Carbene Chemistry 3 Flashcards
Carbenes readily insert into carbon-hydrogen bonds
What is the general mechanism with free carbenes
- (don’t necassarily need the transition state)
- The carbon takes two electron to attack the carbene, resulting in the hydride character developing a partial positive charge on the carbon
- Resulting in the carbene having been inserted into the C-H bond
Carbenes can readily insert into carbon-hydrogen bonds
What is the mechanism with metal carbenes?
- Charge seperated form is more useful as shows an electron deficient carbon centre
- E- density comes from the electron rich C-M bond
Insertion into X-H bonds (X = heteroatom, such as nitrogen, oxygen, or sulfur) follow a different mechanism
What is the mechanism and product for the insertion into a N-H bond
- Once metal carbene is formed, nitrogen LP will attack e- deficient carbon
- (Although the third intermediate has the +tve charge draw on Nitrogen, it is actually spread about its substituents)
- Proton Transfer from N to C on third intermediate - resulting in Rh falling off as a neutral species, which then reacts with the diazo-compound again (catalyst)
The formation of five-membered rings in intramolecular C-H insertions is particularly favourable
- PT to +tve carbon
- 5-membered ring formed due to the steric hinderance of other C-H bonds
Carbene insertion reactions are generally sterospecific - sterochemistry at the reactive centre is retained
Show how for the following example
- Note where the C-C bond forms in the product is where the C-H bond is in the intermediate
- The geometry of the CO2Me isn’t shown because it could be either (diasteroisomers)
Carbenes can undergo 1,2-shifts, where an adjacent group can migrate with its pair of electrons to the electron-deficient carbene centre
What is the mechanism and product
Carbenes can undergo 1,2-shifts, where an adjacent group can migrate with its pair of electrons to the electron-deficient carbene centre
Show the mechanism and product from this carbene precursor
- Extra arrow for the ejection of nitrogen
For this 1,2-shift of a carbene, show which orbitals are involved
Migratory aptitude refers to the relative tendency of a group or atom to migrate from one part of a molecule to another during a reaction
For 1,2-shifts of carbenes, how does the migratory aptitude of different R groups changes?
Migratory aptitude of groups generally follows the order:
H»aryl»alkyl
Rearrangements of alkylidene carbenes can be used to prepare alkynes
The first step involves a lithium-halogen exchange, what is the mechanism?
Rearrangements of alkylidene carbenes can be used to prepare alkynes
What is the mechanism and product once the lithium-halogen exchange has occured
- At the alkylidene carbene, the hydrogen is moves carbon, taking the two electrons with it
- At the same time, the developing positive charge is neutralised by the lone pair of electrons at the carbon centre
This is another example of a rearrangement of alkylidene carbenes to prepare alkynes
What is the mechanism and the product
The Wolff rearrangement involves the conversion of diazoketones to ketenes, where the sterochemistry at the migrating group is retained
What is the mechanism and product
- Silver salts have been found to promote the loss of nitrogen from diazo compounds to give carbines
- Result in the same type of migration occuring
- But this time the carbene is even more electron deficient due to carbonyl group
Draw the mechanism to show the conversion of this molecule to a ketene
The Wolff rearrangement can also be used in the Arndt-Einstert homologation of carboxylic acids
What is the mechanims of the first step?
The Wolff rearrangement can also be used in the Arndt-Einstert homologation of carboxylic acids
What happens once this first step occurs?
The Wolff rearrangement can be used in ring-contraction reactions
What is the mechanism and product?
- Going from a 6-membered ring to a 5-membered ring
Here is another example of the Wolff rearrangement which can be used in ring-contraction reaction
What is the mechanism and product?
- Although increasing ring strain, it works due to the highly reactive nature of the carbines
