Development of N-Heterocyclic Carbene–Copper Complexes for 1,3-Halogen Migration

Abstract

A series of NHC−copper complexes was synthesized and their potential to catalyze 1,3-halogen migration explored. Increasing the steric bulk around the metal drastically improves the lifetime of NHC−CuH species and promotes 1,3-halogen migration of both 2-bromo-and 2-chlorostyrenes through transfer of an aryl halogen to a benzylic carbon with concomitant arene borylation. The NHC-based system displays a broad substrate scope with notable advantages over previously reported phosphine-based catalysts, including complete selectivity for migration versus competing benzylic borylation, increased steric tolerance, efficient aryl chloride migration, and facile formation and characterization of organocopper catalytic intermediates. Experimental evidence and DFT calculations support a mechanism proceeding through dearomatization of a benzyl copper species, followed by a 1,4-halogen shift and borylation of the resulting ArCu(I) intermediate.

Spectroscopic Evidence for a Monomeric Copper(I) Hydride and Crystallographic Characterization of a Monomeric Silver(I) Hydride

“…[23] In conclusion, the very bulky ligand IPr** allows the first spectroscopic characterization of am onomeric copper hydride [(IPr**)CuH, 2], which is not isolable in the solid state, but is found to be in equilibrium in solution with its dimer [(IPr**)CuH] 2 3.T hese results support the hypothesis that copper hydride aggregates dissociate in solution. [12] On the other hand, our data suggest that silver hydrides do not have the tendencyt oa ggregate in the solid state;h owever, the labile nature of the silver-carbene bond, which is typically used in carbene transmetalation reactions, [24] makes them prone to dissociation in solution.…”

“…[10,11] However,t he existence of an equilibrium between am ononuclear LCuH and its polynuclear form (LCuH) n has never been experimentally observed. [12] Fors ilver hydrides,i n2 010 Liu et al isolated cationic [Ag 8 (m 4 -H){Se 2 P(OR) 2 } 6 ] + clusters B containing Ag Ihydride bridges (Ag-m-H-Ag), [13] and in 2013 Sadighi et al described the dimeric silver hydride cation C. [14] Theo nly crystallographically characterized monomeric coinage metal hydrides are gold complexes, [15] complex D being the first ever reported. [16] Herein we report the unprecedented spectroscopic characterization of am onomeric LCuH complex, which is in equilibrium in solution with its dimer [LCuH] 2 ,aswell as the isolation and crystallographic characterization of the first monomeric LAgH complex.…”

Selective Reduction of CO2 to a Formate Equivalent with Heterobimetallic Gold‐ ‐ ‐Copper Hydride Complexes

“…In recent years, there has been increasing interest in the synthesis and catalytic application of coinage metal hydrides (M=Cu, Ag, Au) . The majority of catalytic studies have focused on the in situ generation of terminal metal hydride complexes stabilized by N‐heterocyclic carbene (NHC) ligands . There is, however, a growing appreciation that species possessing three‐center, two‐electron M−H−M bonds may play a role in catalysis.…”

Enantioselective Cu‐Catalyzed Functionalizations of Unactivated Alkenes

“…Notable improvements in 1,3‐halogen migration were achieved with bulky NHC ligands (IPent and IHept) with 2‐bromo‐ and 2‐chlorostyrenes as substrates …”

“…Notable improvements in 1,3-halogen migration were achieved with bulky NHC ligands (IPent and IHept) with 2-bromoand 2-chlorostyrenes as substrates. [36] Ta ntillo, Schomaker,a nd co-workersa lso studied the mechanism of this interesting transformation. [37] Experimental and computational studies revealed interesting mechanistic features of this process.I ts eems unlikely that the reaction involvesc hanges in the oxidation state of copper.…”