2d Flashcards
okay so when we had an alkene and we wanted to perform an epoxidation reaction but one of the substituents was dashed,, how would we draw the epoxide
we drew the epoxide wedged if the other substituent was dashed
bc we would think of sterics
when we think of whether we should draw an epoxide dashed or wedged,, what should we think of
we should think of possible conformers u get from bond rotation
and which conformer is the lowest energy
bc the lowest energy one will be the conformation the molecule will most likely be in
meaning we need to look at that conformer and see what face the substituents are blocking.
we attack on the opposite side theyre blocking
when we have an alkene and an OH group,, how will epoxidation go down
if u have an OH its gonna H bond with the mcpba // epoxidation reagent.
the O on the OH of mcpba is gonna be the one the double bond attacks
the H of the mcpba OH is gonna be H bonded to the mcpba carbonyl
the extra O on the mcpba is gonna be H bonded to the OH substituent.
this is a stabilising interaction and is called a directing epoxidation bc the substituent is guiding the O into place.
if the OH is dashed the O epoxide will probs also be dashed.
if OH directs the epoxidation to be on the same phase as itself using H bonding to stabilise it,, what happens when we add another substituent to that same face which normally does the opposite effect
the OH will direct the epoxidation towards itself
the other substituent will hinder this but not by a lot
so the epoxidation will probs still occur on the same side as the OH.
okay so so far weve been thinking about cyclic molecules with esstricted rotation,, what about aliphatic ones
we need to think of the different conformers of the molecule
steps to finding the most stable confomer
look at the chiral carbon
usually the one after the double bond
this is position 1,, now count across the double bond till u get to 3.
now take 1 and rotate along that bond so the substituents on the chiral centre move round like a propeller. until u find the one that has the least amount of allylic 1,3 strain.
then look at the substituents and which bulky ones are blocking the faces from being epoxidated.
then epoxidate the ones that arent blocked
why do we base epoxidation off the most stable conformer
bc the most stable conformer is the configuration that the molecule will spend most its time in
so epoxidation will obvs reflect this
we want the conformer with the least amount of what
the least amount of A1,3 strain
aka less allylic 1,3 strain.
when we have OH what do we need to rmemeber
there will be stabilising H bonding so epoxidation will probs occur on the same side as the OH
OH of mcpba with the double bond
H of OH mcpba with carbonyl on mcpba
O on the mcpba with H on the OH substituent
