Arenes synthetic applications

The synthetic applications 440) and mechanistic aspects 4411 of intermolecular photocycloaddition reactions of arenes to olefins have been reviewed recently. Intramolecular cycloadditions442a,b) have been studied in the context of the photochemical behaviour of bichromophoric molecules, as to investigate interchromophoric interactions in polyfunctional molecules. Three types of addition products can be formed in the photocycloaddition of benzene to an alkene (4.37)441. ... 

Several chapters deal with the synthesis of and the synthetic applications of organolithium compounds such as orthometallation, arene catalysed lithiation, addition to carbon-carbon double bonds, their reaction with oxiranes, and asymmetric deprotonation with lithium (-)-sparteine. We gratefully acknowledge the contributions of ah the authors of these chapters. 

The synthetic application of vicarious nucleophilic substitution, whereby hydrogen of an electrophilic arene is replaced by an a-functionalized alkyl substituent, has been reviewed 177 the sequence usually involves attack on a nitroalkene by a carbanion containing a leaving group X at the carbanionic centre, /i-elimination of HX from the er-adduct, and rearomatization on subsequent protonation. 

Heterostannenes, preparation and characteristics, 3, 870 Heterosubstituted arenes, metallation, 9, 17 Hexaaryldiplumbanes, preparation, 3, 887 Hexabutyldistannane, preparation, 3, 856 Hexacarbenes, in cobalt(III) complexes, 7, 19 Hexacarbonyl complexes, with molybdenum kinetics and reactivity, 5, 395 photochemistry, 5, 393 solid-support studies, 5, 394 spectroscopic and theoretical studies, 5, 392 Hexadentate iV-substituted triazacyclononane, synthetic applications, 1, 70... 

Complexes of Cr, W, Mo, Fe, Ru, V, Mn and Rh form stable, isolable arene if -complexes. Among them, arene complexes of Cr(CO)3 have high synthetic uses. When benzene is refluxed with Cr(CO)6 in a mixture of dibutyl ether and THF, three coordinated CO molecules are displaced with six-7r-electrons of benzene to form the stable i/fi-benzene chromium tricarbonyl complex (170) which satisfies the 18-electron rule (6 from benzene + 6 from Cr(0) + 6 from 3 CO = 18). Complex formation is facilitated by electron-donating groups on benzene, and no complex of nitrobenzene is formed. Complex formation has a profound effect on reactivity of arenes, and the resulting complexes are used in synthetic reactions. The metal-free reaction products can be isolated easily after decomplexation by mild oxidation using low-valent Cr. Cycloheptatriene also forms a stable complex with Cr(CO)3 and its synthetic applications are discussed below. 

M. Yus, F. Foubelo, Reductive Opening of Saturated Oxa-, Aza- and Thia-Cycles by Means of an Arene-Promoted Lithiation Synthetic Applications, Rev. Heteroatom Chem. 1997, 17, 73-108. 

Akira Suzuki, from Kurashiki University, Japan, is one of the great names of arene chemistry due to his outstanding reaction that bears his name. As expected, his chapter (No. 3) deals with the Suzuki reaction with arylboron compounds and its synthetic applications in arene chemistry. The Suzuki reaction is now becoming all the more important as requirements for organic synthesis will necessarily have to take ecological aspects ( green chemistry ) into account and use of a number of toxic metals is phased out. 

Ipatieff and coworkers carried out the first alkylation with alkenes and branched and normal chain alkanes (except methane and ethane) in the presence of AlCb as the catalyst. The sulfuric acid catalyzed alkylation reaction of arenes and isoalkanes, developed in 1938, is a still widely used industrial process to produce alkylates with high octane numbers. For synthetic applications, however, Friedel-Crafts-type alkylations of alkenes and alkanes have limited value since they tend to give mixtures of products, including oligomers of alkenes. ... 

Arene-alkene photocycloaddition reactions have been reviewed in detail. This review includes a tabular survey, synthetic applications and exhaustive bibliography. Many of the reactions considered in this review were however performed to determine answers to mechanistic questions, therefore they are not necessarily optimized and chemical yields are not indicated. Tables 2 and 3 bring together, from the references cited in the review , those intermolecular photochemical meta cycloadditions which are principally synthetic methods. More concise general treatments of this reaction have since been published, as have other papers mainly concerned with its mechanistic aspects. """ ... 

Synthetic applications of the photochemistry of transition-metal arene complexes are rare, except for the (h -arene)M(CO)j (M = Cr, Mo, W) complexes where photo-substitution of ligands for CO occurs. These complexes also undergo arene exchange upon photolysis ... 

