C-H Bond Activation Flashcards
why oxidative addition more difficult than H-H
less favourable thermodynamics increasing dS and more challenging kinetically due to steric hinderance upon approach of CH bond to MC giving a higher activation energy
how is CH bond broken
s/p-donation weakens CH bond, need efficientt pi-back meaning very e-rich MC
why intramolecular better than inter
thermodynamics = no -ve dS and products stailised by chelating effect
kinetics = CH bond already close to MC so less steric repulsion
CH ligand type
L
Bonding of agostic complexes
depopulation of CH s BMO, population of MLn s* ABMO, not enough pi-back to break CH bond, flow of e- density from CH to MC
geometric consequences of agostic complexes
elongated CH bond, shorter MH bond and distortion of MC bod angle for agostic ligand
characterisation of agostic complexes
structural data with neutron diffraction, 13C NMR shows reduced CH coupling compared to non-agostic bonds, IR spec as weaker CH gives lower stretching freq
characteristics of CH activation via s-bond metathesis
common with d0 as no oxidative addition possible, needs vacant site at MC and polarisation of RH bond as it approached ionic MR’ bond
detection of alkane s-complexes
in situ generation and detection using NMR or evidence from H/D scrambling
synthesis of alkane s-complexes
alkanes = v weak ligands, too easily displaced in soln, produced directly in solid state
Murai reaction
intermolecular C(sp2-)H activation that couples aromatic ketones with alkenes using [Rh]
adv of murai
alkene adds ortho to keto sub due to donation from lp acting as directing group and selective to straight chain alkyl subs
consequences of metal subs
products controlled by sterics, reaction at less hindered alkyl C
catalyst used for alkane dehydrogenation
Ir complex with PCP pincer ligand
