Chapter 23 Recap Pericyclic reactions are a group of concerted ring forming breaking reactions They all depend upon two factors orbital alignment and rehybridization More on both of these later Pericyclic reactions are classified into three types on the basis of how many and what type of bonds are formed and or broken during the reactions o Electrocyclic reactions involve only one molecule Overall the reaction leads to one fewer bond and one more bond than was present in the starting material 3 bonds 2 bonds 1 new bond o Cycloadditions can involve one or two molecules Overall the reaction leads to two fewer bonds and one more bond than were present in the starting material 3 bonds 1 bond 2 new bonds o Sigmatropic rearrangements involve one molecule Overall the reaction leads to the same number of both and bonds as were present in the starting material but these bonds change position Electrocyclic reactions can be best understood by using molecular orbital theory Specifically M O theory can help predict the conditions under which a given reaction will occur and provide insight into the stereochemical outcome of the reaction To get started we will look at everybody s favorite M O diagram hexatriene Don t lie we all know you love it 6 5 4 E 3 2 1 The thing to note in this context is how the orbitals at the ends of the carbon chain relate to one another with respect to their phases If the orbitals at the ends of the chain are in phase with one another the orbital is symmetric S If the orbitals are out of phase with one another the orbital is antisymmetric AS 6 AS 5 S 4 AS 3 S 2 AS 1 S E Before delving too deeply into why this matters we should look at what happens to the orbitals in the context of a pericyclic reaction specifically an electrocyclic reaction alignment or rehybridization alignment rehybridization Things to note o The orbitals are rotating in opposite directions the green arrow is clockwise and the blue is counterclockwise This is called disrotatory o The disrotatory mode of ring closure or ring opening requires a symmetric orbital Conversely check out the reaction shown below alignment rehybridization or alignment rehybridization Things to note o The orbitals are rotating in the same direction blue is counterclockwise and green is clockwise This is called conrotatory o The conrotatory mode of ring closure opening requires an antisymmetric orbital The final consideration is which orbital to use in the context of the electrocyclic reaction Given that electrons are required in order to form the new bond an occupied orbital is necessary Given that these electrons need to have sufficient energy to react the highest energy electrons are required Overall this makes the highest occupied molecular orbital HOMO the best choice In the case of hexatriene 3 the highest occupied molecular orbital is symmetric This means that the disrotatory ring closure is the only possible mode and helps to predict the stereochemistry of the reactions shown below R1 R1 R1 R2 R2 R2 R1 R1 R2 R1 R2 H R1 R1 R2 R2 R1 R2 R1 R2 H R2 R1 H R2 R2 R1 However under different conditions a different outcome can occur R1 R1 R2 R1 R1 R2 R2 hv R2 R2 R2 R1 H R2 R1 R2 hv R2 R2 Because the mode of ring closure is dictated by the nature of the orbital symmetric or antisymmetric the different outcomes of the reactions above mean that a different orbital must be used This is possible because in the presence of light hv an electron can be excited to a higher level thereby changing the highest occupied molecular orbital here the change would be from 3 to 4 6 5 4 E 3 2 1 R1 R2 R1 R2 R1 R1 R2 R1 H R1 R2 R2 H R2 R1 R2 R2 R1 In addition to which orbital is used the number of bonds involved in the reaction is important as this determines whether the HOMO is symmetric or antisymmetric As an example the molecular orbital diagram for butadiene is shown below Here the highest occupied ground state orbital is 2 an antisymmetric orbital 4 3 E 2 1 R1 R1 R1 R2 R2 R2 R1 R2 R2 R1 R1 H R2 R1 R1 R2 R2 R1 R2 R2 R2 R1 R2 R1 R2 R2 R1 R1 R2 As with hexatriene in the presence of light an electron is can be excited to 3 a symmetric orbital This leads to the stereochemical outcome shown below H R1 R1 R1 hv R2 R2 R2 R1 R2 R2 R1 R2 R1 R1 R1 R1 R2 R2 hv R2 R2 R1 R1 R2 R2 R2 R1 R2 R1 R2 Yay Cycloadditions also depend upon orbital symmetry The Diels Alder reaction is an extremely common cycloaddition referred to as 4 2 reactions 4 atoms 2 atoms Unlike electrocyclizations cycloadditions require two orbitals to interact one full orbital and one empty orbital the highest occupied orbital and the lowest unoccupied molecular orbital the LUMO The Diels Alder cycloaddition of butadiene and ethene can be used to examine the orbitals involved o The MO diagram is shown below Note that ethene s orbitals are of higher energy because it is not conjugated In principle either HOMO LUMO pair could be used In practice the HOMO of the diene and the LUMO of the dieneophile are most often the orbitals involved Regardless of which orbitals are paired they must align to provide a bonding interaction in order for the reaction to proceed This type of cycloaddition is symmetry allowed In contrast two molecules of ethene a 2 2 reactions would not be able to react as the orbitals do not lead to a bonding interaction However as with electrocyclizations a thermally disallowed reaction could proceed through photochemical means Exciting one electron to 2 gives the pairing shown below hv HOMO 2 in phase bonding interaction in phase bonding interaction Therefore those cycloadditions that are thermally forbidden are photochemically allowed and vice versa The final category of pericyclic reactions is sigmatropic rearrangement Sigmatropic rearrangements are described on the basis of the number of atoms to either side of the bond that breaks and the bond that forms 2 3 1 3 2 2 1 1 2 The reaction above is therefore a 3 3 sigmatropic rearrangement Note that the numbering only includes those atoms involved in the reaction Atoms can also migrate through this mechanism 2 3 4 3 1 3 LUMO 2 5 1 H1 2 3 4 1 H 1 5 The above reaction involves the movement of a hydrogen atom plus its electrons It is therefore classified as a 1 5 hydride shift