The Application of Pulse Radiolysis to the Study of Ni(I) Intermediates in Ni-Catalyzed Cross-Coupling Reactions

Abstract

Here we report the use of pulse radiolysis and spectroelectrochemistry to generate low-valent nickel intermediates relevant to synthetically important Ni-catalyzed cross-coupling reactions and interrogate their reactivities toward comproportionation and oxidative addition processes. Pulse radiolysis provided a direct means to generate singly reduced [(dtbbpy)­NiBr], enabling the identification of a rapid Ni(0)/Ni­(II) comproportionation process taking place under synthetically relevant electrolysis conditions. This approach also permitted the direct measurement of Ni­(I) oxidative addition rates with electronically differentiated aryl iodide electrophiles (k OA = 1.3 × 104–2.4 × 105 M–1 s–1), an elementary organometallic step often proposed in nickel-catalyzed cross-coupling reactions. Together, these results hold implications for a number of Ni-catalyzed cross-coupling processes.

Mechanistic Understanding of Arylation vs Alkylation of Aliphatic Csp3–H Bonds by Decatungstate–Nickel Catalysis

“…34−37 For this reaction, we have reported a synergic computational and experimental work highlighting a Ni I complex to be the most likely active species, 38 a result supported by recent pulse radiolysis experiments. 39 Another example of C sp3 −H arylation and alkylation has been reported by Martin, who used diaryl ketone as a photocatalyst for the hydrogen atom transfer (HAT) reactions, instead of a metalbased photocatalyst. 28 Another recently developed methodology rests on using a polyoxometalate, W 10 O 32 4− , as an inorganic photo-HAT catalyst for dehydrogenative cross-coupling reactions.…”

“…A strategy developed over the last decade, based on the merging of photoredox and transition-metal catalysis, can possibly solve some of these challenges. − Among the most promising strategies in combined photoredox-transition-metal catalysis is possibly employing complexes based on transition metals that are earth-abundant, such as nickel, as the organometallic catalyst. ,,, This combined photoredox-nickel catalysis has been demonstrated to be an effective strategy for the activation of nonclassic cross-coupling nucleophiles. ,,, Among the most notable examples − is the functionalization of α-amino C sp3 –H bonds using a visible light-activated photoredox catalyst together with a Ni catalyst and an organocatalyst, proposed recently by MacMillan. − For this reaction, we have reported a synergic computational and experimental work highlighting a Ni I complex to be the most likely active species, a result supported by recent pulse radiolysis experiments . Another example of C sp3 –H arylation and alkylation has been reported by Martin, who used diaryl ketone as a photocatalyst for the hydrogen atom transfer (HAT) reactions, instead of a metal-based photocatalyst …”

Controlling Ni redox states by dynamic ligand exchange for electroreductive Csp3–Csp2 coupling

“…Aryl electrophiles are activated by 2e – oxidative addition at Ni, and alkyl electrophiles react through 1e – processes to form alkyl radicals ( 20 , 21 ). Although it initially seemed that activation of each electrophile occurred at a distinct oxidation state of Ni (Ni 0 or Ni I ), a growing body of evidence from electrochemical ( 22 ), photoredox ( 23 ), and chemical ( 24 ) studies suggests that only Ni I (pyridyl) intermediates are accessible under reductive conditions. On the basis of these reports, we hypothesized that XEC is restricted to electrophiles that react with comparable rates at Ni I , whereas electrophiles that are exceedingly reactive (3° alkyl halides) or unreactive (Ar–Cl/OTf) at Ni I are incompatible coupling partners (Fig.…”

Aminomethylation of Aryl Bromides by Nickel-Catalyzed Electrochemical Redox Neutral Cross Coupling

“…On the basis of the studies mentioned above, a proposed mechanism is illustrated in Scheme 2d. Initially, Ni(II)L undergoes a two-electron reduction process at the cathode to afford Ni(0)L, which undergoes a rapid Ni(0)/Ni(II) comproportionation process to form Ni(I)L. 23 The following oxidative addition of Ni(I)L to aryl bromide forms an Ar-Ni(III)L-Br intermediate, which is reduced to Ar-Ni(II)L-Br ( 14) at the cathode. Meanwhile, α-amino carbon radical 15 is produced after oxidation at the anode and deprotonation.…”