Carbene Chemistry 2 Flashcards
What is the basic structure of an oxonium ylide?
- Note the 1,3-dipole
The reactivity of carbenes can often be readily visualised with the help of a zwitterionic resonance form
Draw this, and what does it demonstrate?
- Carbenes are generally electrophilic (6e species) and react well with nucleophiles
- Want to push electron from the lone pair AND at the same time accepting a pair of electrons - complete octet
The Reimer Tiemann reaction is a reaction of dichlorocarbene with phenols
It involves a nucleophile-electrophile reaction
What is the mechanism and product?
- The NaOH will deprotonate phenol
- Substitution with carbene occur at the ortho and para position as the e- density can be stabilised there (resonance forms)
- Negative charge need to be transfered if reaction occurs with neutral species
- (The reaction isnt finished here)
The Reimer Tiemann reaction is a reaction of dichlorocarbene with phenols
- Introduction of a aldehyde group at the ortho position on a phenol
Carbenes react with heteroatom lone pairs to form ylides which can then undergo a range of different reactions. The following example, an oxonium ylide is formed which undergoes a 1,3-dipolar cycloaddition with an activated alkyne
What is the mechanism for the first step?
Carbenes react with heteroatom lone pairs to form ylides which can then undergo a range of different reactions. The following example, an oxonium ylide is formed which undergoes a 1,3-dipolar cycloaddition with an activated alkyne
What happens once the metal carbene is formed?
- The oxonium ylide in this case is a Rhodium bound one which is why it has a slightly different structure
- The C-Rh bond is the most electron rich in the 1,3 dipolar cycloaddition
In this example, the oxonium ylide undergoes a 2,3-sigmatropic rearrangement
What is the mechansim and product
- Again produces a Rhodium bound oxonoim ylide
- Note there are no overall changes of the intermediates
- 2e- go from the electron rich C-Rh bond to the terminal carbon atom in the last step
Carbenes undergo cycloadditions with alkenes to form cyclopropanes
The cyclopropanation of singlet carbenes are concerted, sterospecific reactions
What is the reaction mechanism and product?
The stereospecific nature means that (E)- and (Z)-isomers of alkenes give rise to different steroisomers of cyclopropane
What is the mechanism and product of the following reaction?
Draw a molecular orbital representation of cyclopropanation of alkenes with singlet carbenes
(remember empty p-orbital and 2e- in the sp² orbital
A non-linear approach is adported to enable orbital overlap
Cyclopropanations with triplet carbenes usually give rise to steroisomeric mixtures
What is the mechanism and product of this reaction
- Triplet carbenes (2 radicals with the same spin)
- Cyclopropanations with triplet carebenes usually give rise to stereoisomeric mixtures, why
- This is due to them reacting in a stepwise mechanism due to parallel spin configuration. The intermediate allow for free rotation and spin state mixing which reduces the sterochemical control
Rhodium carbenes can undergo intramolecular cyclopropanations with tethered alkenes - these reactions are sterospecific
What is the reaction mechanism and the product
- Is E-alkene because R is trans to the rest of the molecule
Rhodium carbenes can undergo intramolecular cyclopropanations with tethered alkenes - these reactions are sterospecific
What is the reaction mechanism and the product?
- The reaction of the Z-alkene
Which conformer don’t exist IRL and why
It is impossible when a 3MR attached to a 5MR, where one hydrogen is up and the other hydrogen is down
Carbenes also can react with alkynes to form cyclopropenes
What is the mechanism and product?
What happens to the Rhodium once the C-Rh bond is broken
- Leaves as Rh₂(AOc)₄ = [Rh]
- Acts as a catalyst (regenerated) as well as speeding up reaction + making it sterospecific
