Journal of the American Chemical Society p. 1598 - 1605 (1986)
Update date:2022-07-30
Topics:
Cheung, C. K.
Tseng, L. T
Lin, M.-H.
Srivastava, S.
Noble, W. J.
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.
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