ChemInform Abstract: “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.

Abstract

224ChemInform Abstract A new class of coordinatively unsaturated, monomeric organoscandium compounds has been prepared. (I), obtained from ScCl3(THF)3 and pentamethylcyclopentadienyllithium (xylenes, 140 rc C, 3 d), undergoes metathetical reactions with rganolithium reagents to give the derivatives (IIa)-(IIc) which react rapidly with H2 in hexane to yield the unstable complex (III) and in THF to yield the stable adduct (IV). Rapid exchange of dihydrogen with the hydride ligands of (III) and (IV) occurs even at low temperatures. eactions of (III), generated in situ, with olefins or allene at -80 rc C yield the derivatives (IId), (IIe) and (V). (III), (IIa), (IIb) (R: -Ph), (IIc) and (IV) react with Py to afford the orthometallated complex (VI), which crystallizes in the space group Pna21 with Z=4. Spectroscopic nd crystallographic data for (IIa) and (IId) indicate that the methyl ligand is bonded in an undistorted manner, while the ethyl ligand participates in a β-C-H to Sc "agostic" interaction. (IIa) crystallizes in the space group P212121 with Z=4. The H/D exchange between H2, renes and C-H bonds of alkanes, catalyzed by (III) and (IV) is investigated thermodynamically. (IIa) reacts with a range of hydrocarbons, activating their C-H bonds to afford CH4 and complexes such as (IIf). Kinetics for these reactions are studied. Very small differences n the rates of vinylic C-H bond activation for p-substituted styrenes (X: -CF3, -OMe) and aryl C-H bonds of PhY (Y: -CF3, -H, -Me, -NMe2) as well as the positional nonselectivity for activation of the meta and para C-H bonds of toluene indicate that the Sc center does not nteract substantially in the π-system of these substrates in the transition states for these reactions. Thus for sterically encumbered organoscandium compounds, sp2-hybridized C-H bonds are activated without formation of a π-complex;

Computational study of carbon–hydrogen bond deprotonation by alkali metal superbases

“…We further note that the ADF-EDA analysis of the LiNH 2 /CH 4 transition state yields nearly equal stabilization from orbital and electrostatic interactions, which counteracts the usual often assumption that deprotonation reactions are highly ionic and have little covalent character. This latter result is also consistent with the modest q values computed and experimentally measured for activation of C-H bonds by electrophilic transition metals with nucleophilic activating ligands [69]. 6.…”

“…Experiments by Bercaw for r-bond metathesis with cyclopentadienyl scandium complexes showed insensitivity to substituted arenes, with k X=NMe2 / k X=H % 0.97 and k X=CF3 /k X=H % 0.42 for arene C-H cleavage of C 6 H 5 X (X = H, CF 3 or NMe 2 ) [69]. Herein, in both the Li and Cs amide systems, the calculations suggest that inductive (resonance) effects will be increased (lessened) in the presence of solvent.…”

C−H Activation and Olefin Insertion in d8 and d0 Complexes: Same Elementary Steps, Different Electronics

“…C−H Bond activation and olefin insertion represent two ubiquitous reactions in homogeneous catalysis . The insertion of an olefin into a metal−carbon bond is the key elementary step in olefin polymerization, controlling the regio‐ and stereoselectivity and thereby the properties of the resulting polymer .…”

“…Frontier molecular orbital analysis will be accompanied by and related to calculated energy profiles of C−H activation and olefin insertion reactions. The obtained results will be contrasted with 1‐Sc used as a prototypical d 0 ‐configured complex that is active in both C−H activation and olefin insertion …”

“…[19] The necessity of a π-character in the MÀ C bond probably explains why many late transition-metal complexes that are involved in CÀ H bond activation contain strong πacceptor ligands such as CO. With increasing d-electron count, the π*(MÀ C) orbital can be populated, leading to a preference for oxidative additionreductive elimination pathways over σ-bond metathesis. d 8 Square planar metal alkyl complexes represent another important class of homogeneous catalysts. In particular, cationic α-diimine Ni(II) and Pd(II) methyl complexes and related complexes have been developed as alternative polymerization catalysts to traditional d 0 metallocene or Ziegler-type systems, with the advantage of being tolerant towards olefins bearing polar functional groups.…”