Aryl exchange reactions

Finally, the kinetics of the one-aryl exchange reaction, equilibrium (289)... 

Alkyl and aryl exchange reactions between trialkyl alanes and trialkyl or triaryl boranes have been very fully investigated (188). The exchange of ligands, which occurs very rapidly even at room temperature, occurs through the association products like ... 

SCHEME 4.188 Synthesis of phospholes through an aryl-aryl exchange reaction [316]. 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

Yields in the above reactions can often be improved by the addition of 1 mole of triphenylphosphine directly to the trifluoroacetic acid solution of the reactants immediately before final work-up. It would appear that the triphenylphosphine functions as a scavenger for TTFA released in the metal-metal exchange reaction, thus protecting the final phenol from further electrophilic thallation and/or oxidation. Validation of the metal-metal exchange mechanism was obtained indirectly by isolation and characterization of an ArTlX2/LTTFA complex directly from the reaction mixture. NMR analysis revealed that this complex still possessed an intact aryl-thallium bond, indicating that it was probably the precursor to the transmetallation products, an aryllead tristrifluoroacetate and TTFA. 

A highly diastereoselective exchange reaction between a variety of bis(dimethy-lamino)arylphosphines and (S)-2-(anilinomethyl)pyrrolidine (197a) and its SV-aryl analogues constituted a key step in the synthesis of a series of substituted monodonor diazaphospholidine ligands, 213-220 [90, 91] (Figure 1). 

Another multicomponent synthesis giving N-3-substituted compounds 28 consisted of the sequential reaction of metallated phosphonoacetates, in one pot, with a nitrile then an aldehyde and finally an isocyanate. This was an extensive study of the scope and limitations of the different substituents on all the components. The most important feature was that, for good yields, the isocyanate should bear an electron-withdrawing group - tosyl was the most successful. However, an exchange reaction could be carried out by reaction of the tosyl products with aryl isocyanates under microwave irradiation, giving the N-3-aryl derivatives 29 <06CEJ7178>. 

Compound (tht)AuC6F5 undergoes random aryl exchange with pentafluorophenylpalladium(ii) complexes, while the 3,5-dichloro-2,4,6-trifluorophenylgold complex specifically catalyzes the trans-cis isomerization of [Ar2Pd(tht)2]. The reaction appears to follow an associative mechanism.1... 

Scheme 6.145 Halide-exchange reactions in aryl halides.

In the direct synthesis of aryl terminal alkynes via Pd-catalyzed cross-coupling of aryl halides with ethynylmetals, formation of diarylethynes is one of the potential side reactions. Indeed, the Kumada coupling of 2-iodo-5-methylthiophene (29) with ethynylmagnesium chloride gave the desired 2-ethynyl-5-methylthiophene (30) in only 35% yield, along with 24% of bis(5-methyl-2-thienyl)ethyne (31) [29], The high propensity for H-Mg exchange reaction to occur was blamed for the diarylethyne formation. 

Likewise, the sulfoxide-metal exchange reaction of /3-acetoxy sulfoxides 164 (/3-mesyloxy sulfoxides can also be used), which are prepared from alkenyl aryl sulfoxides 163 and aromatic aldehydes, with a Grignard or alkyllithium reagent at low temperatures gives the allenes 162 in good to excellent yield (Scheme 5.24) [65],... 

These authors propose as the mechanism for this reaction a reversible oxidative addition of the aryl-phosphido fragments to a low valent rhodium species. A facile aryl exchange has been described for complexes Pd(PPh3)2(C6H4CH3)I. The authors [35] suggest a pathway involving oxidative additions and reductive eliminations. The mechanism outlined below, however, can also explain the results of these two studies. 

Copper iodide acts as an efficient reagent for the nucleophilic displacement of 1-haloalkynes. It transforms 1-bromoalkynes (72) into 1-iodoalkynes (73) which, on further treatment with copper(II) bis(arenesulfinate), are converted into the corresponding alkynyl aryl sulfones (74). An electron transfer between 1-haloalkynes and copper(I) salts is believed to take place for the copper-assisted halogen-exchange reaction at the acetylenic carbon atom. 

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