Palladium pincer complex-catalyzed condensation of sulfonimines and isocyanoacetate to imidazoline derivatives. Dependence of the stereoselectivity on the ligand effects

Palladium-catalyzed condensation reactions of sulfonimines with isocyanoacetate were performed using various PCP, SCS, SeCSe and NCN pincer complexes as catalysts. The reactions proceeded rapidly (2 h) at room temperature using only 1 mol % pincer complex catalyst without any additives. The electron-deficient and relatively bulky PCP complex provides imidazoline derivatives with a very high syn diastereoselectivity. The applied PCP catalyst proved to be very robust under the applied reaction conditions, as it could be recovered without any decomposition after the completed catalytic process. The stereoselectivity of the condensation reactions is reversed by employing the electron-rich SeCSe type of complexes. Simple palladium salts, such as palladium acetate, Pd(OAc)(2), catalyze the reaction with a poor stereoselectivity. The stereoselectivity of the PCP complex-catalyzed process does not depend significantly on the steric bulk of the sulfonimine component. Mechanistic studies revealed that the PCP complex-catalyzed reaction proceeds via an eta(1)-coordinated palladium isocyanoacetate pincer intermediate. This intermediate could also be isolated and its structure was determined by X-ray diffraction. The X-ray structure of this reaction intermediate indicates a surprisingly strong carbon-metal bond between the palladium atom and the coordinated isocyanoacetate molecule. Our mechanistic studies show that the pincer complex catalyst does not undergo redox reactions and, thus it retains a +2 oxidation state under the catalytic process.