Arene-transfer reagents

Yordanov, A. T. Roundhill, D. M. Extraction of platinum from aqueous solution into chloroform using a 2-pyridylthio-N-oxide derivatized calix[4]arene as phase transfer reagent. Inorg. Chim. Acta 1997, 264, 309-311. 

The synthesis of enantiopure cyclohexane derivatives has been investigated by hydrogenating arenes by use of chiral auxiliaries bound either to the support of the catalyst or to the substrate, or by use of chiral phase-transfer reagents [15]. Although significant progress has been reported, enantioselectivity is still moderate (maximum 68 % e. e.). 

Epoxides can be reduced under Birch conditions by solvated electrons [34] and by arene radical anions [35] without the presence of low-valent metal complexes. In both cases )5-lithiumoxy organolithium compounds are formed after further reduction with a second equivalent of the electron transfer reagent. These species are stable enough to be trapped by electrophiles at low temperatures. They do not show the typical reactivity patterns of radicals. Thus, these transformations will not be dealt with here in detail. 

Polystyrene-supported diaryliodonium salts 33 can be prepared by the reaction of diacetate 4 with arenes in the presence of sulfuric acid (Scheme 5.19) [55,56]. Polymer-supported aryliodonium salts are useful aryl transfer reagents in Pd(II)-catalyzed cross-coupling reactions with salicylaldehydes [56] and aromatic hydrocarbons [55]. 

In an effort to move away from precious metal catalysts, various reports in recent years have focused on the use of first-row metal catalysts for direct arylations [57-60]. As a representative example of these new developments, we illustrate in Scheme 23.15 the chelate-assisted ortho-C-H arylation of arenes with Fe catalysts [61]. With iron being cheap, nontoxic, and ubiquitous, this protocol is highly attractive for pharmaceutical syntheses. Using the catalyst precursor Fe(acac)j in conjunction with bidentate pyridine ligands, Zn-aryl reagents as aryl transfer reagents and 1,2-dichloroisobutane as the oxidant, excellent yields of the arylated product were obtained. An interesting feature of this reaction is the hydrolysis of the imine moiety after work-up. The reaction conditions tolerate additional functionalities such as cyanides, chlorides, triflates, tosylates, and thiophenes. 

Organozinc complexes have been used for a long time as carbene transfer reagents on double bonds (Simmons-Smith eyelopropanation) and arene rings which leads to their extension to eyelohexatrienes. 

The symmetric series provides functional cyclohexadienes, whereas the non-symmetric one serves to build deuterated and/or functional arenes and tentacled compounds. In both series, several oxidation states can be used as precursors and provide different types of activation. The complexes bearing a number of valence, electrons over 18 react primarily by electron-transfer (ET). The ability of the sandwich structure to stabilize several oxidation states [21] also allows us to use them as ET reagents in stoichiometric and catalytic ET processes [18, 21, 22]. The last well-developed type of reactions is the nucleophilic substitution of one or two chlorine atoms in the FeCp+ complexes of mono- and o-dichlorobenzene. This chemistry is at least as rich as with the Cr(CO)3 activating group and more facile since FeCp+ activator is stronger than Cr(CO) 3. 

This hydroxylation-induced intramolecular migration, known as the NIH shift, was explained by the involvement of arene oxides formed by the attack of electrophilic oxoiron(V) porphyrin on the aromatic ring.753 Intermediate 98 was also suggested to be formed in hydroxylation by the Fenton and related reagents in aprotic media after initial oxidation with an oxoiron(V) complex followed by electron transfer.744 754... 

Aryl-A3-iodanes bearing an electron-deficient alkyl ligand such as aryl(sul-fonylmethyl)-A3-iodanes (Section 3.2.7) and aryl(perfluoroalkyl)-A3-iodanes are relatively stable. A series of (perfluoroalkyl)phenyl-A3-iodanes 96 were synthesized in good yields by treating bis(trifluoroacetoxy)-A3-iodanes with benzene in the presence of triflic acid [47]. The AModanes 96 transfer the perfluoroalkyl groups to a variety of nucleophiles with reductive elimination of iodobenzene. The nucleophiles involve Grignard reagents, alkyllithiums, enolate anions, alkenes, alkynes, trimethylsilyl enol ethers, arenes, phenols, and thiols. In these reactions, the AModane 96 serves as a source of the perfluoroalkyl cation and, in... 

Here, either X = (I T2C(0)-R j and ox = Jones reagent or X — CN and ox = CeIv. It has been emphasized that the addition of nucleophiles to (arene)Mn(CO)3+ complexes does not occur through an initial ET from the nucleophile to the metal center [2]. This represents an additional advantage since such redox reactions frequently lead to the decomposition of the metal complex, a typical example being the reductive deligation of bis(arene)Fe2+ complexes [48]. On the other hand, intramolecular charge transfer from the arene to the metal not only induces an electron deficiency in the arene ring (which is critical for effective attack of the nucleophile), but it also results in an attenuation of the electrophilicity of the metal center so as to avoid undesired ET reactions of the metal with the nucleophile. 

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