Stereochemistry and synthesis Flashcards
Stereoselective reaction
Many reactions can give many stereoisomers and in this instance, one stereoisomer is favoured
Stereospecific reaction
A specific stereoisomer gives a specific stereoisomer product
Why is the Diels-Alder reaction stereoselective?
The reaction occurs in a single step, the transition state has implications on this selectivity
Diels-Alder reactions with cis and trans dienophiles
The stereochemistry of the substituents of the dienophile is maintained. If they begin trans to one another, they will be trans to one another in the final product
What happens to the substituents of the diene?
The substituents will be pushed ‘down’ remaining cis to each other in the final product.
Diastereotopic faces and groups
Faces of aldehydes are diastereotopic. Addition to either face gives a specific diastereoisomer. The same is true of groups in chiral molecules. The substitution of a proton in a chiral molecules will give one of two diastereoisomer
Enantiotopic faces and groups
Faces and groups in achiral molecules whose attack on or replacement will give a specific enantiomer
Guiding attack on the faces of aldehydes
Substituents can block attack to a face of the carbonyl however, rotation about bonds makes attack on either likely. Adding a RMgBr species chelates the aldehyde, holding it in a conformation, guiding the line of attack
Why do we want to use catalytic amounts of chiral ligands when synthesising chiral products?
They’re expensive and large
What is Et2Zn
An organozinc reagents. Acts like a Grignard reagent but is much much slower and shittier
Using organozinc species to get single enantiomer products
Use an amino-alcohol chiral ligand to give a highly reactive, chiral reagent. This then reacts to give the final enantiomer, before dissociating from the product to be used again
Enantiomeric success definition
Measure of the ratio of the enantiomers in the product. Is the % of the major enantiomer - % of the minor enantiomer
Enantiomeric vs diastereomeric transition states of aldehydes
Using a non-chiral reagent, the transition states of the attack to give two enantiomers are also enantiomers and therefore equal in energy. Using a chiral reagent, the transition states form two enantiomers are diastereoisomers and hence, have different energies which gives rise to enantioselectivity
Face assignment of aldehydes
Assign priority like you would a chiral centre. a = highest, c = lowest. Clockwise a,b,c is the Re face, anti-clockwise a,b,c is the Si face
