Topicity, Stereoselective Synthesis and Chiral reagents
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What is needed to generate an enantiomerically pure product
• To generate an enantiomerically pure product an enantiomerically pure reagent originating from the chiral pool is needed • Cannot be produced if all the starting materials, reagents and catalysts are either achiral or racemic
What are the 3 types of chiral reagent that could be used to generate an enantiomerically pure product
• Chiral substrate • Chiral reagent • Chiral catalyst
What would be formed if a Nuc was added to homotopic faces
• Homomeric (identical) products • C2 axis in substrate
What would be formed if a Nuc was added to enantiotopic faces
• Enantiomeric products • No C2 axis- but is a mirror plane
What would be formed if a nuc was added to diastereotopic faces
• No C2 axis or mirror plane • Diastereomeric products formed
Does the stereogenic unit need to be adjacent to the reaction centre
• No it doesn’t need to be directly adjacent to the reaction site to generate diastereotopic faces • But selectivity is usually highest when this is the case • If too remote may get 50:50 mix
What is produced if a substitution reaction occurs to a carbon containing 2 homotopic groups (not involved in reaction)
• C2 axis • Homomeric products • No stereogenic centre
What is produced if a substitution reaction occurs to a carbon containing 2 enantiotopic groups (not involved in reaction
• No C2 axis but mirror plane • Produces enantiomeric product
What is produced if a substitution reaction occurs to a carbon adjacent to a stereocentre
• Diastereomeric product • 2 Stereogenic centres
What are types of enantioselective reactions
• Differentiation of enantiotopic faces with a chiral reagent • Differentiation of enantiotopic groups using catalyst - convert one group • Differentiation of enantiomeric molecules - only one reacts with a certain molecule
What are types of diastereoselective reactions
• Differentiation of diastereotopic faces - produces syn or anti • Differentiation of diastereotopic groups - Intramolecular cyclisation where 2 TS are different energy so only product is formed • Differentiation of diastereomeric molecules - One is not reactive due to orientation of groups
What happens if a racemic reagent/catalyst is added to enantiotopic faces/groups
• No ennatiocontrol- racemic product
What happens if an enantiopure reagent/catalyst is added to enantiotopic faces/groups
• Enantioenriched product • Enantiocontrol
What happens if a racemic reagent/catalyst is added to diastereotopic faces/groups
• Induced diastereocontrol • Enantiopure
What happens if What happens if an enantiopure reagent/catalyst is added to diastereotopic faces/groups
• Double diastereocontrol • Want diastereotopic group + reagent and catalyst to reinforce themselves
Define substrate
• Reactant whose main structural unit is maintained throughout a reaction sequence leading to a target molecule
Define reagent
• Reactant that transfers one or more atoms/groups to or from the substrate
For the simplest case of two competing pathways how is the product ratio (P) defined
• Rate of formation of product 1 versus product 2 • P=k1/k2= exp(-DeltadeltaG(TS)/RT) • Difference between TS Delta G / RT
What does the product ratio depend on
• Temperature • Lower temps generally results in formation of chiral products with greater levels of stereocontrol
What temp are stereoselective reactions often carried out at
• -78 degrees • It is easy to use an ice bath prepared from acetone and dry ice • Maximise ee
Describe P ratio relationship to deltadeltaG(TS) and effect on what is required for high selectivity (P)
• Exponential relationship • So energy differences DeltadeltaG(TS) required for high selectivity is surprisingly small in comparison to strength of C-C bonds
Describe allylboration of aldehydes with achiral reagent
• A racemic homoallylic alcohol is produced • The TS are enantiomeric and equal in energy • Si face attack on the aldehyde in one TS • Re attacked in the other
Describe allylboration of aldehydes with a racemic reagent
• A racemic homoallylic alcohol is produced • TS equal in energy • S reagent gives attack on one face e.g. Re • R reagent gives attack on opposite
Describe allylboration of aldehydes with a chiral enantiopure allylating reagent
• An enantioenriched homoallylic alcohol is produced • Only S or R reagent present • For one face groups are equatorial - lower TS