Dinickel Catalyzed Vinylidene-Alkene Cyclization Reactions

Abstract

A dinickel catalyst promotes reductive cyclization reactions of 1,1-dichloroalkenes containing pendant olefins. The reactions can be conducted with a Zn reductant or electrocatalytically using a carbon working electrode. Mechanistic studies are consistent with the intermediacy of a Ni₂(vinylidene) species, which adds to the alkene and generates a metallacyclic intermediate. β-Hydride elimination followed by C-H reductive elimination forms the cyclization product. The proposed dinickel metallacycle is structurally characterized and its stoichiometric conversion to product is demonstrated. Spin polarized, unrestricted DFT calculations are used to further examine the cyclization mechanism. These computational models reveal that both nickel centers function cooperatively to mediate the key oxidative addition, migratory insertion, β-hydride elimination, and reductive elimination steps.

Keywords: cyclization; electrocatalysis; metal–metal bonds; nickel; vinylidene.

Figure 1.

(a) Exo and endo cyclization modes of 1,6-enynes. (b) Reductive activation of 1,1-dichloroalkenes enable selective vinylidene–alkene cyclization reactions.

Figure 2.

Substrate scope studies. Isolated yields were determined following purification and were averaged over two runs. Standard reaction conditions: substrate (0.2 mmol), ^i-PrNDI (4) (5–10 mol%), Ni(dme)Cl₂ (10–20 mol%), Zn (3.0 equiv), NMP (1.6 mL), THF (6.4 mL), rt or 50 °C, 24 h. ^a substrate (0.2 mmol), ^c-PentNDI (7) (10 mol%), Ni(dme)Cl₂ (20 mol%), Zn (3.0 equiv), NMP (0.8 mL), THF (7.2 mL), rt, 24 h.

Figure 3.

Experiments supporting a stepwise vinylidene addition mechanism.

Figure 4.

(a) Controlled potential electrolysis experiments for the catalytic conversion of 1 to 2. (b) Experimental setup (CE = counter electrode; RE = reference electrode; WE = working electrode) and a proposed one-electron catalytic cycle. (c) Cyclic voltammetry data for complex 5 in the presence and absence of 1 (0.3 M [n-BuN₄]PF₆ in NMP; 100 mV/s scan rate). All cyclic voltammograms were internally referenced to the Cp₂Fe/Cp₂Fe⁺ couple.

Figure 5.

Experiments probing the stoichiometric activation of 1,1-dichloroalkene substrates with (^i-PrNDI)Ni₂(C₆H₆) complex 6. (a) Stoichiometric reductive cyclization of 1 using complex 6. (b) Stoichiometric reaction between complex 6 and substrate 25 to form metallacycle 38. (c) XRD structure of metallacycle 38. Selected bond metrics: Ni1–Ni2: 2.743(1) Å; Ni1–C1: 2.000(6) Å; Ni1–C3: 1.967(4) Å; Ni2–C1: 1.954(5) Å; Ni2–Cl: 2.243(2) Å. (d) Frozen solution EPR spectrum for 38 (105 K; toluene). Simulated parameters: g = [2.378, 2.226, 2.093]. * corresponds to a S = 1/2 impurity. (e) UM06-L spin density plot for 38.

Figure 6.

Comparison of rates and yields for stoichiometric cyclizations to form 26 as a function of oxidation state.

Figure 7.

(a) DFT-calculated reaction pathways for the cyclization of 25 to give complex 38. Relative Gibbs free energies are shown in kcal/mol. Isopropyl groups were modeled as methyl groups. (b) 3D representations of key transition-state structures. Spin states are given in parenthesis. Hydrogens are removed for clarity. Distances are reported in Å.

Figure 8.

DFT-calculated reaction pathways for conversion of 38 to 44/46 as a function of oxidation state. Relative Gibbs free energies are shown in kcal/mol.