Aryl synthesis, regiospecific

The reaction of appropriate 1,3-diketones (302) with hydroxylamine hydrochloride in pyridine (79MI41601) has been reported to result in a regiospecific synthesis of 3-alkyl-5-arylisoxazoles, as has the reaction of an a -bromoenone (307) with hydroxylamine hydrochloride in ethanol in the presence of potassium carbonate (81H(16)145). Regiospecific syntheses of 5-alkyl-3-phenylisoxazoles also result from the reaction of an a-bromoenone (307) with hydroxylamine in the presence of sodium ethoxide (81H(16)145). 3-Aryl-5-methylisoxazoles were prepared from phosphonium salts (304) and hydroxylamine (80CB2852). 

The orthometallation-carbonylation sequence illustrated in Scheme 155 affords a useful regiospecific methoxycarbonylation of the aryl ring of 4-chromanones.232 Unfortunately an extra step is incurred in the synthesis because of the unreactivity of the dimeric complexes (126) toward carbonyl insertion. 

Trialkylaryltin derivatives 30 are converted into fluoro-substituted derivatives 31. 37-38 (4-Mcth-oxyphenyl)trimethylsilane (32) in acetonitrile gives 4-fluoroanisole and (3-fluoro-4-meth-oxyphenyl)trimethylsilane in the ratio 1 2. A -Methyldiethanol esters of arylboronic acids 33 (l-aryl-5-methyl-2,8-dioxa-5-azonia-l-boranuidabicyclo[3.3.0]octanes) are converted into tluoroaromatic compounds with cesium fluoroxysulfate in acetonitrile in the presence of 1,3-dinitrobenzene at room temperature.39-40 Regiospecific synthesis of 2-fluoro-3-0-methyles-trone in 27 % yield occurs upon fluorination of the corresponding arylboronic acid with cesium fluoroxysulfate.41... 

Diaryliodonium fluorides can be used as efficient reagents for the arylation of silyl enol ethers [71, 72]. The reaction of silyl enol ethers 95 with o-nitro-phenyliodonium fluoride 96 results in a regiospecific arylation yielding arylke-tones 97 in good yields (Scheme 43) [72]. This arylation has been applied in a new, regiocontrolled synthesis of carbocycle-fused indoles [72]. 

Ogawa, S. Furukawa, N. Regiospecific ortho lithiation of o-halophenyl p-tolyl sulfoxides and synthesis of met -substituted optically active aryl alcohols./. Org. Chem. 1991, 56, 5723-5726. 

An unusual process that has attracted much interest recently is the photochemical reaction between chloroaromatic and an alkene, with the aryl ring and chloride adding across the double bond. This offers an alternative approach for the synthesis of alkylaromatics, as illustrated in Scheme 3.29. The irradiation of 4-chloroanisole caused the photoheterolysis of the Ar—Cl bond, thus forming a reactive phenyl cation and Cl. The cation added regiospecifically to the double bond of 1-hexene, and the resulting adduct cation was trapped by the chloride anion to yield 4-(2-chlorohexyl) anisole (46) in moderate yield [76]. 

The base-catalyzed condensation of azides with activated methylene compounds is a well-established route to 1/7-triazoles. This is a regiospecific method ideally suited for the synthesis of triazoles with a 5-amino or -hydroxy substituent, and an aryl or carbonyl function at C-4. The general scheme is shown in Scheme 8. The stepwise character of the reaction is best revealed by the anomerism of glycosyl azides during their reaction with... 

Now, these arylthallium compounds are useful, not in themselves, but as intermediates in the synthesis of a variety of other aromatic compounds. Thallium can be replaced by other atoms or groups which cannot themselves be introduced directly into the aromatic ring - or at least not with the same regiospecificity. In this way one can prepare phenols (ArOH, Sec. 24.5) and aryl iodides (Sec. 25.3). Direct iodination of most aromatic rings does not work very well, but the process of thallation followed by treatment with iodide ion gives aryl iodides in high yields. 

Aryl groups separated by moieties other than ethylene also undergo facile photo-oxidative cyclization and this type of reaction has been used to provide the regiospecific synthesis of... 

Regiospecific aryl ketone synthesis. Negishi et al. have reported a regio-specific synthesis of aryl ketones from aryl bromides in which the new C—C bond is specifically at the carbon atom that originally was substituted by bromine. Thus the aryllithium compound (2) is prepared by halogen-metal exchange this reacts with a trialkylborane to form a lithium aryltrialkylborate (3). The reac-... 

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