Transition Metal & Organocatalysis: examples and chiral induction MCQs With Answer

Transition Metal & Organocatalysis: examples and chiral induction MCQs With Answer

by

Introduction

Transition Metal & Organocatalysis are central to modern asymmetric synthesis, especially in drug development where stereochemistry dictates biological activity. This quiz collection focuses on illustrative examples and principles of chiral induction used in catalytic processes, combining transition metal complexes (Rh, Ru, Ir, Pd) with chiral ligands (BINAP, DIOP, TADDOL) and small-molecule organocatalysts (proline, MacMillan imidazolidinones, cinchona derivatives). Questions cover catalytic cycles, mechanistic models of enantioselectivity, ligand design, kinetic and dynamic resolutions, and practical parameters such as TON/TOF and ee. Intended for M.Pharm students, these MCQs deepen conceptual understanding and link theory to practical asymmetric methodologies widely used in pharmaceutical synthesis.

Q1. Which catalytic system is most commonly associated with high enantioselectivity in asymmetric hydrogenation of dehydroamino acids and simple olefins?

  • Ruthenium complexes with BINAP ligands
  • Palladium(0) with triphenylphosphine
  • Copper(I) salts with bipyridine
  • Iron porphyrin complexes

Correct Answer: Ruthenium complexes with BINAP ligands

Q2. In proline-catalyzed aldol reactions, what key intermediate provides enamine activation and directs stereochemistry?

  • Enamine formed between proline and the ketone
  • Iminium ion formed between proline and the aldehyde
  • Lewis acid coordination to the carbonyl
  • Radical intermediate via single-electron transfer

Correct Answer: Enamine formed between proline and the ketone

Q3. Which organocatalyst class utilizes iminium activation to accelerate conjugate additions and Diels–Alder reactions?

  • MacMillan imidazolidinone catalysts
  • Cinchona alkaloids
  • Proline derivatives
  • Phase-transfer quaternary ammonium salts

Correct Answer: MacMillan imidazolidinone catalysts

Q4. Which stereochemical model explains observed facial selectivity in many nucleophilic additions to carbonyls mediated by chiral ligands on metals?

  • Felkin–Anh model
  • Curtin–Hammett principle
  • Marcus theory
  • Hard–Soft Acid–Base (HSAB) concept

Correct Answer: Felkin–Anh model

Q5. Which chiral ligand is well known for enabling high ee in asymmetric hydrogenation and has atropisomeric biaryl structure?

  • BINAP
  • EDTA
  • PPh3
  • TMEDA

Correct Answer: BINAP

Q6. Dynamic kinetic resolution (DKR) differs from simple kinetic resolution by:

  • Converting racemate to a single enantiomer by in situ racemization of the substrate
  • Using only stoichiometric chiral reagents
  • Removing product immediately to stop reaction
  • Performing reactions at cryogenic temperatures exclusively

Correct Answer: Converting racemate to a single enantiomer by in situ racemization of the substrate

Q7. Which metal-catalyzed asymmetric reaction is Noyori most associated with?

  • Asymmetric hydrogenation with Ru–BINAP and diamine ligands
  • Asymmetric epoxidation using Ti–tartrate
  • Sharpless asymmetric dihydroxylation
  • Asymmetric allylic substitution with Pd

Correct Answer: Asymmetric hydrogenation with Ru–BINAP and diamine ligands

Q8. In asymmetric catalysis, enantiomeric excess (ee) is defined as:

  • The percentage difference between the two enantiomers
  • The absolute concentration of the major enantiomer
  • The average of optical rotations of enantiomers
  • The ratio of reaction rate constants kR/kS

Correct Answer: The percentage difference between the two enantiomers

Q9. Which catalytic strategy employs chiral phosphoric acids to induce asymmetry by hydrogen-bonding activation?

  • Chiral Brønsted acid catalysis
  • Transition-metal oxidative addition
  • Radical chain catalysis
  • Phase-transfer catalysis

Correct Answer: Chiral Brønsted acid catalysis

Q10. Which of the following is a common mechanistic explanation for enantioselection by chiral ligands on square planar Pd(II) complexes in allylic substitutions?

