Hydrogenolysis acyl halides

The Pd-catalyzed hydrogenoiysis of acyl chlorides with hydrogen to give aldehydes is called the Rosenmund reduction. Rosenmund reduction catalyzed by supported Pd is explained by the formation of an acylpalladium complex and its hydrogenolysis[744]. Aldehydes can be obtained using other hydrides. For example, the Pd-catalyzed reaction of acyl halides with tin hydride gives aldehydes[745]. This is the tin Form of Rosenmund reduction. Aldehydes are i ormed by the reaction of the thio esters 873 with hydrosilanes[746,747]. 

Acyl halides can be reduced to aldehydes1206 by treatment with lithium tri-f-butoxyaluminum hydride in diglyme at -78°C,1207 R may be alkyl or aryl and may contain many types of substituents, including N02, CN, and EtOOC groups. The reaction stops at the aldehyde stage because steric hindrance prevents further reduction under these conditions. Acyl halides can also be reduced to aldehydes by hydrogenolysis with palladium-on-barium sulfate... 

Trapping of acylpalladium derivatives via hydrogenolysis must be a critical step in the classical Rosenmund reductiont of acyl halides to give aldehydes. Aldehydes can now be... 

Examples of the formylation of aryl halides with synthesis gas catalyzed by palladium complexes are summarized in Equation 19.90. These reactions relied upon the development of ligands with particular steric and electronic properties. The dia-damantyl-n-butyl phosphine shown in the equation, in combination with palladium acetate, leads to the formation of aromatic aldehydes in high yields from electron-rich and electron-poor aryl bromides. Reactions of nitroarenes and 2-bromopyridine provided the aldehydes in low yield, but other examples occurred in satisfactor) yield with only 0.1-0.75 mol % catalyst. The identity of the base is important in this process, and TMEDA was the most effective base. The mechanism of this process was not proposed in the initial work, but is likely to occur by oxidative addition of the aryl halide, insertion of the carbon monoxide into the palladium-aryl bond, and a combination of hydrogenolysis of the acyl intermediate and elimination of hydrogen halide to regenerate palladium(O). The base would then be involved in the hydrogenol5 sis and consumption of hydrogen halide. 

In comparison to the runs in the absence of halide anions, the reaction was accelerated by a factor of 6-7 (chloride/bromide) or 3-4 (iodide). Regioselec-tivity increased in the reverse order, for example, iodide > bromide > chloride. No esters were formed. It was argued that the hydrogenolysis of a Pd-acyl intermediate is advantageously influenced by the halide (Scheme 5.61). The latter probably... 

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