Photorearrangements and excited state reactivity are key players in photochemistry. These processes involve complex molecular transformations triggered by light, leading to unique products and reaction pathways not accessible through thermal chemistry.

Understanding these mechanisms is crucial for harnessing the power of light in organic synthesis and materials science. From di-π-methane rearrangements to photocycloadditions, these reactions offer tools for creating complex structures and novel materials with exciting applications.

Photorearrangements

Mechanisms of common photorearrangements

• Di-π-methane rearrangement involves 1,2-acyl shift followed by 1,2-alkyl shift results in ring contraction or expansion with stereochemistry typically retained (cyclopropylacetone to bicyclo[3.1.0]hexanone)
• Oxa-di-π-methane rearrangement proceeds through biradical intermediates leads to formation of cyclopropanone derivatives in oxygen-containing compounds (benzophenone to phenylcyclopropyl ketone)
• Norrish Type I reaction cleaves α-carbon-carbon bond in ketones forms acyl-alkyl radical pair leads to decarbonylation or recombination products (acetophenone to benzaldehyde and methane)
• Norrish Type II reaction involves intramolecular hydrogen abstraction in γ-hydrogen containing ketones forms 1,4-biradical intermediate results in cyclobutanol formation or fragmentation (2-pentanone to acetone and ethylene)

Principles of photocycloaddition reactions

• [2+2] Photocycloaddition reacts two π-bonds forms four-membered rings follows Woodward-Hoffmann rules for photochemical reactions (ethylene to cyclobutane)
• [4+4] Photocycloaddition reacts two conjugated dienes forms eight-membered rings less common than [2+2] cycloadditions (1,3-butadiene to cyclooctadiene)
• Applications in organic synthesis enable stereospecific formation of cyclic compounds access strained ring systems used in natural product synthesis (artemisinin)
• Applications in materials science include photopolymerization reactions self-healing materials photoswitchable molecules (azobenzene derivatives)

Excited State Reactivity and Orbital Symmetry

Factors in photochemical reactions

• Excited state reactivity
  • Singlet vs triplet excited states determine reaction pathways and product distribution
  • Energy transfer processes enable sensitized reactions (triplet sensitizers)
  • Intersystem crossing influences excited state lifetime and reactivity (heavy atom effect)
• Orbital symmetry considerations
  • Frontier molecular orbital theory predicts allowed and forbidden reactions
  • Correlation diagrams visualize orbital interactions during reaction progress
  • Conservation of orbital symmetry in pericyclic reactions governs stereochemical outcomes (conrotatory vs disrotatory)
• Conformational effects
  • Molecular geometry in excited states impacts reaction pathways and product distribution
  • Steric and electronic factors affect conformational preferences influence regioselectivity and stereoselectivity

Applications of photochemical reactions

• Synthetic utility
  • Access complex molecular structures enables synthesis of natural products (vitamin D)
  • Stereospecific and regioselective transformations allow precise control over product formation
  • Formation of strained ring systems provides unique building blocks for further synthesis (cubane)
• Limitations
  • Scale-up challenges due to light penetration issues require specialized reactor designs
  • Potential side reactions include photooxidation and radical-induced polymerization
  • Substrate-specific reactivity may limit general applicability
• Applications in organic synthesis
  • Total synthesis of natural products utilizes photochemical key steps (paclitaxel)
  • Preparation of pharmaceutical intermediates employs photochemical transformations
  • Construction of polycyclic frameworks achieves complex molecular architectures (dodecahedrane)
• Applications in materials science
  • Photopolymerization for 3D printing enables rapid prototyping and manufacturing
  • Development of photochromic materials creates smart windows and adaptive lenses
  • Design of molecular machines and switches forms basis for nanoscale devices (rotaxanes)