  • Formation of diastereomeric Pd–π-allyl complexes with different reaction barriers
  • Radical recombination directed by ligand anisotropy
  • Direct nucleophile coordination to free ligand
  • Single-electron transfer from ligand to substrate

Correct Answer: Formation of diastereomeric Pd–π-allyl complexes with different reaction barriers

Q11. Which organocatalyst family is commonly used for asymmetric phase-transfer catalysis in alkylation of enolates?

  • Cinchona alkaloid-derived quaternary ammonium salts
  • MacMillan imidazolidinones
  • Proline and derivatives
  • TADDOLs as neutral ligands

Correct Answer: Cinchona alkaloid-derived quaternary ammonium salts

Q12. Which descriptor quantifies the efficiency of a catalyst in terms of product produced per catalyst molecule?

  • Turnover number (TON)
  • Enantiomeric excess (ee)
  • Enthalpy of activation (ΔH‡)
  • Optical purity

Correct Answer: Turnover number (TON)

Q13. Sharpless asymmetric epoxidation uses which components to induce chirality in allylic alcohol epoxidation?

  • Titanium isopropoxide with diethyl tartrate and tert-butyl hydroperoxide
  • Ruthenium catalyst with BINAP and hydrogen gas
  • Proline and aldehyde to form enamines
  • Palladium-catalyzed allylic oxidation

Correct Answer: Titanium isopropoxide with diethyl tartrate and tert-butyl hydroperoxide

Q14. Which factor often improves enantioselectivity in transition-metal catalyzed reactions?

  • Increasing steric differentiation around the metal center via bulky chiral ligands
  • Running the reaction at extremely high temperatures to speed up kinetics
  • Using achiral excess ligand to dilute the chiral effect
  • Replacing ligand with simple halide ions

Correct Answer: Increasing steric differentiation around the metal center via bulky chiral ligands

Q15. MacMillan imidazolidinone catalysts induce asymmetry primarily by which interaction with α,β-unsaturated carbonyl substrates?

  • Formation of an iminium ion that lowers LUMO and enforces facial selectivity
  • Enamine formation raising HOMO of the nucleophile
  • Hydrogen bonding to activate the carbonyl oxygen
  • Metal chelation to form a chiral metal complex

Correct Answer: Formation of an iminium ion that lowers LUMO and enforces facial selectivity

Q16. Which ligand motif is frequently used to create a chiral pocket through multiple hydrogen-bonding interactions in organocatalysis?

  • TADDOL-derived diols
  • Triphenylphosphine oxide
  • Ethylenediaminetetraacetic acid (EDTA)
  • TMEDA (tetramethylethylenediamine)

Correct Answer: TADDOL-derived diols

Q17. What is the main reason chiral counterions can induce enantioselectivity in catalytic reactions?

  • Formation of diastereomeric ion pairs that differ in reactivity or stability
  • They increase reaction temperature uniformly
  • They convert catalysts to achiral species
  • They generate radicals that racemize the substrate

Correct Answer: Formation of diastereomeric ion pairs that differ in reactivity or stability

Q18. Which transition metal is especially effective for asymmetric allylic alkylation when combined with chiral phosphine ligands?

  • Palladium
  • Chromium
  • Nickel in zero oxidation state exclusively
  • Magnesium as an organometallic reagent

Correct Answer: Palladium

Q19. In matched/mismatched effects between substrate and chiral catalyst, the highest enantioselectivity is observed when:

  • The chiral catalyst stereochemistry matches the substrate’s existing stereochemical bias
  • The chiral catalyst is the opposite configuration to the substrate’s bias
  • The substrate is racemic and catalyst is achiral
  • The reaction is run without solvent

Correct Answer: The chiral catalyst stereochemistry matches the substrate’s existing stereochemical bias

Q20. Which statement best describes enantioselective induction by a chiral ligand in a transition-metal catalytic cycle?

  • The chiral ligand creates diastereomeric transition states with different energies, favoring one enantiomer
  • The chiral ligand converts the substrate into a single enantiomer prior to catalysis
  • The chiral ligand always forms covalent bonds with the substrate leading to racemization
  • The chiral ligand functions only as a spectator and does not influence stereochemistry

Correct Answer: The chiral ligand creates diastereomeric transition states with different energies, favoring one enantiomer

Leave a Comment