Benzene: acylation iodination

Proton acids can be used as catalysts when the reagent is a carboxylic acid. The mixed carboxylic sulfonic anhydrides RCOOSO2CF3 are extremely reactive acylating agents and can smoothly acylate benzene without a catalyst.265 With active substrates (e.g., aryl ethers, fused-ring systems, thiophenes), Friedel-Crafts acylation can be carried out with very small amounts of catalyst, often just a trace, or even sometimes with no catalyst at all. Ferric chloride, iodine, zinc chloride, and iron are the most common catalysts when the reactions is carried out in this manner.266... 

The combination of ortho metallation and meta nucleophilic acylation was used to prepare a key intermediate in a synthesis of deoxyfrenolicin (42), as outlined in Scheme 11. The complex of anisole is orf/io-metallated with n-butyllithium and quenched with chlorotrimethylsilane the resulting [(o-(tri-methylsilyl)anisole)Cr(CO)3] (43) is then metallated again, converted to the arylcuprate, and coupled with ( )-2-hexenyl bromide to give the complex of l-trimethylsilyl-2-methoxy-3-(2-hexenyl)benzene (44). Addition of the carbanion from the cyanohydrin acetal of 4-pentenal, followed by the standard iodine oxidation and subsequent hydrolysis of the cyanohydrin acetal to regenerate the carbonyl group... 

The AuCl-catalysed 4 + 2-cycloaddition of benzyne with o-alkynyl(oxo)benzenes produced anthracene derivatives having a ketone in the 9-position, in good to high yields under mild conditions.118 Hypervalent iodine compounds, [5-acyl-2-(trimethyl-silyl)]iodonium triflates, readily yielded acylbenzynes which could be trapped with furan.119 Both DMAD and benzyne reacted with borabenzene to yield substituted borabarrelenes and borabenzobarrelenes, respectively.120... 

As regards the protecting effect, the complex is stable to Lewis acids. Also, no addition of BH3 occurs. As Co2(CO)6 can not coordinate to alkene bonds, selective protection of the triple bond in enyne 137 is possible, and hydroboration or diimide reduction of the double bond can be carried out without attacking the protected alkyne bond to give 138 and 139 [32], Although diphenylacetylene cannot be subjected to smooth Friedcl Crafts reaction on benzene rings, facile /7-acylation of the protected diphenylacetylene 140 can be carried out to give 141 [33], The deprotection can be effected easily by oxidation of coordinated low-valent Co to Co(III), which has no ability to coordinate to alkynes, with CAN, Fe(III) salts, amine /V-oxidc or iodine. 

An alternative method for the formation of acyl hypoiodites, developed by Barton, involves the treatment of the acid with r-butyl hypoiodite. Subsequent white light photolysis in benzene at room temperature gave good yields of it ides from primary, secondary and tertiary acids (equation 21). The method was not applicable in the presence of alcohols. A more recent technique involving hypervalent iodine is due to Suarez primary, secondary or tertiary aliphatic acids are heated to reflux in tetrachloromediane with iodosylbenzene diacetate and iodine resulting in good yields of iodides. The method is mild and, with obvious exceptions such as unprotected alcohols, is tolerant of many functional groups, as illustrated in equation (22). ... 

Many organic compounds react with carboxylic acids, acyl halides, or anhydrides in the presence of certain metallic halides, metallic oxides, iodine, or inorganic acids to form carbonyl compounds. The reaction is generally applicable to aromatic hydrocarbons. Benzene, alkylbenzenes, biphenyl, fluorene, naphthalene, anthracene, acenaphthene, phenanthrene, higher aromatic ring systems, and many derivatives undergo the reaction. 

Iodine in trace amounts has been used as catalyst in Friedel-Crafts acylations of furane and thiophene and of more active members of the benzene series such as anisole and acetanilide. " Oddly enough, it is not effective for benzoylation of anthracene.""... 

Reactions of4-hydroxy-2-cyclobutenones 364 with (diacetoxyiodo)benzene in 1,2-dichioroethane at reflux afford 5-acetoxy-2(5//)-furanones 365 as rearranged products (Scheme 3.146) [466], The formation of these products is explained by ring cleavage in the hypervalent iodine intermediate 366 foiiowed by recycliza-tion of the resulting acyl cation 367 with the carbonyl oxygen (Scheme 3.146). In a similar procedure,... 

Acetonation of 3-0-methyl-D-glucitol affords the isomeric 1,2 5,6-and 2,4 5,6-di-0-isopropylidene derivatives which are differentiated on silica gel layers [hRf 50 and 25, respectively, in benzene-methanol (90 + 10)] with visualization by iodine vapor [85]. The four stereoisomers of l,2 5,6-di-0-trichloroethyUidene-a-D-glucofuranose, which differ in the configuration of the acetal carbons atoms, are resolved on silica gel with ethyl acetate-petroleum ether (b. p. 40—50°) (50 + 50) [86]. Chloroform-acetone (97 + 3) and benzene-ether (90 + 10) have been used to separate mixtures of acylated di-O-isopropylidene-hexitols [87, 88]. 

Aromatic compounds (eqs 16 and 17) are acylated by PhCOCI in the presence of a Lewis acid such as AICI3, TiCU, BF3, SnCU, ZnClz, or FeClz, or of a strong acid such as polyphos-phoric acid or CF3SO3H. Metallic A1 or Fe and iodine (in situ formation of a Lewis acid) can also act as a catalyst. Various solvents that have been used to perform this reaction are CSz, CH2CI2, 1,2-dichloroethane, nitrobenzene, and nitromethane. PhCOCI is less reactive than aliphatic carboxylic acid chlorides (with benzene in nitromethane the relative reaction rates are Ph-COCl MeCOCl = 6 100). As for all electrophilic substitutions, the rate and the regioselectivity of the acylation closely depend on the nature and on the position of the substituents on the aromatic system (eqs 16 and 18 ). The nature of the solvent can also exert a strong influence. ... 

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