Exchange reactions Transmetalation

Exchange reactions (transmetallation) with other heterocyclopentadienes... 

The formation of organometallic compounds from metals is described in Ch. 5. In this section, a few references are given, dealing with the formation of such compounds by an exchange reaction (transmetallation or ligand exchange) imder solid-liquid conditions and the preparation of compoimds containing non-metal elements. 

Organic compounds M—R and hydrides M—H of main group metals such as Mg, Zn, B, Al, Sn, SI, and Hg react with A—Pd—X complexes formed by oxidative addition, and an organic group or hydride is transferred to Pd by exchange reaction of X with R or H. In other words, the alkylation of Pd takes place (eq. 9). A driving force of the reaction, which is called transmetallation, is ascribed to the difference in the electronegativities of two metals. A typical example is the phenylation of phenylpalladium iodide with phenyltributyltin to form diphenylpalladium (16). 

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 few measurements are available that relate to the ion pair acidity of ethylene and some other alkenes. Ethylene is difficult to metallate directly, but vinyl bromides and iodides undergo facile transmetallation with alkyllithium reagents. Applequist and O Brien determined the equilibrium constants of transmetallation exchange reactions as a measure of relative acidity (equations 6 and 7)25. 

The functionalized arylzinc reagents are best prepared either starting from an aryllithium obtained by halogen-lithium exchange followed by a low-temperature (-80°C) transmetalation with ZnBrj or by performing an iodine-magnesium exchange reaction. The latter reaction tolerates temperatures up to -30°C and is more convenient for industrial applications. ... 

Stannylation by transmetallation and halogen-metal exchange reactions... 

The mixed 1,2-bimetallic Zn/Si-reagent 47 is a versatile species which reacts with aldehydes in high diastereoselectivity . It is prepared by a bromine/lithium exchange reaction starting from 48 followed by a transmetalation with ZnCl2 (Scheme 21). The... 

---