Thermal behavior of [2.1.1]propellane: a DFT/ab initio study

Area of Science:

• Computational chemistry
• Quantum chemistry
• Organic chemistry

Background:

• [2.1.1]propellane is a strained bicyclic hydrocarbon.
• Understanding its thermal isomerization is crucial for predicting its reactivity.

Purpose of the Study:

• To investigate the low-energy reaction pathways for the thermal isomerization of [2.1.1]propellane.
• To determine the activation barrier for the most favorable isomerization route.

Main Methods:

• Density functional theory (DFT) calculations.
• Ab initio molecular orbital calculations, including CAS(12,12)PT2N/6-31G(d), QCISD(T)/6-311G(d,p)//QCISD/6-31G(d), and B3PW91/6-311G(d,p)//B3PW91/6-311G(d,p) levels of theory.

Main Results:

• Three potential reaction modes were analyzed: ring opening to 1,2-dimethylenecyclobutane, four-membered ring opening to 1,3-dimethylenecyclobutane, and central bond/side bond cleavage to a carbene.
• The pathway involving central bond and side bond cleavage to form carbene 17 was found to have the lowest activation energy.
• The activation barrier for the isomerization of [2.1.1]propellane to carbene 17 was calculated to be 29 kcal/mol.
• The carbene intermediate (17) is stabilized by hydrogen migration, leading to dienes (18 and 19).

Conclusions:

• The thermal isomerization of [2.1.1]propellane proceeds primarily through a carbene intermediate.
• The calculated activation barrier provides insight into the kinetic stability of [2.1.1]propellane.
• Further stabilization of the carbene via hydrogen migration influences the final product distribution.