Introduction to Stable Carbenes Flashcards
What is a stable carbene?
Carbenes that can be isolated in pure form at or above room temperature
What is a persistent carbene?
Carbenes that can be spectroscopically observed but not isolated
What is a carbene?
Divalent carbon atom with six valence electrons
How does carbene electronic configuration influence geometry?
Triplet carbenes tend to be linear, with sp-hybridised centre with two non-bonding degenerate orbitals. Singlet carbenes tend to be bent with sp2-hybridised centre.
What factors influence carbene electronic configuration?
The singlet-triplet gap vs pairing energy - these will be determined by the energy of the sigma and p-pi orbital relative energies. Hund’s rules used to fill.
How do inductive effects stabilise carbenes?
Sigma EWG substituents favour singlet form by pulling electron density away and stabilising the sigma non-bonding orbital.
How do mesomeric effects stabilise carbenes?
Substituents can be pi-EDG or pi-EWG giving rise to resonance forms, stabilising carbenic centre
What electronic effects are there on carbene structure and stability by substituent variation?
Inductive and mesomeric effects, stabilising effect if substituent helps maintain electroneutrality at the carbenic centre.
What steric effects are there on carbene structure and stability by substituent variation?
Bulky substituents kinetically stabilise all carbenes. Increasing steric bulk of carbene substituents can also broaden the carbene bond angle, favouring triplet state.
Rank the substituent effects in determining carbene structure and stability from most to least dominant.
Mesomeric > inductive > > steric
What are the differences between push-push, pull-pull, and push-pull carbenes?
Push-push (most common) tend to be bent singlet carbenes with nucleophilic centres. Two pi-EDG groups.
Pull-pull tend to be linear singlet carbenes with electrophilic centres. Two pi-EWG groups.
Push-pull generally allene-like singlet carbenes.
What methods can be used to generate dicyclicamino carbenes?
Deprotonation of salt, formation of K2S and carbene from relevant thiourea, extrusion of CO2 by NHC.CO2 adduct, alpha elimination. Precursors can be synthesised in a few ways, often used Bronsted or Lewis acid catalysts to improve rate. Similar techniques can be used for triazoliums, thiazoliums etc.
How does the stability of acyclic carbenes compare to cyclic carbenes? (diamino)
Acyclic typically less stable. Donation from nitrogen is poorer due to rotation, so less p-pi donation leading to lower sigma-p-pi gap.
What methods can be used to generate acyclic amino carbenes?
Deprotonation by non-nucleophilic base. Precursor synthesised by formamide via Vilsmeier reaction and amine addition.
What methods can be used to generate BACs?
Synthetic generation from cyclopropenium salt by non-nucleophilic base deprotonation. Precursors by amine reaction with 1,2,3,3-tetrachlorocyclopropenium.
