Racemization of isobornyl chloride via carbocations: a non-classical look at a classic mechanism.

Henry S. Rzepa and Charlotte S. M. Allan

Interactive Table 1. B3LYP/cc-pVQZ calculated geometries for 3, 6, 8-exo, 8-endo and 9.a
Description: The Meerwein-Montefort(4) transition state 3.
Show: b
Show: c d
Relative ΔG:e 19.1; COI:f 10042/to-257
Description: The non-classical carbocation 6
Show: g
Relative ΔG:e 0.0; COI: 10042/to-373
Description: The Houben-Pfankuch transition state 8-exo
Relative ΔG:e 11.3, COI: 10042/to-256
Description: Stereoelectronically disfavoured 8-endo
Relative ΔG:e 19.6, COI: 10042/to-374
Description: Transition state 9
Relative ΔG:e 63.6, COI: 10042/to-375
Description: Crystal structure of 11

aTo view the interactive components of this table, you will need a Java-enabled Web browser, and appropriate security settings to view the model rendered by the Jmol Java applet. bThe two-electron-3-center region is shown in orange. In this representation, C2-C6 appear connected by a bond, but this is only by virtue of their short internuclear separation, and implies nothing about the electronic structure of the region. cFour bond critical points are shown as light blue atoms, the single ring critical point is shown in darker blue. dThe ELF function, at an isosurface threshold of 0.659. The trisynaptic basin is shown surrounding the transferring hydrogen, colored red. eThe energy, corrected for thermal and entropic contributions relative to molecule 6, in kcal mol-1. fThe persistent object identifier resolves to a digital repository entry. This contains all the archival information necessary for viewing the molecule in a separate program, and has details of the complete calculation itself. gThe transferring methyl carbon is shown in green, along with the dihedral angle subtended to the adjacent carbocationic centre for both it and the alternative (non-migrating) methyl group.

Literature cited

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  21. Arguably, the seminal experiment which does define the very dawn of mechanistic organic chemistry is Beckmann's proposal of a planar enolate replacing a chiral center as the key intermediate in the isomerisation of menthone to isomenthone under the influence of acid, using again the polarimeter as the analytical instrument; Beckmann, E. Annalen, 1889, 250, 322, DOI: 10.1002/jlac.18892500306.