Alkenes, arylation, also

Insertion of aUcynes into aromatic C-H bonds has been achieved by iridium complexes. Shibata and coworkers found that the cationic complex [Ir(COD)2]BF4 catalyzes the hydroarylation of internal alkynes with aryl ketones in the presence of BINAP (24) [111]. The reaction selectively produces ort/to-substituted alkenated-aryl products. Styrene and norbomene were also found to undergo hydroarylation under similar condition. [Cp IrCl2]2 catalyzes aromatization of benzoic acid with two equivalents of internal alkyne to form naphthalene derivatives via decarboxylation in the presence of Ag2C03 as an oxidant (25) [112]. 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

Photocycloaddition reactions of alkyl and aryl 2-thioxo-3//-benzoxazole-3-carboxylates 142 to alkenes afforded stable isolable spirocyclic aminothietanes 143 <02HCA2383> similar reactions with both electron-poor and electron-rich alkenes were also performed on 2-methyloxazolo[5,4-h]pyridine <02EJO4211>. 

This chapter surveys the reduction of saturated alkyl halides to alkanes. Reductive -eliminations of vicinal dihalides to alkenes are also described briefly. Reduction of vinyl and aryl halides is covered in this volume. Chapter 4.5 hydrogenolysis of allyl and benzyl halides is covered in this volume. Chapter 4.7, and reduction of a-halo-substituted carbonyl compounds CX—CO to carbonyl compounds CH—CO is covered in this volume. Chapter 4.8. 

Arylation of alkenes can also be achieved by treatment with an arylpalla-dium reagent, typical generated in situ from an aryl halide or other suitably... 

The Rh(I)/PCy3 catalytic system that was found to be optimal for the C-H addition of heterocycles to alkenes was also identified as an ideal first-generation catalyst for the arylation of nitrogen heterocycles with iodoarenes in the presence of a tertiary amine base. Under typical reaction conditions, the heterocycle (1 equiv.), Arl (2 equiv.), Et3N (4 equiv.), [Rh(coe)2Cl]2 (0.05 equiv.), and PCy3 (0.4 equiv.) are heated (105-150 °C) in THF ( 0.11 M in heterocycle) for 6-18 h. Moderate to good chemical yields (30-78%) are obtained. The reaction is stereoselective for the position that is amenable to NHC formation in all cases (e.g., the 2-position in benzimidazole). Higher yields were obtained with more electron rich iodoarenes, but only a limited number of examples (2) with iodoarenes other than iodobenzene were presented (Scheme 14). 

Additions to Cyclopentenones and Related Systems. (2 + 2)-Cycloadditions are reported following the irradiation of mixtures of alkyl and aryl 2-thioxo-3/f-benzoxazole-3-carboxylates with alkenes. Cycloaddition also occurs to the CS double bond. The photochemical additions of arylalkenes to 3-phenylcyclo-pentenone and 3-phenyl cyclohexenone have been studied. The regio- and stereochemistry observed in the additions has been rationalized in terms of the stability of the intermediate biradicals. Photocycloaddition of allene to the cyclopentenone derivative (6) in methylene chloride solution at — 78°C affords... 

Palladium-catalyzed reductive arylations of alkenes have also been applied to intramolecular additions70 75 77. 

C-H borylation is a widely used methodology for the synthesis of organoboronates [63-65]. Most of the applications have been presented for the synthesis of aryl-boronates. However, functionalization of alkenes has also attracted much interest [66, 67]. In most applications, iridium catalysis was used. However, in case of alkenes, borohydride forms as a side product of the C-H borylation, which undergoes hydroboration with alkenes. This side reaction can be avoided using palladium catalysis under oxidative conditions. In a practically useful implementation of this reaction, pincer-complex catalysis (Ig) was appHed (Figure 4.17) [51]. The reaction can be carried out under mild reaction conditions at room temperature using the neat aUcene 34 as solvent. In this reaction, hypervalent iodine 36, the TFA analog of 29, was employed. In the absence of 36, borylation reaction did not occur. 

The situation is problematic when considering less reactive aryl chlorides or deactivated aryl bromides involved in the rate-determining oxidative addition, since the alkene will also contribute to decelerate the slow oxidative addition by complexation of the reactive Pd°L2(OAc) (Scheme 1.16). To solve this problem, one has to design a new ligand which will make the Pd(0) more reactive or introduce the alkene via a syringe pump, so that a low alkene concentration can be maintained throughout the catalytic reaction. 

P,C-palladacycle precursors and ligated to the monophosphine 11 (Scheme 1.44). Kinetic data are still missing for most steps of the catalytic cycle [63]. The oxidative addition of aryl bromides is probably not rate determining, since the rate of the overall reaction is highly dependent on the structure of the alkene, as evidenced by competitive reactions of two different alkenes with the same aryl bromide in the presence of 5 [55a]. However, the alkene might also favour the reductive elimination in the palladacycle 5, which slowly delivers the active Pd(0) complex into the catalytic cycle [60]. 

The arene substrates are not limited to simple benzene derivatives. A variety of het-eroarenes can also participate in alkene arylations to generate the desired coupling products. Stoichiometric oxidative coupling of aromatic heterocycles such as furan, thiophene, selenophene, A-methylpyrrole, benzofiiran and benzothiophene with a variety of alkenes, including acrylonitrile, styrene and methyl acrylate, have been extensively studied by Fu-jiwara and coworkers [8]. Furan, thiophene, selenophene and A-methylpyrrole are easily alkenylated with alkenes to give 2-alkenylated and 2,5-dialkenylated heterocycles in relatively low yields (3 6%) [8a], while the reactions of benzofuran and benzothiophene with alkenes produced a mixture of 2- and 3-alkenylated products [8b]. 

Similar palladium-catalyzed cascade arylations also occur with acyclic alkenes, including a,p-unsaturated sulfones (Equation 19.153), sulfonamides, phosphine oxides, and phosphonate esters. In contrast, typical conjugated olefins, such as a, 3-unsaturated esters and enones almost exclusively react to form products from Heck reactions. Direct arylations have also been conducted with disubstituted alkynes containing a terminal arene and a large group, such as an aryl or ferf-butyl group (Equation 19.154). ... 

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