Computational Insights into BF3-Catalyzed [2π + 2σ] Cycloadditions: Catalyst, Substrate, Solvent, and Electric Field...

Area of Science:

• Computational Chemistry
• Organic Synthesis
• Medicinal Chemistry

Background:

• Bridged bicyclic scaffolds, like azacycle-fused bicyclo[2.1.1]hexanes, are crucial 3D bioisosteres of benzene in drug design.
• Understanding reaction mechanisms and selectivity is key to synthesizing these valuable scaffolds.

Purpose of the Study:

• To computationally investigate the BF3-catalyzed [2π + 2σ] cycloaddition between dihydropyridines (DHPs) and bicyclo[1.1.0]butanes (BCBs).
• To elucidate the mechanism and regioselectivity of the reaction, particularly the formation of azacycle-fused products.
• To explore strategies for controlling the formation of specific regioisomers (P1 vs. P2).

Main Methods:

• Density Functional Theory (DFT) calculations.
• Coupled-cluster with single, double, and perturbative triple excitations with Complete Basis Set extrapolation (DLPNO-CCSD(T)/CBS) calculations.
• Analysis of catalyst roles, substituent effects, electric fields, and solvent polarity.

Main Results:

• The reaction proceeds via a concerted BCB ring-opening and nucleophilic attack, revising the initial stepwise mechanism.
• BF3 acts as a catalyst to lower activation barriers and reverse intrinsic selectivity, favoring P1.
• Under catalyst-free conditions, P2 is favored due to dispersion interactions; BF3 coordination eliminates this advantage.
• Bulky C3 substituents, electric fields, and solvent polarity can be used to tune selectivity and kinetics, enabling P2 formation.

Conclusions:

• This work provides novel mechanistic insights into BF3-catalyzed cycloadditions involving strained rings.
• A multifactor strategy combining steric, electronic, and environmental factors allows for rational control over reaction selectivity.
• The findings offer a pathway for the targeted synthesis of valuable 3D bicyclic scaffolds for medicinal chemistry applications.