Geometric Equivalents of Enantiomers in Studies of the Stereochemical Course of Substitution at Carbon. Electronic Effects in Nucleophilic Addition to Carbonyl Groups and to Carbocations. Virtual Proof of the Existence of ? Participation by Unstrained Carbon-Carbon Bonds

Article abstract of DOI:10.1021/ja00267a034

The argument is developed that 2,5-(or 1,4-) substituted adamantanes are the geometric equivalents of enatiomers in that stereorandom reactions at the 2-position would be revealed by a 50/50 product composition.Systematic and fairly large deviations from this expectation are encountered in nucleophilic additions to the carbonyl group of 5-substituted adamantanones; electron-withdrawing substituents favor syn approach, and electron-donating groups lead to anti approach.The product distribution correlates well with the strength of the induction: Δρ = -0.39.These findings lend strong support to Cieplak's view of electronic effects in asymmetric induction, which attributes these effects to preferential interaction of the newly developing ?* orbital with the electron-richest anti-periplanar bonds.The equivalence objective is achieved with 5-deuterio substitution, the isomers being distinguishable by 13C (and sometimes 2H) NMR spectroscopy.This probe is then applied to determine the stereochemistry of solvolysis of 2-adamantyl substrates; predominanat retention is found.Large 5-substituent effects are also described in the capture of both tertiary and secondary 2-adamantyl cations.The tertiary ions produce the same mixtures regardless of progenitor; the now very large deviations from 50/50 product distributions are again attributed to the Cieplak effect that for cations translates into ? participation.Thus, the 5-substituted tertiary 2-adamantyl cations constitute an exceedingly sensitive indicator of ? participation; no alternative interpretation is available.Secondary ions, generated from the alcohol and capture with Lucas reagent, give rise to products of largely retained configuration, more so for the Z than for the E isomers as expected.The retention in the secondary ions is almost certainly related to now stronger participation and hence more strongly ?-delocalized ions, but in this case, pyramidal cations or loose ion pairs may also contribute.