Finally, steric effects have been observed to play a significant role in controlling the dia-stereoselectivity of the arene-alkene cycloadditions. Elaboration of this aspect of the reaction again rests on an exciplex model and will be provided in the context of synthetic applications where it has been most frequently encountered. 

The Fries rearrangement of phenyl- and 1-naphthyl esters can be efficiently performed in the presence of hafnium triflate. The method is based on the one previously described for the Friedel-Crafts acylation of arenes with acyl chlorides. The reaction occurs in toluene-nitromethane mixtures at 100°C for 6 h. Several examples of this Fries isomerization with synthetic application are reported in Table 5.5. In all cases, complete regioselectivity is obtained, and 2-acylated phenol or naphthol derivatives are isolated in good yields. 

One of the first eye-catching synthetic applications of arene-chromium chemistry was the synthesis of the sp/ro-sesquiterpenes ( )-acorenone and ( )-acorenone B (rac-7) disclosed by Semmelhack and Yamashita in 1980 [14]. These authors twice exploited the meta-selective nucleophile addition to anisole-Cr(CO)3 derivatives (Scheme 1). Starting from complex rac-1, such a reaction is first used for the regioselective introduction of an acyl sidechain to give 2 after oxidative workup. A few steps later, the nitrile rac-4 (obtained from rac-3 by complexation and separation of the diastereomeric products by preparative HPLC) is deprotonated to form the spiro addition product rac-5, from which the enone rac-6 is obtained after protonation and hydrolysis of the initially formed dienol ether. The final conversion of rac-6 into acorenone B (rac-7) efficiently proceeds over five steps and involves a diastereoselective hydrogenation of an exo-methylene group. 

The oxidation of phenols or o- and p-hydroquinones with stoichiometric [bis(acyloxy)iodo]arenes to the corresponding benzoquinones is one of the most typical synthetic applications of hypervalent iodine reagents (Section 3.1.11). The catalytic version of this reaction was first reported by Yakuraand Konishi in 2007 [52], The reaction of p-alkoxyphenols 50 with a catalytic amount of 4-iodophenoxyacetic acid in the presence of Oxone as the terminal oxidant in aqueous acetonitrile at room temperature affords p-quinones 51 in high yields (Scheme 4.26) [52]. 4-Iodophenoxyacetic acid is a readily available and water-soluble aromatic iodide that has a particularly high catalytic activity in this reaction. 

Polymer-supported (diacetoxyiodo)arenes have found broad synthetic application. Poly[ (diacetoxyiodo)styrene] (4) has been used for the oxidation of alcohols catalyzed by TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) [27-30], or in the presence of KBr [31]. In a specific example, primary alcohols are readily oxidized to methyl esters upon treatment with reagent 4 in the presence of KBr in the acidic aqueous methanol solution (Scheme 5.5) [31]. Likewise, organic sulfides are selectively oxidized to the respective sulfoxides by reagent 4 in water in the presence of KBr [32]. 

Although the reactions of ir-arene complexes are stoichiometric, not catalytic, ttiis addition chemistry has been used in a variety of synttietic applications. Considering the low cost of many of the metals involved in this chemistry, the labor involved in the preparation of substrates used in the synthetic applications, the small scale of most modem syntheses of biologically active materials, and the short syntheses created by the chemistry of these compounds, greater consideration of the synthetic value of these transformations should be given. 

Two of the major goals of alkane functionalization have been the selective conversion of methane to methanol and the replacement of strong terminal alkyl C-H bonds with a functional group. These goals have also included the selective functionalization of a single aliphatic or aromatic C-H bond, directed by either a substituent attached to the arene or controlled by the particular steric or electronic properties of the substrate. Thus, many of the principles that have been developed for the functionalization of small molecules with the goal of producing chemical feedstocks have now been adopted for more intricate synthetic applications. 

Scheme 17.25 C-H a7lation of pyridine N-oxides and N-aminopyridiniums with arenes and synthetic applications.

Synthetic applications of [4+2] photocycloadditions have not been extensively developed. It has been observed, however, that intramolecular photocycloaddition of arenes and allenes proceeds preferentially via the [4+2] reaction mode. This process appears to be reasonably general across a diverse range of allenes attached to aromatic aldehydes and ketones with a variety of tethers. Thns, the protected aniline 36 undergoes para-cycloaddition to give the intriguing bridged polycyclic product 37 (Scheme 15.13) [34]. Mechanistic details of this and related transformations have not yet been reported. 

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