Reductive Elimination to Form C(sp³)–N Bonds from Palladium(II) Primary Alkyl Complexes

Abstract

Reductive eliminations to form alkyl–nitrogen bonds are rare, and examples of this reaction from isolated complexes containing simple, unstabilized primary alkyl groups have not been observed. We report the synthesis of stable neopentylpalladium­(II) anilido and methyleneamido complexes that undergo reductive elimination to form the C­(sp3)–N bonds in N-neopentyl anilines and N-neopentyl imines, respectively. The synthesis and isolation of these complexes were enabled by weak chelation of palladium by P,O ancillary ligands. DFT calculations suggest that neopentylpalladium­(II) complexes undergo reductive elimination by a concerted mechanism resembling a migration of the alkyl ligand to the nitrogen either following initial dissociation of the oxygen donor or in concert with lengthening of the Pd–O bond, depending on the identities of the reacting and ancillary ligands.

DFT Calculations Investigate Competing Pathways to Form Dimeric Neopentylpalladium(II) Amido Complexes: The Critical Importance of Dispersion

“…Ar: 4-F-2-(OCH 3 )-C 6 H 3 . Experimental values from ref . Selected angles in degrees and bond distances in Å. Hydrogen atoms are omitted for clarity.…”

“…Single-point calculations with M06 and B97D functionals (6-311++G­(d,p) basis set) were also carried out at B3LYP-optimized geometries. Calculations assumed 1 atm and 243.15 K to simulate published experiments . Global minima were confirmed by having zero imaginary frequency.…”

“…Meanwhile, orbital interactions within the core vary depending on the distances among the M and L atoms . Transition metal complexes with nitrogen-containing ligands are well-known and have drawn considerable attention for decades in catalyzing versatile reactions such as the palladium-catalyzed amination of aryl halides. − Given available lone-pair electrons, many nitrogen-based ligands may coordinate to an additional metal center, leading to dimeric complexes with a core four-membered ring. For instance, Driver and Hartwig reported reductive elimination from dimeric palladium amido complexes through three-coordinate intermediates (Figure a) …”

“…Recently, our groups reported the synthesis of phosphine-ligated neopentylpalladium­(II) amido complexes that underwent reductive elimination to form C sp3 –N bonds . Our experimental collaborators attempted to synthesize the complex 1 (Ad 2 PCH 2 CH 2 OCH 3 -κ 2 P , O )­Pd­(neopentyl)­NH­(4-F-2-(OCH 3 )-C 6 H 3 , Figure b) containing a flexible P,O ligand and an anilido ligand.…”

“…According to a search of the Cambridge Structure Database, D is the first structurally characterized example of a dimeric phosphine-ligated alkylpalladium­(II) amido complex . Furthermore, the formation of stable dimeric Pd II complexes may be an unproductive decomposition pathway that hinders both fundamental studies of the rare C sp3 –N bond-forming reductive elimination and thus the development of catalytic reactions involving alkylpalladium­(II) amido complexes. − Considering both the unique structure of this complex and its potential impact on further work, we chose to investigate the dimeric Pd II complex D more closely with DFT (density functional theory) and sought to understand its bonding and the factors that control its formation.…”