Formation of CC-type palladacycles with assistance from an apparently innocent NH(CO) functional group

Abstract

A novel cyclometalation pathway to form CC-type palladacycles is reported. Unlike common donor-assisted cyclometalation, the NH(CO) auxiliary group undergoes a deprotonation step to form a palladalactam intermediate. The coordinating nitrogen atom functions as an intramolecular base promoting selective C-H bond cleavage. Without the NH proton, the ortho-N-phenyl C-H is activated instead.

Chasing C,C‐Palladacycles

“…Related metalation reactions on 2-substituted imidazolium derivatives were reported by Lee et al (palladacycles 78-81; Scheme 61). [82] Interestingly, upon the coordination of the ylide, the CÀ H activation could take place in both C(sp 2 )À H and C(sp 3 )À H bonds of the respective aryl or alkyl substituent on the imidazolium ring. Furthermore, the substitution pattern on the amide group present in the substrate was also a key issue, since the tertiary substituted one prevented the C(sp 3 )À H metalation in favor of the six-membered palladacycle 81.…”

Design and synthesis of versatile ligand precursors based on phosphonium ylides for palladalactam formation and catalytic investigation

“…2). 30–34 In exploring palladium complexes with mesoionic NHC ligands, we have concurrently uncovered an intriguing class of palladalactams. 33,34 The precursors of the ligand systems can be easily synthesized through a one-step reaction involving various imidazole derivatives and organic amides.…”

“…30–34 In exploring palladium complexes with mesoionic NHC ligands, we have concurrently uncovered an intriguing class of palladalactams. 33,34 The precursors of the ligand systems can be easily synthesized through a one-step reaction involving various imidazole derivatives and organic amides. Drawing inspiration from Gessner's works, 22 we propose a novel approach where the imidazole derivatives in our ligand system are substituted with diverse phosphines, leading to an unprecedented series of phosphonium chlorides (Scheme 1).…”

Zwitterionic Palladium Complexes: Room-Temperature Suzuki–Miyaura Cross-Coupling of Sterically Hindered Substrates in an Aqueous Medium

“…Zwitterionic metal complexes that exhibit formal charge separation within an overall neutral molecular framework are currently attracting interest. , Over the past few years, a wide scope of zwitterionic cationic metal complexes, in particular those of the platinum-group metals (PGM), have been reported and some of them exhibited promising catalytic properties. − In contrast, zwitterionic complexes with reverse polarity, that is complexes with formal negative charges on the metal centers and positive charges on the ancillary ligands, are less common and well-documented examples were mostly derived from phosphonium ylides or conjugated equivalents. , · Zwitterionic metalate derived from ammonium, − iminium, , and sulfonium ancillaries are also known. Recently, we have reported a unique class of zwitterionic anionic palladium complexes derived from imidazole derivatives (Chart ). , These complexes, including palladalactams and CC-type palladacycles, can be classified as “mesoionic” or “truly zwitterionic”, since no nonzwitterionic connected Lewis structure can be drawn by resonance of the π electrons . They were essentially formed by deprotonation of the methylene protons between the imidazole ring and the CO group of the ligands.…”