Asymmetric Synthesis (sorta more notes) Flashcards
Felkin-Anh Model - what is it, how does it work
Basis: Attack of nucleophile to carbonyl. Draw circle with carbonyl sticking upwards and H downwards and substituents around the side. The nucleophile will attack on other side of LARGE group which likes to be PERPENDICULAR. Rotate so that the nucleophile is opposite the large group, then rearrange into structure. Remember this is for TWO CARBONS. keep nuclophile and large group on PLANE OF PAGE! also large group goes left hand carbon.
Why is Felkin-Anh Model selective/work? And when does it work better? and Why?
It works because large group has preference for equatorial position but only in about a 4:1 ratio, it becomes more selective when there are ELECTRONEGATIVE substituents, due to hyperconjugation between sigma* orbital of C-X substituent and pi* orbital of C=O bond (but no electrons in these orbitals until nucleophile arrives.
When does electron withdrawing substituents not increase selectively of Felkin-Anh Model?
When using chelating metals (e.g. Mg, Zn, Ti, Ce) that bind carbonyl and EWG substituent (O, N, S) in axial position! This is acc advantageous as have access to both stereoisomers now :)
Which chelating metals can be used to make anti Felkin-Anh Products?
Ti, Ce, Zn, Mg, (bind O, N, S)
Addition to benzaldehyde with BuLi and cold temp?
Attack of carbonyl carbon to form ester with BLOCK
Addition to benzaldehyde with Et2Mg and cold temp?
Attack of carbonyl carbon to form ester with WEDGE line (R?)
Addition to benzaldehyde with Et2Zn and 20 degrees?
Addition of Et group and OH with WEDGE line, also need a chiral aminoalcohol to activate
What do you need to add to reaction to react ZnEt2 with aldehyde?
Aminoalcohol (e.g. DAIB or others), need it as it binds to zinc first in dimeric fashion (one zinc binds to N, one to O) this coordinates aldehyde, separation
Benzaldehyde and ZnEt2, toluene, <0 degrees, DAIB/aminoalcohol
Addition of ethane with OH in WEDGE line
