Rebecca Melon Flashcards
Two methods to calculate Lewis acidity
- Gutmann Becket Test
- Childs method
Gutmann Becket test
- The lewis acid in question forms an adduct with triethylphosphineoxide (Et3P=O).
- The result is the removal of electron density from the P atom, which can be obsereved by a deshielding effect in the 31P NMR chemical shift.
- From this a value of Lewis acidity can be calculated
Childs Method
- Measure the downfield chemical shift of the H3 proton on crotonaldehyde, upon coordination to a lewis acid.
- The more downfield the shift of H3, the stronger the lewis acid
Lewis Acid and lewis Base
Lewis Acid - Electron Pair acceptor (non-bonding, low energy sigma* orbital)
Lewis Base - Electron pair donator
Frustrated lewis pair (FLP)
A frustrated lewis pair is a compound or a mixture of two compounds that contains both lewis acidic and lewis basic sites which are prevented from adduct formation due to steric hinderance
Dehydrocoupling
Element-element bond formation
Hard/Soft acids and bases
Hard: Small, highly charged, weakly polarisable
Soft: Large, low charge, strongly polarisable
FLP reaction with alkynes (deprotonation or Nucleophilic attack)
-Reactivity depends on the nature of the phosphine
-More bronsted basic phosphines favour deprotonation
tBu3P = deprotonation
o-tol3P and PPh3 = 1,2 addition
-Can be proved through P NMR (presence of PH coupling)
Main group catalysed B-N dehydrocoupling (requirements)
- A strong base to deprotonate the amine-borane (hydride, amido, alkyl)
- A lewis acidic metal to act as a lone pair acceptor (from amine)
Which dehydrocoupling reaction works best Si-S or Si-O?
S-H bonds are weaker than O-H bonds and so sulphur will work better.
B-N dehydrocoupling: primary vs secondary amines
- Primary amines you can loose up to 2H2 per B-N unit. Typically form borazine
- Secondary amines loose 1H2 per B-N unit. Bulk R groups favour N=B unit where as small R groups favour dimer
