Palladium‐Catalyzed Reactions in Industry, 4[]. Synthesis of New Palladium Catalysts: First Isolation and Characterization of all Intermediates in a Cyclopalladation Reaction

In order to synthesize chiral palladacycles for stereoselective C–C coupling reactions we studied the cyclopalladation of P‐chiral phosphanes 2 and 3. New palladium complexes of the type L2PdX2 (6, 8) and LXPd–μ‐X2–PdXL (5, 9, X = Cl; L = 2, 3) were isolated. A detailed study of the reactivity of all intermediates towards cyclopalladation proved the mechanism of cyclometalation reactions of o‐tolylphosphanes for the first time. Different deuteration experiments clearly demonstrated the higher reactivity of dimeric palladium complexes towards metalation compared to monomeric species. In agreement with this observation only 5 and 9 gave the cyclopalladated products 4 and 10 as revealed by FAB mass spectrometric investigations. Under the described reaction conditions the synthesis of the corresponding palladacycles 4, 10 is not possible because cyclometalation is a reversible process with LXPd–μ–X2–PdXL as thermodynamic more stable products. The results demonstrate the importance of free coordination sites on the metal atom for cyclometalation reactions or more general CH activation processes.