Bürgi-Dunitz trajectory
The Bürgi-Dunitz trajectory (also Bürgi-Dunitz angle ; named after Hans Beat Bürgi and Jack D. Dunitz ) describes the angle of attack of a nucleophile on an electrophilic carbonyl group , which is about 107 °. At the same time, the π bond between carbon and oxygen is broken, while instead a σ bond is formed between the carbon and the nucleophile.
According to the molecular orbital theory here is an overlap of the highest occupied molecular orbital must ( H ighest O ccupied M olecular O rbital) HOMO of the nucleophile, often a free electron pair, with the lowest unoccupied molecular orbital ( L owest U noccupied M olecular O rbital) LUMO of the electrophile, in in this case the π * bond of the carbonyl function (the antibonding orbital of the CO double bond) takes place. The nucleophile attacks linearly, that is, the angle of attack depends on the electrophile . The double bond of the carbonyl group consists of a σ-bond and an energetically higher π-bond (see picture), only the π-bond is important here. The higher the energy level of a binding molecular orbital, the lower the energy level of the associated antibonding molecular orbital, which is why the attack takes place at the π bond, the antibonding π * is the LUMO.
Since oxygen is more electronegative than carbon, its bonding π orbital is larger than that of carbon. For reasons of symmetry, the opposite is true for antibonding orbitals. Thus, the π * orbital (the LUMO) at carbon has the largest coefficient, the attack of the HOMO of the nucleophile takes place there.
The p orbitals that form the π * bond are perpendicular to the plane of the C = O function. The optimal angle of attack for the nucleophile would therefore be 90 ° to the plane of the carbonyl bond, since its LUMO and the HOMO of the nucleophile would then optimally overlap. However, since oxygen has two free electron pairs and a significantly higher electronegativity than carbon, the electron density is greatest at oxygen, and the nucleophile carrying a free electron pair is therefore repelled by the oxygen.
The angle of 107 ° and the Bürgi-Dunitz trajectory is therefore a compromise between the angle for the optimal possible overlap of HOMO and LUMO (90 °) and the repulsive Coulomb force between the high electron density on oxygen and the HOMO of the nucleophile.
literature
- HB Bürgi, JD Dunitz, E. Shefter: In "Geometrical Reaction Coordinates. II. Nucleophilic Addition to a Carbonyl Group". J. Am. Chem. Soc. 95, 1973, 5065-5067.
- HB Bürgi, JD Dunitz, JM Lehn, G. Wipff: In "Stereochemistry of reaction paths at carbonyl centers". Tetrahedron 30, 1974, 1563-1572. doi : 10.1016 / S0040-4020 (01) 90678-7