Competitive 1,2-C Atom Shifts in the Strained Carbene Spiro[3.3]hept-1-ylidene Explained by Distinct Ring-Puckered...

Area of Science:

• Organic Chemistry
• Reaction Intermediates
• Computational Chemistry

Background:

• Spirocyclic compounds present unique structural and reactivity challenges.
• Carbene intermediates are crucial in various organic transformations.
• Understanding rearrangement pathways is key to controlling reaction outcomes.

Purpose of the Study:

• To investigate the reaction pathways and product selectivity of the strained spiro[3.3]hept-1-ylidene carbene.
• To elucidate the mechanisms of competing [1,2]-sigmatropic rearrangements.
• To utilize computational chemistry to explain observed product distributions.

Main Methods:

• Generation of spiro[3.3]hept-1-ylidene via high-vacuum flash pyrolysis (HVFP) of its p-tosylhydrazone sodium salt.
• Analysis of hydrocarbon products using gas chromatography and mass spectrometry.
• Computational modeling to determine structures, energies, and transition states of rearrangements.

Main Results:

• Five hydrocarbons were produced with an 82% overall yield.
• The carbene underwent competing ring-contraction and ring-expansion rearrangements.
• Ring contraction to cyclopropylidenecyclobutane was favored over ring expansion by a factor of 6.7:1.
• Secondary rearrangements of primary products were identified and analyzed.
• Computational analysis revealed four distinct geometric conformations influencing transition-state selection.

Conclusions:

• The study elucidates the rearrangement mechanisms of spiro[3.3]hept-1-ylidene.
• Product selectivity is governed by competing ring-contraction and ring-expansion pathways.
• Carbene conformation plays a significant role in determining reaction outcomes.
• Computational chemistry provides valuable insights into the selectivity of strained carbene rearrangements.