14&15 - PERICYCLIC REACTIONS Flashcards
how do lewis acids affect normal electron demand diels-alder?
accelerate those reactions
improve the regioselectivity as well
also works with protic acids, if they protonate something and make it a stronger EWG, this can make the reaction go faster
lewis acids can block one face or the other to give regioselectivity
what is a normal electron demand cycloaddition?
add an EWG to the dienophile
add an EDG to the diene
this will make the reaction go faster
what is inverse electron demand diels alder?
when EDG on dienophile and EWG on diene, the HOMO and LUMO are not as close
in which cases is positive charge on oxygen okay?
when there are 8 electrons around the oxygen but it’s still a positive charge makes the positive charge okay
how to figure out the regiochemistry of diers-alder? and an example?
figure out which is the most E+ and most nu position and then bring those two together
you can figure out those positions by drawing resonance structures
how can the alder endo rule be explained?
can be explained by secondary orbital overlap: stabilizing but not bonding
in the endo transition state the carbonyl is tucked under the diene and it gives stabilisation
endo group is close to two groups and exo group is close to one group
what influences the position of the equilibrium in thermal vs photochemical reactions?
thermal reactions: position of equilibrium is determined by starting material and product stability
photochemical reactions: wavelength of light can be used to influence position of the reaction (it may not be at equilibrium)
thermal and photochemical reactions happen but don’t happen in the same way
table of conrotatory vs disrotatory for 4n+2 and 4n at thermal and photochemical conditions
what were woodward’s observations of the synthesis of vitamin B12?
each alkene isomer gave rise to a specific cyclic product under thermal conditions
however, under photochemical conditions it reverted to the opposite isomer!
what does stereoselectivity of electrocyclic ring closure/opening depend on?
depends on the number of electrons and whether the reaction is induced thermally or photochemically
photochemical has same mechanism forward and backwards but thermal does not
how does rotation determine whether cis or trans? example with butadiene
this ring closing is a 4n reaction: antarafacial
since you lose pi bond character, the bonds can rotate
they rotate in the same sense= conrotatory
the R groups end up trans
example of ring closing of hexatriene?
distrotatory
4n+2, suprafacial
the center bond has to be cis so that the alkenes are close enough to interact
why is cyclopentene one of the best and how does it form?
why are thermal 4n con and why are thermal 4n+2 dis?
4n+2 are suprafacial
4n are antarafacial
what are the rotations for 4n and 4n+2 under photochemical conditions?
antarafacial for 4n+2, conrotatory
suprafacial for 4n, disrotatory
as you shine light, the HOMO switches to a LUMO, so you have one more node and you change it
what does lindlar’s catalyst do?
reduces alkynes but not alkenes
happens on a Pd surface, which mean you transfer the hydrogen on the same side and you get a cis bond
how does the synthesis of endrianic acid happen?
what are the observations made about cholesterol and its stereoisomer?
what is a sigmatropic shift?
the shift of an atom or group of atoms across a pi system
labelled in the form [m,n] shift where m=atoms shifting across an n atom pi system (n is almost always odd)
what are the most common shifts?
[1,7] (4n) and [1,5] (4n+2) are very common
[3,3] and [2,3] are also very common
concerted [1,3] shift of hydrogen never occurs
some [5,5] shifts have been observed
how to figure out the nomenclature of the shifting?
why is [1,3] antarafacial not allowed but [1,5] and [1,7] works?
when you do [1,5] since its suprafacial, the stereochemistry of the bond that moves is the same
for [1,7], since its antarafacial, the stereochemistry changes, there’s a switch
what is a sigmatropic shift that happens in nature?
how do you combat a problematic 1,5 shift in corey synthesis of prostaglandin?
can photochemical sigmatropic shifts happen?
theoretically possible but rare, because shining light on it does a bunch of other stuff
the observations we see are the opposite of the thermal ones
[1,3] shifts are now possible suprafacially
[1,5] cannot happen because it is now antarafacial
[1,7] can happen suprafacially
how is the synthesis of vitamin B12 an example of the ultimate sigmatropic shift and electrocyclic reaction
involved a photoinitiated [1.16] sigmatropic shift of H and then a 16pi closure
examples of [2,3] shifts?
examples of [3,3] shifts
have a chair conformation transition state
what is the [3,3] shift transition state?
what is the cope rearrangement of divinylcyclopropane?
what are oxy-cope and anionic oxy-cope rearrangements?
what are some examples of a cope rearrangement being used in synthesis?
ateriscanolide and precapnelladiene
what is the driving force for a claisen rearrangement and what is an aromatic claisen rearrangement?
the driving force is the formation of a C=O bond, which is more stable than a C-O bond
