7d Flashcards
whats an enantioselective reaction
a reaction where 2 enentiomers are made but one is made faster // at a greater ratio than the other
how are enantioselective reactions a thing??? like how is one enantiomer made more than the other if enantiomers are known for having the same reactivity and therefore the same ROR and ratio
there has to be smt diastereoisomeric about it
and this is its transition state!!!!
what does epoxidation allow
epoxidation allows us to attack the alkene from the back or the front
u have 2 options!!! so youd get enantiomers, one wedged and one dashed
whats cool about diastereoisomers again
they have diff energies and therefore diff ROR and therefore are made in diff ratios
when we have achiral sm and form smt chiral,, what does that mean
we def had smt diasteromeric in between that made it chiral
okay describe the energy diagram that makes a enantioselective process
u have ur SM which are achiral at the bottom
then u have the ts which is a diastereomeric ts
thenu. have the enantiomeric products
the 2 lines arent overlapping bc they have diff enenrgies
due to the diastereomeric ts
meaning one enantiomeric product is made faster and more is made .
okay when we go from achiral to chiral ,, what do we use to do this
we use either a chiral catalyst,, in reactions that cannot undergo without a catalyst
or we use a chiral oxidising agent !! either an S or R one.
what does a chiral catalyst do to the reaction
it adds chirality
it adds an O to the substrate
okay when we have an alkene with a CH2 and then an OH after this,, and we want to perform an epoxidation reaction using an S-catalyst,, what ts and products do we get
we get the dashed epoxide with an S catalyst attached to the O : this will be SS
then u have the wedged epoxide with the S catalyst attached to the O. this will give us SR
and these are diastereoisomers
aka we get diastereosiomers as products
okay describe the sharpless asymmetric epoxidation
u have ur alkene with CH2 and OH after this!!!
then u need ur
Ti(Oi-Pr)4
t-BuOOH
i-PrO2C- OH wedge - OH wedge - CO2i-Pr (this is the chiral bit and its S config)
the Ti from the Ti(Oi-Pr)4 coordinates to the O of the OH of our starting material. then it also corrdinates to the O from t-BuOOH.
this arrannges the moecule in a way that adds the O from t-BuOOH on one side of the allylic alkene
when we say the alkene with the CH2 and the OH at the end, what is this acc called
an alyllic alcohol!!!
whats an allylic alcohol
when the OH group is bonded to the C adjacent to the double bond.
whats chemoselective mean
when a reagent prefers to react with one fg over the other fg’s.
what does allylic mean
the carbon adjacent to the double bond
okay we dont need to know the jacobsen epoxidation but expoain what happens
manganese complex has a chiral ligand ( and we only have a single enantiomer of this - aka only its S or R)
this means that its able to give asymmetry and stereoselectivity!!
we get an epoxidation where both things are wedged from an alkene. RS
describe sharpless asymmetric dehydroxylation
its when an alkene gets 2 OH’s added to it
both wedged!!!!
what chiral ligand do we use for sharpless asymmetric dihydroxylation
we use (DHQD)2 PHAL
so get both wedged
and we use (DHQ)2 PHAL as the enantiomer of the chiral ligand to get both dashed
this is bc the chiral complexes arent rlly enantiomers,, but they react as if they were. they function as enantiomers so we just use them as they were.
think that one of them already have D in their name so they need to have wedged products
and the one that doesnt have a d in its name needs dashed products
what previous way do we know we can hydrogenate smt
we can do this by using the metal surface and then the alkene approaches it and u have symmetric hydrogenation
what happens during asymmetric hydrogenation
we have an achiral ligand and H is added to both faces of the double bond to form a racemic mixture.
when we have a chiral ligand (but only one enantiomer of it) it only attacks from one side, so we get one product formed more than the other.
in the catalyst /// ligand used in asymmetric hydrogenation,, what atom is chiral
the P!!!
bc it has 3 diff groups and a lone pair!
so P is the chiral one.
are alkenes the only thing that can be hydrogenated,, also ehat is hydrogenation btw
hydrogenation is when u add H to a double bond
alkenes can by hydrogenated but so can carbonylsssss
whenu hydrogenate a carbonyl what do u get
u get an alcohol.
