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 L(2)PdX(2) (6, 8) and LXPd-mu-X(2)-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 Me corresponding palladacycles 4, 10 is not possible because cyclometalation is a reversible process with LXPd-mu-X(2)-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.