C-H activation Flashcards
(39 cards)
E-factor
Mass of waste / mass of products
General C-H activation partial catalytic cycle using a directing group
Draw
Benefits of C-H activation catalysis
Atom efficient
Easier to purify the products (fewer by-products)
Problems with C-H activation catalysis
Need efficient catalysts
Need to understand stoichiometric reactions in order to develop catalysis
Why is activation of C-H bonds attractive?
Many hydrocarbons e.g. methane, benzene are cheap and abundant so could be valuable feedstocks
In an ideal world, we would be able to use C-H activation to selectively functionalise these unfunctionalised alkanes without the need for directing groups
Two methods for activating a C-H bond
- Sigma-bond metathesis
2. Oxidative addition
An overall C-H activation equation (OA)
LnM + R-H —> LnM-H(-R)
Thermodynamic problems for C-H activation
DeltaS = C-H activation is disfavoured entropically (2 molecules --> 1 molecule) DeltaH = reaction must be sufficiently exothermic to overcome the loss of entropy
Why is C-H activation harder than H-H activation?
Energy required to break H-H = 436 kJ mol-1
Energy release on forming M-H = 311 kJ mol-1
Energy required to break H3C-H = 436 kJ mol-1
Energy released on forming M-CH3 = 235 kJ mol-1
i.e. H-H and H3C-H bonds have similar strengths
Less energy is released on forming a metal-carbon than metal-hydrogen bond, so H3C-H activation is less favoured
Bond enthalpies for Ph-H activation
Energy required to break Ph-H bond = 460 kJ mol-1 (more energy required than for general C-H due to breaking aromaticity)
Energy released on formation of Ph-M = 344 kJ mol-1 (more energy released because there are opportunities for donating electron density onto the metal (ring acting as ‘electron pump’) and backbonding from the metal onto the ring (ring acting as ‘electron sink’))
Why does spontaneous complex decomposition occur with M-alkyl complexes?
Because M-C bond is so weak
(and C-H bond is strong)
Draw
Kinetic problems for C-H activation
Issues generally related to sterics
Difficult for metal to approach sp3 C-H bond without encountering severe steric hindrance (draw)
Therefore sp3 C-H bond activation often has a very high activation energy
Stability of product from C-H activation
M-C complexes can undergo beta/gamma-hydride elimination
Intramolecular C-H activation
Easier than intermolecular!
Thermodynamics: entropically neutral (just 1 molecule reacting with itself) and chelate formed (chelate effect = driving force)
Kinetics: complex is pre-disposed to C-H activation because the C-H is already in close proximity to the metal centre
Agostic interactions
If the C-H bond is weakened through interaction with the metal centre but not completely broken
Occurs when the C-H bond of a substituent on a ligand interacts with an unsaturated metal centre (requires an empty orbital on the metal to accept the electron density)
Can get alpha- and beta-agostic interactions (draw)
Electronics of agostic interactions
Similar to the bonding in B2H6
2 electrons are donated from the C-H bond to the metal centre – 3 centre-2 electron bonding
(i.e. contributes 2 electrons to valence electron count)
The metal contributes an empty orbital to the 3c-2e bond
Agostic interactions with non-d0 metals
Metal can backdonate electron density into sigma* C-H (strengthens interaction)
But if the backbonding is too strong, the C-H bond is fully cleaved (‘activated’) and the metal alkyl-hydride complex is formed
If sigma-donation is too weak, the agostic interaction is unlikely to occur
Analytical techniques to detect/determine the presence of agostic interactions
Increase in C-H bond length can be detected by neutron diffraction
Normal C-H = 110 pm, agostic C-H = 113-119 pm
Short M—H contacts
1.8-2.3 A (but this is longer than a normal M-H bond which is 1.5-165 A)
Narrow M-H-C bond angles
90-140 degrees
Reduced coupling constants in 1H NMR due to Karplus relationship
Normal C-H = 120-130 Hz, agostic C-H = 75-100 Hz
Upfield shift in 1H NMR
-5 to -15 ppm (due to hydridic character)
IR stretching frequency lower - consistent with a longer, weaker C-H bond
Normal C-H = 3000-2800 cm-1, agostic C-H = 2700-2300 cm-1
Anagostic interactions
Any M-H-C interactions that do not involve 3c-2e bonds
i.e. not quite an agostic interaction, very limited exchange of electron density between M and H
What are anagostic interactions characterised by?
Long M—H interactions 2.3-2.9 A
Large M-H-C interactions 110-170 degrees
Downfield shift in 1H NMR (i.e. a non-hydridic, H-bonding-type interaction)
Examples of intermolecular C-H activation reactions with isolated products
See flashcards
Common features of intermolecular C-H activation reactions with isolated sp3 products
Low coordination number
Sigma-donor ligands on metal centre e.g. PR3 (R=alkyl)
No net change in oxidation state (complex first undergoes reductive elimination to form really low coordinate, highly reactive species that then undergoes oxidative addition)
Mechanistic studies on Bergman C-H activation reaction
Two possibilities for reaction mechanism
- Reductive elimination/oxidative addition
- Radical process
Draw
Whitesides investigation into his C-H activation reaction
Whitesides knew that the first step involved reductive elimination of neopentane
Wanted to investigate whether this was facilitated by a Cy group in the phosphine ligand undergoing its own C-H activation to form a chelate and stabilise the low-coordinate Pt centre
Found that this didn’t happen - the bidentate phosphine ligand is ‘tied back’ so the Cy can’t swing round to be in close enough proximity for C-H activation
Steric clash between the neopentane ligand and the Cy groups leads to instability, which facilitates neopentane loss
The unstable, low-coordinate Pt complex formed then reacts with the cyclopropane in solution to form a strong Pt-C bond
This product also has less steric repulsion than the neopentane
