Advanced Organic Chemistry Practice Problems 2021 <2024-2026>

Reaction: An enantioenriched chiral phosphoric acid (CPA, 5 mol%) catalyzes the addition of a silyl ketene acetal to an N-Boc isoquinolinium ion generated in situ from 3,4-dihydroisoquinoline and Boc₂O.

Observation: The product, a tetrahydroisoquinoline derivative, is formed in 94% ee.

Questions:
a) Draw the transition state model explaining the facial selectivity.
b) How does the counteranion (chiral phosphate) influence the electrophile’s trajectory?
c) If the reaction is run at 0 °C instead of –78 °C, the ee drops to 72%. Why?

Scenario:
The enolate of 3-methylcyclohex-2-enone is generated with LDA at $-78^\circ\textC$ (kinetic conditions) vs. with t-BuOK at $25^\circ\textC$ (thermodynamic conditions). The enolate is then trapped with $\textCH_3\textI$.

Problems:

2021 Advanced Insight: The thermodynamic enolate is actually the less substituted enolate (the one at the $\alpha$ carbon not in the ring) because the ring has significant angle strain when forced to adopt a planar enolate geometry. The kinetic enolate is the more substituted (exocyclic) one because LDA plucks the most accessible proton.

Practice Problem 5.1: Given the product distribution (70% kinetic, 30% thermodynamic), calculate the $\Delta\Delta G^\ddagger$ between the two transition states using the Arrhenius equation at $298K$.

Monomer: endo-Norbornene dicarboximide bearing a bulky chiral substituent (menthyl group).

Catalyst: Grubbs 3rd generation (RuCl₂(py)₂(H₂IMes)(=CHPh)). advanced organic chemistry practice problems 2021

Tasks:
a) Predict the polymer tacticity (isotactic, syndiotactic, atactic) based on the monomer structure and catalyst. Justify.
b) Draw the first two propagation steps after initiation, showing stereochemistry of the metal carbene.
c) How would switching to a cis-selective Ru catalyst (e.g., Z-selective Grubbs) change the polymer backbone geometry?

Scenario:
Consider $(2E,4Z,6E,8Z)$-deca-2,4,6,8-tetraene.

Questions:

The 2021 Advanced Twist:
You must apply the Woodward-Hoffmann rules using the Hückel–Möbius concept. Count the number of $\pi$ electrons: 8 electrons. Reaction: An enantioenriched chiral phosphoric acid (CPA, 5

Practice Problem 2.1: Draw the transition state for the thermal closure. Why is the disrotatory mode favored despite steric clash of methyl groups?

2021 problems excelled in integrating spectroscopy with synthesis. Rather than isolated spectral analysis, advanced problems presented students with 1H NMR, 13C NMR, and IR data and asked them to propose a synthesis route that would yield a molecule consistent with the data. This "detective work" approach remains the gold standard for advanced proficiency.

The problems trending in 2021 moved away from simple functional group transformations. Instead, they emphasized cryptic stereocenters and non-classical carbocations.