Quantification of nonstatistical dynamics in an intramolecular Diels-Alder cyclization without trajectory computation

Area of Science:

• Organic Chemistry
• Chemical Dynamics
• Computational Chemistry

Background:

• Enyne-allenes are versatile organic molecules with complex reaction pathways.
• Understanding reaction dynamics, particularly non-Intramolecular Capture (non-IRC) pathways, is crucial for predicting chemical reactivity.
• The Schmittel reaction, a [2,3]-sigmatropic rearrangement of enynes, can proceed through various mechanisms.

Purpose of the Study:

• To investigate the reaction mechanism of aryl-substituted enyne-allenes.
• To determine if the Diels-Alder cyclization proceeds through a dynamic non-IRC pathway.
• To develop a protocol for estimating the fraction of molecules exhibiting nonstatistical dynamics.

Main Methods:

• Experimental investigations of enyne-allene reactions.
• Computational studies using Density Functional Theory (DFT) to model transition states and intermediates.
• Analysis of reaction pathways and energy barriers.

Main Results:

• Enyne-allenes with an aryl group at the allene terminus follow a dynamic non-IRC Diels-Alder cyclization.
• Two distinct reaction paths originate from separate C(2)-C(6) (Schmittel) transition states (TS).
• These paths converge to a common diradical intermediate without mixing, preserving initial transition state momentum.

Conclusions:

• The reaction proceeds through a dynamic, nonstatistical pathway where initial transition state momentum influences product formation.
• A protocol was developed to estimate nonstatistical dynamics by analyzing transition state partitioning and experimental ratios.
• The fraction of dynamically reacting molecules is primarily determined by the energy difference between the initial and subsequent transition states.