Organic halides direct oxidative addition

A. Direct Oxidative Addition of Magnesium to Organic Halides. 512... 

Unfortunately, the preparation of functionalized Grignard reagents via direct oxidative addition of magnesium metal to organic halides still suffers from severe limitations. This is mainly due to the intrinsic high reducing potential of magnesium metal. 

Zhu, L., Wehmeyer, R. M., Rieke, R. D. The direct formation of functionalized alkyl(aryl)zinc halides by oxidative addition of highly reactive zinc with organic halides and their reactions with acid chlorides, -unsaturated ketones, and allylic, aryl, and vinyl halides. J. Org. Chem. 1991,56, 1445-1453. 

Recently, mechanistic and computational studies aiming to establish whether organic electrophiles bearing halides on a sp -hybridized carbon could indeed undergo an oxidative addition leading to paUadium(IV) complexes, and to uncover alternative pathways that could account for the formation of the observed products, have appeared in the literature. In a representative study, Echavarren reported DPT calculations comparing the feasibility of a direct oxidative addition of vinyl halides to pallada(II)cycles analogous to those proposed in Dyker s and Catellani s... 

Three methods are commonly employed for the in situ preparation of organopalladium derivatives (i) direct metallation of an arene or heterocyclic compound with a palladium(II) salt (ii) exchange of the organic group from a main group organometallic to a palladium(II) compound and (iii) oxidative addition of an organic halide, triflate or aryldiazonium salt to palladium(O) or a palladium(O) complex. 

The mechanism involves oxidative addition of the halide or triflate to the initial palladiumfO phosphine complex to form a palladium(II) species. The key slow step is a transmetallation, so calk because the nucleophile (R1) is transferred from the metal in the organometallic reagent to the palla dium and the counterion (X = halide or triflate) moves in the opposite direction. The new palladi um(II) complex with two organic ligands undergoes reductive elimination to give the couple product and the palladium(O) catalyst ready for another cycle. 

The order of reactivity of organic halides is I > Br > Cl > F. Although organic iodides are the most reactive, they produce more Wurtz coupling products through the radical oxidative addition process. Thus, it is usually advisable to use the chlorides or bromides, except in the case of aromatic iodides and methyl iodide. 1-Alkenyl Grignard compounds are obtained by the direct reaction of magnesium and vinylic halides in THF [6], but the reaction is not completely stereospecific and the retention of the stereochemistry of haloalkenes varies from 60-90% [7]. The difficulty associated... 

---