The Sharpless epoxidation of an allylic alcohol had a big impact on synthetic chemistry when it was introduced in the 1980s, and led the way for the discovery (design?) of many new asymmetric catalytic systems. Each achieves its chiral magic by control of the geometry at the transition state for the reaction, and the stabilizations (or destabilizations) that occur at that geometry.

The title of this post paraphrases E. J. Corey’s article in 1997 (DOI: 10.1016/S0040-4039(96)02248-4) which probed the origins of conformation restriction in aldehydes. The proposal was of (then) unusual hydrogen bonding between the O=C-H…F-B groups. Here I explore whether the NCI (non-covalent-interaction) method can be used to cast light on this famous example of how unusual interactions might mediate selectivity in organic reactions.

The Pirkle reagent is a 9-anthranyl derivative (X=OH, Y=CF ₃ ). The previous post on the topic had highlighted DIST1, the separation of the two hydrogen atoms shown below. The next question to ask is how general this feature is. Here we take a look at the distribution of lengths found in the Cambridge data base, and focus on another interesting example. 9-anthranyl derivatives.

Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion is partnered by a carbocation long ago entered the standard textbooks (see DOI 10.1021/ed800058c and 10.1021/jo100920e for background reading).

This molecule is not leaving me in peace. It and I first met in 1990 (DO: 10.1039/C39910000765), when we spotted the two unusual π-facial bonds formed when it forms a loose dimer.

The previous post set out a problem in conformational analysis. Here is my take, which includes an NCI (non-covalent interaction) display as discussed in another post. The lowest energies of the four diastereomers A-D , each in two conformations ( 1/2 ) were calculated at the ωB97D/6-311G(d,p)/SCRF=ethanol level, and are shown here relative to A1 (kcal/mol) as free energies.

NCI (non-covalent interactions) is the name of a fascinating new technique for identifying exactly these. Published recently by Johnson, Keinan, Mori-Snchez, Contreras-Garca, Cohen and Yang, it came to my attention at a conference to celebrate the 20th birthday of ELF when Julia Contreras-Garcia talked about the procedure.