Dicationic Palladium(II) Complexes as Active Lewis Acid Catalysts for Polarized Nazarov Cyclization

Two new bidentate phosphonamidite ligands − 1,2-bis((3αS)-3,3-diphenyltetrahydro-2-oxa-6α-aza-1-phosphapentaleno)ethane, L1, and 1,2-bis((3αS)-3,3-diphenyltetrahydro-2-oxa-6α-aza-1-phosphapentaleno)benzene, L2 − were prepared from (S)-α,α-diphenylprolinol and either 1,2-bis(dichlorophosphino)ethane or 1,2-bis(dichlorophosphino)benzene, respectively. Reactions of L1 and L2 with one equivalent of Pd(PhCN)2Cl2 in dichloromethane result in the formation of the neutral complexes Pd(L1-P,P)Cl2, 2, and Pd(L2-P,P)Cl2, 3, in good yields (85% and 92%, respectively). Complexes 2 and 3 were fully characterized by NMR spectroscopy, elemental analyses, and X-ray crystallography. Chloride abstraction from 1 and 2 using two equivalents of AgSbF6 in the presence of excess diethylisopropylidene malonate (DIM) leads to the formation of the respective dicationic palladium complexes [Pd(L1-P,P)(DIM)][SbF6]2, 4, and [Pd(L2-P,P)(DIM)][SbF6]2, 5. As a result of the cationic nature of 4 and 5 and the labile coordination of DIM, these complexes exhibit high activity as electrophilic catalysts for the Nazarov cyclization of electronically polarized substrates. For those substrates in which a quaternary carbon center is generated upon Nazarov cyclization, spirocycle formation occurs via a Wagner−Meerwein rearrangement following cyclization. In these cases, addition of NaBAr4f as a cofactor to the reaction system improves the selectivity to spirocycle formation relative to the simple Nazarov product.