure for deprotection of the anomeric allyl group of carbohydrates (O Scheme 73) [404]. The reaction goes through the intermediacy of a radical addition of a perfluoroalkyl iodide to the double bond followed by Zn-mediated reductive -elimination. 

The differential reactivity of allyltributyltin towards electron-rich versus electron-poor radicals means that it is possible to carry out reaction sequences in which multiple carbon-carbon bonds are formed in a single transformation. The first example of such a sequence was reported by Mizuno and Otsuji [24], They showed that reaction of alkyl iodides with electron-deficient alkenes such as l,l-dicyano-2-phenylethene 33 and allyltributyltin gives good yields of three-component coupling products 34 (Scheme 6). In this process, an electron-rich alkyl radical 35 generated either by photolysis or by AIBN-mediated initiation undergoes selective addition to the electron-deficient alkene. Addition to the alkene occurs selectively since this process is much faster than addition of the alkyl radical to allyltributyltin. How-the resulting adduct radical 36 is now electron deficient, so it adds to allyl-... 

---