Aromatics alkenylation

Reductive coupling of acrylates 521 and imines in the presence of iridium provides azetidin-2-ones bearing aromatic, alkenyl, and alkynyl side chains, with high diastereoselection (Equation 213) <2002OL2537>. The reaction has been proposed to occur through a reductive Mannich addition-cyclization mechanism. 

A species such as (8), formed via transmetallation, often undergoes reductive elimination to complete a very useful coupling reaction. Thus, magnesium,boron, zinc and tin are effective. The major asset of this method is the complete retention of the configuration in the alkene, resulting in products of exceptional isomeric purity. The halides involved include aromatic, alkenyl and allenyl. Several reviews of this method have been published. " ... 

The Peterson reactions of a-sUylbenzyl carbamates provide a method for the preparation of aromatic alkenyl carbamates. Addition of carbonyl compounds to the carbanions derived from the a-sUylbenzyl carbamates 64 with Bu Li leads directly to the corresponding alkenyl carbamates 65 with Z-selectivity (Scheme 2.41)... 

Transition-metal-catalyzed direct arylation of arenes with aryl halides via cleavage of aryl C—H bonds serves as a straightforward method for aryl-aryl bond formation [3], Hence, direct arylation of two aromatic rings tethered by aromatic/alkenyl moieties or heteroatoms (i.e. intramolecular variant of the direct arylation) constitute efficient syntheses of PAHs such as benzofurans, carbazoles, dibenzothiophenes, dibenzosiloles, and phenanthrenes. In this section we overview the progress with subsections categorized by product structures [5]. 

Recent studies by laser-flash photolysis showed that vinyl cations are formed quite rapidly after excitation of alkenyl halides, suggesting that photolysis of aromatic alkenyl halides induces both heterolysis and homolysis of the C-X bond. The synthetically interesting and useful characteristics of these processes are ... 

Ionic reactions occur preferentially when aliphatic alkenyl iodides and aromatic alkenyl chlorides and bromides are irradiated. The trapping of the vinyl cations generated by irradiation readily takes place in nucleophihc solvents such as alcohols. In these cases, enol ethers or acetals are the main products (Equation 11.8 and Equation 11.9).3.20 4,30... 

Figure 3.2 General structures of the common carbonyl-based functional groups. The symbols R, R, R" and R" represent any aliphatic, aromatic, alkenyl or alkynyl group.

The reactions of the second class are carried out by the reaction of oxidized forms[l] of alkenes and aromatic compounds (typically their halides) with Pd(0) complexes, and the reactions proceed catalytically. The oxidative addition of alkenyl and aryl halides to Pd(0) generates Pd(II)—C a-hondi (27 and 28), which undergo several further transformations. 

The transmetallation of various organometallic compounds (Hg, Tl, Sn, B, Si, etc.) with Pd(II) generates the reactive cr-aryl, alkenyl, and alkyl Pd compounds. These carbopalladation products can be used without isolation for further reactions. Pd(II) and Hg(II) salts have similar reactivity toward alkenes and aromatic compounds, but Hg(II) salts form stable mercuration products with alkenes and aromatic rings. The mercuration products are isolated and handled easily. On the other hand, the corresponding palladation products are too reactive to be isolated. The stable mercuration products can be used for various reactions based on facile transmetallation with Pd(II) salts to generate the very reactive palladation products 399 and 400 in rim[364,365]. 

Formation of carboxylic acids ami their derivatives. Aryl and alkenyl halides undergo Pd-catalyzed carbonylation under mild conditions, offering useful synthetic methods for carbonyl compounds. The facile CO insertion into aryl- or alkenylpalladium complexes, followed by the nucleophilic attack of alcohol or water affords esters or carboxylic acids. Aromatic and a,/ -unsaturated carboxylic acids or esters are prepared by the carbonylation of aryl and alkenyl halides in water or alcohols[30l-305]. 

Hydrogenolysis of aryl and alkenyl halides and triflates proceeds by the treatment with various hydride sources. The reaction can be explained by the transmetallation with hydride to form palladium hydride, which undergoes reductive elimination. Several boro hydrides are used for this purpose[680], Deuteration of aromatic rings is possible by the reaction of aryl chlorides with NaBD4681]. 

