Reactions of Arene Complexes

Sigma- and 7t-arene complexes of transition metals are useful catalysts, reagents, and intermediates in organic synthesis. Catalytic applications of these complexes have been reviewed (Andretta et ai, 1970 Harmon et al., 1973 Dolcetti and Hoffman, 1974 James, 1973). This chapter will emphasize the stoichiometric reactions of arene complexes. 

Some of the reactions of arene complexes, notably substitution of the ring, are described in Chapter 8. 

Fagnou and co-workers reported on the use of a palladium source in the presence of different phosphine ligands for the intramolecular direct arylation reaction of arenes with bromides [56]. Later, they discovered that new conditions employing palladium complex 27 promoted the direct arylation of a broad range of aryl chlorides to form six- and five-membered ring biaryls including different functionalities as ether, amine, amide and alkyl (Scheme 7.11) [57]. 

In the palladium-catalyzed coupling reactions of arenes with alkenes, the cr-arylpalladium complexes react with CO to give aromatic acids in AcOH, as shown in Scheme u 97>97a 97c This carboxylation reaction of arenes with CO proceeds catalytically with respect to Pd at room temperature under atmospheric pressure of CO, when K2S2O8 is added as an oxidant and TFA is employed as a solvent. 

The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Because of the instability of hydroxy-substituted arene chromium tricarbonyl complexes, yields of the latter tend to be low. High yields of arene complexes can, however, be obtained by in situ silylation of the crude product of the benzannulation reaction [336]. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. Treatment of the benzannulation products with phosphines also leads to decomplexed phenols [272]. 

TABLE I Reaction of Arenes with 4 g of Cr(CO)6 to Give the Corresponding Complexes (ij6-Arene)Cr(CO)3a... 

Rich and co-workers employed the intramolecular SNAr reaction of arene-ruthenium n-complexes in their total syntheses of K-13 (9) and OF4949-III (7). Treatment of tripeptide complex 27 with potassium ferf-butoxide gave 17-membered macrocycle 28, which was demetalated in acetonitrile to give cycloisodityrosine 29, a precursor to OF4949-III (7) (Scheme 8). 34 ... 

It is also likely that radical cation formation occurs in reactions of very reactive arenes with Pd(II) (see Section II.B.3.b), which would also lead to biaryl formation. That the reactions of arenes with Pd(II) compounds are far from simple is illustrated by the work of Arzoumanidis and Rauch.573,574 In the reaction of Pd(02CCF3)2 with benzene or naphthalene in TFA, a variety of polynuclear complexes, containing both Pd(I) and Pd(II) and arenes, were isolated574 in addition to the usual biaryls. 

The benzannulation affords arene-Cr(CO)3 complexes possessing a plane of chirality resulting from the unsymmetrical arene substitution pattern. This aspect is relevant to stereoselective synthesis, in which enantiopure arene tricarbonyl chromium complexes play a major role [56]. The benzannulation reaction avoids both harsh conditions incompatible with the retention of chiral information and the cumbersome separation of enantiomers, and is thus attractive for the diastereo- and enantioselective synthesis of arene complexes [17b, 57]. 

Synthetically useful reactions of (arene)Mn(CO)3+ complexes with such nucleophiles as methoxide, benzenethiolate, azide, various amines, and anilines are also noteworthy. These complexes have also been extensively studied with regard to nucleophilic addition reactions resulting in thermally stable cyclohexadienyl complexes [2]. 

CT Complexes as Critical Intermediates in Donor/Acceptor Reactions of Arenes... 

The reaction of arenes with aryllead tricarboxylates performed in trifluoroacetic acid, affording biarenes, takes place via a cationic 7t-complex.162 Since azulenes form 7r-donor/acceptor complexes with various 7r-acids, the arylation of 4,6,8-trimethylazulene 108 was attempted with />-methoxyphenyllead triacetate 1 (Equation (135)).163 Only one isomer of 1-arylazulene 109 was formed although in a modest 27% isolated yield. Based on recovered unreacted azulene, the effective yield was 43%. A dimer 110, 3,3 -dianisyl-l,l -biazulene (4% yield), was suggested to result from the one-electron oxidation of the intermediate 4-methoxyphenyl cation in the 7t-complex. 

More recently, cationic intermediates have been observed in the Heck reactions of arene diazonium salts catalyzed by triolefinic macrocycle Pd(0) complexes [17,59], o-iodophenols and enoates to form new lactones [60], and o-iodophenols with olefins (the oxa-Heck reaction) [61 ]. In the first case ions were formed by oxidation of the analyte at the capillary, or by association of [NH4] or Na". In the two other cases ionization occurred through the more typical loss of a halide ligand. The oxa-Heck reaction provides a good example of how these experiments are typically performed and the type of information that can be obtained. The oxyarylations of olefins were performed in acetone, catalyzed by palladium, and required the presence of sodium carbonate as base. Samples from the reaction mixtures were diluted with acetonitrile and analyzed by ESI(+)-MS. Loss of iodide after oxidative addition of o-iodophenol to palladium afforded positively-charged intermediates. Species consistent with oxidative addition, such as [Pd(PPh3)2(C6H50)], and the formation of palladacycles of the type seen in Scheme 8 were observed. Based on this, a mechanism for the reaction was proposed (Scheme 8). 

In an interesting report, 1,2-dibromocyclohexane cleaves the Hg—Sn bond in Hg[Sn(CH2(Bu-f)3)] to form Hg, BrSn(CH2(Bu-f)3) and cyclohexene. Also noteworthy is the formation of carbene complexes (7 -arene)(CO)2Cr CR2 formed in the substitution/elimination reactions of stannyl complexes K[( -arene)(CO)2CrSnPh3] with activated C—X bond in organic dihalogeno derivatives R2CX2 or ionic halides [R2CX]Y . 

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