14&15 - PERICYCLIC REACTIONS

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

add an EWG to the dienophile add an EDG to the diene this will make the reaction go faster

when EDG on dienophile and EWG on diene, the HOMO and LUMO are not as close

when there are 8 electrons around the oxygen but it’s still a positive charge makes the positive charge okay

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

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

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

each alkene isomer gave rise to a specific cyclic product under thermal conditions however, under photochemical conditions it reverted to the opposite isomer!

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

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

distrotatory 4n+2, suprafacial the center bond has to be cis so that the alkenes are close enough to interact

4n+2 are suprafacial 4n are antarafacial

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

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

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)

[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

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