CHE 3332 – Organic Chemistry 2 – Spring 2021 Week 7 Resource Hello everyone! Virtual group tutoring for this class will be every Tuesday from 6-7 pm, and you can reserve your spot by making an appointment at www.Baylor.edu/Tutoring. This week, we’ll

cover Chapter 17: Conjugated Systems. Keywords: conjugation, molecular orbitals, pericyclic reactions, electrophilic addition

Conjugated systems All conjugated systems must have these components: conjugated pi bonds that interact and increase stability as well as overlapping p-orbitals–there can’t be any isolated p-orbital. Examples: This is NOT a conjugated system because p-orbitals don’t overlap & pi bonds aren’t conjugated: Electrophilic Additions Substituents can be added to a 1,3-diene (four carbon conjugated system) under different conditions. A 1,2-addition is favored in cold conditions and a 1,4-addition is favored in hot conditions. A halide adds to the more substituted of the two carbons and a hydrogen adds to the other carbon. Radical Stability Radicals are most stable at the allylic and benzylic position, followed by tertiary, secondary, and primary carbons. A radical on a methyl is the least stable. Allylic Bromination Previously, we used Br2/hv to add bromine to the allylic position. Now, we can also use the NBS reagent to get the same effect.

How to build Polyene Molecular Orbitals Example: butadiene

1. Four carbons in the conjugated system so draw four connected dots and four levels of molecular orbitals labeled pi 1-4. 2. Draw symmetric nodes (dashed lines) starting with zero at the lowest level and increasing a node as you go up. If there’s an odd number of carbons, then you can draw a node through a carbon. 3. Draw phases (+ or -) left to right for each carbon. Switch phase at a node. Always start with “+” 4. Determine number of electrons in conjugated system and draw them in as arrows on orbitals starting from the bottom. This has 2 double bonds so 4 electrons/arrows. Max of 2 electrons or arrows per orbital. 5. Assign HOMO/LUMO. The Highest Occupied Molecular Orbital is the highest orbital that has electrons. The Lowest Unoccupied Molecular Orbital is the lowest orbital without any electrons–above the HOMO. More rules: If the reaction uses light (hv), it moves one electron up an orbital, thereby changing the HOMO/LUMO orbitals. An anion on a structure acts as 2 electrons. A cation doesn’t contribute any electrons.

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Pericyclic Reactions – There are 3 types: cycloaddition, electrocyclic, and sigmatropic. All pericyclic reactions have four traits: 1) concerted so all steps happen at once, 2) involves a ring of electrons moving in a closed loop, 3) reaction goes through a cyclic transition state, 4) the transition state has little charge. 1. Cycloaddition – Diels-Alder Reaction A diene has a four-carbon conjugated system and is electron rich because of its two double bonds. The diene must be cis because the trans form is unreactive and stable. A dienophile is itself electron poor, has a double or triple bond, has an electron withdrawing group on its structure, and likes to be near a rich diene. The diene and dienophile interact to create a cyclic ring. Specifically, the HOMO of the diene reacts with the LUMO of the dienophile. Overall, 3 pi bonds are broken; 2 sigma and 1 new pi bonds are made.

• The dienophile’s electrons line up with the diene to contribute to the ring of electrons:

diene dienophile

• Only one pi bond on the dienophile (in this case, of the triple bond) is needed to cyclize:

• If the dienophile has substituents, retain stereochemistry. Substituents that are cis end up

on the same side on the product. Substituents that are trans end up on opposite sides on the product:

vs

• If the diene is a cyclopentadiene, the product will be a bridged bicyclic ring. Endo substituents will predominate if the dienophile can pi overlap:

• If the diene has a substituent on the #2 carbon, a 1,4-product is made. If the diene has a substituent on the #1 carbon, a 1,2-product is made.

vs 2. Electrocyclic Reaction This reaction does not have a diene and dienophile; one conjugated polyene (something with a conjugated system) cyclizes itself. 1 pi bond turns into a sigma bond and all other remaining pi bonds change location to cyclize. A cyclized ring is always formed, but it’s not always a cyclohexane.

• If there are substituents, you must look at the phases on the HOMO to determine stereochemistry. If the carbons with substituents have the same phase (i.e. both plus or minus), they will be on the same side. If they have different phases (one plus and one minus), they will be on opposite sides:

3. Sigmatropic Rearrangements This reaction does not have a diene and dienophile, just one structure. 1 sigma bond is made at the expense of another sigma bond; the pi bonds change location. There are two iconic sigmatropic rearrangements.

• Cope rearrangement: broken sigma bond formed sigma bond

• Claisen rearrangement:

broken sigma bond formed sigma bond

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Quiz: Draw the product for each reaction.

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2.

3. Answers: All questions sourced from the Wiley Organic Chemistry Second Semester textbook.

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