Homo aryl halides

Early development of the homo-coupling reactions of aryl halides involves the use of stoichiometric amounts of air-sensitive Ni(0) complexes.54 The reaction could also be realized with a catalytic amount of Ni(0) complexes formed in situ when a stoichiometric amount of Zn was present. Besides aryl iodides, tosylates,... 

The rate-determining step in the homo-coupling reaction of aryl halides could be the oxidative step or the reduction of Ni(II) to Ni(I) step. 

NMR spectroscopy, 93. See also Proton NMR integrations Hoechst continuous process, 548 Homo-coupling reactions, aryl halide, 486-487 Homopolymers, 7 Hot-cast prepolymer method, 211 Hot phosgenation, 222 Houvink-Sakurada equation, 286 HTMAB. See Hexadecyltrimethylammonium bromide (HTMAB)... 

The homo-coupling of benzylic halides appears to proceed via benzylnickel intermediates similar to that of aryl halides. The intermediate, benzylnickel halide, was successfully trapped with electron deficient olefins. 

Total basicity is measured by standard acid—base titration techniques. The activity divided by the total basicity should be greater than 90%. If it is not, then the Grignard reagent should be checked for unreacted alkyl or aryl halide, homo-coupled product, hydrolysis products, and oxidation products. 

A three-component reaction based on the umpolung of re-allylpalladium (II) complexes indium metal was reported by Grigg and co-workers (Scheme 8.31) [74]. In this reaction, the electrophilic nature of the n-allyl palladium species generated from aryl halides and allenes is reversed by transmetallation with indium metal. The resultant nucleophilic allylindium reagent subsequently adds to the third component - aldehyde [75] or imine [76] - to give the corresponding homo-allylic alcohol 64 or amine 65 respectively. 

By similar reasoning, we can further conclude that oxidation reactions will be most favorable for the lesser-halogenated isomers. This concept is illustrated in Figure 5(b) where relative rates of oxidation and reduction are plotted as a function of level of chlorination. In practice, the literature has revealed that there may be a (chemical-dependent) zone where oxidation and reduction reactions are equally likely. This zone captures aryl halides with four to six chlorines, and alkyl halides with one to three chlorines per molecule. Figure 5(c) shows a trend between the Gibbs free energy for reductive dechlorination and the HOMO-LUMO gap (HOMO = highest occupied... 

Homo-Coupling of Aryl Halides The Ulhnann Reaction De-halogen-coupling... 

Entry Aryl Halide X R Time (min) Conversion %Y> Yield (%) Homo- Coupling (%)... 

Pd-catalyzed homocoupling of organic electrophiles proceeded under a number of different reaction conditions with various reductants (Table 13). From a practical point of view, the most commonly used ones were amines or potassium carbonate, or their combinations with other reductants, such as isopropanol. These reductants are suitable for the homo-coupUng of a variety of alkenyl and aryl halides. 

The yields of symmetrical biaryls in the Ullmann reaction may vary from approximately a few to more than 90%, depending on the reactant structure and reaction conditions [1-6]. However, in many examples the yields of biaryls were moderate to high. Theoretically, the Ullmann reaction requires two equivalents of the copper metal per one equivalent of the biaryl formed, thus two equivalents of copper(I) halide are generated. Practically, several equivalents, three to tenfold excess, of the copper metal are used in order to reach high conversion of the aryl halides [3,4,5]. The reactivity of the aryl halide with the copper metal in homo-coupling reactions is... 

For the highly reaetive eopper metal forms, e.g. Rieke copper, oxidative addition to aryl halide occurs readily at room temperature or below [27]. At these temperatures oxidative addition of the resultant arylcopper(l) compound to the second molecule of aryl halide, or decomposition to biaryl proceeds very slowly or not at all. The arylcopper(l) compounds can then be homo-coupled or react with the aryl halides to form biaryl as the temperature is raised. In the classical Ullmann reaction employing... 

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