Thallium acetate-bromine

Problem 11.6 Aromatic bromination catalyzed by the Lewis acid thallium acetate, TI(OOCCHj), gives onl> the para isomer. Suggest an explanation for this regio-spccificity. (Hint See Sec, 11,7.)... 

Lithiated polystyrene resins can be obtained either via convenient bromine-lithium exchange reaction using nBuLi starting from 4-bromo-substituted polystyrene 102,111-115 qj. jjy direct lithiation of polystyrene using nBuLi in cyclohexane in the presence of TMEDA 11L116 pjjjj method, however, yields a mixture of para- and meta isomers. The bromination of microporous resins in the presence of the Lewis-acid catalysts was carried out in the dark whereby the degree of functionalisation could conveniently be controlled by the amount of bromine used in the reaction.iii Macroreticular resins were brominated using Br and FeClj or stoichiometric amounts of thallium acetate as Lewis acid catalysts. 2 ... 

Related Reagents. Bromine-r-Butylamine Bromine Chloride Bromine-l,4-Diazabicyclo[2.2.2]octane Bromine-1,4-Dioxane Bromine-Silver(I) Oxide Bromine-Triphenyl Phosphite iV-Bromosuccinimide A7-Bromosiiccinimide-Dimethylformamide 7V-Bromosuccinimide-Dimethyl Sulfide Al-Bromosuccinimide-Sodium Azide Copper(II) Bromide Hy-drobromic Acid Mercury(II) Oxide-Bromine Phosphoms(III) Bromide Pyridinium Hydrobromide Perbromide Sodium Bromide Thallium(III) Acetate-Bromine. 

Related Reagents. Mercury(II) Oxide-Bromine Thallium(I) Carbonate Thallium(III) Acetate-Bromine. 

The iodo acetal could be easily obtained (as a mixture of E and Z isomers, 40/60), by a nickel catalyzed iodine-bromine exchange. This synthon reacted smoothly with the C15 tertiary allylic alcohol in the presence of a catalytic amount of palladium acetate and a stoechiometric amount of either a silver or a thallium salt. The C20 hydroxy-acetal was obtained in 38% yield, as a mixture of E and Z isomers (48/52). Finally retinal was obtained by treatment with dilute HBr in refluxing acetone, as a mixture of E and Z isomers (C(9)=C(io) and C(i3)=C(i4)), Fig. (32). 

The iodination of cross-linked polystyrene has been achieved using iodine under strongly acidic reaction conditions [55] or in the presence of thallium(III) acetate [61], but this reaction does not proceed as smoothly as the bromination. More electron-rich arenes, such as thiophenes [45,62-64], furans [46], purines [65], indoles [66], or phenols [67,68] are readily halogenated, even in the presence of oxidant-labile linkers (Figure 6.2). Polystyrene-bound thiophenes have also been iodinated by lithiation with LDA followed by treatment with iodine [64],... 

Aromatic bromides (3, 286). The definitive paper on electrophilic aromatic bromination with bromine and thallium(lll) acetate has been published. The two most oul.standingfeaturc.s are I) monobromination is observed in almost all cases, and 2) exclusive para substitution is observed with almost all monosubstituted benzenes. Electron-withdrawing groups inhibit bromination of monosubstituted benzenes. It... 

Thallium carboxylates, particularly the acetate and trifluoroacetate, which can be obtained by dissolution of the oxide in the acid, are extensively used in organic chemistry.14 Both Tl metal and Tl1 salts such as the acetylaceton-ate also have specific uses. One example is the use of thallium(m) acetate in controlled bromination of organic substances such as anisole. The trifluoroacetate will directly thallate (cf. aromatic mercuration, Section 18-9) aromatic compounds to give aryl thallium ditrifluoroacetates, e.g., C6H5Tl(OOCCF3)2. It also acts as an oxidant, inter alia converting para-substituted phenols into p-quinones. 

