Organocatalysis: A case study: CO2 activation

why

era of transition metal catalysis is over

• need for organocatalysis for sustainability

• we look specifically into CO₂ activation

  • = use CO₂ as building block for organic materials

Use of CO₂

• renewable resource, but poor reactivity due to kinetic inertness

  • high activation barrier

  • so lower barrier using catalysis

• Direct use of CO₂ is much better than sequestration

  • sequestration = CO₂ capture from the atmosphere

  • but low conc (although too high) so not very efficient

  • using CO2 directly (eg produced by a plant) is much more efficient

• metal catalysis has been explored in depth for a long time but has shortcommings

  • rather toxic

  • expensive (Pd, Rh,…)

  • active catalysts often very sensitive (like Pd to air —> complicates synthesis)

  • even immobilization of homogenous catalyst is sufficient to obtain traceless (metal-free) products

• photosynthesis is good example in nature of CO₂ incorporation, but uses metals

  • through enzymatic catalysis with non toxic, abundant metals like Mg or Fe

  • but harnessing catalytic power of such metals is much more difficult than just using Pd, …

organocatalysts

• cheaper, non-toxic and readily available

• can be derived from biomass and in some cases even be extracted from it

• main drawback is lower catalytic activity

  • solve by using higher catalyst loadings

  • not a problem due to availability and lower cost

Main CO₂ using pathways

non-reductive pathways

• produces cyclic carbonates, cyclic carbamates and quinazolin-2,4-diones

• can be used for the production of polycarbonates or polyurethanes

  • their production is currently not sustainable

  • bisphenol A is hormone disrupting

  • phosgene = toxic dangerous gas

reductive pathwyas

• produces methanol, methane or formyl derivatives

• useful as (liquid) fuelss

• BUT sustainable reductant is not trivial

• holy grail would electrochemistry (but not there yet)

  

Activation modes for CO₂ incorporation in cyclic carbonates

Two pathways

• activation of CO2

  • base attacks on CO2 forming more nucleophilic O that can attack and ring open an epoxid which after ringclosure regenerates the base

• activation of the epoxide

  • bas ring opens the epoxide, epelled oxygen can attack on CO2, base is expelled again after ringclosure

however: SM is epoxides and are still toxic

Dual activation = combine both mechanisms

• funcitonalized IL (ionic liquids)

• NHC (N-heterocyclic carbenes)

• FLP (frustrated lewis acid base pair)