“σ Bond Metathesis” For C-h Bonds of Hydrocarbons And Sc-r (r = H, Alkyl, Aryl) Bonds of Permethylscandocene Derivatives: Evidence For Noninvolvement of The π System In Electrophilic Activation of Aromatic And Vinylic C-h Bonds

A new class of coordinatively unsaturated, monomeric organoscandium compounds, Cp*₂Sc-R (Cp* = (η⁵-C₅Me₅); R = halide, hydride, alkyl, alkenyl, alkynyl, aryl), has been prepared. Cp*₂Sc-Cl is obtained from reaction of scc 1₃(THF)₃(THF = tetrahydrofuran) with LiCp*, and Cp*₂Sc-R (R = CH₃, C₆H₅, C₆H₄CH₃, CH₂C₆H₅) from treatment of Cp*₂Sc-Cl with the appropriate organoalkali reagent. These organoscandium compounds react with dihydrogen rapidly to yield R-H and Cp*₂Sc-H. The tetrahydrofuran adducts Cp*₂ScX(THF) (X = Cl, H, CH₃) are obtained upon treatment of Cp*₂Sc-X with tetrahydrofuran. Rapid exchange of dihydrogen with the hydride ligands of Cp*₂Sc-H and Cp*₂ScH(THF) occurs even at low temperatures. Other alkyl derivatives may be conveniently prepared by treatment of Cp*₂Sc-H (or Cp*₂ScH(THF)) with a-olefins, e.g., Cp*₂ScCH₂CH₃from Cp*₂Sc-H and ethylene. Allene and Cp*₂Sc-H afford Cp*₂Sc(η³-C₃H₅). Cp*₂Sc-R (R = H, CH₃, aryl) reacts with pyridine to yield Cp*₂Sc(C, N-η²-C₅H₄N), which crystallizes in the orthorhombic space group Pna2₁(a = 16.297 (4) A, b = 9.684 (3) A, c = 14.486 (6) A, V = 2286.3 (12) A³, Z = 4). Least-squares refinement led to a goodness-of-fit of 1.74 for 1128 reflections and a value for R of 0.0362 (I > 3σ, 944 reflections). Spectroscopic and crystallographic data for Cp*₂Sc-CH₃ and Cp*₂Sc-CH₂CH₃indicate that the methyl ligand is bonded in a conventional (i.e., undistorted) manner, while the ethyl ligand likely participates in a β C-H to Sc “agostic” interaction. Cp*₂Sc-CH₃crystallizes in the orthorhombic space group P2₁2₁2₁(a = 8.502 (2) A, b = 11.095 (4) A, c = 21.554 (7) A, V = 2033.2 (11) A³, Z = 4). Least-squares refinement led to a value for R of 0.072 (F_o²> 3σ (f02), 1143 reflections). H/D exchange between H₂, arenes, and the primary and secondary C-H bonds of alkanes is catalyzed by Cp*₂Sc-H and Cp*₂ScH(THF). In benzene solution Cp*₂Sc-H, Cp*₂Sc-C₆H₅, and H₂are in equilibrium (ΔH⁰= 6.7 (3) kcal-moL^-1, ΔS⁰- 1.5 (1) eu), indicating that the bond dissociation energy of the Sc-H bond for Cp*₂Sc-H is 1.5 (4) kcal-mol^-1higher than the bond dissociation energy of the Sc-C bond for Cp*₂Sc-C₆H₅. Cp*₂ScH(THF) is also in equilibrium with H₂, Cp*₂Sc-C₆H₅, and THF in benzene solution (ΔH⁰= 18.9 (8) kcal-mol^-1, ΔS⁰- 25 (2) eu). Cp*₂Sc-CH₃reacts with a range of hydrocarbons (R-H), activating their C-H bonds to afford methane and Cp*₂Sc-R (R-H =¹³CH₄, arenes, styrenes, alkynes). The reaction of Cp*₂Sc-CH₃with styrenes is characterized by a moderate enthalpy of activation (ΔH* = 12 kcal-mol^-1) and a large negative entropy of activation (AS* = -36 eu). A kinetic deuterium isotope effect (k_H/k_D= 2.8 (2)) is observed for the reaction of (Cp*-d₁₅)₂Sc-CH₃with C₆Y₆(Y = H,D) at 80 °C. Very small differences in the rates of vinylic C-H bond activation for (p-C₆H₄X)CH=CH₂(X = CF₃, OCH₃) and aryl C-H bonds of C₆H₅X (X = CF₃, H, CH₃, N(CH₃)₂) as well as the positional nonselectivity for activation of the meta and para C-H bonds of toluene indicate that the scandium center does not interact substantially with the π system of these substrates in the transition states for these reactions. Thus, for these sterically encumbered organoscandium compounds, sp²-hybridized C-H bonds are activated without formation of a π complex; rather, the electrophilic scandium center directly attacks the C-H σ bond. A general mechanism for these reactions with C-H and H-H bonds (Cp*₂Sc-R + R-H, =; Cp*₂Sc-R^/+ R-H), termed “σ bond metathesis”, is proposed, which accounts for the systematic decrease in rate with decreasing s character of the reacting σ bonds (R = R’ = H » R = alkyl, R’ = H » R = alkyl, R^/= sp C-H > R = alkyl, R’ = sp²C-H > R = alkyl, R’ = sp³C-H). Both alkyl exchange |Cp*₂Sc-R + R’-I ⇆ Cp*₂Sc-R’ + R-I (R = CH₃, R’ =¹³CH₃; R = CH₂CH₃, R^/= CH₃)) and alkyl coupling {Cp*₂Sc-R + R’⇆Cp*₂Sc-X + R-R’ (R = R’ = CH₃, X = I ; R = C h3, R’ = CH₂C₆H₅, X = Cl, Br)} are observed in the reactions of Cp*₂Sc-R with alkyl halides.