Cross coupling mechanisms

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

Figure 6. Cross-Coupling Mechanism Involving Anionic Pd-Species...

When performing the Kumada-Corriu cross-couplings in the presence of an alkyl iodide such as /-PrI, a radical cross-coupling mechanism operates, allowing the performance of such cross-couplings at ambient temperature with less expensive aryl bromides (Scheme 2-8). The use of powerful Pd catalysts such as Pd(0)/5-Phos or PEPPSI is mandatory for achieving this radical-mediated cross-coupling. 

Aerodynamic whirl. Although the mechanism is not clearly understood, it has been shown that aerodynamic components, such as compressor wheels and turbine wheels, can create cross-coupled forces due to the wheel motion. Figure 5-23b is one representation of how such forces may be induced. 

Another potential problem is due to rotor instability caused by gas dynamic forces. The frequency of this occurrence is non-synchronous. This has been described as aerodynamic forces set up within an impeller when the rotational axis is not coincident with the geometric axis. The verification of a compressor train requires a test at full pressure and speed. Aerodynamic cross-coupling, the interaction of the rotor mechanically with the gas flow in the compressor, can be predicted. A caution flag should be raised at this point because the full-pressure full-speed tests as normally conducted are not Class IASME performance tests. This means the staging probably is mismatched and can lead to other problems [22], It might also be appropriate to caution the reader this test is expensive. 

Even with the limitation on yield implied by the statistical process, cross-dimerization is still useful when one of the reactants is an alkane, because the products are easy to separate, and because of the few other ways to functionalize an alkane. The cross-coupling of an alkane with trioxane is especially valuable, because hydrolysis of the product (10-6) gives an aldehyde, thus achieving the conversion RH RCHO. The mechanism probably involves abstraction of H by the excited Hg atom, and coupling of the resulting radicals. 

Scheme 26 Two different mechanisms for cross-coupling reactions [9]...

A possible mechanism involves formation of a Pd(II) intermediate that can undergo cross coupling with the zinc reagent. 

---