3d Flashcards
what do we need in order to make a stereogenic centre
u need a carbonyl and a nucleophile
and a H to then protonate the O to form OH
when u attack a carbonyl what degree do u attack from
u attack from 107.5
aka the burgi dunitz trajectory
describe a cyclohexanone
its a cyclohexane but with a carbonyl on one of the carbons
the 3 closest carbons including the ketone one are in the same plane!!!!
when u have the cyclohexanone,, and u attack it with a nuc what two diff forms can u get
- u have the OH axial and NUC equitorial
- u have the OH equitorial and the nuc axial
when u have cyclohexanone and u attack it with a bulky nucleophile what do u get,, OH axial and nuc equ or OH eq and nuc axial
when u have a bulky nuc u have the OH axial and nuc equitorial!!
this is bc the axial H’s kinda block the other attack!!!
yhh how do we figure out what side of the cyclohexanone do we attack from and why
we look at the axial groups and the axial down groups
if theres a lot of H axial,,,, then it blocks the nuc from attcking from above!!!!!
so it would rather attack from the bottom!!!!
what about when u have a cyclohexanone with a ch3 on the C adjacent to it
the Ch3 H,, only a singular one will also act like an axial H
which means when we count the number of axil H’s u have 4 axial H’s pointing up.
this obvs blocks the nuc from attacking from the top.
this makes it attack from the bottom!!!!
whats cool about diastereoisomers
its normal for one to be favoured and to be made at a larger amount than the other.
process is nomally stereoselective
how do we knoe if smt is a diastereoisomer
if theres more than one chiral centre
and not all of them changed
aka if the diastereoisoemrs have one chiral centre with the same configuration and then one chiral centre with the opposite configuration.
when we have a carbonyl,, what causes us to have a diastereoisomer
the side we attack the carbonyl on will give us diff isomers
either dashed or wedged
what side do we attack on??? what do we need to draw
we need to draw a newman projection
looking down the CC bond on the sid eof the carbonyl
theres 2 methods of doing this tho
what are the 2 methods of using newman projections to figure out what side of the carbonyl we attack
- donald cram
- kelkin anh
describe the donald cram exp
this is the one we dont use bc we always put the biggest group at the bottom,, ecliped to the group thats bonded to the carbonyl
then u attack between this and the side the smallest group is on
but ofc if this is like a CH3,, why would the biggest group choose to remain eclipesed to it,, aka it wouldnt so this method is a bit weird
describe the kelkin anh exp
u look down the CC bond of the carbonyl and draw the CO upwards and its substituent down.
then u write the biggest group in its own semi circle,, either the left or the right
then u look clockwise to see where all the substituents are
then u draw an arrow from the front of th page to the carbonyl along the bond closest to u.
this is where the nuc will react from.
so its either the M or the S group!! not based on the larger group but on the diff between the S and M.
then look at the moelcule and imagine the nuc attacking along that bond,, then think of what face the OH will be ‘pushed’ onto,, then iffy bc its the opposite of what we would think it is,
felkin anh approach when u have a big bulky group
this is the groupo that has its own semi circle
felkin anh approach when u have an electronegative group
this is the group that has its own semi circle
okay but what about felkin anh when u have a substituent that has the molecules C bound to an O or N
we turn the molecule so that the carbonyl O and the O / N of the substituent are close and can aggregate to the metal Mg from the gringard reagent.
this is called the chelate effect : or the cram chelation control
how do we make another diastereoisomer more selected
by changing the size of the groups!!!! of the substituents
