Arenes side chain substitution

Lead tetraacetate reacts with arenes to lead either to aromatic nucleus substitution or to substitution on the benzylic position of the side chain. Substitution on the nucleus involves electrophilic attack of (AcO)3Pb " to give aryllead tricarboxylates. Subsequently, these aryl species react with acid to afford eventually the corresponding aryl esters. ... 

Oxidation of arenes by Pd(II), which is typically carried out in acetic acid solution, leads to oxidative coupling and ring or side chain substitution, depicted in Scheme 2. 

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

Oxidation of Other Arenes. Aromatic compounds with longer alkyl side chains can be converted to ketones or carboxylic acids. All the previously discussed reagents except Cr02Cl2 usually afford the selective formation of ketones from alkyl-substituted arenes. Oxidation with Cr02Cl2 usually gives a mixture of products. These include compounds oxidized in the P position presumably formed via an alkene intermediate or as a result of the rearrangement of an intermediate epoxide.110,705... 

The first stage (addition-elimination) is well known with CN and a variety of N-, O- and S-nucleo-philes.7-72-75 The detachment of the product arene from the Fe is more difficult. The conversions in equation (22) demonstrate the stability of the arene-Fe bond toward oxidation, as a side chain methyl is converted to a carboxylic acid, and suggest the generality of heteroatom substitution under mild conditions.76... 

In comparison with molecular catalysts, solid catalysts can be isolated from the reaction mixtures by filtration or used in continuous processes this is both environmentally friendly and useful in laboratory-scale experiments. The most important reactions catalyzed by solid superbases are isomerization reactions and the alkylation of substituted arenes in the side chain (Scheme 2). They proceed at room temperature or below with high yield (typically >99%). The surperbase-cata-lyzed alkylation of aromatic compounds complements the acid-type Friedel-Crafts alkylation and acylation, because the latter results in ring alkylation, whereas the former results in side-chain alkylation. 

The general process begins with Cr(CO)3L3, in which the L unit can be CO (most common),MeCN, " o-alkylpyridine, ammonia, and other donor ligands (equation 91). The rate (reaction temperature) is related to the nature of L the most reactive readily available source of Cr(CO)3 is ( ] -naphthalene)Cr(CO)3, which undergoes favorable arene exchange under mild conditions with many substituted arenes. " The most general and convenient procedure employs a mixture of THF and di-n-butyl ether at reflux. " A variety of polar and nonpolar aprotic solvents has been used and, for some purposes such as complexation of a-amino acids with aromatic side chains, water-THF mixtures are effective. 

Polymerization of phthalocyanines in water occurs for derivatives substituted with oligo (ethylene oxide) side-chains (27c).167 168 In the lyotropic mesophases in water supramolecular polymers are present, and a comparative aggregation study between tetraphen-ylporphyrins and phthalocyanines proved the polymerization of the phthalocyanines to be stronger.168 The strong arene—arene interactions and the flatness of the aromatic core in the phthalocyanines causes them to aggregate more strongly, also mediated by the additional hydrophobic effect. 

A series of 5-(2-methoxyphenyl)pent-l-enes (65), substituted at the a-, P-, or y- positions with either an OH or OSiMe3 group, undergo an intramolecular meta photocycloaddition reaction to give a product (66) having endo stereochemistry at the substituted carbon atom. The diastereoselectivity is a consequence of minimisation of steric interactions between the side chain substituent and the ortho-MQO group of the arene unit. A molecular mechanics study of the regio- and stereoselectivity of cation-dependent [2 + 2]-photo-... 

Solution N-propylbenzene is a compound made up of aromatic and alkane units. Hence, it belongs in the category of alkylbenzenes, which, in turn, are part of the group of compounds known as arenes (compounds that contain both aliphatic and aromatic units). Such compounds show two sets of chemical properties. The ring undergoes the electrophilic substitution characteristic of benzene, whereas the side chain undergoes the free-radical substitution characteristic of alkanes. Each should modify the other. Except for oxidation and hydrogenation these are the reactions to be expected for arenes. 

Similarly, chromium-complexed benzylic cations are also stabilized and organic reactions based on the benzylic cation species have been developed. For example, planar chiral o-substituted benzaldehyde dimethylacetal chromium complexes 4 were treated with 3-buten-l-ol in the presence of TiCl4 to give tet-rahydropyran derivatives with high diastereoselectivity (Eq. 5) [5]. The chromium-complexed benzylic oxonium ion 6 would be also generated and subsequent intramolecular cyclization afforded the cyclization product 7. Furthermore, the chromium-complexed benzyl alcohol derivative having electron-rich arene ring at the side chain produced tetrahydroisoquinoline skeleton by treatment with Lewis acid with stereochemical retention at the benzylic position (Eq. 6) [6]. 

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