Nitrene Chemistry Flashcards
What are Nitrenes
- Nitrogen version of carbenes
- They are neutral, monovalent (3-2= 1), electron-deficient, 6-electron species
- In general, are electrophilic
- Are approximated as sp-hybridised and exist in triplet or singlet form
Draw the generate structure of a singlet and triplet nitrene
Nitrenes can be generated from azides (there are some parallels with carbene generation from diazo compounds)
What is the mechanism and product?
- Curly arrow starts from the N-N bond
- If you supply energy (heat/photons) causes decomposition of acids to give nitrenes
Alkyl azides are not the most stable species, therefore adding an acyl group will increase the stability
What is the mechanism and the product
- The formal negative charge on N will be delocalised across the acyl group
- Once it becomes an acyl nitrene, it mangnifies the electron deficiency so are pretty reactive species
Metal Nitrenes can be prepared from amides, carbamates or sulfonamides containing a NH₂ group
- The two hydrogens are removed from the nitrogen group
- Rh groups used in sub-stoichiometric quantities as a catlyst
- Magnesium oxide is a base & iodine (III) compound is an oxidant
The same way that carbenes react with alkenes to form cyclopropanes, nitrenes react with alkenes to form aziridines
- The sulfonyl group is EWG, hence making the nitrene more reactive than it would usually be
- 2e- come from the N-[Rh] bond to attack the carbonyl
- Note this is a stereospecific reaction
Similar to carbenes, nitrenes readily insert into carbon-hydrogen bond stereospecifically
What is the mechanism and product?
- 5-MR ring forms as two moeties are close together and avoids ring strain
Nitrenes can readily insert into carbon-hydrogen bonds stereospecifically
What is the mechanism and product?
- Formation of metal nitrene has been skipped
- In this mechansim, because we don’t have a free nitrene, the e- density comes from the N-[Rh] bond
- The [Rh] has not been shown falling off
- Here we form a 6-MR, due to the presence of large atom (S + N), results in longer bonds, which results in the favouring of larger rings AND +tve charge is stabilised more on that carbon
In this case, does a 5-MR or a 6-MR form?
- A 5-MR as less strained, as the C-H bond to form a 6-MR is further away
- Both bond point up, as the C-O points up and it is more favourable to react with the C-H bond which is up
- The product has an enantiomer of the 5-MR being on C1+2 rather than C1+6
Acyl azides undergo 1,2-shifts in the Curtius rearrangement which may proceed via a transient free acyl nitrene
(This reaction is similar to the Wolff rearrangement)
- This mechanism has been abbreviated
- Good way to put a C-N bond where there was a C-Carbonyl bond
Draw the curly arrows to show a concerted mechansim in the Curtius rearrangement
