3× Axial vs 3× Equatorial: The ΔGGA Value Is a Robust Computational Measure of Substituent Steric Effects.
J Am Chem Soc
; 143(34): 13573-13578, 2021 09 01.
Article
em En
| MEDLINE
| ID: mdl-34465097
ABSTRACT
We define ΔGGA as the free energy change for the formal equilibrium [13]G-H + 1-X-adamantane â [13]G-X + adamantane, where [13]G-H is the C13H22 fragment of all-trans graphane with 3-fold symmetry. This compares with a situation where the group X is equatorial to three cyclohexane rings with one where it is axial to three rings. ΔGGA values vary from 2.9 (CN) to 145.7 kJ mol-1 (CCl3), and this wide range means that ΔG can be calculated with confidence. ΔGGA values for Me, Et, i-Pr, and t-Bu form a regular series, 34.9, 63.3, 101.6, and 142.0, and clearly reflect the steric size of the groups. We propose a model where the six axial hydrogens surrounding X on [13]G-X provide a nearly circular constriction on the substituent close to its point of attachment but which does not extend far above this. We compare these results with A values and with calculations on 2- and 7-substituted [1(2,3)4]pentamantanes. We show that electronic effects on ΔGGA values are negligible but that they correlate well with computed cone and solid angles subtended by the substituent.
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01-internacional
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MEDLINE
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En
Ano de publicação:
2021
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Article