Bürgi-Dunitz angle

File:Burgi-Dunitz-angle-2D.png

Diagram showing the Bürgi-Dunitz angle, where Nu is a nucleophile

File:Nucleophile-HOMO-carbonyl-LUMO-overlap-3D-balls.png

Molecular orbital diagram of the attack of a nucleophile on a carbonyl LUMO

The Bürgi-Dunitz angle describes the angle of attack of a nucleophile at a carbonyl center. The angle was named after H. B. Bürgi and J.D. Dunitz, its principal investigators.

The Bürgi-Dunitz angle depends not upon the nature of the nucleophile but, instead, is an intrinsic property of carbonyl centers. Originally described vaguely as 105 ± 5°, the angle has come to be recognized as 107°. This result, the product of SCF-LCAO-MO calculations, is quite close to the tetrahedral angle (109.5°) and supports the existence of a tetrahedral intermediate in such reactions.

The angle is a product of the overlap between the LUMO of the carbonyl function and HOMO of the nucleophile. The π*-orbital is perpendicular to the C=O bond, and optimally so, too, would be the angle of nucleophilic attack, which would produce the most energetically optimal overlap between its HOMO and the LUMO of the carbonyl center. However, the lone pairs of oxygen and the atom’s high electronegativity interact with that of the basic nucleophile force the angle of attack to 107°.

While it may appear inconsequential, the Bürgi-Dunitz angle was integral to the understanding the stereocontrol of hindered carbonyl centers and was an important factor in the perfection of the Cram-Felkin-Anh (Nguyen) model.