“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

Mark E. Thompson, Steven M. Baxter, A. Ray Bulls, Barbara J. Burger, William P. Schaefer, John E. Bercaw, Michael C. Nolan, Bernard D. Santarsiero

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Abstract

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 (Fo2> 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 (kH/kD= 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.

Original languageEnglish (US)
Pages (from-to)203-219
Number of pages17
JournalJournal of the American Chemical Society
Volume109
Issue number1
DOIs
StatePublished - Jan 1 1987
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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