Carbene Chemistry 1 Flashcards
What are Carbenes?
- Carbenes are neutral, divalent, 6-electron species
- In general they are electron-deficient (electrophilic)
- (divalent - ion/molecule which has a valence of 2)
The first type of carbene is a triplet carbene
Draw its general structure
- Trigonal Planar (sp²)
- 1 electron goes in the p-orbital and one goes in the sp² orbital - both with the same spin
- Similar behaviour to a radical
The second type of carbene is a singlet carbene
Drae it general structure
- Trigonal Planar (sp²)
- Two-paired electrons in the sp² orbital, leaving the p-orbital empty
- Similar behaviour to lone pair of electrons
Triplet carbenes are often lower in energy for simple carbenes, but…
this depends on the nature of the substituents
How can you stababilise a singlet carbene?
In dichlorocarbene, the singlet state is stabilised by electron donation from the adjacent chlorine atoms
Why are N-Heterocyclic carbenes examples of relatively stable carbenes
What properties does this give them?
- As a result of significant electron donation from the adjacent nitrogen atoms
- p-orbitals on nitrogen are the same symmetry as the empty p-orbitals of carbon
- N-heterocyclic carbenes are actually nucleophilic (electron-rich), and are good ligands for transition metals (this is a contrast as carbenes are ususally electrophilic)
You can make a carbene through α-elimination reactions
What is the mechanism and product
You can also make a carbene using β-elimination (E2)
What is the mechanism and product?
What is the reason for these large aromatic substituents of the carbene?
- The big aromatic groups with the methyl substituents provide a lot of steric hindrance
- This protects the n-heterocyclic carbine from side reactions (e.g. dimerising with another)
KN(SiMe₃)₂ will act similarly to LDA in the following reaction
What is the mechanism and product?
Forms an alkylidene carbene
Draw another resonance form from this compound
(you can see the formation of the carbene and loss of dinitrogen a lot more easily on the resonance form on the RHS)
What is the formation of a carbene from this diazo compound
- Liberation of dinitrogen is a strong driving force
How can we reduce the reactivity of the diazo compound?
- If you make R1 and R2 electron withdrawing
- In the RHS resonance, there is a negative charge on carbon, so if we delocalise or put a strong EWG on R1 and R2, we stabilise the structure - less hazardous
α-diazocarbonyl compounds can be prepared from diazomethane and acid chlorides
What is the mechanism and product?
- Nucleophilic acyl substitution reaction
Which resonance form of the two products is most useful mechanistically?
Because that resonance form is the one which demonstrates the most reactivity in the mechanism which are drawn
Why is this resonance form also quite stable
Because of the delocalisation of the negative charge into the carbonyl compound
Diazo compounds can be prepared by diazo transfer reactions
What is the mechanism and product
- Make sure you have the following resonance form shown on TsN₃ as shown otherwise there will be a barrier for attack
- Nitrogen is used to deprotonate the second hydrogen
- Note that the Ts is removed usin e- density from the nitrogen not the negative charge on carbon
Carbenes can also be generated from tosylhydrazones in the Bamford-Stevens reaction
What is the mechanism and product?
- First step uses the reaction of condensation of amines with carbonyl compounds to form imines
Another way of generating α-diazocarbonyl compounds
Starting with an α-ester and treating it with triethylamine we go through the same process as the bamford-steven reactions but produce
Free carbenes are very reactive and often unselective in their reactions
We can moderate and control their reactivity by…
…using metal carbenes, based upon ususally Rhodium or Copper
It removes some of the electron deficiency - so we can tune the reactivity
What does the following mean?
Simplified way of showing two resonance forms
Here is an example of using metal carbenes as a way to tune their reactivity
what is the mechanism and product?
- The rhodium carbene makes the carbene electron deficient, due to the large acetate ligands providing steric shielding - reduced reactivity
How are metal carbenes stabilised?
metal carbenes are stabilised by σ-donation of the carbene lone pair into an empty d-orbitals, and π-donation from a filled metal d-orbital into the empty carbene p-orbital
Stabilises the carebene so its not as reactive
What are the resonance foorms that can be used as an aid to predict/visualise metal carbene reactivity?
