Unraveling the Bürgi-Dunitz Angle with Precision: The Power of a Two-Dimensional Energy Decomposition Analysis

Understanding the geometrical preferences in chemical
reactions
is crucial for advancing the field of organic chemistry and improving
synthetic strategies. One such preference, the Bürgi-Dunitz
angle, is central to nucleophilic addition reactions involving carbonyl
groups. This study successfully employs a novel two-dimensional Distortion-Interaction/Activation-Strain
Model in combination with a two-dimensional Energy Decomposition Analysis
to investigate the origins of the Bürgi-Dunitz angle in the
addition reaction of CN– to (CH3)2CO. We constructed a 2D potential energy surface defined
by the distance between the nucleophile and carbonylic carbon atom
and by the attack angle, followed by an in-depth exploration of energy
components, including strain and interaction energy. Our analysis
reveals that the Bürgi-Dunitz angle emerges from a delicate
balance between two key factors: strain energy and interaction energy.
High strain energy, as a result of the carbonyl compound distorting
to avoid Pauli repulsion, is encountered at high angles, thus setting
the upper bound. On the other hand, interaction energy is shaped by
a dominant Pauli repulsion when the angles are lower. This work emphasizes
the value of the 2D Energy Decomposition Analysis as a refined tool,
offering both quantitative and qualitative insights into chemical
reactivity and selectivity.