The Burgi-Dunitz Trajectory

  • Overview: Describes the geometric angle at which a nucleophile approaches an electrophile during bond formation.
  • Relevance: Influences stereoelectronic effects in reactions like nucleophilic addition to carbonyl compounds.

The Bürgi-Dunitz trajectory is a well-studied concept in organic chemistry that describes the angle of attack by a nucleophile on a carbonyl carbon. This angle is typically around 107°, measured between the trajectory of the nucleophile and the plane of the carbonyl group. This specific angle minimizes steric hindrance while maximizing overlap between the nucleophile's highest occupied molecular orbital (HOMO) and the carbonyl carbon's lowest unoccupied molecular orbital (LUMO), facilitating an efficient reaction.

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Traditionally, the BD angle has been rationalized as the result of a compromise between maximum orbital overlap for the stabilizing HOMO(nucleophile)-LUMO(C=O) (i.e. θ = 90°) and minimum repulsion with the electron density in the carbonyl π-bond for θ > 90°.

More recently, Activation Strain Model (ASM) and Energy Decomposition Analysis (EDA) methods allowed for a detailed breakdown of the forces influencing the BD angle, such as strain energy, interaction energy, Pauli repulsion, and orbital contributions. For cyanide reacting with acetone, studied across various attack angles ranging from 90° to 140°, indicated that the optimal BD angle of 111.6° corresponds to the transition state with the lowest energy.

The role of Pauli repulsion in shaping the BD trajectory. By minimizing the overlap between the nucleophile's highest occupied molecular orbital (HOMO) and the carbonyl π-bond, the obtuse approach reduces destabilizing repulsion. This insight challenges previous assumptions that emphasized orbital overlap as the dominant factor. Furthermore, enhanced interactions between the nucleophile HOMO and carbonyl π*-LUMO were observed, favoring the BD trajectory. Electrostatic interactions also contributed to the stabilization but to a lesser extent.

Rodríguez, H. A., Bickelhaupt, F. M., & Fernández, I. (2023).
Origin of the Bürgi‐Dunitz Angle. ChemPhysChem, 24, e202300379.