On treatment of both quinoline and pyridine /V-oxides with bromine and thallium(iil) acetate, y-bromo-derivatives are formed pyridine JV-oxides are the less reactive substrates/ The reaction of 8-methoxyquinoline iV-oxide with acetic anhydride and methanol yields 84% of 2,8-dimethoxyquinoline. Similarly, the reaction of the 6-methoxy-iV-oxide and ethanol and acetic anhydride gives 2-ethoxy-6-methoxyquinoline, but, under the same conditions, the 7-methoxy-N-oxide is unchanged/ The Reissert compounds (149) derived from substituted quinolines, on treatment with thallium(lll) nitrate and trimethyl orthoformate, undergo either smooth ring-contraction or oxidative debenzoyl-ation, depending on the substitution pattern in the aromatic ring (Scheme 61)/ ... 

OXIDATION, REAGENTS Acetyl nitrate. Bis(tri-n-butyltin)oxide. Bromine-Hexameth-ylphosphoric triamide. f-Butyl perbenzoate. Ceric ammonium nitrate. N-chlorosuc-cinimide-Dimethyl sulfide. Chromic add. Chromic anhydride. Chromic anhydride-Acetic anhydride. Chromic anhydride-Hexamethylphosphoric triamide. 2,3-Dichloro-5,6-dicyano-l,4-benzoquinone. Dimethyl sulfoxide. Dimethyl sulfoxide-Trifluoro-acetic anhydride. Diphenylseleninlc anhydride. Iodine tris(trifluoroacetate). Lead tetraacetate. N-Methylmorpholine -N-oxide. p-NitrobenzenesulfonyI peroxide. Oxygen, singlet. Palladlumfll) chloride. Peroxybcnzimidic acid. Phenylseleninyl chloride. N-Phenyl-l,2,4-triazoline-3,5-dione. Potassium chromate. Potassium superoxide. Pyri-dinium chlorodiromate. Salcomine. Silver carbonate-Celite. Sodium hypochlorite. Sulfuryl chloridc-Silica gel. Thallium(III) acetate. ThaUium(III) nitrate. Triphenyl phosphite ozonide. Trltyl tetrafluoroborate. Uranium hexafluoride. 

Pure 2-bromobiphenylene is obtained in high yield by reaction of biphenylene with bromine in the presence of thallium(III] acetate [153]. 

Carboxylic acids (RCOOH) can be converted to norhalides (RBr or RI) by the Hunsdiecker reaction (silver salt and bromine) or by some modification of this (mercuric oxide and bromine, lead tetra-acetate and iodine, or thallium salt and bromine). 

Thallation of thiophens in the 2-position with thallium(m) trifluoro-acetate in trifluoroacetic acid is complete within a few minutes at room temperature. The thallium derivative reacts in situ with aqueous potassium iodide solution to give a convenient and high-yield synthesis of iodo-thiophens. A mixture of thallium(m) acetate has been shown to be a mild and efficient reagent for electrophilic aromatic bromination. Thiophen yields 2-bromothiophen in 82% yield and very little dibromothiophen. 3-Methylthiophen appears to be selectively brominated in the 2-position and 2-methylthiophen in the 5-position in 70—75% yield. The direct thiocyanation of thiophen and some alkylthiophens with thiocyanogen under various conditions using a variety of Friedel-Crafts catalysts has... 

Poole and Dhanesar investigated the thallium(i)-catalysed electrophilic bromination of diphenyl ether and l,3-bis(3-pheno)yphenoxy)benzene in their study of the preparation of cyanophenyl ethers that could be used as polar and thermally stable liquid phases in gas chromatography. Under mild conditions, bromination of diphenyl ether with thallium(i) acetate catalyst produces exclusively the para-substituted product. Formation of ortho- and para-substituted products could be achieved under more vigorous conditions, such as higher temperature and increased concentration of bromine and thallium(i) salts (Scheme 20.22). Similarly, the substitution pattern of l,3-bis(3-pheno)yphenoxy)benzene bromination product could be controlled by reaction conditions. 

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