Pretransmetalation Intermediates in Suzuki–Miyaura C–C and Carbonylative Cross-Couplings: Synthesis and Structural Authentication of Aryl- and Aroylnickel(II) Boronates

Abstract

Covalently tethering a supporting ligand to a boron nucleophile allowed typically elusive, kinetically unstable Suzuki–Miyaura (SM) pretransmetalation species to be tamed. This strategy enabled the synthesis and X-ray crystallographic characterization of an organonickel(II) boronate pretransmetalation intermediate. Performing reactions in the presence of carbon monoxide delivered pretransmetalation intermediates in carbonylative SM reactions–aroylmetal boronates. This enabled the X-ray crystal structure of an aroylnickel(II) boronate (and an aroylpalladium(II) boronate) to be secured.

Pretransmetalation Intermediates in Suzuki–Miyaura Cross-Couplings: Synthesis of Csp3-, Csp2- and Csp-Organopalladium(II) Boronates

“…1 H NMR spectroscopic analysis of organopalladium(II) boronates 5a – 5f in toluene- d 8 suggested that the respective O–Bpin-uncoordinated structures are readily accessible in solution at ambient temperature (Figures S13, S19, S25, S31, and S37). This is consistent with similar observations with analogous arylpalladium(II) boronates …”

“…The reaction mechanism has been comprehensively investigated, and it is generally acknowledged that transmetalation is the most complicated elementary step of the catalytic cycle . In this regard, typically transient pretransmetalation intermediates in palladium-catalyzed SM couplingsorganopalladium(II) boronatesremain particularly difficult to study due to their inherent kinetic instability (Figure A). − …”

“…Alkylpalladium(II) boronates 5c and 5d were prepared from benzyl bromide and iodomethane, respectively. Interestingly, subjecting propargyl bromide to the standard conditions afforded allenylpalladium(II) boronate 5e , while the reaction of benzoyl chloride delivered benzoylpalladium(II) boronate 5f …”

“… A summary of selected bond lengths is provided in Table . We secured the structure of benzoylpalladium(II) boronate 5f as part of a previous study exploring pretransmetalation intermediates in carbonylative SM reactions . Molecules 5b – 5d are the first C sp - and C sp 3 -organopalladium(II) boronates that have been isolated.…”

“…To date, all of the kinetically stable, mononuclear SM pretransmetalation intermediates that have been isolated feature structures that display a secondary interaction between a second boronate-tethered phosphine ligand and the oxygen atom within the M–O–B linkage. , In an effort to synthesize a complex in which this would not be possible, we prepared palladium(II) bromide dimer 9 (Figure ). Heating complex 9 , which is derived from oxidative addition to an aryl bromide bearing a covalently tethered phosphine ligand, under our standard conditions furnished arylpalladium(II) boronate 10 .…”