A -Arylation complexes

Iron porphyrin complexes with axial (7-alkyl and (7-aryl groups have been prepared and fully characterized by several groups (17,18). Addition of a chemical oxidant to (19, 20), or electrochemical oxidation of (21), the low-spin iron(III)-alkyl (-aryl) porphyrins results in transient formation of an iron(IV) (7-alkyl (a-aryl) complex that undergoes reductive elimination to give the iron(II) N-substituted product as shown in Scheme 2. The iron(IV) intermediate has been directly observed by low temperature lH NMR spectroscopy (22) and spectroelectrochemistry (21). 

One-electron oxidation of phenyl iron(III) tetraarylpor-phyrin complexes with bromine in chloroform at —60°C produces deep red solutions whose H and H NMR spectra indicate that they are the corresponding iron(IV) complexes. For the low-spin aryl Fe porphyrins the electron configuration is (dxyf(dxz,dyzf, with one tt-symmetry unpaired electron, and for the low-spin aryl Fe porphyrins the electron configuration is d, yf- d, zAyzf with two TT-symmetry unpaired electrons. The aryl Fe porphyrins are thermally unstable, and upon warming convert cleanly to A-phenylporphyrin complexes of Fe by reductive elimination. This process has been investigated by electrochemical techniques, by which it was shown that the reversible (at fast scan rates) one-electron oxidation of a-aryl complexes of PFe was followed by an irreversible chemical reaction that yielded the Fe complex of the A-phenylporphyrin, which could then be oxidized reversibly by one electron to yield the Fe complex of the A-phenylporphyrin. (If the Fe complex of the N-phenylporphyrin is instead reduced by one electron, the Fe complex of the A-phenylporphyrin is formed reversibly at... 

Cyclopentadienyl-substituted vanadium(III) compound (93) is also allowed to undergo the substitution reaction to give the a-alkyl, a-alkynyl, or a-aryl complexes (94, Scheme 52). 2,132-137 [nbu4][V(C6C15)4], which is prepared... 

Pd(III) and Pt(III) Species. Only a limited number of persistent (f OM Pd(III) and PtIII species is known to date. These involve some sterically protected Pt (a-Aryl) complexes, a couple of Pd(III)-carbolides and [Pd(CN)4(Cl)2] in KCl/NaCl host lattices. All Pd -R and Pt -R alkyl complexes are highly instable, and tend to decompose via —C bond... 

VII.6.a-ARYL Complexes oe Pt(IV) eormed in Thermaland Photochemical Reactions of Aromatic Hydrocarbons WITH Pt(IV) Halide Complexes... 

The experimental data reported in the previous sections leave no doubt of the intermediate formation of aryl and alkyl platinum derivatives in the reactions of Pt(ll) and Pt(IV) chloride complexes with arenes and alkanes in aqueous solutions. In the 1980s, it was discovered that the reaction of H PtCCwith arenes in aqueous carboxylic acids produces stable a-aryl complexes of platinum(IV) which can be isolated in crystalline form (see, for example, [43]). 

Heating a solution of an aromatic compound (for example, benzene or toluene) with Na2PtCl4 in aqueous trifluoroacetic acid affords a a-aryl complex of platinum(II which is much less stable in comparison with a corresponding complex of platinum(IV). However this complex was identified after the oxidation with H2PtClg (at room temperature) to produce the latter derivative [48d] ... 

The mechanism of this process has been investigated [9]. The reaction exhibits the features of electrophilic substitution electron-releasing substituents in the aromatic nucleus accelerate the metalation. Usually a-aryl complexes of palladium(ll) are not stable and cannot be isolated. However, dialkyl sulfides stabilize the complexes which can be isolated as yellow crystals [10] ... 

The coupling of electron-neutral, electron-rich, and electron-deficient triaryl phosphates (92) has been achieved using a Ni(II)-(a-Aryl) complex/ A-heterocyclic carbenes catalyst system in dioxane at 110 °C, in the presence... 

Substitution in position 4 displays a more complex influence. Cyclization of the 4-methyl- and 4-ethyl-thiosemicarbazones of phenylpyruvic acid and of the 4-methylthiosemicarbazone of phenyl-glyoxylic acid (103) was readily achieved (104), whereas it was not possible to cyclize the analogous 4-methyl derivatives of pyruvic and glyoxylic acids. It thus appears that cyclization is hindered by substitution in position 4 and that this unfavorable effect can be partly relieved by the known favorable effect of an aryl or aralkyl group in the a-position. 

The NMR spectra of the corresponding A-aryl derivatives (168d, 169d) are fairly complex (several E/Z isomers), but it was concluded that tautomers 168 and 169 are both present. The equilibrium strongly depends on the polarity of the solvent, the concentration, the temperature, and the nature of the substituent R. 

Nucleophilic substitutions on an aromatic ring proceed by the mechanism shown in Figure 16.17. The nucleophile first adds to the electron-deficient aryl halide, forming a resonance-stabilized negatively charged intermediate called a Meisenlieimer complex. Halide ion is then eliminated in the second step. 

Aryl complexes Raubenheimer HG, Kruger GJ, Van Lombard A, Linford L, Viljonen JC (1985) Organometallics 4 275... 

A greatly enhanced chemoselective formation of phenol is observed for alkoxy(alkenyl)carbene complexes compared to alkoxy(aryl)carbene complexes. This behaviour reflects the ease of formation of the rf-vinylketene complex intermediate E starting from alkenylcarbene complexes for aryl complexes this transformation would require dearomatisation. 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

The reaction between aryl halides and cuprous cyanide is called the Rosenmund-von Braun reactionP Reactivity is in the order I > Br > Cl > F, indicating that the SnAt mechanism does not apply.Other cyanides (e.g., KCN and NaCN), do not react with aryl halides, even activated ones. However, alkali cyanides do convert aryl halides to nitrilesin dipolar aprotic solvents in the presence of Pd(II) salts or copper or nickel complexes. A nickel complex also catalyzes the reaction between aryl triflates and KCN to give aryl nitriles. Aromatic ethers ArOR have been photochemically converted to ArCN. 

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