5d Flashcards
alkylation of enolates,, what does that mean
were adding an alkyl chain to the enolate
enolates are flat??
yes
when we have a flat enolate and we can add smt dashed or wedged,, why does it sometimes not matter
bc if theres no other chiral centres
the one chiral centre being dhased or wedged wont matter
bc theyll be enantiomers!!! bc all the chiral centres have changed
youll get a racemic mix bc they have the same energy
when theres no stereoselectivity,, how can we make it stereoselective
by forming diastereoisomers kinda
u basically want to add a chiral centre to the molecule
a chiral auxilialy!!!
whats a chiral auxilialy ? and give an example
smt u add to a molecule to allow it to have a chiral centre!!
think of a 5 membered ring with a N,, carbonyl, O , O then a isopropyl group coming out of the bottom left side (this is dashed)
and the N is bonded to the rest of the molecule were attaching it to
what does adding a chiral auxilialy do
it makes a reaction more stereoselective
bc u will now have a diastereoisomer,, we just need to figure out what the ratio is between them
it makes it a temporary diastereoisomer.
what can do chiral auxilialy do to the molecule before anything acc happens
it can make bonds rotate, to get substituents far away from the auxiliary!!
what do we need to remeber about O-
it cant be bare,, if we used LDA,, well have lithium bonded // associated to it.
what happens once the enolate is formed
that conformation is locked!!!!
the double bond can no longer rotate!!!
aka so if the bond rotated so the substituents would be far from the auxiliary,, it now remains in that position
if were using the auxiliary we think we are,, what wisde will the enolate attack the electrophile from
the auxiliary is dashed meaning its pointing away.
meaning that the electrophile will be added to the front to avoid the bulky group!!!
once weve used the enolate and added the alkyl,, what do we then need to do and how do we do this
we then need to remove the auxiliary,,
we do this by cleaving the bond by reacting H-O-O- as a nucleophile which will attack the carbonyl and cleave off the auxiliary!! whihc is also bound to the carbonyl.
we only get the wedged conformation!!!!!
what happens when we want to get the dashed conformer even tho the auxiliary is blocking the back side from enolate attack
we rotate the bond!!!!
bc by rotating it we see that both its non rotated and rotated form are both S chiral centres!!
so for this one we start with the group we added via enolate as our original substituents
and we attack the previous original substituent using the enolate we form!! bc in this way the new substituent is added to the wedge,, but its still S conformation!!! so it still workksssssss
if the stereochem only allows the dashed form but u want the wedge,, what do u do
u switch the substituents around
and then u rotate the bond
but make sure its still the same R or S as u wantttt the product to be!!!
even tho u have a product with super high yield,, how do u increase it even more using the auxiliary we had before
we could make the branching larger by adding another methyl!!
butttt
this isnt favoured bc its made using an aa,, so chnaging that is very spenny.
so we add 2 methyl groups on the other bottom end which sterically push the methyl groups further in the way of the enolate,, preventing any possible backward attack!!!!!
