Reductive Eliminations to Form C-X Bonds from Acyl Complexes

Reductive Eliminations to Form C-X Bonds from Acyl Complexes 

Reductive elimination to form carbon-heteroatom bonds from acyl complexes occurs more readily than it does from alkyl or aryl complexes. This fast rate is consistent with the trends noted previously—namely, reductive elimination is faster from complexes containing groups that are sp hybridized, that are electrophilic, and that can coordinate to the metal center during tiie reductive elimination. Reductive elimination of acid halides has been proposed as a step in the Monsanto acetic acid process, and experimental evidence has been obtained for this type of reductive elinunation from acetylrhodium halide complexes generated in solution (Equation 8.66). - A closely related incorporation of KZO into add halides catalyzed by Wilkinson s catalyst provides further evidence for reversible addition and elimination of add halides involving Rh(I) and Rh(III), respectively.  

In addition, reductive elimination of palladium and nickel complexes to form esters (Equations 8.67 and 8.68), amides, and tiiioesters has been reported. - The reductive eliminations of esters and amides were observed during mechanistic studies on the palladium-catalyzed formation of esters and amides from aryl halides, carbon monoxide, and alcohols or amines. This catalytic process is presented in Qiapter 17 (carbonylation processes). The reductive elimination of thioesters from nickel complexes were studied, in part, to understand the C-S bond-forming process of acetyl coenzyme A synthase. Prior to this work, an iron-mediated synthesis of p-lactams had been reported that appears to occur by reductive elimination to form the amide C-N bond of the lactam.  

Reductive elimination is the product-forming step in some of the most important catalytic cycles, including hydrogenation, the Monsanto acetic acid process, and various types of cross-couplings. For this reason, detailed studies of this process have been conducted. Hrese studies have revealed examples of reductive eliminations to form H-H and C-H bonds, as well as reductive eliminations to form C-G and C-X bonds (in which X = halide, amide, alkoxide, thiolate, and phosphide). The mechanisms of these processes include the same pathways as have been deduced for oxidative addition (i.e., concerted, ionic, and radical), because reductive elimination is the same as oxidative addition, but in the reverse direction. 

Basato, M. Morandini, E Longato, B. Bresadola, S. hwrg. Chem. 1984,23,649. 

---