 | Acta Crystallographica Section D Acta Crystallographica Section D BIOLOGICAL CRYSTALLOGRAPHY |
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![[Figure 1]](dw5075fig1mag.jpg)
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Figure 1
Physical organic chemistry concepts regarding the trajectory of the approach of a nucleophile to an electrophilic center. (a) The Bürgi–Dunitz angle (αBD) describes the trajectory of approach of a nucleophile (the lysine N∊ atom in the TAL reaction) to an electrophilic center (the substrate carbonyl C atom in TAL). Atoms and groups of a carbonyl R1C(O)R2 are positioned on a blue-grey plane and an approaching N atom is poised above. The αBD is estimated as ∠N∊⋯C=O (arrow). (b) The αBD originates in HOMO–LUMO overlap. After Bürgi, Lehn et al. (1974  ) and Fleming (2010), a side-on cartoon view of the approach of the two lobes of the p-type highest occupied molecular orbital (HOMO) of nitrogen (green) is shown at three hypothetical positions (αBD = 135, 90 and 45°). Poses are relative to the four lobes of the antibonding π*-type lowest unoccupied molecular orbital (LUMO) of formaldehyde, H2C=O, the simplest carbonyl electrophile (standard atom colors). Per current theory, carbinolamine C—N and C—O single bonds result when the nitrogen trajectory follows an obtuse value of αBD, allowing HOMO–LUMO overlap by wavefunctions (lobes) of like sign (shading) such that this attractive potential overcomes other repulsive potentials (Bürgi, Dunitz et al., 1974; Fleming, 2010). An approximate optimum computed for addition to formaldehyde of the simplest nucleophile, hydride anion (Hψ), is indicated by the orange dashed line (αBD ≃ 107°, SCF calculations; Bürgi et al., 1973). |
![[Figure 1]](dw5075fig1.jpg)
| BIOLOGICAL CRYSTALLOGRAPHY |
ISSN: 1399-0047
