When Weaker Can Be Tougher: The Role of Oxidation State (I) in P- vs N-Ligand-Derived Ni-Catalyzed Trifluoromethylthiolation of Aryl Halides

Abstract

The direct introduction of the valuable SCF3 moiety into organic molecules has received considerable attention. While it can be achieved successfully for aryl chlorides under catalysis with Ni0(cod)2 and dppf, this report investigates the Ni-catalyzed functionalization of the seemingly more reactive aryl halides ArI and ArBr. Counterintuitively, the observed conversion triggered by dppf/Ni0 is ArCl > ArBr > ArI, at odds with bond strength preferences. By a combined computational and experimental approach, the origin of this was identified to be due to the formation of (dppf)NiI, which favors β-F elimination as a competing pathway over the productive cross-coupling, ultimately generating the inactive complex (dppf)Ni(SCF2) as a catalysis dead end. The complexes (dppf)NiI–Br and (dppf)NiI–I were isolated and resolved by X-ray crystallography. Their formation was found to be consistent with a ligand-exchange-induced comproportionation mechanism. In stark contrast to these phosphine-derived Ni complexes, the corresponding nitrogen-ligand-derived species were found to be likely competent catalysts in oxidation state I. Our computational studies of N-ligand derived NiI complexes fully support productive NiI/NiIII catalysis, as the competing β-F elimination is disfavored. Moreover, N-derived NiI complexes are predicted to be more reactive than their Ni0 counterparts in catalysis. These data showcase fundamentally different roles of NiI in carbon–heteroatom bond formation depending on the ligand sphere.

Halide Abstraction Competes with Oxidative Addition in the Reactions of Aryl Halides with [Ni(PMenPh(3−n))4]

“…This mechanism differs from that established for [Pd(PPh 3 ) 4 ], which proceeds via [Pd(PPh 3 ) 2 ] and a concerted three‐centre transition state 17. Concerted three‐centre transition states have been computationally characterised for oxidative addition to Ni 0 complexes bearing bidentate phosphine ligands 9, 12, 14, 18, 19, 20. We present evidence from computational studies that Ni I and Ni II products both do indeed arise via [Ni(PEt 3 ) 3 ]; Ni I is obtained via an open‐shell singlet transition state and Ni II is formed from an S N 2‐type oxidative addition event (Figure 1 (b)).…”

Divergent Reactivity of a Dinuclear (NHC)Nickel(I) Catalyst versus Nickel(0) Enables Chemoselective Trifluoromethylselenolation

“…Ultimately, biaryls are obtained upon reductive elimination. The resulting [L n Ni 0 ] species may then undergo comproportionation with [Ni II (X) 2 ] to [Ni I ] n 5a. We speculated that the lack of significant conversion with ArBr and ArCl may be associated with the intermediacy of [Ni I ] n , rather than [Ni 0 ].…”

A Simple Nickel Catalyst Enables Broad E-Selective Alkyne Semihydrogenation

“…[19,20] Both electron-poor and electron-rich tolane derivatives bearing trifluoromethyl, methoxy,c hloro, and nitrile substituents 10 d-10 g were all convertedw ith equallyg ood resultsi nt erms of yields and stereoselectivity (> 82 %y ield, E/Z > 99:1). For aniline 9c to be successfully converted, the nickel(II)/dppf complex had to be pre-formed, because the presenceo ft he aniline inhibited the formation of the active catalyst.…”

“…With regards to the most atom-economic reducinga gent dihydrogen (H 2 ), only few catalysts for the challenging E-selective alkyne semihydrogenationh ave been disclosed,e ach with au nique substrate scope profile.Here,w es how that ac ommerciallya vailablen ickel catalyst facilitates the E-selective alkyne semihydrogenation of a wide variety of substituted internal alkynes.T his results in a simple and broadly applicable overall protocol to stereoselectively access E-alkenes employing H 2 ,w hichc ould serve as ag eneral methodf or synthesis.Scheme2.E-selective alkyne semihydrogenationw ith anickel(II) complex.Scheme3.Nickel-catalyzed semihydrogenationofd iaryl alkynes.[a] Preformed NiI 2 /dppf complex [20] had to be used. With regards to the most atom-economic reducinga gent dihydrogen (H 2 ), only few catalysts for the challenging E-selective alkyne semihydrogenationh ave been disclosed,e ach with au nique substrate scope profile.…”

“…[a] Preformed NiI 2 /dppf complex [20] had to be used. [b] Conditions: 10 mol %N iI 2 , 12 mol %d ppf, 40 bar H 2 ,100 8C, 24 h.…”