what does using a chiral ligand on an achiral molecule do when we hydrogenate smt
the chirality of the ligand makes one enantiomer of the product favoured over the other
so we get an enantioselective reaction!!! where the TS is diastereomeric,, aka diff in energy,, allowing diff ratios of enantiomers to be made.
if u have a cyclohexane with MgBr,, aka a gringard reagent attached to it,, and u react it with an aldehyde what happens
the aldehyde takes the place of the MgBr
and u have an alochol bc the aldehyde will be hydrogenated!!!
what reacts the same as gringard reagents
dialkyl zinc compounds!!!
aka R2-Zn
the zinc compounds are slightly less reactive tho!!!
due to the dialkyl zinc compounds being less reactive than gringard agents,, to react them with an aldehyde,, what do we need to do
we need a catalysttttt
what type of catalyst do we need in the dialkyl zinc aldehyde reaction
we need amino alcohol catalyst
aka like smt like DAIB
describe DAIB
catalyst needed for dialkyl zinc to react with aldehydes
its like a bent rectangle with NMe2 top right,, OH bottom right and we have a hut with 2 methyl groups joining the 2 middle carbons
and u then have a methyl coming from one of the middle carbonssss of the framework
okay when u have a MgBr and an aldehyde what is the product
the alldehyde attaches to where MgBr was,, and an alochol forms
but this will now be a chiral centre meaning u can have dahsed or wedged .
normally a racemic mix is madeeeeeee. aka 50:50 ratio of both enantiomers
okay so if we want to make one enantiomer over the other when we have a gringard and an aldehyde,, what do we normally think to do
we normally think of making the gringard asymmetric ,, aka making it chiral
but we cant do this bc the gringard is too reactive
whats a downside to aldehyde and dialkyl zinc reactions
they occur so so slowly
like they basically dont react unless u have a catalyst with them. (aka an amino alcohol ligand) which speeds up the reaction.
we just need to make sure the catalyst is asymmetric aka chiral.
think of DAIB,, the folded rectangle aka cyclohexane with NMe2 top right, OH bottom right,, Me in the centre and the bridge over the central carbons with the 2 Me sticking out ,, how many stereocentres does it have and what do we call them to simplify things
it has 4 stereogenic centres
but they all associate with eachother
so theyre all S or all R
so we just say DAIB has 2 forms,, S or R. and we say theyre basically in a racemic mix
bc the mix is 55:45 which is basically racemic.
when u have DAIB and a dialkylzinc ,, what happens
a dimer forms (Me2Zn with DAIB) and the dimer and monomeric form are in equilibrium!!!
the dimer is made up of a monomer,, and if the monomer is S or R,, the dimer can be what
it can be SS RR SR or RS righttt !! bc if the dimer is made up of 2 monomers,, and the monomer is either R or S,, u have those options available!! makes sense girl youve got this.
what dimer form is super stable and doesnt do anything
the RS form!!!!
its super stable and doesnt do anythingggg
okay so lets say u have an enantiomer mixx of 45:55
and u have RR + SS <–> 2S + 2R <–> 2RS
and the 2RS are super stable.
what do the 45 and 55 do
the 45 enantiomer,, lets say S,, takes 45 of the the R enantiomer
to form RS
meaning no SS can be formed bc all the S is used up
and only a few RR can be made,, bc obvs majority of it went to making RS/SR.
so the minor enantiomer uses the major enantiomer.
and bc RS is catalytically inactive,, and theres no SS,, we just have RR dimer!!
so we produce one type of enantiomer product bc of this !!!!!
what does the enantiomeric excess thing mean
if u have an e.e of 0 it means u have a racemic mixture
if u have an ee of 100 it means one enantiomer is formed and the other one isnt.
how do we do amplification of stereochemistry!!!
we have aldehyde + R2Zn
which give us a racemic mix but one enantiomer is slightly slightly more abundant than the other one.
and this enantiomer is used as a catalyst or smt like that ,, and its used 6 times in diff reactions and each time its ee slowly increases
the autocatalytic reaction can lead to a larger enantiomeric excess of one of the enantiomerssss!!
