Single Electron Transfer to Diazomethane-Borane Adducts Prompts C-H Bond Activations

Abstract

While (Ph₂ CN₂ )B(C₆ F₅ )₃ is unstable, single electron transfer from Cp*₂ Co affords the isolation of stable products [Cp*₂ Co][Ph₂ CNNHB(C₆ F₅ )₃ ] 1 and [Cp*Co(C₅ Me₄ CH₂ B(C₆ F₅ )₃ )] 2. The analogous combination of Ph₂ CN₂ and BPh₃ showed no evidence of adduct formation and yet single electron transfer from Cp*₂ Cr affords the species [Cp*₂ Cr][PhC(C₆ H₄ )NNBPh₃ ] 3 and [Cp*₂ Cr][Ph₂ CNNHBPh₃ ] 4. Computations showed both reactions proceed via transient radical anions of the diphenyldiazomethane-borane adducts to effect C-H bond activations.

Keywords: DFT; borane; diazomethane; one-electron reduction; radical anions.

Scheme 1

Interactions of main group systems with N₂‐fragments.

Figure 1

POV‐Ray depiction of the anion of a) 1 and b) 2. The cation and hydrogen atoms (except NH) are omitted for clarity. C: black, N: blue, F: pink, B: yellow‐green, Co: green, and H: grey.

Scheme 2

Reactions of Ph₂CN₂ with B(C₆F₅)₃ and Cp*₂Co, and with BPh₃ and Cp*₂Cr.

Figure 2

POV‐Ray depiction of the anion of a) 3 and b) 4. The cation and hydrogen atoms (except NH) are omitted for clarity. C: black, N: blue, and B: yellow‐green.

Figure 3

POV‐Ray depiction of the anion of 5. The cation and hydrogen atoms (except NH) are omitted for clarity. C: black, N: blue, and B: yellow‐green.

Scheme 3

Reactions of (C₁₂H₈)CN₂ with BPh₃ and Cp*₂Cr.

Figure 4

DFT‐computed free energy profile (kcal mol⁻¹, at 298 K and 1 mol L⁻¹ reference concentration) for the formation of anion of 3 and 4. Selected bond lengths are given in angstroms while selected C: grey, H: white, N: blue, and B: pink.

Figure 5

POV‐Ray depiction of the anion of a) 8 and b) 9. The cation and hydrogen atoms (except the NH and the Al‐bound CH) are omitted for clarity. C: black, N: blue, Al: Cyan, and F: pink.

Scheme 4

Reactions of (C₁₂H₈)CN₂, Al(C₆F₅)₃ and Cp*₂Fe with Ph₃SnH.