Direct Detection of an Acyclic Dimeric Intermediate in Charge Transfer-Induced [2π + 2π] Cycloaddition

Area of Science:

• Organic Chemistry
• Photochemistry
• Spectroscopy

Background:

• Radical cation-mediated [2+2] cycloadditions are known reactions.
• The acyclic radical cation dimer intermediate has been spectroscopically elusive.
• This intermediate is crucial for understanding head-to-head cyclobutane product formation.

Purpose of the Study:

• To experimentally detect the acyclic radical cation dimer intermediate.
• To elucidate the role of this intermediate in [2+2] cycloadditions.
• To understand the driving forces behind product selectivity.

Main Methods:

• Broadband femtosecond transient absorption spectroscopy.
• Photoexcitation of a charge transfer complex between a dendritic-bridged tripyridiniumtriazine cation (dBTPT+3) and n-vinylcarbazole.
• Electronic structure calculations (ωB97XD/6-311G(d,p)).

Main Results:

• The first experimental signature of an n-vinylcarbazole cation radical dimer was observed.
• The dimer intermediate absorbs around 640 nm.
• Calculations show a preference for a head-to-head cis-conformation due to through-space charge transfer.

Conclusions:

• Weak Coulombic interactions within the dimer intermediate drive observed product selectivity.
• This study provides direct experimental evidence for the proposed reaction mechanism.
• The findings advance the understanding of radical cation-mediated cycloadditions.