Aqueous media, reactions iodination

In fact, reaction 4.105 also represents an example of a condensation reaction. A prior redox reaction in non-aqueous medium also often occurs, e.g., in the highly sensitive analysis of peroxides with HI in acetic acid, both under absolutely water-free conditions, where iodine is quantitatively liberated and is subsequently titrated. For much work on non-aqueous redox titrations by Tomicek s school published mainly in the Czech literature, see ref. 17. 

The choice of the solvent and of the electrophile is very important since the reaction can be carried out under kinetic or thermodynamic control. The possibility of equilibrating a cyclic intermediate strongly influences the regio- and stereochemistry of the reaction. The presence of a base in an aqueous medium generally results in kinetic control of the cyclization process18, while reversible conditions are favored by iodine in acetonitrile19. In addition, A -iodosuccin-imide in chloroform, iodine in chloroform and iodine in tetrahydrofuran/pyridine are considered to give cyclizations under kinetic control. On the other hand, the use of AT-bromosuccin-imide or bromine affords lower selectivity. 

Iodination of phenols that have free ortho- and / ara-positions, in an alkaline medium, by iodine, leads not only to the desired iodo derivatives, but also to reddish-brown polymeric by-products, e.g., Lautemann s Red from phenol462,463 and aristol from thymol.464 These unpleasant side reactions are reduced by working in methanolic alkali465 or in concentrated aqueous or methanolic ammonia.466 A nearly quantitative yield of 2,4,6-triiodophenol is obtained by slowly adding a concentrated solution of I2 in KI to phenol in concentrated aqueous ammonia.467... 

The NO2 radicals obtained from nitrite ions in acidic solution add to olefins regioselectively. The resulting radical is oxidized in situ by CAN to provide nitroolefins, even labile ones, in high yield and short reaction times.32 The second reaction in Fig. 5 shows another preparation by the addition of nitryl iodide, from potassium nitrite and iodine, followed by a base-induced elimination of hydrogen iodide. A radical mechanism is probably not involved. This mild one-pot method, a valuable alternative to other published techniques, is accomplished in a non-aqueous medium and gives good yields and regioselectivities even with sterically hindered substrates. 

Iodine shows a very strong catalytic activity in aqueous medium, which is much faster than any catalyst reported for the synthesis of 3,3 -aiylmeth-ylenebis-(4-hydroxycoumarin). The powerful catalytic activity of iodine is further substantiated by less reaction time as well as high product yields for the desired transformation. 

Dilithium tetrachlorocuprate is recommended as an additive for cross coupling of Grignard compounds with tosylates even allylic and benzylic acetates give good yields . a-Methylene-ketones, -carboxylic acids and -lactones have been prepared via sulfides and sulfoxides. A convenient and general synthesis of acetylene derivatives from boranes via the reaction of iodine with lithium 1-alkynyltriorganoborates has been published ar-Nitrostyrenes can be easily obtained by a Wittig synthesis with formaldehyde in an aqueous medium . A new synthesis of unsym. ketones by reaction of dialkyldiloroboranes with lithium aldimines has recently been published . Metallo aldimines have also served for the synthesis of a variety of other compound classes such as a-hydroxyketones, a-keto acids, nitriles, and for the asym. synthesis of a-amino acids . Polycondensations of malononitriles with benzylic chlorides have been carried out quantitatively under mild conditions in dimethyl sulfoxide with triethylamine as acid acceptor . Carbonyl compounds can react with dibromoacetonitrile to yield a-bromo esters with additional carbon atom . ... 

This is the favored procedure for preparing 2,4,6-trinitrobenzaldehyde. Reaction (A) is run at room temperature in pyridine, using iodine as a catalyst, or in a alcohol-acetone mixture with anhydrous sodium carbonate as a catalyst. Reaction (B) proceeds in a strong aqueous hydrochloric acid solution overall yields are 39 to 52 percent. Reaction (A) can be run in an aqueous medium in the presence of light with a yield of 80 percent and less by-product formation than in the other methods. Other types of aromatic nitroso compounds, for example nitrosobenzene and nitrosotoluene, form nitrones according to the equation ... 

