Pericyclic reactions Flashcards
What are syn and anti isomers?
Syn = cis, anti = trans
What is conrotatory?
rotation of the lobes in the same direction
What is disrotatory?
rotation of the lobes in the opposite direction
For 2, 6, and 10e- complexes, what component is used for Woodward-Hoffmann?
(4q+2)x
For 4, 8, and 12e- complexes, what component is used for Woodward-Hoffmann?
(4r)x
What do a and s refer to for Woodward-Hoffmann?
antarafacial and suprafacial (NOT ANTI AND SYN)
How are a and s decided for Woodward-Hoffmann?
if the lobes are on opposite side (a), if the lobes are on the same sides (s)
How many e- per double bond?
2e-
What is the Woodward-Hoffmann rule?
To be symmetry allowed under thermal conditions, the sum of (4q+2)s and (4r)a must be an odd number (zero+1)=1
How to work out HOMOs and LUMO drawings?
no. of atoms in the reaction = no. of MOs
MO number -1= no. of inversion centres to decide orientation of lobes
Effect of EDG on HOMO and LUMO?
EDG raises energy of HOMO and LUMO, makes the lobe where the EDG is larger in the LUMO, and the lobe next to it larger in the HOMO
Examples of EDG?
OMe, OSiMe3, NMe2
Effect of EWG on HOMO and LUMO?
EWG lowers energy of HOMO and LUMO, makes the lobe next to where the EWG is larger in both the LUMO and HOMO
Examples of EWG?
Ketone, ester, amide, nitrile, NO2
Effect of conjugated group on HOMO and LUMO?
CONJ raises energy of HOMO and lowers the LUMO, bringing them closer together, makes the lobe next to where the CONJ is only very slightly larger in both the HOMO and LUMO
Example of CONJ group?
phenyl, aryl
How do you know if something reacts as its HOMO or LUMO?
Electron rich reacts as homo (EDG), electron poor reactions as LUMO (EWG)
What can catch you out with HOMO LUMO?
double bonds from ketones etc will have pi orbitals too so must be considered
What are endo and exo diastereoisomers?
Exo = the R group and bridge are on the same side
Endo = the R group and bridge are on opposite sides (more favoured TS)
Why isn’t 2-2 cycloaddition possible with heat (unless ketene)?
light promotes one e- from the HOMO to the LUMO forming pseudo ‘HOMO’ and ‘LUMO’
How do HOMOs and LUMOs react with pseudo ones?
e.g. LUMO reacts with ‘LUMO’, ‘LUMO’ is the same as the LUMO, just flipped around so the lobes line up for the reaction
What is the exception for 2-2 cycloaddition with heat?
ketenes
