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Buchwald Hartwig

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The elegance of the Buchwald-Hartwig protocol is that the conjugate acids of the heteroatom reagents are used and that the acid formed from this proton and the halide is neutralized by an added base. Prepared Pd complexes were employed as a catalyst for the Buchwald-Hartwig amination of aryl haildes. The obtained results were compared with those for the structurally related oxyamino-thioaminocarbene and the related NHC complex. Catalysts were efficient in the amination of bromobenzene demonstrating activities similar to those of the NHC complex. 

The Buchwald-Hartwig amination reaction is shown below.

Buchwald-Hartwig Amination

Mechanism

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In Buchwald-Hartwig reaction a carbon-nitrogen bond is formed. Cross coupling reactions share this general label because of the use of similar catalysts and because they are considered to exhibit the same general mechanism. The R-X molecule is first oxidatively added to the metal. Then the transmetalation step follows where the R group is transferred from Y to the metal and the X group leaves the metal coordination sphere. The final step is the reductive elimination where R and R'are bound and the metal catalyst recovers its initial state. The existence of this general scheme does not explain all the mechanistic details. The catalyst is usually formed and its precise form is often unknown in particular concerning the number and nature of the ligands.

The generally accepted overall mechanism for cross coupling reactions are shown below. 

Buchwald Hartwig Cross Coupling Mechanism

For Heck reactions using reagents without a carbon metal bond, syn addition and $\beta$-hydride elimination take place instead of transmetalation. The formation of MX or HX respectively renders the reaction thermodynamically favorable.

The standard mechanism for palladium catalyzed C-C cross coupling reactions are shown below. 

Cross Coupling Reaction