Iodine deactivation

An estimate of (j) may be obtained by studying the temperature dependence of high temperatures and low light intensities, all the formyl radicals formed in primary process I can be expected to dissociate and to convert subsequently into hydrogen molecules. This seems to be the case, since Calvert et al observed that attains a limiting value at high temperatures, which can be taken as equal to i. On this basis, Calvert et determined the value of 0.81 for i at 3130 A, which is in sharp contrast with that obtained in the presence of iodine. The discrepancy seems to support the suggestion that iodine deactivates the excited acetaldehyde molecules. 

Photo-induced ITP has recently been developed in solutimi by Wolpers and Vana [55]. The polymer—iodine bond was directly cleaved by photo-irradiation (DC process in Scheme lb), and accumulation of the iodine deactivator (generated by DC) was avoided by continuously supplying Polymer" using a conventional radical initiator. It would be interesting to apply this photo-induced ITP to surface-initiated polymerizatiOTi in the future. 

The mix of inductive and resonance effects varies from one halogen to another but the net result is that fluorine chlorine bromine and iodine are weakly deactivating ortho para directing substituents... 

Since the formation of the Grignard compound takes place at the metal surface, a metal oxide layer deactivates the metal, and prevents the reaction from starting. Such an unreactive metal surface can be activated for instance by the addition of small amounts of iodine or bromine. 

In contrast to the hydrogenation of imines, where addition of acids and/or iodine often has a beneficial effect, here additives of this type were found to deactivate the catalyst. 

Only a few detailed studies of the reaction mechanism of the homogeneous hydrogenation of imines have been published until now. A generalization seems to be very difficult for two reasons. First, rather different catalyst types are effective and probably act by different mechanisms. Second, the effect of certain additives (especially iodide or iodine and acid/base) is often decisive for ee and rate, but a promoter in one case can be a deactivator in another case. 

Iodine monofluoride, prepared in situ from the elements, has been recommended for the iodination of deactivated substrates without the need for Friedel-Crafts catalysts (90JOC3553) and may find application with heterocyclic compounds. The corresponding chloride has been used frequently [82CJC554 83ACS(B)345 84CHE492, 84M11 86JHC1849 ... 

Fluorine has been used for the generation of extremely strong electrophilic halogenating agents in electrophilic iodination and bromination of deactivated aromatic substrates in highly acidic reacton media. Polyhalogenation of more activated aromatic substrates is also possible (Fig. 90) [231-233]. 

Alcohols, (a) Monohydric The common impurities in alcohols are aldehydes or ketones, and water. [Ethanol in Chapter 3 is typical.] Aldehydes and ketones can be removed by adding a small amount of sodium metal and refluxing for 2 hours, followed by distillation. Water can be removed in a similar way but it is preferable to use magnesium metal instead of sodium because it forms a more insoluble hydroxide, thereby shifting the equilibrium more completely from metal alkoxide to metal hydroxide. The magnesium should be activated with iodine (or a small amount of methyl iodide), and the water content should be low, otherwise the magnesium will be deactivated. 

It is widely employed as a disinfectant in medicine (Povidone-iodine) because of its mildness, low toxicity, and water solubility. According to the U.S. Pharmacopeia, l ovidone-iodme is a free-flowing, brown powder dial contains from 9-12% available iodine. 11 is soluble in water and lower alcohols. When dissolved in water, the uncomplexed free iodine level is very low however, tine complexed iodine acts as a reservoir and by equilibrium replenishes the free iodine lo the equilibrium level. This prevents free iodine from being deactivated because the free form is continually available at effective biocidal levels from this large reservoir. PVP will interact with other small anions and resembles serum albumin and other proteins in this regard. It can be salted in with anions such as NaSC.N or out with NasSOa much like water-soluble proteins. 

Barluenga et al.565 have reported the selective monoiodination of arenes with bis (pyridine)iodonium(I) tetrafluoroborate [I(py2)BF4] in excess superacids (2 equiv.) [Eq. (5.210)]. Comparable results were found for activated compounds with both HBF4 and triflic acid, whereas triflic acid was more effective in the iodination of deactivated aromatics. For example, nitrobenzene and methyl benzoate are unreactive in HBF4 but give the corresponding iodo derivatives in triflic acid (83% and 84% yields, respectively, in 14 h). Iodination of phenol required low temperature (-60°C). 

The rate of diffusive separation, k, was determined from separate experimental measurements of iodine radical diffusion rates in the high pressure diffusion limited regime (19). The rate of excited state deactivation, k i, was calculated from the measured quantum yields at high densities where G> = kd/k i (18). It was assumed that k i is proportional to the inverse diffusion coefficient, D 1 (19,23) as both properties are related to the collision frequency. 

Finally, we discuss briefly the results of Donovan and Husain126 on the relaxation of I(52Pi->i), for which AE = 7603 cm-1. The problem is even more complex than the foregoing, because the splitting is large and the extent of vibrational excitation accompanying electronic deactivation is not known. Atomic iodine in the 5 2Pi state can be produced by flash photolysis of various iodides,... 

---