Insertion of CO₂ Mediated by a (Xantphos)Ni^I–Alkyl Species

Abstract

The incorporation of CO2 into organometallic and organic molecules represents a sustainable way to prepare carboxylates. The mechanism of reductive carboxylation of alkyl halides has been proposed to proceed through the reduction of NiII to NiI by either Zn or Mn, followed by CO2 insertion into NiI‐alkyl species. No experimental evidence has been previously established to support the two proposed steps. Demonstrated herein is that the direct reduction of (tBu‐Xantphos)NiIIBr2 by Zn affords NiI species. (tBu‐Xantphos)NiI‐Me and (tBu‐Xantphos)NiI‐Et complexes undergo fast insertion of CO2 at 22 °C. The substantially faster rate, relative to that of NiII complexes, serves as the long‐sought‐after experimental support for the proposed mechanisms of Ni‐catalyzed carboxylation reactions.

Nickel‐Catalyzed Conversion of Enol Triflates into Alkenyl Halides

“…Our hypothesis is that it coordinates ZnI 2 salts that are generated as part of the initial Ni II reduction. [21][22][23] This hypothesis is consistent with the observation that addition of exogenous ZnI 2 inhibits the reaction, but that reactivity can be restored by addition of DMAP. [20] Although 2a could be obtained in good yield with 5mol %N i (entry 10), incomplete conversions were obtained for many substrates when the scope of the iodination was investigated.…”

“…Theh alide exchange was found to be compatible with av ariety of common functional groups, including amines (4), carbamates (5,13), pyridines (20), alkenes (12), dienes (10,14), esters (19), ketals (6, 14), and enones (11). Chemoselective halogenation of alkenyl triflates was observed in preference to aryl triflates (15, 20), aryl chlorides (9, 21), and aryl boronates (22). However,competitive halide exchange was observed in the presence of aryl bromides and iodides.…”

“…Formost substrates,complete conversions were achieved with 5mol %N iand without the need for DMAP,a lthough addition of DMAP improved the yields for substrates where incomplete conversion was observed with only cod (14-16;T able 2). In addition, for 1arylvinyl triflates (19)(20)(21)(22)t he use of Ni(cod) 2 (Method C) provided cleaner reaction profiles for the bromination and iodination reactions. [25] To demonstrate the scalability of the reaction, the bromide 13 b was prepared in 95 %y ield on 1mmol scale using abenchtop setup.…”

Nickel‐Catalyzed Conversion of Enol Triflates into Alkenyl Halides

“…The precise role of DMAP in the iodination remains unclear; our hypothesis is that it coordinates ZnI2 salts that are generated as part of the initial Ni(II) reduction. 21,22,23 This is consistent with the observation that addition of exogenous ZnI2 inhibits the reaction, but that reactivity can be restored by addition of DMAP. 20 Although 2a could be obtained in good yield with 5 mol % Ni (Table 1, entry 10), incomplete conversions were obtained for many substrates when the scope of the iodination was investigated.…”

“…The halide exchange was found to be compatible with a variety of common functional groups, including amines (4), carbamates (5, 13), pyridines (20), alkenes (12), dienes (10,14), esters (19), ketals (6, 14), and enones (11). Chemoselective halogenation of alkenyl triflates was observed in preference to aryl triflates (15, 20), aryl chlorides (9, 21), and aryl boronates (22); however, competitive halide exchange was observed in the presence of aryl bromides and iodides. 20 Although the Ni-catalyzed halogenation exhibits good functional group tolerance, the iodination, bromination, and chlorination did not perform equally well.…”