Cyclometallation of indole derivatives: Cyclopalladation of gramine and 1-methyl gramme and CO insertion

Article abstract of DOI:10.1016/S0022-328X(96)06630-2

Li₂PdCl₄, in methanol, with gramine 1 and 1-methyl gramine 2 gave the dimeric complexes 1a and 2a with chlorine bridges and having the indole nucleus metallated at position 2. The corresponding bromo derivatives 1b and 2b were isolated from the reactions of Pd(OAc)₂ with 1 and 2, followed by treatment with LiBr. Complexes 1a,b and 2a,b gave the monomeric, cyclometallated derivatives 3a-d by reaction with PPh₃ in methylene chloride. When the cyclopalladation of 1-methyl gramine 1 was attempted by reacting PdCl₂, PPh₃ and 2 in methanol in the presence of AcONa, an unexpected compound 4 was obtained. A structural determination of 4 showed it consisted of the known [Pd(PPh₃)Cl₃]^- anion packed with the dimethyl-bis(3-methylene-1-methyl indole) ammonium cation. When compounds 1a and 2a were reacted with carbon monoxide at atmospheric pressure in methanol or ethanol and in the presence of NEt₃, the corresponding 2-carboalkoxy indole derivatives 5 and 6 were isolated in high yields.