Tetrahalobenzynes, however, react with a variety of aromatic compounds to afford tetrahalobenzobarrelene derivatives in good yields, frequently in the range of 55 to 75%. The dehalogenation of a variety of alkenyl chlorides with alkali metals in tetrahydrofu-ran containing tert-butyl alcohol suggested this approach to the dechlorination of tetrachlorobenzobarrelenes. 

The addition of a 2-methyl-2-penten-4-yl radical to the QDI (based on p-phenylene diamines [PPDs] thus producing the corresponding PPD radical) is highly exothermic. The reaction not only stabilizes the relatively unstable alkenyl radical, but also results in the aromatization of the diimino-cylcohexadienyl ring. The enthalpy of reaction for this reaction is calculated (using MOPAC/AMl Hamiltonian ) to be about —40 kcal/mol. 

This section will describe the Friedel-Crafts alkylation reactions of aromatic hydrocarbons with alkenylchlorosilanes containing short chain alkenyl groups such as allyl and vinyl. The reaction will be discussed in terms of the substituent effect on silicon and the arene rings. 

Chloroacylation of terminal aryl, alkyl or alkenyl alkynes [Le. the addition of RC(=0)-C1 across the CC triple bond] with aromatic acyl chlorides was catalysed by [IrCl(cod)(lPr)] (5 mol%) in good conversions (70-94%) in toluene (90°C, 20 h). Z-addition products were observed only, hitemal alkynes were umeactive. Surprisingly, a phosphine/[lr(p-Cl)(l,5-cod)]2 system under the same conditions provides decarbonylation products (Scheme 2.34) [117]. 

Subshtuted 3-alken-l-ynes can be hydroaminated with primary or secondary aliphahc or aromatic amines at the alkynyl sites or at the alkynyl and at the alkenyl sites in the presence of Hg(ll) salts. However, the reachon is essentially stoichiometric in nature, even if the mercury compound can be recycled without apparent loss of achvity [262-264]. 

A second group of aromatic substitution reactions involves aryl diazonium ions. As for electrophilic aromatic substitution, many of the reactions of aromatic diazonium ions date to the nineteenth century. There have continued to be methodological developments for substitution reactions of diazonium intermediates. These reactions provide routes to aryl halides, cyanides, and azides, phenols, and in some cases to alkenyl derivatives. 

Electrochemically generated nickel is very selective for the reduction of aromatic nitro compounds into anilines, in which alkenyl, alkynyl, halo, cyano, formyl, and benzyloxy groups are not affected.84 Sodium sulfide has been used for the selective reduction of aromatic nitro group in the presence of aliphatic nitro groups (Eq. 6.44).85... 

Selective cyclization of an alkenyl imine is catalyzed by trimethylsilyl triflate (Scheme 76).329 /-Butyldimethylsilyl triflate ( BuN SiOTf) catalyzes imino Diels-Alder reactions of TV-phenyl-aromatic aldimines to afford exo adducts preferentially.330 When A1C13 is used instead of Bufv SiOTf, endo adducts are obtained predominantly. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

Feldman and Eastman have suggested that the kinamycins may by reductively activated to form reactive vinyl radical (25) and orf/to-quinone methide (26) intermediates (Scheme 3.2c) [16]. The authors provided convincing evidence that the alkenyl radical 25 is generated when the model substrate dimethyl prekinamycin (24) is exposed to reducing conditions (tri-n-butyltin hydride, AIBN). Products that may arise from addition of this radical (25) to aromatic solvents (benzene, anisole, and benzonitrile) were isolated. The ort/io-quinone methide 26 was also formed,... 

Aromatic imines can be ortho-arylated and alkenylated using ruthenium catalysis (Equation (129)).1... 

Nitrogen functionality also assists the alkylation of ortho-Cr-H bonds of aromatics, as shown in Equations (10)—(12). In the case of aromatic imines, Ru3(GO)i2 exhibits a high catalytic activity.8-10 This reaction gives the alkylation product together with the alkenylation product in the reaction with triethoxyvinylsilane. Rhodium catalysts show the same activity to give the alkylation product.11,12,12a For example, the Rh(i)-catalyzed reaction of the imine of aromatic ketones with methyl acrylate... 

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