Arenes with side chains

In addition to the trivial names already mentioned, a number of more specific group names for arenes with side chains are also retained. 

An even more unusual situation was observed with 3-pentylpyrogaUol[4] arene. These side chains interact in a gear-like fashion and form a surface that is highly ordered across the aqueous phase. The surface behavior of the 3-pentylpyrogallol[4]arenes exhibits both ion transport and a remarkable nanotube structure. These are discussed in Section 8. 

In 1980 Sonogashira reported a convenient synthesis of ethynylarenes - the Pd-catalyzed cross-coupfing of bromo- or iodoarenes with trimethylsilylacetylene followed by protiodesilylation in basic solution [15]. Prior to this discovery, formation of terminal acetylenes required manipulation of a preformed, two-carbon side chain via methods that include halogenation/dehydrohalogenation of vinyl- and acetylarenes, dehalogenation of /1,/1-dihaloalkenes, and the Vils-meier procedure [ 14]. With the ready availability of trialkylsilylacetylenes, the two-step Sonogashira sequence has become the cornerstone reaction for the construction of virtually all ethynylated arenes used in PAM and PDM synthesis (vide infra). 

Benzene derivatives with saturated or unsaturated C-containing side chains are arenes. Examples are cumene or isopropylbenzene, C(,H CH(CH,)2, and styrene or phenylethene, Cf,H,CH=CH2. 

An arene ruthenium complex with polymerizable side chains for the... 

AN ARENE RUTHENIUM COMPLEX WITH POLYMERIZABLE SIDE CHAINS FOR THE SYNTHESIS OF IMMOBILIZED CATALYSTS... 

These experimental observations were explained by invoking the formation of the most stable benzylic carbocation that then undergoes rearrangement via an intermediate phenonium ion72 (Scheme 4.5). This mechanism accounts for cc-p isotope scrambling, and the low amount of isopropylbenzene is in harmony with the involvement of the primary cation. Other observations, in turn, suggest the involvement of the n complex of the arene and the side-chain carbocation.73-75... 

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... 

The electronic effect of perfluoroalkyl substituents on the absorption spectra of arenes is relatively small (e.g., Xmax 3-perfluorooctylthiophe-ne = 229 nm 3-octylthiophene = 235 nm). Thus, the anomalously low absorption maximum of 504 is the effect of twisting around the backbone. It is apparent that the difference in size of the side chains is sufficiently large to cause twisting of the conjugated backbone of 504 due to steric interactions between the perfluoroalkyl substituents and the adjacent repeat unit. 504 exhibits green fluorescence (Xmax = 512 nm) in solution with a maximum blue shifted by 58 nm relative to POT (570 nm). Accordingly, 504 shows a Stokes shift of ca. 1.4 eV (186 nm) compared to only 0.6 eV (126 nm) for POT. 

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. 

Further functionalizations are obtained via the electron transfer— radical cation fragmentation pathway a typical example is side-chain nitration by irradiation of methyaromatics with tetranitromethane. Aromatics form charge-transfer complexes with C(N02)4 irradiation leads to electron transfer and fragmentation of the C(N02)4 radical anion to yield the triad [Ar + C(NO)J N02], followed by combination between the arene radical cation and the trinitromethanide anion. Thus, cyclohexadienes are formed that generally eliminate and rearomatize at room temperature yielding ring-functionalized products [234] (Sch. 21). 

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