A solution of iodine in aqueous iodide has an intense yellow to brown colour. One drop of 0.05M iodine solution imparts a perceptible pale yellow colour to 100 mL of water, so that in otherwise colourless solutions iodine can serve as its own indicator. The test is made much more sensitive by the use of a solution of starch as indicator. Starch reacts with iodine in the presence of iodide to form an intensely blue-coloured complex, which is visible at very low concentrations of iodine. The sensitivity of the colour reaction is such that a blue colour is visible when the iodine concentration is 2 x 10 " 5 M and the iodide concentration is greater than 4x 10 4M at 20 °C. The colour sensitivity decreases with increasing temperature of the solution thus at 50 °C it is about ten times less sensitive than at 25 °C. The sensitivity decreases upon the addition of solvents, such as ethanol no colour is obtained in solutions containing 50 per cent ethanol or more. It cannot be used in a strongly acid medium because hydrolysis of the starch occurs. 

A mixture of 1,4-dioxane and water is often used as the solvent for the conversion of aldehydes and ketones by H2Se03 to a-dicarbonyl compounds in one step (Eq. 8.117).331 Dehydrogenation of carbonyl compounds with selenium dioxide generates the a, (i-unsaturated carbonyl compounds in aqueous acetic acid.332 Using water as the reaction medium, ketones can be transformed into a-iodo ketones upon treatment with sodium iodide, hydrogen peroxide, and an acid.333 Interestingly, a-iodo ketones can be also obtained from secondary alcohol through a metal-free tandem oxidation-iodination approach. 

Iodine-catalysed hydroperoxidation of cyclic and acyclic ketones with aqueous hydrogen peroxide in acetonitrile is an efficient and eco-friendly method for the synthesis of gem -dihydroperoxides and the reaction is conducted in a neutral medium with a readily available low-cost oxidant and catalyst.218 Aryl benzyl selenoxides, particularly benzyl 3,5-bis(trifluoromethyl)phenyl selenoxide, are excellent catalysts for the epoxidation of alkenes and Baeyer-Villiger oxidation of aldehydes and ketones with hydrogen peroxide.219 Efficient, eco-friendly, and selective oxidation of secondary alcohols is achieved with hydrogen peroxide using aqueous hydrogen bromide as a catalyst. Other peroxides such as i-butyl hydroperoxide (TBHP), sodium... 

Of all the methods described for the synthesis of thiazole compounds, the most efficient involves the condensation of equimolar quantities of thiourea and a-halo ketones or aldehydes to yield the corresponding 2-aminothiazoles (Scheme 167) (l888LA(249)3l). The reaction occurs more readily than that of thioamides and can be carried out in aqueous or alcoholic solution, even in a distinctly acid medium, an advantage not shared by thioamides which are often unstable in acids. The yields are usually excellent. A derived method condenses the thiourea (2 mol) with the non-halogenated methylene ketone (1 mol) in the presence of iodine (1 mol) or another oxidizing agent (chlorine, bromine, sulfuryl chloride, chlorosulfonic acid or sulfur monochloride) (Scheme 168) (45JA2242). 

Hypoiodites are used for qualitative tests for methyl ketones (Lieben test). For this purpose, a compound to be tested is stirred with an aqueous solution of sodium hydroxide (80 mol/mol of methyl ketone). Iodine (4.5 mol of 12) is added portionwise with stirring, and the mixture is set aside for 20 min at 25 °C before acidification. In the presence of a methyl keto group, a yellow heavy precipitate of iodoform settles at the bottom of the test tube. Iodoform can be identified easily not only by its characteristic smell but also by its melting point (120-123 °C) [1173], This test applies not only to methyl ketones but to any compound that can be converted in the reaction medium into a species containing the COCH3 group, for example, isopropyl or ethyl alcohol. 

The extraction of Sbis with benzene from 5 M H2SO4 medium in the presence of small excess of KI enables to determine Sb by the sensitive amplification method. From the washed benzene extract Sbl3 is re-extracted with water. In this aqueous extract, the iodide is oxidised to iodate with bromine. After removal of excess of bromine, the I03 ions react with KI to yield iodine which is determined by its colour reaction with starch (see Section 25.2.1). Thus 18 iodine atoms are formed for each Sb atom (in Sbl.i). The molar absorptivity e in this method is 3.0T0 (a = 2.6). 

The choice of an appropriate solvent as the reaction medium is of primary importance and usually involves a compromise among various unfavorable factors. Because of the high chemical reactivity of the halogens and of the polyhalides, solvents which are easily susceptible to halo-genation or which effect solvolysis reactions are to be avoided. This, of course, precludes the use of aqueous solutions for many syntheses, with the exception of those of some polyiodides since iodine is fairly unreactive with water. 

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