Benzene derivatives palladium reactions

Benzoic acid and naphthoic acid are formed by the oxidative carbonylation by use of Pd(OAc)2 in AcOH. t-Bu02H and allyl chloride are used as reoxidants. Addition of phenanthroline gives a favorable effect[360], Furan and thiophene are also carbonylated selectively at the 2-position[361,362]. fndole-3-carboxylic acid is prepared by the carboxylation of 1-acetylindole using Pd(OAc)2 and peroxodisulfate (Na2S208)[362aj. Benzoic acid derivatives are obtained by the reaction of benzene derivatives with sodium palladium mal-onate in refluxing AcOH[363]. 

The main steps in the currently accepted catalytic cycle of the Heck reaction are oxidative addition, carbopalla-dation (G=G insertion), and / -hydride elimination. It is well established that both, the insertion as well as the elimination step, are m-stereospecific. Only in some cases has formal /r/ / i--elimination been observed. For example, exposure of the l,3-dibromo-4-(dihydronaphthyloxy)benzene derivative 16 and an alkene 1-R to a palladium source in the presence of a base led to a sequential intra-intermolecular twofold Heck reaction furnishing the alkenylated tetracyclic products 17 in good to excellent yields (Scheme 9). " In the rate-determining step, the base removes a proton in an antiperiplanar orientation from the benzylic palladium intermediate. The best amine base was found to be l,4-diazabicyclo[2.2.2]octane, which apparently has an optimal shape for this proton abstraction. 

During their work on the arylation of aromatic compounds by substitution, Fujiwara, et al. observed biaryl formation when aromatic compounds were placed in the presence of olefin-palladium complexes and silver nitrate.80 Developing this reaction as a method for biphenyl synthesis, these authors showed that the more stable the olefin-palladium complex was, the lower the yield. Ethylene dichloropalladium proved to be the best choice, when used with silver nitrate. However, the reaction required stoichiometric amounts of both catalysts (Scheme 10.47). Benzene derivatives substituted by electron-donating or -withdrawing groups reacted as well, but a mixture of regioisomers was produced, except for nitrobenzene, which only gave m,m -dinitrobiphenyl. 

It is well known that palladium chloride is an active catalyst for the cyclization of acetylene to form cyclobutadiene as well as benzene derivatives. In this reaction an intermediate complex was isolated which has a palladium carbon a-bond, the formation of which was explained by an insertion mechanism, not by concerted cyclotrimerization. When this complex obtained from butyne and palladium chloride was decomposed by various means, 5-vinyl-l,2,3,4,5-penta-methylcyclopentadiene and 5-(l-chlorovinyl)-l,2,3,4,5-pentamethylcyclopenta-diene were obtained in addition to hexamethylbenzene... 

C-H o-bond activation of hydrocarbons by transition metal complexes is of considerable importance in modern organometallic chemistry and catalytic chemistry by transition-metal complexes [1], because a functional group can be introduced into alkanes and aromatic compounds through C-H o-bond activation. For instance, Fujiwara and Moritani previously reported synthesis of styrene derivatives from benzene and alkene via C-H o-bond activation of benzene by palladium(ll) acetate [2]. Recently, Periana and his collaborators succeeded to activate the C-H o-bond of methane by the platinum(ll) complex in sulfuric acid to synthesize methanol [3], Both are good examples of the reaction including the C-H o-bond activation. 

Reduction of benzene derivatives carrying oxygen or nitrogen functions in ben-zylic positions is complicated by the easy hydrogenolysis of such groups, particularly over palladium catalysts. Preferential reduction of the benzene ring in these compounds is best achieved with ruthenium or rhodium catalysts, which can be used under mild conditions. For example, mandelic acid is readily converted into the cyclohexyl derivative 29 over rhodium-alumina, whereas with palladium, hydrogenolysis to phenylacetic acid is the main reaction (7.18)... 

The precursor aryl-malonamide 21 is prepared in a three-step procedure from 2,6-diethyl-toluidine. A technically feasible cross<oupling reaction has been developed for the synthesis of aryl malononitrile 20 starting from benzene derivative 19 and malononitrile. The optimized procedure with PdQ2/tricyclo-hexylphosphine and sodium tert-butoxide as base in refluxing xylene [81] was improved even further using palladium dichloride/triphenylphosphine as catalyst and sodium hydroxide as base in l-methyl-2-pyrrolidone at 125-130 °C [82]. The aryl-malononitrile 20 is hydrolyzed to the aryl-malonamide 21 in cone, sulphuric acid. 

Early in 1992, Negishi et al. reported a palladium-catalyzed cascade involving alkynes toward benzene derivatives [38], Recently, Blond et al. reported a palladium-catalyzed cascade reaction toward strained aromatic polycycles 100 [39] (Scheme 6.22). Bromoenediynes 98 undergo A-exo-dig cyclocarbopalladation and subsequent 5-exo-dig cyclization to afford the vinylpalladium intermediate 99, which possibly undergoes a 6n-electrocyclization and a syn 3-H elimination to furnish the products observed. 

Catalytic [2+2+2] cycloaddition of diyne 2.42 to allenes (Scheme 2.59, route (b) leads to cyclic condensed methylencyclohexadienes 2.169, which are often isomerized to the corresponding benzene derivatives Cycloaddition of diyne 2.42 with alkenes route (c) leads to derivatives of 1,3-cyclohexadiene 2.170, which may be used as substrates for further transformations [4]. Different transition metals M such as ruthenium, palladium, rhodium, iridium, nickel, and cobalt were used as catalysts for this reaction. 

The chiral siloxycyclopropane 106 undergoes carbonylative homocoupling to form the 4-ketopimelate derivative 108 via the palladium homoenolate 107 without racemization. The reaction is catalytic in CHCI3, but stoichiometric in benzene[93]. 

The bromine atoms in 2,5-dibromo-l,3,4-thiadiazole 54 undergo a palladium-catalyzed Stille reaction with the organostannyl derivative 55 (Equation 7) <1998CEJ2211>. The thiadiazole 54 was co-polymerized with diethynyl benzene 56 (Equation 8) and diethynyl pyrrole in a Sonogashira cross-coupling reaction <2005MM4687>. 

Benzyne, which is generated in situ from 2-(trimethylsilyl)phenyl triflate and KF, acts as an alkyne congener in distannation in the presence of palladium-/ r/-alkyl isocyanide complex.157 A variety of substituted benzyne derivatives inserts into the Sn-Sn bond to give l,2-bis(stannyl)benzenes (Equation (59)). The reaction fails to occur in the presence of other palladium catalysts such as Pd(PPh3)4. 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

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